Endohedral Fullerenes - ACS Publications - American Chemical Society

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Endohedral Fullerenes Alexey A. Popov,*,† Shangfeng Yang,*,‡ and Lothar Dunsch*,† †

Department of Electrochemistry and Conducting Polymers, Leibniz-Institute for Solid State and Materials Research (IFW) Dresden, D-01171 Dresden, Germany ‡ Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion & Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China 4.3.3. Methano-clusterfullerene 4.3.4. Oxide Clusterfullerenes 4.3.5. Sulfide Clusterfullerenes 4.3.6. Cyano-clusterfullerenes 4.4. Noble Gas and Nonmetal Endohedral Fullerenes 5. Theoretical Studies of Electronic and Molecular Structure of EMFs 5.1. Chemical Bonding in EMFs 5.1.1. Metal-Cage Bonding 5.1.2. Intracluster Interactions in Clusterfullerenes 5.1.3. Metal−Metal Bonding in EMFs 5.2. Isomerism in Endohedral Metallofullerenes: Stability of the Charged Carbon Cage 5.3. Violation of the Isolated Pentagon Rule (IPR) in Endohedral Fullerenes 5.4. Cage Form Factor 5.5. Gibbs Energy Considerations 6. Spectroscopic Properties and Electronic Structure of EMFs 6.1. NMR Spectroscopy 6.1.1. 13C NMR Spectroscopy 6.1.2. Multinuclear NMR Spectroscopy 6.2. ESR Spectroscopy 6.2.1. Monometallofullerenes M@C82 (M = Sc, Y, La) 6.2.2. Nonlanthanide Monometallofullerenes with Other Cages 6.2.3. Rare-Earth Monometallofullerenes 6.2.4. Carbide Clusterfullerenes Sc3C2@C80 and M2C2@C82 (M = Sc, Y) 6.2.5. Dimetallofullerenes 6.2.6. Dimetallic Endohedral Heterofullerenes M2@C79N 6.2.7. Nitride Clusterfullerenes (NCFs) 6.2.8. Radical Ions of Oxide Clusterfullerene Sc4O2@C80 6.3. Vibrational Spectroscopy 6.3.1. Mono- and Dimetallofullerenes 6.3.2. Nitride Clusterfullerenes (NCFs) 6.3.3. Carbide Clusterfullerenes 6.3.4. Other Clusterfullerenes 6.4. UV−vis-NIR Absorption Spectroscopy 6.4.1. Carbon Cage Isomerism and Charge State

CONTENTS 1. Introduction 2. Synthesis of Endohedral Fullerenes 2.1. Synthesis of the Conventional Endohedral Metallofullerenes (EMFs) 2.2. Synthesis of Nitride Clusterfullerenes (NCFs) 2.2.1. The Trimetallic Nitride Template (TNT) Process 2.2.2. The Reactive Gas Atmosphere Route 2.2.3. The Solid Nitrogen Source 2.2.4. Chemically Adjusting Plasma Temperature, Energy, and Reactivity (CAPTEAR) Method 2.3. Ion Bombardment 2.4. High Pressure Method 2.5. Other Methods 3. Solubility, Extraction, and Separation of Endohedral Fullerenes 3.1. Solubility and Extraction of Endohedral Fullerenes 3.2. Separation by Sublimation 3.3. Chromatographic Separation 3.4. Extended Separation by Recycling HPLC 3.5. Separation by Chemical and Electrochemical Methods 4. Molecular Structures of EMFs 4.1. Introductory Notes 4.2. Classical EMFs 4.2.1. Monometallofullerenes 4.2.2. Dimetallofullerenes 4.2.3. Trimetallofullerenes 4.3. Clusterfullerenes 4.3.1. Nitride Clusterfullerenes (NCFs) 4.3.2. Carbide Clusterfullerenes © 2013 American Chemical Society

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Received: July 24, 2012 Published: May 2, 2013 5989

dx.doi.org/10.1021/cr300297r | Chem. Rev. 2013, 113, 5989−6113

Chemical Reviews 6.4.2. Influence of the Metal 6.4.3. Optical Gaps 6.4.4. Molar Absorptivity 6.5. Photophysical Properties of Endohedral Fullerenes 6.5.1. Luminescence Spectroscopy of Endohedral Er Fullerenes 6.5.2. Photoexcited Charge and Energy Transfer in EMF-Based Donor−Acceptor Systems 6.5.3. Charge Carrier Mobility in Solid EMFs 6.5.4. Nonlinear Optic Properties, Singlet Oxygen Generation, and Plasmon Excitations 6.6. High-Energy Spectroscopy 6.6.1. Lanthanide-Based M@C82 6.6.2. Nonlanthanide M@C82 6.6.3. Monometallofullerenes M@C60 6.6.4. Dimetallofullerenes 6.6.5. Nitride Clusterfullerenes (NCFs) 6.6.6. Carbide Clusterfullerenes 6.7. Electron Microscopy of Endohedral Fullerenes 6.7.1. STM/STS and Related Techniques 6.7.2. HRTEM Studies of EMFs 7. Electrochemistry and Spectroelectrochemistry of Endohedral Fullerenes 7.1. Monometallofullerenes 7.1.1. Pristine Monometallofullerenes 7.1.2. Derivatives of Monometallofullerenes 7.2. Dimetallofullerenes 7.2.1. Pristine Dimetallofullerenes 7.2.2. Derivatives of Dimetallofullerenes 7.3. Nitride Clusterfullerenes (NCFs) 7.3.1. M3N@C80 7.3.2. NCFs with Other Carbon Cages 7.3.3. Derivatives of NCFs 7.3.4. Irreversibility of the Reduction of NCFs 7.4. Carbide Clusterfullerenes 7.5. Sulfide, Oxide, and Cyano Clusterfullerenes 7.6. Endohedral Electrochemistry 7.7. Gas-Phase Electron Affinity of EMFs 8. Chemical Properties of Endohedral Fullerenes 8.1. Conventional Endohedral Fullerenes 8.1.1. Photochemical Disilylation and Carbene Addition Reactions 8.1.2. Cycloaddition Reactions 8.1.3. Radical Addition Reactions 8.1.4. Water-Soluble Derivatives 8.1.5. Supramolecular Complexes of EMFs with Macrocyclic Compounds 8.2. Heterofullerenes M2@C79N (M = Y, Gd) 8.3. Clusterfullerenes 8.3.1. Diels−Alder Reactions 8.3.2. Prato Reactions 8.3.3. Bingel−Hirsch Reactions 8.3.4. Radical Addition Reactions 8.3.5. [2 + 2] Cycloaddition Reactions 8.3.6. Photochemical Reactions 8.3.7. Azide Addition to Sc3N@C80 8.3.8. Organometallic Complexation of Sc2C2@ C82 8.3.9. Water-Soluble Derivatives

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8.3.10. Supramolecular Complexes of Nitride Cluster Fullerenes 9. Magnetic Properties of Endohedral Fullerenes 9.1. SQUID Studies of Conventional Metallofullerenes 9.2. XMCD Studies of Conventional Metallofullerenes 9.3. Nitride Clusterfullerenes 9.4. Single Molecular Magnetism in Endohedral Fullerenes 10. Potential Applications of Endohedral Fullerenes 10.1. Biomedical Applications 10.1.1. MRI Contrast Agents 10.1.2. X-ray Contrast Agents 10.1.3. Radiotracers and Radiopharmaceuticals 10.1.4. Antitumor Activity of [Gd@C82(OH)22]n Nanoparticles 10.1.5. Antimicrobal Activity of Sc3N@C80Polymer Film 10.2. Applications in Organic Photovoltaics 10.2.1. Endohedral Fullerenes As New Acceptors in PSCs 10.2.2. Endohedral Fullerene-Based Donor− Acceptor Dyads 10.2.3. Photoelectrochemistry (PEC) Cells Based on Endohedral Fullerenes 10.3. Endohedral Fullerene Peapods 10.4. Other Potential Applications 11. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References Note Added in Proof

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1. INTRODUCTION One of the attractive properties of the hollow carbon clusters, known as fullerenes, is the possibility to use them as robust containers for other species. Although an IUPAC recommendation proposed the term “incar fullerene” for fullerenes encapsulating atoms, ions, or molecules, the specification “endohedral” is nowadays used throughout for fullerenes with species incorporated in the carbon cage. This term originates from a combination of Greek words (“endon” − within), and (“hedra” − face of a geometrical figure). In application to fullerenes encapsulating other species, the term “endohedral” was introduced in 1991 by Cioslowski1 and independently by Schwarz and Krätschmer.2 Anticipated first in 1985,3 endohedral metallofullerenes (EMFs) attracted a booming increase of attention in the first decade after the discovery of macroscopic fullerene production in 1990.4 Many metal atoms were put inside fullerenes during this period, many new EMF molecules were reported, and thus the basis for the further advance in the field was grounded. However, the yields of EMFs in the 1990s were still rather low (on the order of 1% of the empty fullerenes), and there was virtually no control over the molecular composition of the products (other than the choice of what metal was used). Extended HPLC protocols required for isolation of composi-

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delivered invaluable information about the structure and dynamics of many EMF molecules. Extended knowledge on the molecular structures of EMFs served as a basis for advanced computational studies, which allowed the formulation of the general principles governing the stability of EMFs and carbon cage isomerism (section 5). Nowadays, the structures of new EMFs can be routinely predicted from the first principles (knowing only a formula of the molecule) with the high reliability rivaling that of singlecrystal X-ray diffraction studies. The field of chemical derivatization of EMFs has flourished in the past decade (section 8). Many cyclo- as well as radical addition reactions of EMFs are described forming a basis for the targeted synthesis of EMF-based functional materials. The applications for EMFs as MRI contrasting agents and as electron-accepting blocks in photovoltaic devices are now considered as the most promising. Importantly, the reactivity and addition patterns of EMFs are significantly different from those of empty fullerenes. Numerous electrochemical studies of EMFs showed the large diversity in their redox behavior (section 7). Many clusterfullerenes exhibit electrochemically irreversible but chemically reversible reductions, which points to the reversible structural transformations following the charge transfer, but complete description of the charge transfer mechanism of clusterfullerenes still remains a challenge.18,19 Besides, the unique ability of the carbon cages to stabilize unusual metal clusters can be further extended to manipulation of the electronic and spin states of the endohedral species by means of endohedral redox reactions, which is the main subject of endohedral electrochemistry (section 7.6).20 In this work, we provide a detailed, consistent overview of the different aspects of the endohedral fullerene research focusing primarily on the development of the field after 2000. A state of the art in the EMF research at the moment of 2000 was described in a fundamental contribution by Shinohara in 200021 and in a monograph edited by Akasaka and Nagase published in 2002.22 Plenty of more or less specialized reviews and book chapters devoted to EMFs were published in the past decade, and here we mention only a few of them. The general progress in the field (often with particular author’s accounts) was discussed in refs 23−29. Nitride clusterfullerenes were a subject of a detailed review by Yang and Dunsch in 20076 and by Echegoyen et al. in 2009,26,30 oxide clusterfullerenes were reviewed by Stevenson in 2011,31 whereas new advances in the field of clusterfullerenes were discussed in the ref 32. Structural aspects of EMFs were reviewed in refs 33−36. Analyses of the computational studies, electronic structure, and bonding in EMFs were given in refs 35, 37−42. Chemical properties of EMFs were discussed in refs 6, 23, 26, 33, 43 and were a subject of a dedicated review by Lu, Akasaka and Nagase in 2011.44 Electrochemical behavior of EMFs was described in refs 45 and 46, and the basic account on the endohedral electrochemistry appeared in 2011.20 Finally, applications of EMFs in organic photovoltaic47 and especially their biomedical applications were extensively reviewed.48−55

tionally and isomerically pure EMFs were under development, and availability of pure EMF samples was very limited. A deeper understanding of the principles governing EMFs production slowly developed, and in the last 15 years the field of EMF research has advanced into a new level. In 1999 it was found that the presence of small amounts of nitrogen gas in the arc-burning reactor resulted in metal-nitride cluster fullerenes (NCFs) with the composition M3N@C2n (M = Sc, Y, Gd−Lu; 2n = 68−96).5,6 Importantly, the yields of NCFs were much higher than those of conventional EMFs, and they exhibited improved kinetic stability due to larger HOMO−LUMO gaps. Sc3N@C80 is de facto the most abundant fullerene after C60 and C70. The discovery of NCFs was soon followed by the elucidation of the molecular structure of the first carbide clusterfullerene, [email protected] This finding emphasized an ambiguity of the structural assignments of polymetallofullerenes, and the high abundance of carbide clusterfullerenes was then established after extended structural studies of a series of EMFs, many of which were initially thought to be dimetallofullerenes. In the past few years, new types of EMFs with methano-,8 oxide-,9 cyano-,10 and sulfide-metal clusters11 were discovered, showing the versatility of the EMFs in stabilizing unusual species that cannot exist outside the carbon cage. Significant progress was achieved in the synthetic and separation procedures of EMFs. Judicious choice of the nitrogen source resulted in the invention of the procedures, allowing suppression of empty fullerene formation and leaving NCFs as the major fullerene products of the synthesis (section 2.2).12 Separation techniques based on the different reactivity and electron-accepting properties of empty fullerenes and EMFs were developed (section 3),13−15 allowing the use of nonchromatographic approaches to simplify EMF separation. Advanced synthetic approaches and the progress in separation techniques dramatically improved the situation with availability of the EMF samples, which resulted in more dedicated and detailed studies of their structural, electronic, physical, and chemical properties. In the 1990s the field of the EMFs remained in the shadow of the empty fullerenes, which often resulted in the blind transfer of the guidelines, structural and chemical properties revealed for the empty fullerenes onto EMFs. Accumulation of the evidence of the lawbreaking behavior of EMFs resulted in the understanding that the structural, chemical, and electronic properties of EMFs are different from those of empty fullerenes, which is largely attributed to the electron transfer from the metal atom(s) to the carbon cage. For instance, the isomeric structures of the carbon cages in EMFs are different from those of empty fullerenes. Moreover, strong interaction with the metal and metal-to-cage electron transfer can stabilize pentagon adjacencies in the fullerene cage. As a result, the famous Isolated Pentagon Rule, which is very strict for nonderivatized empty fullerenes, is invalidated for EMFs. In 2000, two independent studies showed that EMFs with the cage violating IPR can exist,16,17 and a number of non-IPR EMF structures was described in the following years (see section 5.3). Enormous progress was achieved in the structural studies of EMFs (section 4). Chemical derivatization and cocrystallization techniques circumvented the problem of rotation of the fullerene molecules in the crystal, which earlier precluded the use of single-crystal X-ray diffraction for the structure elucidation. Conventional and paramagnetic NMR also

2. SYNTHESIS OF ENDOHEDRAL FULLERENES Similar to the production of empty fullerenes involving the generation of a carbon-rich vapor or plasma in He or Ar inert gas atmosphere, three strategies have been developed to date for preparing macroscopic amounts of endohedral fullerenes. This includes vaporization of graphite (laser ablation,3,56−59 arc 5991


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discharge,60−62 resistive heating,63 radiofrequency furnace,64 and plasma-torch65), implantation of the atoms through the walls of the already existing carbon cages (ion bombardment,66−68 high pressure treatment,69−73 explosion-based implantation74) and chemical routes via opening the orifices in the fullerenes.75−77 In particular, the standard DC arc discharge methods are the most popular ones routinely used to date.

For the DC arc-discharge method, the yield of EMFs varies sensitively with He gas pressure during the arc synthesis. An optimum He pressure for EMFs synthesis depends on arc conditions (the size of a composite rod, DC current, the arc gap of the two electrodes, etc.), and the size of the chamber as well, and is normally close to the He pressure used for the synthesis of higher empty fullerenes such as C82 and C84.89 In addition to the high-temperature heat treatment, the “in situ activation”90 and “back-burning”86 techniques are also found to be crucial to an efficient synthesis of EMFs. An enhancement of the yield of EMFs was reported by Mieno et al. under gravitation-free arc discharge conditions as compared with the normal gravitational condition because the gravitation-free conditions suppress thermal convection of hot gas in the arc region and thus enable a long-duration high temperature reaction of carbon clusters suited to metallofullerene production, but the efforts for this synthesis are out of standard lab conditions.91 Particularly, for some air (moisture)-sensitive EMFs, in order to avoid their degradation during the soot handling, Shinohara et al. developed a modified DC arc-discharge apparatus consisting of a synthesis chamber and a collection chamber, equipped with an anaerobic sampling and collection mechanism of raw EMF-containing soot.92 The DC arc-discharge method is simple and cost-effective. However, it is a rather chaotic and violent process which does not allow taking in situ probes of the formed products. Hence, it is difficult to study the formation mechanism of EMFs in DC arc-discharge experimentally. Moreover, the standard arc synthesis of endohedral fullerenes described in the past resulted in very low yields. Generally EMFs are found in a yield of 2% or less in the fullerene soot.21 For instance, Figure 2 illustrates the

2.1. Synthesis of the Conventional Endohedral Metallofullerenes (EMFs)

The first 10 years of studies on the synthesis of conventional endohedral metallofullerenes (EMFs) have been reviewed in ref 21. In brief, laser ablation was the first method which indicated that EMFs might exist.3 Typically, a target composite rod or disc composed of metal-oxide/graphite mixture with a highstrength pitch binder is placed in a furnace at 1200 °C.78 A frequency-doubled Nd:YAG laser at 532 nm is focused onto the target rod in an Ar gas flow. EMFs and empty fullerenes are produced by the laser vaporization and then flow through the tube with the Ar gas carrier to be finally trapped on the quartz tube wall near the end of the furnace.56,79−81 The laser ablation method is suited to the study of the growth mechanism of fullerenes and EMFs. However, the apparatus including the laser source is very expensive, and the EMF synthesis rate is very low. The use of this method for the bulk production of EMFs is thus not feasible. It was the contact arc method proposed by Krätschmer and Huffman that made macroscopic amounts of EMFs available since the late 1991.4,60−62 Figure 1 shows an example of the

Figure 2. HPLC profile of a carbon disulfide extract of the soot containing dysprosium metallofullerenes with a 5PYE column. Regions A, B, C, and D contain dysprosium metallofullerenes. Reproduced with permission from ref 93. Copyright 2000 American Chemical Society.

Figure 1. Schematic view of a Krätschmer−Huffman generator used at IFW-Dresden, which is the representative DC arc discharge apparatus used in many groups nowadays.

HPLC profile of a carbon disulfide extract of the soot containing dysprosium metallofullerenes, indicating that the yield of the dysprosium metallofullerenes (e.g., Dy@C82 (I) in region A) is much lower than that of empty fullerenes C60 and C70.93

Krätschmer-Huffman generator which is the most popular DC arc discharge apparatus for the large-scale synthesis of EMFs.82−86 Metal-oxide (or metal)/graphite composite rods are used as positive electrodes (anodes), which are first subject to a high-temperature (above ca. 1600 °C) pretreatment.87,88 At such high temperatures, various metal carbides can be formed in the composite rods, which is crucial for the efficient production of EMFs since uniformly dispersed metal atoms or ions in a composite rod give EMFs in a higher yield. The rods are then arced in the direct current (300−500 A) spark mode under conditions of the 50−100 Torr flow of He used as a cooling gas, and the soot thus produced is collected for further handling.

2.2. Synthesis of Nitride Clusterfullerenes (NCFs)

2.2.1. The Trimetallic Nitride Template (TNT) Process. The new world of nitride clusterfullerenes (NCFs) was introduced by a synthesis which occurred by chance. While it was accepted in the fullerene community to avoid nitrogen as a cooling gas, this element was the key for a new type of clusterfullerene. The first class of novel endohedral fullerenes with encapsulated metal/nonmetal cluster was introduced as the trimetallic nitride endohedral fullerenes, the Sc3N@C80 5992


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being the first and most abundant member.5 This structure was discovered in 1999 at a yield 3−5%, significantly higher than that of all other endohedral fullerenes, when a small portion of nitrogen gas was introduced into the Krätschmer-Huffman generator used for vaporization of graphite rods containing metal oxides.5 This method was named the “trimetallic nitride template” (TNT) process as proposed by Dorn et al.5 On the basis of the TNT method, several nitride clusterfullerenes were synthesized, such as ErxSc3−xN@C80 (x = 0−3)5,94,95 and AxSc3−xN@C68 (x = 0−2; A = Tm, Er, Gd, Ho, La).16 In the latter case, the Sc3N cluster is trapped in a non-IPR C 68 -D 3 (6140) cage, as determined by X-ray crystallography and NMR spectroscopy.16,96 It makes this fullerene particularly interesting as will be discussed further below. Furthermore, the following nitride cluster structures have been also synthesized by the TNT method over a next decade: Sc3N@C78,97 Lu3N@C80,98 Lu3−xAxN@C80 (x = 0−2; A = Gd, Ho),98 Y3N@C2n (2n = 80−88),99,100 Tb3N@C2n (2n = 80, 84, 86, 88),101,102 CeSc2N@C80,103 Gd3N@C2n (2n = 78−88).104−108 2.2.2. The Reactive Gas Atmosphere Route. The standard arc synthesis of endohedral fullerenes described in the past resulted in very low relative yields. Generally EMFs are found in a yield of 2% or less in the fullerene soot and are surpassed by 10-fold higher amounts of empty fullerenes.21 The first description of the Sc3N@C80 fullerene formation by the TNT process claimed to get a higher yield of the cluster fullerene in the soot extract ranging from 3 to 5%.5 While in the original work air traces were used as a source of nitrogen (replaced then by molecular nitrogen), an improved route for nitride cluster fullerene has to be searched based on other selective nitrogen sources. The breakthrough was achieved by the development of the reactive gas atmosphere method in Dresden group.12,109 By introducing NH3 as the reactive gas into the Krätschmer− Huffman generator of local design, the NCFs were produced for the first time as the main fullerene products in the soot, while the relative yield of the empty fullerene and conventional metallofullerenes was less than 5%.110−112 For instance, the Dy3N@C2n (39 ≤ n ≤ 44) clusterfullerenes were synthesized with the relative yields reaching up to 98% of all of the fullerenes, overwhelmingly higher than those of the empty fullerenes (∼ 2%) (see Figure 3).111 This way it was been demonstrated for the first time that the endohedral fullerenes can be the main products in the fullerene soot mixture. This dramatically facilitates the isolation of EMFs since only one separation step even by a simple chromatographic technique is often sufficient. In applying this synthesis method, a prerequisite for a successful application of endohedral fullerenes is fulfilled. On the basis of the preferential production of the NCFs, several large families of clusterfullerenes M3N@C2n (M = Ho, Tb, Gd, Dy, Tm; 38 ≤ n ≤ 44), and specifically the clusterfullerenes with cages larger than C80, were isolated by our group.111−114 It was important that several new nitride cluster structures have been synthesized by the reactive gas atmosphere route including GdxSc3−xN@C80 (I, II, x = 0− 2),115,116 DySc2N@C76,117 MSc2N@C68 (M = Dy, Lu),118 Lu2ScN@C68,118 LuxY3−xN@C80 (I, x = 0−2),119 and the first EMF with a non-IPR C70 cage, [email protected] The reactive gas atmosphere method was also used in the group of Echegoyen to produce NCFs with large lanthanide atoms, resulting in the families of M3N@C2n clusterfullerenes (M = La, Ce, Pr, Nd; 2n

Figure 3. Chromatogram of a Dy 3 N@C 2n fullerene extract (combination of two 4.6 × 250 mm Buckyprep columns; flow rate 1.6 mL/min; injection volume 500 μL; toluene as eluent; 40 °C). The peak around 5 min is due to the hydrocarbon byproducts. (a) Clusterfullerenes (Dy3N@C2n, 45 ≤ n ≤ 49); (b) dimetallofullerenes (Dy2@C2n, 45 ≤ n ≤ 57); (c) trimetallofullerenes (Dy3@C2n, 40 ≤ n ≤ 50). Reproduced with permission from ref 111. Copyright 2005 American Chemical Society.

reaches up to 96 for isolable clusterfullerenes and 110 for clusterfullerenes observed only in the mass spectra).46,121,122 Following the success of using NH3 as the reactive gas for the synthesis of NCFs, other reactive gases have been employed in the arc-discharge synthesis of EMFs as well. Methane was used as a reactive gas afforded the first isolation of Sc3CH@C80 in our group,8 while the use of SO2 in the group of Echegoyen resulted in the alternative synthesis of sulfide clusterfullerenes, Sc2S@C2n (2n = 40−50).123 Interestingly, the reactive gas atmosphere method was also found useful for the synthesis of nonclassical empty fullerenes: addition of CCl4 into the reactor leads to formation of a series of chlorofullerenes C2nClx (2n = 50, 54, 56, 60, 64, 66; x = 4−12)124−128 with non-IPR carbon cages, while non-IPR C64H4 was formed with the use of CH4.129 2.2.3. The Solid Nitrogen Source. Historically, in a first attempt to modify the synthesis of NCFs, a solid nitrogen containing compound which does not contain oxygen was applied in 2004.12 The choice was calcium-cyanamide (CaNCN) which was added to the metal oxide/graphite powder mixture. For all synthesis procedures a carbon/metal ratio of 12.5 was used for the production of the fullerene soot. The addition of calcium-cyanamide to the graphite mixture in the same extent as that of the metal content was applied for clusterfullerene synthesis. The reaction scheme was proposed for the solid state reaction, including formation of gaseous nitrogen in the side process.12 This way, the Sc3N@C80 clusterfullerene was synthesized with an enhanced relative yield and a selectivity ranging from 3 to 42%.12 Besides Sc3N@C80, another NCF, Sc3N@C78, was produced in a sufficient amount to be separated by HPLC. The fullerenes C60 and C70 which were up till that moment the main component of the soot in all arc burning processes appeared to be the low-content byproducts of the reaction. It was obvious that the new nitrogen source causes a strong increase of the selectivity of the endohedral NCFs. The disadvantage of the new fullerene production was the low reproducibility of the 5993


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C70, N2@C70, and P@C60 were also synthesized using this ion implantation method.137,140,141 Ion bombardment was also used to implant alkali metal ions (especially Li+) into C60 by Campbell et al.67,68,142,143 The synthesis however suffered from the difficulties with the characterization of the final products, whose identity could not be properly proved. Recently Li@C60 produced by ion bombardment was isolated by Sawa, Tobita and co-workers in a form of its cationic salt, [Li+@C60](SbCl6).144

fullerene yield. The reason for the variation of the selectivity is not clear, but traces of water and/or hydrocarbons were expected to influence the yield strongly. The concept of using the solid nitrogen-containing compound for the synthesis of nitride clusterfullerenes was further developed recently as the new “selective organic solid (SOS)” route by Yang and Dunsch et al.130 Using two different guanidinium salts (guanidium thiocyanate (CH5N3·HSCN) and guanidinium hydrochloride (CH5N3·HCl)) as the nitrogen-rich source, a series of nitride clusterfullerenes based on different metals such as Dy, Sc, Y, Gd, Lu, and mixed metals Sc/Dy, Sc/Gd, Sc/Lu, and Lu/Ce were synthesized, and their yields were found to be comparable to those in the “reactive gas atmosphere” route. Thus the “SOS” route appears to be a general route for the synthesis of NCFs, which promises both high selectivity of NCFs and high reproducibility of the fullerene yield. Recently, two other nitrogen-containing organic compounds, urea131 and melamine,132 were successfully used for selective synthesis of NCFs. Importantly, since the solid source of guanidium thiocyanate (CH5N3·HSCN) contains sulfur as well, a new class of endohedral sulfide clusterfullerenes, M2S@C82 (M = Sc, Y, Dy, and Lu), was simultaneously synthesized along with the corresponding nitride clusterfullerenes.11 This result suggests that the formation of clusterfullerene structure is a practical and beneficial way for the synthesis of new endohedral fullerenes. 2.2.4. Chemically Adjusting Plasma Temperature, Energy, and Reactivity (CAPTEAR) Method. In order to selectively synthesize NCFs without compromising their yield and to study the optimal formation parameters of NCFs, in 2007 Stevenson et al. developed a new method titled “Chemically Adjusting Plasma Temperature, Energy, and Reactivity” (CAPTEAR).133 The CAPTEAR concept is based on the hypothesis that different types of carbonaceous nanomaterials (e.g., nanotubes, empty-cage fullerenes, NCFs) have preferred temperatures of formation and stability in a given chemically reactive plasma.133 The CAPTEAR process with Cu(NO3)2·2.5H2O uses an exothermic nitrate moiety to suppress empty-cage fullerene formation, whereas Cu functions as a catalyst additive to offset the reactive plasma environment and boost the Sc3N@C80 NCF production.134 In this method, the introduction of copper nitrate hydrate forms NOx vapor generating solid reagents, air, and combustion, and in this way the temperature, energy, and reactivity of the plasma environment could be tuned. With the extent of temperature, energy, and reactive environment stoichiometrically varied, the optimal conditions for selective NCF synthesis has been achieved.133 The results indicate that the percentages of C60 and Sc3N@C80 are inversely related, whereas the percentages of C70 and higher empty-cage C2n fullerenes are largely unaffected. CAPTEAR approach combined with the air flow was found to be convenient for the synthesis of scandium-oxide clusterfullerenes.9,135

2.4. High Pressure Method

In 1993 Saunders and Cross have discovered that noble gases can be encapsulated into empty fullerenes by a high-pressure treatment.69 A small amount of C60/C70 mixture was heated at 600 °C in 3He at a pressure of ∼2500 atm, resulting in the formation of ∼0.1% He-containing fullerene molecules (3He@ C60, 3He@C70).69 In a similar way, other noble-gas atoms including Ne, Ar, Kr, Xe as well as He2, Ne2, and HeNe “dimers”, or CO and N2 molecules had also been incorporated into fullerenes.70,71,73,145−148 It was proposed that, as the size of an included noble gas atom increases, it is able to interact favorably with more carbon atoms of C60 at the same time. 2.5. Other Methods

After numerous attempts to develop a chemical route to endohedral fullerenes, the task was finally accomplished by Komatsu et al.,77 who have developed a complex multistage chemical protocol to create an orifice in the fullerene cage, to insert species into the carbon cage by a pressure treatment, and then to close the orifice. By this method, H2@C60 was first synthesized in 2005,77 followed by reports on He@C60,70,149 (H2)1,2@C70150, and [email protected] Recently, Peng and Wang et al. developed an explosionbased method for preparing He-containing endohedral fullerenes, namely, He@C60 and [email protected] The fullerene C60, an explosive, and a noble gas were first mixed in an enclosed space. A flying plate within the apparatus is used to convert the explosion energy into kinetic energy of the gas molecules, which in turn bombard the fullerenes. Through adjusting the quantity of explosive, the gas molecules can be accelerated to a sufficiently high energy to penetrate the fullerene surface and form endohedral fullerenes. While isolation of He@C60 from C60 was not possible, the authors succeeded in isolation of He2@C60 with the estimated yield of ∼0.4%.

3. SOLUBILITY, EXTRACTION, AND SEPARATION OF ENDOHEDRAL FULLERENES 3.1. Solubility and Extraction of Endohedral Fullerenes

A unique π-conjugated closed cage structure makes fullerenes soluble in a large variety of organic solvents such as CS2 or substituted benzenes (toluene and o-dichlorobenzene are the most widely uses ones). Since fullerenes can have special interactions with solvents (for instance, via π−π interactions) the study of their solubility can provide new information on the mechanisms of solute−solvent interactions. The solubility of C60 and other fullerenes is of both practical and fundamental interest. Ruoff et al.152 determined the room temperature solubility of pure C60 in 47 solvents and found that the solubilities cover a wide range from 0.01 mg/mL in methanol to 50 mg/mL in 1-chloronaphthalene. The highest solubility of C60 was achieved in solvents with a large refraction index, a dielectric constant of around 4, a large molecular volume, a Hildebrand solubility parameter equal to 10 cal1/2 cm−2/3, and a

2.3. Ion Bombardment

A special class of nonmetal endohedral fullerenes comprises nitrogen136 or phosphorus137 atoms which are encaged in C60 or C70 cage. The synthesis of N@C60 was first accomplished by Weidinger et al. by bombarding C60 with nitrogen ions from a conventional plasma discharge ion source which was carried out in a vacuum of 10−5 mbar.136 The yield of N@C60 after atomic nitrogen ion implantation is about 10−5 to 10−4%.136,138 N2@ C60 had been also detected at a similar yield.139 Likewise, N@ 5994


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isomers of EMFs because the sublimation temperatures of isomers might be very close. Importantly, sublimation enthalpies (ΔHsub) of EMFs are expected to be higher than those of empty fullerenes because of the nonuniform charge distribution. In 2010 Stibor et al. measured sublimation enthalpy of Er3N@C80 by Knudsen effusion mass spectrometry,177 and as far as we know, this is the only determination of sublimation enthalpy of EMF reported so far. ΔHsub(Er3N@C80) was found to be 237 ± 7 kJ/mol in the temperature range of 862−1119 K, which is noticeably higher than the values measured for empty fullerenes of comparable size (e.g., ΔHsub = 210 ± 6 kJ/mol for C84) and much higher than for C60 (ΔHsub = 166 ± 2 kJ/mol).177

tendency to act as a moderate strength nucleophile. Taking advantage of the different interaction of fullerenes with various solvents, a rational choice of solvents capable of purifying the fullerenes by cheaper and scalable methods seems possible.152 Endohedral fullerenes also exhibit a considerable solubility in various solvents such as CS2, toluene, and o-dichlorobenzene (o-DCB); however the solubility of endohedral fullerenes is generally different from that of the empty fullerenes because of their different electronic structures as discussed in section 5. Considerable solubility of empty and endohedral fullerenes in organic solvents makes their extraction from the primary soot possible. Solvent extraction is the most common method and frequently used in the first step of fullerenes separation from the soot. In contrast to empty fullerenes, most EMFs have an anisotropic charge distribution (large dipole moments153). It is therefore possible to use this difference to facilitate the extraction of EMFs from the primary soot and achieve a partial separation from the empty fullerenes already at this stage of extraction. Progress has been made in extracting EMFs from the soot by a wealth of approaches, including two-step solvent extraction,85,154 mixed solvent extraction,153 polar solvent extraction,155−157 high-temperature high-pressure extraction,90,158,159 and solid-phase extraction.160 As a common and simple method, Soxhlet extraction has been the workhorse in this field from the very beginning since it is safe, convenient, and commercially available. A hot solvent or ultrasonic extraction is normally employed to increase the efficiency of solvent extraction in Soxhlet method.161,162 Commonly used solvents for extraction include toluene or CS2; however, in many cases, the extraction is not complete, and up to nearly half of EMFs or even more still remains in the residual soot even after CS2 extraction.163−166 Among the other solvents, N,Ndimethylformamide (DMF) should be mentioned as a widely used solvent for extraction of EMFs.153,154,156,164,167,168 The mechanism of special action of this solvent (as well as other amine solvents such as aniline167 or pyridine169) is based on the formation of the charge-transfer to EMFs, which are present in solution in anionic state.170−172 The difference in electrochemical properties of EMFs and empty fullerenes (see section 7 for more details) can be also used for their separation. In particular, since many EMFs can be more easily oxidized than empty fullerenes, the oxidation with subsequent separation based on the different solubility of neutral and charged species can be employed.165,173

3.3. Chromatographic Separation

Chromatography is widely used for separating of mixtures of organic compounds. The method is based on the different distribution of compounds between a stationary and a mobile phase in a chromatographic column. The differences in distribution are determined by such properties as a boiling point, polarity, an electric charge (for ionic compounds), a size of the molecule, and so forth. In a separation run, the mobile phase moves across the column, in effect washing (eluting) compounds at a different rate. Complete separation of EMFs has been accomplished only by high performance liquid chromatography (HPLC),178−183 which has been demonstrated to be one of the most powerful and frequently used techniques for separation of EMFs. The HPLC technique applying different modified HPLC columns can even afford the separation of structural cage isomers of various EMFs. However, the purification of EMFs via HPLC also meets serious difficulties, not only because of the large variety of the fullerene species in the extract and their similarity in terms of the gradual changes of the cage size and shape, but also because of the often limited content of EMFs in the raw soot. Furthermore, the solubility of EMFs in standard HPLC solvents is often even lower than that of higher empty fullerenes. Thus, HPLC separation is normally suitable only for the purification of EMFs on a milligram scale, and a full cycle of the preparation of EMFs starting from the arc-discharge synthesis and finishing with highly purified isomers of EMFs is an extremely labor- and time-consuming procedure. Detailed HPLC separation of the EMFs such as monometallofullerenes M@C82 and carbide clusterfullerenes Sc2C2@ C82 (thought to be “Sc2@C84” at that moment) has been described in ref 21. In the scheme of the separation and purification of the first NCF, Sc3N@C80, a four-step HPLC was applied earlier, with the application of several different commercially available HPLC columns (PBB, Buckyclutcher etc.) in each step.5 Later, many new nitride cluster fullerenes such as Sc3N@C68 and Sc3N@C78 were isolated by multistep HPLC.16,97 Thanks to the highly selective synthesis of nitride cluster fullerenes by the reactive gas atmosphere method as described in section 2, a facile isolation of nitride cluster fullerenes have been achieved by a single-step HPLC in our group.110,111 Figure 4 illustrates a typical isolation scheme of the Sc3N@C2n NCFs synthesized by the reactive gas atmosphere method utilizing a combination of two Buckyprep columns.110,120 Owing to the high relative yields of Sc3N@C2n (2n = 68, 78, 80), which is clearly seen in the chromatogram, the non-IPR Sc3N@C68 as well as Sc3N@C78 were readily isolated by singlestep HPLC to a high purity.110 Likewise, several Dy3N@C2n

3.2. Separation by Sublimation

While solvent extraction is suited to large-scale extraction of EMFs, the sublimation method has been developed to enrich EMFs with the advantage of obtaining ‘solvent-free’ extracts.56,165,174−176 In a sublimation method, as in the case of empty fullerenes, the raw soot containing EMFs is heated in He gas or in a vacuum up to 400 °C where EMFs such as La@C82 and Y@C82 start to sublime. The EMFs then condense in a cold trap, leaving the soot and other nonvolatiles in the sample holder. Thus, by this method, “insoluble” fullerenes and EMFs can be isolated from the carbon soot. However, a complete separation of metallofullerenes has not been achieved to date by sublimation. Although gradient sublimation has been shown to give a clean route to partial purification of EMFs, it could be only used for a crude separation of EMFs from the empty fullerenes in the soot, but in general it does not work for the separation of 5995


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Figure 4. Chromatograms of a Sc3N@C2n fullerene extract mixture synthesized by the “reactive gas atmosphere” method (a) and the isolated Sc3N@C68 (b) (combination of two 4.6 × 250 mm Buckyprep columns; flow rate 1.6 mL/min; injection volume 100 μL; toluene as eluent (mobile phase); 40 °C). A: Sc3N@C78; B: Sc3N@C80 (I); C: Sc3N@C80 (II). Inset: positive ion LD-TOF mass spectrum of the isolated Sc3N@C68. Reproduced with permission from ref 110. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 5. (a) Recycling HPLC chromatograms of two fractions A and B containing GdxSc3−xN@C80 (Ih, x = 0−2) which was first isolated by conventional HPLC as the first-step separation (10 × 250 mm Buckyprep-M column; flow rate 5.0 mL/min; injection volume 5 mL; toluene as eluent; 25 °C). (b) Chromatograms of fractions A and B in the final cycle by recycling HPLC based on a). Reproduced with permission from ref 115. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

NCFs were facilely isolated by the single-stage HPLC with the purity higher than 95% (see Figure 3).111 Such a facile isolation is attributed to both the high relative yields of the clusterfullerenes and the judicious choices of the production and separation conditions. 3.4. Extended Separation by Recycling HPLC

The separation of the mixed metal NCFs AxB3−xN@C2n, for which the encaged nitride cluster comprises two different metal ions (A and B), is even more difficult and tedious since the components of the mixture have the same carbon cage and are different only in the composition of the nitride cluster. A successful isolation by using recycling HPLC has been accomplished in our groups for DySc2N@C76,117 MSc2N@ C68 (M = Dy, Lu) and Lu2ScN@C68,118 LuxY3−xN@C80 (I, x = 0−2),119 LuxSc3−xN@C80 (I, x = 0−2),184 Ih and D5h cage isomers of GdxSc3−xN@C80 (x = 0−2)115,116 and HoxSc3−xN@ C80 (x = 1, 2)185 as well as TiSc2N@C80(I)186 and TiY2N@ C80(I).187 Besides, by using recycling HPLC, separation of the cage isomers which relies on multistep HPLC is expected to be simplified. Figure 5 shows recycling HPLC chromatograms of two fractions A and B containing GdxSc3−xN@C80-Ih (x = 0−2) which were first isolated by conventional HPLC as the first-step separation. After 13 cycling steps, Gd2ScN@C80-Ih and GdSc2N@C80-Ih are completely separated and isolated with a high purity as confirmed by laser desorption time-of-flight (LDTOF) MS analysis. Likewise, Sc3N@C80-Ih is isolated from the fraction B which also contains the minor product of Gd2ScN@ C80(II).115 Recycling HPLC has been also widely used for separation of the conventional EMFs, 93,188−191 carbide clusterfullerenes,192,193 and sulfide clusterfullerenes.11,194 For instance, in the early works of Shinohara et al., a series of isomers of different cages of dysprosium (Dy) EMFs (e.g., Dy@C82 (I), Dy2@C80 (I), Dy2@C82 (I), Dy2C2@C82 (I, II, III), Dy2@C86 (I, II), Dy2@C88 (I, II), Dy2@C90 (I, II, III), Dy2@C92 (I, II, III), Dy2@C94 (I, II) were isolated from the empty fullerenes

(note that some of the structures denoted here as “Dy2@C2n” can be also carbides Dy2C2@C2n−2).93 3.5. Separation by Chemical and Electrochemical Methods

Electrochemical studies of EMFs revealed that their redox potentials are substantially different from those of empty fullerenes, which can be efficiently used for the separation of EMFs from the latter. In 2003 Bolskar and Alford designed a separation protocol based on the electrochemical as well as chemical oxidation of EMF extracts by AgPF6, AgSbCl6, and tris(4-bromo-phenyl)aminium hexachoroantimonate.173 The fact that reduction potentials of many conventional EMFs (such as La@C82 and La2@C80) are substantially more positive than those of empty fullerenes was used by the group of Akasaka to develop a separation technique based on the electrochemical reduction of the fullerene extract in o-DCB at the potential, at which EMFs are already reduced, whereas empty fullerenes still remain in their neutral form.195 After evaporation of o-DCB, anionic EMFs could be dissolved in acetone/CS2 mixture, whereas neutral empty fullerenes remained insoluble. The early electrochemical study of fullerenes revealed that the fullerene cage isomers can have substantially different oxidation potentials.196,197 In light of such differences, Echegoyen et al. reported a simple separation of two isomers of Sc3N@C80 with Ih(7) and D5h(6) carbon cages via selective chemical oxidation of the D5h isomer, whose first electrochemical oxidation potential was found to be shifted cathodically with respect to the oxidation potential of the Ih isomer by 270 mV.198 A similar difference in the first electrochemical 5996


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oxidation potential was also found for the Ih and D5h isomers of [email protected] By taking advantage of the extraordinary kinetic chemical stability of nitride cluster fullerenes with respect to the other fullerenes in Diels−Alder (D-A) reactions with a cyclopentadiene-functionalized resin, a single-step isolation of NCFs based on selective chemical reactivity was reported by Dorn et al.15 A reactive resin (cyclopentadiene-functionalized styrene-divinylbenzene resin) was first prepared. While empty fullerenes and conventional metallofullerenes react with the asprepared resin by D−A cycloaddition, the less reactive nitride cluster fullerenes pass through the column and relatively pure nitride cluster fullerenes could be obtained.15 The same group has also reported a nonchromatographic separation of Sc- and Lu-based NCFs from raw soot based on the low reactivity of NCFs in D−A reaction with 9-methylanthracene.199 Recently, a further method named “SAFA” (stir and filter approach) was proposed by Stevensen et al.13 Without using any chromatography equipment, the authors used cyclopentadienyl and amino functionalized silica to selectively bind contaminant fullerenes (empty fullerenes and non-nitride clusterfullerenes) and claimed to obtain the purified nitride clusterfullerenes under optimum conditions.13 This method could be also used to separate Sc3N@C80-Ih and Sc3N@C80-D5h isomers.200 It was reported in 1997 that the separation of C60 from other empty-cage fullerenes (e.g., C70) can be achieved using Lewis acids.201 In 2009, Stevenson et al. found that metallic nitride clusterfullerenes (NCFs) and oxometallic fullerenes (OMFs) react quickly with an array of Lewis acids such AlCl3, AlBr3, and FeCl3, whereas empty cage fullerenes are largely unreactive.14 Experimentally determined rate constants exhibited linear correlation to the electrochemical band gaps of corresponding EMFs, resulting thus in the following reactivity order: Sc4O2@ C80-Ih > Sc3N@C78 > Sc3N@C68 > Sc3N@C80-D5h > Sc3N@ C80-Ih. The authors showed that by manipulating the Lewis acids, molar ratios, and kinetic differences within the families of OMF and MNF metallofullerenes, one can either achieve a purification of Sc3N@C80-Ih without HPLC at all or obtain a phase enriched with Sc4O2@C80-Ih, Sc3N@C78, Sc3N@C68, and Sc3N@C80-D5h suitable for facile HPLC separation.14 In 2012, Shinohara et al. reported that complexation with TiCl4 can be also used for almost quantitative separation of mono- and dimetallofullerenes from empty fullerenes.202 In the subsequent study the same group found that efficient separation can be achieved for EMFs whose oxidation potential is lower than 0.62−0.72 V versus Fe(Cp)2+/0 couple.203 VisNIR absorption spectroscopy showed that electron transfer took place from EMFs to TiCl4.

insufficiently developed, which altogether resulted in the lack of availability of isomerically pure EMF samples especially for higher cage sizes. Second, the use of standard structural tools met serious limitations due to the intrinsic properties of EMFs. In addition to the low availability of single crystals of EMFs of diffraction quality, single-crystal X-ray diffraction studies were severely hampered by the rotational disorder of the fullerene molecule in their crystals, which precluded direct determination of the carbon cage structures. As a result, before 2000s, only one work was published in which carbon cage structures of EMF were determined by this method.5 Because of the lack of single-crystal data, Shinohara and co-workers performed structural studies of EMFs based on the powder Xray diffraction data obtained with synchrotron irradiation with subsequent Rietveld/MEM analysis.36,204 Although not as reliable as single-crystal X-ray studies, for some EMFs this method provided the first structural characterization. 13 C NMR spectroscopy, another common structural tool in the fullerene chemistry, requires significant amounts of isomerically pure samples (which were not readily available for many new EMFs in 1990s). Because of the low natural abundance of 13C isotope, its relatively small gyromagnetic ratio, and long relaxation time in fullerenes, 13C NMR spectroscopy requires extended capacity in the spectroscopic studies not available in most of the laboratories. Besides, the method gives only the symmetry of the carbon cage, and hence the structure remains ambiguous when several isomers of the same symmetry are possible (in this regard, theoretical studies can be useful to choose the most appropriate isomers). Finally, many EMFs are paramagnetic, which also limits the use of 13C NMR for their structural elucidation. Among the other spectroscopic techniques, vibrational spectroscopy is the most structure sensitive tool, but it does not provide structural information directly. Usually, to determine the structure based on its vibrational spectra, it is necessary to use theoretical modeling of the spectra of several possible structural isomers. Clearly, the success of such studies strongly depends not only on the availability and quality of the EMF samples, but also on the reliability of theoretical methods and on the range of considered isomers. Therefore, any reliable vibrational spectroscopic structural study of EMFs should (i) consider as broad a range of isomers as possible and use some criteria to distinguish which isomers are really reasonable and which structures can be neglected (see section 5 on the discussion of such criteria); (ii) the spectra should be computed at a reasonably high level of theory to give reliable predictions of vibrational spectra. The latter condition usually implies DFT calculations with the basis sets of at least DZVP quality (TZVP or TZVPP are better). In the 1990s the routine use of DFT for computations of the vibrational spectra of EMFs was still too expensive. Thus, although several vibrational spectroscopic studies of EMFs have been reported, they were often not aimed in structure elucidation. A further problem in experimental vibrational studies of EMF are minor impurities of linear hydrocarbons and PAHs resulting from the HPLC columns, since such impurities exhibit much stronger IR bands than EMFs (in Raman spectra, signals from EMFs are usually stronger). In general, the purity of a fullerene sample is often analyzed by mass-spectrometry and HPLC with respect to the presence of other fullerenes and not to the total chemical composition. In many cases careful further purification is required before a vibrational spectroscopic study.

4. MOLECULAR STRUCTURES OF EMFS 4.1. Introductory Notes

Structural elucidation is a necessary step of any research work aimed at the synthesis of new compounds, and EMFs are not exclusions from this rule and require precise determination of isomeric structures of the carbon cages. In spite of the booming rise of the research activities in the field of EMFs starting from the early 1990s, the progress in the structural studies in the first decade of extensive EMF research was rather modest.21 The main obstacles which hindered further development at that moment can be classified into two groups: first, the yields of EMFs were modest whereas separation techniques were 5997


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Table 1. Molecular Structures of EMFs as Obtained by Structural Studies C2n C60 C66 C68 C70 C72

C74 C76 C78

cage isomer Ih C2v(4348) C2v(4059) D3(6140) C2v(6073) C2v(7854) C2(7892) Cs(10528) D2(10611) C2(10612) D3h(1) Td(1) Cs(17490) D3h(5)

C2(22010) C80

C2v(3)

C2v(5) D5h(6)

Ih(7)

Li@C60 Sc2@C66 Sc2@C66 M3N@C68 Sc2C2@C68 Sc3N@C70 Sc2S@C70 Sc2S@C72 M2@C72 La@C72 M@C74 Lu2@C76 DySc2N@C76 M2@C78 Ti2C2@C78 Sc3N@C78 M3N@C78 Sc3NC@C78 La@C80 Yb@C80 Sm@C80 Sc2C2@C80 Ce2@C80 M3N@C80-II

M2@C80

Sc3N@C80-I M3N@C80-I

C79N C82

″Ih(7)″ C2(5) Cs(6)

C3v(7)

C3v(8)

C2v(9)

method: metal/refsa

EMF

Sc3CH@C80 Sc4O2@C80 Sc4O3@C80 Sc3C2@C80 Sc4C2@C80 Sc3CN@C80 M2@C79N MII@C82-II (III) MIII@C82-II MII@C82-I M2@C82-I M2C2@C82-I Sc2S@C82 Sc2O@C82 La@C82 Ca@C82-III MII@C82-II Sc2C2@C82-III M2@C82-III M2C2@C82-III Sc2S@C82 M2S@C82 MII@C82-III MIII@C82-I

144

378,379

(X-ray, NMR), X-ray (Drv) NMR,17 X-ray (powder)17,299 QC300 NMR: Sc,16 X-ray: Sc,96 (Vib + QC): Sc,110 (DySc2, LuSc2, Lu2Sc)118 (NMR + QC)360 (Vib + QC)120 (QC + Echem)373 X-ray194 NMR: La,289 Ce,293 Pr;294 X-ray (Drv): La;290,291 HRTEM/EELS: Pr295 X-ray (Drv)247 NMR: Ca,231 Yb;235 X-ray: Ba;244,245 X-ray (Drv): La;246,380 (Vib + QC): Eu;381,382 UV−vis: Sr267 NMR308 (Vib + QC)117 NMR: La,296 Ce;298 X-ray (Drv): La,297 Ce298 NMR,356 QC (+NMR),357,358 HRTEM359 X-ray,97,383 X-ray (Drv),384 (Vib + QC)385 (Vib + QC): Dy,337 Tm,337 Y;338 X-ray: Gd;105 NMR: Y339 (Vib + UV−vis + QC)375 X-ray (Drv)248 (NMR + QC),236 X-ray250 NMR,233 X-ray233,251 X-ray (Drv),362,386 NMR,362 X-ray350 NMR,285 X-ray284 NMR: Sc,334 LuxSc3−x,184 Y;99 X-ray: Sc,335 Tb,102 Tm;323 X-ray (Drv): Sc;336 Vib (+QC): Sc,331 (GdxSc3−x),116 (DyxSc3−x, LuxSc3−x),184 Tm,112 HoxSc3−x185 NMR: La,270 Ce,283 Pr;287 X-ray (powder): La;271 X-ray: Ce;284 X-ray (Drv): La,274−281 Ce;277,283,387 NMR,5 X-ray,5,320 X-ray (Drv),321,388−397 (Vib + QC)330 NMR: CeSc2,103 CeLu2,329 (LuxSc3−x, LuxY3−x),119 HoxSc3−x;185 X-ray: Gd,108 Tb,102 Dy,322 Tm,323 Lu,320 (CeSc2),103 (ErSc2),95 (GdSc2, TbSc2, Gd2Sc),324 LaSc2325 X-ray (Drv): Y,326,327 Lu328 Vib (+QC): Y,330 Gd,114 (Y, Tb, Ho, ErxSc3−x),12 (GdxSc3−x),115 Dy,398 Tm,112,399 (YxLu3−x, LuxSc3−x),119 (CeLu2),329 (TiSc2),186 (NdxSc3−x, DyxSc3−x),184 HoxSc3−x185 (Vib + QC)8 X-ray,9 NMR371 X-ray135 NMR: 1D,319 2D;349 X-ray (Drv);319 X-ray (powder)400 (NMR, Vib + QC)368 (X-ray, NMR, Vib + QC)10 X-ray: Tb315 (NMR + QC): Ca,232 Tm,230 Sm,234 Yb;236 UV−vis: Yb,401 Sm,190 Eu;238 X-ray: Sm,234,261 Yb262 X-ray (Drv): La;257 NMR: La;241 UV−vis: Pr402 (NMR + QC): Ca,232 Tm,230 Yb;236 X-ray (powder): Eu;227 X-ray: Sm,261 Yb;262 UV−vis: Yb,401 Sm,190 Eu238 X-ray: Er;301 UV−vis: Er,305 Tm306 X-ray (Drv): Sc;354 NMR: Sc,354,403 Y;193,364 UV−vis: Y,193 Er,305 ErY,192 Dy93 (UV−vis + QC),123 X-ray372 X-ray370 X-ray (Drv)249 (NMR + QC)232 X-ray: Sm,261 NMR;352,404 2D-NMR;349 X-ray;350 X-ray (Drv);353,405 (Vib + QC)37,406 X-ray: Er;302 NMR: Y,193 Sc;303 UV−vis: Y,193 Er,305 (Tm, HoTm)306 NMR: Y;193,364,407 X-ray (powder): Y;304 UV−vis: Er,305 (ErY),192 Dy93 (UV−vis + QC);11,123 (Vib + QC),11 X-ray372 (Vib + QC + Vis-NIR): (Y, Dy, Lu)11 NMR: Ca,232 Tm,230 Yb;236 X-ray: Yb;262 X-ray (powder): Eu;227 UV−vis: Yb,401 Sm,190 Eu238 NMR: Y,239 La,206 Ce,207 Pr;240 5998


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Table 1. continued C2n

cage isomer

method: metal/refsa

EMF 242

C86 C88

Cs(39663) C2(11) C1(12) C2(13) D3d(19) D2d(23) Cs(51365) C1(51383) D3(19) D2(35)

C90

C1(21)

C84

C92

C2(40) C2(42) C2(45) C2v(46) Cs(24) C1(42) D3(85)

C94 C96 C100

T(86) C3v(134) D2(186) D5(450)

C104

D3d(822)

M2@C82-II M2C2@C82-II M3N@C82 MII@C84-IV/II Yb@C84-III MII@C84-II/I Sm@C84-III Sc2C2@C84 M3N@C84 Y2C2@C84 M3N@C86 M3N@C88 Sm2@C88 Gd2C2@C88 Lu3C2@C88 Sm2@C90 Gd2C2@C90 Sm@C90-I Sm@C90-II Sm@C90-IV Sm@C90-III Sm@C92-II Sm@C92-I M2C2@C92 Sm2@C92 La3N@C92 M@C94-I La3N@C96 La2@C100 Y2C2@C100 Sm2@C104

243

NMR-2D: La, Ce X-ray (Drv): Sc,256 Y,254 La,252,253,408−414 Ce,255 Gd,229 Dy258 X-ray (powder): Sc,224 Y,222 La,225 Gd226 X-ray: La,259 Gd260 UV−vis: Er,305 (Tm, HoTm)306 X-ray (Drv): Sc;355 NMR-2D: Sc;355 NMR: Sc,403 Y;193 UV−vis: Sc,403 Er,305 Y,193 Dy93 X-ray: Gd;106 NMR: Y;99 Vib: Gd104 (NMR + QC): Yb;236 UV−vis: Sm265 (NMR + QC)236 (NMR + QC): Yb;236 X-ray: Sm265 X-ray265 NMR: 1D,7 2D;349 (Vib + QC);415 X-ray350 X-ray: Tb,101 (Tm, Gd);341 NMR: Y;99 Vib: Gd104 (NMR + QC)364,365 X-ray: Tb,102 Gd;342 NMR: Y;99 Vib: Gd104 X-ray: Tb;102 NMR: Y;100 Vib: (Y, Gd),104 Lu340 X-ray309 UV−vis309,363 (Vib + QC)340 X-ray309 UV−vis309,363 X-ray266 X-ray266 X-ray266 X-ray266 X-ray264 X-ray264 X-ray: Gd;363 (Vib + QC): Y;416 NMR: Y364 X-ray309 (QC + Echem)345 X-ray: (Ca, Tm),263 Sm264 (QC + Echem)345 X-ray313 QC364 X-ray310

a

Abbreviations of the methods used for the structures elucidation of EMFs: X-ray: single crystal X-ray diffraction of the nonderivatized EMF; X-ray (Drv) − the same, but for the derivative of EMF; X-ray (Powder) − synchrotron X-ray diffraction studies of powder samples with subsequent Rietveld/MEM analysis; NMR − 13C NMR; Vib − vibrational spectroscopy (IR and/or Raman); QC − quantum-chemical calculations; UV−vis − UV−vis-(NIR) absorption spectroscopy; HRTEM/EELS − high resolution transmission electron microscopy coupled with electron energy loss spectroscopy; Echem − electrochemistry.

vibrational and absorption spectroscopic studies of EMFs will be given in section 6. A qualitatively new situation in the structural studies of EMFs has been developing since the late 1990s. Improved synthetic and separation techniques (see sections 2 and 3) largely solved the problems of the samples availability. In the single-crystal X-ray diffraction studies, two strategies have been developed, which effectively circumvented the problem of the rotational disorder. On one hand, rotation of fullerene molecules in a crystal can be hindered by the use of cocrystallizing agents such as Ni- or Co-octaethylporphyrines.205 This technique was developed and extensively used in the group of A. Balch and M. Olmstead (U California, Davis), and by now dozens of EMF structures are determined this way as discussed below.35 On the other hand, chemical derivatization can also hinder the rotation of EMF molecules in the crystals, and the structures of many EMFs have been determined by single-crystal X-ray diffraction studies of their derivatives such as in the studies performed by Akasaka and coworkers (University of Tsukuba, Japan).44

Absorption spectra of EMFs in the visible and near-IR range are usually dominated by the π−π* excitations of the carbon cage and are highly structure sensitive, but as in the case of vibrational spectroscopy, direct structural information is not readily available from the spectra. Moreover, reliable prediction of electronic absorption spectra is not as straightforward as it is now for IR vibrational spectra (prediction of the Raman spectra is still difficult now because of the resonance effects in the experimental Raman spectra of EMFs). Despite the outlined limitations, both vibrational and UV− vis-NIR absorption spectroscopies have found frequent applications in the structural studies of EMFs since it had been recognized that the spectra of EMFs with the same carbon cages and the metal atoms in the same valence state are very similar (see section 6.4). As a result, it is possible to use vibrational or absorption spectra to confirm the isomeric structures of new EMFs by comparison to the spectra of isostructural EMFs (but with different metal atoms or clusters) whose structures are already described in the literature and which are determined by other methods. Further details on 5999


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Figure 6. Molecular structures of selected mono- and dimetallofullerenes: (a) C6H3Cl2 adduct of La@C72-C2(10612); (b) C6H3Cl2 adduct of La@ C74-D3h(1); (c) Yb@C80-C2v(3); (d) La@C82-C2v(9); (e) Yb@C82-C2(5); (f) Yb@C82-Cs(6); (g) Sm@C82-C3v(7); (h) Tm@C94-C3v(134); (i) La2@ C72-D2(10611); (j) Lu2@C76-Td(1); (k) La2@C78-D3h(5); (l) La2@C80-Ih(7); (m) Sc2@C82-C3v(8); (n) La2@C100-D5(450); (o) Sm2@C104D3d(822). Carbon atoms are gray except for the atoms in the adjacent pentagon pairs, which are shown in red; metal atoms are dark orange (La), blue (Yb), violet (Sm), lime (Tm), light green (Lu), and magenta (Sc). The lines connecting metal atoms in Lu2@C76 and Sc2@C82 denote covalent bonds between them. Atomic coordinates for visualization of the structures are either DFT-optimized (a, b, d, i, j, k, l, m, n) or from the cif-files accompanying original single-crystal X-ray diffraction studies (c, e, f, g, h, o, see Table 1). For the latter, only the sites with the highest occupancies are shown.

In the field of NMR spectroscopy, structural studies gained significantly from the use of bulk electrolysis of solutions of paramagnetic EMFs, which yields their diamagnetic forms (usually monoanions) suitable for further 13C NMR spectroscopic studies.206 NMR studies have been also successfully performed for paramagnetic EMFs in which paramagnetism originated from encapsulated lanthanide atoms (such as CeIII in Ce2@C72,78,80).207,208 In addition to the carbon cage symmetry, such studies can also provide information on the binding sites of paramagnetic metal atoms by the analysis of the temperature-dependence of the 13C chemical shifts (see section 6.1). Finally, it is worth mentioning that continuous development of computer hardware has also contributed to the improved availability of high-level computational studies of EMFs (see section 5). Such computations are often of strong help in the structure elucidation of EMFs by discriminating possible isomers via their energy, providing the general guidelines on the fullerene isomerism, or assisting in experimental spectroscopic studies.37,209−213 It is necessary to clarify that in an ideal situation, elucidation of molecular structure implies (i) determination of the carbon cage isomer and (ii) determination of the metal or cluster position with respect to the carbon cage. In fact, the position of the metal atoms often remains unclear even when the cage structure has been determined by singe-crystal X-ray diffraction

because there can be many possible metal positions with close energies and low barriers of interconversion. In the following, we will consider that the molecular structure of EMF is determined, if at least its carbon cage structure is identified. In Table 1, we have compiled the list of all known EMFs whose molecular structures have been determined either unambiguously (with the use of single-crystal X-ray diffraction studies) or with a high degree of certainty (with the use of 13C NMR spectroscopy and other spectroscopic techniques in combination with first principle quantum chemical calculations). Representative molecular structures of different classes of EMFs are shown in Figures 6−9. For generality, the Table 1 lists all single-crystal X-ray diffraction studies of EMFs and their derivatives reported to date. For some EMFs like Sc3N@C80, whose chemical properties have been extensively explored the number of available structures can exceed 10, but for many others only 1−2 reports are available. For the EMFs studied only by 13C NMR spectroscopy, we have also listed the compounds whose structure elucidation based solely on 13C NMR spectra might be ambiguous but whose isomers’ assignment could be clarified with the use of first principle calculations. Finally, in some cases the isomeric structures were not determined in early works, but can be determined now based on the spectroscopic information from original publications and the studies of isostructural EMFs with 6000


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successive loss of C2 species from the carbon cage which still preserved the encapsulated La atom.215 Although further structural studies of EMFs by various techniques including extended X-ray absorption fine structure (EXAFS) spectroscopy,216−218 high resolution transmission electron microscopy (HRTEM),219 scanning tunneling microscopy (STM),87,220 and surface-induced dissociation experiments221 indicated that endohedral location of metal atoms is the most probable for metallofullerenes, the results were still insufficiently reliable because of the unavailability of macroscopic amounts of the solid-state sample of EMFs at that time. The final evidence of the endohedral nature and detailed endohedral structures of the EMFs was not obtained until the synchrotron X-ray diffraction experiments on purified Y@C82 powder samples by Takata et al. in 1995.222 The experimental data were analyzed in an iterative way with the combination of Rietveld analysis and the maximum entropy method (MEM). MEM electron density distribution of Y@C82 showed that a high-density area is present inside the C82 cage, and the density maximum at the interior of the C82 cage corresponds to the yttrium atom.222 That study has also revealed that the yttrium atom does not reside at the center of the C82 cage but is shifted to one site of the carbon cage, as also suggested theoretically.223 Similar results were also obtained in a synchrotron X-ray diffraction study with MEM analysis of Sc@C82 by Takata et al.224 The nearest-neighbor Sc−C distance estimated from the MEM map is 2.53(8) Å, which is very close to a theoretical value (2.52− 2.61 Å). The X-ray result indicates that the carbon cage of Sc@ C82 has C2v symmetry.224 Synchrotron X-ray powder diffraction study with subsequent Rietveld/MEM analysis was also used to determine the structures of monometallofullerenes La@C82,225 Gd@C82,226 and two isomers of Eu@C82 (I and III).227 Similar to major isomers of other M@C82 EMFs, C2v symmetry was assigned to the carbon cages of La@C82, Gd@C82, and Eu@ C82(III), whereas Eu@C82(I) was determined to have Cs(6) symmetric fullerene cage. Whereas for Y and Sc MEM/Riteveld analysis reveals a fixed position of the metal atoms inside the fullerene,222,224 a floating motion inside the carbon cage was suggested for La, Gd, and Eu.225−227 However, the latter result was questioned in more recent studies.228,229 For the neutral EMFs, the use of 13C NMR spectroscopy is limited to the monometallofullerenes with divalent metals since there is no spin on the carbon cage in such EMFs. So far, 13C NMR studies have been reported for Tm@C82 (I−III),230 Ca@ C2n (2n = 74,231 82 (I−IV)232), Sm@C80-C2v(3),233 Sm@C82C2(5),234 and Yb@C2n (2n = 74,235 80, 82(I−III), 84(II− IV) 236 ). Interpretation of the spectra of Tm@C 82 is complicated by the paramagnetism of TmII, and only cage symmetries (but not their numbers) of Cs and C2 isomers could be determined (C82 has several IPR isomers of each symmetry type).230 On the other hand, for the C2v(9) isomer of Tm@C82, assignment is unambiguous since there is only one C2vsymmetric IPR isomer of C82.230 CaII and YbII are diamagnetic, and hence the spectra of corresponding M@C2n are easier to analyze. Nevertheless, unambiguous structural assignment based on the 13C NMR data is possible only for M@C74D3h(1), Yb@C80-C2v(3), Sm@C80-C2v(3), and M@C82−C2v(9) (M = Ca, Yb).231−233,236 For the isomers of Yb@C82 and Yb@ C84, DFT calculations of the relative energies and chemical shifts of possible isomers were also performed, which afforded assignment of the isomeric structures of other Yb@C82 isomers to C2(5) and Cs(6).236 The close resemblance between the spectra of Ca@C82 and Yb@C82 indicates that the same

different metals reported later. Such structures have been also included in the table. In a conclusion to these introductory remarks, it should be pointed out that still many structures of EMFs, especially those of mono- and dimetallofullerenes, remain unknown. In the following parts of this section, we will review the stateof-the-art in the studies of molecular structures of different classes of EMFs, which also requires clarification of the nomenclature. It is a common practice to label the isomers of fullerenes by their symmetry (to be precise, by the highest symmetry possible for the given topology of atoms) and the number of the isomers in accordance with the FowlerMonolopoulos spiral algorithm.214 Usually, a short form of numbering system is used, in which only IPR isomers are numbered. However, an increasing number of the non-IPR isomers found for EMFs requires the use of the extended notation, which includes all possible isomers for a given number of carbon atoms. For instance, the IPR isomer of C80 usually labeled as Ih(7) is Ih(39712) in the full notation. However, the full numbering system has been rarely used in numerous studies of IPR isomers. To simplify the discussion and comparison to the results of many previous works, we use in this review a 2fold numbering system which is de facto employed in many publications: the short notation is retained for all IPR isomers, while the full notation is adopted for the non-IPR isomers.37,208,213 This system will not result in confusion, at least for the fullerenes with more than 60 carbon atoms, since the isomers with small numbers in the full numbering system (less than ∼1000) have too many pentagon adjacencies and are unstable. Hence, in the following text, the isomers with small numbers (less than 1000) are necessarily IPR isomers, while the isomers with large numbers (more than several thousands) are non-IPR isomers. When labeling the cage isomers of EMFs, the symmetry and the number of the given cage isomer will follow the molecule after the hyphen. For instance, “La@C82-C2v(9)” means that the La atom is encapsulated in the C2v(9) IPR isomer of C82. It is worth noting that the actual symmetry of an EMF molecule can be lower than the highest possible symmetry of its carbon cage because of the encapsulation of the metal or cluster. In many experimental works the isomers of EMFs were labeled by Roman numbers according to their HPLC retention times. This “nomenclature” is widespread in the literature and is often the only way to distinguish the isomers when their molecular structures are not known. When appropriate, in the table 1 we indicate these numbers for different isomers. However, it should be kept in mind that such labeling can result in confusion when different research groups use somewhat different conditions of synthesis and separation so that different isomers can have the same number and vice versa. 4.2. Classical EMFs

4.2.1. Monometallofullerenes. Starting from the first proposed metallofullerene LaC60+ observed in the gas phase in 1985,3 the nature of the metal-fullerene arrangement was under debate for several years. Just like the close-shaped structure of fullerenes met some skepticism in 1980s, the endohedral nature of the metal atom in metallofullerenes was challenged by some researchers till the mid 1990s. The first strong evidence in favor of the endohedral nature of La@C82 was its enhanced stability in the gas phase demonstrated by laser photofragmentation.215 Instead of fragmenting into La and C82 species as might be expected for the exohedral complex, La@C82 exhibited only 6001


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C2v(9) and Gd@C82-C2v(9) cocrystallized with NiII(OEP).259,260 In 2012, significant progress was also achieved in the structural elucidation of the isomers of MII@C82 EMFs (MII denotes divalent metals Sm, Yb). Co-crystallization with NiII(OEP) allowed determination of the cage structures of three isomers of Sm@C82, which were proved to have C2(5), Cs(6), and C3v(7) cage structures (Figure 6e,f).234,261 Soon after that, the isomers of Yb@C82 with C2(5), Cs(6), and C2v(9) cages were characterized by the same method (Figure 6g).262 The X-ray data confirmed the previous assignments based on the 13C NMR spectra. Thus, the structures of four MII@C82 isomers (from the five reported) are now known unambiguously, and assignment of the structures for other MII metals (Ca, Tm) can be done based on their UV−vis-NIR absorption spectra. The list of structurally characterized monometallofullerenes has been recently extended by M@C94−C3v(134) (M = Ca, Tm, Sm) (Figure 6h),263,264 three isomers of Sm@C84 (C2(11), C2(13), and D3d(19)),265 four isomers of Sm@C90 (C2(40), C2(42), C2(45), and C2v(46)),266 and two isomers of Sm@C92 (Cs(24) and C1(42)).264 For single-crystal X-ray diffraction studies, all these EMFs have been cocrystallized with NiII(OEP). Importantly, since the carbon-cage isomerism of EMFs is to a large extent determined by the formal charge of the metal, unambiguous structural characterization of the EMF structure with one metal simplifies characterization of other EMFs with the same composition and valence state of the metal. For instance, on the basis of the structure of Yb@C80-C2v(3), it is reasonable to suggest the same structure for Ca@C80, Ba@C80, and Sr@C80, which were isolated but not structurally characterized.84 Likewise, structural determination of M@C74D3h(1) (M = Ca,231 Ba,245 Yb235) allows the assignment of the D3h(1) cage isomer to Sr@C74 and [email protected] Such suggestions can be additionally solidified by the comparison of the vibrational and absorption spectra. A special monometallofullerene to be mentioned in the end of this section is Li@C60+. For the first time it was obtained in 1996 by irradiating C60 with the beam of Li+,67 and till now it remains the only metallofullerene produced in isolable amounts by the metal insertion into the already available fullerene. Nevertheless, the unambiguous confirmation of the preservation of C60-Ih carbon cage and endohedral metal position was obtained for Li@C60 by single-crystal X-ray diffraction study of its salt [Li@C60+](SbCl6−) only in 2010.144 4.2.2. Dimetallofullerenes. Dimetallofullerenes have been obtained in synthesis in parallel to monometallofullerenes since the very first years of EMF research, but their yields are usually considerably lower.21 Besides, after a decade of intense studies of EMFs it was recognized that M2@C2n can sometimes correspond to carbide clusterfullerenes M2C2@C2n−2,7 and it is not possible to distinguish two classes of structures based on the mass-spectrometry data. In the past decade, some “dimetallofullerenes” (such as “M2@C84”, “Ti2@C80”, “Sc2@ C82”) have been reassigned to carbides (see section 4.3.2 below). The list of well characterized dimetallofullerenes is thus much smaller than that of monometallofullerenes and clusterfullerenes. Very recently, Nishibori, Shinohara, and coworkers reported a powder X-ray studies of 24 EMFs and found a linear correlation between the number of carbon atoms in the fullerene cage and the volume it occupies in the cell.268 On the basis of this correlation, the authors proposed a “structural

structures can be also assigned to C2 and Cs isomers of Ca@C82 isomers. As can be deduced from the UV−vis absorption spectra, C2(5), Cs(6), and C2v(9) isomers are also formed for Tm@C82237 and other MII@C82 (MII = Sm,190 Eu238). The 13C NMR spectrum of another isomer of Ca@C82 was also reported, and its structure was tentatively assigned to C3v(7), although the C3v(8) isomer could not be fully excluded.232 On the basis of the 13C NMR spectra and results of DFT calculations, the structures of three isomers of Yb@C84 were determined as C2(11), C1(12), and C2(13).236 Monometallofullerenes with trivalent atoms have a spin on the carbon cage and cannot be studied by 13C NMR spectroscopy in their pristine form. The problem was circumvented by their electrochemical reduction to the anionic form to make the carbon cage spin-free (the whole EMF molecule can be still paramagnetic depending on the nature of the encapsulated metal atom). This way, the structures of M@ C82-C2v(9) (M = Y,239 La,206 Ce,207 Pr240) and La@C82Cs(6)241 have been elucidated. For La@C82− and Ce@C82−, the structural assignment was further verified by 2D INADEQUATE NMR measurements.242,243 The position of the metal atom in Ce@C82 was determined by the analysis of the temperature-dependence of 13C NMR shifts induced by the interaction with 4f1 electron of Ce (see section 6.1).243 The first elucidation of the molecular structure of a monometallofullerene by single-crystal X-ray diffraction was reported in 2004, when Jansen et al. succeeded in the preparation of the diffraction-quality crystals of Ba@ C 74 ·Co II (OEP) (cobalt(II) octaethylporphyrin). 244,245 Although the structure exhibited a strong disorder, the carbon cage symmetry could be reliably identified as D3h(1). The same cage isomer was also assigned in 2005 to La@C74 based on the single-crystal X-ray study of the EMF derivative, La@ C74(C6H3Cl2) (Figure 6), formed in the process of the soot extraction by 1,2,4-trichlorobenzene (importantly, “missing” metallofullerenes La@C74 is too reactive in the pristine form and cannot be isolated as such).246 Single-crystal X-ray diffraction studies of dichlorophenyl adducts were also used to determine the molecule structures of the non-IPR La@C72C2(10612)247 and IPR La@C80-C2v(3)248 and La@C82C3v(7).249 All these EMFs are too reactive to be studied in the nonderivatized form, and their formation in the arc discharge burning could be revealed only because their reaction with trichlorobenzene during extraction yields stable adducts. Interestingly, Yb@C80-C2v(3) (Figure 6c) is more stable than its La analogue, and its molecular structure has been recently confirmed by single-crystal X-ray study of Yb@ C80·NiII(OEP)·CS2·1.5C6H6.250 Likewise, single-crystal X-ray studies of Sm@C80 cocrystallized with NiII(OEP)·and bis(ethylenedithio)tetrathiafulvalene also proved that it has a C2v(3) cage structure.233,251 Till 2012, single-crystal X-ray diffraction studies of the most abundant monometallofullerene M@C82-C2v(9) (Figure 6d) have been successful only for the derivatized EMFs. In 2004 Aksaka et al. reported the structure of the adamantylidene (Ad) adduct La@(C82-C2v(9))Ad,252 followed in 2006 by the structure of the Bingel adduct;253 the structures of Ad-adducts were also solved for Y@C82-C2v(9),254 Ce@C82-C2v(9),255 Gd@ C82-C2v(9),229 Sc@C82-C2v(9),256 and La@C82-Cs(6).257 Singlecrystal X-ray structure was also reported for the [6,6]-open adduct of Dy@C82-C2v(9) bearing a triphenylphosphonium substituent.258 Very recently Akasaka et al. reported the first single-crystal X-ray studies of the nonderivatized La@C826002


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NMR spectroscopic studies also allow determination of the metal position because the temperature-dependent shifts are more pronounced for carbon atoms located at close distances to Ce atoms.298 Sc2@C66 was one of the first reported non-IPR fullerenes (since there are no IPR fullerenes for C66, the non-IPR nature of this EMF is unavoidable).17 In the original report in 2000, the C2v cage symmetry of Sc2@C66 was determined by 13C NMR spectroscopy.17 On the basis of the Rietveld/MEM analysis of the synchrotron X-ray diffraction data of the powder sample, the authors determined the number of the isomer as 4348 with two pairs of adjacent pentagons.17,299 This assignment was questioned in the computational study of Kobayashi and Nagase, who have shown that the isomer Sc2@ C66-C2v(4059) with two pairs of 3-fold fused pentagons is much more stable, and its computed NMR spectrum fits the experimental data very well.300 For M2@C82 dimetallofullerenes, up to three isomers can be isolated. In 2002 molecular structures of two isomers of Er2@ C82 with Cs(6)301 and C3v(8)302 cages have been determined by single-crystal X-ray diffraction studies of the samples cocrystallized with NiII(OEP). In both structures, metal atoms are disordered among many positions along the belt of 10 continuous hexagons. C3v(8) cage isomer was determined for one isomer of Y2@C82193 and Sc2@C82303 (Figure 6m) by 13C NMR spectroscopy. Y2@C82-C3v(8) was also studied by powder X-ray diffraction with Rietveld/MEM analysis, and in addition to the structure of the carbon cage, this study has revealed that Y atoms are disordered in many positions similar to La2@ C80.304 Importantly, UV−vis-NIR absorption spectra of M2@ C82-C3v(8) (M = Sc, Y) are almost identical to those of M2C2@ C82-C3v(8) (M = Sc, Y),193,303 the spectra of three isomers of Er2@C82 are virtually identical to those of Er2C2@C82,305 and likewise, they are also similar to the spectra of the isomers of Tm2@C82 with divalent metal.306 First, it means that these EMFs have the same cage isomers of C82, which agrees with the results of the structural studies of M2C2@C82 (see section 4.3.2). Second, it implies that the formal charge of the cage in M2@C82 EMFs is −4. Therefore, metal atoms such as Sc, Y, or Er are in their divalent state, and there should be a M−M bond in M2@C82, which was indeed confirmed by theoretical studies.303,307 In 2010 Shinohara and co-workers reported the isolation and 13 C NMR structural characterization of Lu2@C76-Td(1) (Figure 6j).308 Spectroscopic data also strongly suggested that the metal atoms are circulating freely inside the cage. Three disamarium EMFs, Sm2@C88-D2(35), Sm2@C90C1(21), and Sm2@C92-D3(85), were isolated and characterized by single-crystal X-ray diffraction in 2011.309 The largest isolated and structurally characterized dimetallofullerene to date is Sm2@C104-D3d(822) (Figure 6o).310 Its structure was determined by a single-crystal X-ray diffraction study of Sm2@C104·2NiII(OEP)·C6H5Cl. In comparison to Er2@C82 and other Sm2@C88,90,92, metal atoms in Sm2@C104 are remarkably ordered (major position has 0.74 fractional occupancy).310 The second largest dimetallofullerenes isolated to date is [email protected] Structural studies of this EMF have not been performed yet, but computational studies show that it is likely to have the D5(450) carbon cage.312 Recently, its La analogue, La2@C100, was characterized by single-crystal X-ray diffraction, and its carbon cage structure was proved to be D5(450) (Figure 6n).313

diagnostic diagram” which is claimed to be able to distinguish carbide clusterfullerenes from dimetallofullerenes based solely on the lattice cell parameters. La2@C80 was first isolated in 1991,269 and the Ih(7) isomer was proposed for its carbon cage based on theoretical calculations,210 but the first structure elucidation based on 13 C NMR and 139La NMR was reported in 1997.270 The NMR study has also shown that La atoms are freely rotating inside the cage. In 2001, La2@C80 was also studied by means of synchrotron X-ray powder diffraction with Rietveld/MEM analysis.271 In agreement with the results of NMR studies, this study has shown the Ih(7) cage structure and distribution of La atoms over the whole cage forming regular dodecahedron with the longest La···La distance of 3.84(2) Å.271 Theoretical studies demonstrated that La atoms in La2@C80-Ih(7) can occupy two almost isoenergetic positions with overall D 2h or D 3d symmetries in which La atoms are facing either the center of hexagons or the single carbon atoms on the junction of three hexagons (Figure 6l).210,272,273 Starting from 2006,274 at least eight derivatives of La2@C80-Ih(7) were characterized by singlecrystal X-ray diffraction.274−281 The major isomer of Ce2@C80282 is isostructural to La2@C80 and has also the Ih(7) cage as shown by 13C NMR spectroscopy and a single-crystal X-ray diffraction studies of its disilirane adduct283 as well as Ce2@C80 cocrystallized with NiII(OEP).284 Likewise, 13C NMR spectrum of the mixture of La2@C80, CeLa@C80, and Ce2@C80 was reported in 2005 and also proved the Ih(7) cage structure for all three EMFs. A second isomer of Ce2@C80, with D5h(6) cage, was characterized by 13C NMR spectroscopy in 2009.285 On the basis of the temperature-dependent shifts in the spectrum, it was also proposed that Ce atoms circulate two-dimensionally along a band of 10 contiguous hexagons. The conclusion on the preferable localization of Ce atoms near the belt of hexagons was recently confirmed by a single-crystal X-ray diffraction study.284 Although D5h(6) is a plausible cage structure for the minor isomer of La2@C80 as well, to our knowledge, structural studies of this EMF have never been reported. Pr is also known to form [email protected] The 13C NMR spectra of Pr2@C80 and LaPr@C80 exhibited characteristic temperature dependence due to the effect of paramagnetic Pr and clearly indicated the Ih(7) cage structure of these two EMFs.287 In addition to M2@C80, La and Ce also form isostructural pairs of dimetallofullerenes for M 2 @C 72 and M 2 @C 78 compositions (Figure 6i,k). La2@C72 was isolated for the first time in 1998,288 and its non-IPR D2(10611) cage structure was elucidated in 2003 based on the 13C NMR study and DFT calculations.289 In 2008 it was also confirmed by single-crystal X-ray diffraction studies of its mono- and bis-adamantylidene adducts.290,291 The structure of Ce2@C72-D2(10611), first isolated in 2001,292 was elucidated by means of 13C NMR spectroscopy in 2008.293 Recently Porfyrakis et al. reported on the isolation and UV−vis as well as 13C NMR spectroscopic characterization of “PrSc@C80”.294 An HRTEM/EELS study then proved it to be Pr2@C72, whose 13C NMR is consistent with the D2(10611) carbon cage.295 La2@C78 was first isolated in 2004, and its carbon cage was assigned to the D3h(5) isomer by means of 13C NMR spectroscopy.296 In 2008 its structure was proved by singlecrystal X-ray diffraction study of adamantylidene adduct.297 In the same year, the structure of Ce2@C78 was elucidated by 13C NMR and by single-crystal X-ray study of the adamantylidene adduct.298 Importantly, for Ce-based dimetallofullerenes, 13C 6003


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Figure 7. Molecular structures of nitride clusterfullerenes: (a) Sc3N@C68-D3(6140); (b) Sc3N@C70-C2v(7854); (c) Sc3N@C78-D3h(5); (d) Sc3N@ C80-Ih(7); (e) DySc2N@C76-Cs(17490); (f) M3N@C78-C2(22010); (g) M3N@C82-Cs(39663); (h) M3N@C84-Cs(51365); (i) M3N@C86-D3(17); (j) M3N@C88-D2(35); (k) La3N@C92-T(86); (l) La3N@C96-D2(186). In (e−j), M denotes Y or lanthanides. Carbon atoms are gray except for the atoms in the adjacent pentagon pairs, which are shown in red; nitrogen is blue, metal atoms are magenta (Sc), dark green (Y, Gd, Dy, etc.), or dark orange (La).

The first detection of metallo-heterofullerenes La@C81N and La 2 @C 79 N formed by fast atom bombardment mass fragmentation of the adducts La@C82(NCH2Ph) and La2@ C80(NCH2Ph) was reported by Akasaka et al. in 1999,314 but it was only in 2008 when dimetallo-heterofullerenes M2@C79N (M = Y, Tb) could be synthesized in the arc-discharge synthesis, isolated, and structurally characterized by Dorn, Balch, and co-workers.315 Isolation of Gd2@C79N was also reported in 2011.316 Single-crystal X-ray diffraction has shown that a Tb2 unit is encapsulated in the pseudo-Ih(7) carbon cage; the exact position of the nitrogen atom, which replaces one of the carbon atoms in the C80 cage, could not be elucidated by the X-ray diffraction study, but a DFT studies showed that it is likely to be located on the pentagon−hexagon−hexagon junction.315 4.2.3. Trimetallofullerenes. Well-characterized conventional EMFs are limited so far to molecules containing one or two metal atoms. Although several EMFs with three encapsulated metal atoms have been reported (Er3C74,317 Tb3C80,157 Y3C80,318 or Dy3C98311), their characterization is mostly limited to mass-spectrometry data, and there has been no experimental evidence showing whether the isolated species with three metal atoms are conventional trimetallofullerenes or carbide clusterfullerenes (as now proved for Sc3C2@C80319). DFT studies showed that Y3C80 is most likely a conventional trimetallofullerene Y3@C80-Ih(7) since this structure is much more stable than Y3C2@C78 isomers.318 On the contrary, Dy3C98 is likely to be a carbide with C96-D2(186) cage since Y3C2@C96-D2(186) is found to be 50 kJ/mol lower in energy than the most stable trimetallofullerene isomer Y3@C98C2(166).318

NMR spectroscopy.5 The Sc atoms were found to form an equilateral triangle with a nitrogen atom in the center of the planar cluster. A single-crystal X-ray study was also performed for the o-xylene solvate of Sc3N@C80-Ih(7).320 Because of its high availability for the further studies of chemical reactivity, numerous single-crystal X-ray diffraction studies of Sc3N@C80Ih(7) derivatives were reported since the first such study of its Diels−Alder cycloadduct in 2002.321 The family of M3N@C80-Ih(7) NCFs is so far the most extended and most studied group of EMFs. For nonderivatized NCFs, single-crystal X-ray diffraction studies were reported for M = Gd,108 Tb,102 Dy,322 Tm,323 and Lu320 as well as for mixed-metal MxL3−xN@C80 -Ih(7) NCFs with ErSc2 N,95 CeSc 2 N, 103 GdSc 2 N, 32 4 TbSc 2 N, 324 LaSc 2 N, 32 5 and Gd2ScN324 nitride clusters (except for Lu3N@C80 studied as a solvate with o-xylene, all other structures were cocrystallized with Ni(OEP) or Co(OEP)). Single-crystal X-ray structures of the derivatives were studied so far only for Y3N@C80326,327 and [email protected] In the row of lanthanides, the studies revealed that the M3N cluster is planar for Dy,322 Tm,323 and Lu320 and is pyramidal for metals with larger ionic radii such as Tb102 and Gd.108 In the mixed metal NCFs formed by metal atoms of different size, such as Ce and Sc or Gd and Sc, crystallographic as well as vibrational and theoretical studies show that nitrogen atoms is displaced from the center of the cluster toward atoms with smaller radii (i.e., Sc−N bonds are shorter and Gd−N bonds are longer than in Sc3N@C80 and Gd3N@C80, respectively).103,115,116,324 The highly symmetric carbon cage C80-Ih(7) facilitates 13C NMR studies of the most abundant isomer M3N@C80-Ih(7). The fact that only two lines were observed in the spectra of Sc3N@C80,5 CeSc2N@C80,103 CeLu2N@C80,329 LuxSc3−xN@ C80,119 HoxSc3−xN@C80,185 and LuxY3−xN@C80 (x = 0−3)119 indicates that the M3N cluster rotates fast at the NMR time scale. For Ce- and Ho-containing NCFs, a paramagnetic shift of 13 C lines was observed in the NMR spectra (see also section 6.1).103,185,329

4.3. Clusterfullerenes

4.3.1. Nitride Clusterfullerenes (NCFs). In 1999 the molecular structure of the first isolated nitride clusterfullerene, Sc3N@C80, was determined by single-crystal X-ray diffraction (in the form of cocrystals with Ni(OEP), Figure 7) and 13C 6004


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Figure 8. Molecular structures of carbide clusterfullerenes: (a) Ti2C2@C78-D3h(5); (b) Sc2C2@C80-C2v(5); (c) Sc3C2@C80-Ih(7); (d) Sc4C2@C80Ih(7); (e) Sc2C2@C82-C3v(8); (f) Sc2C2@C84-D2d(23); (g) Y2C2@C84-C1(51383); (h) Lu3C2@C88-D2(35); (i) Gd2C2@C88-D2(35); (j) Y2C2@C92D3(85); (k) Y2C2@C100-D5(450). Carbon atoms in the fullerene cage are gray except for the atoms in the adjacent pentagon pairs, which are shown in red; whereas carbon atoms of the endohedral C2 units are shown in orange. Metal atoms are magenta (Sc), dark green (Y, Gd), or light green (Lu).

almost 50 times lower than that of Sc3N@C68, and this NCF was first isolated in 2007.120 Assignment of its carbon cage to the C70-C2v(7854) isomer (Figure 7b) was based on extended DFT calculations and vibrational spectroscopy.120 Importantly, the shape of the Sc3N cluster in Sc3N@C70 is strongly distorted from the equilateral triangle, the Sc−N−Sc angles being 150° and 105°. For non-Sc NCFs, the carbon cages smaller than C80 are rarely accessible. M3N@C68 can be synthesized only in the form of mixed metal NCF with Sc as a second metal. This way, LuSc2N@C68, Lu2ScN@C68, and DySc2N@C68 could be isolated in sufficient amounts to afford their characterization by UV−vis-NIR and IR spectroscopies,118 all showing almost identical carbon cage patterns to that of [email protected] A special NCF to be mentioned is DySc2N@C76 as neither Sc3N@C76 nor Dy3N@C76 could be isolated as nonmixed NCFs.117 Vibrational spectroscopy and extended DFT calculations allowed assignment of its cage structure to the Cs(17490) with two pairs of adjacent pentagons (Figure 7e).117 The structure of M3N@C78 is a nice example of an influence of the cluster size on the carbon cage isomerism. Spectroscopic studies of Dy3N@C78 and Tm3N@C78 in comparison to Sc3N@C78 showed that it was unlikely that lanthanide-based NCFs had a C78-D3h(5) carbon cage.337 DFT calculations showed that because of the larger cluster size, encapsulation of Y3N into the C78-D3h(5) cage forces a pyramidalization of the cluster, whereas the non-IPR C2(22010) cage with two adjacent pentagon pairs has a more suitable shape of the cage and provides the structure of Y3N@C78 with planar cluster and lower energy (Figure 7f).337 IR spectra computed for the Y3N@ C78−C2(22010) perfectly matched experimental data for Dy3N@C78 and [email protected] In 2009, the molecular structure of Gd3N@C78-C2(22010) was proved by single crystal X-ray diffraction.105 Recently, Y3N@C78-C2(22010) was isolated and its isomeric structure was verified by UV−vis and vibrational spectroscopy.338 13C NMR spectrum of Y3N@ C78-C2(22010) was reported by Dorn et al.339

The carbon cage vibrational pattern is very similar in the vibrational spectra of all M3N@C80-Ih(7) NCFs, including M = Sc,330,331 Y,322 Gd,114 Tb,12 Dy,322 Ho,12 Er,12 Tm,112 Lu,119 as well as mixed metal NCFs TiSc2N@C80,186 ScYErN@C80,332 YxSc3−xN@C80,333 NdxSc3−xN@C80,184 GdxSc3−xN@C80,115 DyxSc3−xN@C80,184 HoxSc3−xN@C80,185 ErxSc3−xN@C80,12,329 LuxSc3−xN@C80,119 and LuxY3−xN@C80119 (x = 1−2). This fact allows a reliable assignment of the carbon cage isomer for those M3N@C80-Ih(7) NCFs, whose structures are not elucidated by single-crystal X-ray diffraction or 13C NMR. The structure of the second, less abundant isomer of Sc3N@ C80 with D5h(6) cage symmetry was first assigned in 2003 by 13 C NMR spectroscopy334 followed by single-crystal X-ray study in 2006.335 Recently, a single-crystal X-ray structure of Sc3N@C80(CF3)18, trifluoromethylated derivative of Sc3N@ C80-D5h(6), was reported.336 The list of other structurally characterized M3N@C80-D5h(6) NCFs is much shorter than that for the Ih(7) isomer and includes Tb3N@C80-D5h(6)102 and Tm3N@C80-D5h(6),323 both cocrystallized with Ni(OEP) and studied by single-crystal X-ray diffraction, as well as Y3N@C80-D5h(6)99 and LuxSc3−x@C80D5h(6) (x = 1−3)184 characterized by 13C NMR spectroscopy. Vibrational spectroscopic studies showing close similarity of the spectral pattern of all M3N@C80-D5h(6) NCFs were performed for Sc3N@C80-D5h(6),331 Tm3N@C80-D5h(6),112 and mixed MxSc3−x@C80-D5h(6) (x = 1−3) NCFs with M = Gd,116 Ho,185 and Lu.184 In addition to Sc3N@C80, the family of Sc-based NCFs includes Sc3N@C68, Sc3N@C70, and Sc3N@C78. The non-IPR cage structure with D3(6140) symmetry and three pairs of adjacent pentagons (Figure 7a) was first proposed for Sc3N@ C68 in 2000 based on the 13C NMR and computational data.16 In 2003 this structure was confirmed by single-crystal X-ray diffraction study of Sc3N@C68·Ni(OEP)·2C6H6.96 Sc3N@C78D3h(5) (Figure 7c) was first isolated in 2001 and characterized by 13C NMR and single-crystal X-ray diffraction of Sc3N@ C78·Co(OEP)·1.5C6H6·0.3CHCl3.97 The yield of Sc3N@C70 is 6005


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compound was reanalyzed by powder synchrotron X-ray diffraction with Rietveld/MEM analysis, which at that time showed an additional density in the center of the cage corresponding to the C2 unit.351 In 2007 the molecular structure of Sc2C2@C82-C3v(8) was determined by a singlecrystal X-ray diffraction study of the adamantylidene adduct,353 followed in 2011 by the X-ray study of the nonderivatized Sc2C2@C82-C3v(8) cocrystallized with Co(OEP).350 In 2008 Sc2C2@C82-C3v(8) was also analyzed by 2D INADEQUATE NMR study, and a signal from acetylide unit was found at 253 ppm.349 The first 13C NMR spectrum of Sc2C2@C82-Cs(6) pointing to the Cs cage symmetry was reported in 2000, but at that moment it was still thought to be “Sc2@C84”. In 2011 Akasaka et al. reanalyzed the 13C NMR spectrum of a 13C-enriched sample and identified a signal from the acetylide unit at 244.4 ppm.354 In the same work, single crystal X-ray study of the pyrrolidine derivative of Sc2C2@C82-Cs(6) was performed and unambiguously confirmed the structure of this isomer.354 Recently, complete structural characterization of Sc2C2@C82C2v(9) was fulfilled by means of 13C NMR spectroscopy (1D and 2D INADEQUATE) as well as single-crystal X-ray diffraction study of its pyrrolidine adduct.355 Ti2C2@C78 was originally thought to be Ti2@C80, and in 2001 its 13C NMR spectrum was interpreted as a mixture of C80-D5h(6) and C80-Ih(7) isomers in the 3:1 ratio.356 In 2005, computational studies have shown that Ti2C2@C78 is much more stable than Ti2@C80, and the carbide clusterfullerene Ti2C2@C78-D3h(5) also better fits the experimental 13C NMR spectrum (Figure 8a).357,358 The acetylide unit in Ti2C2@C78 was visualized by an HRTEM study.359 In 2006 Wang, Lu, Shinohara and co-workers reported the isolation of “Sc2C70” and its characterization by 13C NMR spectroscopy and DFT calculations.360 On the basis of a combination of experimental and computational data, the structure was assigned to the non-IPR Sc 2 C 2 @C 68 C2v(6073),360 albeit recent theoretical study showed that the non-IPR Sc2@C70-C2v(7854) is more energetically stable.361 In 2011 one of the “Sc2C82” isomers was shown to be Sc2C2@C80 with C2v(5) cage structure (Figure 8b), as proved by a singlecrystal X-ray diffraction study of the adamantylidene adduct.362 A 13C NMR spectroscopic study of Sc2C2@C80 revealed that the motion of Sc2C2 cluster is temperature dependent: at room temperature the spectrum corresponded to a Cs-symmetric structure because the cluster is tightly fixed, but at 413 K rotation of the cluster is sufficiently fast to average the signals of carbon atoms and produce the spectrum corresponding to the C2v(5) cage.362 In 2011 the structure of the nonderivatized Sc2C2@C80-C2v(5) was also analyzed by single-crystal X-ray diffraction.349 The close correspondence of the absorption spectra of the structurally characterized Sm2@C88 and Sm2@C90 to the spectra of Gd2C90 and Gd2C92 indicates that the latter are probably carbide clusterfullerenes Gd2C2@C88-D2(35) (Figure 8i) and Gd2C2@C90-C1(21), respectively.309 The largest carbon cage among structurally characterized carbide clusterfullerenes is found in Gd2C2@C92-D3(85), whose structure was elucidated by single-crystal X-ray diffraction study of Gd 2 C 2 @ C92·NiII(OEP)·2C6H6.363 Recently, a family of Y2C2@C2n carbide clusterfullerenes was isolated by Dorn et al., and the cage structures of some of the compounds, including Y2C2@ C92-D3(85) (Figure 8j), were determined by 13C NMR spectroscopy.364 The structure of Y2C2@C100-D5(450) was

In contrast to Sc, yttrium and lanthanides form isolable M3N@C2n NCFs with 2n > 80. In particular, the structures with C82, C84, C86, and C88 cages can be isolated for yttrium99,100 and lanthanides from Gd to Lu (but excluding Yb).101,102,107,112,114,340 Molecular structures were determined by single-crystal X-ray diffraction for Gd3N@C82-Cs(39663),106 M3N@C84-Cs(51365) (M = Tb,101 Tm,341 Gd341), M3N@C86D3(19) (M = Tb,102 Gd342), and Tb3N@C88-D2(35),102 all cocrystallized with Ni(OEP) (Figure 7g−j). For Y3N@C2n (2n = 80−88), 13C and 39Y NMR spectra were also reported and were found to be consistent with the cage isomers determined crystallographically for other metals.99,100 For larger lanthanides (La, Ce, Pr, Nd), the distribution of the cage sizes shifts to larger cages. Thus, for Nd-based NCFs, Nd3N@C88 is the main product of the synthesis,343 while for La the distribution is further shifting to La3N@C92 and La3N@C96 as the largest isolable NCF.122 Neither 13C NMR spectroscopic nor crystallographic studies have been performed so far for any M3N@C2n with 2n > 88. On the basis of theoretical computations213,344 and electrochemical gaps,345 C92-T(86) and C96-D2(186) cage isomers (Figure 7k−l) were tentatively assigned to M3N@C92 and M3N@C96 (M = La, Ce, Pr), respectively. The structural assignment for La3N@C92-T(86) should be considered as questionable since extended computational studies proved that the isomer La3N@C92-C2(36) is 125−150 kJ/mol lower in energy (albeit it has smaller HOMO−LUMO gap and does not fit electrochemical data).346 Interestingly, while encapsulation of the La3N cluster inside the carbon cages smaller than C86 was not observed,122 metallic nitride azafullerene La3N@C79N was detected by mass spectrometry347 and its extended DFT computations were reported.348 4.3.2. Carbide Clusterfullerenes. In 2001 a 13C NMR spectroscopic study of “Sc2@C86” has shown that this molecule exhibits 11 lines in the range of C-sp2 atoms.7 Such a spectral pattern is not consistent with any of the IPR isomers of C86. At the same time, the spectrum resembles that of D2d(23)-C84. Hence, “Sc2@C86” was proposed to be a carbide clusterfullerene, Sc2C2@C84 (Figure 8), and its structure was also confirmed by powder synchrotron X-ray studies with Rietveld/MEM analysis, which revealed additional electron density due to the rapidly rotating C2 unit in the center of the cage.7 In 2008 the structure of Sc2C2@C84-D2d(23) was further verified by 2D INADEQUATE NMR study; 13C NMR signal from the acetylide unit at 249 ppm could be also detected in that work.349 Finally, single-crystal X-ray diffraction study of Sc2C2@C84-D2d(23) was reported in 2011 by Akasaka et al.350 Soon after that finding the encapsulation of acetylide unit was found to be a common feature of metallofullerenes (Figure 8). 13C NMR spectroscopic study of three isomers of “Y2@C84” in 2004 have shown that these EMFs were actually isomers of Y2C2@C82 with Cs(6), C3v(8), and C2v(9) carbon cages.193 A close correspondence of the absorption spectra of Y2C2@C82 isomers to those of “Sc2@C84” and “Dy2@C84” indicated that the latter were also carbide clusterfullerenes Sc2C2@C82 and Dy2C2@C82 with the same cage isomers. Analogous carbon cage isomers can be also assigned to three isomers of Er2C2@ C82.305 The structure of Y2C2@C82-C3v(8) was also studied by powder synchrotron X-ray studies with Rietveld/MEM analysis.304,351 The assignment of the major isomer of Sc2C84 to Sc2C2@ C82-C3v(8) (Figure 8e) was performed in 2006 by Akasaka et al. by means of 13C NMR spectroscopy.352 In the same year the 6006


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proposed in the same study based on the DFT calculations (Figure 8k). In contrast to the D2d symmetric cage in Sc2C2@ C84, only C1 symmetry of the fullerene cage was found in Y2C2@C84 by 13C NMR spectroscopy.364 A recent DFT study showed that Y2C2@C84 likely has a non-IPR C1(51383) carbon cage (Figure 8g).365 Structural and computational studies of carbide clusterfullerenes show that the shape of the M2C2 cluster strongly depends on the carbon cage and can vary from almost linear (Ti2C2@ C78 or Y2C2@C100) to butterfly like (M2C2@C82-C2v(3), Gd2C2@C92) or orthogonal (in Sc2C2@C84, C2 unit is perpendicular to the Sc···Sc axis) (Figure 8). Gradual changes in the Y2C2 cluster shape with the cage size were described by Dorn et al. as the result of “nanoscale fullerene compression.”.364 The family of carbide clusterfullerenes is not limited to EMFs with two metal atoms. “Sc3C82” was thought to be a trimetallofullerene Sc3@C82 since its first isolation in 1992.366 However, in 2005 by means of a 13C NMR spectroscopic study of its diamagnetic anion and by single-crystal X-ray diffraction study of the adamantylidene adduct of “Sc3@C82” Akasaka and co-workers have shown that “Sc3@C82” is in fact a carbide clusterfullerenes Sc3C2@C80 with icosahedral Ih(7) cage (Figure 8c).319 In the Sc3C2 cluster, scandium atoms form a triangle, while the C2 unit can adopt different configurations either inplane or perpendicular to the Sc3 plane.319,367 Another carbide clusterfullerene, Lu3C2@C88, has been isolated in 2011 and characterized by Raman spectroscopy and DFT computations; its structure was proposed to be D2(35) as in the corresponding NCF Lu3N@C88 (see above).340 The number of encapsulated metal atoms in carbide clusterfullerenes can be as high as four as found for Sc4C2@ C80.368 By means of 13C NMR spectroscopy, its carbon cage was assigned to Ih(7). DFT computations have shown that scandium atoms form a tetrahedron with the acetylide unit close to its center (Figure 8d).369 4.3.3. Methano-clusterfullerene. The icosahedral Ih(7) carbon cage was proposed for the methano-clusterfullerene Sc3CH@C80 based on the absorption and Raman spectroscopic studies, which revealed close resemblance between the spectra of Sc3CH@C80 and [email protected] The proposed structure was also supported by DFT calculations (Figure 9a).8 4.3.4. Oxide Clusterfullerenes. The family of oxide clusterfullerenes discovered by Stevenson et al. in 2008 includes so far Sc4O2@C80 (the most abundant),9 Sc4O3@C80,135 and [email protected] All three compounds were characterized by single-crystal X-ray diffraction studies with NiII(OEP) as a cocrystallizing agent, which have shown that the C80 cage in Sc4O2@C80 and Sc4O3@C80 has Ih(7) symmetry,9,135 while Sc2O@C82 has Cs(6) cage isomer (Figure 9d−f).370 In Sc4O2,3 clusters, Sc atoms form a distorted tetrahedron, while μ3oxygen atoms are located above its faces.9,135 In Sc2O, a Sc− O−Sc angle of 157° has been found.370 13C and 45Sc NMR spectroscopic study for Sc4O2@C80 reported in 2012 showed that the cluster is rotating fast at the NMR time scale giving only two 13C signals as in many other EMFs with the C80-Ih(7) carbon cage (Table 1).371 4.3.5. Sulfide Clusterfullerenes. Sulfide clusterfullerenes were discovered in 2010 and were shown to have different structures than the most abundant oxide clusterfullerenes.11,123 UV−vis-NIR absorption spectra of M2S@C82 (M = Sc, Y, Dy, Lu) resembled closely those of M2C2@C82-C3v(8) which indicated that M2S@C82 also have the C3v(8) cage isomer

Figure 9. Molecular structures of selected clusterfullerenes: (a) Sc3CH@C80-Ih(7); (b) Sc3NC@C78-C2(22010); (c) Sc3NC@C80Ih(7); (d) Sc4O2@C80-Ih(7); (e) Sc4O3@C80-Ih(7); (f) Sc2O@C82Cs(6); (g) Sc2S@C70-C2(7892); (h) Sc2S@C72-Cs(10528); (i) Sc2S@ C82-C3v(8). Carbon atoms in the fullerene cage are gray except for the atoms in the adjacent pentagon pairs, which are shown in red. Endohedral carbon atoms (in Sc3CH and Sc3NC clusters) are shown in orange, nitrogen is blue, oxygen is red, sulfur is yellow, Sc atoms are magenta.

(Figure 9i).11 This conclusion was also supported by DFT calculations and  for Sc2S@C82  by IR spectroscopy.11 The structures of two isomers of Sc2S@C82 isolated by Echegoyen et al.123 have been determined by single-crystal X-ray diffraction as Cs(6) and C3v(8);372 Sc2S clusters are bent with the Sc−S− Sc angles of ca. 114° and 97°, respectively. In 2012, Sc2S@C72Cs(10528) with non-IPR cage was isolated and structurally characterized by single-crystal X-ray diffraction (Figure 9h).194 Very recently, Echegoyen et al. reported the isolation and spectroscopic and electrochemical characterization of Sc2S@ C70.373 Two independent computational studies showed that the most probable cage isomer of Sc2S@C70 is a non-IPR C2(7892) (Figure 9g).373,374 4.3.6. Cyano-clusterfullerenes. The Ih(7) cage isomer of Sc3NC@C80, isolated in 2010 by Lu, Shu, Wang and coworkers, was assigned by a single-crystal X-ray diffraction study of Sc3NC@C80·NiII(OEP)·1.5C6H6 and 13C/45Sc NMR spectroscopy (Figure 9c).10 The Sc3NC cluster is planar with the nitrogen atom in the center and C atom residing on one side of the triangle formed by the Sc atoms. Recently the same group also reported isolation of Sc3NC@C78−C2(22010) (Figure 9b), whose structure was elucidated based on UV−vis and vibrational spectroscopy and DFT computations.375 4.4. Noble Gas and Nonmetal Endohedral Fullerenes

Since fullerenes encapsulating noble gas (Ng) atoms and other nonmetal atoms or molecules are obtained starting from the already defined carbon cages (usually C60 or C70), which are preserved in the endohedral structures, the problem of the structure determination for such endohedral fullerenes is not as crucial as it is for endohedral metallofullerenes. On the other hand, encapsulation of noble gases and other atoms into the 6007


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analysis, which shows that the frontier MOs of the EMFs can be described as MOs of the fullerene cores with additional electrons borrowed from metal atoms.418,419 The contributions of metal atoms to these MOs are almost negligible in the majority of EMFs (some exceptions however exist and they will be discussed later). Suggested first for monometallofullerenes such as La3+@ C823−, the ionic model is also used to describe the electronic structure of endohedral fullerenes with more complex encapsulated species, such as dimetallofullerenes (e.g., (La3+)2@C806−), nitride clusterfullerenes (e.g., (Sc3+)3N3−@ C806−), carbide clusterfullerenes (e.g., (Sc3+)2C22−@C824−), and all other EMFs.6,21,26 In favor of the ionic conjecture, the results of XPS studies (see Section 6.6) revealed that the spectra in the range of metal levels usually correspond well to the spectra of metal oxides in the corresponding oxidation states.58 The ionic conjecture is further supported by other techniques such as UV−vis-NIR absorption spectroscopy and electrochemistry (spectra and redox potentials of isostructural EMFs with the same isomeric cage but different metal atoms are usually very similar if these metal atoms are in the same oxidation state). 21,26,420 Experimentally measured electrochemical gap values for a series of M3N@C2n compounds (with the formal transfer of six electrons) show a good correlation to the DFT-computed gaps between the LUMO+2 and LUMO+3 orbitals of the corresponding empty fullerenes.345 Computational studies have also shown that the stability order of the isomers of EMFs corresponds well to the stability order of the isomers of empty fullerenes in the appropriate charge states (this correspondence will be discussed in more detail in the next section).120,213,337 The spatial distribution of electrostatic potential (ESP) inside fullerenes also agrees well with the ionic model of bonding. Nagase and co-workers223,421,422 have shown that ESP has large negative values inside negatively charged carbon cages. This results in the strong stabilization of the metal cations if they are placed inside fullerene anions thus favoring the formation of EMFs. Importantly, the minimum of the electrostatic potential in C823−-C2v(9) was found in the position where the metal ion resides in M@C82 as predicted by calculations223,423 and shown by experimental X-ray diffraction studies (Figure 10).36,259,260

fullerene proceeds with very low yields and separation of the Ng@C2n molecules from the corresponding empty fullerenes is often very tedious. Kr@C60 was the first Ng@C60 compound isolated in 1999 in sufficient amounts and purity to allow its spectroscopic characterization.71 The 13C NMR line showed only one signal which was shifted downfield from that of empty C60 by 0.39 ppm. UV−vis-NIR absorption spectra of C60 and Kr@C60 are also very similar except for the slight red shift of some bands for the latter. In 2002, the sample enriched with ca. 30% of Xe@C60 was obtained and characterized by NMR spectroscopy, which showed a downfield shift of 0.95 ppm of a 13 C signal with respect to C60 and a single line in 129Xe NMR spectrum at −8.89 ppm vs Xe dissolved in benzene (which gives a chemical shift of 179.24 ppm relative to 129Xe gas).72 For Ar@C60, a downfield shift of 0.17 ppm was observed in the 13 C NMR spectrum reported in 2006.376 Single 13C NMR signals observed in the spectra of all Ng@C60 molecules show that noble gas atoms reside in the center of the carbon cage. The same conclusion was done based on the single-crystal Xray diffraction study of C60 enriched with 9% of Kr@C60 and cocrystallized with Ni(OEP).377 The study revealed an additional electronic density due to Kr atoms residing in the center of C60 cages. Similar to Ng@C60, H2@C60 also exhibits very weak interaction with the carbon cage. The only 13C NMR signal of H2@C60 is downfield shifted from the C60 value by only 0.078 ppm.77 In 1H NMR, a peak at −1.44 ppm was observed which is shifted 5.98 ppm upfield from the signal of dissolved H2.77 Likewise, for N@C60 and P@C60, highly isotropic ESR spectra indicate that encapsulated N and P are in their atomic state and exhibit only weak interaction to the cage.136−138 In 2011, Kurotobi and Murata reported the singlecrystal X-ray structure of [email protected]

5. THEORETICAL STUDIES OF ELECTRONIC AND MOLECULAR STRUCTURE OF EMFS 5.1. Chemical Bonding in EMFs

The variability of the clusters that can be encapsulated in EMFs and flexibility of the carbon cage isomerism in dependence on the endohedral species discussed in the previous section raise the question of the reasons for this diversity. In this section we give an overview of the electronic structure and bonding in EMFs and discuss the factors which determine the stability of the isomers of EMFs. These questions are largely addressed by computational studies, which thus provide an important insight into the structures and properties of EMFs. 5.1.1. Metal-Cage Bonding. Since the isolation of the first EMFs in early 1990s, the electronic structure of these compounds was of high interest to the fullerene community because it is closely correlated to the stability of EMFs. The high electron affinity of fullerenes naturally suggests the salt-like ionic structure of EMFs, in which metal cations are encapsulated in the negatively charged carbon cages (e.g., La3+@C823−). Thanks to the radical nature of M@C82 (M = La, Y), the ionic conjecture soon received confirmations by ESR spectroscopic studies.60 Namely, relatively small hyperfine coupling constants of the metal atoms (1.25 G for La@C8260 and 0.48 G for Y@C82417) observed in the ESR spectra indicated that the spin density in M@C82 is presumably localized on the fullerene cage. This finding corresponds well to the structures, in which valence electrons of the metal atoms are almost completely transferred to the carbon cage.60 The same conclusion also follows from the molecular orbital

Figure 10. Electrostatic potential maps of (a) C82 3−-C2v(9) (reproduced with permission from ref 223. Copyright 1998 Elsevier Science B.V.), (b) C844−-D2d(23), and (c) C806−-Ih(7) (reproduced with permission from ref 422. Copyright 1996 Elsevier Science B.V.). In each map, the 40 contour lines are in the range of 20 kcal/mol from the highest negative value.

At the same time, it was also found that the electrostatic potential inside C806− has no distinguishable minima, and hence there are no distinct bonding sites for metal atoms or clusters in C80-Ih(7) (Figure 10). This is indeed confirmed by several computational studies of La2@C80-Ih(7),210,272,273,424 which show that there are two almost isoenergetic minima for positions of La atoms with D2h and D3d molecular symmetries, 6008


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and each minimum is multiplied by the group symmetry operations of Ih group. As a result, La2 unit exhibits almost free rotation inside the C80 cage, and the charge density of La atoms forms a pentagonal dodecahedron as revealed by a synchrotron radiation powder diffraction study.271 Likewise, computational and experimental studies of other EMFs based on C80-Ih(7) cage (such as Sc3N@C80-Ih(7)383,425−428) usually reveal almost free rotation of endohedral clusters. Spatial distribution of ESP was also been used to explain the positions and dynamics of the metal atoms in derivatives of EMFs.33,277 All the facts outlined above resulted in the wide acceptance of the ionic conjecture for EMFs by the scientific community. However, although the ionic model explains many spectroscopic and structural properties of EMFs, the transfer of a large integer number of electron from an endohedral atom or cluster to the fullerene cage is only formal and should not be understood literally. There are numerous studies which show that the ionic model is oversimplified. While the vibrational (IR, Raman) and UV−vis-NIR spectra of EMFs with the same isomeric cage but different metals are indeed very similar and in many cases are well reproduced by DFT or TD-DFT calculations (e.g., vibrational spectra of all M3N@C80 NCFs are almost identical except for the cluster-based modes), analogous calculations for the charged empty cages do not match experimental spectra of EMFs for the fullerene-based vibrations or excitations.37,110,385,429 This fact results in the conclusion that the electronic state of the cage cannot be considered as purely ionic, and the covalent contribution to the metal-cage bonding must be also taken into account. Likewise, while the analysis of the frontier MO energy levels in the EMFs and the corresponding empty cages can be indeed interpreted by an electron transfer from the metal to the cage, many experimental430,431 and theoretical383,421,423,432,433 studies clearly show a substantially nonzero population of nd-levels of the endohedral metal atoms. The mixing of the MOs of the cluster and fullerene cage as well as the corresponding change of the orbital energies in EMF as compared to the MOs of the empty cage is the most apparent for Sc3N@C78 (Figure 11).383,385 In many other EMFs the metal contribution to individual π-MO orbitals of the cage is small and can remain hidden in a standard MO analysis, but the analysis of the system in its whole complexity, e.g., the analysis of the electron density (rather than individual orbitals), can reveal significant degree of covalency in metal-cage interactions.37,428,433−435 In a most illustrative way such an analysis can be done with the use of a promolecule deformation density approach,37,383,428,433,436 i.e., by visualizing the changes in the electron density distribution Δρ = [ρ(Mol) − ρ(ref)], where ρ(Mol) is the electron density in the molecule under study, while ρ(ref) is the electron density of the reference system (see a review by Koritsanszky and Coppens in ref 437). Obvious choices of the reference system in the studies of EMFs can be either (i) noninteracting metal atoms or clusters (e.g., Sc3N) and corresponding C2n molecules or (ii) metal or cluster cations (e.g., Sc3N6+) and appropriately charged C2n anions (e.g., C2n6−) (Figure 12). In particular, when two 6-fold charged ions were taken as a reference, a considerable concentration of the electron density was found at the Sc atoms in all Sc3N@C2n (2n = 68, 78, 80) EMFs,37,428 the effect referred by some researchers as “back-donation”.434,435 The cluster-cage bonding in Sc3N@C78 and Sc3N@C80 was also analyzed using the energy decomposition method,383 and a strong orbital mixing

Figure 11. (a) PBE/TZ2P Kohn−Sham MO energy levels in Sc3N@ C78-D3h(5), C78-D3h(5), and C786−-D3h(5). MOs Sc3N@C78 and C78 are given in the same scale, while MO levels of C786− are shifted to match the energies of the neutrally charged species (based on the data from ref 385); (b) MO isosurfaces for a2′ orbital with considerable Sc−C bonding contribution in Sc3N@C78 (left), and corresponding orbital in C78 (right).

Figure 12. Difference densities of C3v conformer of Sc3N@C80: (a) [ρ(C806−) − ρ(C80)]; (b) [ρ(Sc3N@C80) − ρ(C80) − ρ(Sc3N)]; (c) [ρ(Sc3N@C80) − ρ(C806−) − ρ(Sc3N6+)]; (d) [ρ(Sc3N@C80−) − ρ(Sc3N@C80)]. (a−c) are plotted with an isodensity value of 0.008 au, (d) is plotted with an isodensity value of 0.0016 a.u; brown denotes regions with positive Δρ, green − negative. In (d) the surplus electron in the Sc3N@C80− anion that results in the redistribution of the electron density of Sc atoms is shown. Thus the positive and negative lobes cancel each other upon integration, and the change of the Sc charges is negligible, whereas the spin density in the anion radical is localized on the Sc3N cluster (not shown). This effect is dubbed the spatial spin-charge separation. Reproduced with permission from ref 428. Copyright 2008 American Chemical Society.

and an electronic reorganization were found after the Sc3N6+ cation was encapsulated inside C78,806− cages. 6009


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as covalent if the total energy density at BCP, Hbcp, is negative.446 The in-depth study of the metal-cage and intracluster bonding by QTAIM/DFT was reported in 2009 for the four major classes of endohedral metallofullerenes, including monometallofullerenes, dimetallofullerenes, metal-nitride clusterfullerenes, metal-carbide clusterfullerenes, as well as Sc3CH@C80 and Sc4Ox@C80 (x = 2, 3).307 Recently, QTAIM studies were also reported for Y3@C80,318 M2S@C82 (M = Sc, Y),11 TiSc2N@C80,436 and Sc3NC@C2n clusterfullerenes,447 as well as for analysis of the metal−metal bonding in EMFs.38 The analysis of the BCP indicators showed that both the intracluster and the metal-cage interactions in EMFs were characterized by the negative total energy density, which means that bonding in EMFs exhibits a high degree of covalency.307 Furthermore, the electron density in EMF molecules was found to be a complex topological object, with many critical points, unpredictable number of the bond paths, and large bond ellipticities, similar to the bonding situation in complexes of transition metals with π-carbocyclic ligands.448 In this respect, the analysis of the metal-cage bonding based only on the properties of the bond paths was found to be insufficient, and the metal-cage delocalization indices, δ(M, C), were also analyzed (by definition, δ(M, C) is the number of the electron pairs shared by the atoms M and C, which is very similar to the “bond order” in Lewis definition). It was found that in the majority of EMFs, δ(M, C) values are a smooth function of M− C distances and do not exceed 0.25; however, when summed up over all metal-cage interactions, values of δ(M, cage) close to 2−3 were obtained.307 While QTAIM atomic charges of the metal atoms were found to be approximately two times smaller than their formal oxidation states, the total number of the electron pairs shared by the metal atoms with the EMF molecule, Δ(M), was found to be close to the typical valence of the given element (≈3 for Sc, Y, and La, and ≈4 for Ti).307 Thus, an analysis of the Δ(M) values circumvents an apparent contradiction between the ionic model and the substantial degree of covalency in metal-cage interactions revealed by more comprehensive analyses. The surplus electrons (e.g., 6 electrons in M3N@C2n compounds) are indeed available to the carbon cage in EMFs, but these electrons are not completely transferred to the cage; instead, they are shared between the endohedral cluster and the carbon cage (note that sharing of electrons is the essence of the covalent interaction). In some works the bonding in EMFs was also studied via analysis of the electron localization function (ELF).11,38,318,449 The ELF was first introduced by Becke et al.450 and in alternative formulation by Savin et al.451 to visualize the electron localization in atoms, molecules, and crystals and is in some respects similar to the Laplacian of the electronic density (e.g., ELF is homeomorphic to Laplacian452). The topological analysis of ELF yields basins, which are characterized by a synaptic order (usually a number of atoms, the bonding of which is described by the basin, e.g., monosynaptic basins are typical for lone electron pairs, disynaptic basins  for bonds between two atoms, trisynaptic basins  for three-center bonding, etc.) and an electronic basin population (an integral of the electron density within the basin, for valence basins it roughly corresponds to twice the bond order).453 Thus, chemical bonds in terms of ELF can be visualized as polysinaptic valence basins (Figure 13). While in the empty fullerene molecule only disynaptic valence carbon−carbon basins V(C,C) are found, the formation of the metal-cage bond

Computed atomic charges are also much smaller than the values expected for the purely ionic bonding. For instance, net Bader, Mulliken, and NBO charges of the Sc3N cluster in Sc3N@C80 are +3.50, +2.88, and +2.99, respectively, at the B3LYP/6-311G* level of theory;428 another group reported the net Sc3N charges of +1.28 (Mulliken), +1.10 (Hirshfeld), +0.94 (Voronoi) at the BP/TZVP level of theory with ZORA relativistic corrections.438 The charges strongly depend both on the method of theory used to compute wave functions and on the electron density partitioning, but all reported values are significantly smaller than +6 expected for the purely ionic (Sc3N)6+@C606−. In summary, with respect to molecular orbitals, the mechanism of the metal (cluster)-cage interactions in EMFs can be described as the formal transfer of an appropriate number of electrons from the endohedral species to the carbon cage with subsequent coordination of the metal cations by the cage as a “ligand” and reoccupation of the metal nd orbitals (of course, this is only one of many possible mechanisms illustrating the bonding situation; in reality all electrons are indistinguishable and hence have no “history”). Note that in most cases there are no localized orbitals that could be ascribed to the metal-cage bonding. Instead, this kind of interaction occurs through the overlap of many π-orbitals of the cage carbon with nd orbitals of the metal atoms. One of the consequences of this type of bonding is a spin-charge separation found in the radical anions of the EMFs with the metal-localized LUMO (such as Sc3N@C80, La2@C72,78,80, Ti2C2@C78).428,436,439 In these anions, the spin density is mostly localized on the metal atoms; however the changes of the metal charges are rather small as compared to the neutral state of the EMF molecule. Instead, the surplus charge is mostly delocalized over the carbon cage (Figure 12).428 Since the metal-cage bonding in EMFs is best revealed in the analysis of the whole electron density of the molecule, for the quantitative description of the metal-cage interactions in EMFs it is appealing to use the methods based on the analysis of the electron density. Nowadays, one of the most refined and wellestablished methods for the analysis of the topology of the electron density is Bader’s quantum theory of atoms in molecules (QTAIM).440,441 For EMFs, QTAIM was first used by Kobayashi and Nagase in 1998 in the analysis of the bonding in Sc2@C84, Ca@C72, and [email protected],442 These studies have shown that, although the bond critical points (BCPs: critical points of the electron density, in which the density has a minimum along one axis and has a maximum in the plane perpendicular to this direction) could be found between Sc or Ca and certain carbon atoms of the cage, the values of the electron density at BCPs (ρbcp) were rather small (0.05 au for Sc−C and 0.01−0.02 au for Ca−C), and the Laplacian of the density (▽2ρbcp) at metal−carbon BCPs was always positive. The authors concluded that such values for these descriptors are signatures of the ionic character of the metal-cage bonding. However, as summarized by Macchi and Sironi443 and Gatti,444 an analysis of the bonding of transition metal compounds by QTAIM (see, e.g., ref 445) reveals that for the bonds involving transition metals ▽2ρbcp values are usually positive and ρbcp values are small because of the diffuse character of electron distribution,443 and hence these descriptors alone cannot properly describe the bonding situation. The analysis of the energy density appears more useful than the analysis of the electron density alone. In particular, Cremer and Kraka suggested that bonding between the atoms can be considered 6010


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to the formal values, metal atoms in EMFs still bear a sufficiently high positive charge to induce strong Coulomb repulsion when two or more metals are encapsulated inside one carbon cage.42,454 For instance, estimation of Coulomb repulsion energy between metal atoms based on the QTAIMbased interacting quantum atom (IQA) analysis yields the values of 9 and 14 eV for La2 and Y3 units in La2@C80 and Y3@ C80, respectively,38 which is comparable to the dissociation energy of the strongest covalent bonds (e.g., 9.8 eV in N2455). Thus, the propensity of fullerenes to encapsulate clusters with nonmetals has clear physical background: repulsive metal− metal interactions in clusterfullerenes can be compensated by attractive interactions with negatively charged species such as N3−, C22−, S2−, O2−, etc. (here formal charge states are listed; see section 4 for different types of clusterfullerenes). Topological analysis indeed shows that the nonmetals in clusterfullerenes localize a substantial excess of electron density: typical QTAIM charges are −1.7 to −1.9 for nitrogen in M3N@ C2n, −1.3 to −1.5 for a C2 unit in M2C2@C2n, ca. −1.2 for sulfur in M2S@C82, and in −1.3 to −1.5 for oxygen in Sc4O2,3@ C80.11,307,370,447 Besides, according to the QTAIM analysis, intracluster interactions also exhibit a significant degree of covalency, which is exemplified by negative total energy densities and considerable metal/nonmetal delocalization indices.307,436,447 For instance, δ(M, N), δ(M, C2), and δ(M, S) values in respective clusterfullerenes span the range of 0.7− 0.8, whereas δ(Sc, O) indices in oxide clusterfullerenes are 0.5− 0.6.11,307 An interplay between ionic and covalent contributions in the intracluster bonding in Sc4O2@C80 was recently analyzed by means of IQA-QTAIM,38 which allows partitioning of the interatomic interaction energies into physically sound ionic and covalent terms.456−458 The study showed that the net Sc−Sc and O−O interactions are strongly repulsive (ΣESc−Sc = 69 eV, EO−O = 8 eV), whereas net Sc−O interactions are attractive (ΣESc−O = −127 eV), resulting for the whole Sc4O2 cluster in the net stabilization energy of −50 eV. Interestingly, ionic Sc− O interaction energy, −109 eV, is much larger than the covalent term, −17 eV, but the former is largely compensated by the ionic Sc−Sc term, resulting in comparable covalent and ionic contributions to the intracluster interaction energy (−20 and −30 eV, respectively).38 5.1.3. Metal−Metal Bonding in EMFs. Analysis of the formal charges in EMFs reveals that group-III metals (Sc, Y, lanthanides) do not adopt their highest 3+ oxidation state in Y3@C80,318 Sc4O2@C809 as well as in a certain fraction of dimetallofullerenes, including Lu2@C76,308 M2@C79N,315,316 several M2@C82 structures (e.g., Sc2@C82,303 Y2@C82,193 Er2@ C82301,302,305). The lower oxidation state implies that the outer valence electrons are not completely transferred from the metal to the carbon cage, thus yielding a possibility of the covalent metal−metal bonding. In this case, HOMO usually has pronounced M−M bonding character.193,303,459 Furthermore, even when metal atoms in dimetallofullerenes are in the 3+ state (e.g., in La2@C2n and Ce2@C2n), the LUMO of these EMFs is usually a M−M bonding orbital,290,293,296,428,460−462 and hence metal−metal bonds can be formed as a the result of the electrochemical or chemical reduction.38 EMFs thus provide an interesting situation  on the one hand, metal atoms are strongly repulsive due to Coulomb interaction; on the other hand, they can form intermetallic covalent bonds. Although at the first sight description of the interactions between highly repulsive positively charged metal atoms in

Figure 13. Illustration of metal-cage and metal−metal bonding in EMFs with the use of electron localization function (ELF): (a-d) ELF (isovalue 0.72) in Y3@C80+ (a, b), Y2@C82 (c) and Y3N@C80 (d); color code for basins: yellow = valence V(C,C), light green = core C(C), light blue = valence V(Y,C,C), orange = core C(Y), dark blue/ green = valence V(Y,Y,Y), V(Y,Y) and V(N,Y,Y). In (b−d), valence V(C,C) and core C(C) basins are not shown. Valence trisynaptic V(Y,C,C) basins are real-space representations of the yttrium-cage bonding. Reproduced with permission from ref 318. Copyright 2010 American Chemical Society.

is seen in ELF as a spatial extension of some V(C,C) basins in the vicinity of the metal atoms and their transformation into the trisynaptic V(M, C, C) basins (Figure 13). In some cases, monosynaptic valence carbon basins are also formed for the cage atoms experiencing the strongest interaction with the metal atoms (populations of such basins are usually rather small, ca. 0.2 e). The number of the V(M, C, C) basins is rather large for each metal atom, showing that the metal atoms are bonded to the electron density of the large fragment of the cage.11,318 To conclude the section on the metal-cage bonding in EMFs, it is necessary to distinguish two definitions of the atomic (or cluster) charges in EMFs: (i) a “formal” charge and (ii) an “actual” charge. The formal charge (the same as an oxidation state or a valence state) is an integer and implies that the metal (cluster)−cage bonding is purely ionic. Formal charges do not describe the actual electron distribution in the endohedral fullerenes, but they are useful for understanding the spectroscopic and structural properties of endohedral fullerenes. For instance, compounds with the same formal cage and metal charges exhibit very similar absorption and vibrational spectra. In the next section we will show that the formal charge of the cage determines the isomeric structure of a given EMF molecule as well. The “actual” charge should represent the real electronic structure of the EMF molecule, but unfortunately there is no unique way to partition the electron density between the atoms. Depending on the definition of the charge and on the method of theory used to evaluate the wave function, the charges can vary in a large range in the same molecule as discussed above for Sc3N@C80. However, whatever method is used, computed charges are always considerably smaller than the formal charges, which is a clear manifestation of the covalent contribution to the metal-cage interactions. 5.1.2. Intracluster Interactions in Clusterfullerenes. In the previous section it was shown that EMFs are characterized by a transfer of valence electrons from metal atoms to the carbon cage. Although a large degree of covalency in metal-cage d-π bonding reduces the actual atomic charges in comparison 6011


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5.2. Isomerism in Endohedral Metallofullerenes: Stability of the Charged Carbon Cage

endohedral dimetallofullerenes in terms of covalent bonding might look questionable, analysis of the metal−metal interactions by different approaches, including MO analysis,38,307,315,459,463−465 QTAIM,38,307 and ELF38,307,318 topological analysis, consistently showed that covalent bonds between metal atoms in EMFs do exist and that their parameters are not much different from those of metal−metal bonds in organometallic compounds.466−471 Moreover, the M− M bond critical points in EMFs are characterized by negative values of the density Laplacian,307 which is a usual situation for covalent bonds between the first row elements, but is rarely observed for metal−metal bonds. At the same time, IQA analysis showed that the covalent bonding contribution to the total metal−metal interaction energy (ca. −1 to −2 eV) is much smaller than the repulsive ionic term (ca. 5−9 eV), which results in the net repulsion between the metal atoms.38 Since Coulomb repulsion is the dominant term in metal−metal interactions, metal atoms in EMFs show a tendency to be as far from each other as possible within the fullerene cage leading to unprecedently long metal−metal bonds. For instance, covalent bonding between La atoms in charged states of La2@C100 can be established at the La−La distances of ca. 5 Å (for comparison, single-bond covalent radius of La is 1.80 Å472).38 Comprehensive MO analysis showed that the valence state of metal atoms in dimetallofullerenes is determined by an energy matching between the carbon cage orbitals and the lowestenergy valence MO of the free metal dimer, which usually has (ns)σg2 character.38 Since the energy of the (ns)σg2 orbital in the M2 dimer correlates with the ns2(n−1)d1 → ns1(n−1)d2 excitation energy of the free metal atom, it is this energy, rather than the third ionization potential of the metal (as it is known for monometallofullerenes473,474), that largely determines the valence state of metal atoms in dimetallofullerenes. Namely, ns2(n−1)d1 → ns1(n−1)d2 excitation energies are increasing in the row La − Sc/Y − Lu as 0.33 − 1.43/1.36 − 2.34 eV, respectively,475 and the energies of (ns)σg2 MO in corresponding M2 dimers are decreasing in the same row.38 As a result, in dimetallofullerenes La is always trivalent, Lu tends to adopt a divalent state, whereas the valence state of Sc and Y is more sensitive to the energy of carbon cage MOs.38 Since the M−M bonding orbitals in di-EMFs have hybrid spd character361,459,463 and inherit significant ns-component from the (ns)σg2 MO of corresponding metal dimers,38 the presence of such M−M bonds can be verified straightforwardly by ESR spectroscopy of the cation-radicals via large metal-based hyperfine coupling constants. Likewise, when the M−M bonding is absent in the neutral state of the EMF but appears in the anionic state (i.e., when M−M bonding orbital is LUMO as in the La2@C80), ESR spectra of anion-radicals can be used to prove formation of the M−M bonds (see section 6.2). A special M−M bonding situation is found in Y3@C80. MO analysis showed that the highest 2-fold occupied orbital of Y3@ C80 has metal−metal bonding character and involves all three Y atoms, so that bonding can be described as a three center−two electron bond.318 QTAIM analysis did not show the presence of direct Y−Y bond paths in [email protected] Instead, the nonnuclear attractor (i.e., the maximum of the electron density not associated with the nuclei, also referred to as “pseudoatom”) was found in the center of the molecule, and each Y atom forms a bond path to the pseudoatom. This way, pseudoatom in Y3@ C80 mimics nitrogen atom in Y3N@C80.

Structural studies of EMFs show that their carbon cage isomers are usually different from those of the isolated empty fullerenes, which can be explained by the metal-to-cage electron transfer as mentioned in the previous section. The fact that charging can affect relative stabilities of the fullerene isomers was first reported by Fowler and Manolopoulos in 1992.476 In 1995 Fowler and Zerbetto performed an in-depth study of the isomers of C60, C80, and C84 in different charge states using the semiempirical QCFF/PI method.477 The authors have found that charging dramatically changes the relative stabilities of the fullerene isomers. For instance, Ih(7) isomer of C80, being the least stable structure for (+2) and (0) charge states, was found to be the lowest energy structure for (−4) and (−6) states.477,478 In agreement with this finding, it was shown that the most stable isomer of La2@C80 has Ih(7) cage, as opposed to the D2-symmetric hollow C80.210 Variation of the relative energies of C82 isomers in the negatively charged states was analyzed by Kobayashi and Nagase at the HF/3-21G//AM1 level to rationalize experimentally observed isomers of M@ C82.223,479 The authors have found that while C2(3) is the lowest energy isomer of the neutral C82, C2v(9) cage is the most stable for both C822− and C823− as well as for M@C82 (M = Ca, Sc, Y, La, etc.).223,479 In the recent DFT studies the C3v(8) isomer was shown to be the most stable one for the tetraanion C824−,11,37,438 in line with the finding that this cage is the most abundant isomer of Sc2C2@C82,351,352 Y2C2@C82,193,304 and M2S@C82 (M = Sc, Y, Dy).11,123 The most probable structure of the carbide Sc2C2@C68 with non-IPR C2v(6073) cage was also proposed based on the analysis of the stability in the row of C2v isomers of C684−.360 The most detailed studies of the relative stabilities of EMF isomers were performed for the family of nitride clusterfullerenes (NCFs). The electronic structure of NCFs may be conceived as a result of a formal 6-fold electron transfer from the (M3+)3N3− cluster to the fullerene. The simple use of this formal 6-fold electron transfer in M3N@C2n to predict the most suitable cage isomers of NCFs was proposed by Poblet and coworkers.211,480 They have supposed that only the fullerenes with a considerable gap between LUMO+2 and LUMO+3 (in the C2n6− hexaanion and presumably in M3N@C2n these orbitals become, respectively, HOMO and LUMO) should be considered as suitable hosts for nitride clusters.211 With the application of this criterion, only C60, C78-D3h(5), C80-Ih(7), and C80-D5h(6) were found to be suitable cage isomers among all IPR fullerenes in the C60−C84 range. Indeed, besides C60, which is too small to host a Sc3N cluster, only these and no other IPR cage isomers were found for Sc3N@C2n clusterfullerenes. Later this work was expanded by considering also all IPR isomer in the C86−C100 range,480 and the isomers with the largest orbital gaps found for C86, C88, and C100 hexaanions (D3(19), D2(35), and D5(450), respectively) were those proved to exist in Tb3N@C86, Tb3N@C88, and La2@C100 by singlecrystal X-ray diffraction studies.102,313 The orbital gap criterion was also applied in the studies of carbide clusterfullerenes M2C2@C2n which have the formal 4-fold electron transfer from the cluster to the cage, and the preferable formation of EMFs with C82-C3v(8) carbon cage was rationalized by the largest gap.438 Obviously, the orbital gap criterion discriminates the cages based on their kinetic stability, but it does not answer the question on their thermodynamic stability. 6012


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A different approach to the search of the suitable cage isomers of NCFs is based on the thermodynamic stability of the hexaanions.117,120,213,337 It was suggested that the suitable cage isomers of M3N@C2n should be found among the most stable isomers of C2n6−; in the view of the growing number of EMFs violating the IPR, the non-IPR isomers were also included in the search. The good correlation found between the relative energies of the C2n6− isomers computed at the DFT and semiempirical AM1 levels of theory120 allowed screening of all possible isomers at the cheap AM1 level (optimization of fullerene structure at the AM1 levels takes normally only ca. 1− 2 min at the standard office computer). Only 10−20 lowest energy C2n6− isomers (and corresponding M3N@C2n isomers) were then studied at the higher  and more computationally demanding  DFT level of theory. By screening through the large number of the C706−, C766−, and C786− isomers, the molecular structures of non-IPR Sc3N@C70-C2v(7854),120 DySc2N@C76-Cs(17490),117 and M3N@C78-C2(22010) (M = Dy, Tm)337 were proposed. The structural guesses were then confirmed by comparison of the experimental and DFTcomputed vibrational spectra; besides, the C2(22010) cage isomer in M3N@C78 NCFs has been confirmed in 2009 by a single crystal X-ray diffraction study of [email protected] Recently, the reports on the search of the most stable isomers of M3N@ C88 (M = La, Gd, Lu)481 and La3N@C92346 were published. The full search of the most favorable cage isomers of Sc3N@ C2n and Y3N@C2n (2n = 68−98) at the PBE/TZ2P level was reported in 2007 (for C68−C88, both IPR and non-IPR isomers were considered).213 This work has proved that the isomers shown to exist by single-crystal X-ray studies are always found among the lowest energy isomers of C2n6−. For M3N@C82, the study has predicted the C82-Cs(39663) isomer as one of the most probable structures, and later it was indeed confirmed by a single-crystal X-ray study of [email protected] It is remarkable that C2n6− isomers with high thermodynamic stability (i.e., low relative energy) quite often exhibit high kinetic stability (i.e., large HOMO−LUMO gap). As a result, the IPR isomers proposed in the group of Poblet211,480 in most cases coincide with predictions based on the relative stability of the hexaanions.213 The broad range of fullerenes studied in ref 213 enabled the authors to follow the general trends in their stabilities. Figure 14a plots the absolute energies of the most stable C2n6− isomers normalized to the number of atoms as a function of the fullerene size. The energy decreases smoothly with the growth of the fullerene size, which can be explained by (i) the decrease of the curvature of the cage, which results in a smaller curvature-induced strain, and (ii) the decrease of the on-site Coulomb repulsions of six surplus electrons in C2n6− with the increase of the cage size. However, there is a significant deviation from the smooth curve found for C80-Ih(7) and C80D5h(6) isomers. These isomers are 185 and 98 kJ/mol more stable than they might be if they were like all other fullerenes (i.e., like those which obey the smooth decay in the normalized energy). Note that similar deviations of the overall smooth dependence were also found for C60 and C70 in the DFT study of the noncharged fullerenes.482 The kinetic model developed by Curl et al.482 shows that such deviations as found for C60 and C70 lead to a high yield of corresponding fullerenes. Thus, the enhanced stability of the two C806− isomers explains the increased yield of M3N@C80 compared to all other cage sizes. Besides, it explains why the C80-Ih(7) isomer is always the most preferable structure in other clusterfullerenes with formal 6-fold

Figure 14. (a) Normalized energies of the most stable C2n6− isomers (black dots) and their fit with the exponential decay function (blue line). Normalized energies for C806−-Ih(7) and C806−D5h(6) isomers are shown as red dots; (b) the number of isomers with a given number of adjacent pentagon pairs (APPs) among the 10 lowest energy C2n6− isomers plotted as a function of 2n. Reproduced with permission from ref 213. Copyright 2007 American Chemical Society.

electron transfer to the cage, such as La2@C80, Sc4O2,3@C80,9 or [email protected] The extraordinary stability of the Ih(7) and D5h(6) isomers of C806− was explained in refs 37 and 213 using the concept of hexagon indices established by Raghavachari483 to quantify the distribution of pentagon-induced curvature in fullerenes. According to Raghavachari,483 the neighbor index of a hexagon is the number of hexagons to which it is adjacent, and every fullerene isomer can be characterized by a set of indices hk (k = 0−6), where hk is the number of hexagons with neighbor index k (for IPR isomers, only four indices (h3, h4, h5, h6) are sufficient because h0, h1, and h2 are equal to 0).483,484 To minimize the steric strain, the dispersion of the indices should be minimized, i.e., the indices of all hexagons should be as close to each other as possible. Hence, the lowest strain is expected for those structures, in which all indices are equal and pentagons are distributed in the most uniform way. For fullerenes C2n with 2n < 120 this condition is fulfilled only for C60-Ih(1), C80-Ih(7), and C80-D5h(6).484 Thus, the exceptional stability of two C806− isomers can be explained by the favorable uniform distribution of the pentagons. For other IPR fullerenes, (h3, h4, h5, h6) indices minimizing the strain are (80 − 2n, 3n − 90, 0, 0) for C2n with 2n ≤ 80, and (0, 70 − n, 2n − 80, 0) for 6013


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C2n with 2n ≥ 80.484 It was shown in ref 213 that the list of the least-strained IPR isomers perfectly corresponds to the lowest energy IPR isomers of C766−−C906−. Interestingly, it appears that the relative energies of the IPR C2n6− isomers follow the rationalization of the stability based on the steric strain much better than the relative energies of the uncharged fullerenes, which usually violate the requirement of the least-strain. In 2010 this fact was clarified by Poblet and co-workers.212 The authors suggested that the relative stabilities of the isomers of multiply charged fullerene anions are largely determined by the on-site Coulomb repulsion. Since the negative charge in anions is mostly localized on the pentagons because of their nonplanarity, the lowest-energy structures should be those in which the maximal separation (and hence the most uniform distribution) of the pentagons is achieved. The authors have constructed the inverse pentagon separation index and showed that its correlation with the relative stabilities of the isomers is improving with the increase of the charge. Thus, minimization of the on-site Coulomb repulsion and minimization of the steric strain are achieved by fulfillment of the same condition, namely, the maximal separation (and hence the most uniform distribution) of the pentagons. If for the noncharged fullerenes the steric strain factor can be outweighed by other factors such as stability of the π-system (and hence the lowest strain condition is usually violated), with the increase of the negative charge the Coulomb repulsion sooner or later outweighs all other factors. Therefore, to predict the lowest energy isomers of the EMFs with high formal charge of the cage (such as M3N@ C2n) one can apply the same arguments as developed in early 1990s to find the isomers with the lowest steric strain. In summary, (i) the stability of the isomers of appropriately charged empty fullerenes provides a good estimation of the relative stability of the cage isomers of endohedral fullerenes, (ii) the relative energies of the fullerene isomers are dramatically affected by the charge, and (iii) the lowest energy isomers for highly charged cages are those in which pentagons are distributed in the most uniform way.

C1(168785).491 The state-of-the-art in the studies of non-IPR fullerenes has been reviewed in 2009.492 The tendency to violate the IPR in endohedral fullerenes can be explained by the changes of the relative energies of the fullerene isomers with the increase of the cage charge. It was shown already in 1995 that for C60 the non-IPR C2v(1809) isomer with two pairs of adjacent pentagons is substantially less stable than the IPR isomer in the neutral state, but with the increase of the negative charge on the cage, the difference in the relative energies is diminishing, reaching almost zero for (−4) and inversing for (−6) states.477 Semiempirical and DFT calculations for the whole set of 6332 isomers of C68 in 0, (−2), (−4), and (−6) charge states showed that the lowest energy isomers of C684− and C686− are C2v(6073) and D3(6140)493 in agreement with the isolation of these isomers for Sc2C2@C68360 and Sc3N@C68,16,96 respectively. By plotting the relative energies of the isomers of the same charge versus the number of pentagon adjacencies, the authors have found that the penalty for a pentagon adjacency is decreasing from 92 kJ/mol in C68 to 70 kJ/mol in C686− at the AM1 level, confirming that the increase of the charge stabilizes pentagon adjacencies.493 An explanation of the stabilization of the pentagon adjacencies in metallofullerenes was proposed by Slanina et al.494 The authors noted that the pentalene (a unit of two fused pentagons) is a 8π antiaromatic system in the neutral state, but becomes 10π aromatic in the dianionic state.495 In a similar fashion, when the metal in the non-IPR EMF is coordinated to the pentagon pair, it donates two electrons to the pentalene unit to make it aromatic. This reasoning agrees with the fact that the maximum number of pentagon pairs found in EMFs is equal to half of the formal charge of the cage: three pairs for (6−) charge as in Sc3N@C6816,96 or Sc3N@C70,120 two pairs for (4−) state as in Sc2C2@C68,360 and one pair for (3−) and (2−) states as in La@C72247 and [email protected] A correlation of the total number of adjacent pentagon pairs in the lowest-energy isomers of C2n6− (2n = 68−88) with the cage size was revealed in ref 213. Figure 14b shows that, with the increase of the cage size, the number of pentagon pairs in the most stable isomers is decreasing: while three pairs are common for C686− and C706−, two pairs are preferable for C726−−C786−, and one pair is preferable for C806−−C866−. Relative energies of the non-IPR isomers with respect to the IPR isomers are also increasing with the size of the fullerene, and formation of non-IPR fullerenes beyond C84 should not be common. Indeed, the largest cage reported to date for any nonIPR fullerene is C84-Cs(51365).101,341 With the increase of the cage size a more uniform distribution of pentagons over the fullerene is possible, and hence, localization of such a steric strain and surplus charge in pentagon adjacencies should become more unfavorable than for the smaller cages. At the same time, coordination of metal atoms to pentagon adjacencies results in their additional stabilization, and it cannot be excluded that stable non-IPR isomers of EMFs can be found for larger cages such as predicted recently for Gd2@ C98-C1(168785).491

5.3. Violation of the Isolated Pentagon Rule (IPR) in Endohedral Fullerenes

The specific feature of endohedral fullerenes is the violation of the isolated pentagon rule (IPR). This rule imposes a strict limitation on the possible isomers of empty fullerenes, but encapsulation of a metal or a cluster inside the cage often results in the violation of the IPR, affording thus the non-IPR endohedral fullerenes. Such possibility was first suggested by Kobayashi et al. in theoretical studies of Ca@C72 isomers,485 and the first experimental evidence was provided in 2000 when Sc2@C66-C2v(4348)17 and Sc3N@C68-D3(6140)16 were isolated. Since that time, a lot of non-IPR endohedral fullerenes were reported, including Sc2C2@C68-C2v(6073),360 Sc3N@C70C2v(7854),120 Sc2S@C70-C2(7892),373 Sc2S@C72-Cs(10528),194 La@C72-C2(10612),247 La2@C72-D2(10611),289,290 DySc2N@ C76-Cs(17490),117 M3N@C78-C2(22010) (M = Y, Dy, Tm, Gd), 105,337,338 Sc 3 NC@C 78 -C 2 (22010), 375 Gd 3 N@C 82 C s (39663), 10 6 M 3 N@C 84 -C s (51365) (M = Tb, Gd, Tm).101,341 On the basis of computational studies, the nonIPR isomers were also proposed for Sc2@C70-C2v(7854),361 Sc2@C76-Cs(17490),213 Ca@C72-C2(10612) or C2v(11188),485,486 the minor isomer of Yb@C74 (C1(13393) or C1(14049)),487 M2@C74 (C2(13295) and C2(13333), M = Sc, La),488 M@C76 (C2v(19138) and C1(17459), M = Yb, Ca, Sr, Ba), 4 8 9 , 4 9 0 Y 2 C 2 @C 8 4 -C 1 (51383), 3 6 5 Gd 2 @C 9 8 -

5.4. Cage Form Factor

In the previous section it was established that the relative stability of the carbon cage in the charged state is the main factor determining isomeric distribution of the EMFs. However, this factor is not the only one which determines whether an EMF isomer is stable or not. Another important parameter which strongly influences the stability of the EMF molecules 6014


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structure and the lowest energy structures already compete in stability with the Y3N@C80-D5h(6) isomer.213 It means that their formation might be, in principle, possible. Note that Yang and Dunsch reported the isolation of the third isomer of Dy3N@C80,398 whose structure is still not known, but C1(28325) is a likely candidate.213 On the other hand, if the fullerene is too large, efficient coordination of the nitride cluster is also not possible,213 which limits the size distribution of NCFs (for Sc-NCFs, Sc3N@C80 is the largest isolable structure, while for La the largest NCF isolated so far is La3N@C96122). Another way how the cage form factor can influence the isomer distribution is most apparent for non-IPR cages. In all non-IPR EMFs, pentagon adjacencies are stabilized by coordination to the metal atoms. According to the principles discussed in the previous section, pentagon pairs should be distributed in a most uniform way. In line with these guidelines, the most stable C726− isomer D2(10611) has an elongated shape with two pentagon pairs located on the opposite poles of the molecule. This isomer is confirmed for isolated M2@C72 EMFs (M = La,289 Ce293), but the nitride clusterfullerenes with C72 have never been found. The reason is that the location of the pentagons pairs as well as the elongated shape of the cage is not compatible with the triangular shape of the M3N cluster. Metal atoms bear significant positive charge and hence strongly repel each other, and this factor imposes a limitation on their relative positions. For instance, the optimum form of the M3N cluster is an equilateral triangle with metal atoms at the vertexes and nitrogen in the middle, and its deformation increases repulsive interaction between metal atoms.42 Sc3N cannot be coordinated to two pentagon pairs of C72-D2(10611) and preserve its regular triangular shape at once, and hence Sc3N@C72 with the D2(10611) cage is very unstable.213 The most stable isomers of Sc3N@C72 are thus based on the unstable carbon cages and are not formed in noticeable amounts during the synthesis. The same reasoning explains why M3N@C74 NCFs have never been observed.213 Likewise, the yield of Sc3N@C70, in which the cluster has to be significantly deformed, is much lower than that of other Sc-based NCFs.120 Moreover, M3N@C76-Cs(17490) could be synthesized in appreciable yield only with the mixed DySc2N cluster, which can adopt a suitable geometry to coordinate two pentagon pairs of the non-IPR C76-Cs(17490) cage and still have suitable metal−nitrogen bond-lengths (neither Sc3N@C76 nor Dy3N@C76 are isolated).117 From the discussion above, it should be rather obvious that the cage form factor is much more important for the clusterfullerene stability than for conventional EMFs. While for the former the shape and the size of the cluster play a limiting role in the cage size distribution, for mono and dimetallofullerenes these factors are not so crucial and a much broader range of structures can be obtained.263,266,309,310,363

can be defined as the cage form factor. That is, the carbon cage topology in terms of its inner space and the distribution of the metal-bonding sites also plays an important role in stabilization of the metal clusters inside the fullerene. An example of the influence of the cage form factor, which changes the preferable cage isomer in the M3N@C78 system, was already mentioned in section 4. In particular, the most stable isomer of C786− is D3h(5),337 and this cage is indeed found in M2@C78 (M = La,296 Ce298) and Sc3N@C7897 (note however that the position of the metal atoms is different: in M 2 @C 78 metal atoms are close to the poles of the cage,296−298,462 while in Sc3N@C78 metal atoms are bonded to the pyracelene units at the cage equator97,383,385). However, when the larger metal atoms are considered (such as Y or Lu), the size of the M3N cluster turns out to be so large that the C78D3h(5) cage does not provide enough space to accommodate it. DFT calculations showed that the Y3N cluster is pyramidal in Y3N@C78-D3h(5), which points to the large steric strain in the system.337 For M3N@C78 EMFs, this results in the preference of another isomer, non-IPR C78-C2(22010), whose flattened shape provides sufficient room for the large cluster.105,337,338,375,447 Although in the empty hexaanionic form, C786−-C2(22010), this isomer is less stable than C786−-D3h(5) by 59 kJ/mol, Y3N@C78-C2(22010) is more stable than Y3N@C78D3h(5) by 84 kJ/mol.337 Importantly, the inner space in C80-Ih(7) is also rather limited, and for the nitride clusterfullerenes with large lanthanides it results in the preference of larger cages.121,343 Even when this isomer is still preferable (such as for Y3N@ C80), the increase of the internal strain already influences the structure (e.g., pyramidalization of the Gd3N cluster108), chemical327,333,496−498 and spectroscopic properties322 and can be visualized by comparing the relative energies of C806−, Sc3N@C80 and Y3N@C80 isomers (Figure 15). C806−-Ih(7) is more stable than any other C806− isomer (except for D5h(6)) with the gap of more than 200 kJ/mol, and almost the same situation is found for the isomers of [email protected] However, other isomers (with more suitable cage shape) are significantly stabilized for Y3N@C80 with respect to the Y3N@C80-Ih(7)

5.5. Gibbs Energy Considerations

Stability factors described in the previous sections determine relative energies of the EMF isomers at T = 0 K, whereas the synthesis of EMFs in the arc proceeds at high temperatures exceeding 2000 K. Since isolated structures roughly correlate to their stability, at least partial thermodynamic control of the products can be postulated. Hence, equilibrium composition computed for the reaction temperature may correspond to the real distribution of isomers better than their zero-temperature relative energies. Equilibrium composition of the mixture of isomers can be computed via Gibbs energies, which in due turn

Figure 15. Relative energies of C806− and M3N@C80 (M = Sc, Y) isomers with the same carbon cages computed at the PBE/TZ2P level of theory. Experimentally available isomers are encirclied in red. C1(28325) is a possible structure of the third isomer of Dy3N@C80. On the basis of the numerical data from ref 213. 6015


a

6016

Li

C

LuxSc3−xN@C80, LuxY3−xN@C80 (x = 0−3) M3N@C80-Ih(7) (M = Sc, Y, Lu) Sc3N@C80-Ih(7) Ca@C74 Sc2C2@C82 (II, III) Sc2C2@C82-Cs(6) Sc2C2@C82-C3v(8) Sc2C2@C80-C2v(5) Lu2C2@C80-C2v(5), Lu2@C80-C2v(5) [Ce@C82]− [Ce@C82]− [Ce@C82(Ad)]− Ce2@C72 Ce2@C78, Ce2@C78(Mes2Si)2CH2 Ce2@C80-Ih(7) Ce2@C80-Ih(7), Ce2@C80(Mes2Si)2CH2 Ce2@C80-D5h(6) [6,6] and [5,6] Ce2@C80(CH2)2NTrt Ce2@C80(Ad) LaxCe2−x@C80-Ih(7) (x = 0−2) CeSc2N@C80-Ih(7) CeLu2N@C80-Ih(7) [Pr@C82]−-C2v(9) LaPr@C80, Pr2@C80 Pr2@C72 Sm@C80-C2v(3) Sm@C82-C2(5) Tm@C82 (I, II, III) [Li+@C60]SbCl6− [Li+@C60]PF6−, [Li+@PCBM]PF6− Li@C60(OH)18

compound

13

C pyramidalization T1 relaxation, VT-NMR solid state VT-NMR VT-NMR solid state VT-NMR VT-NMR VT-NMR VT-NMR VT-NMR VT-pNMR 2D INADEQUATE pNMR VT-pNMR VT-pNMR VT-pNMR VT-pNMR VT-pNMR VT-pNMR VT-pNMR VT-pNMR pNMR pNMR VT-pNMR pNMR VT-pNMR pNMR pNMR VT-pNMR VT-pNMR

method/effecta

C NMR Studies of EMFs 119 516 517 231 524 354 349 362 528 207 243 255 293 298 519 283 285 274,277 276 518 103 329 240 287 294 233 234 230 144 378 525

ref

Sc

Y La

45

89 139

N

14

nuclei M3N@C80 (M = Sc, Y, Lu) LuxSc3−xN@C80, LuxY3−xN@C80 (x = 0−3) Sc2C2@C82 (I, III) Sc2C2@C82-Cs(6) Sc2C2@C80-C2v(5) Sc2C2@C82 (II, III) Sc3N@C68 Sc3N@C80 Sc3N@C80 Sc3N@C80 CeSc2N@C80 LuxSc3−xN@C80-Ih(7), D5h(6) (x = 0−2) Sc2S@C82 Sc2S@C72 Sc3NC@C80 Sc2@C82-C3v(8) Sc4O2@C80-Ih(7) Y3N@C2n (2n = 80, 84, 86) La2@C80 [6,6] and [5,6] La2@C80(CH2)2NTrt La2@C80(Dep2Si)2CH2 [5,6] La2@C80(C2(CN)4O) La2@C80(Dep2Si(CH2)-CHtBp), 2 isomers La2@C72 (La@C72-C2(10612))C6H3Cl2, 3 isomers (La@C74-D3h(1))C6H3Cl2, 2 isomers (La@C80-C2v(3))C6H3Cl2, 3 isomers [La@C82-C2v(9)]− (La@C82−C3v(7))C6H3Cl2, 2 isomers

compound

method/effecta

VT-NMR VT-NMR VT-NMR VT-NMR VT-NMR

solid state VT-NMR VT-NMR, T1 VT-pNMR

metal-dependence, VT-NMR metal-dependence, theory VT-NMR VT-NMR VT-NMR solid state VT-NMR

When the method is not indicated, the measurements were performed in solution at fixed temperature; VT − variable temperature, pNMR − paramagnetic NMR.

7

13

nuclei

Table 2. Multinuclear and Nonconventional 526 527 523 354 362 524 16 5 517 103,526 103 184 11 194 10 303 371 99 270 274,277 281 280 275 289 247 246 248 206 521

ref

Chemical Reviews Review


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Figure 16. Enlarged view of the metal-coordinated hexagon in the C3 isomers of Sc3N@C80-Ih(7) (a), Lu3N@C80-Ih(7) (b), Y3N@C80-Ih(7) (c) with DFT-optimized interatomic distances (Å, black) and POAV angles (red); the THJs are highlighted as black spheres. (d) Correlation between the experimental 13C chemical shifts of LuxY3−xN@C80 and LuxSc3−xN@C80 (x = 0−3) and the averaged POAV angles of the carbon atoms on THJs. (e) Correlation of Δ(13C shift) and Δ(POAV) for two types of carbon atoms in the MMCFs. Reproduced with permission from ref 119. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

grow with the increase of the temperature since the entropy includes a term R ln(σ), where σ is a symmetry number. The larger the symmetry of the carbon cage (and in the MEF model only the cage symmetry counts499), the more negative this contribution in the entropy. Analysis of the symmetry factor for isomers of Gd3N@C82 as well as for some other fullerene cages is given in ref 510. In addition to the composition of isomeric mixtures, thermodynamic analysis can address the question of the relative yield of EMFs with different metals. Slanina et al. reported such an analysis for various monometallofullernes,511−514 and recently also for Sc2S@C82/[email protected] These studies revealed that the relative yields of EMFs with different metal atoms can be rationalized employing saturated vapor pressures of metals and their ionization potentials.

can be estimated using statistical thermodynamic approaches. The most widely used one is the rigid rotor-harmonic oscillator (RRHO) approach, which requires the knowledge of molecular structure and vibrational frequencies to compute corresponding partition functions. Since endohedral species can exhibit large amplitude motions at high temperatures, a modification of RRHO called “free encapsulate model” (FEM) was also proposed.499 For the molecules with a small HOMO−LUMO gap and/or at high temperatures, electronic partition function can be also substantially different from unity, and hence lowest excitation energies should be also taken into account. Exhaustive analysis of the influence of the temperature on equilibrium composition of isomeric mixtures of EMFs was performed by Slanina et al. in a series of publication.487,499−509 In 2010 Gibbs energy analysis showed that while Sc2O@C82C3v(8) is the lowest energy isomer at low temperature an increase of the temperature beyond 1000 K results in the preference of another isomer, Sc2O@C82-Cs(6), which was the only isomer found experimentally.370 It should be noted that real temperatures of the arc in EMF synthesis and the temperatures at which interconversion between different EMF isomers is still possible are not known. Besides, the relative energies of the isomers, which have a crucial influence upon the temperature dependence of the isomeric composition, are usually taken from DFT calculations. Precision of such caculations remains unknown since experimental values on the relative energies of isomer of EMFs are not available so far, and hence it is not possible to validate DFT results quantitatively. When the effect of the temperature is taken into account in the analysis of the isomeric distribution, two further considerations should be kept in mind. First, the concentration of the structures unstable at T = 0 K should increase with the temperature as follows from the simple inspection of the Boltzmann term, exp(−ΔE/RT), where ΔE is the relative energy and R is the universal gas constant. That is, with the increase of the temperature the role of ΔE decreases. Second, conentration of the low-symmetric isomers should

6. SPECTROSCOPIC PROPERTIES AND ELECTRONIC STRUCTURE OF EMFS 6.1. NMR Spectroscopy

6.1.1. 13C NMR Spectroscopy. The vast majority of NMR spectroscopic studies of EMFs were dedicated to the determination of the cage structures by 13C NMR spectroscopy in solution. Such studies were already reviewed in the section 4 on the molecular structures of EMFs and will not be considered in this section. However, in addition to the cage symmetry, 13C NMR spectroscopy can give valuable information on the metalcage interactions and dynamics of the metal atoms and clusters inside the carbon cages. In this section, we will give a survey of such 13C NMR studies (Table 2). A systematic 13C NMR study of a series of YxLu3−xN@C80Ih(7) and LuxSc3−xN@C80-Ih(7) NCFs has shown that the chemical shifts of two types of cage carbon atoms exhibited systematic downshift with the shrinking of the encaged nitride cluster.119 This downshift showed a perfect linear correlation with the π-orbital axis vector (POAV) pyramidalization angles of the carbon atoms in DFT-optimized structures and was more 6017


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NMR studies of Tm@C82 isomers,230 LaPr@C80,287 Pr2@ C80,287 and Sm@C82-C2(5)234 were also reported. The paramagnetic contribution to the chemical shift has two major parts: (i) Fermi contact (“through bond”) and (ii) pseudocontact (“through space”) terms, which scale with temperature as T−1 and T−2, respectively. The temperature dependence of the measured chemical shifts allows the discrimination of Fermi and pseudocontact terms: extrapolation of the values to the T−1 = 0 or T−2 = 0 limits should give the diamagnetic shifts, which can be then compared to the corresponding chemical shifts in diamagnetic analogues (e.g., in La-containing EMFs). It was shown that for all Ce EMFs, the paramagnetic shift is dominated by the pseudocontact term, which agrees well with the strongly localized, “buried” nature of unpaired 4f1 electron in Ce3+ ions.103,207,243,255,285,293,298,329 Therefore, the chemical shift (δ) can be presented as a sum of only two terms: δ = δdia + cpc·T−2, where cpc is a characteristic constant for a given carbon atom and depends on its orientation and distance to the paramagnetic center. Therefore, it is possible to estimate the relative position of paramagnetic metal ions and different cage carbon atoms from the temperature dependence of the chemical shifts. In particular, enhanced shifts of the carbon atoms in adjacent pentagons in the non-IPR Ce2@C72-D2(10611)293 or in metal-coordinated hexagons in [Ce@C82]−-C2v(9)207,243 and Ce2@C78-D3h(5)298 (signal “8” in Figure 17) point to the localized position of Ce

pronounced for pyrene-type carbon atoms (Figure 16). It was concluded that with the increase of the cluster size, the metal atoms change their coordination site from the pentagon/ hexagon edge in Sc3N@C80 to the center of the hexagon in Y3N@C80, which increases pyramidalization of the pyrene carbon atoms.119 Measurements of relaxation rates R1 of carbon atoms in Sc3N@C80, Y3N@C80, and Lu3N@C80 (all with Ih(7) isomer) were reported by Klod et al.516 The study revealed that NCFs have higher relaxation rates in comparison to those of empty fullerenes indicating that the nitride cluster acts as an internal relaxation reagent. While the enlarged cage size increases the relaxation of the cage carbons, the negative charge on the cage increases the electron density at the cage, thus decreasing R1. The study of the temperature dependence of T1 showed that the diffusion is fast compared to the rotation of the molecule at higher temperatures.516 An interesting example of the study of the cluster dynamics by 13C NMR spectroscopy was reported by Akasaka et al.362 Sc2C2@C80-C2v(5), whose structure was unambiguously determined by single-crystal X-ray diffraction, exhibited a much larger number of 13C NMR lines at room temperature than expected for the C80-C2v(5) cage. However, at temperatures above 410 K, a C2v(5)-symmetric pattern was observed in the spectra. These facts point to the frozen position of the Sc2C2 cluster at room temperature and its rapid rotation at higher temperatures.362 Variable temperature 13C NMR spectroscopy was also used to study dynamics of acetylide unit in 13C enriched Sc2C2@C82-C3v(8).349 The signal of the carbon atoms in C2 unit is by ca. 5 times broader than for the cage carbon atoms and exhibits considerable broadening with the increase of the temperature due to the spin-rotation relaxation.349 Similar broadening and temperature-dependent shifts of the signals corresponding to carbon atoms of the acetylide unit was recently reported for Sc2C2@C82-Cs(6).354 In the solid state, an extended 13C NMR study of Sc3N@C80 at various temperatures with and without magic angle spinning (MAS) was reported by Martindale et al.517 The study was focused on the determination of spin−lattice relaxation times and dynamics of the Sc3N cluster as well as the whole Sc3N@ C80 molecule. In contrast to C60, the measurements showed an activated behavior of the molecular reorientations over the whole temperature range. Principal values of the 13C chemical shift anisotropy tensor (220, 180, and 30 ppm for σ11, σ22, and σ33, respectively) were determined from the powder measurements at 30 K.517 Even when the carbon cage in an EMF is diamagnetic (i.e., the formal charge of the cage is even), the metal ions can be intrinsically paramagnetic, such as it is typical for many rareearth metals in 2+ and 3+ valence states. In the presence of endohedral paramagnetic centers, the lines in 13C NMR spectra are strongly broadened, usually to such an extent that the measurement of NMR spectra becomes impossible. However, in certain cases, 13C NMR spectra could still be measured but exhibited a temperature-dependent paramagnetic shift. The most detailed studies of paramagnetic 13C NMR spectra were performed for a series of Ce-containing classical EMFs Ce@ C82−,207,243 Ce2@C72,293 Ce2@C78,298 Ce2@C80,285,518,519 LaCe@C80,518 and their derivatives,255,276,283,298 as well as nitride clusterfullerenes CeSc2N@C80103 and [email protected] 13 C NMR spectroscopic studies of Pr@C82− anion,240 Pr2@ C72,294 HoxSc3−xN@C80(Ih),185 Sm@C80-C2v(3),233 and VT-

Figure 17. 13C NMR spectra (125 MHz) of Ce2@C78 in CS2 at 283− 303 K. The relative integrated intensity ratio of the lines marked with an open circle and a solid circle is 2:1. Reproduced with permission from ref 298. Copyright 2008 The Royal Society of Chemistry.

atoms. In Ce@C82-C2v(9), a variable temperature (VT) 13C NMR study revealed a preferable localization of the Ce atom close to the hexagon on the C2 axis.207,243 Likewise, only six carbon atoms exhibited strong temperature dependence of the δ(13C) values in the bis-silylated adduct of Ce2@C80-Ih(7), which was interpreted as a localized coordination of the Ce atoms to hexagons.283 In contrast, uniformly small cpc values of Ce2@C80-Ih(7)283 and CeLu2N@C80-Ih(7)329 agree well with the three-dimensional rotation of Ce2 and CeLu2N clusters 6018


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inside the icosahedral C80 cage. For Ce2@C80-D5h(6), twodimensional circulation of Ce atoms along a band of 10 hexagons was found.285 While 13C lines in CeM2N@C80 (M = Sc, Lu) in comparison to Sc3N@C80 are shifted by no more than 2 ppm, much larger paramagnetic shifts were found recently for HoxSc3−xN@ C80(Ih, x = 1−2).185 Each Sc-to-Ho substitution induced the shifts of 25−40 ppm for corannulene-type and 60−70 ppm for pyrene-type carbon atoms. 6.1.2. Multinuclear NMR Spectroscopy. A nonzero nuclear spin of metal as well as nonmetal atoms of the encaged species in EMFs enables the studies of their structure and dynamics by multinuclear NMR. In particular, 3Li, 14N, 45Sc, 89 Y, and 139La NMR spectroscopic studies of EMFs have been reported (Table 2). 139 La NMR Spectroscopy. In 1997, Akasaka et al. reported the 139La NMR spectrum of pure La2@C80-Ih(7), which exhibited only one peak at δ = −403 ppm in the whole studied temperature range (258−363 K), pointing to the free circulation of La atoms.270 The studies of the peak-width dependence on the temperature revealed two main regimes of the spin relaxation: in the 258−305 K range, the line width is narrowing with increasing temperature due to the quadrupoledominated relaxation. However, at higher temperatures, a dramatic increase of the line width was observed. This phenomenon was interpreted as a result of an additional magnetic field produced by circulation of La3+ ions and a corresponding spin-relaxation mechanism.270 Similar broadening of the 139La signal was also found in a VT-NMR study of the bis-silylated derivative La2@C80(Dep2Si)2CH2 (δ ≈ −365 ppm, T = 183−303 K, see Figure 18),281 two isomers of La2@

La chemical shifts in La2@C72 and La2@C80 were reported by Slanina et al.494 and Jin et al.520 Solution 139La NMR spectra were also reported for La2@C72 (δ = −576 ppm),289 [La@C82]− (δ = −470 ppm),206 (La@C72C2(10612))C6H3Cl2 (3 isomers, δ = −603, −605, and −619 ppm),247 (La@C74-D3h(1))C6H3Cl2 (2 isomers, δ = −513 and −511 ppm),246 (La@C80-C2v(3))C6H3Cl2 (3 isomers, δ = −493, −500, and −488 ppm),248 (La@C82−C3v(7))C6H3Cl2 (two isomers, δ = −456 and −468 ppm).521 Thus, depending on the carbon cage and the position of the addend, the 139La signal can vary in a broad range of ca. 250 ppm. A complex 139La NMR spectrum of the solid La-EMF extract with hexamethylphosphoramide was reported by Koltover et al. in 2004, but no further details could be given in that work.522 45 Sc NMR Spectroscopy. 45Sc NMR spectroscopy is the frequently used noncarbon NMR technique in the studies of EMFs. Chemical shifts of Sc atoms in EMFs exhibit significant variation in dependence on the carbon cage, cluster structure, and valence state of Sc (Table 3), thus making 45Sc NMR Table 3. 45Sc NMR Chemical Shifts in Diamagnetic Sc-Based EMFs δ(ScII) Sc4O2@C80-Ih(7) Sc2@C82-C3v(8) Sc2C2@C80-C2v(5) Sc2C2@C82-Cs(6) Sc2C2@C82-C3v(8) Sc3N@C68-D3(6140) Sc3N@C78-D3h(5) Sc3N@C80-Ih(7) LuxSc3−xN@C80-Ih(7), x = 1, 2 Sc3N@C80-D5h(6) LuxSc3−xN@C80-D5h(7), x = 1, 2 Sc3NC@C80 Sc2S@C82-C3v(8) Sc2S@C72-Cs(10528)

292/285 430

δ(ScIII) a

129/138a

ref 371 303 362

130/170b; 150 (HTc) 200/245b; 220 (HTc) 225 90b 79 200 200 195−200

523 16 d d 5 184

212 195

184 184

280/360b 290 183

10 11 194

354,523

a

The values measured in CS2/o-DCB. bExact values were not given and are estimated from the figures in the original papers. cThe values measured at higher temperatures (>400 K) after coalescence of two signals. dUnpublished results of the authors.

spectroscopy a very sensitive probe of the internal state of EMFs. A large quadrupole moment of Sc nuclei (I = 7/2) results in rather broad 45Sc NMR lines, but their width shows appreciable temperature dependence and can be used in the studies of the dynamics of encaged Sc atoms. The first such study was reported for two isomers of Sc2C2@C82 in 1996 (at that time the studied compounds were thought to be Sc2@ C84).523 At 363 K and at lower temperatures down to 298 K the 45 Sc NMR spectra of isomer I (presumably with the Cs(6) cage) exhibited two signals of equal intensity with the distance of 50 ppm (Figure 19). At 383 K the signals coalesced, and the single line with an intermediate chemical shift showed a narrowing during the further temperature increase up to 433 K. These results were confirmed in a recent study of structurally characterized Sc2C2@C82-Cs(6).354 On the contrary, the isomer III with C3v(8) cage showed only one line at ca. 225 ppm in the

Figure 18. (a) 139La NMR spectra of La 2@C80 (Dep 2Si) 2CH2 measured at different temperatures; (b) temperature dependence of the 139La NMR peak width. Reproduced with permission from ref 281. Copyright 2007 The Royal Society of Chemistry.

C80(Dep2Si(CH2)-CHtBp) (δ ≈ −392 and −397 ppm, T > 280 K),275 and presumably in cycloadduct of La2@C80 with tetracyanoethylene oxide (δ ≈ −323 ppm, T > 280 K).280 At the same time, a broadening was not observed in the La2@ C80(CH2)2NTrt derivative (δ = −464 ppm and −412 ppm for [6,6] and [5,6] isomers, respectively) pointing to the fixed position of La atoms in the latter.274,277 Theoretical studies of 6019


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Figure 19. 45Sc NMR spectra of Sc2C2@C82-C3v(8) (left) and Sc2C2@C82-Cs(6) (right) measured in CS2 and o-dichlorobenzene solution at different temperatures. Reproduced with permission from ref 523. Copyright 1996 American Chemical Society.

In CeSc2N@C80, the 45Sc NMR signal has shown a distinct temperature dependence (at room temperature it is located at 260 ppm), which was assigned to the pseudocontact interactions between Sc and the Ce-localized unpaired spin.103 Recently, temperature-dependent 45Sc chemical shifts were reported for Ih and D5h cage isomers of [email protected] In line with the larger magnetic moment of Ho, cpc values in HoSc2N@C80 were found to be several times higher than in [email protected] Solid state 45Sc NMR measurements of Sc3N@C80 at low temperatures (20−330 K) were performed by Martindale et al.517 At 65 K, the first order quadrupole powder pattern was revealed with a quadrupolar coupling constant e2·q·Q/h = 67.9 MHz. At higher temperatures, a motional narrowing of the signal was observed. The 45Sc relaxation rate was shown to be 6 orders of magnitude faster than that for 13C indicating that the spin-quadrupole relaxation is much more efficient than the chemical shift anisotropy mechanism.517 For Sc3N@C68, a 45Sc NMR signal at ca. 90 ppm was first observed in 2000.16 Measurements of the 45Sc NMR spectra of Sc3N@C68 and Sc3N@C78 in our group gave the signals at 79 and 200 ppm, respectively (unpublished results). Recently, the 45Sc NMR spectra of Sc2S@C82-C3v(8),11 Sc2S@C72-Cs(10528),194 and Sc3CN@C80-Ih(7)10 were reported. Sc2S@C82 exhibits one line at 290 ppm in CS2 pointing to the fast rotation of the Sc2S cluster at room temperature,11 whereas in Sc2S@C72 position of the 45Sc signal is shifted to 183 pm.194 For Sc3NC@C80, two signals in a 2:1 intensity ratio were found at 298 K (Figure 20a), which suggests the rigid structure of the Sc3NC cluster with two types of nonequivalent Sc atoms in good agreement with single-crystal X-ray diffraction study.10 The first 45Sc NMR spectrum of an EMF with divalent Sc, Sc2@C82−C3v(8), was published recently by Akasaka and co-

whole temperature range. The changes in the spectra of the isomer I were attributed to the fixed position of two Sc atoms at low temperature; the activation energy of hopping between them was determined from the temperature dependence of the rate constants to be 80 kJ/mol523 (62 kJ/mol in ref 354). The study of the temperature dependence of the line width of both isomers revealed another process with activation energy of ca. 0.1 eV, which was interpreted as the reorientation of the whole molecule in solution. The quadrupole coupling constant (e2·q·Q/h) was estimated to be 60−30 MHz.523 Studies of the temperature dependence of the line width in the solid state 45 Sc NMR spectra of Sc2C2@C82-C2v(9) and Sc2C2@C82-C3v(8) (single lines were observed for both structures) gave activation energies of 6.4 and 6.6 kJ/mol, respectively.524 The authors associated the corresponding process with the rotational motion of the Sc2C2 clusters inside the cages.524 Two 45Sc NMR lines with a distance of 30 ppm coalescing above 353 K were observed recently in the VT-NMR study of Sc2C2@C80C2v(5); the free energy of the inversion barrier was estimated to be 63 kJ/mol.362 The first 45Sc NMR spectrum of Sc3N@C80-Ih(7) with a single line at ca. 200 ppm was reported in 1999.5 The study of 45 Sc NMR spectra of the series of LuxSc3−xN@C80 (x = 0−2) NCFs with Ih(7) and D5h(6) carbon cages by Yang et al. showed that the 45Sc chemical shift in these NCFs stays in the range of 195−212 ppm and is almost independent of the number of Lu atoms in the nitride cluster.184 In 2006, by studying the temperature dependence of the 45Sc signal in o-dichlorobenzene, Dorn et al. determined the barrier of the Sc3N cluster rotation to be 75 ± 8 meV (7.2 ± 0.8 kJ/ mol).103 Almost the same value, 79 ± 6 meV, was determined for [email protected] It should be noted that rotational motion of the carbon cage also contributes to the spin relaxation and hence has some influence on the determined “barriers”. 6020


Chemical Reviews

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nuclei broadens the signals and makes 14N NMR studies hardly useful. On the other hand, the low natural abundance of 15N isotope precludes the broad use of 15N NMR spectroscopy in routine studies of nitrogen-containing fullerenes. However, it has been shown by Dorn et al. that the rarely applied 14N NMR spectroscopy can be a useful tool in the studies of nitride clusterfullerenes.526 Sc3N@C80, Y3N@C80, and Lu3N@C80 exhibited surprisingly narrow 14N NMR signals at 395.9, 381.7, and 292.0 ppm, respectively, which was attributed to the symmetric environment of the nitrogen atoms in M3N clusters. From the temperature dependence of the 14N line width, the authors determined the activation energies to be 75 ± 8, 122 ± 10, and 186 ± 14 meV, respectively, and suggested that these values can be assigned to rotation barriers of the M3N clusters inside the carbon cage.526 Narrow 14N signals were also observed for the mixed metal clusters in LuxSc3−xN@C80 and LuxY3−xN@C80 (x = 0−3) series, and the δ(14N) values were found to be a linear function of x in each family (Figure 21).527 DFT calculations were

Figure 20. 45Sc NMR spectra of (a) Sc3NC@C80-Ih(7), reproduced with permission from ref10. Copyright 2010 American Chemical Society; (b) Sc4O2@C80-Ih(7), reproduced with permission from ref 371. Copyright 2012 American Chemical Society.

workers.303 While 45Sc NMR signals of EMFs with trivalent Sc are usually observed at 150−300 ppm, the signal of Sc2@C82 is shifted to ca. 430 ppm.303 In the oxide clusterfullerene Sc4O2@C80 with mixed-valence state of Sc, 45Sc NMR spectroscopy distinguishes two types of Sc atoms with chemical shifts of 285 and 135 ppm (Figure 20b). On the basis of DFT calculations, they were assigned to ScII and to ScIII, respectively.371 89 Y NMR Spectroscopy. Although the 89Y isotope is 100% abundant in nature, the 89Y NMR studies are usually hampered by long spin−lattice relaxation times. The only 89Y NMR study of EMFs was reported by Dorn et al. in 2009.99 The authors succeeded in measuring the spectra of Y3N@C80-Ih(7), Y3N@ C84-Cs(51365), and Y3N@C86-D3(19). While only one 89Y signal was observed for Y3N@C80 and Y3N@C86 with a rotating Y3N cluster, three such signals were found in Y3N@C84 in which the cluster is fixed. 89Y chemical shifts were shown to be highly sensitive to the subtle changes in the electronic environment of yttrium atoms. The δ(89Y) values cover the range over 200 ppm: 191.63 ppm in Y3N@C80, 62.65 ppm in Y3N@C86, and 104.32/65.33/−19.53 ppm in Y3N@C84. In the latter, the strongly shielded signal at −19.53 ppm is assigned to the Y atom coordinated to the adjacent pentagon pair.99 Recently, Dorn et al. reported the values of 1JY−C coupling constants describing interaction of Y atoms and acetylide C2 units in a series of Y2C2@C2n carbide clusterfullerenes.364 7 Li NMR Spectroscopy. A single 7Li NMR signal at high filed (δ = −10.5 ppm vs LiCl in D2O) was reported for the [Li+@ C60]SbCl6− salt in o-DCB/acetonitrile.144 The upfield shift compared to that of free Li+ was attributed to the shielding effect of the π-conjugated system of the fullerene. In o-DCB solution of [Li+@C60]PF6−, the 7Li signal was detected at δ = −11.2 ppm, and the line was shifted to −10.7 ppm and −12.3 ppm in the [5,6] and [6,6] isomers of [Li+@PCBM]PF6−, respectively.378 Even more negative 7Li NMR signal, −15 to −19 ppm, was found in the Li@C60-based fullerenol with brutto composition Li@C60(OH)18.525 14 N NMR Spectroscopy. Although the two stable nitrogen isotopes have a nonzero nuclear spin, the use of NMR spectroscopy is mostly focused on the 15N isotope (I = 1/2), since it is believed that the quadrupolar moment of 14N (I = 1)

Figure 21. 14N NMR spectra of LuxSc3−xN@C80 and LuxY3−xN@C80 (x = 0−3) in CS2 at room temperature. An asterisk marks a signal of molecular nitrogen. Reproduced with permission from ref 527. Copyright 2011 American Chemical Society.

performed to interpret the experimentally measured spectra, and the main factors affecting 14N chemical shifts in nitride clusterfullerenes were analyzed using the Ramsey theory both in localized and canonical molecular orbitals. It was shown that 14 N chemical shifts in M3N@C80 and related systems are solely determined by nitrogen-localized orbitals. The variation of 14N chemical shifts in different compounds can be traced back to the px,y,z atomic orbitals of nitrogen and their contribution to the paramagnetic shielding. As a result, the changes in the nitrogen shielding could be clarified by the simple analysis of the density of states projected on the nitrogen p-orbitals and its variation in different chemical environments.527 6.2. ESR Spectroscopy

To be eligible for ESR spectroscopic studies, EMFs should be paramagnetic. Therefore, the EMFs studied by ESR spectroscopy can be classified into two groups: (i) EMFs, which are paramagnetic in their pristine states, and (ii) diamagnetic EMFs, which are transformed into paramagnetic forms via either chemical or electrochemical redox reactions. The first group includes presumably monometallofullerenes, usually based on Sc, Y, and La, as well as their exohedral derivatives 6021


Chemical Reviews

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Table 4. Isotropic Hyperfine Coupling Constants of Metal Atoms in Radicals of EMFs.a a(M)b Sc@C82-I C2v(9) Sc@C82(Ad): isomers a−d Sc@C82-II Cs(6) Sc@C82-II Cs(6) Sc@C84 [Sc2C2@C82-Cs(6)]− Sc3C2@C80-Ih(7)

[6,6]-Sc3C2@C80(Ad) [5,6]-Sc3C2@C80(Ad) Sc3C2@C80(Pyr) Sc3C2@C80(Pyr)2 Sc3C2@C80-bisfulleroid [Sc3N@C68]+ [Sc3N@C68]− [Sc3N@C80-Ih(7)] −

[Sc3N@C80(CF3)2]− [Sc3N@C80(CF3)2]3− [Sc3N@C80(CF3)10]− [Sc3N@C80(CF3)12]− [5,6]-pyrrolidino-Sc3N@C80− TiSc2N@C80-Ih(7) [Sc3NC@C80] − [Sc4O2@C80]+ [Sc4O2@C80] − Y@C82-I C2v(9)

Y@C82(Ad)-a Y@C82(Ad)-b Y@C82(Mes2Si)2CH2-I Y@C82(Mes2Si)2CH2-II Y@C82-II (Cs(6)) [Y2@C82-Cs(6)]− Y2@C79N Y2@C79N-pyrrolidin-A Y2@C79N-pyrrolidin-B [Y2C2@C82-Cs(6)]− [Y3N@C80(C4H9N)]−

La@C76 La@C78 La@C80-I La@C80-II La@C82-I C2v(9)

La@C82CH2(GeDep2)2: I−III La@C82(CPh2): A−D La@C82(CH2)2NCH3-I La@C82(CH2)2NCPh3-I La@C82((CH2)2NCH3)2-III La@C82-exTTF

3.82 3.63 3.67−5.36 1.74 1.16 3.78 0.48, 0.97 6.22 3 × 6.51 6.22 → 6.80 3 × 6.26 2 × 7.33, 1.96 ∼2 × 7.9, ∼1.7 2 × 4.82, 8.60 8.21, 4.82, 4.73 2 × 4.00, 6.73 3 × 1.28 3 × 1.75 3 × 55.6 3 × 55.5 3 × 55.4 2 × 9.34, 10.70 2 × 10.8, 49.2 0.6, 11.1, 21.5 0.6, 7.4, 8.1 2 × 33.4, 9.6 a(45Sc) 1 ms

662 663,664 646

Dy@C82 Dy@C82 [5,6] Sc3N@C80-pyrrolidine

MPc P3HT ferrocene

LB film LB film dyad

C60-pyrrolidine

ferrocene

dyad

[5,6] Sc3N@C80-pyrrolidine

2-TPA-pyr

dyad

C60-pyrrolidine

2-TPA-pyr

dyad

[5,6] Sc3N@C80-pyrrolidine C60-pyrrolidine [5,6] Sc3N@C80-pyrrolidine

N-TPA-pyr N-TPA-pyr ZnP (at ∼32.7 Å)

dyad dyad dyad

[5,6] Sc3N@C80-pyrrolidine

ZnP (at ∼45.9 Å)

dyad

[6,6]-open Sc3N@C80-PCBE

ZnP

dyad

[6,6]-open Ce2@C80-PCBE

ZnP

dyad

[6,6]-open La2@C80-PCBE

ZnP

dyad

[5,6] La2@C80-pyrrolidine La@C82-pyrrolidine

exTTF exTTF

dyad dyad

C60-pyrrolidine

exTTF

dyad

La@C82−PCBE [6,6]-open Lu3N@C80-PCBE

TPP PDIf

dyad dyad

[5,6] La2@C80-pyrrolidine

TCAQf

dyad

Sc3N@C80⊂bisporphyrin Sc3N@C80⊂bisporphyrin Lu3N@C80⊂bisporphyrin La@C82-pyrrolidine-Py [5,6] La2@C80-pyrrolidine-Py C60-pyrrolidine-Py [6,6]-open Lu3N@C80-PCBH

TPP ZnP TPP ZnP ZnP ZnP P3HT

complex complex complex complex complex complex blend

646

647 647 647 647 648 648 279 387

279

649 575 666 411 650

665

651 651 651 576 576 576 652

a

PCBX here is [6,6]-open phenyl-C80-butyric acid alkyl ester, in PCBE the alkyl group is 2-oxyethyl spacer, H in PCBH stands for hexyl (see also Figure 37), Py is pyridyl functional group. bMPc − metallophthalocyanines, P3HT − poly(3-hexylthiophene), TPA − triphenylamine, ZnP − zinc tetraphenylprphyrine, exTTF − π-extended tetrathiafulvalene, TPP − tetraphenylporphyrine, PDI − 1,6,7,12-tetrachloro-3,4,9,10-perylenediimide, TCAQ − 11,11,12,12-tetracyano-9,10-anthra-p-quinodimethane. cLB film − Langmuir−Blodgett film, dyad − molecular system in which EMF and donor fragments are covalently linked via a bridge, complex − host−guest complex of NCF with bisporphyrins or a coordination complex between Py group on a fullerene and Zn in ZnP; blend − EMF blend with conjugated polymer. dCSS − charge separation state; when available, lifetimes are listed for different solvents; DMF − N,N-dimethylformamide, THF − tetrahydrofurane. eReversed charge separation state (cationic state of EMF). f PDI and TCAQ are electron acceptors in these dyads.

effect”.279 One year later the same group also reported the synthesis of a series of donor−acceptor conjugates comprised from Sc3N@C80 or Y3N@C80 linked to tetrathiafulvalene, pthalocyanine or ferrocene, but their photophysical studies were not reported.596

In two other reports, photophysical properties of molecular donor−acceptor conjugates of Sc3N@C80 bridged to donors via pyrrolidine linker were studied in dependence on the donor− acceptor distance.647,648 In 2009 Pinzon et al. reported the synthesis of two dyads, in which triphenylamine (TPA) was linked to [5,6] Sc3N@C80-fulleropyrrolidine either via the 6042


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Figure 37. Selected EMF-based donor−acceptor conjugates: (a) [5,6] Sc3N@C80-pyrrolidine-ferrocene, reproduced with permission from ref 671. Copyright 2008 Wiley; (b) two [5,6] Sc3N@C80-pyrrolidine-TPA dyads with different way of linking to the pyrrolidine ring, reproduced with permission from ref 647. (c) two [5,6] Sc3N@C80-pyrrolidine-ZnP dyads with different Sc3N@C80-to-ZnP distances: 33 Å (1) and 46 Å (2), reproduced from ref 648. Copyright 2011 The Royal Society of Chemistry; (d) [6,6]-open La2@C80-PCBE-ZnP and [6,6]-open Sc3N@C80-PCBEZnP dyads, reproduced from ref 279. (e) La@C82−PCBE-H2Por dyad (3 isomers), reproduced from ref 411. Copyright 2011 Wiley; (f) [5,6] La2@ C80-pyrrolidine-exTTF, taken from ref 649. (g) [6,6]-open Lu3N@C80-PCBH (left, reproduced from ref 652. Copyright 2009 Nature. and [6,6]open Lu3N@C80-PCBEH (right, reproduced from ref 670. Copyright 2009 Wiley) used in blends with P3HT.

Figure 38. Energy diagram for Ce2@C80-PCBE-ZnP dyad with different de-excitation pathways in nonpolar solvents (toluene and THF, left) and polar solvents (benzonitrile and DMF, right). Reproduced with permission from ref 387. Copyright 2010 American Chemical Society.

nitrogen or via the carbon atom of the ring (Figure 37b,c).647 A nitrogen-linked conjugate (which has longer distance between Sc3N@C80 and TPA) was found to be much more thermally stable and exhibited a longer lifetime of the photoinduced charge-separated state than the carbon-linked system (>3 and 2.2 ns, respectively, in benzonitrile). Importantly, the recombination of the charge-separated states in Sc3N@C80based dyads was several times slower than in their C60 analogues. In nonpolar solvents (such as toluene or CS2) neither Sc3N@C80 nor C60-based dyads underwent photo-

excited charge transfer; instead, fullerene-based singlet-excited states decayed into the triplet excited states via intersystemcrossing.647 Long-range charge transfer in two Sc3N@C80-zinc tetraphenylporphyrine (ZnP) conjugates linked via long chains (center-to-center donor−acceptor distances of ca. 33 and 46 Å, Figure 37c) was studied by Wolfrum et al. in 2011.648 When the dyads were excited at 420 nm, near the wavelength of the Soret band of ZnP (428 nm), the ZnP-based fluorescence was noticeably quenched in comparison to the fluorescence of the free ZnP, especially in polar solvents. The fluorescence lifetime 6043


Chemical Reviews

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between the excited state of TPP and EMF (Figure 37).411 The authors proposed that in this dyad the energy transfer should be the dominant mechanism because the energy of the first excited doublet state of La@C82, 0.88 eV, is below the estimated energy of the ion pair (La@C82)•−-PCBE-(TPP)•+, 1.05 eV.411 Whereas the vast majority of fullerene-based donor−acceptor dyads are constructed with the fullerene moiety playng the role of the acceptor, in the recent study Akasaka, Guldi et al. proposed a “paradigmatic change” by linking Lu3N@C80-PCBE to an electron acceptor and light harvester 1,6,7,12-tetrachloro3,4,9,10-perylenediimide (PDI).650 Transient photophysical studies proved that when excited at 530 nm, the PDI singlet excited state decayed fast (lifetime less than 1 ps) into an anionradical state of PDI, and NIR absorption features of the Lu3N@ C80+ cation were observed in the transient spectra. (Lu3N@ C80)•+-PCBE-(PDI)•− is metastable and decays rapidly with lifetimes of 120 ps (in toluene) or less (in polar solvents). Interestingly, when the dyad was excited at 387 nm, only Lu3N@C80-based triplet state was observed.650 Supramolecular Complexes. In 2011 two groups independently reported formation of supramolecular complexes of NCFs with bisporphyrins and their photophysical studies. Guldi, Boyd and co-workers studied a complexation of Sc3N@C80 and Lu3N@C80 by calixarene scaffold bearing bisporphyrins (TPP and ZnP) in solution.651 Efficient quenching of bisporphyrin fluorescence was found in host−guest complexes, and transient spectroscopic studies revealed that the quenching is due to the charge transfer with formation of the cationic states of NCFs and anionic bisporphyrins.651 Echegoyen, Ballester and coworkers reported the complexation of Sc3N@C80 with cyclic Zn-bisporphyrins having aryl groups in meso positions and connected by flexible hexyldioxo spacers.667 Fluorescence of Zn-porphyrin moieties was quenched when the complexes were formed, which was assigned to the charge transfer upon photoexcitation.667 Binding constants of Sc3N@C80-bisporphyrin complexes determined in both works by UV−vis and fluorescence titration were at the order of 105 M−1.651,667 In 2013 Akasaka, Guldi and co-workers reported photophysical studies of coordination complexes between ZnP and fullerenes C60, La2@C80, and La@C82 functionalized by a pyridyl group through a pyrrolidine linker.576 Studies of the absorption spectra of ZnP at different concentrations of the reagents allowed determination of the binding constants which were found to be near 104 for all three fullerenes. Transient absorption spectra showed that photoexcitation of the complexes at 420 nm (i.e., at ZnP) induced electron transfer with formation of (ZnP)•+ and radical anion of corresponding fullerene. The fastest charge separation process was found in the complex of La@C82, whereas the longest lifetime of the ion pair was found for [email protected] Fukuzumi et al. studied photoinduced electron transfer between Li+@C60 and various donors in PhCN solution by femto- and nanosecond laser flash photolysis.668 Electron transfer rate constant determined from transient absorption spectra were considerably higher for Li+@C60 than for C60 in analogous reactions. The higher reactivity of Li+@C60 in the triplet excited state, 3(Li+@C60)*, in comparison to that of 3 C60* was explained by a much higher reduction potential of the former. Interestingly, reorganization energy of the electron transfer of Li+@C60 was found to be 0.28 eV larger than in C60.668

of the ZnP singlet excited state was also reduced from 2.1 ns in free ZnP to 0.5 and 1.6 ns in dyads with shorter and longer Sc3N@C80-ZnP distances, respectively. Transient absorption spectroscopic studies unambiguously proved that the 1*(ZnP) state in the dyads decayed via (Sc3N@C80)•−-linker-(ZnP)•+ charge separated states with unprecedentedly long lifetimes of 1.0 μs (Sc3N@C80-ZnP distance 33 Å) and 1.2 μs (Sc3N@C80ZnP distance 46 Å).648 In 2010 Guldi et al. reported the first synthesis and photophysical studies of a dyad based on the dimetallofullerene, Ce2@C80-Ih(7), which was linked to ZnP via a PCBM-like 2oxyethyl butyrate spacer (PCBE hereafter) (Figure 37d).387 In the (Ce2@C80)-PCBE-(ZnP) dyad, the fluorescence of ZnP was effectively quenched both in polar (DMF and benzonitrile) and nonpolar solvents (toluene and THF) pointing to a charge/energy transfer. However, the changes in the transient absorption spectra in polar and nonpolar solvents were noticeably different. The interpretation of this phenomenon was made by comparing the transient spectra to the absorption spectra of electrochemically generated ZnP•−, (Ce2@C80PCBM)•− and (Ce2@C80-PCBM)•+. The authors proved that in the nonpolar media, the singlet excited state of the ZnP moiety, Ce2@C80-PCBE-1*(ZnP), evolves via EMF-anionic (Ce2@C80)•−-PCBE-(ZnP)•+ ion pair, while in polar media the charge transfer direction is reversed and proceeds via unprecedented metastable EMF-cationic ion pair (Ce2@ C80)•+-PCBE-(ZnP)•−, which then quickly recombines with the formation of triplet state of the EMF, 3*(Ce2@C80)-PCBEZnP (Figure 38).387 An analogues solvent-dependent behavior was found for the La2@C80-PCBE-ZnP dyad, whereas a charge separation upon photoexcitation of the Sc3N@C80-PCBE-ZnP dyad proceeded via the (Sc3N@C80)•−-PCBE-(ZnP)•+ ion pair independent of the solvent polarity.279 This concept was further corroborated in the study of [5,6] La2@C80-pyrrolidine conjugated with the strong electron acceptor TCAQ (11,11,12,12-tetracyano-9,10-anthra-p-quinodimethane).665 Photoexcitation of this dyad resulted in the formation of (La2@ C80)+-pyrrolidine-TCAQ•− ion pair with the lifetime from 80 to 230 ps both in polar and nonpolar solvents followed by the charge recombination with formation of the triplet state of [email protected] In 2010 Martin, Guldi, Akasaka and co-workers also reported a synthesis and photophysical studies of [5,6] La2@C80pyrrolidine conjugates with covalently linked π-extended tetrathiafulvalene (exTTF) (Figure 37f).649 No quenching of exTTF fluorescence was found when the dyad was excited at 430 nm. Transient absorption spectra showed that upon photoexcitation at 387 nm in THF and toluene both La2@C80 and exTTF moieties are excited into their singlet states, which then decayed fast (within ∼20 ps) into the nanosecond-lived charge-separated state (La2@C80)•−-pyrrolidine-(exTTF)•+.649 Photophysical studies of an analogues dyad with La@C82 as a fullerene component were reported in 2012.575 Photoexcitation at 387 nm resulted in the excited states of both La@C82 and exTTF moieties, which then evolved within few picoseconds into the radical ion pair state (La@C82)•−-pyrrolidine-(exTTF)•+. The charge recombination in this pair was rather slow with the lifetimes of 2.4 ns in THF and 1.1 ns in cyclohexylisonitrile.575 Note that these lifetimes are much shorter than in the C60pyrrolidine-exTTF dyad (Table 8).666 Efficient quenching of the tetraphenylporphyrine (TPP) fluorescence in the La@C82−PCBE-TPP dyad reported by Akasaka, Guldi and co-workers indicated the strong interaction 6044


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Lu3N@C80-P3HT Blends. Another direction in the use of NCFs for OPV devices was explored by Drees and coworkers.652,669 The authors synthesized a series of [6,6]-open PCBM-like derivatives of Lu3N@C80 with different terminal alkyl groups in the ether fragment and used Lu3N@C80-PCBH (H stands for hexyl) to prepare blends with conjugated polymer P3HT.652 Transient spectroscopic studies showed that in such blends the singlet excited state of P3HT has ultrashort lifetime of only 0.55 ps, decaying fast to the charge-separated state with the P3HT-localized radical cation and Lu3N@C80based radical anion. This radical ion pair state was remarkably stable in the film with the lifetime exceeding 1 ms. Photocurrent efficiency of the OPV with P3HT/Lu3N@C80PCBH active layer exhibited noticeably higher power conversion efficiency (PCE = 4.2%) than analogues P3HT/ C60-PCBM device (PCE = 3.4%). The reason of the improved performance of the EMF-based device is in its higher open circuit voltage (VOC =0.81−0.89 V) than in C60-based device (VOC =0.63 V), stemming from the negative shift of the first reduction potential of Lu3N@C80-PCBH by 0.28 V compared to the C60-PCBM value.652 In the follow-up publication the same authors studied other factors which might further improve PEC of the P3HT/Lu3N@C80-PCBH based OPV cell such as film processing and donor/acceptor ratio.669 An in depth study of the factors influencing the short circuit current (JSC) in bulk heterojunction solar cells based on P3HT/ Lu3N@C80-PCBEH (EH stands for 2-ethyl-hexyl) was recently reported by Liedtke et al.670 The authors prepared P3HT/ Lu3N@C80-PCBEH and P3HT/C60-PCBM with a comparable donor/acceptor volume ratio and found that the EMF-based system exhibits ca. two times weaker quenching of P3HT photoluminescence and smaller JSC when compared to C60based analogue. Combining optical and ESR spectroscopic methods, the authors demonstrated the formation of the triplet excited state of P3HT in photoexcited P3HT/Lu3N@C80PCBEH blends, which were absent in the P3HT/C60-PCBM system.670 6.5.3. Charge Carrier Mobility in Solid EMFs. Conductivity and charge-carrier mobility of solid EMFs may be important factors in their applications, including those in photovoltaics. Two series of charge-carrier mobility measurements in EMFs are reported to date (Table 9). In the mid 2000s, mobility in sublimed films of Dy@C82,672 Ce@C82,673 Pr@C82,674 and La2@C80675 were measured in a field-effect transistor (FET) mode. The measurements were performed for sublimed films of EMFs and showed low electron mobilities on the order of ∼1 × 10−4 cm2 V−1 s−1, attributed to low crystallinity of the samples. Conductance properties of N,Ndimethylformamide extracts of Y@C2n and La@C2n extracts were reported in 2003 by Kareev et al.676,677 Electrical properties of thin films of La@C82 and Li@C60 prepared by sublimation in vacuum were reported by Popok et al. in 2008.678 Dielectric activity of solid films of La@C82 augmented with differential scanning calorimetry measurements was studied by Iwasa et al.679 Transport properties of Dy@C82 were also studied by Kubozono et al.680 In 2011−2012 electron mobilities in La@C82Ad681 and La@ C82·NiII(OEP)259 single crystals as well as drop-casted polycrystalline films of La@C82Ad,681 La2@C80,682 Sc3N@ C80,682 and Sc3C2@C80682 were determined by time-resolved microwave conductivity (TRMC) measurements. The electron mobility of single-crystalline derivative La@C82Ad along the c axis crystal direction was determined to be as high as 10 ± 5

Table 9. Electron Mobility in EMFs EMF

methoda

sample

C60 C60 C60

film (sublim) film (sublim)b single-crystal

FET FET TOF

Dy@C82 Ce@C82 Pr@C82 La2@C80 La2@C80

film (sublim)b film (sublim)b film (sublim)b film (sublim)b drop-casted filmc drop-casted filmc drop-casted filmc single-crystal

Sc3N@C80 Sc3C2@C80 La@C82Ad

La@C82Ad La@C82Ad La@C82Ad La@C82 C60Ad La@ C82·NiII(OEP)

b

nanorod drop-casted filmc drop-casted filmc drop-casted filmc drop-casted filmc single-crystal

μ (cm2 V−1 s−1)

ref 684 685 686

FET FET FET FET TMRC

0.08 0.5 μe 0.5 ± 0.2 μh 1.7 ± 0.2 8.9 × 10−5 ∼1 × 10−4 1.5 × 10−4 1.1 × 10−4 5.0 × 10−3

TMRC

5.7 × 10−3

682

TMRC

0.13

682

TMRC

10 ± 5 (long axis) 0.2 ± 0.1 (45°) 0.7 ± 0.3 (90°) >10 (long axis) (7 ± 3) × 10−2

681

(8.0 ± 2.4) × 10−2 (6.0 ± 2.0) × 10−2 (5.0 ± 1.5) × 10−3 0.9 (c axis)

681

TMRC TMRC TOF TOF TOF TMRC

672 673 674 675 682

681 681

681 681 259

0.3 (a axis) 0.1 (30°, 45°) a FET − field effect transistor; TMRC - time-resolved microwave conductivity; TOF − current-mode time-of-flight. bSublimed films from. cDrop-casted films from CS2 solution; for single-crystals, anisotropy was determined and the values for different crystal directions are listed.

cm2 V−1 s−1, which is the largest mobility value for organic semiconductors measured by TMRC.681 Mobility exhibited significant anisotropy, and the value along other crystal directions could be more than order of magnitude smaller. Moreover, TMRC measurements of the La@C82Ad film dropcasted from CS2 solution showed a drop of mobility to (7 ± 3) × 10−2 cm2 V−1 s−1, and similar values of drift mobilities were obtained for the films of La@C82Ad and La@C82 by time-offlight technique. Thus, crystallinity of the samples was proved to be of paramount importance in affecting the carrier mobilities. Besides, due to the radical nature of La@C82Ad, DFT computations showed a band gap of only 0.005 eV, showing that La@C82Ad can be considered as semimetal.681 Co-crystal of La@C82 with NiII(OEP) studied in the singlecrystalline state also exhibited relatively high and anisotropic electron mobility of 0.1−0.9 cm2 V−1 s−1 (in dependence on the crystal direction).259 DFT study showed that the crystal has no gap (i.e., the compound exhibits metallic character and thus is highly conducting). High mobility of the paramagnetic EMF was recently revealed in the comparative TMRC study of La2@ C80, Sc3N@C80, and Sc3C2@C80 in the form of drop-casted films. While the electron mobility in paramagnetic Sc3C2@C80 is 0.13 cm2 V−1 s−1 (which is the highest value for EMF polycrystalline films), in the diamagnetic La2@C80 and Sc3N@ C80 the values are ca. 20 times smaller.682 The values listed in the Table 9 show that crystallinity of the sample plays a crucial role on the transport properties and hence after optimization, 6045


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photon absorption cross sections of M@C82 EMFs (M = Sc, Y, La) EMFs increase with the increase of the metal size (i.e., form Sc to Y to La).689 Optical properties of M@C82 EMFs (M = Gd, Ce, La, and Y) in dimethylformamide solution were studied by Alidzhanov et al.659 A visible-range luminescence accompanied by enhanced Raman scattering was found for these samples. These phenomena were observed only when nanosized anionic complexes of EMFs were formed and were related to the nanoplasmon excitations in the cluster of EMFs. A photoemission study of Y3N@C80 was reported by Bharadwaj and Novotny in 2010.635 When excited at 633 nm in xylene solution, Y3N@C80 exhibited a fluorescence peak centered at 710 and assigned to the LUMO → HOMO transition with an excited state lifetime of 240 ns. Such a long lifetime of a singlet excited state is rather surprising (compare to ∼48 ps in Sc3N@C80 and Lu3N@C80;646,651 the triplet state lifetime of Sc3N@C80 is 109 ns651) and should be taken with caution. Confocal photoluminescence microscopy was used to study the emission of single Y3N@C80 molecules spin-coated on different transparent surfaces. The photophysical properties of Y3N@C80 were noticeably dependent on the substrate, the best results being obtained with poly (methyl methacrylate). The quantum yield of Y3N@C80 at 635 nm was estimated to be less than 0.05. However, when Y3N@C80 molecules were coupled to gold nanoparticles, the photoluminescence efficiency was plasmon-enhanced by 2 orders of magnitude.635 In 2012 Fukuzumi et al. reported photophysical studies of [email protected] The compound exhibited no measurable fluorescence at room temperature in solution, whereas both fluorescence and phosphorescence could be detected in deaerated 2-methyltetrahydrofuran glass at 77 K. Ultrafast photodynamics of the intersystem crossing from the singlet to the triplet state was followed by femtosecond laser flash photolysis and transient absorption spectroscopy. The rate constant of the S1 → T1 intersystem crossing in Li+@C60, 8.9 × 108 s−1, is ca. two times larger than that of C60 in PhCN, whereas the triplet state lifetime of Li+@C60, 48 μs, is similar to C60 value (49 μs).668

much higher values can be obtained for properly prepared thin films on special substrates as it was found for C60.683 6.5.4. Nonlinear Optic Properties, Singlet Oxygen Generation, and Plasmon Excitations. In 1998 Gu et al. reported a study of the third-order nonlinear optical properties of Dy@C82 in CS2 solution.653 The second-order polarizability at 532 nm was found to be much higher than that of empty fullerenes. The large nonlinear optical response of Dy@C82 was explained by the resonant enhancement and the electron transfer from the endohedral Dy atom to the C82 cage. Similar conclusions were made by Heflin et al. in photophysical studies of Er2@C82(III).654 The authors have found that a nonlinear response of EMF is 2−3 orders of magnitude higher than that of empty fullerenes. The enhancement of the third-order nonlinear optical susceptibility of Er2@C82 was attributed to the electron transfer from metal atoms to the cage.654 In 1999 Khudyakov et al. reported on the picosecond laser photolysis studies of the La@C2n extract (main component was La@C82) in ortho-dichlorobenzene at an excitation wavelength of 528 nm.687 The decay of the transient absorption signal followed a biexponential law with relaxation times of 35 ± 3 ps and 1100 ± 200 ps, while the lifetime of the optical anisotropy was found to be 19 ± 5 ps and agreed with the microscopic model describing rotation of La@C82 molecule.687 Transient spectroscopic properties of La@C82 and La2@C80 on the nanosecond time scale were studied in 2000 by Fujitsuka et al.656 Transient spectra of both EMFs showed a two-step decay with characteristic lifetimes of 83 ns/2.9 μs for La@C82 and 150 ns/∼40 μs for La2@C80. In La@C82, the slow decay was assigned to the lowest excited state with quartet spin multiplicity.656 Note that transient absorption studies of La@ C82 and La2@C80 derivatives reported by Guldi et al. showed that the lifetimes of the first excited states (doublet and singlet, respectively) did not exceed 100 ps, whereas the lifetimes of the quartet and triplet states evolved after intersystem crossing were at the order of nanoseconds.575,576,649 Thus, the assignment of the μs-slow decays in photoexcited La@C82 and La2@C80 found by Fujitsuka et al.656 remains unclear so far. Transient absorption spectra of Gd2@C80 under 800 nm excitation at different pulse durations were measured by Yaglioglu et al.657 Substantial dependence of the spectra and nonlinear optical coefficient (which was much larger than that of empty fullerenes) on the pulse duration were found and explained by a theoretical model combining two-photon and excited-state absorptions.657 Tagmatarchis et al. reported in 2001 that M@C82 (M = Dy, Gd, La) and Dy2@C2n (2n = 84, 86, 88, 90, 92, 94) can efficiently generate singlet oxygen under photolytic conditions depending on the metal type and the number of encapsulated atoms.658 At the same time Yanagi et al. determined the total quenching rate constants of singlet oxygen by La@C82, Ce@ C82, and Ce2@C80 and showed that the quenching efficiency of EMFs is comparable to that of the well-known 1O2 quencher βcarotene.688 In 2009 Phillips and co-workers showed that Sc3N@C80 exhibited a singlet-oxygen generation ability under irradiation in the visible range.633 In 2004 Xenogiannopoulou et al. studied the nonlinear optical response of Dy@C82(I), Dy2@C82(I), and Er2@C92(IV) by means of the optical Kerr effect technique.655 The second polarizability of Dy@C82 was higher than that of empty C82, while the encapsulation of two metal atoms reduced the second polarizability.655 A theoretical study of Hu et al. has shown that the third-order nonlinear optical polarizabilities and the two-

6.6. High-Energy Spectroscopy

UV−vis-NIR absorption spectroscopy provides an information on the low-energy electron excitations and describes mostly π−π* transition of the carbon cage. To probe the valence and core states of the carbon cage as well as of the endohedral species, higher energy excitations have to be studied. By highenergy spectroscopy in this section we imply different kinds of absorption and emission spectroscopy with excitation energies ranging from UV to X-ray, including UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), valence-band photoelectron spectroscopy (VB-PES), resonant photoelectron spectroscopy (ResPES, also known RIXS as resonant inelastic X-ray scattering; note that ResPES is a special case of VB-PES, while the latter in due turn is a type of UPS/XPS), X-ray absorption spectroscopy (XAS), (near)-edge X-ray absorption fine-structure (NEXAFS aka XANES and EXAFS), electron energy loss spectroscopy (EELS), measurements of the photoionization cross-section in the gas phase, etc. The use of these spectroscopic techniques and their combinations in different energy ranges enables a detailed description of the electronic structures of EMFs and the formal oxidation states of the metal atoms. Moreover, structural information on the metal-cage bonding sites can be obtained by 6046


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Table 10. High-Energy Spectroscopic Studies of EMFs and Formal Oxidation States of Metal Atoms in EMFs EMF

q

methods

q

EMF

methods

Ca@C82 (III, IV) Ba@C74

2+ 2+

UPS,725 XANES245

Gd@C60 Gd@C82

3+ 3+

Sc@C82

“2+”

UPS734

Gd@C82(OH)x

3+

Sc2C2@C82-C3v(8) (aka “Sc2@C84III”) Sc3N@C80 Sc3N@C78 Ti2C2@C78

3+

XPS,754 UPS,746,754,756 EELS,755 XAS756

Gd2@C92@SWNT

3+

3+ 4+

XPS,5,431 UPS,431 XAS431 UPS,630 XPS630 EELS,356 UPS629,758,759

Tb@C82 Dy@C60 Dy@C82-C2v(9) (I)c

3+ 3+ 3+

Ti2C2@C82 (I + II) Y-EMF mixturea (Kx)Y@C82

4+

EELS,757 UPS758

Dy@C82-Cs(6) (II)

3+

3+ 3+

XPS,58 XANES,216 EXAFS216,217 UPS,736 XPS736

Dy2@C88-II KxDy@C82

Y2C2@C82-C3v(8)

3+

UPS745

Dy3N@C80-Ih(7)

3+ 2+/ 3+ 3+

Y2@C82-C3v(8) La@C82

3+ 3+

Dy3N@C80@SWNT Ho@C82

3+ 3+

La@C82(OH)x KxLa@C82 La2@C78 La2@C80 Ce@C82b

3+ 3+

Er@C82-C2v(9) (I) Er2@C82-C2v(9) (I) Er2C2@C82-C2v(9) (I) Er2@C82-C3v(8) (III) Er2C2@C82-C3v(8) (III)

3+ “3+” 3+ “3+” 3+

UPS,720 UPS,747 UPS,747 XAS,305 XAS,305

Ce2@C80 Ce2@C72

3+ 3+

UPS745 XPS,58 UPS,690,691 VB-XPS,691 XANES,430 ResPES,430 EXAFS,218 NIXSW694,695 VB-PES693 UPS,692 XPS692 UPS629 UPS,744 XANES,742 EXAFS,742 VB-PES693 VB-PES,697 XPS,696,697 XAS,697 EXAFS,605 XANES,605,697 PFY-XAS,732 ResXES,732 NIXSW,698 photoionisation (in gas)727,730 UPS,744 XPS,282 XAS743 XAS,708 ResPES708

UPS,751 XAS,751 XPS,751 (resonant) XPD753 ResPES752 XPS,718 XAS719

Er2@C90 Tm@C82-C2v(9)

3+ 2+

CeSc2N@C80

3+

Ce 3d XPS103

Tm@C82-Cs(6)

2+

Pr@C82

3+

XPS,286 photoionisation (in gas)728

Tm3N@C80

3+

Pr2@C80 Nd@C82 Sm@C2n (2n = 74 −84) Sm@C82@SWNT Eu@C60 Eu@C82-C2v(9) (III)

3+ 3+ 2+

XPS286 XPS624 EELS473,474

Lu@C82-C2v(9) Lu2@C80-C2v(5) Lu2@C82-C2v(9)

3+ 2+ 2+

XAS719 XPS,721 VB-PES,721 XAS,722 EELS,721,722 ResPES,708 UPS724 XAS,722 XPS,722 VB-PES,722 EELS,722 UPS724 UPS,749 XPS,399,749 XAS/EELS,749 VBPES399 XPS,587 UPS726 UPS744 UPS726

2+ 2+ 2+

EELS/HRTEM699,700 EXAFS,737 XANES739 EELS238

M2C2@C82-C3v(8) M2C2@C82-C2v(9)

3+ 3+

UPS: (M = Sc, Y, Lu, Er)746,747 UPS: (M = Sc, Y, Lu)726,746

3+ 3+

UPS,741 ResPES741 UPS,701,702,707 XANES,228 EXAFS,703 EELS,704 XAS,706 ResPES,706−708 EELS/HRTEM704,705 EELS,760 XAS,709,710 XPS711 (x = 12, 20, 22, 26) EELS/HRTEM761 XPS,716 UPS717 XANES,740 EXAFS740 XPS,712 XANES,713 EXAFS,713 Mössbauer,714 XAS,719,748 photoionisation (in gas)729 EXAFS715 XAS,748 ResPES748 XANES680

XAS192 XPS747 XPS747 UPS747 UPS747

a c

Mixture of Y-EMFs with prevailing Y@C82-C2v(9). bEither pure C2v(9) isomer or a mixture of C2v(9) and Cs(6) isomers with ca. 80% of C2v(9). Mixture of isomers with prevailing C2v(9) isomer.

controversial and indicated the presence of close Y−Y contacts at the distance of 4 Å and rather short Y−C distances at 2.35 Å. These results were interpreted as an indication of the exohedral position of Y.216 In 1993 Park et al. reported EXAFS study of a sample containing a mixture of Y@C82 and Y2@C82 with some empty fullerenes.217 The spectrum was considerably different from that measured by Soderbolm et al. and could be well fitted considering two carbon shells with a Y−C distances of 2.4 and 2.9 Å and with six carbon atoms in each shell, which corresponds to the endohedral position of Y atoms.217 These early studies of the nonseparated EMF samples were important to give the first insight into the valence state of endohedral metal atoms; however more solid data were obtained from the purified samples which became available soon.

the use of EXAFS. Table 10 lists the studies of EMFs by highenergy spectroscopy, the formal charge of metal atoms and spectroscopic techniques used in these studies. Thanks to the sensitivity and selectivity of core-level electron spectroscopy, the first high-energy spectroscopic studies of EMFs could be performed before their separation from empty fullerenes. In 1992, Weaver and co-workers measured XPS spectra of La-EMF mixtures in the region of La 3d3/2 and 3d5/2 core levels and found it to be similar to the spectra of La trihalides, which confirmed a formal La3+ charge state proposed earlier by ESR spectroscopy.58 A trivalent state of Y in Y-EMF mixture was also conformed by XPS in the same study.58 Soderbolm et al. studied a fullerene extract enriched with Y@ C82 by XANES and EXAFS.216 The position of the absorption peak in the Y K-edge spectrum pointed to the trivalent state of the metal in Y@C 2n . 216 However, EXAFS data were 6047


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least-squares deviations of less than 0.005 Å.605 In 2005 Schulte et al. measured PES, XAS and NEXAFS spectra of Ce@C82 on clean and silver-terminated Si(111) surfaces.697 The valenceband PES showed a similarity to the spectra of La@C82, and Ce 3d NEXAFS spectra were consistent with the 3+ valence state as in the earlier report of Shibata et al.605 A NIXSW study of Ce@C82 on Ag(111) did not reveal any sign of ordering induced by the interaction with the surface.698 Pr@C82. In 1996 Ding and Yang measured XPS spectra of Pr@C82 and Pr2@C80 in the 3d core level region of Pr.286 The spectra of two EMFs were very similar indicating the same valence state of Pr. A comparison to the analogous spectra of Pr trihalides proved the trivalent state of metal in Pr@C82 and [email protected] Nd@C82. A trivalent state of Nd in Nd@C82 was determined by Ding et al. in 1996 by comparing the XPS spectra of Nd@ C82 and NdCl3 in 3d and 4d core levels regions of Nd.624 Sm@C82. Okazaki et al. isolated a series of Sm@C2n monometallofulleres (2n = 74−84), including three isomers of [email protected],473,474 A divalent state of Sm in all these EMFs was proved by Sm M4,5 EELS spectroscopy. The authors pointed out that for all lanthanides forming divalent EMFs (Sm, Eu, Tm, Yb) the third ionization potential exceeds 23 eV, whereas the metals adopting a trivalent state in EMFs have smaller third ionization potentias.473,474 The divalent state of Sm was also found in Sm@C82@SWNT peapods by means of EELS measured in high-resolution transmission electrone microscope (HRTEM).699,700 Gd@C82. UPS spectra of Gd@C82 were reported by Hino et al. in 1996.701,702 In agreement with the formal transfer of three electrons to the cage, the spectra were found to be similar to those of La@C82 and likewise exhibited a SOMO-derived lowenergy feature with the spectral onset of 0.3 eV below Fermi level.701,702 In 1999 Giefers et al. reported XANES/EXAFS study of Gd@C82 in the 10−300 K temperature range.703 The valence state of Gd3+ was assigned from the XANES spectrum by comparison to the spectrum of GdF3. Suenaga et al. assigned a Gd3+ valence state in Gd@C82 by comparing the Gd M4,5 (3d → 4f) near-edge fine structure in the EELS spectrum of Gd@ C82 microcrystals to that of Gd2O3 measured by XAS and Gd3+ predicted theoretically.704 Similar spectra were also obtained in the EELS/HRTEM studies of Gd@C82@SWNT peapods; note that in these studies, the valence state of single Gd atoms could be determined.704,705 In 2004 Pagliara et al. used C 1s ad Gd 4d XAS as well as ResPES measurements across the Gd 4d → 4f absorption edge to analyze the electron transfer from Gd to the carbon cage.706 The ResPES spectrum was dominated by the emission of Gd 4f electrons at about 10.5 eV accompanied by weak features above 30 eV assigned to 5p−1 emission of Gd; no features due to Gd 6s2 or Gd 5d6s states could be observed in agreement with the assigned Gd3+ state.706,707 However, Golden et al. pointed out that the magnitude of resonant enhancement of Gd 4f emission in ResPES spectra was significantly lower than in metallic Gd, which indicated the presence of hybridization between valence states of Gd and carbon cage.708 By studying valence photoelectron spectra and pre-edge and resonance regions in Gd 4d → 4f XAS spectra of Gd@C82 and various Gd@C82(OH)x derivatives (x = 12, 20, 22, 26), Tang et al. have shown that the exohedral functionalization can substantially affect the electronic state of endohedral metal atoms.709−711 The local structure of Gd-cage bonding site was studied by EXAFS703 and XANES.228 The best fit to the experimental

6.6.1. Lanthanide-Based M@C82. In the view of the highest yield of M@C82 monometallofullerenes in the standard EMF synthesis, the majority of the spectroscopic studies was devoted to this group of EMFs. The most detailed studies were performed at La@C82, Ce@C82, Gd@C82, Dy@C82, and Tm@ C82 as discussed below. La@C82. In 1993 Hino et al. reported a UPS spectroscopic study of La@C82 and C82.690 A spectral onset of La@C82 was found at 0.2 eV below the Fermi level, whereas that of empty C82 is deeper than 1.1 eV. A low-energy part of the difference of the UPS spectra of La@C82 and C82 was deconvoluted into two components with the intensity ratio of 1:2 suggesting that the number of transferred β-spin electrons is twice that of α-spin electrons, resulting in a three-electron transfer.690 Similar results were also obtained in 1994 by Poirier et al., who identified a low-energy feature in the UPS spectrum centered at 0.64 eV as a SOMO-derived band;691 however a more pronounced difference between the UPS spectra of C82 and La@C82 was revealed. By measuring the VB-XPS spectrum at 1486.6 eV these authors proposed that there was essentially no La 5d character in the occupied states of La@C82 since the La 5d band was absent. Finally, a La 5p band was identified in the VB-XPS spectrum and a strong mixing of La 5p states with σ states of C82 cage was suggested.691 In 1997 Kessler et al. reported a XANES and ResPES study of La@C82 with excitation energies corresponding to the La 3d→4f transitions.430 In contrast to the results of Poirier et al.,691 these authors have found a strong resonant enhancement of La 5d emission near the Fermi level when the incident energy matched the transition from the La 3d3/2 level at 848 eV. From the ratio of the La 5p and La 5d peaks in the ResPES spectrum, the authors estimated that the population of the La 5d state in La@C82 to be close to 1/3.430 Ton-That et al. studied potassium-doped La@C82 by UPS and XPS spectroscopies and did not find any changes in the valence state of La upon doping up to a level of [email protected] Recently, Wang et al. reported a VB-PES study of La@C 82 and several La@C 82 (OH) x compositions (x = 12, 18, 24, 32).693 In 1995 Nomura et al. studied the structure of the local environment of La in La@C82 by La LIII-edge EXAFS.218 La was found to be located near a hexagon on the C2 axis of the cage in the off-center position with the nearest La−C distances of 2.47 and 2.94 Å.218 Ton-That et al. studied the ordering of La@C82 molecules in monolayers deposited on Ag(111) and Cu(111) surfaces by means of incident X-ray standing wave (NIXSW) technique at the C-1s and La-3d5/2 peaks.694,695 The measurements showed that EMF molecules adopt a preferential orientation with respect to substrates, whereas La atoms exhibit a significant ordering with respect to the height distribution, but the lateral ordering is weak. Ce@C82. In 1996 Ding et al. measured UV−vis-NIR and XPS spectra of Ce@C82 in the range of the Ce 3d3/2 and 3d5/2 core levels.696 Since characteristic feature of Ce4+ was not observed in the spectrum, this valence state was excluded. Ce 3d XPS peaks of Ce@C82 were found to be similar to those of Ce trihalids, and the formal state 3+ was assigned to Ce in Ce@C82 although the Ce2+ state could not be completely ruled out.696 In 2003 Shibata et al. confirmed the Ce3+ valence state in Ce@C82 by Ce LIII-edge XANES and determined the local structure around the Ce ion by Ce LIII-edge EXAFS.605 The distances from the Ce atom to the two nearest sets of carbon atoms (6 atoms in each set with Ce coordinating the hexagon of C2v(9) cage) were determined to be 2.473(9) and 2.743(9) Å with the 6048


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EXAFS data in the report of Giefers et al. was obtained by considering two shells of carbon atoms near Gd with Gd−C distances of 2.49 and 2.95 Å implying coordination of the Gd atom to a hexagon of the carbon cage.703 The weak temperature dependence of EXAFS parameters pointed to the strong binding of Gd to the fullerene cage.703 Liu et al. compared the XANES spectrum of Gd@C82-C2v(9) to the spectra computed for four different positions of Gd inside the cage and found the best fit for the configuration, in which Gd is facing the hexagon located on the C2 axis of the C2v(9) cage.228 Dy@C82. The XPS spectrum of a Langmuir−Blodgett film of Dy@C82 in the core-level range of Dy 4d were reported in 2000 by Huang and Yang.712 Comparison to the XPS spectrum of DyCl3 revealed that Dy adopts in Dy@C82 a formal 3+ state.712 In 2001 Iida et al. measured Dy LIII-edge XANES and EXAFS spectra of the isomeric mixture of Dy@C82 (the prevalence of the C2v(9) isomer).713 A valence state of Dy was determined by XANES to be 3+ in agreement with XPS data. The best fit to EXAFS data was obtained for Dy@C82-C2v(9) with Dy placed in the off-center position and coordinated to the hexagon with the nearest Dy−C distances of 2.52 and 2.86 Å.713 The valence state of Dy3+ in Dy@C82 was also confirmed in 2000 by 161Dy Mössbauer spectroscopy.714 In 2003 Kubozono et al. found an additional feature in the XANES spectrum of potassium-doped Dy@C82 which was not present in the spectrum of [email protected] This feature was assigned to Dy2+ which indicated that doping can change the valence state of endohedral metal atoms.680 So far it is the only example of such a behavior since no changes in the valence states were found for other EMFs doped with potassium. Takabayashi studied the location of Dy atom in the minor isomer Dy@C82-Cs(6) by Dy LIII-edge EXAFS.715 The best fit to the experimental data was obtained when Dy was placed near the C−C bond formed by two fused hexagons with the nearest Dy−C distances of 2.35, 2.75, and 3.18 Å; however, an exact position of this bond could not be determined.715 Eu@C82. In 2004 Sun et al. isolated a series of Eu@C2n (2n = 74−90) EMFs.238 The chemical shift of the major isomer Eu@ C82 (III) with presumably the C2v(9) carbon cage measured in Eu M4,5-edge EELS spectra proved the Eu2+ state of the metal atom.238 Tb@C82. A trivalent state of Tb in Tb@C82 was determined by Huang et al. in 2000 by comparing the XPS spectra of Tb@ C82 and TbCl3 in 3d and 4d core levels regions of Tb.716 UPS spectra of Tb@C82 measured by Iwasaki et al. also confirmed 3+ state of Tb and were almost identical to the spectra of La@ C82.717 Ho@C82. A trivalent state of Ho in Ho@C82 was proposed by Huang et al. in 1999 based on the comparison of the XPS spectra of Ho@C82 and HoCl3·3H2O in the 3d core level region of Ho.718 Besides, M4,5-edge XAS spectra of Gd@C82, Dy@C82, and Ho@C82 compared to the results of the atomic multiplet calculation have shown that in these EMFS all metal atoms are in their trivalent state.719 Er@C82. UPS spectra of Er@C82-C2v(9) (i.e., isomer I) were measured by Miyazaki et al. in 2010.720 The spectral onset of 0.4 V was found to be similar to other MIII@C82 and much smaller than that in MII@C82, which points to the 3+ state of Er in Er@C82. The low-energy part of the experimental spectrum was reproduced by a spectrum simulated for the trianion C823−C2v(9).720 The M5 edge XAS spectrum of Er@C82−C2v(9) reported in 2008 by Shinohara and co-workers also confirmed a 3+ oxidation state of Er (Figure 39).192

Figure 39. Er-M5 edge XAS spectra of Er@C82, Er2@C82, and Er2C2@ C82, all with C2v(9) carbon cage. Experimental spectra are shown as circles, solid lines are computed spectra for free Er3+. Reproduced with permission from ref192. Copyright 2008 American Chemical Society.

Tm@C82. In 1997 Pichler and co-workers reported a detailed spectroscopic study of Tm@C82-C3v(8) by XPS/VB-PES and EELS.721 The XPS spectrum of Tm@C82 in the region of Tm 4d core level was distinctly different from that of metallic Tm and exhibited pronounced a similarity to the spectrum calculated for Yb3+ with 4f13 ground state configuration. Thus, a Tm2+-4f13 state was assigned to Tm@C82. A further proof of the divalent state of Tm was obtained by analysis of the 4f multiplet structures VB-PES spectrum.721 Comparing the spectra to the multiplet profiles computed for 4f12 → 4f11 (Tm3+) and 4f13 → 4f11 (Tm2+) photoemission (Figure 40a), the authors unambiguously proved the 4f13 state of Tm; no admixtures of the 4f12 → 4f11 multiplet could be detected. Finally, the Tm 4d excitation spectrum of the Tm@C82 measured by EELS in transmission mode was also consistent with the divalent state of Tm.721 Using the same methods, these authors have also proved a divalent state of Tm in

Figure 40. 4f photoemission spectra of (a) Tm metal and Tm@C82 (reproduced with permission from ref 721. Copyright 1997 The American Physical Society) and (b) Tm3N@C80 (reproduced with permission from ref 399. Copyright 2005 American Chemical Society). Also shown are calculated photoemission multiplets for 4f12 → 4f11 (Tm3+) and 4f13 → 4f12 (Tm2+). 6049


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another isomer of Tm@C82 with Cs(6) carbon cage.722 Besides, a more extended spectroscopic study was presented for two isomers in that work, which included also UPS, C 1s photoemission and absorption as well as analysis of the EELS-derived loss function and optical conductivity.722 Doping of Tm@C82-Cs(6) with potassium up to the level of K6Tm@ C82 was followed by UPS and XPS spectroscopies and did not show any changes in the valence state of Tm.723 In 2004 Golden et al. reported a ResPES study of Tm@C82 at photon energies crossing the Tm 4d → 4f absorption.708 Resonant enhancement of the Tm f13 → f12 multiplet was observed when the excitation energy matched the 4d → 4f absorption threshold, but no new features were detected pointing to the weak hybridization of the valence states of Tm and the fullerene.708 In 2005 Hino et al. measured the UPS spectra of three isomers of Tm@C82724 and pointed out a close similarity of the spectra to those of Ca@C82 isomers measured earlier.725 Importantly, no characteristic features of the SOMO-derived bands, which are typical for MIII@C82, were observed in the spectra of [email protected] The authors used Hartree−Fock calculations of the density of states of C82 dianions to identify the cage isomers. Besides, reassignment of the Tm@C82-C3v(8) isomer studied by Pichler et al.721 to the C2-symmetric cage was proposed.724 Lu@C82. In 2000 Huang and Yang isolated Lu@C82 and determined a 3+ valence state of Lu in this EMF by comparing the XPS spectra of Lu@C82 and LuCl3 in the Lu 4f region.587 UPS spectroscopic study of Lu@C82 was reported in 2013.726 Gas-Phase Photoionization Studies. In 2005 and 2008 Mitsuke and co-workers reported gas-phase photoionization studies of Ce@C82727 and Pr@C82728 in the photon energy ranges of 90−160 and 100−150 eV, respectively. For both compounds, total photoabsorption cross sections were determined and broad peaks observed at 120−140 eV were assigned to the 4d → 4f giant dipole resonance of encapsulated metal atoms. The same group also studied photoion yields of Dy@C82+ and Dy@C822+ in the energy range from 24.5 to 39.5 eV including Dy 4d → 4f absorptions.729 Müller et al. also studied the photoionization cross-section of the Ce@C82+ ion in the gas phase by synchrotron radiation.730 Spectral fingerprints of the 4d excitation of Ce atoms encapsulated in a fullerene have been observed and the anticipated redistribution of decay probabilities of a caged atom versus its free counterpart has been demonstrated.730 Theoretical study of the photoabsorption of Ce@C82 was reported by Chen et al.731 PFY-XAS and ResXES. The electronic structure of Ce@C82 and Pr@C82 was studied by partial fluorescence yield X-ray absorption spectroscopy (PFY-XAS) and resonant X-ray emission spectroscopy (ResXES) at the LIII absorption edge.732,733 A nearly three electron transfer was confirmed, and a satellite structure corresponding to the back-electron transfer induced by the core−hole potential in the final state was observed in the XAS and ResXES spectra of both EMFs.732,733 6.6.2. Nonlanthanide M@C82. Sc@C82. A UPS spectroscopic study of Sc@C82 was reported by Hino et al. in 1999.734 The authors have found a very close resemblance of the spectra of Sc@C82, La@C82, and Tb@C82, including the presence of the low-energy feature near the Fermi level assigned to SOMO of MIII@C82 EMFs.734,735 Based on the difference between the spectra of La@C82 and Sc@C82, the data were interpreted at that moment as the evidence of the 2+ state of Sc. In fact, the

3+ state provides a more appropriate explanation of the spectroscopic data and agrees better with the recent experimental and computational results.423,549 Y@C82. Ton-That et al. measured UPS and Y 3d XPS spectra of purified Y@C82 in 2006.736 UPS spectra were found to be consistent with those of other MIII@C82 EMFs and likewise exhibited a SOMO-derived spectral onset at 0.3 eV below Fermi level.736 The Y 3d core level XPS spectrum of Y@C82 was found to be similar to that of Y triiodide proving thus a 3+ state of Y. These authors have also shown that doping with potassium up to a level of K2.3Y@C82 did not affect the valence state of Y.736 Ca@C82. In 2001 Hino et al. reported the UPS study of the isomers II and IV of [email protected] In contrast to EMFs with trivalent metal atoms, a rather high shift of the spectral onset from the Fermi level was found, which is consistent with the two-electron transfer and a formal valence state of Ca2+. DFT calculations of different isomers were performed to identify molecular structures, and the C2 cage was found to provide the best fit to the spectra of the isomer III in agreement with the results of NMR measurements, while Cs symmetry was assigned to the isomer IV.725 6.6.3. Monometallofullerenes M@C60. Although La@C60 was the first EMF detected in the mass spectra back in 1985,3 isolation of M@C60 has met some difficulties mainly because these EMFs were not soluble in standard solvents used for the fullerene extraction. The problem was solved by using aniline as a solvent, which afforded several M@C60 EMFs for the spectroscopic studies.167 Eu@C60. In 1999 Inoue et al. reported a Eu LIII-edge EXAFS study of the soot enriched with [email protected] Two nearest Eu−C distances determined from EXAFS data were 2.34 and 2.84 Å with an error less than 0.01 Å. These values indicated that Eu is inside the carbon cage (in the outside position the second Eu− C distance would be near 3.73 Å) and is displaced from the center of C60 by 1.4 Å737 in good agreement with the results of DFT study.738 One year later the same groups isolated Eu@C60 in a pure form and characterized it by UV−vis-NIR and Eu LIIIedge XANES spectroscopies.739 By comparison to the XANES spectra of Eu2O3 and EuS, the authors have shown that the oxidation state of Eu in Eu@C60 is 2+. Dy@C60. In 2001 Kanbara et al. measured the Dy LIIINEXAFS spectrum of Dy@C60 and determined a 3+ state of Dy by comparison to the spectrum of Dy2O3.740 An EXAFS study has shown that Dy is located inside the carbon cage with the nearest Dy−C distances of 2.39 and 2.68 Å. It was not possible to distinguish whether Dy is coordinated to the pentagon or a hexagon, but in both cases the metal atom is displaced from the center of C60 cage by 1.25−1.30 Å.740 Gd@C60. In 2008 Sabirianov et al. measured UPS and ResPES spectra and performed DFT+U calculations of the electronic structure of [email protected] In UPS spectra, a good agreement was found between the experimental and computed density of states. Gd 4f states were found in the range of the binding energy from −10 to −11 eV. In a ResPES study, photoelectron intensity was measured at the binding energy of −6 eV (with respect to Fermi level). Resonant enhancement was observed at the energies corresponding to the Gd 5s, Gd 5p1/2 and Gd 5p3/2 shallow cores pointing to the hybridization between the valence states of Gd and the carbon cage.741 6.6.4. Dimetallofullerenes. M2@C80. Hino et al. reported UPS spectrum of La2@C80 in 1997; however, no distinct features could be distinguished in the spectrum.702 A formal 6050


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main groups of features and multiplet shapes were found to be characteristic for a trivalent state of Ce.708 The ResPES spectra measured across the Ce N4,5-edge showed resonant enhancement of the states near 1 eV (Ce f1 → f0 transitions) when the incident energy was matching a giant resonance at 123 eV. No Ce 5d states were identified in the ResPES spectra indicating the relatively weak hybridization between the Ce 5d and valence states of the carbon cage.708 M2@C82. Hino et al. reported a UPS study of Y2@C82-C3v(8) and Y2C2@C82-C3v(8).745 The spectra were found to be almost identical and were fitted well by the density of states computed for the tetraanion C824−-C3v(8). Nevertheless, based on the difference spectrum obtained by subtracting the UPS spectra of Y2C2@C82 from Y2@C82, the authors supposed that the cage charge of Y2@C82 is 6− and Y adopts a trivalent state.745 DFT calculations showed that the divalent state of Y inY2@C82 is more appropriate.307 The UPS spectrum of Lu2@C82-C2v(9) was also found to resemble the spectra of M2C2@C82-C2v(9) carbide clusterfullerenes746 suggesting a divalent state of Lu in Lu2@C82 just like in [email protected] Similar UPS results were also recently reported for Er2C2@C82 and Er2@C82 isomers with C3v(8) and C2v(9) carbon cages,747 and also for Lu2@C82C2v(9).726 Other M2@C2n. Dy M4,5-edge XAS and valence band 3d → 4f ResPES spectra of Dy2@C88−II reported by Bondino et al. in 2006 showed that Dy adopts the 3+ state in this EMF.748 The M4,5-edge XAS spectrum of Er2@C90 compared to the results of the atomic multiplet calculation have shown that Er is in the trivalent state.719 It is not known yet whether Dy2@C88 and Er2@C90 are real dimetallofullerenes or carbide clusterfullerenes Dy2C2@C86 and Er2C2@C88. Okimoto et al. showed that Er M5 edge XAS spectra of Er@C82, Er2@C82, and Er2C2@C82 with C2v(9) carbon cage are virtually identical and are perfectly fitted Er3+ multiplet (Figure 39), which indicates a trivalent state of Er in [email protected] However, UV−vis spectroscopy shows the 4− charge state of the cage in Er2@C82;305 DFT calculations favor a 2+ state of Er as well.626 This fact emphasizes that the assignment of the valence state of the lanthanides in EMFs based on M4,5 edge (3d → 4f) XAS data should be taken with caution since the difference in the valence states may be caused not only by population of 4f-states. In particular, Er2+ in Er2@ C82 and Er3+ in Er2C2@C82 seem to have the same f11 configuration (and hence identical M4,5 edge XAS pattern), whereas an extra electron in M2@C82 EMFs presumably occupies s and p-orbitals (which then form an intermetallic bond).459 6.6.5. Nitride Clusterfullerenes (NCFs). Sc3N@C78,80. The first XPS study of Sc3N@C80 was measured by Stevenson et al. in the original paper on the synthesis and isolation of this NCF.5 The authors observed a characteristic Sc 2p multiplet and N 1p peak at the energy close to that in scandium nitride.5 A detailed spectroscopic study of Sc3N@C80 by UPS, XPS, and XAS was reported by Alvarez et al. in 2002.431 Peak positions in Sc 2p photoemission spectra of Sc3N@C80 and Sc2C2@C82 were found to be very similar pointing to a formal Sc3+ state. Quantitative determination of the effective valency was done by analyzing the Sc L2,3 (2p → 3d) XAS pattern and comparing it to the multiplets predicted for Sc3+ and Sc2+. The best fit to the experimental spectra was obtained by a superposition of the two multiplets resulting in a state with an effective charge of +2.4.431 A well-structured UPS spectrum of Sc3N@C80 with relatively narrow peaks was consistent with the high symmetry of the C80-Ih(7) carbon cage and its closed-

charge of 3+ was assigned to La in La2@C80 by Kubozono et al. in 2001 based on the K-edge XANES spectrum and a comparison to that of La2O3.742 The same authors performed an EXAFS study of La2@C80 in the temperature range from 40 to 395 K.742 The data were modeled considering the coordination of La atoms to hexagons, which resulted in two nearest La−C distances of ca. 2.40 and 2.95 Å and a La−La distance of ca. 3.89 Å, which were only weakly dependent on temperature.742 VB-PES spectra of La2@C80 were reported by Wang et al. in 2010.693 Ding and Yang have found that the XPS spectrum of Ce2@ C80 in the range of Ce 3d core levels is virtually identical to that of Ce@C82696 and Ce trihalides, which allowed them to assign a 3+ state to Ce in [email protected] XAS spectrum of Ce2@C80 in the Ce 3d → 4f excitation region was reported recently by Ishikawa et al.743 In 2008 Hino et al. remeasured the UPS spectra of Ce2@C80 and La2@C80 and compared them to those of Lu2@C80 and [email protected] Well structured spectra of Ce2@C80 and La2@C80 were found to be virtually identical in line with the same C80-Ih(7) cage structure and a formal cage charge of −6 in these EMFs. On the other hand, the spectrum of Lu2@C80-C2v(5) was distinctly different from the spectra of M2@C80-Ih(7) and exhibited a close resemblance with the spectra of Lu2C2@C80. Thus, in line with the data of the 13C NMR and UV−vis-NIR spectroscopic study, the same C2v(5) isomer and a formal charge of −4 should be assigned to the carbon cage in Lu2C2@C80 and Lu2@C80, which implies that Lu adopts a divalent state in [email protected] La2@C78. In 2007 Hino et al. reported a UPS study of La2@ C78 in comparison to Ti2C2@C78 which also has the D3h(5) carbon cage.629 Although M@C82 EMFs with the same carbon cage and metal charge usually exhibit almost identical UPS spectra,735 for these two molecules a UPS pattern was found to be noticeably different (Figure 41). This phenomenon was explained by DFT calculations, which showed a strong hybridization of the metal and fullerene states.629 Ce2@C72. A detailed study of the electronic structure of Ce2@C72 by means of XAS ResPES spectroscopies was reported by Golden et al.708 In the XAS spectrum measured at the Ce M4,5 (3d → 4f) and N4,5 (mainly 4d → 4f) edges, the

Figure 41. UPS spectra of Sc3N@C78, Ti2C2@C78, and La2@C78 (all with D3h(5) carbon cage) measured at photon energy hν = 40 eV. Reproduced with permission from ref 630. Copyright 2012 American Chemical Society. 6051


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6.6.6. Carbide Clusterfullerenes. Sc2C2@C82. The electronic structure of Sc2C2@C82 was first studied by Takahashi et al. in 1995 by means of XPS and UPS.754 In the XPS spectra, the features of Sc2C2@C82 were found in the region of Sc 2p core level at the intermediate energies between Sc2O3 and metallic Sc. The data were interpreted as an indication of an intermediate, divalent valence state of Sc.754 A divalent state of Sc was also suggested by Suenaga et al. based on the position of Sc L-edge peaks in the EELS spectra.755 In 2000 Pichler et al. assigned a trivalent state of Sc in Sc2C2@C82 by comparing the Sc 2p → 3d XAS spectrum with the calculated ionic multiplet spectra.756 An effective charge state of +2.6 was derived by fitting the experimental XAS spectrum with the multiplet structures predicted for 3d0 → 2p53d1 (Sc3+) and 3d1 → 2p53d2 (Sc2+) absorptions.756 Ti2C2@C2n. In 2001 Cao et al. isolated Ti2C2@C78 (first identified as “Ti2@C80”) and studied the valence state of Ti by L2,3-edge EELS.356 A comparison to the spectra of TiO2, Ti2O3, TiO, and TiC did not afford an unambiguous assignment because of the different peak shapes and positions. The authors suggested that the valence should be less than 2.356 Similar conclusions on the valence state of Ti were achieved by Cao et al. in the EELS study of two isomers of [email protected] The UPS spectra of Ti2C2@C78 and Ti2C2@C82 were studied by Hino et al. in 2004, but the interpretation of the spectra was still based on the dimetallofullerenes conjecture.758,759 In 2007 the spectra of Ti2C2@C78 were reanalyzed taking into account its carbide nature and compared to La2@C78 with the same D3h(5) carbon cage.629 The spectra were found to be significantly different, and DFT calculations have shown that Ti is strongly hybridized with the carbon cage.629 Other M2C2@C82 EMFs. The UPS spectrum of Y2C2@C82C3v(8) was reported by Hino et al. in 2005.745 The authors pointed out a close similarity of the spectra to those of Sc2C2@ C82 (at that time still thought to be “Sc2@C84”) and assigned a 4− charge to the carbon cage and 3+ to Y.745 A very close resemblance of the UPS spectra was also reported by Hino et al.746 for two groups of carbide clusterfullerenes: (i) M2C2@ C82-C2v(9) with M = Sc, Y, Lu and Lu2@C82-C2v(9); (ii) M2C2@C82-C3v(8) with M = Sc, Y, Lu, Er. At the same time, the spectra of monometallofullerenes with the same carbon cages but different charge states (Sc@C82-C2v(9) and Tm@C82C3v(8)) were found to be distinctly different.746 Likewise, UPS spectra of the carbide Lu2C2@C80-C2v(5) and dimetallofullerene Lu2@C80-C2v(5) were found to be almost identical.744 In 2012 Hino et al. reported UPS study of Er2C2@C82 and Er2@ C82 with C2v(9) and C3v(8) carbon cages,747 and in 2013 this group also published a UPS study of Lu@C82, Lu2@C82, and Lu2C2@C82, all with the C2v(9) cage isomer.726

shell hexaanionic electronic configuration. The authors have also studied the influence of potassium doping on the electronic structure and did not find variations in the valence state of Sc up to the K4.9Sc3N@C80 level of doping.431 Recently, Hino et al. reported a UPS and XPS study of Sc3N@C78-D3h(5) and showed a significant difference of the UPS spectra near the Fermi level of Sc3N@C78, La2@C78, and Ti2C2@C78, all with the same carbon cage in the same formal charge (Figure 41).630 Tm3N@C80. The electronic structure of Tm3N@C80-Ih(7) was studied by Liu et al. and Krause et al. by means of XPS, VBPES, and XAS/EELS.399,749 Tm 4d core-level XPS showed a shift of the Tm 4d main line of Tm3N@C80 from the position observed for Tm@C82 and was found to be very close to the Tm3+ 4d line.749 Likewise, a multiplet pattern Tm 4d core-level excitation spectrum of Tm3N@C80 measured by EELS in transmission unambiguously proved a trivalent state of Tm and was distinctly different from Tm@C82 with divalent Tm.749 Doping with potassium up to K/C80 = 6 did not change the valence state of Tm749 similar to Sc3N@C80431 and Dy3N@ C80.750 A quantitative estimation of the effective valence of Tm in Tm3N@C80 was done by means of VB-PES.399 The spectrum measured with the incident photons of Al Kα 1486.6 eV showed a 4f photoemission multiplet which was perfectly reproduced by the 4f12 → 4f11 profile of Tm3+ with only small admixture of 4f13 → 4f12 of Tm2+ (Figure 40b), so that the effective valence of Tm was estimated as +2.9.399 Dy3N@C80. In 2005 Shiozawa et al. reported a detailed study of Dy3N@C80-Ih(7) by high energy spectroscopy.751 Depicted by He I UPS and C 1s XAS, the occupied electronic states of the carbon cage in Dy3N@C80 resemble those of Sc3N@C80431 and Tm3N@C80749 studied earlier. Likewise, information on the unoccupied states of the fullerene was obtained from the shakeup valence-band excitations studied by core-level photoemission spectroscopy with Al K radiation. The effective valence of Dy was studied by Dy 4f and Dy 4d XPS as well as Dy 4d-edge XAS. The multiplet originating mostly from the trivalent states accompanied by pre-edge shoulders assigned to the divalent states were observed in Dy 4d XPS spectra. A detailed analysis of the Dy 4f multiplet structure observed in the valence band (VB) XPS spectra and comparison to the atomic multiplet calculations for 4f9 → 4f8 (Dy3+) and 4f10 → 4f9 (Dy2+) photoemission allowed the estimation of the effective valency of Dy in Dy3N@C80 as 2.8.751 The valence state of Dy in Dy3N@C80 was not affected by potassium doping up to the intercalation ratio of K/C80 = 11.9750. At the same time, the Dy 4d-4f ResPES spectroscopy of Dy3N@C80@ SWNT peapods has shown an effective valence state of Dy to be near 3.0.752 In 2009 Greber et al. reported a scanning tunneling microscopy and X-ray photoelectron diffraction (XPD) study of sub- and monolayer of Dy3N@C80 deposited on the Cu(111) surface.753 C 1s-XPD and N 1s-XPD showed that the carbon cages are well ordered facing the copper surface by the hexagon, while the nitrogen atom is located near the geometrical center of the carbon cage. Resonant XPD measured at the Dy M5 absorption edge showed that the Dy3N cluster is also partially ordered with respect to the copper substrate. Experimental data were modeled by two orientations of the cluster, one inclined and one parallel to the substrate.753 CeSc2N@C80. Wang et al. reported an XPS study of CeSc2N@C80 in 2006.103 The spectrum in the region of Ce 3d core levels showed a characteristic pattern of Ce3+ similar to that in Ce@C82 and Ce2@C80; the Ce4+ feature was absent.103

6.7. Electron Microscopy of Endohedral Fullerenes

Different kinds of electron microscopy (in particular, scanning tunneling microscopy/spectroscopy, STM/STS, and high resolution electron transmission microscopy, HRTEM) were among the methods widely used to characterize EMFs from the very early stages of their research. However, in the 1990s these methods were predominantly applied to characterize the morphology of surface layers of EMFs on different substrates. A detailed overview of such a work is beyond the scope of this review. Instead, we focus on the STM/STS studies of the electronic structure of EMFs on a single-molecule basis and a direct visualization of the location and dynamics of single metal atoms in EMFs by HRTEM. 6052


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6.7.1. STM/STS and Related Techniques. Through a measurement of differential conductance (dI/dV), STS allows determination of the density of states near the Fermi level which corresponds in molecular systems to the frontier MO energies. Moreover, measurements of conductance maps at different bias voltages provide information on individual molecular orbitals and bonding states. In certain cases inelastic electron tunneling spectra (IETS, defined as d2I/dV2) can be obtained and give important information on the electron− phonon coupling in EMFs. Conventional Metallofullerenes. A STM/STS study of single Ce@C60 and La@C60 molecules deposited on the highly oriented pyrolytic graphite (HOPG) surface was reported by Klingeler et al. in 2001.762 A metallic behavior was found for La@C60 at room temperature, while below 28 K a bandgap of ca. 40 meV emerged. For Ce@C60, a semiconducting behavior was found.762 A combined STM and UPS study of La@C82 on Si(111) surface was reported in 2003 by Ton-That et al.763 STS showed that the bands corresponding to HOMO and LUMO both approach Fermi level pointing to metallic or semimetallic nature of the La@C82 film.763 In the same year, Taninaka et al. used STM/STS to study the electronic properties of multilayer La2@C80 and La@C82 islands assembled on hydrogenterminated Si(100)-2 × 1-H surface.764 Intramolecular structures were observed in STM images of the monolayers but were lost in a larger number of layers. STS determined rather large gap of 1.3−1.5 eV for La2@C80, while the gap of La@C82 was only 0.5 eV. The STS pattern was correlated to the density of states computed for the C806− and C823− cages.764 In 2005 the same group reported an analogous study of multilayer La2@C72 islands on the same Si(100)-2 × 1-H surface.765 In agreement with the D2(10611) carbon cage isomer determined by 13C NMR, the molecules of La2@C72 were found to have an ellipsoid shape. Averaged STS spectra taken on top of multilayer islands revealed onsets of occupied and empty states at −1.0 and 0.4 eV, respectively, pointing to a relatively high bandgap of 1.4 eV. However, the measurements at specific positions at the center of La2@C72 molecules showed an additional small feature near the zero sample bias. With the help of DFT calculations, this feature was assigned to LUMO of La2@C72 localized on La atoms, while the major bands below −1.0 eV and above 0.4 eV derived from the carbon cage states.765 In 2004 Fujiki et al. reported a STM/STS study of two Ce@ C82 isomers deposited on Si(111)-7 × 7 surface.766 At a low concentration (ca. 0.02 ML), EMF molecules were found to be absorbed on the surface as single molecules without the formation of clusters. They were strongly bonded to the surface and did not exhibit motions at 295 K but were randomly oriented with respect to the surface. Frontier MO levels of both isomers were determined by STS from the dI/dV curves. HOMO and LUMO energies of Ce@C82(I) were found to be −0.55 and 0.50 eV, respectively, while the Eg gap was estimated to be 0.7 eV from the onsets of the HOMO and LUMO peaks. For Ce@C82(II), the HOMO was also found near −0.5 eV, while the LUMO peak was observed at higher energy (1.0−1.3 eV), resulting in a higher Eg gap of 1.5 eV.766 Detailed STM and IETS study of Ce@C82 on the Cu(111) substrate augmented with DFT calculations was reported in 2013.767 The study revealed a strong dependence of the electronic and vibrational properties of Ce@C 82 molecules on their orientations on the substrate surface.

Ohashi et al. have shown that electron injection from a STM tip induces the polymerization of Ce2@C80 deposited on the Si(111)-7 × 7 surface, while Lu2@C76 was nonreactive under the same conditions.768 Strózė cka et al. used STM to measure the conductance of single Ce2@C80 molecules on a Cu(111) surface and showed that conductance of EMFs in contact regime was five times lower than that of C60 molecules under the same conditions.769 These results were interpreted with the help of DFT calculations which showed that in the energy range of conductance measurements (0−0.5 eV) C60 has delocalized cage-copper MOs, which serve as main conductance channels. On the contrary, MOs of Ce2@C80 in this energy range are mainly localized on Ce and hence show a worse transmission.769 Recently, Strózė cka et al. reported an IETS study of [email protected] Experimentally gathered d2I/dV2 curves showed pronounced features characteristic for inelastic electron tunneling. Especially distinct peaks were found between 6 and 11 meV and near 60−65 meV. On the basis of DFT calculations of the electron-vibration cross sections of Ce2@ C80, the authors suggested that the low-energy features corresponded to the vibrational motions of Ce atoms inside the carbon cage. A strong influence of such motions on the conductance was explained taking into account a preferential localization of MOs near Fermi level on Ce atoms. A corresponding Ce-based resonance peak crossing Fermi level was found in the STS spectra of [email protected] STM/STS measurements of Pr@C82 on Si(111)-(7 × 7) surface were performed by Hosokawa et al. in 2004.606 The stripe patterns observed for individual Pr@C82 molecules in STM images of the isomer I at different bias voltages were similar to those of Dy@C82 and indicated that Pr@C82-I molecules are not rotating on the substrate. HOMO- and LUMO-derived peaks in the STS curve were observed at −0.4 ad 0.7 eV, respectively, and the bandgap was estimated to be 0.7 eV.606 In 2005 Leigh et al. measured STM patterns of single Nd@ C82 molecules on the Ag/Si(111) surface.771 Well-defined MO structures could be resolved in the range of bias voltage from −2 to +2 V. The patterns were assigned to individual MOs by DFT calculations, and two isomers of Nd@C82 could be distinguished by their STM images.771 In 2005 Grobis et al. performed STS studies of inelastic and elastic tunneling of isolated Gd@C82 molecules on Ag (001) substrate (Figure 42).772 To probe the spatial and energetic distribution of the EMF wave functions, the authors measured STS spectra at different positions of a single Gd@C82 molecule and found six distinct molecular states at the bias voltage of

Figure 42. Constant current STM topographs (65 Å × 65 Å) of two Gd@C82 and one C60 molecules measured at bias voltages of 0.1 V (a) and 2 V (b); current is 1 nA in both figures. Reproduced with permission from ref 772. Copyright 2005 The American Physical Society. 6053


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−0.85, 0.10, 0.65, 1.00, 1.32, and to +1.66 V, each exhibiting a unique spatial pattern. IET spectra obtained by numerical differentiation of the dI/dV curves revealed three inelastic tunneling channels at 43, 60, and 70 mV corresponding to the increase in differential conductance of ca. 5, 15, and 2%, respectively. The 60 meV channel was well localized at a particular region of the Gd@C82 molecule and was assigned to specific vibrations of the carbon cage by means of DFT calculations of the electron−phonon coupling in the empty C824− cage.772 In 2010 Jiang et al. reported DFT-calculated IET spectra of Gd@C82 and showed that the predicted IETS pattern was strongly dependent on the position of the metal atom inside the cage; besides, interaction with the Ag substrate also influenced the spectra.773 Recently Zhao et al. performed STM studies of Gd@C82(I) on Cu(111) and Cu(100) surfaces.774 STS spectra showed weak dependence of the electronic structure on the metal support (e.g., LUMO was near 1.0 V for majority of samples), but a significant spatial variation of dI/ dV spectra was found for single molecules, dimers, and ordered layers, pointing to the importance of the intermolecular interactions.774 A series of STM studies of Gd@C82@SWNT peapods was reported by Shinohara and co-workers starting from the first synthesis of such peapods in 2000 (see section 10.3).775−778 By means of extended STM/STS study with atomic resolution, in 2002 Lee et al. revealed a local bandgap modulation of the nanotubes near the position of encapsulated Gd@C 82 molecules.777 For instance, while the empty (11,9) tube showed a bandgap of 0.43 eV, the gap was narrowed in the peapod to 0.17 eV near the sites of Gd@C82. This phenomenon was tentatively explained by elastic strain caused by EMF molecules or the charge transfer between EMFs and nanotubes.777 Narrowing of the bandgap in Gd@C82@SWNT peapods resulted in their ambipolar behavior with both p- and n-type characteristics exhibited by a bundle of peapods in a field-effect transistor.779 Note that C60-filled peapods exhibited only p-type characteristics under the same conditions.779 In 2003 Kimura et al. reported axially and peripherally resolved STS measurements of Gd@C82@SWNT peapods around the Gd sites and found substantial local interactions between Gd@ C82 molecules and the outer carbon nanotubes, although the nature of these interactions could not be explained at that moment.778 In 2011 by combined STM and DFT study, Ohashi et al. showed that the bandgap modulation depends on the orientation of Gd@C82 molecules inside SWNT.780 In particular, DFT studies have shown that inside the (20,2) nanotube Gd@C82 molecules are in the off-center position with the nearest wall-fullerene distances of ca. 3.2 Å, while mechanical deformations of the nanotube by encapsulation of EMF molecules are negligible. Computations have also shown that the encapsulation of Gd@C82 opens the bandgap of the originally metallic (20,2) nanotube and the valence band top is located near the energy levels of Gd@C82, implying a substantial hybridization between the EMF and the nanotube. STS spectra measured at regular intervals along the nanotube axis revealed intensity peaks with the period of ca. 1.1 nm, but peak positions were different depending on the bias voltage. This indicated that regularly positioned Gd@C82 molecules had random orientations. DFT simulations of STS images with different orientations of Gd@C82 molecules inside the nanotube confirmed this suggestion.780 The orientation switching of Tb@C82 molecules on an octanethiol monolayer self-assembled on the Au(111) substrate

was studied in 2005 by Yasutake et al. by means of STM/ STS.781 At 68 K in such a double-barrier tunnel junction architecture, STS exhibited a zero-current region from −0.87 to +0.50 V, assigned to the Coulomb gap. When the bias voltage was swept from −1.8 to +1.8 V and vice versa at 13 K, STS characteristics exhibited hysteresis including a negative differential conductance. This phenomenon was assigned to the switching of the orientation of Tb@C82 molecules caused by the interaction between the molecular dipole moment and the external electric field. Such a hysteresis was not observed at 68 K presumably because of the rotation of the molecules at this temperature.781 Interestingly, STM/STS studies of Tb@C82 on an alkanethiol self-assembled monolayer with hexanethiol, octanethiol, and decathiol showed that the electronic properties of Tb@C82 were strongly dependent on the alkane chain.782 In 2003 Wang et al. reported single-molecule resolved STM studies of a Dy@C82(I) sub-monolayer on an Ag monolayer deposited above Si(111) substrate.783 The interpretation of the experimental results was based on DFT calculations. Measuring the dI/dV maps at different bias voltages the authors could identify hybrid metal-cage states, whose characteristic features were well seen near 2.0 V and were bright rings or spots near the positions of the metal atoms. Occupied cage MOs were detected at negative bias voltage as a standard pattern known already for empty fullerenes and consisting of several curved stripes, while purely metal-localized states could not be detected by STM.783 Orientation of Dy@C82 molecules on the Au(111) surface and its dependence on the coverage was studied by high-resolution STM in 2012.784 In 2010 Iwamoto et al. studied individual Lu@C82 molecules localized on alkanethiol self-assembled monolayers at 65 K.785 STM images exhibited a well-defined stripe pattern of Lu@C82, which were used together with DFT calculations to determine the orientation of Lu@C82 molecules with respect to the surface. STS measurements revealed the HOMO−LUMO gap of 0.47 eV. STS spectra were also taken at the grid of points distributed over and near a single Lu@C82 molecule with the bias voltage of −0.5 and 1.1 V. A significant spatial variation of conductance was detected in both cases. In particular, the enhanced conductance near the Lu position at the negative voltage was assigned to a hybrid metal-cage HOMO-2 localized on Lu and the nearest carbon atoms. The authors have also shown that the application of a high electric field and tunneling current resulted in the switching of orientations of the Lu@C82 EMF molecules.785 STM/STS study of Lu2@C76 was reported by Umemoto et al. in 2010.308 Experimental spectra were reasonably reproduced by DOS of C766−-Td cage; however, a more recent theoretical study has shown that the formal charge of the cage in Lu2@C76 is −4.459 Clusterfullerenes. In 2007 Leigh et al. studied self-assembly of Er3N@C80 and Sc3N@C80 on different surfaces by variable temperature STM.786 Intermolecular resolution with a variety of molecular orbital patterns was observed for Er3N@C80 on Si(001) already at room temperature, while resolution of such patterns on the Ag/Si(111) surface required cooling down to liquid nitrogen temperature.786 In 2010 Huang et al. reported on an STM/STS study of Sc3N@C80 chemisorbed on Cu(110)-(2 × 1)-O surface.787 At positive bias voltage of ca. 3.5−5.0 V, the authors could identify a series of delocalized atom-like “superatom” molecular orbitals similar to those observed earlier for C60.788,789 In the dimers or trimers of Sc3N@C80, these states form hybridized superatom molecular 6054


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6.7.2. HRTEM Studies of EMFs. The advantage of HRTEM in the field of EMFs is the possibility to visualize individual metal (especially lanthanide) atoms as dark spots inside the fullerene because of the high atomic contrast. This way, not only the position of metal atoms with respect to the cage can be seen, but also the dynamics of the endohedral metal atoms and the whole EMF molecules can be studied. Besides, HRTEM can be combined with EELS, and hence the identity and the valence state of single atoms can be determined. A vast majority of TEM studies of EMFs are performed for peapods, i.e., single-wall carbon nanotubes (SWNT) encapsulatiing fullerene molecules. Low contrast makes SWNTs semitransparent in TEM, and hence the details of the encapsulated molecules can be well studied. At the same time, SWNTs serve as stable containers enabling dispersion and immobilization of the studied compounds in a single-molecule form suitable for HRTEM measurements. Conventional Metallofullerenes. In 2004 Gloter et al. studied the dynamics of Ca@C82 molecules in Ca@C82@ SWNT peapods by time-resolved HRTEM and showed that HRTEM allows the detection of individual Ca atoms in the peapods.793 An HRTEM study of La@C82@SWNT peapods by Suenaga et al. in 2003 allowed the detection of La atoms and hence the determination of the orientation of dipoles.794 A statistical analysis had shown that the orientation of the dipoles along the nanotube axis was much more preferable than a perpendicular arrangement. With respect to neighboring EMF molecules in the peapod, a head-to-tail arrangement was found to be much more abundant than any other.794 The HRTEM study of Sc@ C82 and La@C82 peapods by Warner et al. revealed a substantial difference in the EMF dynamics.795 Inside SWNTs, Sc@C82 molecules were rotating freely on the measurement time scale (2 s), and hence individual Sc atoms could not be detected. The rotation was partially frozen inside multiwall carbon nanotubes thus allowing the observation of Sc-derived contrasts. On the contrary, La@C82 exhibited ratchet-like rotational motion with long intervals between the rotation events (up to 50 s), and hence individual La atoms could be well seen.795 In 2011, Warner et al. reported on the HRTEM study of La@C82@SWNT peapods with SWNTs of sufficiently large diameter to accommodate 2D packed La@C82 molecules (Figure 44).796 A prolonged exposure of these peapods to the 80 keV electron beam resulted in the gradual coalescence of La@C82 molecules under formation of the inner tube and nanocrystals of La.796 La2@C80@SWNT peapods with 1.4 nm nanotubes were synthesized for the first time and studied by HRTEM by Smith et al. in 2000.797 Positions of individual La atoms in La2@C80 molecules could be identified. Importantly, the sole fact that the La atoms can be seen in the time scale of the HRTEM measurements (0.8 s in this work) indicates that the rotation of the La2 cluster in La2@C80 is hindered in the peapods as compared to the free rotation reported for La2@C80 in solution270 or in crystalline state.271 In 2004 Khlobystov et al. observed the rotational motion of Ce@C82 molecules inside single-wall carbon nanotubes.798 Ce atoms were clearly seen as dark spots at the edges of C82 cages. The authors concluded that dipole interactions of EMF molecules inside the same nanotube as well as in neighboring nanotubes in a bundle affect relative orientations of Ce@C82 molecules and may be responsible for cooperative translational oscillations.

orbitals with clear bonding and antibonding character (Figure 43). The assignment of these states was confirmed by DFT

Figure 43. STS (dI/dV) spectra measured at different positions of a triangular Sc3N@C80 trimer. Inset: STM topography of a trimer, where red circle, green triangle, and blue square indicate the positions where the dI/dV spectra were recorded. (b−f) dI/dV mapping of the triangular Sc3N@C80 trimer at different energies (bias voltages are indicated in the figures) showing hybrid SAMOs of bonding (b, c) and antibonding (e, f) character. The measurement bias voltages are indicated in the figures. Reproduced with permission from ref787 . Copyright 2010 The American Physical Society.

calculations.787 Recently the same group also reported that under conditions of low-temperature STM, single Sc3N@C80 molecules deposited on the Cu(110)-(2 × 1)-O surface exhibit conductance switching at the low bias voltage (100 mV).790,791 STM imaging accompanied by DFT computations showed that inelastic scattering of tunnelling electrons resulted in the excitation of the Sc−N stretching vibrations, which then induced switch-like rotations of the Sc3N cluster between three pairs of chiral conformations inside the immovable carbon cage.790 STM study by Williams and co-workers showed that the junction based on the Gd3N@C80 sandwiched between two Au contacts exhibits conductance peak at zero bias, which can be an indication of the Kondo effect caused by interaction between localized spin of Gd ions and π-electrons of the carbon cage.104 The same group also reported IETS spectra of [email protected] STM/STS study of Ti2C2@C78 on Cu(111) surface was reported by Fukui et al. in 2008.792 Both at 100 K and at room temperature the authors observed bias-dependent molecular patterns on the surface of Ti2C2@C78 molecules, indicating that molecules are not rotating. These pattern were reproduced by DFT calculations of Ti2C2@C78-D3h(5). Interestingly, the STS study showed that the first layer of Ti2C2@C78 on Cu(111) exhibited metallic behavior; however, the second layer already showed semiconducting properties with the bandgap of ca. 0.6 eV.792 6055


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off-center dark spots in most of the metallofullerene cages, thus showing the EMF molecules nonrotating inside SWNT. Gd@ C82 molecules were regularly distributed inside the nanotube at the distance of 1.4−1.5 nm from each other.800 “Elementselective single atom imaging” of Gd@C82@SWNT was accomplished in the same year by the EELS/HRTEM technique, which enabled an identification of the nature and valence state of individual atoms seen in HRTEM.705 Dynamics of metal atoms in Gd@C82@SWNT and Ca@C82@SWNT under conditions of the irradiation-induced cage breaking was studied in 2006.801 It was shown that Gd tends to leave the cage through the defect, while the Ca ion remains inside the cage. In 2003 Suenaga et al. reported an HRTEM study of Gd2@ C92@SWNT peapods at room temperature and at 100 K.761 Gd atoms were detected in HRTEM images as dark spots, which were somewhat elongated at room temperature. The shape of the Gd contrast was ascribed to the motion of Gd atoms inside the carbon cage. Modeling of HRTEM patterns showed that Gd contrast could hardly be distinguished if the motion amplitude exceeded 4 Å, while room temperature and 100 K data were fitted well by considering amplitudes of ca. 2 Å and 1 Å, respectively.761 The dynamics of Dy atoms in Dy@C82@SWNT peapods was studied by Chuvilin et al. using aberration-corrected (AC) HRTEM with the 80 kV electron beam.802 The breaking of C82 cages and their coalescence leading to the formation of the inner nanotube encapsulating Dy clusters was initiated by motions of Dy atoms inside Dy@C82. The details of this process on a second time scale were observed by HRTEM. The mechanism involving the ionization of endohedral Dy3+ by incident electrons with formation of very reactive Dy4+ responsible for the cage breaking was proposed.802 Clusterfullerenes in Peapods. A HRTEM study of Sc2C2@ C82@SWNT peapods based on the isomer I of Sc2C2@C82 (presumably with Cs(6) cage symmetry) was reported by Suenaga et al. in 2003.755 Individual Sc atoms could be resolved in HRTEM images pointing to the frozen rotation of both EMF molecules as a whole and endohedral clusters inside the cages. This finding agrees well with 45Sc NMR measurements, which showed two nonequivalent Sc atoms for Sc2C2@C82(I).523 Sato et al. reported in 2006 a HRTEM study of Ti2C2@C78@ SWNT peapods and showed that HRTEM images of Ti2C2@ C78 taken with atomic resolution have streaks between the Ti.359 Such streaks were not observed for dimetallofullerene and were assigned to the C2 units. Experimentally gathered images were reproduced well by simulations of the HRTEM pattern performed for DFT-optimized Ti2C2@C78-D3h(5).359 In 2011, Khlobystov and co-workers managed to encapsulate Sc3N@C80 as well as its pyrrolidine adduct bearing an alkyl chain with a dithiolane group into SWNT and obtained images of the Sc3N cluster positions with respect to the carbon cage by AC-HRTEM.803 Irradiation of the derivative by the 80 keV electron beam inside the nanotube resulted first in breaking the attached groups followed by disruption of the fullerene cage and finally in the formation of graphene nanoribbons with sulfur-terminated edges.804 In 2007 Sato et al. studied Er3N@C80@SWNT peapods by AC HRTEM.805 The Er atoms in Er3N@C80-Ih(7) and the rotation of the Er3N cluster inside the cage on the time scale of the measurements were unambiguously visualized, and the orientation of the cluster with respect to the cage could be

Figure 44. (a) HRTEM image of the La@C82@SWNT peapod in its initial state. The positions of the diameter measurements are marked by colored lines. (b−d) HRTEM images during rotation and coalescence of the La@C82 molecules corresponding to total exposure times of (b) 590 s, (c) 830 s, and (d) 2040 s. (e) Variation of the apparent carbon nanotube diameter with total exposure time. The plot colors correspond to measurements at the positions of the lines marked in part a. Reproduced with permission from ref 796. Copyright 2011 American Chemical Society.

In 2010 Nicholls et al. studied peapods based on the EMF which was initially sought to be “PrSc@C80” by means of HRTEM (using low energy beam with aberration correction), high-angle annular dark-field imaging technique of a scanning transmission electron microscope, as well as EELS.295 “PrSc@ C80” was reinterpreted to be Pr2@C72, and the longest Pr−Pr distance in one molecule was determined to be 5.5 Å.295 Aberration-corrected HRTEM (AC-HRTEM) study of the dynamics of the single Pr atoms’ release from Pr2@C72 into the inner space of multiwall nanotube peapods and formation of PrC2 nanocrystals under 80 kV electron beam irradiation was reported by Warner et al. in 2011.799 In 2001 Okazaki et al. reported a combined HRTEM/EELS study of Sm@C82@SWNT peapods.700 In the initial peapod, Sm atoms could be seen in a perfectly ordered row of Sm@C82 molecules inside the nanotube; however, a prolonged irradiation by the electron beam (120 keV) resulted in the formation of the intercage bonds (after ca. 10 min) followed by coalescence of EMF molecules and their conversion into much longer nanocapsules after ca. 20 min of the exposure. An EELS study has shown that this process was accompanied by the change of the Sm valence state from +2 to +3.700 A series of HRTEM studies of Gd@C82@SWNT peapods was reported by Iijima, Shinohara, and co-workers starting in early 2000s.705,800 The peapods were first synthesized and studied by HRTEM in 2000 by Hirahara et al.800 The authors have succeeded in detecting positions of individual Gd atoms as 6056


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when the first isomerically pure samples became available.420,460,553,806,807 Since then, virtually any newly isolated EMF and many of their derivatives were characterized electrochemically, in particular by cyclic voltammetry or pulse voltammetric techniques (differential pulse voltammetry, DPV, and square-wave voltammetry, SWV). The electrochemical properties of EMFs have been the subject of several comprehensive reviews, including recent publications of Echegoyen et al.,26,46 Dunsch et al.,6,20 and Akasaka et al.24 For extended details the reader is referred to these reviews. Table 11 includes an exhaustive list of redox potentials of EMFs and their derivatives available by the submission data of this paper (February 2013). Further in this section we will review the state of the art in the electrochemistry of EMFs, discuss the peculiarities of different classes of EMFs and their derivatives, and emphasize a special situation available in some EMFs  the endohedral redox activity.20 Unless specifically noted, redox potentials discussed in this section are measured in odichlorobenzene (o-DCB) and referred to versus ferrocene Fe(Cp)2+/0 (Fc+/0) couple.

deduced (Figure 45). The distance between Er atoms in the Er3N cluster were determined to be ca. 3.5 ± 0.3 Å.805

7.1. Monometallofullerenes

7.1.1. Pristine Monometallofullerenes. The frontier molecular orbitals (MOs) of monometallofullerenes are essentially carbon cage MOs, and therefore the electrochemical activity of such EMFs is mainly determined by the properties of their carbon cages. Like empty fullerenes, monometallofullerenes usually exhibit several reversible reduction steps; besides, one to two oxidation steps can be also accessed in ambient conditions. The electronic state of the endohedral metal atoms remains barely constant irrespective of the charge of the whole EMF molecule. As a result, redox potentials of EMFs with the same carbon cage and different metal atoms in the same formal charge state are virtually identical within a few tens of mV as was first demonstrated by Suzuki et al. in the studies of a series of M@C82 (M = Y, La, Ce, Gd) (Figure 46).420 However, the small variations of the redox potentials in M@C82 EMFs shows a good correlation to the ionic radii of the metals420,808 and their third ionization potential.808 M@C2n EMFs with trivalent metal atoms result in the paramagnetic state of the carbon cage. An open-shell electronic structure of the carbon cage in such EMFs leads to small electrochemical gaps (the difference between first oxidation and first reduction potentials), mainly because the first oxidation potential (Table 11) is much more negative compared to empty fullerenes. For instance, in archetypical MIII@C82-C2v(9) EMFs, the electrochemical gap does not exceed 0.50 V, with the first oxidation potential (E1/2+/0 = 0.07 V) near that of the Fe(Cp)2+/0 couple.402,420,806 MIII@C82-Cs(6) (M = La,553 Pr402) EMFs are even easier to oxidize (E1/2+/0 = −0.07 V), and their electrochemical gap is only 0.40 V. Reduction potentials of EMFs are at least comparable to or even more positively shifted than those of empty fullerenes in spite of the significant negative charge distributed over the carbon cage in EMFs (for instance, E1/20/−(C82-C2(3)) = −0.69 V can be compared to E 1/2 0/− (La@C 82 -C 2v (9)) = −0.42 V and E1/20/−(La@C82-Cs(6)) = −0.47 V).420 The fact that EMFs remain good electron acceptors was also reported by a gasphase study of their electron affinities.809 A special peculiarity of the MIII@C82 family is the second reduction step either as a two-electron transfer (M = Y, Gd, Tb, Er)420,553,808,810 or as two one-electron reductions with the potential distance of less than 0.2 V (M = La, Ce, Pr, Nd) (Figure 46);402,420,806,808

Figure 45. (a) A TEM image of the Er3N@C80-Ih(7) molecules aligned inside the (18,2) SWNT. (b) Sequential TEM images of the Er3N@C80 molecules 1, 2, and 3 (left, center, and right panels, respectively) marked with triangles in (a); (c) contrast profiles along the dotted lines in the TEM images of the Er3N@C80 molecules 1 (28 s), 2 (71 s), and 3 (2 s), from which the Er−Er distances are estimated to be 0.35 ± 0.03 nm. Reproduced with permission from ref 805 . Copyright 2007 American Chemical Society.

7. ELECTROCHEMISTRY AND SPECTROELECTROCHEMISTRY OF ENDOHEDRAL FULLERENES Fascinating electronic properties of EMFs and especially the phenomena that appear due to the interaction of the carbonbased π-system and entrapped metal ions and clusters raise the question of their behavior in the charge transfer processes. Electrochemistry and spectroelectrochemistry provide a direct access to the energetics and mechanisms of the electron transfer in molecules, and hence electrochemical properties of EMFs were in the focus of the researchers since the mid-1990s, 6057


Chemical Reviews

Review

Table 11. Redox Potentials of All EMFs and Their Derivatives Reported to Datea EMFb C60-Ih(1) C70-D5h(1) C82-C2(3) Li@C60 Sc@C82-C2v(9) Y@C82-I C2v(9) Y@C82-I C2v(9) La@C82-I C2v(9) La@C82-II Cs(6) La@C82-II Cs(6) Ce@C82-I C2v(9) Pr@C82-I C2v(9) Pr@C82-I C2v(9) Pr@C82-II Cs(6) Nd@C82-I C2v(9) Gd@C82-I C2v(9) Gd@C82-I C2v(9) Tb@C82-I C2v(9) Dy@C82-I C2v(9) Ho@C82-I C2v(9) Er@C82-I C2v(9) Lu@C82-I C2v(9) Ca@C76 Ca@C82-II Ca@C82-III C2(5) Ca@C84-II Sm@C74 Sm@C76 I Sm@C80 Sm@C80-C2v(3) Sm@C82-I Sm@C82-II C2v(9) Sm@C82-II C2v(9) Sm@C82-III Sm@C82-C2(5) Sm@C82-IV Sm@C84-I Sm@C84-II Sm@C84-III Sm@C86 Sm@C90-I C2(40) Sm@C90-II C2(42) Sm@C90-III C2(45) Sm@C92-I Sm@C92-II Sm@C94-I Sm@C94-II Sm@C94-III Sm@C96 Eu@C74e Yb@C74-II Yb@C76-I Yb@C76-II Yb@C78 Yb@C80 C2v(3) Yb@C82-I Cs(6) Yb@C82-II C2(5) Yb@C82-III C2v(9) Yb@C84-II Yb@C84-III Yb@C84-IV Yb@C80 C2v(3) Yb@C82-I Cs(6)

ref 420 420 420 144 256 553 420 806 553 257 420 808 402 402 808 420 836 808 808 808 808 808 812 812 812 812 813 813 813 233 813 813 837 813 234 813 813 813 813 813 813 813 813 813 813 813 813 813 813 381 197 197 197 197 197 197 197 197 197 197 197 236 236

ox-II

1.07 1.07 1.07 1.08 1.08 1.08 1.05 1.08

0.85

ox-I

red-I −1.12 −1.09 −0.69 −0.98 −0.35 −0.34 −0.37 −0.42 −0.47 −0.54 −0.41 −0.39 −0.39 −0.48 −0.35 −0.39 −0.25 −0.36 −0.37 −0.36 −0.37 −0.38 −0.61 −0.65 −0.59 −0.64 −0.52 −0.45 −0.59 −0.85 −0.65 −0.27 −0.28 −0.54 −0.84 −0.22 −0.66 −0.39 −0.63 −0.47 −0.62 −0.54 −0.53 −0.61 −0.40 −0.51 −0.56 −0.32 −0.52 −0.79 −0.52 −0.46 −0.68 −0.48 −0.57 −0.33 −0.60 −0.33 −0.63 −0.49 −0.46 −0.89 −0.62

+1.21 +1.19 +0.72 −0.39 0.15 0.10 0.10 0.07 −0.07 −0.07 0.08 0.11 0.07 −0.07 0.14 0.09 0.20 0.11 0.13 0.13 0.13 0.11

0.43

0.42

0.77

0.78

0.23

0.34 0.34 6058

red-II

red-III

red-IV

gapc

−1.50 −1.48 −1.04 −1.44 −1.29 −1.34 −1.34 −1.37 −1.40 −1.47 −1.41 −1.35 −1.35 −1.39 −1.28 −1.38 −1.25 −1.35 −1.07 −1.20 −1.35 −1.17 −0.99 −0.96 −0.74 −0.90 −0.98 −0.85 −0.98 −1.23 −0.99 −0.59 −0.63 −0.76 −1.01 −0.57 −0.87 −0.7 −1.08 −0.78 −0.87 −0.86 −0.86 −0.93 −0.61 −0.84 −0.87 −0.53 −0.85 −1.19 −0.96 −0.83 −1.02 −0.79 −0.95 −0.65 −0.76 −0.67 −0.88 −0.68 −0.72 −1.27 −0.92

−1.95 −1.87 −1.58 −1.83

−2.41 −2.30 −1.94 −2.36

2.33 2.28 1.41 0.62

−2.22 −2.22 −1.53 −2.01

−2.47 −2.47 −2.26 −2.40

−1.53 −1.46 −1.46 −1.99 −1.46 −2.22

−2.25 −2.21 −2.21

0.44 0.47 0.49 0.40 0.47 0.49 0.49 0.46 0.41 0.49 0.48 0.45 0.47 0.50 0.49 0.50 0.49

−1.60 −1.48 −1.54 −1.57 −1.55 −1.30 −1.27 −1.55 −1.43 −1.58 −1.76 −1.59 −1.54 −1.52 −1.27 −1.51 −1.45 −1.12 −1.58 −1.71 −1.31 −1.42 −1.30 −1.37 −1.29 −1.11 −1.33 −1.35 −1.19 −1.29 −1.71 −1.55 −1.46 −1.59 −1.46 −1.55 −1.58 −1.33 −1.56 −1.26 −1.57 −1.34 −1.87 −1.81

−2.03

−1.92 −2.01 −2.14 −2.00 −1.97 −1.90 −1.70 −1.65 −1.96 −1.74 −1.92 −2.07 −1.89 −1.80 −1.88 −1.66 −1.90 −1.81 −1.31 −1.81 −1.71 −1.78 −1.65 −1.75 −1.63 −1.28 −1.69 −1.60 −1.49 −1.50 −2.12 −1.99 −1.89 −2.01 −1.83 −1.90 −1.81 −1.73 −1.90 −1.64 −1.79 −1.54 −2.13 −2.01

1.28

1.29

commentsd DPV

DPV DPV DPV DPV DPV

ACN/toluene

ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene DPV ACN/toluene ACN/toluene ACN/toluene ACN/toluene DPV ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene

1.02

1.23 0.96

ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene ACN/toluene DPV


Chemical Reviews

Review

Table 11. continued EMFb Yb@C82-II C2(5) Yb@C82-III C2v(9) Yb@C84-I Yb@C84-II C2(13) Yb@C84-III C1(12) Yb@C84-IV C2(11) Sc@C82-C2v(9) Sc@C82Ad-2a Sc@C82Ad-2b Sc@C82Ad-2b Sc@C82Ad-2b Y@C82Ad-2a Y@C82Ad-2b Y@C82(Mes2Si)2CH2 I Y@C82(Mes2Si)2CH2 II La@C72(C6H3Cl2)-A La@C72(C6H3Cl2)-B La@C72(C6H3Cl2)-C La@C74(C6H3Cl2)-A La@C74(C6H3Cl2)-B La@C74(C6H3Cl2)-IA La@C74(C6H3Cl2)-IB La@C74(C6H3Cl2)-IC La@C74(C6H3Cl2)-IIA La@C74(C6H3Cl2)-IIB La@C74(C6H3Cl2)-IIC La@C80(C6H3Cl2) La@C82(Mes2Si)2CH2 La@C82Ad La@C82[CH(COOEt)2]2 La@C82[C(COOEt)2] La@C82[CBr(COOEt)2]-A La@C82[CBr(COOEt)2]-B La@C82[CBr(COOEt)2]-C La@C82(C6H5CH2)-a La@C82(C6H5CH2)-b La@C82(C6H5CH2)-c La@C82(C6H5CH2)-d La@C82(CHClC6H3Cl2)-b La@C82(CHClC6H3Cl2)-d La@C82−PCBM-Por-1 La@C82−PCBM-Por-2 La@C82−PCBM-Por-3 La@C82−PCBM-1 La@C82−PCBM-2 La@C82−PCBM-3 La@C82Cp* La@C82(C6H4)2NO2 La@C82-pyrrolidine (N−C8H17-2exTTF) La@(C82-Cs(6))Ad-a La@(C82-Cs(6))Ad-b La@(C82-C3v(7))C6H3Cl2-a La@(C82-C3v(7))C6H3Cl2-b Ce@C82Ad-a Ce@C82Ad-b Gd@C82(C6H4)-I Gd@C82(C6H4)-II Gd@C82Ad Dy@C82-I C2v(9) Dy@C82-methano-(CO2Me) (C− CO2Me-PPh3)

ref

ox-II

ox-I

red-I

red-II

red-III

red-IV

gapc

−0.98 −0.78 −1.06 −1.16 −0.94 −1.14 −1.29 −1.43 −1.39 −1.40

−1.50 −1.60 −1.63 −1.50 −1.76 −1.70

−1.87 −1.90

−1.51 −1.37 −1.36

−1.84 −1.70

−1.37 −1.36 −1.29 −1.36 −1.38 −1.36 −1.38 −1.40 −1.34 −1.39 −1.48 −1.43 −1.71 −1.44 −1.57 −1.19 −1.31 −1.32 −1.33 −1.02 −1.40 −1.42 −1.15 −1.39 −1.07 −1.41 −1.41 −1.36 −1.37 −1.38 −1.38 −1.71 −1.39 −1.32

−1.64 −1.64 −1.70 −1.62 −1.93 −1.92 −2.32

1.24 1.07 0.88 1.41 0.98 1.04 0.50 0.48 0.46 0.47 0.47 0.52 0.48 0.45 0.45 1.44 1.42 1.41 1.35 1.32 1.35 1.32 1.36 1.18 1.14 1.28 1.43 0.43 0.50 0.40 0.36 1.04 1.06 1.08 0.93 1.16 1.01 1.20 1.15 1.23 0.51 0.52 0.47 0.44 0.45 0.44 0.47 0.64 0.34

408 575

0.38 0.61 0.12 0.46 0.22 0.19 0.15 0.09 0.09 0.05 −0.04 −0.02 0.05 −0.10 −0.03 0.44 0.42 0.46 0.30 0.24 0.30 0.24 0.30 0.23 0.24 0.23 0.36 −0.07 −0.01 0.08 0.08 0.38 0.23 0.26 0.25 0.21 0.17 0.15 0.24 0.25 −0.03 −0.04 −0.02 0.02 0.02 0.01 0.02 0.25 −0.04/{0.05}

−0.86 −0.46 −0.76 −0.95 −0.76 −0.85 −0.35 −0.39 −0.37 −0.42 −0.43 −0.54 −0.43 −0.55 −0.42 −1.00 −1.00 −0.95 −1.05 −1.08 −1.05 −1.08 −1.06 −0.95 −0.90 −1.05 −1.07 −0.50 −0.49 −0.32 −0.28 −0.66 −0.83 −0.82 −0.68 −0.95 −0.84 −1.05 −0.91 −0.98 −0.48 −0.48 −0.45 −0.42 −0.43 −0.43 −0.45 −0.39 −0.38

257 257 521 521 255 255 815 815 229 258

−0.12 −0.20 0.65 0.66 0.01 0.02 0.26 0.38 0.06 0.46

−0.57 −0.60 −1.12 −1.11 −0.41 −0.42 −0.97 −0.55 −0.62 0.03

−1.48 −1.49 −1.42 −1.42 −1.36 −1.35

−1.94 −1.92 −1.72 −1.74

−1.48 −0.69

−1.76 −1.27

258

0.28

−0.18

−1.16

−1.57

236 236 236 236 236 236 256 256 256 256 256 254 254 578 578 247 247 247 246 246 380 380 380 380 380 380 248 578 252 409 253 253,410 253 253 413 413 413 413 413 413 411 411 411 411 411 411

0.90 0.53 0.68 0.48

0.10

1.01

0.85

{0.58} {0.57} {0.61} 1.18

6059

−1.86 −1.97 −2.06

−1.92 −2.32

−2.00

−1.89 −1.75 −1.79

−1.47 −1.48 −1.50 −1.21 −1.74 −1.81 −1.34 −1.74 −1.75 −1.71 −1.71 −1.71 −1.74 −2.22

{−2.07} {−2.06} {−2.07}

−1.66

−1.64

0.45 0.40 1.77 1.76 0.40 0.40 1.23 0.93 0.64 0.43 0.36

commentsd DPV

DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV ACN/toluene ACN/toluene DPV ACN/toluene, vs Ag/AgCl ACN/toluene, vs Ag/AgCl


Chemical Reviews

Review

Table 11. continued EMFb Sc2@C82-III C3v(8) Sc2@C82-III C3v(8) Er2@C82-III C3v(8) La2@C72-D2(10611) Ce2@C72-D2(10611) La2@C78-D3h(5) Ce2@C78-D3h(5) La2@C80-Ih(7) Ce2@C80-Ih(7) La2@C80-D5h(6) Ce2@C80-D5h(6) Gd2@C79N-Ih(7) La2@C72Ad-a La2@C72Ad-b La2@C72Ad-c La2@C72Ad-d La2@C72Ad-e La2@C72Ad-f La2@C72(Ad)2 La2@C78Ad (M1) La2@C78Ad (M2) La2@C78Ad (M3) La2@C78Ad (M4) Ce2@C78(Mes2Si)2CH2 La2@C80Ad La2@C80(CClPh) La2@C80(CClPh)Ad-a La2@C80(CClPh)Ad-b La2@C80(CClPh)Ad-c [6,6]-o La2@C80-PCBM [6,6]-o La2@C80-PCBM-ZnP [5,6] La2@C80-pyrrolidine (N-trityl) [5,6] La2@C80-pyrrolidine (N-trityl) [6,6] La2@C80-pyrrolidine (N-trityl) [5,6] La2@C80-pyrrolidine (N-ethyl-2Ph) [5,6] La2@C80-pyrrolidine (N−C8H17-2exTTF) [5,6] La2@C80(C2(CN)4O) La2@C80(Mes2Si)2CH2 La2@C80(Dep2Si)2CH2 La2@C80(Dep2Si(CH2)-CHtBp)-A La2@C80(Dep2Si(CH2)-CHtBp)-B Ce2@C80Ad Ce2@C80(Mes2Si)2CH2 [5,6] Ce2@C80-pyrrolidine (N-trityl) [6,6] Ce2@C80-pyrrolidine (N-trityl) [6,6]-o Ce2@C80-PCBM [6,6]-o Ce2N@C80-PCBM-ZnP Sc3N@C68-D3(6140) Sc3N@C78-D3h(5) Sc3N@C78-D3h(5) Sc3N@C78-D3h(5) Sc3N@C78-D3h(5) Y3N@C78-C2(22010) Dy3N@C78-C2(22010) Gd3N@C78-C2(22010) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7)

ref 169 303 169 290 293 296 298 460 283 285 285 316 290 290 290 290 290 290 291 297 297 297 297 298 276 278 278 278 278 279 279 274 277 277 649 649 280 281 281 275 275 276 283 277 277 387 387 542 820 342 817 817 338 18 105 331 198 497 461 335 333

ox-II

0.75 0.82 0.62 0.79 0.95 0.95 0.78 0.66 0.66 0.64 0.67 0.60 0.48 0.67 0.57 0.63 0.69 0.61 0.63 0.50 0.86 0.93 1.11 0.91 0.89 0.88/{0.65} 0.56 0.63 0.95 0.59 0.58

0.89 0.62 0.99 0.91 0.85/{0.63} 0.85 0.68

0.53 [1.00] 1.09

ox-I

red-I

0.07 0.05 0.19 0.24 0.18 0.26 0.25 0.56 0.57 0.22 0.20 0.51 0.15 0.12 0.15 0.10 0.02 0.12 0.02 0.23 0.16 0.21 0.13 −0.04 0.49 0.52 0.46 0.45 0.48 0.49 0.52/{0.33} 0.16 0.23 0.55 0.20 0.19/{0.00} 0.64 −0.06 −0.03 0.11 0.13 0.47 −0.07 0.22 0.56 0.48 0.55/{0.35} 0.33 0.12 0.21 0.33 0.55 0.25 0.47 0.47 0.62 0.59 0.59 0.62 0.57 0.67 6060

−0.87 −1.10 −0.87 −0.68 −0.81 −0.40 −0.52 −0.31 −0.39 −0.36 −0.40 [−0.96] −0.68 −0.80 −0.76 −0.79 −0.82 −0.82 −0.86 −0.43 −0.46 −0.48 −0.44 −0.81 −0.36 −0.26 −0.48 −0.41 −0.41 −0.36 −0.39 −0.45 −0.45 −0.51 −0.44

red-II

red-III

−1.29

−1.85

−1.26 −1.92 −1.86 −1.84 −1.86 −1.72 −1.71 −1.72 −1.76 [−1.98] −1.99

red-IV

−2.28 −2.23 −2.13

−2.16

−2.00 −2.06 −2.07 −2.05 −2.08 −1.82 −1.83 −1.83 −1.78

gapc

commentsd

0.94 1.15 1.06 0.92 0.99 0.66 0.77 0.87 0.96 0.58 0.60 1.47 0.83 0.92 0.91 0.89 0.84 0.94 0.88 0.66 0.62 0.69 0.57 0.77 0.76 0.78 0.94 0.86 0.89 0.85 0.91 0.61 0.68 1.06 0.64

SWV, pyridine

DPV DPV DPV

SWV, pyridine

DPV

DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV DPV

−1.78 −1.47 −1.66 −1.63 −1.54 −1.76 −1.79 −1.71 −1.71 −1.65 −1.70

−2.33 −1.67 −1.87 −1.81 −1.89 −2.19 −2.15 −2.22 −2.30 −2.19 −2.22

−0.45

−1.73

−2.23

0.64

DPV

−0.21 −0.76 −0.70 −0.50 −0.53 −0.43 −0.73 −0.51 −0.55 −0.42 −0.43 [−1.45] [−1.54] [−1.56] −0.84 −0.66 [−1.62] [−1.54] [−1.53] −1.24 −1.26 [−1.29] −1.22 [−1.27] −1.24

−1.32

−1.64

−1.76 −1.75 −1.74 −1.76 [−2.05]

−2.25 −2.34 −2.23 {−1.91}

0.85 0.82 0.73 0.61 0.66 0.90 0.80 0.73 1.11 0.90 0.98 1.78 1.66 1.77 1.17 1.21 1.87 2.01 2.00 1.86 1.85 1.88 1.84 1.84 1.91

DPV DPV DPV DPV DPV DPV DPV DPV DPV

[−1.91] −1.23 −1.06 [−1.99] [−1.93] [−1.89] −1.62 −1.62 [−1.56] −1.59

−2.37 [−2.32] −1.90

−1.64

−1.79

−1.40 −1.23

−2.28 −2.28

−2.25

−1.73

−2.45

ACN/toluene

5−20 V/s

DPV


Chemical Reviews

Review

Table 11. continued EMFb

ref

Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7)

342 279 818 818

Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Sc3N@C80-Ih(7) Y3N@C80-Ih(7) Y3N@C80-Ih(7) Y3N@C80-Ih(7) Y3N@C80-Ih(7) Pr3N@C80-Ih(7) Nd3N@C80-Ih(7) Gd3N@C80-Ih(7) Tb3N@C80-Ih(7) Dy3N@C80-Ih(7) Ho3N@C80-Ih(7) Er3N@C80-Ih(7) Tm3N@C80-Ih(7) Tm3N@C80-Ih(7) Lu3N@C80-Ih(7) Lu3N@C80-Ih(7) Lu3N@C80-Ih(7) Lu3N@C80-Ih(7) Lu3N@C80-Ih(7) Lu3N@C80-Ih(7) TiSc2N@C80-Ih(7) TiY2N@C80-Ih(7) YSc2N@C80-Ih(7) Y2ScN@C80-Ih(7) ScYErN@C80-Ih(7) LuY2N@C80-Ih Lu2YN@C80-Ih CeLu2N@C80-Ih CeLu2N@C80-Ih Sc3N@C80-D5h(6) Lu3N@C80-D5h(6) Tm3N@C80-D5h(6) Dy3N@C80-D5h(6) Y3N@C80-D5h(6) Gd3N@C82-Cs(39663) Nd3N@C84-Cs(51365) Gd3N@C84-Cs(51365) Pr3N@ C86-D3(17) Nd3N@ C86-D3(17) Gd3N@C86-D3(17) Y3N@C88-D2(35) La3N@C88-D2(35) Ce3N@C88-D2(35) Pr3N@C88-D2(35) Nd3N@C88-D2(35) Gd3N@C88-D2(35) Gd3N@C88-D2(35) Lu3N@C88-D2(35) La3N@C92-T(92) Ce3N@C92-T(92) Pr3N@C92-T(92) La3N@C96-D2(186) Ce3N@C96-D2(186)

817 817 817 817 817 817 497 342 819 333 121 121 107 342 18 342 497 323 399 335 342 819 652 823 328 186,436 187 333 333 332 819 819 329 329 335 335 323 18 342 46 121 107 121 121 46 100 122 121 121 121 107 100 340 122 345 345 122 122

ox-II 1.09

ox-I

red-I

0.57 0.59 0.70 0.69

−1.30 −1.27 −1.09 −1.10

0.77

−1.11 −0.76 −1.26 −0.68 −0.82 −0.98 [−1.41] [−1.39] −1.43 −1.37 [−1.41] [−1.42] [−1.44] [−1.38] [−1.37] [−1.45] [−1.42] [−1.43] [−1.31] [−1.40] [−1.48] −1.44 −1.42 [−1.42] −1.39 −0.94 −1.11 −1.26 −1.36 −1.55 −1.42 −1.42 −1.39 [−1.43] [−1.33] [−1.41] [−1.45] [−1.40]

0.97 0.76

[1.11]

0.43 0.66 0.63 0.54 0.53 0.49 0.45 0.44

0.53 0.67

0.64 0.63 0.64 0.75 0.59 0.63 0.58 0.59 0.70 0.60 0.63 0.65 0.68 0.64 0.60 0.61 0.64 [0.64] 0.61 0.16 0.00 0.67 0.71 0.64 0.66 0.62 0.01 0.01 0.34 0.45 0.39 0.40 0.38 0.38 0.31 0.32 0.31 0.36 0.33 0.03 0.21 0.08 0.09 0.07 0.06 0.05 0.02 0.36 0.32 0.35 0.14 0.18

[−1.53] [−1.44] [−1.37] [−1.48] [−1.46] [−1.39] −1.43 [−1.36] −1.30 −1.34 −1.36 −1.43 −1.39 −1.32 −1.44 −1.48 −1.46 [−1.54] [−1.50] 6061

red-II −1.66 −1.45 −1.46 −1.51 −1.34 −1.62 −1.05 −1.23 −1.43 [−1.83] −1.91 −1.89 [−1.84] [−1.89] [−1.86]

gapc

commentsd

−2.09 −2.10

1.87 1.86 1.79 1.79

−1.85

1.88

−1.82

2.23

−1.68

1.58

SWV DPV ACN/toluene ACN/toluene, DPV ACN/toluene THF o-DCB pyridine CH2Cl2 DMF/toluene CV

red-III

red-IV

−2.27

[−2.13]

[−1.86] [−1.80] [−1.78] [−1.76]

−1.88 [−1.80] −1.83 −1.58 [−1.79] −1.61 −1.87 −1.97 −1.88 −1.85 −1.88 [−1.92]

[−2.26] −2.16 −2.21 −1.80 −2.19

−2.17

[−1.85] −1.87 −2.02 −1.76 −1.80 −1.79 −1.72 −1.70 [−1.67] −1.57 −1.72 −1.75 −1.74 −1.71 −1.65 −1.64

[−1.77] [−1.84]

−2.15

−2.40 −2.54

2.05 2.02 2.07 2.12 2.00 2.05 2.02 1.97 2.07 2.05 2.05 2.08 1.99 2.04 2.08 2.05 2.06 2.08 2.00 1.10 1.11 1.93 2.07 2.09 2.08 2.04 1.40 1.44 1.68 1.86 1.84 1.80

SWV DPV

SWV SWV DPV

DPV DPV SWV SWV SWV

1.91 1.75 1.69 1.79 1.82 1.82 1.46 1.57 1.38 1.43 1.43 1.49 1.44 1.34 1.80 1.80 1.81 1.68 1.68


Chemical Reviews

Review

Table 11. continued EMFb Pr3N@C96-D2(186) Sc3N@C78 Sc3N@C78-methano-C(CO2Et)2 Sc3N@C80 [5,6] Sc3N@C80-pyrrolidine (N-ethyl) [5,6] Sc3N@C80-Diels−Alder [5,6] Sc3N@C80-pyrrolidine (N-methyl2-ferrocenyl) [5,6] Sc3N@C80-pyrrolidine (N-methyl2-(4-diphenylaminophenyl)) [5,6] Sc3N@C80-pyrrolidine (N-benzyl4-diphenylaminophenyl) [5,6] Sc3N@C80-pyrrolidine (N-methyl2-chain-ZnP) 1 [5,6] Sc3N@C80-pyrrolidine (N-methyl2-chain-ZnP) 2 [5,6] Sc3N@C80-pyrrolidine (N-trityl) [6,6] Sc3N@C80-pyrrolidine (N-trityl) [5,6] Sc3N@C80(C6H4) [6,6] Sc3N@C80(C6H4) [6,6]-o Sc3N@C80-methano-fluorene [6,6]-o Sc3N@C80-methano-C(CO2Et)2 [6,6]-o Sc3N@C80-methano(pyrrolidine-N-carbonyl)2 [6,6]-o Sc3N@C80-dimethano-C (CO2Et)2-CH(CO2Et) [6,6]-o Sc3N@C80-methano-(CHPh) Sc3N@C80 [6,6]-o Sc3N@C80-PCBM [6,6]-o Sc3N@C80-PCBM-ZnP Sc3N@C80(Mes2Si)2CH2 Sc3N@C80(CF3)2 Sc3N@C80(CF3)4 Sc3N@C80(CF3)10 Sc3N@C80(CF3)12 [5,6] Y3N@C80- pyrrolidine (N-ethyl) [6,6] Y3N@C80- pyrrolidine (N-ethyl) [6,6] Y3N@C80- pyrrolidine (N-methyl2-ferrocenyl) [6,6]-o Y3N@C80-methano-C(CO2Et)2 [6,6]-o Y3N@C80-methano-C(CO2Et)2 [6,6]-o Y3N@C80-methano-exTTF [6,6]-o Y3N@C80-methanoanthraquinone [5,6] Er3N@C80-pyrrolidine (N-ethyl) [6,6] Er3N@C80- pyrrolidine (N-ethyl) [6,6]-o Er3N@C80-methano-C(CO2Et)2 Gd3N@C80 [6,6]-o Gd3N@C80-methano-C (CO2Et)2 [6,6]-o Gd3N@C80-dimethano-C (CO2Et)2 Lu3N@C80 [6,6]-o Lu3N@C80-PCBM [6,6]-o Lu3N@C80-PCBH [6,6]-o Lu3N@C80-PCBE [6,6]-o Lu3N@C80-PCBE-DPI Lu3N@C80 [6,6]-o Lu3N@C80-methano-C(CO2Et)2 [6,6]-o Lu3N@C80-methano(pyrrolidine-N-carbonyl)2 [6,6]-o Lu3N@C80-dimethano-C (CO2Et)2-CH(CO2Et) [5,6] Lu3N@C80-pyrrolidine (N-trityl) [6,6] Lu3N@C80-pyrrolidine (N-trityl) [6,6]-o Lu3N@C80-methano-CHPh

ref

ox-II

122 820 820 497 497 497 671

0.53

647

[1.06]

647

ox-I

red-I

red-III

red-IV

gapc

[−1.51] [−1.54] [−1.42] [−1.29] −1.18 −1.16 −1.14

[−1.86]

[−1.56] −1.57 −1.54 −1.53

[−2.32] −2.29 −2.26 −2.25

1.65 1.64 1.56 1.87 1.80 1.78 1.75

{0.39}

−1.10

−1.50

−2.23

1.49

[0.63]

{0.32}

−1.21

−1.59

−2.29

1.53

648

[0.44]

{0.25}

−1.22

−1.62

{−2.00}

−2.32

1.66

648

[0.40]

{0.28}

−1.18

−1.56

{−1.98}

−2.27

1.58

−1.54 −1.35 −1.50 −1.29 −[1.96] [−1.90] [−1.93]

−2.38 −2.35 −2.21 −2.23 [−2.41] [−2.22] [−2.41]

1.45 1.50 1.76 1.90 1.89

1.09

0.14 0.12 0.14 0.59 0.62 0.62 0.61/{0.15}

red-II

825 825 392 392 823 823 823

[0.92] 0.83 1.03 1.08 1.05

[0.34] 0.42 0.52 0.56 0.52

−1.14 −1.06 −1.11 −1.08 [−1.24] [−1.34] [−1.37]

823

1.14

0.62

[−1.38]

[−1.95]

[−2.37]

2.00

0.50 0.59 0.52 0.55/{0.37} 0.08 0.43 0.55 0.86 0.95

[−1.48] −1.27 −1.33 −1.34 −1.45 −1.16 −1.06 −0.84 −0.95 −1.30

[−2.01] −1.66 −1.91 −1.92

[−2.40]

−1.65 [−1.55] −1.32 −1.38 −1.65

−2.14 −2.03 −2.11 −1.98 −2.36

1.98 1.76 1.85 1.89 1.53 1.59 1.61 1.70 1.90

[−1.41]

[−1.77]

[−1.44] [−1.28] [−1.34]

[−1.91] [−1.77] [−1.72]

[−2.22]

2.03 2.18 2.01

838 279 279 279 388 544 394 394 394 497 497 596 497 327 596 596

[1.08] 1.09 1.15 1.10/{0.64} 1.66

0.65 0.57/{0.07}

0.90

0.60 0.59 {0.23} 0.67

−2.26

1.98

0.64 0.64 0.60 0.58 0.58

−1.28

−1.63

−2.33

1.92

[−1.44] [−1.39]

[−1.86] [−1.83]

[−2.13] [−2.17]

2.02 1.97

822

0.59

[−1.40]

[−1.88]

1.99

652 652 652 650 650 823 823 823

[1.11] 1.15 1.07

0.64 0.56 0.56 0.55 0.57 [0.64] 0.62 0.59

−1.42 −1.51 −1.50 −1.46 {−0.86/−1.08} [−1.42] [−1.45] [−1.49]

−1.94 −1.41 [−1.80] [−1.88] [−1.92]

−1.87 [−2.26] [−2.22] [−2.23]

2.06 2.07 2.06 2.01 1.98 2.08 2.07 1.98

823

1.13

0.62

[−1.45]

[−1.89]

[−2.21]

2.07

0.59

−1.13 [−1.28] [−1.49]

−1.42 [−2.03] [−1.95]

−2.43 [−2.31] [−2.32]

2.08

1.10

6062

DPV DPV

20 V/s

497 497 497 107 822

825 825 824

commentsd

SWV SWV SWV DPV DPV


Chemical Reviews

Review

Table 11. continued EMFb Lu3N@C80-Ih(7) [5,6]-o Lu3N@C80(Dep2Si) [6,6] Lu3N@C80(Dep2Si) Lu3N@C80(Mes2Si)2CH2 Lu3N@C80(Dep2Si)2CH2 Gd3N@C84 Gd3N@C84-[C(CO2Et)2] Sc2C2@C80-C2v(5) Sc2C2@C80-C2v(5) Sc2C2@(C80-C2v(5))-Ad a Sc2C2@(C80-C2v(5))-Ad b Sc2C2@(C80-C2v(5))-Ad c Sc2C2@(C80-C2v(5))-Ad d Sc2C2@(C80-C2v(5))-Ad e Sc2C2@C82-C3v(8) Sc2C2@C82-C3v(8) Sc2C2@C82-C3v(8) Sc2C2@(C82-C3v(8))Ad Sc2C2@C82-Cs(6) Sc2C2@(C82-Cs(6))-pyrrolidine (Ntrityl) Sc2C2@C82-C2v(9) Sc2C2@(C82- C2v(9))-pyrrolidine (Ntrityl)-2a Sc2C2@(C82- C2v(9))-pyrrolidine (Ntrityl)-2b Sc2C2@(C82- C2v(9))-pyrrolidine (Ntrityl)-2c Sc3C2@C80-Ih(7) Sc3C2@C80-Ih(7) Sc3C2@C80-Ih(7) Sc3C2@C80Ad Lu3C2@C88-D2(35) Sc2O@C82-Cs(6) Sc4O2@C80-Ih(7) Sc2S@C70-Cs(7892) Sc2S@C72-Cs(10528) Sc2S@C82-C3v(8) Sc2S@C82-C3v(8) Sc2S@C82-Cs(6) Sc3NC@C80-Ih(7)

ref

ox-II

ox-I

red-I

red-II

red-III

−1.39 −1.43 −1.52 −1.55 −1.61 [−1.37] [−1.43] −1.26 −0.74 −0.96 −0.87 −0.89 −0.86 −0.75 −0.95 −0.97 −0.94 −0.79 −0.93 −1.25

−1.83 −1.72 −1.73 −2.01 −2.15 [−1.76] [−1.77] −1.88 −1.33 −1.69 −1.56 −1.48 −1.53 −1.46 −1.38

−2.16 −1.94 −1.99

red-IV

gapc

commentsd

0.93 0.72 0.64 0.65

0.61 0.27 0.43 0.06 0.08 0.32 0.28 −0.12 0.41 0.14 0.28 0.27 0.28 0.11 0.16 0.53 0.47 0.38 0.42 0.33

355 355

0.67 0.70

0.25 0.27

−0.74 −1.13

−0.96 −1.56

0.99 1.40

DPV

355

0.73

0.39

−0.84

−1.19

1.23

DPV

355

0.51

0.12

−1.07

−1.62

1.19

DPV

−0.04 −0.06 −0.03 −0.11 0.31 0.35 0.00 0.14 0.64 0.47 [0.52] 0.39 0.60

−0.32 −0.50 −0.50 −0.55 −1.34 −0.96 −1.10 [−1.44] −1.14 [−1.03] [−1.04] −0.98 −1.05

−1.42 −1.64 −1.64 −1.74 −1.70 −1.28 −1.73 [−1.87] −1.53 −1.16 −1.19 −1.12 −1.68

0.36 0.44 0.47 0.44 1.65 1.31 1.10 1.58 1.78 1.50 1.56 1.37 1.65

SWV, pyridine DPV

328 328 328 839 839 822 822 169 362 386 386 386 386 386 169 829 353 353 354 354

830 816 816 319 340 372 371,831 373 194 11 372 372 10

0.10 0.70

0.72 0.79 [0.65] [1.21] [0.96] [0.65]

−1.15 −1.12 −1.30 −1.60

−2.58 [−2.38] −1.71

−1.60 −1.63

−2.04

−2.20

−1.67 −1.82 −1.84 −2.15 −1.74 −2.35 [−1.99] −2.24 −1.61 −1.63 −1.73

[−2.45]

2.00 1.70 1.95 1.61 1.69 1.69 1.71 1.14 1.15 1.10 1.15 1.16 1.14 0.86 1.11 1.50 1.41 1.17 1.35 1.58

DPV DPV DPV DPV DPV

SWV, pyridine DPV

SWV, pyridine

DPV

SWV

a Unless otherwise noted, the solvent is o-dichlorobenzene, the values are obtained by cyclic voltammetry and referred to versus the Fe(Cp)2+/0 redox pair; for irreversible processes, peak potentials are listed in square brackets, the values determined by pulse methods (differential pulse voltammetry, DPV, or square wave voltammetry, SWV) are listed in italics (and the method is indicated in the last column), redox potentials of nonfullerene addends (e.g., ZnP) are given in curly brackets; for derivatives, redox potentials of the parent fullerenes are listed one more time if they were determined in the same work. bThe families of EMFs are listed in the following order: (1) monometallofullerenes, (2) derivatives of monometallofullerenes, (3) dimetallofullerenes, (4) derivatives of dimetallofullerenes, (5) nitride clusterfullerenes, (6) derivatives of nitride clusterfullerenes, (7) carbide clusterfullerenes and derivatives, (8) other clusterfullerenes. Within each group, compounds are listed in the order of the increase of the cage size, and then according to the number of the metal atom in the periodic table; however, for MII@C2n families (Ca, Sm, Yb) − potentials of the whole family are listed consequently; when several isomers of the same EMF are possible, the isomer is indicated by the group symmetry of the carbon cage and by its roman number (empirical numeration based on retention times of the isomers); unless otherwise noted, derivatives correspond to the main isomer (e.g., C82-C2v(9) for MIII@C82, C80-Ih(7) for M2@C80 and M3N@C80); if several isomers of the derivative were characterized, labeling of the structures corresponds to system used in original publications (i.e., A, B, C, ... or M1, M2, M3, ... etc). For derivatives of C80-based EMFs, “[6,6]-o” and “[5,6]-o” denotes [6,6]-open and [5,6]-open cycloadducts, respectively. cThe electrochemical gap is defined as E(ox-I) − E(red-I). When the first oxidation of the derivative corresponds to the addend, then the first oxidation potential assigned to the fullerene core is used. dACN/toluene − mixture of acetonitrile and toluene (usually 1:4), THF − tetrahydrofuran. ePotentials are estimated based on the Ag/AgCl potentials listed in the original paper and can be ofset by ca. 0.05 V.

whereas typical potential difference of successive single-electron reductions in fullerenes is ca. 0.35−0.40 V. An in situ ESR spectroelectrochemical study of the reduction steps of Y@C82 did not reveal any intermediate paramagnetic state that would correspond to the dianion Y@C822−.810 Interestingly, both for reduction and oxidation the open-shell structure of MIII@C82

EMFs results in the large potential difference (ca. 1 V) of the first and the second step, and this is consistent with the fact that the first and the second steps affect different molecular orbitals (for the closed-shell compounds, i.e., empty fullerenes, both steps proceed with participation of the same MO). 6063


Chemical Reviews

Review

derivative strongly affects the redox potentials: oxidation potentials of three −CBr(COOC2H5)2,253 four benzyl,413 and two −CHClC6H3Cl2413 monoadducts of La@C82-C2v(9) are shifted positively by 0.16−0.31, 0.08−0.18, and 0.17−0.18 V, respectively, while reduction potentials are shifted in the negative directions from the values of the bare La@C82 by 0.24−0.41, 0.26−0.63, and 0.49−0.56 V, respectively. Thus, the electrochemical gap in the monoadducts is usually increased to more than 1 V.253,410,413 Noteworthy, first reduction steps of −CBr(COOC2H5)2 and −CHClC6H3Cl2 monoadducts are irreversible and accompanied by detachment of the addends. Some La@C2n (2n = 72, 74, 80, etc.) EMFs have not been isolated in their bare form and are characterized only as their −C6H3Cl2 monoadducts.246,247,380,521 The analysis of the influence of the addends on the redox properties of parent EMFs is thus not possible. Redox properties of these singlebonded adducts are similar to those of La@C82 exhibiting large electrochemical gaps in most cases exceeding 1.2 V.380 A different situation occurs with groups bonded to the fullerene cage by two single bonds (silyl578 and Bingel409 bisadducts, pentamethylcyclopendadiene,414 carbene,229,252,254,255 and benzyne815 adducts). In such derivatives, open-shell electronic structure of the carbon cage is retained, and the shift of the redox potentials should be ascribed to the reduction of the fullerene π-system and to the electron-withdrawing or donating nature of the addends. In particular, bis-silyl and carbene adducts have an electron-donating nature, which results in negative shifts of both reduction and oxidation potentials of MIII@C82 by ca. 0.1−0.2 V.229,252,254−256,258,578 Small negative shifts (less than 0.05 V) of the redox potentials were also found in pentamethylcyclopentadiene as well as three PCBM-like monoadducts of La@C82−C2v(9).411,414 The electrochemical gap of such derivatives is not strongly affected by a derivatization and remains in the range of 0.5 V. Positive shifts of the first reduction potential of La@C82-C2v(9) was observed for its Bingel bis-adduct, while the first oxidation potential retained its value.409 Unusual redox properties were observed by Lu et al. for two isomers of benzyne adducts Gd@C82(C6H4).815 The first reduction potentials of the derivatives were negatively shifted by 0.72 and 0.30 V, while oxidation potentials were shifted to more positive values by 0.06 and 0.18 V, resulting thus in electrochemical gaps of 1.23 and 0.93 V.815 This behavior is not typical for cycloadducts of MIII@C82 and can point to the formation of single-bonded derivatives. Specific derivative of La@C82-C2v(9) with two benzynederived moieties and one NO2 group was recently characterized by Akasaka et al.408 Although this compound has a closed-shell electronic structure, its electrochemical gap is only 0.64 V, and its first reduction potential is by 0.03 V more positive than that of La@C82, while the oxidation potential is cathodically shifted by 0.18 V.408

Figure 46. Cyclic voltammograms of (a) La@C82, (b) Y@C82, (c) Ce@C82, and (d) Gd@C82 measured in o-DCB at a scan rate of 20 mV/s. Reproduced with permission from ref 420. Copyright 1996 Elsevier Science Ltd.

Electrochemical studies of MIII-monometallofullerenes are limited so far to MIII@C82 EMFs with C2v(9) and Cs(6) cages. The only study of MIII-EMFs with other carbon cages, La@C86 and two isomers of La@C90, was published in 1999 by Yamamoto.811 First reduction/oxidation potentials (−0.42/− 0.03 V, −0.49/−0.03 V, −0.45/−0.06 V for La@C86 and the two isomers of La@C90, respectively) as well as small electrochemical gaps of all three La-EMFs were found to be close to those of [email protected] M II@C2n EMFs with divalent metals (MII = Ca,812 Sm,233,234,813 Eu,292,381 Tm,237,292 Yb197,236) have a carbon cage in the closed-shell electronic state, and the oxidation proceeds at significantly higher potentials than for MIII@C2n. Unfortunately, exact oxidation potentials for the majority of MII@C2n EMFs were not reported yet. Reduction steps are in the similar potential range or somewhat more negative than those in MIII@C2n and strongly depend on the isomeric structure of the carbon cages. As was shown in the studies of the extended families Ca@C2n (2n = 76, 82, 84),812 Sm@C2n (2n = 82−96),813 and Yb@C2n (2n = 74−84)197,236 (Table 11), the difference of the first reduction potentials of isomeric monometallofullerenes can exceed 0.4 V. In sharp contrast to the values discussed above for MIII@C82 EMFs, the difference of the first and the second reduction steps of MII@C2n EMFs is close to 0.3−0.4 V in most cases,197,236,237,381,812,813 and in this respect these EMFs are rather similar to empty fullerenes. 7.1.2. Derivatives of Monometallofullerenes. Chemical derivatization of EMFs can significantly modify their redox potentials by changing the π-system of the fullerene. In the discussion of the derivatives of EMFs with the paramagnetic state of the carbon cage, it is necessary to distinguish two principally different situations: one possibility is the functionalization of MIII@C2n EMFs by the groups forming a single bond to the carbon cage (i.e., −CF3,814 −CBr(COOC2H5)2,410 −CH2C6H5,413 −CHClC6H3Cl2,413 −C6H3Cl2246,247,380,521). In this case, the number of added groups in the derivatives is usually odd, and the carbon cage of the derivative becomes diamagnetic. The transformation of the open-shell structure of the bare EMF molecule to the closed-shell structure of the

7.2. Dimetallofullerenes

7.2.1. Pristine Dimetallofullerenes. The family of dimetallofullerenes with an unambiguously determined molecular structure is rather limited, and detailed electrochemical studies were performed only for M2@C72,78,80 (M = Ce and La) and their derivatives.283,285,290,296,298,460 Anderson et al. also reported electrochemical studies of Sc2@C82 and Er2@C82 isomers,169 but the measurements were performed in pyridine which was later proved to reduce at least some of the EMFs;816 besides, it was recently proved that Sc2@C82−I is in fact a 6064


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carbide clusterfullerene [email protected] It is still not known if other EMFs studied by Anderson et al. were genuine dimetallofullerenes, but Er2@C82−III and Sc2@C82−III with similar electrochemical properties seem to be genuine dimetallofullerenes with a C3v(8) carbon cage. Recently, Akasaka et al. proved the structure of Sc2@C82-C3v(8) and determined its redox potentials and electrochemical gap, which was found to be 1.15 V.303 The first report on the electrochemical study of La2@C80Ih(7) was published in 1995 by Suzuki et al.460 The authors have found that this EMF exhibits two reversible reduction and two reversible oxidation steps in o-dichlorobenzene (o-DCB) at room temperature. The values of the first reduction and oxidation potentials, −0.31 and +0.56 V vs Fe(Cp)2+/0 couple, showed that La2@C80 is a good acceptor and a rather good donor of electrons (albeit not as good as MIII@C2n EMFs) with a comparably low electrochemical gap. Ab initio calculations revealed that the LUMO of the molecule forms a localized domain between two La atoms and its low energy is responsible for the high reduction potential of the La2@C80. Such a form of the LUMO also results in the large gap between the first and the second reduction step, 1.40 V, because two electrons localized in the restricted spatial domain experience a strong Coulomb repulsion.460 A similar shape of the LUMO is also found in all other La2@ C2n EMFs as well as isostructural Ce2@C2n analogues resulting in a good electron accepting ability of all these dimetallofullerenes. However, in spite of the localization of the LUMO on the dimetallic cluster, a particular cage structure still has noticeable influence on the reduction potentials: La2@C72-D2(10611) is harder to reduce than La2@C80-Ih(7) by 0.37 V,290 while La2@ C78-D3h(5) has an intermediate reduction potential of −0.40 V.296 Oxidation potentials of La2@C72 and La2@C78 are also shifted negatively in comparison to that of La2@C80, so that electrochemical gaps of all La2@C2n EMFs remain relatively low (less than 1 V). A particularly small electrochemical gap was found for La2@C80-D5h(6), whose reduction potential (−0.36 V) is close to that of the Ih(7) isomer, while the oxidation potential is found at +0.22 V.285 Thus, the electrochemical gap of La2@C80-D5h(6), 0.58 V, is only slightly larger than that of the paramagnetic La@C82. The replacement of La by Ce results in the negative shift of the first reduction potentials by ca. 0.1 V, while oxidation potentials of Ce2@C2n are virtually the same as in La analogues.283,285,293,298 Gd2@C79N. Closing the section devoted to electrochemical properties of dimetallofullerenes, it is worth mentioning the study of the dimetallo-heterofullerene Gd2@C79N-Ih(7) reported recently by Dorn and co-workers.316 The compound has an interesting electronic structure with the spin density localized between two Gd atoms and a corresponding SOMO orbital “buried” below the cage-based MOs. As a result, the oxidation potential of Gd2@C79N is +0.51 V, close to the value of La2@C80 (+0.56 V) and other C80-Ih(7) based EMFs (see next section), while its reduction potential (−0.96 V) is much more negative than that of M2@C80-Ih(7) EMFs.316 7.2.2. Derivatives of Dimetallofullerenes. Derivatization of dimetallofullerenes was extensively studied by Akasaka and co-workers, and redox potentials are now available for their adamentylidene,276,290,291 pyrrolidine,277 bis-silyl275,281,298 and some other adducts.275,278,387 In all these derivatives redox potentials are shifted negatively with respect to the nonderivatized EMFs, but the magnitude of the shifts strongly depends on the nature of the addends and, to a lesser extent, on

their position. An addition of carbene induced relatively small variations of the redox potentials: on average, the shift in the monoadducts were in the range of −0.1 V.276,278,290,297 For six isomers of La2@C72Ad290 and four isomers of La2@C78Ad,297 the variation of potentials in dependence on the addends’ position was in the narrow range of 0.1 V, showing that the influence of the addition site on redox properties is rather weak. The addition of the second adamentilydene group shifts the redox potentials to more negative values by ca. 0.1 V as was shown for the bis-adduct of [email protected] Likewise, three isomers of the bis-carbene adduct of La2@C80-Ih(7) with Ad and CClPh groups exhibits negative shifts of reduction and oxidation potentials by 0.10−0.17 and 0.08−0.11 V, respectively.278 For pyrrolidine adducts (only derivatives of La2@C80Ih(7) and Ce2@C80-Ih(7) were studied so far), the shifts of the reduction are ca. −0.08 V for [5,6] isomers and −0.15 V for [6,6] isomers.277 The difference between [5,6] and [6,6] isomers is more pronounced in the anodic range: oxidation potentials of [5,6] adducts are more negative than the oxidation potentials of M2@C80-Ih(7) and [6,6] adducts by ca. 0.3 V.277 The strongest negative shifts of the redox potentials induced by derivatization were found for silyl adducts: the first reduction potentials of M2@C2n(Mes2Si)2CH2 (Mes = mesityl) are shifted by −0.29 V for Ce2@C78,298 −0.45 V for La2@C80Ih(7),281 and −0.34 V for Ce2@C80-Ih(7);283 similar large shifts are found for oxidation potentials. Likewise, for two diastereomers of carbosilylated adduct La2@C80(Dep2Si(CH2)CHtBp) the first reduction step was shifted cathodically by −0.19 and −0.22 V (here Dep and tBp is 2,6-diethylphenyl and 4-tert-butylphenyl, respectively),275 while in the bis-silylated adduct La2@C80(Dep2Si)2CH2 the first reduction step is 0.39 V more negative than that of [email protected] Importantly, while reductions of silylated adducts are usually reversible, oxidation is irreversible and results in retro-cycloaddition. As outlined in the previous paragraph, the derivatization of dimetallofullerenes leads in most cases to the negative shift of redox potentials with respect to the parent EMFs, mainly due to the electron-donating nature of the added groups. However, recently Akasaka and co-workers isolated the [5,6] cycloadduct of La2@C80-Ih(7) with tetracyanoethylene oxide, which exhibited a reversible reduction and irreversible oxidation steps shifted anodically by 0.10 and 0.08 V, respectively.280 7.3. Nitride Clusterfullerenes (NCFs)

7.3.1. M3N@C80. Electrochemical studies of nitride clusterfullerenes, the most abundant class of EMFs, were started in 2004, when Krause and Dunsch isolated isomerically pure Sc3N@C80-Ih(7) for the first time and reported its cyclic voltammetry.331 The oxidation potential of Sc3N@C80, +0.56 V, is close to the values found for M2@C80-Ih(7) and Gd2@ C79N, while the first reduction potential at −1.29 V is not far from that of C60 (−1.15 V). Thus, the electrochemical gap of Sc3N@C80-Ih(7), 1.85 V, is significantly larger that the gaps of mono- and dimetallofullerenes and is approaching the values of empty fullerenes. Importantly, in contrast to mono- and dimetallofullerenes, reductions of Sc3N@C80 are electrochemically irreversible at moderate voltammetric scan rates.331 In 2005 Echegoyen et al. studied the mixture of Ih(7) and D5h(6) isomers of Sc3N@C80 and found the oxidation potential of the minor D5h(6) isomer to be 0.27 V more negative than that of the major Ih(7) cage structure198 (note that a similar difference of oxidation potentials was found later for D5h(6) and Ih(7) isomers of M2@C80285). This difference in the redox 6065


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properties was used to remove the D5h(6) isomer by a special chosen oxidation agent, resulting in the isomerically pure Ih(7) isomer. Cyclic voltammetry studies of Sc3N@C80-Ih(7) at different scan rates showed that the electrochemical reversibility of the first reduction step can be achieved at the scan rate of 6 V/s, the second reduction step requires 10 V/s to be reversible, while the third reduction step is reversible at 20 V/s (Figure 47).198 The bulk electrolysis of the solution with subsequent

orbitals of lanthanides. As a result, the carbon cage contribution to the LUMO is dominating for non-Sc M3N@C80, and DFT computations show that the net spin population of the Y3N cluster in Y3N@C80− is less than 16%.344,428 In redox properties, this situation results in the cathodic shift of the first reduction potential of non-Sc M3N@C80-Ih(7) NCFs to ca. −1.4 V (vs ca. −1.25 V for Sc3N@C80). In this respect, a significantly negative reduction potential of ScYErN@C80, −1.55 V, reported by Chen et al.,332 is quite surprising and requires further studies of the Sc-based mixed-metal NCFs. The same group has also succeeded in the isolation of the whole series of YxSc3−xN@C80-Ih(7) and reported their DPV, which showed that the reduction potential of Y2ScN@C80 (−1.36 V) is only 0.01 V more positive than reduction potential of Y3N@ C80 (−1.36 V), while YSc2N@C80 is easier to reduce by 0.1 V (−1.26 V) similar to Sc3N@C80 (−1.24 V).333 Importantly, reductions of non-Sc M3N@C80 such as Dy3N@C80 are also electrochemically irreversible and their reversibility was not reached even at scan rates of 70 V/s.18 The first oxidation of non-Sc M3N@C80-Ih(7) NCFs (E1/2 = +0.65−0.70 V), is somewhat more positive than that of Sc3N@C80 (ca. +0.6 V) and is usually reversible. Computational studies have shown that the HOMO in M3N@C80 is localized on the carbon cage, and hence the electronic state of the nitride cluster is not affected by oxidation.344 The only exclusion is CeLu2N@C80, whose oxidation potential was shifted to a cathodic range by 0.6 V.329 This large value points to a new situation in the charge state and will be discussed in section 7.6 below. While the negative shift of reduction potentials of M3N@C80 NCFs in comparison to Sc3N@C80 is a solid fact, small variations of reduction potentials within the series of non-Sc M3N@C80 NCFs is more ambiguous. It was proposed that the changes of the reduction potentials of isostructural EMFs should correlate with the electronegativity (χ) of the encaged metals.26 Unfortunately, uncertainties in the reported potentials are too large for a detailed discussion of this factor. Redox potentials of Sc3N@C80 reported by different groups (e.g., measured with different equipment with somewhat different conditions) vary in the range from −1.22 to −1.29 V, and hence the analysis of the differences in redox potentials smaller than 0.05 V can hardly be meaningful unless the data are measured under the same conditions in the same group and on the same equipment. Besides, since reduction of M3N@C80 is irreversible, the reported values are usually peak potentials rather than thermodynamically justified E1/2 values. To limit the analysis to the values measured in one group, here we list electronegativity of metals and reduction potentials of M3N@ C80-Ih(7) NCFs reported by Echegoyen and co-workers: Er (χ = 1.24/ Epc = −1.42 V),497 Y (1.22/−1.41 V),497 Gd (1.20/− 1.44 V),107 Nd (1.14/−1.42 V),121 Pr (1.13/−1.41 V).121 These data show that reduction potentials of non-Sc M3N@C80 NCFs do not correlate with the electronegativity of metals. Ti-based mixed-metal NCFs, TiSc 2 N@C 80 -I h (7) and TiY2N@C80-Ih(7), exhibit specific electrochemical properties which are drastically different from those of all other M3N@C80 NCFs. First, the first reduction is reversible at moderate scan rate (for TiSc2N@C80, three reversible reductions are observed even at a small scan rate of 20 mV/s, see Figure 48).436 Second, redox potentials are also substantially different. The first reduction steps are shifted to −0.94 V for TiSc2N@C80 and −1.11 V for TiY2N@C80 (i.e., Ti-based NCFs are easier to reduce).187,436 On the contrary, their oxidation steps are shifted cathodically to 0.16 V for TiSc2N@C80 and 0.00 V for TiY2N@

Figure 47. Cyclic voltammograms of Sc3N@C80-Ih(7) at scan rates of (a) 100 mV/s, and (b) 6 V/s (1), 10 V/s (2), and 20 V/s (3). The different scan rates for each reduction reflect the scan rate necessary to achieve full electrochemical reversibility. Reproduced with permission from ref 198. Copyright 2005 Amercian Chemical Society.

reoxidation of the anion to the neutral state yielded the initial material proving that reduction of Sc3N@C80, although electrochemically irreversible, is chemically reversible.198 In 2007 Zhang et al. reported that the reduction of Sc3N@C80 in dichloromethane and acetonitrile/toluene mixtures is reversible, but the measurements were performed at low concentrations of the sample so that the peaks heights could hardly be estimated.817 Soon after that Echegoyen and coworkers reinvestigated the electrochemical behavior of Sc3N@ C80 in acetonitrile/toluene and showed that reduction steps are irreversible in the same way as they are in o-dichlorobenzene.818 Extended work in the field of synthesis, isolation, and characterization of M3N@C2n NCFs with different metals resulted in an accumulation of the large amount of data on their redox properties.6,26 Redox potentials are now available for M3N@C80-Ih(7) NCFs with a broad range of non-Sc metals, including Y,497 Nd,121 Pr,121 Gd,107 Tb,342 Dy,18 Ho,342 Er,497 Tm,323,399 Lu,335,342 as well as with mixed-metal clusters such as CeLu 2 N, 3 2 9 Y x Lu 3 − x N, 8 1 9 Y x Sc 3 − x N, 3 3 3 ScYErN, 3 3 2 TiSc2N,186,436 TiY2N.187 ESR spectroscopic studies of the Sc3N@C80 anion as well as computational studies indicated that reduction of Sc3N@C80 proceeds via the occupation of the cluster-based LUMO.198,344,428,461,543 This behavior appears to be the unique feature of Sc and is not found for NCFs with Y and lanthanides. The reason is the higher electronegativity of Sc, whose 3d orbitals have lower energies than the 4d orbitals of Y and the 5d 6066


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at 0.21 V342 (0.12 V820) as well as the gap of 1.77 V342 (1.66 V820) is more similar to Sc3N@C68 and Sc3N@C80. For non-Sc NCFs, redox properties were reported by several groups for a series of compounds within the range of cage sizes from C78 to C96 (availability of the different cage sizes for a given metal is determined by its size), including M3N@C78C2(22010) (M = Y,338 Dy,18 Gd105), M3N@C82-Cs(39663) (M = Gd46), M3N@C84-Cs(51365) (M = Nd,121 Gd107), M3N@ C86-D3(17) (M = Pr,121 Nd,121 Gd46), M3N@C88-D2(35) (M = Y,100 La,122 Ce,121 Pr,121 Nd,121 Gd,107 Lu340), M3N@C92T(92) (M = La,122 Ce,345 Pr345), and M3N@C96-D2(186) (M = La,122 Ce,122 Pr122). Especially extensive studies by Echegoyen and co-workers, who isolated and characterized electrochemically the families of NCFs with La, Ce, Pr, Nd, and Gd (see Figure 49 for CVs of Gd3N@C2n family),46,105,107,121,122,345

Figure 48. Cyclic voltammogram of TiSc2N@C80 measured in o-DCB at 20 mV/s. Dotted vertical bars denote reversible redox potentials of Sc3N@C80 from refs 331 and198. Reproduced with permission from ref 436. Copyright 2010 American Chemical Society.

C80. As a result, electrochemical gaps of Ti-based NCFs are around 1.1 V, which is almost twice smaller than that for all other M3N@C80 NCFs. The reasons for this special behavior are discussed in section 7.6 below. Electrochemical studies of the second, less abundant isomer of M3N@C80 with D5h(6) carbon cage symmetry are not so extended at this moment, and the first reduction and oxidation potentials are reported so far for M = Sc,335 Dy,18 Tm,323 and Lu;335 the first oxidation potential is also reported for Y3N@ C80-D5h(6).342 In close similarity to the redox properties of M2@C80 isomers discussed above, the oxidation potential of D5h(6) isomers of M3N@C80 NCFs are shifted from their Ih(7)counterparts to the cathodic range by 0.25−0.30 V. This difference in oxidation potentials of two isomers was also well reproduced by DFT computations coupled to COSMO solvation model.344 The first reduction potential of Sc3N@ C80-D5h(6) is 0.07 V more negative than that of the Ih(7) isomer,335 while for other metals the difference between the Ih and D5h isomers is only 0.01−0.03 V. 7.3.2. NCFs with Other Carbon Cages. Although M3N@ C80 NCFs are the most abundantly produced structures (at least for Sc, Y, and Gd−Lu), formation of NCFs with other carbon cages is also observed, and nowadays many of such structures are isolated and characterized by electrochemical methods. A detailed electrochemical and in situ ESR/Vis-NIR spectroelectrochemical study of Sc3N@C68 was reported by Dunsch and coworkes.541,542 The compound exhibited two oxidation steps at 0.33 and 0.85 V, which were reversible even at very low scan rates of 3 mV/s.542 On the contrary, two reduction steps with peak potentials of −1.45 and −2.05 V were electrochemically irreversible, and the reversibility was not significantly improved up to a scan rate of 75 V/s.541 The electrochemical gap of Sc3N@C68, 1.78 V, is ca. 0.1 V smaller than that of Sc3N@C80-Ih. Electrochemical properties of Sc3N@C78-D3h(5) were studied by Zhang et al.,817 Cai et al.,820 and Chaur et al.342 According to Zhang et al.,817 the electrochemical gap of Sc3N@ C78 is only 1.2 V, and both reduction and oxidation of the compound are reversible in o-DCB and toluene/CH3CN mixture. However, the studies of other groups revealed that the redox behavior of Sc3N@C78 in o-DCB with irreversible reduction at −1.56 V342 (−1.54 V820), and reversible oxidation

Figure 49. Cyclic voltammograms of the Gd3N@C2n family measured in o-DCB at 100 mV/s. Reproduced from ref 46. Copyright 2008 Elsevier Science Ltd.

gave redox properties which are to some extent similar to the properties of M3N@C80-Ih(7). The first (and sometimes also the second) oxidation step is usually reversible, while reduction steps are electrochemically irreversible. Variation of redox potentials of different NCFs with the same carbon cage hardly exceeds 0.1 V and is usually smaller. With the variation of the carbon cages, reduction potentials also do not show significant changes, while oxidation potentials seem to shift cathodically with the increase of the cage size, so that the electrochemical gap tends to be smaller for larger cages, from 2.0 V for M3N@ C78-C2(22010)18,105 to ca. 1.7 V for [email protected] The redox properties of M3N@C88-D2(35) NCFs fall apart from the others in that this is the only group of NCFs which exhibits electrochemically reversible reductions (except for La3N@C88, whose reduction is irreversible122). The oxidation potential of M3N@C88 NCFs are also significantly negatively shifted (to ca. 0.02−0.09 V for Ce−Lu100,107,121,340 and 0.21 V for La122) in comparison to the values for other NCFs of similar size (0.31−0.36 V for M3N@C86,46,121 0.32−0.36 V for M3N@C92122,345). Hence, the electrochemical gap of M3N@ C88-D2(35) NCFs (1.4−1.5 V) is the smallest among all NCFs. In a recent computational study, Xu et al. have shown that the most stable isomer of La3N@C88 has a Cs(81734) carbon cage rather than a D2(35) proved for NCFs with smaller metal 6067


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Figure 50. Cyclic voltammograms of (a) Sc3N@C80-Ih(7), (b) N-tritylopyrrolidino-[5,6]-Sc3N@C80, (c) N-tritylopyrrolidino-[6,6]-Sc3N@C80, (d) Lu3N@C80-Ih(7), (e) N-tritylopyrrolidino-[5,6]-Lu3N@C80, (f) N-tritylopyrrolidino-[6,6]-Lu3N@C80. All measurements are done in o-DCB at a scan rate of 100 mV/s. Reproduced with permission from ref 825. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

atoms such as Lu and Gd.821 If the isolated La3N@C88 has different isomers structure, it might explain why its electrochemical properties are different from those of all other M3N@ C88 NCFs. Electrochemical properties of NCFs can be well rationalized by the results of DFT calculations, which show that both the HOMO and LUMO in these molecules (as well as the spin densities in their charged states) are localized on the carbon cages.344,428 Good correlations between the measured electrochemical gaps of NCFs and the DFT-computed differences between the energies of LUMO(+2) and LUMO(+3) of the corresponding empty fullerenes were used by Poblet and coworkers to confirm the formal 6-fold electron transfer from the cluster to the carbon cage in NCFs.345 The same authors used electrochemical the gap of La3N@C92 in comparison to the results of DFT calculations to confirm the C92-T(92) cage isomer and sort out other alternatives.345 7.3.3. Derivatives of NCFs. Dedicated studies of the chemical properties of NCFs (mainly M3N@C80-Ih(7)) were started in 2002 and resulted in dozens of derivatives, of which many have been studied electrochemically (Table 11). There are two aspects in which derivatization can affect electrochemical properties of NCFs: (i) the reversibility of redox processes can be changed in the derivatives, and (ii) the redox potentials can be shifted depending on the nature of the addends and their addition pattern. The most extended information is available so far on the electrochemical properties of the monocycloadducts of M3N@ C80-Ih(7). The highly symmetric icosahedral carbon cage has only two types of C−C bonds usually designated as [5,6] (pentagon/hexagon edge) and [6,6] (hexagon/hexagon edge), and hence only two isomers of the monocycloadducts are possible for M3N@C80 if the sigma carcass is not “open”. As discussed in detail in section 8, the prevalence of the [5,6] or [6,6] isomer of the cycloadduct depends on the metal size. The first electrochemical study of M3N@C80 cycloadducts (M = Sc, Y, Er) was reported in 2006 by Echegoyen and co-workers.497 The authors succeeded in the isolation of both the [5,6] and [6,6] isomers of N-ethyl-pyrrolidino-M3N@C80 (M = Y, Er)

and showed that they have distinctly different redox properties: [6,6] isomers exhibited irreversible reductions similar to nonfunctionalized NCFs, whereas three reversible reduction steps were found for [5,6] isomers.497 Likewise, [5,6] isomers of the Diels−Alder and pyrrolidine monoadducts of Sc3N@C80 showed also reversible reductions at a scan rate of 100 mV/s ([6,6] cycloadducts of Sc3N@C80 were not isolated). The oxidation of all pyrrolidine-M3N@C80 adducts was irreversible and accompanied by retro-reaction. The irreversible reduction behavior was also found for [6,6]-open methanofullerene derivatives of Y3N@C80 and Er3N@C80 obtained by a BingelHirsch reaction.497 On the basis of these results, the authors concluded that the reversibility of electrochemical reductions can be used to distinguish [5,6] and [6,6] isomers of M3N@C80 adducts.497 This conclusion was then corroborated by many further electrochemical studies of M 3 N@C 80 cycloadducts.327,596,647,648,652,671,822−824 However, the first electrochemical studies of [6,6] closed adducts of Sc3N@C80 reported recently by Echegoyen and co-workers showed that the rule is not valid for pyrrolidine and benzene monoadducts of Sc3N@ C80 since both [5,6] and [6,6] isomers exhibit reversible reductions (Figure 50).392,825 At the same time, [6,6]-open methanoadducts of Sc3N@C80 obtained in a Bingel-Hirsch reaction show irreversible reductions similar to [6,6]-open adducts of non-Sc NCFs. 279,823 In the anodic range, pyrrolydino-M3N@C80 adducts show an irreversible oxidation at ca. 0.3−0.4 V, which was assigned to the addend and resulted in a partial retro-cycloaddition.497 On the contrary, the first oxidation step of open [6,6] methanoadducts is usually reversible.327,497,596,823 Bis-silylation as well as addition of two CF3 groups lead to “1,4” adducts (i.e., addends are attached to carbon atoms of one cage hexagon in para position).388,826 While Sc3N@C80(CF3)2 exhibited three completely reversible reductions (similar behavior was also found for Sc3N@C80(CF3)x derivatives with x up to 12394) and two reversible oxidation steps,544 the reduction of Sc3N@C80(Mes2Si)2CH2 was only partially reversible and was accompanied by a retro-cycloaddition, whereas its oxidation was completely irreversible.388 6068


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pattern on reduction potential can be equally or even more important than the number of the added CF3 groups.827,828 The information available so far on addition patterns of Sc3N@ C80(CF3)x derivatives and their electrochemical properties is not sufficient for a detailed analysis so far, but it might be expected to be of similarly high importance as for empty fullerenes. 7.3.4. Irreversibility of the Reduction of NCFs. Extended electrochemical studies of NCFs outlined above reveal that their reductions are usually electrochemically irreversible. This feature is rather surprising and is not observed for empty as well as mono- and dimetallofullerenes. Dedicated studies have shown that the reduction of NCFs and their derivatives, albeit electrochemically irreversible, is chemically reversible (i.e., initial compounds can be obtained back in the pristine form after complete electrochemical cycle). That is, the reaction with the solvent or the decomposition of NCFs upon reduction can be excluded. In this respect, the reasons of the irreversible reduction behavior have to be looked for in the reversible internal rearrangements of the NCFs upon the electron transfer. On the basis of this assumption, Zalibera et al. proposed a double-square scheme which included equilibrium between pristine and structurally modified NCFs as the followup process (Figure 51a).18 Tarabek et al. fitted kinetic

Electrochemical studies of the NCF derivatives with carbon cages other than C80-Ih(7) are reported so far only for monomethano (malonate) adducts of Sc3N@C78820 and [email protected] Both structures showed irreversible reductions and a reversible oxidation, similar to their parent NCFs. The influence of the addends on the redox potentials of NCFs is rather complex and is not a simple function of the electron-donating or electron-withdrawing nature of the addends. For instance, pyrrolidine adducts of C60 usually have their first reduction potentials negatively shifted by ca. 0.1 V, whereas in M3N@C80 pyrrolidino-adducts the shift is usually positive and markedly dependent on the addition site. Namely, the reduction potential of pyrrolidine-[5,6]-Sc3N@C80 is usually 0.10−0.15 V more positive than in Sc3N@C80Ih(7).497,647,648,671 Even a more positive shift, +0.23 V, is reported for [6,6] Sc3N@C80-pyrrolidine (N-trityl).825 Likewise, positive shifts of +0.18 and +0.21 V, respectively, are reported for the [5,6] and [6,6] isomers of Sc3 N@ C80(C6H4),392 and a shift of +0.13 is found for the [5,6] Diels−Alder adduct of [email protected] Thus, despite the reduction of the fullerene π-system by exohedral addition, a positive shift is observed in all closed-cage monocycloadducts of Sc3N@C80 (to compare E1/2 potentials of reversibly reducible derivatives, we used E1/2 values of Sc3N@C80 determined by Elliott et al.198 at high scan rate). Limited information available so far for closed-cage cycloadducts of other M3N@C80 (M = Y, Er, Lu) shows that the first reduction of their [5,6] isomers is also positively shifted; however, the reduction potentials of [6,6] isomers are similar to those of M3N@C80 (i.e., the effect of [5,6] and [6,6] addition sites is reversed compared to Sc3N@C80).497,596,825 In [6,6]-open methano-adducts, the fullerene π-system is preserved and the redox potentials of NCFs are less affected by functionalization. Both reduction and oxidation potentials of methano-Sc3N@C80 and methano-Lu3N@C80 adducts are slightly negatively shifted (by ca. −0.05 V),279,652,823 whereas for Y3N@C80 and Gd3N@C80 the shift is positive but also small.596,822 Likewise, redox potentials of the Bingel adducts of Sc3N@C78820 and Gd3N@C84822 are virtually identical to those of parent fullerenes. In the bis-silylated Sc3N@C80(Mes2Si)2CH2 derivative the shift of the reduction potential is strongly negative, −0.23 V.388 At the same time, the addition of two CF3 groups to the same position as in Sc3N@C80(Mes2Si)2CH2 shifts the reduction potential to a positive direction by +0.10 V.544 Interestingly, the oxidation potential of Sc3N@C80(CF3)2 is shifted negatively by −0.16 V, so that the electrochemical gap is thus reduced to 1.59 V versus ca. 1.85 V in [email protected],826 Further additions of CF3 groups to Sc3N@C80 increase its electron-accepting ability, shifting the first reduction of Sc3N@C80(CF3)x anodically by +0.20 V (x = 4), +0.42 V (x = 10), and +0.31 V (x = 12).394 These data show that the polyaddition of CF3 groups can be used to tune electron accepting properties of Sc3N@C80, and the most positive first reduction potential of NCFs and their derivatives is achieved so far for Sc3N@C80(CF3)10. Oxidation potentials of Sc3N@C80(CF3)x (x > 2) are also shifted positively so that the electrochemical gaps tend to increase and reach 1.90 V for x = 12. Redox data reported for Sc3N@ C80(CF3)x derivatives corresponds to one isomer of a given composition, whereas several isomers are usually formed in the synthesis for each number of addends x.394 It should be noted that electrochemical studies of multiple isomers of C60(CF3)x and C70(CF3)x revealed that the influence of the addition

Figure 51. (a) Double-square reaction scheme of the electrochemical redox reactions of Dy3N@C80; (0), (1), and (2) corresponds to the neutral form, monoanion, and dianion, respectively. The stable species generated by the structural rearrangement are marked by asterisks. The arc arrows show the main reaction pathways (ECEC). Reproduced from ref 18. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Experimental (upper curve) and simulated (lower curve) cyclic voltammograms of Lu 3 N@C 80 measured in o-DCB at 50 mV/s. Adapted from ref 819. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

parameters and could reproduce experimentally measured cyclic voltammograms by the results of simulation (Figure 51b).819 However, the nature of the structural modification remained unclear. Hypothesis of the structural changes of the endohedral cluster (in particular, its pyramidalization18) was not supported by computational studies of the charged states.344,428,541 Besides, a different electrochemical behavior of the [5,6] and [6,6] isomers of the cycloadducts was also left without explanation. 6069


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reduction and oxidation potentials in a negative direction by 0.33 and 0.09 V, respectively.354 Recently Akasaka et al. showed that the addition of the adamantylidene to Sc2C2@C80-C2v(5) shifts both the reduction and oxidation potentials cathodically, but the magnitude of the shift was noticeably different for the five isomers of the monoadduct.386 Electrochemical studies of three isomers of the Sc2C2@C82-C2v(9)-pyrrolidine monoadducts revealed a significant influence of the addition site on the redox potentials.355 The first reduction potentials of all three derivatives are more negative than that of the pristine clusterfullerene by 0.10−0.39 V, whereas the shifts of the oxidation potentials vary from −0.13 V to +0.14 V; electrochemical gaps of all the cycloadducts are higher by 0.20−0.41 V. Two paramagnetic carbide clusterfullerenes with M3C2 clusters exhibit noticeably different redox properties. Sc3C2@ C80-Ih(7) has a small electrochemical gap (0.47 V) and is similar in its electrochemical properties to MIII@C82 monometallofullerenes. Namely, it appears to be both a good electron donor (E1/2+/0 = −0.03 V) and a good electron acceptor (E1/20/− = −0.50 V) with the gap between the first and second reduction steps exceeding 1 V, and all redox processes are electrochemically reversible.816 Similar properties were also reported for its [6,6]-open carbene adduct.319 On the other hand, electrochemical properties of Lu3C2@C88 are more reminiscent of EMFs with a closed-shell structure in that its electrochemical gap is as high as 1.65 V.340 This value is even higher than that of Lu3N@C88 with the same D2(35) carbon cage. While reduction potentials of Lu3C2@C88 and Lu3N@C88 are very similar within 0.05 V, the oxidation potential of Lu3C2@C88 is more positive that that of the NCF by almost 0.3 V. This surprising finding was attributed to the localization of the single-occupied molecular orbital on the carbide unit.340

In 2008, ESR spectroelectrochemical studies of Sc3N@C68 provided additional information on the possible nature of the follow-up reaction accompanying the reduction of NCFs.541 Although the radical-anion and radical-cation could be detected by ESR upon reduction and oxidation, respectively, the integral intensity of the radical-anion was an order of magnitude lower in the same experimental conditions. This fact indicated that the main product of the reduction was diamagnetic, and a reversible formation of the single-bonded [Sc3N@C68]22− dimer was proposed by the authors.541 For empty fullerenes, such dimers are not formed in solution but are well characterized in the solid phase. Recently, an extended computational study has shown that dimerization of NCF radical-anions is substantially more energetically favored than that of empty fullerenes.19 Besides, that study also revealed that electronically preferable dimerization sites of Sc 3 N@ C80(CF3)2− as well as radical-anions of pyrrolidine adducts of M3N@C80 (M = Sc, Y) are close to the addends, and hence their dimerization is often sterically hindered. Finally, singlebonded anionic dimers of [6,6]-pyrrolidino-Y3N@C80 were found to be much more stable than those of [6,6]-pyrrolidinoSc3N@C80. All these facts have clear parallels with the experimentally observed irreversible/reversible reductions of NCFs and their derivatives, and hence the equilibrium between monomeric and dimeric forms of anion-radicals shifted toward dimers appears to be a plausible explanation of the electrochemically irreversible reduction behavior of NCFs.19 The dimerization hypothesis can also explain why TiSc2N@C80436 and TiY2N@C80187 exhibit reversible reductions. Namely, both Ti-based NCFs are radicals in the neutral state, and their reduction results in the diamagnetic anions, which are not prone to dimerization. 7.4. Carbide Clusterfullerenes

7.5. Sulfide, Oxide, and Cyano Clusterfullerenes

The first data on the redox properties of carbide clusterfullerenes were reported by Akasaka et al.829 and Anderson et al.169 when these EMFs still were not distinguished from dimetallofullerenes (i.e., Sc2C2@C80-C2v(5) was believed to be Sc2@C82−I, Sc2C2@C82 isomers was thought to be “Sc2@C84”). Likewise, Sc3C2@C80 was still considered to be “Sc3@ C82”.816,830 The first reduction of Sc2C2@C82-C3v(8) at −0.94 V is irreversible and followed by two fully reversible reduction steps at more negative potentials, while its first oxidation at 0.47 V is reversible.353 Very similar redox potentials were recently reported for Sc2C2@C82-Cs(6).354 A somewhat smaller electrochemical gap, 1.15 V, was reported for Sc2C2@C80-C2v(5).362 Its oxidation potential is close to the values of Sc2C2@C82 isomers, while the reduction step is shifted to a positive direction by ca. 0.2 V.362 The recently characteized Sc2C2@C82C2v(9) exhibits two reversible reduction and two reversible oxidation steps,355 and its electrochemical gap, 0.99 V, is the smallest among electrochemically characterized Sc2C2@C2n carbide clusterfullerenes. Thus, all dimetallo-carbide clusterfullerenes characterized so far have a relatively high electrochemical gaps of ∼1 V or more. Redox properties of only few chemical derivatives of Sc2C2@ C82 clusterfullerenes are reported so far, and hence a systematic analysis of the influence of derivatization is not yet possible. Addition of the carbene group to Sc2C2@C82-C3v(8) shifted its reduction potential anodically by 0.15 V, while the oxidation potential of the derivative was less positive by 0.09 V.353 On the other hand, pyrrolidine addition to Sc2C2@C82-Cs(6) shifted its

Electrochemical data on other clusterfullerenes are rather scarce. Redox potentials of the sulfide clusterfullerene Sc2S@ C82-C3v(8)11 were found to be almost identical to those of Sc2C2@C82-C3v(8), and the first reduction of the sulfide structure was likewise irreversible followed by two reversible steps.11 More recently, redox potentials were also reported for Sc2S@C82-Cs(6) and the isostructural Sc2O@C82-Cs(6) (Figure 52).372 The first oxidation and reduction potentials of these clusterfullerenes are similar to those of another clusterfullerene with C82-Cs(6) carbon cage in a −4 charge state, Sc2C2@C82Cs(6). The influence of the endohedral cluster was more pronounced for the second reduction: for the sulfide structure, the second reduction step was very close to the first one (−0.98 and −1.12 V, respectively), while for Sc2O and Sc2C2 clusters the gap between the first and the second reduction steps exceeded 0.3 V.11,372 Electrochemcial study of Sc2S@C72Cs(10528) revealed that this sulfide clusterfullerene exhibits reversible redox behavior both in the anodic and cathodic ranges.194 Its electrochemical gap, 1.78 V, is noticeably larger than the values reported for both Sc2S@C82 isomers. For Sc2S@C70-C2(7892), only the first oxidation step is electrochemically reversible, whereas four reductions are electrohemcially irreversible.373 Sc2S@C70 also has the least positive oxidation and well as the most negative reduction potentials in the whole sulfide clusterfullerene family. Oxide clusterfullerene Sc4O2@C80-Ih(7) exhibits two reversible oxidation and two reversible reduction steps at low voltammetric scan rates (20 mV/s).371,831 This electrochemical 6070


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Experimentally, endohedral redox process can reveal itself in an unusual electrochemical behavior (shift of the redox potentials from the analogues molecules, increased reversibility) and in specific parameters of the ESR spectra of cation- or anion-radicals, such as large metal-based coupling constants and a significant shift of the g-factor from the free electron value. The possibility of endohedral redox processes can be also predicted theoretically by means of frontier orbital analysis. However, a more reliable prediction can be gained from the studies of the EMF molecules in different charge states since frontier orbitals of the neutral molecule are not always those which are active in the electron transfer. To exhibit an endohedral redox activity, the EMF molecule should have a rather complex electronic structure of the encapsulated species, and it has never been observed for monometallofullerenes, whose frontier molecular orbitals are localized on the carbon cage. However, the endohedral redox activity is possible already for dimetallofullerenes, which have either the LUMO (La2@C2n) or HOMO (Y2@C82) localized on the metal atoms.291,296,307,424,428,460 Reduction of La2@C80Ih(7) reported in 1995 was the first example of the endohedral redox process (originally proposed based on the computational study),460 and in cavea localization of the spin in the anionradical La2@C80− was later confirmed experimentally by ESR spectroscopy (isotropic 139La coupling constant is 396 G and a g-factor is 1.984).530,832 Likewise, endohedral reduction is expected for other dilanthanofullerenes (La2@C72, La2@C78, La2@C100) and their Ce analogues.285,291,296,298,424,428 In the family of NCFs, unambiguous experimental proof of the endohedral reduction was reported for Sc3N@C80. ESR studies of the anion-radical Sc3N@C80− show an enhanced 45Sc hyperfine coupling constant (ca. 56 G) and a significant shift of the g-factor (1.998−1.999).198,543,544 These findings also agree with numerous computational studies proving the localization of the LUMO (and spin-density in the anion-radical) on the Sc3N cluster.344,383,428,461 Trifluoromethylation of Sc3N@C80 results in a gradual shift of the reduction from the Sc3N cluster to the carbon cage as revealed by a decrease of the 45Sc coupling constants in anion-radicals with the increase of the number of CF3.394,544 An interesting situation was observed for Sc3N@C80(CF3)2 at the third electron transfer: the trianionradical exhibited large 45Sc coupling constants of one Sc atom (49.2 G), which agreed with DFT-predicted presumable localization of the spin density on one of the three Sc atoms.544 Hence, the third reduction of Sc3N@C80(CF3)2 can be described as in cavea reduction of one particular Sc ion. The unusual electronic properties can be also bestowed on metal atoms by placing them in a mixed-metal cluster. DFT PBE0 computations suggested the CeIII valence state in CeLu2N@C80 with one localized 4f-electron (its presence was also proved by a characteristic paramagnetic shift of the 13C NMR spectra),329 which is a standard state of Ce in EMFs. The corresponding MO (essentially 4f1 AO) is 0.37 eV below the level of HOMO. However, the electrochemical study showed that the oxidation potential of CeLu2N@C80 was 0.6 V less positive than the oxidation potentials of all other M3N@C80 molecules. Computational study revealed that the removal of the 4f1 electron from Ce under formation of the CeIVLu2N@ C80 state is more energetically favorable than the oxidation of the carbon cage (which occurs in all other M3 N@C 80 NCFs).329 Thus, CeLu2N@C80 is so far the only known EMF in which an endohedral CeIII exhibits a redox activity and can be reversibly oxidized to a CeIV state.329

Figure 52. Cyclic voltammograms of Sc2S@C82-C3v(8), Sc2S@C82Cs(6), and Sc2O@C82-Cs(6) measured in o-DCB at a scan rate of 100 mV/s. Reproduced with permission from ref 372. Copyright 2011 American Chemical Society.

behavior is rather unusual for clusterfullerenes which normally tend to have electrochemically irreversible reductions. The electrochemical gap of Sc4O2@C80, 1.10 V, is much smaller than that of EMFs with C80-Ih(7) carbon cage and fullerene based reduction and oxidations. Analysis of the redox potentials as well as spectroelectrochemical studies (see section 7.6) revealed that this behvaiour is due to the localizaion of the frontier orbitals on the oxide cluster.371 Both the reduction and oxidation of the cyano clusterfullerene Sc3NC@C80-Ih(7) are reversible.10 The oxidation potential at 0.60 V is close to the values reported for Sc3N@C80, whereas the reduction of the cyano clusterfullerene (E1/2 = −1.05 V) is shifted cathodically by 0.20 V and the electrochemical gap is thus 0.2 V smaller than in Sc3N@C80. These data can be rationalized by the analysis of the frontier molecular orbitals of Sc3NC@C80. While its HOMO is localized on the carbon cage (like in Sc3N@C80, and hence oxidation potentials are similar), the LUMO is localized on the endohedral cluster, which explains the difference in the reduction potentials of Sc3NC@ C80 and [email protected] 7.6. Endohedral Electrochemistry

Encapsulation of metal atoms and cluster inside the fullerene cage can result in unique electrochemical properties possible only for EMFs. Although the carbon cage prevents direct contact of endohedral species with the electrode and the environment in general, it can be formally “transparent” for electrons so that the endohedral species can be electrochemically active.20 In other words, redox processes can occur inside the carbon cage (hence in cavea) changing the valence state of the endohedral species and leaving the formal charge of the carbon cage intact.20 Thus, not only carbon cages can stabilize unusual structures inside the fullerene, they can also stabilize their different charge and spin states! 6071


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A similar situation was found in Ti-based NCFs, TiSc2N@ C80 and [email protected],436 DFT computations show that Ti in these NCFs is in the TiIII state (this conclusion is also proved by ESR spectroscopy), and that both oxidation and reduction proceed via the change of the valence state of the endohedral Ti, which becomes TiII in TiM2N@C80− and TiIV in TiM2N@ C80+.436 Thus, Ti in TiM2N@C80 is the first example of an EMF with an endohedral redox activity both in the reduction and oxidation. It results in the shift of the redox potentials of TiM2N@C80 from those of other M3N@C80 and leads to a considerably smaller electrochemical gap (1.10 V) of both TiNCFs, which is however still much larger than the electrochemical gap of ca. 0.5 V found for paramagnetic MIII@C82 or Sc3C2@C80. Besides, the in cavea (endohedral) reduction of the TiIII atom in TiM2N@C80 (rather than reduction of the carbon cage) results in the reversible reductions since the anion TiM2N@C80− with divalent Ti, unlike anions of all other NCFs, is diamagnetic. Localization of HOMO and LUMO on the oxide cluster was predicted in Sc4O2@C80 (Figure 53),307,465 and its recent

confirmed localization of the spin density on the Sc3NC cluster in this radical.598 Interestingly, spin populations of Sc atoms in Sc3NC@C80− are rather small, and spin density is mainly localized on the NC group. An interesting phenomenon of the spin-charge separation was found in computational studies of the electron density distribution of EMFs with in cavea (endohedral) redox activity and their charged states.428,436 The occupation of the metalbased LUMO and the localization of the spin density in the thus formed radical anions imply that the surplus electron is localized on the endohedral cluster. Hence, atomic charges of the metal atoms should be changed accordingly. However, the analysis of DFT-computed atomic charges did not reveal any noticeable changes of the metal atoms in anions of such EMFs in comparison to the charges in neutral molecules.307,428 This effect persisted for different definitions of atomic charges (Mulliken, Bader, NBO) and thus it does not stem from the artifact of the electron partition scheme and should have a physical background. The analysis of the differences of the electron densities of the neutral and charged states revealed that the addition of an electron to EMF molecules resulted in a complex redistribution of the electron density on metal atoms, including both regions of the fullerene with an increase and a depletion of the electron density (Figure 12d).428,436 These regions balanced each other upon integration giving thus small changes of the atomic charges. Thus, while the electron transfer results in the localization of the spin on the metal atoms, the change of the charge occurs at the carbon cage. Localization of the spin density on the endohedral cluster in EMFs with endohedral redox activity and fast dynamics of the endohedral clusters raises the question of their mutual influence. DFT studies have shown that the spin density distribution in such EMFs is quite flexible, and even a slight reorientation of the cluster results in considerable changes of the spin populations of the metal atoms.428,436,597 This conclusion was further corroborated by DFT-based Born− Oppenheimer molecular dynamics, which provided details of the spin density dynamics (dubbed as the spin f low).436,597 It was found that the spin flow takes place both between the metal atoms in the cluster as well as between the cluster and the carbon cage. For instance, the variation of spin populations of individual Sc atoms in Sc3N@C80− can be as high as 60% over the picosecond time scale. Fourier transformation of the time dependencies of the spin populations resulted in spin flow vibrational spectra, which reveal the major spin flow channels.436

Figure 53. (a) Frontier molecular orbitals of Sc4O2@C80 and enlarged view of the Sc4O2 cluster showing two types of Sc atoms in different valence states; (b) cyclic and square-wave voltammograms of Sc4O2@ C80 measured in o-DCB at room temperature. Reproduced with permission from ref 371. Copyright 2012 American Chemical Society.

7.7. Gas-Phase Electron Affinity of EMFs

Electrochemistry remains so far the main method of studying the electron-accepting properties of EMFs. However, reduction potentials determined in solution include solvation energies, which depend on the solvent and on the size of the fullerene molecule as well as the charge distribution in EMF molecules and ions. The values free from the solvent influence (e.g., electron affinity, EA) can be measured in the gas phase. The number of the EA values of EMFs known so far is rather limited (Table 12), and all except for EA of Ca@C60833 were determined from high temperature ion-molecular equilibria studied by Knudsen-cell mass-spectrometry.809,834,835 Furthermore, the samples studied in those measurements were EMF mixtures, and when more than one isomer exists for a given EMF, assignment of the measured EA value to a specific isomer is not possible. Comparison of EAs and E1/2 potentials of empty

electrochemical and spectroelectrochemical study proved that both reduction and oxidation of Sc4O2@C80 are endohedral redox processes.371 Both radical cation and anion exhibited 45 Sc-based hyperfine structure with large coupling constants proving that the spin density in both radical is localized on the Sc4O2 cluster. Thus, electrochemical behavior of Sc4O2@C80 is entirely determined by its redox-active endohedral oxide cluster. On the basis of DFT computations, the endohedral redox activity may be expected in some other clusterfullerenes, including Sc3C2@C80 (oxidation and reduction),367 Lu3C2@C88 (oxidation),340 Sc3NC@C80 (reduction),10 Sc4C2@C80 (oxidation),369 Sc4O3@C80 (reduction).307,465 Recent ESR and DFT study of the chemically generated radical anion of Sc3NC@C80 6072


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Table 12. Electron Affinities of EMFsa,b EMF

EA

ref

EMF

C60 C70 Ca@C60 La@C80 Gd@C60 Gd@C74 Gd@C76 Gd@C78 Gd@C80 Gd@C82

2.689 ± 0.008c 2.765 ± 0.010c 3.2 ± 0.1c 3.32d 2.85d 3.24d 3.2 ± 0.1 3.26d 3.3 ± 0.1 3.3 ± 0.1

840 841 833 842 842 809 809 809 809 809

C80-D2 C82-C2 Sc3N@C80 ErSc2N@C80 Er2ScN@C80

3.19 3.16 2.81 2.76 2.73

EA 0.04 0.04 0.05 0.05 0.07

834 834 834 834 834

Sc2@C80 “Sc2@C82”e Sc2C2@C82e

3.20 ± 0.04 3.21 ± 0.04 3.10 ± 0.04

834 834 834

± ± ± ± ±

ref

a

All values in eV. bUnless the otherwise noted, EAs are determined via the ion-molecular equilibria studied by Knudsen-cell mass-spectrometry. Determined by photoelectron spectroscopy of anions. dDetermined from single measurement, so uncertainties are not known. eIt is not known if the value corresponds to Sc2@C82 or Sc2C2@C80. c

particular, following the brief summary of the chemical reactions of NCFs by Dunsch and Yang in 2007,6 a general overview of the chemical functionalization of endohedral fullerenes was first given by Echegoyen et al. in 2009.26 In 2010 Akasaka et al. reviewed the chemical functionalization of conventional EMFs such as M@C82−C2v (9), M2@C 72D2(10611), M2@C78-D3h(5), M2@C80-Ih(7), and M2@C80D5h(6), focusing on the positions and dynamic behavior of the metal atoms encapsulated in pristine and functionalized fullerene cages.33 In 2011 Akasaka et al. reviewed the chemical functionalization and supramolecular chemistry of M@C82,23 while the most comprehensive and systematic review on the chemistry of all kinds of EMFs was presented recently by Lu, Akasaka and Nagase.44 In the same year, an overview of functionalization of EMFs was published by Cardona,846 and computational aspects of EMF reactivity were reviewed by Osuna et al.40 In the following section we will briefly discuss all the reported chemical functionalization studies of endohedral fullerenes, focusing in particular on the chemical functionalization of clusterfullerenes; all chemical reactions of EMFs studied so far are summarized in Table 13.

fullerenes and EMFs shows that solvation indeed affects the relative values. For instance, the EA of Sc3N@C80 is 2.81 ± 0.05 eV,834 which is 0.14 eV higher than the EA of C60 (2.69 ± 0.01 eV); at the same time, Sc3N@C80 is a weaker electron acceptor than C60 in o-DCB solution if judged by their first reduction potentials (C60 is easier to reduce by ca. 0.12 V, see Table 11).

8. CHEMICAL PROPERTIES OF ENDOHEDRAL FULLERENES Exohedral chemical functionalization of endohedral fullerenes has a great significance toward their potential applications. Although a large number of experimental studies on the chemical functionalization of empty fullerenes such as C60 and C70 have been already reported,843 methods developed for the exohedral functionalization of endohedral fullerenes are not so extended, and include photochemical reactions, Diels−Alder reactions, Prato reactions, Bingel-Hirsch reactions, radical addition reactions, and some other addition reactions.23−26,32,33,44,208,844,845 The chemical reactivity of fullerenes is mostly determined by their π-system, and in many reactions fullerenes behave as extended polyalkenes with rich addition chemistry. In the view of the multiple addition sites available at each carbon cage, the question of the regioselectivity of the chemical derivatization is one of the most challenging in fullerene chemistry. In EMFs, π-system is modified by the electron transfer from the metal atoms or clusters and by an inhomogeneous distribution of the excess electron density over the fullerene surface. This raises an intriguing question of the mutual influence of the endohedral atoms or clusters and the exohedral groups on the addition pattern and physicochemical properties of the EMF derivatives. Thus, it is natural to expect that chemical properties of EMFs can be different from those of empty fullerenes. Besides the feasibility of synthesizing new functional materials with multiple potential applications, the chemical functionalization appears particularly important for endohedral fullerenes because it provides an alternative way to elucidate the molecular structure of endohedral fullerenes for which the X-ray crystallography characterization is difficult in the pristine form (see section 4). Although the first chemical reactions of EMFs have been reported starting from 1995, an explosive growth of the field of the chemical modification of EMFs began in the middle of the first decade of this century. In the past decade, chemical functionalization of endohedral fullerenes has been attracting great attention among the fullerene community and the achievements in this field have been summarized in several reviews in the last few years. In

8.1. Conventional Endohedral Fullerenes

8.1.1. Photochemical Disilylation and Carbene Addition Reactions. The first exohedral functionalization of EMFs was undertaken by Akasaka et al. in 1995 by a photochemical reaction of La@C82-A (i.e., C2v(9) isomer) with disilirane, leading to the formation of a 1:1 adduct.579 An interesting finding is that La@C82-A reacts thermally or photochemically with disilirane, whereas for empty fullerenes such a reaction could be realized only photochemically. The facile thermal addition of disilirane to La@C82-A can be rationalized by its stronger electron-accepting as well as electron-donating properties compared to empty fullerenes.579 Similar photochemical studies were also accomplished by functionalizing La@C82-A with digermirane572 or diphenylmethylene.573 To investigate the reactivity changes induced by the metal trapped within the same C82 cage, Akasaka et al. carried out the exohedral functionalization of Gd@C82, Ce@C82 and Y@C82 and Pr@C82-A with disilirane, indicating that all of them were functionalized not only photochemically but also thermally as in the case of [email protected],578,847 Likewise, the minor isomers La@C82−B and Pr@C82−B (both with Cs(6) carbon cage) also underwent both thermal and photochemical silylation.402,580 Later on, the same group also reported similar reactions based on the anions and cations of M@C82 (M = Y, La, Ce), revealing that the gain or loss of electrons by ionization was useful for 6073


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Table 13. Chemical Reactions Utilized to Functionalize EMFs reaction class [3 + 2] cycloaddition

1,3 dipolar addition of azomethine ylides (Prato reaction) bis-silylation

[1 + 2] cycloaddition

carbosilylation digermirane addition carbonyl ylide addition carbene addition

carbanion addition (Bingel− Hirsch reaction) “unconventional” Bingel− Hirsch reaction products

[2 + 2] cycloaddition [4 + 2] cycloaddition addition of singlebonded groups

organometallic chemistry water-soluble derivatives Lewis-acid complexation supramolecular complexation

a

EMFsa

reaction type

radical malonate addition zwitterion addition nucleophilic reactions of anions azide addition silylene addition benzyne addition Diels−Alder reaction photochemical perfluoroalkylation thermal radical perfluoroalkylation phenylation (w. C6H3Cl2) radical/photochemical benzyl addition nucleophilic addition of glycine esters hydrogenation complexation with Re3 cluster hydroxylation, carboxylation, PEGylation

853

533

895

Y@C82, Y2@C79N, La@C72(C6H3Cl2), La@C82,574−576 La2@C80,274,277,576,649,665 Ce2@C80,277 Gd@C82,852,853 Sc2C2@C82-Cs(6),354 Sc2C2@C82-C2v(9),355 Sc3C2@C80,531,532 Sc3N@C78,384,881 Sc3N@ C80,333,391,498,596,647,648,671,877,879−881 Sc3N@C80-D5h(6),335 Y3N@C80,326,497,596,878,881 YxSc3−xN@C80,333 GdSc2N@C80,894 GdxSc3−xN@C80,498 Gd3N@C80,881 Er3N@C80,497,645,877 Lu3N@C80825,881 Y@C82,578 M@C82+ (M = Y, La, Ce),848 La@C82,578−580 La2@C80,281,829 Ce2@C78,298 Ce2@C80,283 Ce2@C80-D5h(6),285 Pr@C82,402 Gd@C82,847 Sc2C2@C82,829 Sc3C2@C80,816 Sc3N@C80,388,461 Lu3N@C80839 La2@C80275 La@C82572 La2@C80280 Li+@C60,378 Sc@C82,256 Y@C82,254 La@C82,252,411,573 La@C82-Cs(6),257 La@C82(Cp*),414 La2@C72,290,291 La2@C78,297 La2@C80,276,278,279,896 La2@C80(CClPh),278 Ce@C82,255 Ce2@C80,276,387 Gd@C82,229 Tb@C82,849 Sc2C2@C80C2v(5),362,386 Sc2C2@C82-C3v(8),353 Sc3C2@C80,319 Sc3N@C80,885,897 Y3N@C80,897 Lu3N@C80885 La@C82,253 Sc3N@C68,883 Sc3N@C80,823 Sc3N@C80(CF3)14,394 Y3N@C80,327,596,878 Gd3N@C80,822 Gd3N@C84,822 Er3N@C80,497 Lu3N@C80,823 Gd2@C79N316 singly bonded adducts: La@C82253,409,410 [6,6]-closed adducts: Gd@C60,863 Sc3N@C78820 Sc3N@C80,885 Lu3N@C80885 Dy@C82258 Lu3N@C80-Ih (2−),824 Sc3N@C80-Ih (3−)838 [5,6] and [6,6] azafulleroids of Sc3N@C80397 Lu3N@C80328 La@C82,412 Gd@C82,815 Sc3N@C80392,396 La@C82,414,577 La@C82(Ad),414 Y2@C79N,316 Gd2@C79N,316 Sc3N@C80,321,335 Sc3N@C80-D5h(6),335 Gd3N@C80,875 Lu3N@C80,335 Lu3N@C80-D5h(6)335 La@C82−Cs(6),581 Sc3N@C80394 Y@C82,814 Y2@C80,814 Ce@C82,857 Gd@C82,858 Gd2@C80,858 Sc3N@C80,336,393−395,826 Sc3N@C80-D5h(7),336,826 Er3N@ C80394 La@C72-C2(10612),247 La@C74-D3h(1),246,380 La@C80-C2v(3),248 La@C82-C3v(7)521 La@C82,413 Sc3N@C80,389 Sc3N@C80(CF3)10,394 Lu3N@C80389 Gd@C82856 Er2@C82640 Sc2C2@C82−C3v(8)405 Li@C60,525 Gd@C60,863 Gd@C82,710,711,760,856,898−901 M@C82 (M = La, Ce, Gd, Dy, Er),861 Ho@C82,859,902 Ho2@C82,902 Sc3N@C80,886 GdxSc3−xN@C80,887 Gd3N@C80,888−891,893 Lu3N@C80892 Sc3N@C2n (2n = 68, 78, 80),14 Sc4O2@C8014

calix[8]arene

Dy@C82,867

crown ethers (thia-, aza-) tetrathiafulvalene calix[4] pyrrole bisporphyrin spin-exchange w. organic donors

La@C82868,869 Li@C60379 La@C82,872,873 Sc3N@C80,651,667 Lu3N@C80651 La@C82,870 La2@C80871

If more than one isomer exists, isomeric structure of the major isomers is omited (i.e., Ih(7) for M3N@C80 and M2@C80, C2v(9) for M@C82).

the difference of their reduction potentials. A similar study on the reaction of Ce2@C80 with disilirane reported by the same group revealed that the free random motion of two Ce atoms in Ce2@C80 was hindered at specific positions by exohedral disilylation functionalization.283 Another dimetallofullerene, Ce2@C78, showed unexpectedly high regioselectivity of the silylation yielding only one 1,4-monoadduct.298 Using the photochemical method, Akasaka et al. reported in 2004 the highly regioselective addition of 2-adamantane-2,3[3H]-diazirine to La@C82 and isolated the first paramagnetic carbene monoadduct of La@C82 with adamantylidene (Ad).252 Since 2008 the same group has studied adamantlylidene addition reactions of a series of EMFs including Gd@C82, Ce@ C82, and Y@C82, and showed that these EMFs react in a similar

controlling the stability and reactivity of the fullerene. The cations of M@C82+ (M = Y, La, Ce) reacted with disilirane readily even at room temperature in the dark, indicating that oxidation is an effective method for increasing the disilylation reactivity of EMFs.848 On the contrary, the anions did not react with disilirane either thermally or photochemically, suggesting that the reactivity of M@C82 toward disilirane was dramatically decreased by reduction.848 In another comparative study reported by Akasaka et al., La2@C80 and “Sc2@C84” (whose the structure was later corrected as Sc2C2@C82352) were used as reactants and it was found that La2@C80 reacts with disilirane to afford a 1:1 adduct while the thermal addition of disilirane to Sc2C2@C82 is suppressed.829 These results were understood by considering 6074


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way with formation of two M@C82Ad isomers.229,254,255 Moreover, single-crystal X-ray studies of these carbene derivatives enabled the unambiguous determination of the location of the metal within M@C82 (M = Gd, Ce, Y) and showed that the metal always tends to locate under the hexagonal ring along the C2 axis.229,254,255 Such an off-center metal position and the resulting inhomogeneous charge distribution over the carbon cage are believed to play a critical role in determining the chemical properties of M@C82 EMFs. Analogous reaction of La@C82-Cs(6) with adamantylidene reported by Akasaka et al. in 2008 also showed a high regioselectivity with formation of only two monoadducts. Similar to (M@C82−C2v(9))-Ad adducts, adamantylidene is added to C−C bonds located close to the La atom. It was found that the electronic properties of the two monoisomers were very similar to those of the pristine La@C82-Cs(6), in agreement with the formation of “open” adducts which retail the π-system of the fullerene intact.257 Recently, Akasaka et al also reported Ad addition to Sc2@C82-C2v(9).256 Unlike other MIII@C82 EMFs, Sc@C82 formed four monoadducts, of which one was characterized by single-crystal X-ray diffraction. The group of Akasaka et al. has also extended the study of photochemical carbene addition to the class of dimetallofullerenes. The photochemical reaction of M2@C80 (M = La and Ce) with adamantylidene was studied in 2008, affording the [6,6]open adducts with the long M−M distance. In the molecular structures of M2@C80Ad, two metal atoms are collinear with the spiro carbon atom of the adamantylidene group.276 Photochemical reactions of La2@C78 (D3h) with adamantylidene afforded four isomers of the monoadduct (La2@C78Ad), whereas the thermal reaction gave seven isomeric monoadducts.297 The carbene addition was revealed to occur at both [5,6] and [6,6] bonds around the pole and equator of the cage. The X-ray and theoretical studies of one of the isomers showed an open structure with the two La atoms on the C3 axis of the D3h cage of [email protected] The similar reaction was also studied on a non-IPR dimetallofullerene La2@C72-D2(10611) affording six isomers of monoadducts and more than fifteen bisadducts.290 Interestingly, the X-ray analyses of the major monoadduct isomers revealed that the fused-pentagon sites were very reactive toward carbene addition; however, the carbon atoms forming the [5,5] junctions are not reactive because of the bonding to the encaged metal.290 Single-crystal X-ray study of the La2@C72(Ad)2 bisadduct showed that the two adamantylidene groups are covalently bonded to the [5,6]bonds adjacent to the two fused-pentagon bonds in a symmetric open-cage structure.291 It was concluded that the fused-pentagon sites in La2@C72 were more reactive than other sites, as only mono- and bis-adducts were formed.290,291 Another kind of carbene addition reaction of EMF was reported in 2002 by Gu et al., who has succeeded in the room temperature synthesis of the multiadducts of Tb@C82 with α-diazocarbonyl compounds mediated by a copper(I) catalyst (Cu(MeCN)4PF6), to form Tb@C82[CH2(C6H5)CO2CH3]n (n = 1−6).849 Carbene addition to Li+@C60 with formation of [5,6]-open and [6,6]-closed PCBM-like adducts was reported by Matsuo et al. in 2012.378 8.1.2. Cycloaddition Reactions. Diels−Alder Reaction. In 2005 Akasaka et al. reported the Diels−Alder addition of cyclopentadiene (Cp) to La@C82, affording only one 1:1 monoadduct (La@C82Cp) in 44% yield with a quite high selectivity (Scheme 1A).577 Interestingly, this addition reaction was found to be reversible and the product La@C82Cp

Scheme 1. (A) Schematic Representation of the Reversible Reaction between La@C82 and Cyclopentadiene; (B) Schematic Representation of the Reaction between Sc3N@ C80 (I) and 6,7-Dimethoxyisochroman-3-one (1) via a [4 + 2] Diels-Alder Cycloadditiona

a

The X-ray crystal structure of the monoadduct 3 is also shown on the left. Reproduced with permission from refs 577 and 321. Copyright 2002, 2005 American Chemical Society.

decomposed to La@C82 and Cp even at 298 K in toluene. Such a retro-reaction proceeded much faster than that of C60Cp because of its lower activation energy.577 More recently, the same group substituted cyclopentadiene with 1,2,3,4,5-pentamethylcyclopentadiene (Cp*) and obtained a stable monoadduct (La@C82Cp*) via the Diels−Alder cycloaddition reaction.414 X-ray structural characterization of La@C82Cp* showed two orientations in the crystal structure, and 60% of the monoadduct forms a dimer in the solid state. The length of the C−C bond connecting two C82 cages in the La@C82Cp* dimer (1.606 Å) was only about 0.1 Å longer than the typical C−C single bond length. Such a dimerization in the solid state was explained by the high POAV angles (11.0°) and a spin density (0.025) of the intercage bonding sites. Contrary to La@ C82Cp, the retro-reaction of La@C82Cp* can be controlled by changing the reaction temperature. In the same work the authors also succeeded in the synthesis of the bis-adducts of La@C82 with Cp* and adamantylidene.414 Prato Reaction. The Prato reaction via 1,3-dipolar cycloaddition of azomethine ylides with alkene850 established by Prato and Maggini in 1993 has now turned out to be one of the most used methods of fullerene derivatizations because of its good selectivity and feasibility of adding a wide range of addends and functional groups.851 In 2004 Akasaka et al. reported the first Prato reaction of EMF by addition of an azomethine ylide to La@C82-A in toluene574and found that the addition reaction was very efficient and, to some extent, regioselective. A monoadduct and a bisadduct with an abundance ratio of ∼1:0.4 were isolated by HPLC and characterized by several spectroscopic methods, revealing that the introduction of pyrrolidines to La@C82-A does not alter the octet EPR signal and the endohedral character of the encaged metal atom varies the electronic structure of La@C82-A. In the 6075


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undec-7-ene (DBU).410 In the following year, the mixture of monoadducts was separated by preparative HPLC from multiadducts resulting in the isolation of five monoadducts (mono-A, -B, -C, -D, and -E) which were characterized in detail.253 Such a product distribution was found to be very different from those of empty fullerenes as well as previously reported EMFs, which tend to form cycloadducts. Among these five monoadducts, mono-A was most abundantly produced and isolated in the yield of 40% based on the consumed La@C82. The structure of mono-A was unambiguously determined by the X-ray crystallographic analysis as shown in Figure 54a.

same year, Gu et al. reported the addition of another type of azomethine ylide generated by the reaction of sarcosine and formaldehyde to Gd@C82 affording the mono- through octoadducts.852 This was different to the direct reaction of sarcosine with Gd@C82 which afforded only the monoadduct.852 Furthermore, the same group studied the Prato reaction of M@C82 (M = Gd, Y) with different azomethine ylides which were generated from the reactions of aldehydes with sarcosine and found that multiple adducts formed in all cases precluding thus the definitive structural characterization of the products. The authors concluded that, by choosing the aldehydes with different substituted groups, the number of the pyrrolidinerings added on M@C82 (M = Gd, Y) could be tuned.853 Prato reaction of La@C82 was also used to introduce functional groups for further photophysical studies.575,576 In 2006 Akasaka et al. reported the Prato reaction of a dimetallofullerene La2@C80 with 3-triphenylmethyl-5-oxazolidinone (NTrt) yielding both [5,6]- and [6,6]-adducts (Scheme 2A).274 The same reaction of Ce2@C80 was reported in Scheme 2. (A) Schematic Representation of the 1,3-Dipolar Cycloaddition of La2@C80 and 3-Triphenylmethyl-5oxazolidinone (NTrt); (B) Depiction of the 1,3-Dipolar Cycloaddition of the N-Ethyl Azomethine Ylide to Sc3N@ C80a

Figure 54. ORTEP drawings of a monoadduct (a) and bisadduct (b) of La@C82 via Bingel-Hirsch reactions. Reproduced with permission from refs 410 and 409. Copyright 2005, 2006 American Chemical Society.

Addition of the bromomalonate group takes place at the C8 atom of La@C82-C2v(9), on the apex between two hexagons and one pentagon. Because of the formation of a single bond, the C82 cage undergoes a deformation, while the La atom is located at a fixed position far from the bromomalonate group.410 Another monoadduct, Mono-E, was suggested to be a cycloadduct similar to conventional Bingel adducts.843 More recently, the same group found that, while the aforementioned reaction of La@C82 with malonate in the presence of DBU proceeded very slowly at room temperature, the reaction proceeded much faster and finished in a few hours when the temperature was increased up to 60 °C and as a result a bisadduct, namely La@C82[CH(COOC2H5)2]2, was successfully isolated.409 Interestingly, the bisadduct La@C82[CH(COOC2H5)2]2 crystallizes to form a dimer at 90 K whose X-ray crystallographic structure is shown in Figure 54b. Addition of a malonate takes place at the C7 and C13 atoms of La@C82, which are located at the apex of two hexagons and one pentagon. Notably, C7 is also the addition site for the Bingel monoadduct of La@C82 (mono-A) as discussed above. This result strongly suggests that these Bingel bisadducts and monoadducts are formed via a similar formation mechanism.409 Other cycloaddition reactions of conventional EMFs have been also reported by different groups. In 2004 Gu et al. reported the [2 + 2] cycloadducts of benzyne which was generated in situ by the reaction of isoamyl nitrite with anthranilic acid to Gd@C82, resulting in two monoadducts in a ratio of 8:1 which were isolated by HPLC and characterized by

a

Reproduced with permission from refs 274 and 879. Copyright 2005, 2006 American Chemical Society.

2009.277 The structure of the [6,6]-adducts was fully determined by means of X-ray crystallographic analysis. 139La and 13C NMR spectroscopies revealed that the two metal atoms were fixed in both the [5,6] and [6,6]-adducts in contrast to the random circulation of the metal atoms in the pristine M2@C80Ih(7) dimetallofullerenes.274,277 Bingel−Hirsch Reactions. The Bingel−Hirsch reaction is another most widely applied reactions in fullerene chemistry. The first attractive Bingel adduct of EMFs was Gd@ C60[C(COOC2H5)]10 reported by Bolskar et al. in 2003 via multiple cycloaddition of bromomalonates to [email protected] Gd@C60[C(COOC2H5)]10 was further hydrolyzed to the water-soluble form, Gd@C60[C(COOH)2]10, which has been proved to have favorable properties as a magnetic resonance imaging (MRI) contrast reagent (see also section 10.1).854,855 A singly bonded monoadduct of La@C82 (La@C82CBr(COOC2H5)2) was synthesized by Akasaka et al. in 2005 based on the Bingel−Hirsch reaction of La@C82 with diethyl bromomalonate in the presence of 1,8-diazabicyclo[5.4.0]6076


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Figure 55. (a) Ball-and-stick depiction of Gd@C60[C(COOH)2]10, illustrating a possible arrangement of 10 C(COOH)2 addends on a single C60 cage (light blue, C; red, O; white, H; dark blue, Gd). Reproduced with permission from ref.863 2003 American Chemical Society. (b) Schematic structure model of pegylated-hydroxylated NCFs Gd3N@C80[DiPEG(OH)x] with several hydrogen-bonded water molecules. Reproduced with permission from ref 889. Copyright 2010 American Chemical Society.

Using the fluorous-phase partitioning method (or liquid− liquid extraction), in 2002 Shinohara et al. synthesized the first perfluoroalkylated paramagnetic EMF derivative  La@ C82(C8F17)2.581 The authors concluded that the perfluoroalkyl addends should be located in 1,2 rather than in 1,4 positions derived from the attack at the [6,6] double bond adjacent to La. As a result, the π electron system of the EMF is preserved to a large extent and steric strain in the carbon cage is partially released. The solubility of EMFs in perfluorocarbon solvents can be drastically increased by functionalizing them with long and heavily fluorinated alkyl “ponytails”.581 Another fluoroalkylated derivative of EMF was prepared by Kareev et al. in 2005 via a high temperature reaction of the Y@ C82-enriched EMF extract and silver(I) trifluoroacetate (AgCF3CO2).814 Trifluoromethylation was chosen in that study because CF3 derivatives are usually less prone to hydrolysis than fluorofullerenes and are more soluble in common organic solvents. Interestingly, the addition of an odd number of CF3 groups to paramagnetic Y@C82 rendered the two isomers of Y@C82(CF3)5 diamagnetic, and three possible structures for these two isomers of Y@C82(CF3)5 have been suggested by 19F NMR spectroscopic analysis and theoretical calculations.814 Similar pentakis-adducts were also formed in the trifluoromethylation of Ce@C82857 and Gd@ C82.858 Besides, the authors reported that minor admixtures of Y2@C80 and Gd2@C80 were converted into monoadducts, M2@ C80CF3, but the structures of these derivatives were not elucidated.814,858 8.1.4. Water-Soluble Derivatives. Water-solubilization of endohedral fullerenes is the prerequisite for their potential biological and medical applications which will be discussed in detail in section 10. The first water-soluble EMF derivative (Ho@C82(OH)x) was synthesized by Wilson et al. through the electrophilic addition of nitronium tetrafluoroborate in the presence of arenecarboxylic acid in a nonaqueous medium.859 The biodistribution of Ho@C82(OH)x was examined in detail, pointing to the their potential applicationss as new diagnostic or therapeutic radiopharmaceuticals.860 Following the same method, Shinohara et al. synthesized in 2000 Gd@C82(OH)x and measured its R1 relaxivity (46 L mmol−1 s−1 at pH = 7.5), which appeared to be much larger than that of the commercial Gd-MRI contrast agents such as Gd-DTPA (4.3 L mmol−1 s−1). This finding demonstrated good prospects and encouraged the further intense studies of the water-soluble multihydroxyl lanthanoid EMFs.760 Later on, the same group synthesized a series of lanthanoid metallofullerenols, M@C82(OH)n (M = La,

mass spectrometry and electrochemistry.815 The authors found that the monoadducts dominated the reaction and the exohedral addend had strong reductive effect on the EMF.815,856 In 2007 Yang et al. successfully synthesized a Dy@C82 derivative bearing a single phosphorus substituent in a 90% yield by the regio- and chemoselective reaction of Dy@C82 with dimethyl acetylenedicarboxylate (DMAD) and triphenylphosphine (Zwitterion approach).258 Interestingly, contrary to the product with a [6,6]-fused structure in the reaction of C60, the reaction of the Dy@C82 EMF led exclusively to the product with a [6,6]-bridged structure as revealed in the single-crystal X-ray study.258 More recently, Akasaka et al. reported the first preferential addition of benzyne to the [5,6]-bond of La@C82 to form closed cyclobutene rings between the substituents and the cage.412 Unexpectedly, an NO2 group which was originated from the trace of NO/NO2 radicals in isoamyl nitrite was also found in the benzyne adducts.412 8.1.3. Radical Addition Reactions. The first radical addition reaction of EMFs was reported by Suzuki and Kato et al in 1995. An excess of diphenyldiazomethane was added to a toluene solution of La@C82 in an ESR tube, affording the diphenylmethano-La@C82 adducts La@C82-(CPh2)n (n = 0− 3).573 In 2005 Akasaka et al. synthesized the radical addition adduct of La@C74(C6H3Cl2) by extracting the soot with 1,2,4trichlorobenzene (TCB) under reflux, leading to the discovery of the missing EMF La@C74-D3h(1).246 One year later, this group also discovered La@C72-C2(10612) following the same strategy of synthesizing La@C72(C6H3Cl2),247 and more recently this procedure afforded the isolation and single-crystal X-ray characterization of the C6H3Cl2 adducts of the previously unknown EMFs La@C80-C2v(3)248 and La@C82-C3v(7).521 In 2008 the same group carried out the radical coupling reactions of La@C82-C2v(9) in toluene or 1,2-dichlorobenzene under photoirradiation. 4 13 The benzyl monoadducts La@ C82(CH2C6H5) were synthesized via the thermal reaction of La@C82 with 3-triphenylmethyl-5-oxazolidinone in toluene or simply photoirradiation of La@C82 in toluene without the existence of 3-triphenylmethyl-5-oxazolidinone. These reactions were also applicable to other paramagnetic EMFs such as La@ C82-Cs(6) and Ce@C82-C2v(9). Furthermore, they found that the photoirradiation of La@C82-C2v(9) in 1,2-dichlorobenzene in the presence of α,α,2,4-tetrachlorotoluene also afforded the monoadducts La@C82(CHClC6H3Cl2). On the basis of the Xray structural analysis of La@C82(CHClC6H3Cl2), the authors concluded that the radical coupling reactions took place at the cage carbons that have high spin densities.413 6077


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from the extracts of soot.868 In the same year this group also reported the synthesis of a 1:1 host−guest complexes of La@ C82-C2v with unsaturated thiacrown ethers as a result of electron transfer.869 Another organic donor molecule, N,N,N′,N′tetramethyl-p-phenylenediamine (TMPD) which can form a stable radical cation, was also applied successfully for the complexation with La@C82, which behaved as the reversible intermolecular spin-site exchange systems at complete equilibrium in solution.870 Similar reaction with TPMD was recently reported for [email protected] In 2010 Shinohara, Tagmatarchis et al. reported on the formation of the 1:1 noncovalently bonded complex of La@C82 with isophthaloyl-bridged porphyrin dimer.872 In 2011, inclusion complex of La@C82 with cyclodimeric copper porphyrin was synthesized by Tashiro, Aida et al.873 The ring-closing olefin metathesis of its side-chain olefinic termini was used to transform the bisporphyrin host into the caged form, which switched the coupling of the La@C82 and copper spins from ferrimagnetic to ferromagnetic.873 In 2011 Fukuzumi et al. studied supramolecular complexes of Li+@C60 with tetrathiafulvalene calix[4]pyrroles.379 An electron transfer from the hosy moiety to Li+@C60 was observed and could be controlled by the choice of the anion or cation.

Ce, Gd, Dy, and Er) and studied systematically their performances as magnetic resonance imaging (MRI) contrast agents (see section 10.1. for more details).861 In 1999 Yang et al. successfully synthesized the endohedral metallofullerol Pr@ C82Om(OH)n (m ≈ 10 and n ≈ 10) through the reaction of Pr@C82 with a concentrated nitric acid and a subsequent hydrolysis process.862 Analogous to the same reaction of C60, NO2 radical addition is considered to be a crucial step for the formation of metallofullerols via aqueous chemistry.862 The water-soluble carboxylated EMF derivative Gd@C60[C(COOH)2]10 (Figure 55a) was prepared by Bolskar et al. through the hydrolysis of the Gd@C60[C(COOCH2CH3)2]10 polycycloadduct synthesized via Bingel reaction as discussed in the section 8.1.2.863 The authors also studied Gd@C60[C(COOH)2]10 as a new MRI contrast agent by in vivo MRI biodistribution measurements, relaxometry, and dynamic light scattering. The biodistribution results gathered by comparing the M@C 2n(OH) x species with Gd@C 60 [C(COOH)2 ] 10 complement the light scattering and relaxivity measurements, revealing an aggregation for the polyhydroxyl fullerene derivatives but not for the carboxylated fullerene derivative.854,855 The solubilization and separation of Gd-based EMFs by chemical oxidation was also demonstrated by the same group.173 Several other types of water-soluble derivatives of Gd@C82 EMFs have been synthesized by different groups. In 2004 Gu et al. reported the nucleophilic addition of glycine esters to Gd@ C82, affording the noncycloaddition products with up to eight glycine ethyl ester groups or four glycine methyl ester added to the carbon cage.856 In 2006 Wang et al prepared an amino acid derivatives Gd@C82Om(OH)n(NHCH2CH2COOH)l (m ≈ 6, n ≈ 16 and l ≈ 8) by a direct reaction of Gd@C82 with an excess of an alkaline solution of beta-alanine and concluded its high efficiency as MRI contrast agents based on both water proton relaxivity analysis and MRI phantom studies.864,865 Later on, Wang et al synthesized Gd@C82O2(OH)16(C(PO3Et2)2)10 as an organophosphonate functionalized derivative of Gd@C82, which had a much higher longitudinal water proton relaxivity (37.0 mM−1 s−1) than the commercial Omniscan (Gd-DTPA BMA, 5.7 mM−1 s−1) and carboxylated Gd@C82 (16.0 mM−1 s−1) at 0.35 T.866 8.1.5. Supramolecular Complexes of EMFs with Macrocyclic Compounds. Supramolecular complexes of EMFs with macrocyclic compounds such as calixarenes, crown ethers and porphyrins were also synthesized by several groups. In 2002 Yang et al. first prepared a host−guest supramolecular complex of EMF Dy@C82 and p-tert-butylcalix[8]arene (C8A) ([Dy@C82−C8A]).867 The yield of the [Dy@ C82−C8A] complex was only 33%, much lower than that of the [C60-C8A] complex (100%). This was explained by the larger size of Dy@C82 (8.0 Å) than that of C60 (7.1 Å) and the entropy effect which is associated with the loss of the rotational freedom of EMF upon complexation with the calixarene.867 In 2006 Akasaka et al. studied the complexation of the La@C82C 2v (9) with other macrocyclic compounds, such as 1,4,7,10,13,16-hexaazacyclooctadecane, 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaaza-cyclooctadecane, monoaza-18crown-6 ether, 18-crown-6 ether, and p-tert-butylcalix[n]arenes (n = 4−8).868 In solution, La@C82 formed complexes only with azacrown ethers, and the complex formation was accompanied by the electron transfer to the La@C82 as the result of the low reduction potentials of the EMF. The authors successfully applied such a complexation to the selective extraction of EMFs

8.2. Heterofullerenes M2@C79N (M = Y, Gd)

It can be expected that due to the paramagnetic state of heterofullerenes M2@C79N and the presence of the nitrogen atom on their carbon cage, these EMFs should have specific chemical properties. Only a few types of chemical reaction were reported so far for M2@C79N. Dorn et al. found that Gd2@ C79N has rather low chemical reactivity in spite of its unpaired electron.316 For instance, it does not dimerize (in contrast to C59N) and reacts neither with the spin trap 5,5-dimethyl-1pyrroline-1-oxide (DMPO), nor with toluene or anisole in the presence of p-toluenesulfonic acid (which leads to arylation of C59N). Gd2@C79N also has low Diels−Alder reactivity, but can be functionalized by cyclopropanation with diethyl bromomalonate in the presence of DMF (Bingel−Hirsch reaction). The monoadduct was isolated and characerized by mass-spectrometry. On the basis of DFT computations, the authors proposed that carbon atoms next to the nitrogen are the most probable addition sites.316 Wang et al. found that Y2@C79N-fulleropyrrolidines can be formed in Prato reaction with N-ethylglycine and 4diphenylamino-benzaldehyde.533 Reaction resulted in a dozen isomers of monoadducts, of which two major ones were isolated and characteized by ESR spectroscopy. 8.3. Clusterfullerenes

Since the discovery of Sc3N@C80 in 1999 as the first NCF which has the higher yield than that of all other EMFs, clusterfullerenes has been the fastest-growing family of endohedral fullerenes. While the family of clusterfullerenes consists of nitride clusterfullerenes (NCFs), carbide clusterfullerenes (CCFs), hydrocarbide clusterfullerenes (HCCFs), oxide clusterfullerenes (OCFs), sulfide clusterfullerenes (SCFs), and carbonitride clusterfullerenes (CNCFs),32 most studies of chemical functionalization of clusterfullerenes have been focused on NCFs mainly due to the advances of their high-yield synthesis.25,26,32,44,844,845 8.3.1. Diels−Alder Reactions. The first symmetric derivative of the NCF was obtained in the reaction between Sc3N@C80-Ih(7) and 13C labeled 6,7-dimethoxyisochroman-3one in 1,2,4-trichlorobenzene solution reported by Dorn et al. 6078


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in 2002 (Scheme 1B).390 The proposed mechanism stated that the reaction occurred via a [4 + 2] Diels−Alder cycloaddition to the [5,6] ring junction.390 This hypothesis was proved in the same year by an X-ray crystallographic study (see Scheme 1B), revealing that the Sc3N unit was positioned away from the site of the addition.321 A computational study by Campanera et al. has also shown that the [5,6] bond of Sc3N@C80-Ih(7) is the most preferable for the Diels−Alder addition.874 In 2005 Stevensen et al. reported the synthesis of the bisadduct of Gd3N@C80 by Diels−Alder reactions with 6,7-dimethoxyisochromanone at isolated yields of 10−20%.875 Utilizing the extraordinary kinetic chemical stability of NCFs with respect to the other fullerenes in Diels−Alder reactions with a cyclopentadiene (Cp)-functionalized resin, Dorn et al. developed in 2005 a single-step separation method for NCFs. While empty fullerenes and conventional metallofullerenes react with the cyclopentadiene-funtionalized styrene-divinylbenzene resin via Diels−Alder cycloaddition, the less reactive NCFs pass through the resin-filled column yielding relatively pure nitride cluster fullerenes.15 In 2006 Stevensen et al. used cyclopentadienyl and amino functionalized silica to selectively bind contaminant fullerenes (empty fullerenes and non-nitride cluster fullerenes) and obtained the purified NCFs (see also section 3.5).13 With the availability of the second isomer of M3N@C80D5h(6), the chemical reactivity of M3N@C80-D5h (M = Sc, Lu) was studied by Dorn et al. in 2006 and compared to that of M3N@C80-Ih (M = Sc, Lu).335 In particular, the reactivity of M3N@C80-D5h in Diels−Alder cycloaddition was analyzed in the interaction with the cyclopentadiene-functionalized resin. The results indicated an enhanced reactivity of the D5h isomers toward Diels−Alder and 1,3-dipolar tritylazomethine ylide cycloaddition reactions in comparison to the Ih isomers. This finding was rationalized by the smaller energy gaps of M3N@ C80-D5h revealed in a cyclic voltammetric study and theoretical molecular orbital (MO) calculations, suggesting that the isomerism of the carbon cage plays a more important role in determining the HOMO level than the internal metallic cluster.335 Recently, Osuna et al. reported extended computational study of kinetic and thermodynamic aspect of Diel-Alder addition to Ih(7) and D5h(6) isomers of M3N@C80 (M = Sc, Lu, Gd).876 The authors showed that higher reactivity of the D5h(6) isomer has kinetic reasons and is not related to the presence of pyracylenic [6,6] bonds since the lowest energy monoadducts have [5,6] structure for both Ih(7) and D5h(6) isomers. 8.3.2. Prato Reactions. In 2005, chemical reactions of NCFs based on the pyrrolidino derivatizations via the Prato reaction have been demonstrated independently by two groups.877−879 Echegoyen et al. studied the addition of the N-ethylazomethine ylide to Sc3N@C80-Ih(7) and showed that the formation of the pyrrolidine ring occurred regioselectively at the same [5,6] bond as in the case of Diels−Alder addition (Scheme 2B).879 Under the same conditions, the N-ethyl fulleropyrrolidine derivative of Y3N@C80-Ih(7) was also successfully synthesized and characterized by both NMR and X-ray crystallography.878 Importantly, Echegoyen et al. found that the regioselectivity of this type of cycloaddition reactions is controlled by the trimetallic nitride cluster. While Sc3N@C80 tends to direct the addition to the [5,6] ring junction, the Y3N@C80-Ih(7) prefers to form cycloadducts at the [6,6] bond.878 In the same year, Dorn et al. also reported the synthesis of the N-methyl fullereopyrrolidine derivatives of

M3N@C80 (M = Sc, Er) by reacting NCFs with Nmethylglycine and 13C-formaldehyde, and also observed formation of the [5,6]-adducts of the [email protected] Later, Martiń and Echegoyen et al. reported that N-ethyl Sc3N@C80fullereopyrrolidine undergoes an efficient retro-cycloaddition when heated in o-DCB under reflux in the presence of maleic anhydride.880 In 2006 Echegoyen et al. reported an unexpected and quantitative isomerization of the N-ethyl [6,6]-pyrrolidinoY3N@C80 to the [5,6] regioisomer; an analogous isomerization was also found in the case of Er3N@C80 (I).497 The crystallographic study of the [5,6]-pyrrolidino-Y3N@C 80 showed that the nitrogen atom was slightly displaced (by 0.13 Å) out of the plane of the three Y atoms.326 In the same year, Dorn et al. reported that the Prato reaction of Sc3N@C80Ih with N-triphenylmethyl-5-oxazolidinone yields two different monoadducts characterized by NMR spectroscopy and X-ray crystallography. The [5,6] isomer was shown to be the thermodynamically stable product, while the [6,6] adduct was the kinetic one and interconverted into the [5,6] isomer at elevated temperatures.391 A theoretical study of the interconversion between [5,6]- and [6,6]-pyrrolidino-M3N@C80 cycloadducts was reported by Poblet et al.496 The Prato reaction of mixed metal NCFs (MMNCFs) was also reported. In 2007, Wang et al. studied the regiochemistry of ScxGd3−xN@C80 (x = 0−3) in the 1,3-dipolar addition reaction with N-ethylglycine and formaldehyde.498 The regioisomers of the pyrrolidinofullerene monoadducts were assigned by their HPLC retention times. A thermal treatment of the reaction products was also carried out to compare the relative thermostability of the [6,6]- and [5,6]-regioisomers. Based on the experimental data and theoretical study, it was proved that the [5,6] adduct was favored for the smallest cluster of the series (Sc3N), whereas the [6,6] adduct was dominant in the case of the largest cluster (Gd3N). For mixed clusters, both adducts were formed, but their thermal treatment at 180 °C resulted in the isomerization to the [5,6] adduct. On the contrary, [5,6]-pyrrolidino-Gd3N@C80 isomerized into the [6,6] adduct upon heating. Thus, the regioselectivity of the ScxGd3−xN@C80 (x = 0−3) in exohedral cycloaddition reactions exhibited the strong dependence on the size of the encaged cluster.498 Similar results were later reported by the same group in the study of the ScxY3−xN@C80 (x = 0−3) series. While Sc3N@C80 and Sc2YN@C80 gave only the [5,6]pyrrolidine regioisomers, a critical change in fullerene regioselectivity occurred starting from the ScY2N@C80, which yielded the [6,6] adduct as a minor regioisomer, and then completed for the Y3N@C80, which formed the [6,6] regioisomer as the major product.333 Aroua and Yamakoshi reported kinetic study of Prato reaction M3N@C80 (M = Sc, Lu, Y, Gd).881 The study revealed that initial addition occurred at the [6,6] edge then followed by thermal rearrangement to a [5,6] adduct. Kinetic of the rearrangement was shown to depend on the cluster size, the larger M3N clusters leading to lower reaction rates. Furthermore, for Sc3N@C80 and Lu3N@ C80 rearrangement was complete, whereas for Y3N@C80 and Gd3N@C80 an equilibrium mixture of [5,6] and [6,6] isomers was obtained. The isomerization barriers and entropies were estimated based on the kinetic data.881 The Prato reaction was utilized to couple M3N@C80 (M = Sc, Y) NCFs with organic donor moieties for the formation of intramolecular electron donor−acceptor dyads. In 2008, a ferrocenylpyrrolidine adduct of Sc3N@C80 (Ih) was successfully 6079


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Scheme 3. Cyclopropanation Reaction of Y3N@C80a

a

Reproduced with permission from ref 878. Copyright 2005 American Chemical Society.

́ Torres and Echegoyen et al. via synthesized by Guldi, Martin, the 1,3-dipolar cycloaddition of azomethine ylides generated by sarcosine and ferrocene carboxaldehyde.671 In 2009 Echegoyen et al. synthesized two isomeric [5,6]-pyrrolidine-Sc3N@C80 conjugates containing triphenylamine (TPA) in different positions as the new donor system and synthesized two isomeric [5,6]-pyrrolidine TPA-Sc3N@C80 conjugates. The authors found that the dyad with the N-connected TPA donor had a significantly better thermal stability and a longer lifetime of the photoinduced charge separated state than the corresponding 2-substituted system.647 Later on, the same groups carried out the systematic synthesis, electrochemical, theoretical and photophysical studies of a series of donor− acceptor dyads based on M3N@C80 (M = Y, Sc) and different donors (tetrathiafulvalene, phthalocyanine, or ferrocene), which were synthesized not only by 1,3-dipolar cycloaddition reactions of azomethine ylides but also by Bingel−Hirsch reactions.596 Recently, dyads based on the zinc tetraphenylporphyrine connected to the pyrrolidine moiety of the [5,6]pyrrolidine-Sc3N@C80 via long linkers were synthesized by Wolfrum et al.648 Details of the photophysical studies of such dyads are discussed in section 6.5.2. The Prato reaction of NCFs other than M3N@C80-Ih(7) were reported so far only for Sc3N@C78384 and Sc3N@C80D5h(6).335 In 2006, Dorn et al. synthesized two mono pyrrolidine-adducts of Sc3N@C80-D5h(6) via the 1,3-dipolar cycloaddition of tritylazomethine ylide and characterized them by NMR spectroscopy, but their structures were not completely elucidated.335 The authors proposed that the addition occurred to the pyracylene moieties at the [6,6] bonds, but this suggestion was discarded in a recent computational study, which proved that Prato addition to the D5h(6) isomer also occurs at the [5,6] bond.882 In 2007, two N-tritylpyrrolidino monoadducts of Sc3N@C78-D3h(5) were synthesized by Dorn et al. utilizing the Prato reaction via 1,3-dipolar cycloaddition of tritylazomethine ylides.384 The proposed addition sites are two different [6,6] junctions of the Sc3N@C78 cage that are offset from the horizontal plane defined by the Sc3N cluster. The structure of one adduct was confirmed by single-crystal X-ray study.384 Two groups also studied the Prato reaction of carbide clusterfullerenes. In 2010 Wang et al. reported the synthesis of the Sc3C2@C80 fulleropyrrolidine by the reaction of Sc3C2@C80 with N-ethylglycine and 13C-enriched paraformaldehyde. The [5,6]-ring junction was determined to be the reaction site for the Sc3C2@C80 fulleropyrrolidine monoadduct, and the endohedral Sc3C2 cluster was predicted to be deformed by the pyrrolidine addition. Interestingly, it was found that the pyrrolidine addend changed the spin density distributions and

altered the paramagnetic properties of Sc3C2@C80 fulleropyrrolidine.531 Recently, the same group reported the isolation of several bis-adducts and their characterization by ESR spectroscopy and DFT calculations.532 In 2011, Akasaka et al. showed a remarkably high regioselectivity of the Prato reaction of the Sc2C2@C82-Cs(6) and characterized the only pyrrolidine adduct formed by single-crystal X-ray diffraction.354 In 2012, the same group studied the Prato reaction of Sc2C2@ C82-C2v(9).355 Three monoadducts isomers were isolated, and the structure of the major [5,6]-closed isomer was determined by single-crystal X-ray crystallography. Two other isomers presumably have [6,6]-open motif. 8.3.3. Bingel−Hirsch Reactions. The first monomethanofullerene derivative of EMF was synthesized by Echegoyen et al in 2005 via the Bingel−Hirsch cyclopropanation reaction of Y3N@C80-Ih with excess diethyl bromomalonate and 1,8diazobicyclo-[5.4.0]undec-7-ene (DBU) (Scheme 3).878 Surprisingly, the same cyclopropanation reaction for Sc3N@C80-Ih failed to give any identifiable adduct, but for Y3N@C80 an unexpected [6,6]-open adduct was isolated. These results confirmed that the regioselectivity of the cycloaddition reactions of NCFs is controlled by the trimetallic nitride cluster, as already revealed for the 1,3-dipolar cycloaddition reactions discussed above.878 A similar conclusion was also found by the same group in a more recent work: the electrochemically generated dianions of Lu3N@C80-Ih react readily with benzyl bromide forming a [6,6]-open methano derivative of Lu3N@Ih-C80(CHC6H5), whereas Sc3N@C80-Ih dianion is not reactive toward the electrophile. This phenomena was explained by their difference on the HOMO which is more highly localized on the fullerene cage for [Lu3N@C80-Ih]2− but more localized on the inside cluster for [Sc3N@C80-Ih]2−.824 Very recently, the same authors showed that trianion of Sc3N@C80 exhibits high reactivity toward benzyl bromide in agreement with theoretically predicted localization of the spin density in [Sc3N@C80-Ih]3− on the carbon cage.838 In 2006 Echegoyen et al. synthesized the first methanofullerene derivative of Er3N@C80 (Er3N@C80C(CO2C2H5)2) by the same strategy and showed that the cyclopropanation reaction occurred regioselectively at the [6,6] bond like that of Y3N@ C80.497 One year later, Echegoyen et al. reported the synthesis of another malonate monomethanofullerene, Y3N@C80-C(CO2CH2Ph)2, by using bromo-dibenzylmalonate and elucidated its structure by X-ray crystallography as a fulleroid.327 That is, in Y3N@C80-C(CO2CH2Ph)2 the [6,6] bond at the site of addition opened up and one of the yttrium atoms resided close to the addition site. Such position of the Y3N cluster in the [6,6]-open adduct is strikingly different from that in the 1,36080


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methano-bridged monoadducts of Sc3N@C80-Ih were isolated and characterized: one contains two ester moieties, whereas another monoadduct contains only one ester group and a hydrogen atom on the central carbon of the addend. NMR spectroscopy of the two monoadducts suggested that the addition occurred regioselectively at a [6,6] bond on the surface of Sc3N@C80-Ih, which resembles the corresponding Bingel− Hirsch adducts discussed above. The same conditions were also successfully applied to Lu 3 N@C 80 to produce similar products.885 One year later the same group reported the synthesis of a single, soluble regioisomer of the dibenzyl adduct Sc3N@C80(CH2C6H5)2 which was formed in high yield by photochemical activation of benzyl bromide. The X-ray crystallographic analysis of Sc3N@C80(CH2C6H5)2 confirmed unambiguously its 1,4-addition pattern on the hexagon ring. Under the same conditions, the dibenzyl adduct of Lu3N@C80Ih was also synthesized.389 Trifluoromethylation. Using the high-temperature Rf radical addition methodology based on the reaction with CF3I, in 2007 Strauss, Boltalina et al. reported the first trifluoromethylation reactions of two isomers of Sc3N@C80 (Ih and D5h) affording a series of trifluoromethylated derivatives. The products were preliminarily separated based on their different solubility in organic solvents. One of the fraction included predominantly Sc3N@C80(CF3)2 and small amounts of Sc3N@C80(CF3)4,6.826 An important finding of this study is that under their experimental conditions Sc3N@C80-Ih and Sc3N@C80-D5h react with CF3 radicals at essentially the same rate at 520 ± 10 °C. A bis-adduct was isolated by HPLC and 19F NMR study indicated a 1,4-addition pattern. DFT calculations on the two most stable optimized structures of Sc3N@C80(CF3)2 revealed that two of the three Sc atoms are bonded to the cage C atoms that are in para position with respect to the CF3-bearing carbon atoms. The addition of two CF3 groups resulted in a decrease of the optical gap of Sc3N@C80-Ih by 0.34 eV and a small increase of the optical gap of Sc3N@C80-D5h by 0.03 eV.826 Detailed electrochemical and in situ spectroelectrochemical study of the mono- and dications and the mono-, di-, and trianions of Sc3N@C80(CF3)2 was reported in 2010 by Popov et al.544 Trifluoromethylation reactions of NCFs were further developed by two groups. In 2009 Strauss, Boltalina et al. synthesized Sc3N@Ih-C80(CF3)n (n = 14, 16) by heating Sc3N@C80-Ih with Ag(CF3CO2) and determined their molecule structures by single crystal X-ray crystallography, which revealed their unprecedented addition patterns. Of particular interest is the observation of the multiple cage sp3 triplehexagon junctions, no cage disorder and little or no endohedral atom disorder, an isolated aromatic C(sp2)6 hexagon in Sc3N@ C80(CF3)16 as well as two negatively charged isolated aromatic C(sp2)5− pentagons bonded to one of the Sc atoms.393 In 2011 the same group reported the synthesis and isolation of several new Sc3N@(C80-Ih)(CF3)n (n = 4, 8, 10, 12, 14, 16), which were characterized by spectroscopic (MS, 19F NMR, UV−vis/ NIR, ESR), structural and electrochemical methods. Singlecrystal X-ray studies were also reported for Sc3N@C80(CF3)10 and Sc3N@C80(CF3)12. They showed a significant distortion of the Sc3N cluster shape from the lateral triangle. Furthermore, reactions of Sc3N@(C80-Ih)(CF3)10 with C6H5CH2Br and a Bingel−Hirsch reaction of Sc3N@(C80-Ih)(CF3)14 with diethyl malonate revealed that both reactions occur more readily with Sc3N@C80(CF3)n than with underivatized Sc3N@C80. Such an enhanced chemical reactivity of trifluoromethylated Sc3N@C80 may become adopted as an important route for future designs

dipolar cycloadducts, in which the Y3N cluster is oriented so that two of the metal ions straddle the [5,6] site.327 In 2009, the Bingel−Hirsch reaction was used to synthesize a series of donor−acceptor dyads of Y3N@C80 with organic donor moieties.596 In 2010 Echegoyen et al. showed that under modified reaction conditions, the Bingel−Hirsch reaction proceeds readily also with Sc3 N@C 80-I h . The authors synthesized a series of methano adducts of Sc3N@C80 and Lu3N@C80 and showed that in both NCFs the reaction yields [6,6]-open adducts.823 In 2011 it was also shown that trifluoromethylation of Sc3N@C80 increases its reactivity in the Bingel−Hirsch reaction. In particular, Sc3N@C80(CF3)14 readily reacts with diethyl malonate selectively to form a monoadduct in those conditions, when pristine Sc3N@C80 does not react at all.394 A Bingel−Hirsch cyclopropanation of Sc3N@C78-D3h was carried out by Dorn et al. in 2008 by reacting an excess of diethyl bromomalonate in the presence of DBU in odichlorobenzene, which afforded a single monoadduct and a dominate symmetric bis(ethyl malonate) adduct.820 The authors have also found that under the same cyclopropanation reaction conditions Sc3N@C80-Ih does not react at all, indicating that Sc3N@C78-D3h has significantly higher reactivity than Sc3N@C80. Importantly, for Sc3N@C78 closed cycloadducts are formed,820 while M3N@C80-Ih always form [6,6]open adducts. Besides, Dorn et al. also reported the synthesis of the diethyl malonate adducts of the non-IPR Sc3N@C68D3(6140) with up to penta-adducts formed.883 The monoadduct Sc3N@C68[C(COOC2H5)2] was isolated and characterized by HPLC, mass spectrometry, NMR spectroscopy, and DFT calculations, suggesting that the cyclopropanation of Sc3N@C68 occurs regioselectively at the [6,6]-junction close to the unique fused-pentagon junction with the formation of [6,6]-open adduct.883 In 2008 Echegoyen et al. compared the reactivity of Gd3N@ C80, Gd3N@C84, and Gd3N@C88 in bromodiethylmalonate cycloaddtion reactions.107 For Gd3N@C80, both the monoadduct and the bisadduct were isolated, whereas the same reaction of Gd3N@C84 under identical conditions yielded only one monoadduct suggesting its lower reactivity for the cyclopropanation reaction. More interestingly, Gd3N@C88 showed no sign of reaction under the same conditions even after a gradual increase of the temperature and longer reaction time, and this lower reactivity was attributed to the lower degree of pyramidalization of this larger cage. According to these results, the authors concluded that Gd3N@C80 appears to be the most reactive NCF, whereas Gd3N@C88 is completely nonreactive under Bingel reaction conditions, suggesting a gradual decrease in the reactivity of such NCFs as the size of the carbon cage increases.822 More recently, Poblet, Echegoyen et al. studied the Bingel−Hirsch reactions of the non-IPR Gd3N@C82 both experimentally and theoretically.884 Under the same conditions used for the Bingel−Hirsch cyclopropanation reaction of Gd3N@C84, they found that, similar to the case of Gd3N@C84, for Gd3N@C82 only a monoaddition when reacted with diethylbromomalonate. The computational study suggested that in both NCFs an addition occurs at near adjacent pentagon pair with the formation of [5,6] closed cycloadducts.884 8.3.4. Radical Addition Reactions. Addition of Carbon and Benzyl Radicals. In 2007 Dorn et al. reported the first reactions of NCFs with carbon radicals generated from diethyl malonate catalyzed by manganese(III) acetate. 885 Two 6081


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of various NCF-based materials for practical applications. In the same work, the trifluoromethylation of Er3N@(C80-Ih) with CF3I affording Er3N@(C80-Ih)(CF3)n (n = 12−14) as well as the photochemical perfluoroisopropylation of Sc3N@C80-Ih affording Sc3N@C80(i-C3F7)n (n ≤ 12) were also reported.394 In 2011 Yang and Troyanov et al. synthesized the trifluoromethylated Sc3N@C80(Ih) derivatives by reacting with CF3I at 400 °C. In addition to the reported isomers of Sc3N@(C80-Ih)(CF3)n (n = 14, 16), they isolated four new isomers of Sc3N@Ih-C80(CF3)14 and one new isomer of Sc3N@(C80-Ih)(CF3)16 and characterized their structures by X-ray crystallography.395 A detailed comparison of all the isolated isomers revealed a strong influence of the exohedral CF3 addition pattern on the behavior of the Sc3N cluster inside the C80-Ih fullerene cage.395 Later in the same year, these authors have successfully synthesized and isolated the first multi-CF3 derivative of Sc3N@C80-D5h with 18 CF3 groups, which was characterized by X-ray crystallography.336 Interestingly, Sc3N@(C80-D5h)(CF3)18 has some common features with Sc3N@(C80-Ih)(CF3)14 such as the presence of “empty” pentagons (only two), sp3 THJs (two), and isolated C−C double bonds and benzenoid rings (Figure 56). The study addressed many changes in the molecular structure of Sc3N@ C80-D5h induced by trifluoromethylation, such as smaller angular deviations from trigonal symmetry in the derivative (Sc−N−Sc angles are 110−129° versus 107−132° in pristine Sc3N@C80-D5h), coordination of Sc atoms to one [5,6] C−C and two [6,6] C−C bonds (a reversed situation is observed in the pristine NCF), as well as the change of the position of the Sc3N plane with respect to the (former) D5 symmetry axis of the carbon cage (see Figure 56).336 In summary, the interplay of the CF3 addition patterns with cage distortions and the position of the encaged Sc3N cluster studied in the aforementioned reports clearly revealed the mutual influence between the internal Sc3N cluster orientation and the CF3 radical addition pattern,336,393−395,826 which is crucial for the understanding the structure−reactivity relationship of the trifluoromethylated NCFs. 8.3.5. [2 + 2] Cycloaddition Reactions. The first synthesis of [2 + 2] benzyne cycloaddition products of Sc3N@C80-Ih were reported by Echegoyen et al., affording two monoadducts with a four-membered ring attached to the cage surface on both the [5,6]- and [6,6]-ring fusions as revealed by single-crystal X-ray analysis.392 Both adducts contain planar cyclobutenyl units in which the fullerene C−C bond is elongated to 1.65−1.66 Å. Orientations of the Sc3N cluster with respect to the addition sites are different in [5,6] and [6,6] adducts, but in both structures Sc atoms avoid close contacts to the sp3 carbon atoms.392 More recently, Wang, Yang, Balch et al. synthesized an unprecedented open-cage endohedral fullerene derivative involving a 13-membered ring via the [2 + 2] cycloaddition reaction of Sc3N@C80-Ih with 4,5-diisopropoxybenzyne generated in situ from the reaction of 2-amino-4,5-diisopropoxybenzoic acid with isoamyl nitrite.396 Surprisingly, while the reaction gave only the expected [2 + 2] benzyne adducts under an inert atmosphere, the [5,6]-regioisomer reacted further under an aerobic atmosphere. A single crystal X-ray study revealed that oxygenation produced an intriguing open-cage endohedral fullerene with the largest orifice yet encountered for endohedral fullerenes.396 8.3.6. Photochemical Reactions. Using the same photochemical method as applied for the disilylation of conventional

Figure 56. Molecular structures of trifluoromethylated Sc3N@C80 derivatives as determined by single-crystal X-ray diffraction: (a) Sc3N@(C80-Ih)(CF3)10; (b) Sc3N@(C80-Ih)(CF3)12; (c) Sc3N@(C80Ih)(CF3)14-I; (d) Sc3N@(C80-Ih)(CF3)16; (e) Sc3N@(C80-D5h)(CF3)18; (f) Sc3N@(C80-Ih)(CF3)14-III. In (a−d) 50% ellipsoids are shown for selected atoms; in (e,f) projections are given perpendicular to Sc3N planes. Reproduced with permission from refs 394 and 393 (Copyright 2009, 2011 American Chemical Society) and ref 336 (Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA).

EMFs, Akasaka et al. reported in 2005 the first disilylation of Sc3N@C80 by 1,1,2,2-tetrakis(2,4,6-trimethylphenyl)-1,2-disilirane and compared the chemical reactivity of Sc3N@C80 with [email protected] Their results indicated that Sc3N@C80 has a much lower thermal reactivity toward disilirane than La2@C80, which was rationalized by comparing the energy and spatial distribution of their LUMOs. In a later report by the same group, the photochemical reaction of Sc3N@C80 with 1,1,2,2tetramesityl-1,2-disilirane was studied, affording a bis-silylated product Sc3N@C80(Mes2Si)2CH2 with both 1,2- and 1,4cycloadducts isolated and completely characterized by NMR measurements and single crystal X-ray structure analysis.388 The 1,2-addition was shown to proceed to the [5,6] bond, but this adduct was thermodynamically less stable and isomerized to the 1,4 adduct upon heating. Importantly, experimental results and theoretical calculations showed that the circular motion of Sc3N cluster in Sc3N@C80 was restricted by exohedral addition.388 Photochemical disilylation of Lu3N@ C80 by (R2Si)2CH2 (R = Mes, Dep) was studied in 2012.839 For each R, reaction produced a mixture of two isomeric products. The major isomers were assigned to the 1,4(AA) adduct (1,4addition to the pentagon/hexagon/hexagon junction). Photo6082


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showing significantly higher longitudinal and transverse relaxivities (r1 and r2) compared to commercially available Gd-chelates.889 In 2008 MacFarland reported the synthesis of water-soluble derivatives Gd3N@C80[N(OH)(CH2CH2O)nCH3]x (n = 1, 3, 6; x = 10−22) titled by the authors as “Hydrochalarone” by attaching multiple oligoethylene glycol groups through nitrogen chemistry.890 In 2009 Dorn et al. reported the synthesis of carboxylated water-soluble derivatives of NCFs by modifying Gd3N@C80 with carboxylic acid terminal groups.891 The synthesized Gd3N@C80(OH)∼26(CH2CH2COOM)∼16 (M = Na or H) exhibited the longitudinal and transverse relaxivities r1 and r2 of 207 and 282 mM−1 s−1 (per C80 cage) at 2.4 T, respectively, which are 50 times larger than those of Gd3+ poly(aminocarboxylate) complexes such as commercial Omniscan and Magnevist.891 More recently, the same group reported the functionalization of the 1 7 7 Lu-radiolabeled NCF ( 177Lu xLu 3−xN@C 80) which was first hydroxylated and carboxylated to form 177LuxLu3−xN@ C80(OH)z(CH2CH2COOM)y (M = H, Na; z ∼26, y ∼16) and then conjugated to a fluorescent tag [tetramethyl-6carboxyrhodamine (TAMRA)]-labeled interleukin-13 (IL-13) peptide.892 Their results demonstrated that the radiolabeled 177 Lu ions are not readily removed from the fullerene cage and therefore this compound could be used for potential targeting with a conjugated IL-13 protein.892 An analogues protocol was used by the same group to conjugate IL-13 protein with [email protected] A new type of water-soluble derivatives of NCFs, GdxSc3−xN@C80-BioShuttle-conjugate (abbreviated as Gdcluster@-BioShuttle), was recently reported by Braun et al.894 The compound was synthesized by combined chemical methods, i.e., added functional modules, the derivatized endofullerene cargo as well as the peptide-based modules of the NLS address and the transmembrane transport component by solid phase peptide synthesis. Their results revealed that Gdcluster@-BioShuttle showed a high proton relaxation (>500fold) and high signal enhancement at very low Gd concentrations compared to the commonly used contrast agents such as Gd-DTPA.894 8.3.10. Supramolecular Complexes of Nitride Cluster Fullerenes. In 2011, two groups reported the formation of supramolecular complexes of nitride clusterfullerenes with bisporphyrins. Echegoyen and Ballester et al. synthesized 1:1 inclusion complexes of Sc3N@C80 with cyclic β-pyrrole unsubstituted meso-tetraphenyl bisporphyrin in which porphyrin units are connected by two 2,3-hexadiynyl-1,6-dioxo or two hexyl-1,6-dioxo spacers.667 Depending on the spacer, different orientations of porphyrin moieties were realized in the soldstate complexes. At the same time, Boyd, Guldi et al. reported the complex formation of Sc3N@C80 or Lu3N@C80 and acyclic bisporphyrins bonded to calixarene scaffold.651 Both groups also determined the stability constants of the complexes in solution and analyzed the photoexcited electron transfer (see section 6.5.2. for more details).651,667

chemical disilylation reaction was also carried out on Sc2C2@ C82 (which was wrongly assigned as Sc2@C84 at that time)829 and for [email protected] Akasaka et al. reported another type of photochemical reaction based on a CCF in 2005. Irradiation of a 1,2,4trichlorobenzene/benzene solution of Sc3C2@C80 and excess molar amount of 2-adamantane-2,3-[3H]-diazirine in a degassed sealed tube at room temperature using a highpressure mercury arc lamp resulted in the cycloadduct Sc3C2@ C80(Ad).319 The adduct was purified by preparative HPLC, and single crystals were obtained for the X-ray crystallography study, revealing its carbide nature.319 Adamantylidene addition was also reported by Akasaka et al. for Sc2C2@C82-C3v(8)353 and Sc2C2@C80-C2v(5).362,386 8.3.7. Azide Addition to Sc3N@C80. Very recently, Wang, Balch et al. reported azide addition to Sc3N@C80(Ih).397 In the dark, the reaction of Sc3N@C80 with 4-isopropoxyphenyl azide in o-dichlorobenzene afforded formation of two products, which were identified by NMR spectroscopy as [5,6]-open and [6,6]-open azafulleroids. The structure of the [6,6]-open adduct was also elucidated by single-crystal X-ray diffraction. When reaction was proceeded under photoirradiation, only [6,6]-open adduct was formed. Heating of the azafulleroids in solution resulted in their isomerization.397 8.3.8. Organometallic Complexation of Sc2C2@C82. The first study of the coordination chemistry of EMFs was reported by Yeh et al.405 The authors studied complexation of Sc2C2@C82-C3v(8) with trinuclear Recarbonyl complex [(μH)3Re3(CO)11(NCMe)]. After 3.5 h in chlorobenzene at reflux, the [(μ-H)3Re3(CO)9(η2,η2,η2-Sc2C2@C82)] adduct was obtained in 57% yield, and its structure was determined by single-crystal X-ray diffraction. The Re3 triangle is positioned over the hexagon on the 3-fold axis of the carbon cage in such a way that each Re atom is coordinated to a [6,6] edge.405 8.3.9. Water-Soluble Derivatives. The synthesis of watersoluble derivatives of NCFs aimed at their medical applications was accomplished in several reports. In 2002, a hydroxylated derivative of Sc3 N@C80 -I h has been synthesized from polyanionic radical intermediates by refluxing a toluene solution of Sc3N@C80 with sodium metal under an argon atmosphere, which yielded a black polyanionic radical species precipitated from solution. Oxidation of this product by air and exposure to water resulted in the formation of the water-soluble adduct Sc3N@C80(OH)∼10(O)∼10.886 Following the same procedure, Wang et al. synthesized in 2007 two water-soluble hydroxylated derivatives of Gd-based mixed metal NCFs [ScxGd3−xN@ C80Om(OH)n (x = 1, 2; m ≈ 12; n ≈ 26)].887 Their properties of ScxGd3−xN@C80Om(OH)n as MRI contrast agents were investigated, and the R1 values of Sc2GdN@C80O12(OH)26 and ScGd2N@C80O12(OH)26 were determined to be 20.7 and 17.6 mM−1 s−1, respectively.887 In addition to the hydroxylation of NCFs discussed above, Fatouros, Dorn et al. functionalized in 2006 Gd3N@C80 by poly(ethylene glycol) units to improve its water solubility and biodistribution. 888 The synthesized derivative Gd 3 N@ C80[DiPEG5000(OH)x] (Figure 55b) offer great potential for serving as MR imaging contrast agents because their T1-T2 relaxivity is approximately 40 times greater than conventional gadolinium-containing MR imaging contrast agents (see section 10.1.).888 More recently the same group optimized the synthesis of such hydroxylated and pegylated Gd3N@C80 derivatives, Gd3N@C80[DiPEG(OH)x], with four different chain lengths of PEG (molecular weights: 350−5000 Da), all

9. MAGNETIC PROPERTIES OF ENDOHEDRAL FULLERENES Magnetic properties of EMFs are determined by several factors. First, transfer of an odd number of electrons to the carbon cage (e.g., three electrons in MIII@C82) results in the paramagnetic 6083


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Table 14. Effective Magnetic Moments of EMFs (in μB) Determined by SQUID Measurements a

EMF La@C82

Ce@C82

Gd@C82

Gd3N@C80d

Tb@C82 Tb3N@C80d Dy@C82 Ho@C82f

Ho3N@C80d Er@C82 ErSc2N@C80d Er2ScN@C80d Er3N@C80d Er3N@C80d Tm3N@C80d

Curie−Weiss μeff 0.38 0.12 1.91c 1.83 0.67 3.25c 3.36 6.90 6.66 6.91 7.9 ∼6.7e ∼4.7e 7.51

Brillouin T, K

5−40 4−25 200−300 >190 2−20 200−300 >190 5−200 >40 >2 0.2−1.2 3 >200 >20

9.25

>20

6.30 6.33

>20 >20

7.06 9.8 7.3 6.7 3.4 ∼9.6

>20 4−50 4−50 4−50 2−300 120−300

μeff

theor μeffb

T, K

0.2

L

S

2

∼0.8 ∼2.5 7.70 6.74 6.95

2 >60 3−200 >8 40

6.80 6.0 8.48 6.41 5.43 5.55 6.7 6.37

55 2−160 75 2−5 12−50 40 2−320 125

∼3.5

2.54

3

0.5

7.94

0

3.5

9.72

3

3

10.64 10.60

5 6

2.5 2

9.58

6

1.5

9.62

5

2

2

ref 904 568 568 910 568 568 910 910 903 718 716 916 594 594 716 913 716 921 718 716 913 716 914 914 914 915 323

Cage isomers of all EMFs are either C82-C2v(9) or C80-Ih(7); bTheoretical μeff is computed as g·[J(J + 1)]−1/2 for an isolated M3+ ion in its ground state; for the free electron (L = 0, S = 1/2), μeff = 1.73 μB. cMeasurements are performed for solvate M@C82·(CS2)1.5. dMagnetic moments are normalized to the number of atoms. eExact values are not listed in the original paper and are estimated from the figures. fA mixture of Ho-EMFs with Ho@C82 as a main component. a

Table 15. Magnetic Properties of EMFs Determined by XMCD Measurements Ce@C82-C2v(9) Ce2@C80-Ih(7) Gd@C82-C2v(9) Dy@C82-C2v(9)

Dy2@C88-Ic Dy2@C88-IIc DySc2N@C80-Ih(7) Ho@C82-C2v(9) Er@C82-C2v(9) Er2@C82-C2v(9) Er2C2@C82-C2v(9)c Er2C2@C82-Cs(6) ErYC2@C82-Cs(6) Er2@C90

J (L/S)a

μeffb

T, K

theor μeff/J

ref

CW CW CW CW CW CW 3.9 (2.7/1.2)c 3.1 (1.9/1.2) 1.0 (0.3/0.7 3.4 (2.3/1.1) 3.3 (2.2/1.1) 3.3d 3.8 (2.8/1.0)c CW CW CW CW CW 3.5 (2.8/0.7)c

0.82 1.00 0.75 0.51 6.8 9.5 5.8 4.8 1.9 5.2 5.0 5.0 5.3 8.5 8.5 8.4 7.8 7.8 4.8

6−20 20−60 6−20 20−60 10−40 10−25 6 4 300 4 4 2 6 10−40 10−40 10−40 10−40 10−40 6

2.54/2.5

743 743 743 743 912 912 719 748 748 748 748 919 719 192 192 192 192 192 719

7.94/3.5 10.64/7.5

10.6/8 9.58/7.5

a J is defined as a sum of L and S, which are determined from XMCD measurements according to sum rules. bμeff is determined either from Curie− Weiss dependence of the total magnetization (denoted as CW in the second column), or from J values as μeff = gJ[J(J+1)]0.5, where gJ is 20/15 (Dy3+), 1.25 (Ho3+), 1.2 (Er3+); cThe values of S are taken from ab initio calculations. dFor DySc2N@C80, experimentally determined value is magnetic moment at saturation msat = gJ•J = 4.4 μB, which is divided here by gJ(Dy3+) = 20/15 to get J.

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measured by ESR spectroscopy, which showed for powder and frozen CS2 solution the cage spin quenched via dimerization yielding the spin of S = 7/2 per Gd@C82 molecule.536 When the dimerization was suppressed in diluted solution, the measurement revealed the dominance of the S = 3 state with antiferromagnetic coupling of Gd3+ and carbon cage spins. The S = 4 state with ferromagnetic coupling was found to be 14.4 cm−1 higher in energy.536 For the state with S = 3, the theoretical magnetic moment is 6.93 μB, which is rather close to the experimentally determined values. Coupling of the metal and the cage spins in M@C82, especially in Gd@C82, was also studied theoretically by several groups. B3LYP calculations by Senapati et al. showed that antiferromagnetic coupling in Gd@C82 is more preferable by 16.1 cm−1,906 but their computations were performed for the wrong position of Gd inside the cage.907 For the more experimentally relevant structure of Gd@C82, Mizorogi and Nagase showed that the S = 4 state is ca. 3.5 cm−1 more stable than S = 3 at the B3LYP level with effective core potential for Gd.908 Sebetci and Richter used full electron treatment of Gd with Hubbard U correction and took into account a spin−orbit coupling and noncollinear effects.909 The authors have shown that the antiferromagnetic coupling is more preferable and explained this effect by a small hybridization of the unoccupied 4f-spin-down states of Gd with the highest occupied π-states of the carbon cage.909 Gd@C82 is a relatively simple case since the orbital angular momentum (L) in the half-filled 4f-shell is equal to zero. For other lanthanides, the nonvanishing orbital moment adds to the spin moment resulting in a more complex situation. In 1999 and 2000 Huang et al. reported comparative SQUID studies of Gd@C82, Ho@C82, Tb@C82, Dy@C82, and [email protected],718 Similar to Gd@C82, at low applied field (H = 0.1−0.5 T), the magnetization behavior of all M@C82 followed Curie−Weiss Law at temperatures above 20 K. Below 20 K, the reciprocal magnetic susceptibility plot versus temperature showed a significant deviation from the straight line for all M@C82 EMFs except for Gd@C82. Parameters determined from the linear fitting showed rather large negative Weiss temperatures increasing with the orbital angular moment of M3+ (Gd@C82 (L = 0): −1.8 K, Tb@C82 (L = 3): −6.6 K, Dy@C82 (L = 5): −10.4 K, Ho@C82 (L = 6): −5.9 K, Er@C82 (L = 6): −20.7 K).716 Large Weiss temperature can be an indication of the influence of antiferromagnetic intermolecular interactions. However, dependence of the Weiss temperature on the angular momentum in the M@C82 series shows that magnetic anisotropy can be also a likely explanation in this case. Effective magnetic moments of M@C82 molecules were determined by the Curie−Weiss Law as well as from the fitting isothermal magnetization by Brillouin function. The latter method gave temperature-dependent μeff values, which were increasing with temperature and leveled off above 50−100 K. Both methods gave comparable effective magnetic moments (Table 14) which were noticeably smaller (by 15 to 40%) than theoretically expected values for free M3+ ions. To some extent, quenching of the magnetic moment can be explained by an antiferromagnetic coupling to the carbon cage spin as already described for Gd@C82, but reduction of the magnetic moment for Ho@ C82 and Er@C82 is much stronger. The carbon cage crystal field and the hybridization between the metal and the fullerene may partially quench the orbital angular momentum of the M3+ ions. The pronounced L-dependence of the μeff reduction is also in line with this hypothesis since large L values lead to a stronger

state of the molecule. The associated magnetic moment is localized in this case on the carbon cage. Second, metal ions with localized partially filled semicore shells (such as 4f shell in lanthanides) are intrinsically paramagnetic, and EMF molecules do inherit magnetic properties to some extent from endohedral species. Third, if there are several paramagnetic metal ions in one EMF molecule, their exchange interactions strongly affect the resulting magnetic moment of the molecule. Finally, intermolecular exchange interactions can be also important for the crystalline state. All or some of these factors can be present in one EMF, and the resulting magnetic properties are a result of their complex interplay. Some of the magnetic properties were already discussed in section 6.2 devoted to the ESR spectroscopy of EMFs. The paramagnetism of EMFs also results in the improved proton relaxivity of the surrounding media, and this property will be discussed in the section 10.1 on the medical applications of EMFs. In this section we will mostly describe results of SQUID (Table 14) and X-ray magnetic dichroism (XMCD, Table 15) measurements aimed at the determination of the effective magnetic moments of EMFs. The main conclusion which follow from the measurements described below is that (i) solvent-free EMFs usually exhibit paramagnetic behavior without hysteresis in magnetization curves and (ii) effective magnetic moments are often reduced in comparison to those of free lanthanide ions, which is to a large extent explained by crystal field induced anisotropy and hybridization with the π-electrons of the carbon cage. 9.1. SQUID Studies of Conventional Metallofullerenes

The first dedicated study of magnetic properties of EMFs dates back to 1995 when Funasaka et al. reported on the isolation of macroscopic amounts and SQUID measurements of Gd@ C82903 and [email protected] The magnetization (M) of powder samples was measured at magnetic fields (H) from −5.5 to +5.5 T in the temperature range of 3−300 K. For both EMFs a paramagnetic behavior was found. The inverse susceptibility followed a Curie−Weiss law with Weiss temperatures (θ)/ effective magnetic moments (μeff) of −0.65 K/6.90 μB for Gd@ C82 and 0.89 K/0.38 μB for La@C82. The magnetization of Gd@C82 measured at different temperatures gave a single universal curve in M vs H/T coordinates which was fitted by a Brillouin function with parameters J = 3.38 and g = 2.0 and hence giving an effective magnetic moment of g·[J(J+1)]1/2 = 7.70 μB. The SQUID study of Gd@C82 with similar μeff values determined from the Brillouin function fitting (6.74 μB) and from Curie−Weiss law (6.66 μB at temperatures above 40 K) was reported by Huang et al. in 1999.718 Thus, effective magnetic moments of Gd@C82 determined from the Curie− Weiss law and from the Brillouin function were somewhat different and were slightly smaller than that of free Gd3+ ion in its 8S7/2 state (J = S = 7/2, g = 2.0, μeff = 7.94 μB). The reduction of the effective magnetic moment in Gd@C82 can be understood taking into account that the total spin of Gd@C82 should result from either a summation of the metal ion (S = 7/ 2) and carbon cage (Scage = 1/2) spins, or from their difference. Müller et al. have also reported SQUID study of Gd@C82 and found that the dependence of magnetization on magnetic field and temperature was best described by a Brillouin function assuming a total angular momentum J = 7/2 and g = 2.905 The same function also described the magnetization of Eu@C74 and Eu@C82 pointing to the divalent state of Eu with the same electronic configuration as that in Gd3+.905 As discussed above in section 6.2, the magnetic properties of Gd@C82 were also 6085


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Figure 57. Top panels: M4,5 XAS spectra of Gd@C82 and Dy@C82 recorded with the sample magnetization parallel (open circle) and antiparallel (solid circle) to polarization vector of the incident radiation at normal incidence with respect to the sample surface. Bottom panels: the difference between the two spectra shown in the respective top panels is the XMCD; the insets show the integral of the respective XMCD spectrum indicating the integral over the M5 edge (ΔA5) and the integral over the M4 edge (ΔA4) used to determine L through sum rules. Reproduced with permission from ref 719. Copyright 2004 The American Physical Society.

solvents below their freezing point revealed that the Curie constant was dependent on the magnetic field applied during cooling. Thus, the magnetic field induced preference of certain orientations of Ce@C82 molecules is a result of a magnetic anisotropy of the Ce3+ ion. Interestingly, no field cooling dependence of the Curie constant was found in similar conditions for Gd@C82 in agreement with completely isotropic nature of the Gd3+-f7 configuration. In the solid solvent-free film, Ce@C82 exhibited a Curie−Weiss behavior above 190 K with the Curie constant of 1.42 cm3 mol−1 K (μeff = 3.36 μB), which is close to the C = 1.33 cm3 mol−1 K value found for the CS2 solvate. However, in contrast to the solvate, the Weiss temperature of the solvent-free Ce@C82 was only θ = −1 K showing that antiferromagnetic interactions in Ce@ C82·(CS2)1.5 were induced by the special arrangement of the solvent molecules. Magnetization curves of Ce@C82 were measured at 2, 60, and 200 K. At 200 K the magnetization curve was well fitted by the sum of two Brillouin curves with J = 5/2, g = 6/7 and S = 1/2, g = 2.0023 (i.e., a sum of the f electron of Ce3+ and one π-electron of the carbon cage). At 60 K the best fit was provided by only one Brillouin curve with J = 5/2 and g = 6/7. At 2 K, the magnetization curve was well below the curve expected for free Ce3+ ion and corresponded to only 1/3 of its magnetic moment. To explain the quenching of the magnetic moment at low temperatures, the susceptibility contribution of the f-electron spin of Ce3+ was obtained as a difference between χ(Ce@C82) and χ(La@C82) and then fitted to the crystal-field Hamiltonian of tetragonal symmetry. The fitting showed that the energy splitting between three Kramers doublets of the Ce3+−2F5/2 state are 103 and 316 K, and therefore only the ground energy doublet contributed to the magnetization at 2 K.910 In 2007 Ito et al. reported on the SQUID study of the solvent-free microcrystalline [email protected] The authors have

anisotropy and a stronger quenching of the orbital momentum.716 An extended SQUID and ESR study of La@C82·(CS2)1.5 and Ce@C82·(CS2)1.5 crystallosolvates was reported in 2000 by Nuttal et al.568 In the 200−300 K temperature range, the susceptibility of La@C82·(CS2)1.5 followed the Curie−Weiss Law with strong antiferromagnetic interactions (θ = −130 K) and a Curie moment C = 0.46 cm3 mol−1 K (μeff = 1.91 μB), which is close to the value expected for the localized paramagnetic electron (μeff = 1.73 μB) within the experimental uncertainties. The susceptibility strongly deviated from the Curie−Weiss Law at the 100−180 K, and at ca. 150 K ESR line width exhibited abrupt change assigned to the orientational phase transition in the crystal. In the slow-cooling regime (several hours), the Curie−Weiss behavior was retained below 100 K, albeit with a much smaller Curie moment (C = 0.025 cm3 mol−1 K). On the contrary, in the fast cooling regime, the Curie moment and Weiss temperature of the high-temperature phase were retained below 100 K. For Ce@C82·(CS2)1.5, the Curie−Weiss behavior was also observed in the 200−300 K range corresponding to a Curie moment of C = 1.33 cm3 mol−1 K (μeff = 3.25 μB) corresponding well to the sum of the momenta of a single f electron on Ce3+ and an electron of the carbon cage (C = 0.385 + 0.803 = 1.188 cm3 mol−1 K). A strong antiferromagnetic interaction manifested itself in a large negative Weiss temperature, θ = −110 K. A phase transition similar to that found for La@C82 was also observed for Ce@ C82·(CS2)1.5, and in the low temperature phase remaining Curie moments of 0.06 and 0.25 cm3 mol−1 K (0.69 and 1.41 μB) were observed after fast and slow cooling.568 In 2003 Inakuma et al. examined the field induced changes in the magnetic susceptibility of Ce@C82 solution as well as temperature and field dependence of the magnetization of the solid films.910 The Curie−Weiss analysis of Ce@C82 in different 6086


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found that above 200 K the susceptibility follows the Curie− Weiss law with a strong ferromagnetic coupling (Weiss temperature θ = −309 K). The spin density per Sc@C82 molecule estimated from the Curie constant was 0.56 in this temperature range. In the 150−175 K range, the susceptibility exhibited a sharp decrease correlated to the orientational phase transition in the crystal. In the 60−110 K range, the susceptibility followed again the Curie−Weiss law but with a smaller Weiss temperature (θ = −107 K) and a spin density per molecule estimated to be 0.25. The values for the lowtemperature phase were somewhat dependent on the cooling rate.911

XMCD studies of Er@C82-C2v(9), Er2@C82-C2v(9), Er2C2@ C82-C2v(9), Er2C2@C82-Cs(6), and ErYC2@C82-Cs(6) at 1.9 T in the temperature range of 10−40 K were reported in 2008 by Okimoto et al.192 The temperature-dependent magnetization at 1.9 T measured in a cooling process was fitted by the Curie− Weiss law. A large Weiss temperature of −18.8 K obtained for Er@C82 points to the antiferromagnetic-like intermolecular interactions caused by the open-shell configuration of the carbon cage. In dierbium EMFs with closed-shell cage configurations Weiss temperatures were reduced to −3 ∼ −7 K. Effective magnetic moments determined from the fit were in the range of 8.4−8.5 ± 0.3 μB for all Er-EMFs with C82-C2v(9) carbon cage and 7.8 ± 0.3 μB for ErC2@C82 and ErYC2@C82 with Cs(6) carbon cage. Thus, the carbon cage imposed a significant effect on the magnetic moment, while the presence of one or two magnetic ions or the carbide unit in the EMF molecule did not affect the values.192 In 2011 Ishikawa et al. studied temperature dependence of the XMCD spectra of Ce@C82 and [email protected] Curie−Weiss plot showed significant deviation from linearity, and the temperature ranges of T < 20 K and T > 20 K had to be analyzed separately. Below 20 K, fitted Weiss temperatures were 2.2 ± 0.6 K for Ce@C82 and 0.8 ± 0.5 K for Ce2@C80, while magnetic moments of ca. 0.8 μB were substantially smaller than expected for free Ce3+ (μ = 2.54 μB).743

9.2. XMCD Studies of Conventional Metallofullerenes

In several works, local element-specific magnetism of lanthanide-based EMFs was studied by XMCD. The first such study was reported in 2004 by Dhesi et al.719 Magnetic properties of Gd@C82, Dy@C82, Ho@C82, and Er2@C90 were studied by XMCD at the M4,5 absorption edge (3d → 4f excitations) at T = 6−300 K and with an applied magnetic field up to 7 T (Figure 57). The authors did not find any anisotropy in magnetization curves pointing to the complete paramagnetic behavior in agreement with earlier SQUID studies. A sum rule analysis of the XCMD spectra showed that the orbital momentum of metal atoms was quenched by ca. 50% in comparison to the free M3+ ions (Table 14). A theoretical study has also revealed a proportional reduction of the spin momenta, which altogether resulted in a drastic drop of the total magnetic moments. To explain this phenomenon, the authors performed calculations taking into account a crystal field splitting and hybridization with the π-orbitals of the carbon cage. Both factors were found to give a similar reduction of magnetic moments; however for Gd (L = 0) a crystal field splitting was not able to explain the reduction of the magnetic moment.719 In 2006, similar results were achieved by Bondino et al. in the XMCD study of Dy@C82 and two isomers of [email protected] At 4 K and a magnetic field of 6.9 T, both mono- and dimetallofullerenes exhibited similar spin moments, while the orbital momentum of Dy@C82 was ca. 15% smaller. Both spin and orbital moments are ca. 55% smaller than expected for the free Dy3+ ion giving thus substantially reduced local magnetic moments in Dy-EMFs (4.2 μB in Dy@C82 and 4.5 μB in Dy2@ C88 at 4 K). The study of isothermal magnetization curves and fitting of the curves with Brillouin function showed that at 6.9 T magnetic moments were not completely saturated and hence are underestimated by 2−5%. Other reasons of the reduced magnetic moments of Dy3+ ions in EMFs, including carbon cage crystal field and substrate effect, were also proposed.748 In 2007 Kitaura et al. reported an XMCD study of the monometallofullerenes Gd@C 82 and Dy@C 82 and the corresponding M@C82@SWNT peapods.912 Magnetic moments were deduced from the fitting of the experimental spectra with those computed theoretically. The temperature dependence of magnetization was then fitted by the Curie− Weiss Law giving Weiss temperatures of −4.6 ± 1.5 K for Dy@ C82 and 3.5 ± 0.9 K for Gd@C82. Magnetic moments obtained from the Curie−Weiss Law, 9.5 ± 0.4 μB in Dy@C82 and 6.8 ± 0.5 μB in Gd@C82, are much larger than computed by sum rule in other XMCD studies719,748 and are similar to those estimated from SQUID measurements. Interestingly, magnetic moments of the metal atoms inside the peapods, 11.4 ± 0.4 μB for Dy@ C82@SWNT and 7.2 ± 0.3 μB for Gd@C82@SWNT, were higher than those in bulk EMFs.912

9.3. Nitride Clusterfullerenes

The first determination of magnetic moments of nitride clusterfullerenes was reported by Wolf et al. in 2005.113,913 The authors studied the temperature-dependent magnetization of Ho3N@C80 and Tb3N@C80 by SQUID. Both compounds were found to be paramagnetic and their effective magnetic moments, 17 μB for Tb3N@C80 and 21 μB for Ho3N@C80, were estimated by fitting the magnetization by the Langevin function.113 Fitting of the data with Brillouin functions fixed to the g-factors of the free Tb3+ (g = 1.5) and Ho3+ (g = 1.25) yielded J = 12, μ = 18 μB for Tb3N@C80 and J = 16, μ = 20 μB for [email protected] Magnetic moments of the nitride clusters are significantly smaller than the values expected for collinear alignment of the moments of three independent M3+ ions (namely, 27 and 30 μB for Tb and Ho, respectively). The authors explained this phenomenon by considering the ligand fields of the nitrogen resulting in the preferred alignment of the magnetic moments along the metal−nitrogen bonds (Figure 58). Studying the family of mixed-metal NCFs ErxSc3−xN@C80 in 2008, Tiwari et al. have found a paramagnetic behavior for all NCFs with negligible Weiss temperatures, however effective magnetic moment per Er atom was gradually decreasing with the increase of the number of Er atoms in the molecule, from 9.8 ± 0.5 μB in ErSc2N@C80 to 6.7 ± 0.2 μB in [email protected] The authors proposed that the reason for such a behavior is the quenching of the angular momentum due to the crystal-field effect from the Er3+ ions.914 Note that the same trend might be observed in the case of a noncollinear coupling of the momenta of individual Er atoms as proposed by Wolf et al. in Ho3N@C80 and [email protected],913 In another SQUID study of Er3N@C80, Smirnova et al. reported that the total effective magnetic moment of the molecule determined either from temperaturedependent susceptibility (at 2−300 K) or isothermal magnetization measurements (at 2 K) is only 10.2 μB (i.e., only 3.4 μB per Er),915 which is even smaller than the value found by Tiwari et al.914 and corresponds to the complete quenching of the 6087


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showed that Gd3N@C80 has two almost isoenergetic magnetic states, one is a collinear configuration with a spin magnetic moment 21 μB and another one, 10 meV higher in energy, is a noncollinear state with spin moment of 14.1 μB.918 9.4. Single Molecular Magnetism in Endohedral Fullerenes

Until recently, all studies of the magnetic properties of endohedral fullerenes showed that EMFs exhibit only paramagnetic behavior. A principally new situation in magnetic behavior of lanthanide-based EMFs was discovered in 2012.919 Both XMCD and SQUID studies of the mixed-metal NCF DySc2N@C80(Ih) revealed a characteristic butterfly shape hysteresis of magnetization curves at temperatures below 6 K (Figure 59), which points to the long relaxation time of magnetization and shows that DySc2N@C80 behaves as a mononuclear single-molecular magnet.919

Figure 58. The magnetization values M(H,T) of Tb 3N@C 80 (symbols) normalized to saturation. The solid line is a Brillouin function with J = 12 and a g factor of 1.5. Inset: proposed configuration of the individual atomic magnetic moments m (arrows) in the (R3N) cluster (R = Ho, Tb). Reproduced with permission from ref 913. Copyright 2004 Elsevier BV.

orbital momentum (J ≈ S = 3/2 per Er3+ ion). In sharp contrast to Er3N@C80, for Tm3N@C80 Zuo et al. found a paramagnetic behavior with the total magnetic moment of the molecule at T > 150 K being equal to three times the magnetic moment of an isolated Tm3+ ion without any degree of quenching.323 Temperature dependence of the susceptibility gave Weiss temperature of θ = −88 K pointing to the strong antiferromagnetic interactions, which were proved to be of intramolecular nature since dilution of Tm3N@C80 with C60 did not change the θ value. At low temperature magnetic moment decreased dramatically, and the authors proposed that spin vectors in Tm3N@C80 are oriented along Tm−N bonds as in Tb3N@C80 and Ho3N@C80 but are radiating either away or toward N atom.323 In 2011 Chen et al. reported on-chip SQUID measurements of Gd3N@C80 at the temperatures down to 0.1 K.916 Magnetization curves exhibited no hysteresis in the whole temperature range confirming a completely paramagnetic behavior. A plot of inverse susceptibility vs temperature showed a linear dependence with an abrupt change at 1.2 K. Experimental data were fitted by the Heisenberg model. The low temperature data (T < 1.2 K) were well described by three Gd3+ ions in their ground 8S7/2 state (μeff = 7.9 μB) with weak antiferromagnetic exchange interaction (Jij = 7.6 mK). At T > 1.2 K the data were better described by considering that one of the Gd ions is in the excited 2+ state with 7F6 configuration (L = S = 3, J = 6, μeff = 9.7 μB).916 In 2012 Jánossy studied magnetic properties of Gd3N@C80 using SQUID and highfrequency ESR techniques.594 SQUID studies showed that at low temperature effective magnetic moment corresponds to three coupled S = 7/2 spins with total μeff = 21.9 μB, whereas at higher temperature spins are uncorrelated and μeff of the molecule is approaching the values of 13.7 μB. Experimental ESR and SQUID data were fitted to the Hamiltoin including Heisenberg exchange (J = −15 GHz) and zero-field splitting with uniaxial single ion anisotropy D = 11.5 GHz.594 Magnetic coupling of Gd ions in Gd3N@C80 was also studied theoretically by several groups.116,917,918 In agreement with lowtemperature experimental data, Lu et al. found a weak antiferromagnetic exchange interaction in a collinear spin system studied at the PBE+U level.917 Antiferromagnetic coupling was also predicted in the scalar-relativistic PBE study by Yang et al.116 Noncollinear DFT+U calculations

Figure 59. Magnetization curves of DySc2N@C80 measured at T = 2 K by XMCD and SQUID showing a characteristic butterfly shape hysteresis pattern. Reproduced with permission from ref 919. Copyright 2012 American Chemical Society.

Below 3.5 K, relaxation of magnetization was described by a double-exponential decay. The slower process was thermally activated. From the temperature dependence of the relaxation time, effective barrier for the thermally driven relaxation was estimated to be 24 ± 0.5 K at the field of 0.3 T, which is comparable to 44 K reported for [Pc2Dy]− at 0.35 T.920 At the same time, exponential prefactor for DySc2N@C80 has exceptionally large value of 1 ± 0.1 s, which is 4 orders of magnitude larger than determined for [Pc2Dy]−. Dilution of the sample with 10−20 equiv of C60 increased zero-filed relaxation time at T = 2 K by almost an order of magnitude.919

10. POTENTIAL APPLICATIONS OF ENDOHEDRAL FULLERENES 10.1. Biomedical Applications

Biomedical applications are a developing field that holds special promise for fullerene-based products, since relatively small amounts of the material are required for effective dosing. The nontoxicity of the carbon cage makes endohedral fullerenebased drugs potentially feasible for medical applications.860,922 The internal metal of an endohedral fullerene is effectively isolated from its surrounding environment, giving the endohedral fullerene a distinct advantage of high stability and low toxicity over the metal chelate complexes commonly used in radio-medicine and diagnostic radiology. 10.1.1. MRI Contrast Agents. Magnetic resonance imaging (MRI) is one of the most common techniques for the diagnostic examination of human patients, using a magnetic field and pulses of radio wave energy to make pictures of organs 6088


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Table 16. Relaxivities r1 of Gd-Based Endohedral Fullerenes in Comparison with That of the Commercial MRI Contrast Agent Gd-DTPAa compound Gd@C82(OH)40

Gd@C82(OH)16 Gd@C60[C(COOH)2]10 Gd@C60(OH)27 Gd@C82O6(OH)16(NHC2H4CO2H)8 Gd@C82(OH)22±2 Gd@C82O6(OH)16(NHCH2CH2COantiGFP)5 Gd@C82O2(OH)16(C(PO3Et2)2)10

Gd3N@C80[DiPEG5000(OH)x]

ScGd2N@C80O12(OH)26 Sc2GdN@C80O12(OH)26 Gd3N@C80-hydrochalarone Gd3N@C80(OH)∼26(CH2CH2COOM)∼16 (M = Na or H)

Gd3N@C80[DiPEG(OH)x]b

Gd−DTPA

r1 (mM−1 s−1)

magnetic field (T)

year

67 81 31 19.3 6.8−24.0 14.1−83.2 9.1 8.1 37.7 (pH 2) 61 (pH 7) 12 37 38.9 19.9 102 143 32 20.7 17.6 205 154 207 76 107−227 139−237 41.9−68.2 3.8 3.9 3.8

0.47 1.0 4.7 4.7 1.4 1.4 1.5 0.35 4.7

2001

898

2007 2005 2005 2006

926 929 929 864

2008

928

0.35 0.35 2.4 9.4 0.35 2.4 9.4 14.1 14.1 0.47 0.35 2.4 9.4 0.35 2.4 9.4 0.47 1.0 4.7

2008 2008

864,865 .866

2006

888

2007 2007 2008 2009

887 887 890 891

2010

889

ref

987

a DTPA = diethylenetriaminepentaacetic acid. bDifferent mol. weight of PEG was used (350, 750, 2000, 5000 Da) and r1 vaues are given between the lowest and highest values for these compounds.

and structures inside the body.923 The visibility of the internal body structures in MRI can be drastically improved by the use of MRI contrast agents, which act by the change of the proton relaxation times in tissues and body cavities. Nowadays the most commonly used commercially available MRI contrast agents are gadolinium (Gd) chelate complexes such as GdDTPA (DTPA: diethylenetriamino-pentaacetic acid) which uses the Gd3+ (S = 7/2) ion to enhance the relaxation rate of water protons. The application of endohedral fullerenes as MRI contrast agents has been proposed since 1996 and was extensively evaluated in recent years.21,48−51,55,924−926 Table 16 summarizes the relaxivities r1 of all reported Gd-based endohedral fullerenes in comparison with those of the commercial MRI contrast agent Gd-DTPA. In 2001 Shinohara et al. prepared the water-soluble hydroxylated Gd@C82 EMF (Gd@C82(OH)40) and measured its longitudinal and transverse relaxivities r1 and r2 as an MRI contrast agent based on an in vitro MRI study.898 The measured relaxivity r1 of Gd@C82(OH)40 calculated from the longitudinal relaxation rate of water (81 mM−1 s−1 at pH = 7.5, 1.0 T) is much higher than that of Gd-DTPA (3.9 mM−1 s−1).898 Later, the same group studied systematically a series of lanthanoid metallofullerenols, M@C82(OH)n (M = La, Ce, Gd, Dy, and Er) in order to obtain information on the proton relaxation mechanism of these novel forms of MRI contrast agents and found that both the r 1 and r 2 of these metallofullerenols (0.8−73 and 1.2−80 (mM−1 s−1), respec-

tively) are much higher than those of the corresponding free ions and lanthanoid-DTPA compounds.861 Illustrated in Figure 60 (II) are the Phantom NMR images of various metallofullerenols as well as those of lanthanoid ions and lanthanoidDTPA complexes, confirming that an extremely strong contrast enhancement was observed for Gd compounds, whereas other compounds at the same concentration showed only a slight enhancement of MRI signals as compared with pure water. Furthermore, the MRI signal intensities of all of the studied metallofullerenols are much stronger than those of the corresponding metal ions and DTPA complexes.861 Hydroxylated Gd@C82 EMF with a lower number of hydroxyl groups and other types of water-soluble derivatives based on Gd@C82 were synthesized, and their performance as MRI contrast agents was studied by different groups later on. In 2006 Zhao, Lei and co-workers prepared and characterized Gd@C82(OH)22 and studied its imaging efficiency in mice.927 In 2008 Gao and Zhao et al. synthesized nanoparticles of Gd@ C82(OH)22±2 with ordered microstructures and demonstrated the 12-fold enhancement of its relaxivity r1 in vitro/vivo compared to Gd-DTPA. They also found that Gd@ C82(OH)22±2 nanoparticle with a size below 100 nm could successfully escape the reticuloendothelial system (RES) uptake in vivo.928 Wang et al. synthesized several new Gd@C82-based water-soluble derivatives including Gd@C82O6(OH)16(NHC2H4CO2H)8, Gd@C82O6(OH)16(NHCH2CH2COantiGFP)5, Gd@C82O2(OH)16(C(PO3Et2)2)10, which all show much 6089


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T, respectively, which is markedly higher than that of gadodiamide. This result is confirmed by the in vitro imaging as demonstrated in Figure 60 (II), in which the T1-weighted MR images of aqueous solutions of Gd3N@C80[DiPEG5000(OH)x] (inner ring) and gadodiamide (outer ring) were compared, revealing that at equivalent image intensities, the corresponding concentrations for the Gd3N@C80[DiPEG5000(OH)x] agent are at least 30 times smaller.888 More recently, the same groups synthesized a series of water-soluble PEG functionalized and hydroxylated Gd3N@C80 by varying the molecular weights of PEG (350−5000 Da) and measured their relaxivities at three magnetic field strengths.889 Their result revealed that Gd3N@C80[DiPEG(OH)x] has high relaxivities r1 for the 350/750 Da PEG derivatives, 237/232 mM−1 s−1 at a clinical-range magnetic field of 2.4 T, which is ca. 60 times higher than that of commercially used Gd3+ ion chelated contrast agents such as Gd-DTPA (Table 16). Such significantly enhanced relaxivity r1 compared to commercially available Gd-chelates enables Gd3N@C80[DiPEG(OH)x] as an effective class of MRI contrast agent.889 Several other types of Gd3N@C80-based water-soluble derivatives were also synthesized, and their performance as MRI contrast agents was evaluated by different groups. In 2008 MacFarland et al. synthesized a series of novel derivatives of Gd3N@C80 termed hydrochalarones, Gd3N@C80-hydrochalarone (Gd3N@C80-Rx, where R = [N(OH)(CH2CH2O)nCH3]x, n = 1,3,6 and x = 10−22), and reported that hydrochalarone-6 as the optimal member of this series exhibits the high relaxivity r1 of 205 mM−1 s−1.890 In 2009 Dorn et al. reported the synthesis of Gd3N@C80(OH)∼26(CH2CH2COOM)∼16 (M = Na or H) and determined their longitudinal and transverse relaxivities r1 and r2 to be 207 and 282 mM−1 s−1 (per C80 cage) at 2.4 T, respectively, which are 50 times larger than those of such Gd3+ poly(aminocarboxylate) complexes as commercial Omniscan and Magnevist.891 Given that the yield of Gd3N@C80 is quite low, Gdcontaining mixed metal NCFs with higher yield were also considered, and their properties as MRI contrast agents were studied in two reports. In 2007 Wang et al. synthesized two water-soluble hydroxylated derivatives of Gd/Sc mixed metal NCFs [ScxGd3‑xN@C80Om(OH)n (x = 1, 2; m ≈ 12; n ≈ 26)], for which the relaxivities r1 were determined to be 20.7 and 17.6 mM−1 s−1 for Sc2GdN@C80O12(OH)26 and ScGd2N@ C80O12(OH)26, respectively.887 Important recent developments in applications of EMFs for MRI include the conjugation of EMFs with the biological targeting modules (e.g., special proteins), which would deliver EMFs-based agents to only special kinds of cells. This approach can drastically improve the specificity of EMF-based contrast agents and at the same time reduce required amounts. In 2010 Braun et al. used GdSc2N@C80 to synthesize a modular system abbreviated as “Gd-cluster@-BioShuttle”, comprising three functional modules: GdSc2N@C80 as a contrast agent was covalently linked to the nuclear address module, which was in due turn linked to the peptide facilitating the passage across cell membranes.894 In vitro studies of Gd-cluster@-BioShuttle for imaging human breast cancer cells revealed that Gd-cluster@ -BioShuttle exhibited high proton relaxivity and high signal enhancement at very low Gd concentrations compared to the commonly used contrast agents such as Gd-DTPA, resulting thus in a 500-fold gain of sensitivity.894 The synthesis of Gdcluster@-BioShuttle system targeting c-myc mRNA-expressing cells and facile intracellular transport of this system were

Figure 60. (I): Phantom NMR images of various metallofullerenols (together with those of lanthanoid ions and lanthanoid-DTPA complexes) solutions at 1.0, 0.5, and 0.1 mmol metal/L. Reproduced with permission from ref 861. Copyright 2003 Amercian Chemical Society. (II): T1-weighted MR image (700/10) of aqueous solutions of Gd3N@C80[DiPEG5000(OH)x] (inner ring, concentration decreasing in clockwise direction: 0.2020, 0.0101, 0.0505, 0.0252, 0.0126, 0.0063, 0.0032, and 0.0016 mmol/L) and gadodiamide (outer ring, concentration decreasing in clockwise direction: 5.0, 3.0, 1.0, 0.70, 0.50, 0.30, 0.10, and 0.050 mmol/L). Note the substantially lower concentrations required for Gd3N@C80[DiPEG5000(OH)x] for achieving equivalent image intensities to gadodiamide. Reproduced with permission from ref 888. Copyright 2006 Amercian Chemical Society.

enhanced relaxivity r1 compared to Gd-DTPA (see Table 16).864−866 Xing et al synthesized Gd@C82(OH)16 and performed its in vivo studies.926 Despite the high relaxivity, the polyhydroxyl EMF derivatives induce spontaneous aggregation of erythrocytes when they are in contact with blood and exhibit excess RES uptake. To this problem, the carboxylated EMF derivative Gd@C60[C(COOH)2]10 synthesized by Bolskar et al. in 2003 shows a favorable (non-RES localizing) biodistribution with a comparable relaxivity r1 (4.6 mM−1 s−1 at 20 MHz and 40 °C) to that of Magnevist.854 In 2005, Merbach et al. reported that the aggregates of both water-soluble Gd@C60[C(COOH)2]10 and Gd@C60(OH)27 formed in aqueous solution can be disrupted by the addition of salts including phosphate and sodium halides.929 The measured relaxivities r1 of Gd@C60[C(COOH)2]10 and Gd@C60(OH)27 are 6.8−24.0 and 14.1− 83.2 mM−1 s−1, respectively, which are dependent on the concentration of salts (see Table 16).929,930 In 2007 Bolskar and Wilson et al. proposed that the rapid exchange of water molecules, which are confined in the interstices of the aggregates, with the bulk contributed to the high relaxivity of the aggregated of Gd@C 60 (OH) 27 and Gd@C 60 [C(COOHyNa1−y)2]10.931 Since 2006, the application of NCFs as MRI contrast agents has attracted more attention not only because of their higher yield but also due to the higher relaxivity compared to conventional EMFs. As discussed already in section 9, up to three magnetic ions can be encaged in NCFs; hence even higher proton relaxivities can be expected provided that the magnetic moments of the three metal ions are coupled ferromagnetically.113,913 In 2006 Fatouros, Dorn et al. functionalized Gd3N@C80 with poly(ethylene glycol) units and then hydroxylated the derivative (Gd3N@C80[DiPEG5000(OH)x]), which shows an improved water solubility and biodistribution.888 They then carried out in vitro and in vivo imaging studies of Gd3N@C80[DiPEG5000(OH)x] and measured its relaxivity r1 of 102, 143, and 32 mM−1 s−1 at 0.35, 2.4, and 9.4 6090


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reported recently by Svitova et al.132 In 2011, Fillmore, Shultz and co-workers conjugated hydroxylated/carboxylated Gd3N@ C80 to the fluorescence-labeled interleukin-13 (IL-13) peptide, targeting human brain tumor cells. Further studies of the conjugate proved its targeting ability and enhanced cellular uptake when compared to Gd3N@ C80(OH)z(CH2CH2COOM)y alone.893 10.1.2. X-ray Contrast Agents. Because of the large cross section of the lutetium (Lu) atoms, Lu3N@C80 was reported by Dorn et al. in 2002 to provide a good X-ray contrast.98 Such a contrast effect was not found for C60, providing evidence that the contrast cannot be attributed to the carbon cage. Accordingly, the authors proposed that mixed-metal species (e.g., Lu3−xAxN@C80, where A is Gd or Ho, and x = 0−2) may prove useful as a multimodality contrast agent (e.g., X-ray and MRI). Using a contrast agent that provides enhancements for a variety of spectroscopic methods would simultaneously provide directly comparable images with minimum exposure to the patient.98 10.1.3. Radiotracers and Radiopharmaceuticals. Another potential application for endohedral fullerenes in biomedicine is in the field of nuclear medicine with radiotracers and radiopharmaceuticals. At present radiopharmaceuticals generally use small quantities of drugs containing specially chelated radioisotopes of metals for imaging or therapeutic applications, but these drugs have the problem of in vivo kinetic instability, which can result in the release of small amounts of toxic radioactive metal ions. However, because of their resistance to metabolism and high kinetic stability, endohedral fullerenes provide a unique alternative to chelating compounds. The first study on radiotracer applications of EMFs was reported in 1995 by Kobayashi et al.932 An emulsion of 140Lalabeled La@C82 and La2@C80 in 5% polyvinylpyrrolidine was injected in the heart of rats. After 24 h radioactivity in each organ was analyzed by γ-ray spectroscopy, and large radioactivity due to 140La-EMFs were found in blood and liver.932 The application of holmium-based endohedral fullerene 166 Hox@C82(OH)y as radiotracers and therapeutic radiopharmaceuticals was studied by Wilson et al. in 1999.902 The authors first hydroxylated a purified Hox@C82 mixture (x = 1, 2) and then prepared 165Hox@C82(OH)y by a 165Ho[n,γ]166Ho neutron-activation of the fullerenol mixture. Biodistribution studies of 166Hox@C82(OH)y as a radiotracer in BALB/c mice over a 48-h period showed a selective localization of the 166 Hox@C82(OH)y tracer in the liver but with slow clearance, as well as its uptake by bone without clearance. Moreover, a metabolism study of 166Hox@C82(OH)y in Fischer rats indicated 20% excretion of intact 166Hox@C82(OH)y within 5 days. In conclusion, the authors proposed that 166Hox@ C82(OH)y can be used as radiotracers to monitor the fate of fullerene-based materials in animals and therefore may be useful components in drug design.902 More recently, Dorn et al. conjugated the 177Lu-radiolabeled NCF (177LuxLu3−xN@C80) to a fluorescent tag [tetramethyl-6-carboxyrhodamine (TAMRA)]-labeled interleukin-13 (IL-13) peptide. Their results demonstrated that the radiolabeled 177Lu ions are not readily removed from the fullerene cage after the ß-decay process, and it could be therefore used for radiotherapeutic and radiodiagnostic applications.892 Recently Nagasaki et al. reported the use of Gd@C82 for neutron capture therapy.933 The authors solubilized Gd@C82 nanoparticles in water via complexation with poly(ethylene glycol)-block-poly(2-(N,N-diethylamino)ethyl methacrylate)

and proved a low cytotoxicity of this complex in vitro. Neutron irradiation of colon-26 adenocarcinoma cells with nanoparticles considerably decreased the cell viability compared to the unirradiated samples, indicating the emission of γ rays and the conversion electrons upon the neutron capture reactions of 155 Gd and 157Gd.933 Neutron activation and even transmutation of endohedral metal atoms in EMFs were reported by several other groups, but their medical applications are not studied yet.934−939 In 2011 Shultz et al. reported that functionalized Gd3N@C80 conjugated to radiolabeled Lu chelated by tetraazacyclododecane tetraacetic acid (the system is denoted hereafter as 177LuDOTA-f-Gd3N@C80) can be used for effective brachytherapy.940 In such system, Gd3N@C80 offers longitudinal tumor imaging, whereas 177Lu-DOTA acts as a radiotherapeutic. Treatment of mice with implanted human glioblastoma cells by 177 Lu-DOTA-f-Gd3N@C80 allowed both imaging and effective therapy with extended survival time of treated mice. The key features of 177Lu-DOTA-f-Gd3N@C80 as a successful theranostic agent are high relaxivity, its extended distribution throughout the tumor, and its prolonged retention.940 In 2012 the efficacy of this system was also demonstrated in two orthotopic xenograft brain tumor models of glioblastoma multiforme. The in vivo stability of the agent was verified over a 7 day period allowing for the delivery of an efficious brachytherapy dose.941 10.1.4. Antitumor Activity of [Gd@C82(OH)22]n Nanoparticles. Anticancer activity of [Gd@C82(OH)22]n nanoparticles has been extensively studied since 2005.901,942−951 In 2005 Zhao et al. reported that Gd@C82(OH)22 nanoparticles with the average size of ca. 22 nm exhibited a high antineoplastic activity in mice against H22 hepatoma.901 At the same time, this study proved the low toxicity of nanoparticles in vivo and in vitro, showing that the antitumor activity is not due to toxic effects to cells.901 Subsequent studies showed that Gd@C82(OH)22 nanoparticles can regulate the oxidative stress in tumor cells in vivo and act as scavengers of reactive oxygen species, which can be a reason of their antitumor activity.942,943 In 2009 Zhao, Chen et al. found that at high concentrations Gd@C82(OH)22 nanoparticles enhanced the immune responses and stimulated immune cells to release more Th1 cytokines, thus helping the immune system to eliminate tumor cells.944 It was also shown that Gd@ C82(OH)22 can protect cells against oxidative damage by scavenging the reactive oxygen species; the work included both in vitro studies of H2O2-induced cytotoxicity as well as ESR spin trap technique of the free radical scavenging.952 In 2010 Liang and Zhao showed that Gd@C82(OH)22 nanoparticles help to solve the problem of acquired tumor resistance to chemotherapeutic cisplatin.945 In the same year these authors reported that Gd@C82(OH)22 nanoparticles act as efficient angiogenesis inhibitors by simultaneously down-regulating multiple angiogenic factors.946 The anticancer efficiency of nanoparticles in vivo was comparable to that of the clinic anticancer drug paclitaxel, but without pronounced side effects. In 2012 these authors showed that Gd@C82-fullerenol nanoparticles significantly inhibit cancer metastasis.948 The mechanism of the antimetastasis activity was shown to be not in the cytotoxicity but in the inhibition of the matrix metalloproteinase production. The formation of a thick fibrous cage served as a physical barrier cutting the communication between cancer- and tumor-associated macrophages.948 In 2012 it was shown that Gd@C82(OH)22 can effectively block tumor growth 6091


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in human pancreatic cancer xenografts as was shown by experiments at animal, tissue, and cellular levels.953 In the same work, molecular dynamics (MD) simulations were applied to explain the action of Gd@C82(OH)22 at the molecular level. MD simulations were also used to model the interaction between Gd@C82(OH)22 and a small protein domain.954 Several reviews on the properties and biological effects of Gd@C82(OH)22 and other functionalized fullerenes, in particular their use for cancer therapy, were recently published.52−54,955 10.1.5. Antimicrobal Activity of Sc3N@C80-Polymer Film. In 2009 Stevenson, Phillips et al. discovered the singlet oxygen sensitization by [email protected] The NCF was then embedded into polystyrene-block-polyisoprene-block-polystyrene copolymer pressure sensitive adhesive films. Stemming from in situ generation of 1O2, such films exhibited animicrobal activiy evaluated in a number of essays.

Figure 61. J−V curves of P3HT/Lu3N@C80-PCBH (triangles) PCE = 4.2%, Voc = 810 mV, Jsc = 8.64 mA cm−2 and FF = 0.61 and P3HT/ C60-PCBM (squares and dashed lines) PCE = 3.4%, Voc = 630 mV, Jsc = 8.9 mA cm−2 and FF = 0:61 blend devices. Filled symbols show the dark curves and open symbols show devices under simulated Air Mass 1.5 (100 mW cm−2). Reproduced with permission from ref 652. Copyright 2009 Nature.

10.2. Applications in Organic Photovoltaics

The photovoltaic (PV) effect is the generation of an electric potential by means of photoirradiation. If the radiation source is the sun, the PV device is called a solar cell, which can therefore convert sunlight into electrical power and deliver this power into a suitable load in an efficient manner.956 An organic photovoltaic cell (OPV, or organic solar cell) is a photovoltaic cell that uses organic semiconductors (conductive organic polymers or small organic molecules) as photoactive molecules for light absorption and charge transport. The low cost of the organic semiconductor materials, the ease of fabrication of solar cells, and the compatibility with mass printing techniques make organic solar cells very promising in commercial applications. In particular, polymeric solar cells (PSCs) based on composites of an electron-donating conjugated polymer and an electronaccepting fullerene are quite promising for the realization of a low-cost, printable, portable and flexible renewable energy source. The most efficient architecture to build PSCs is the bulk heterojunction (BHJ) structure comprising an interpenetrating network of a conjugated polymer donor such as poly(3hexylthiophene-2,5-diyl) (P3HT) and a soluble fullerene acceptor system which is typically [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the photoactive layer.957−961 10.2.1. Endohedral Fullerenes As New Acceptors in PSCs. In 2009 Drees et al. reported the first synthesis of Lu3N@C80 methano derivatives and their use as novel acceptor materials in PSCs.652 Their results revealed that an optimized high open circuit voltage of 890 mV can be obtained for 1-(3hexoxycarbonyl)propyl-1-phenyl-[6,6]-Lu3N@C81 (Lu3N@ C80−PCBH), which is the highest value for P3HT/fullerene PSC devices and 260 mV above the reference devices made with the commonly used C60-PCBM acceptor. Thus a power conversion efficiency (PCE) of >4% was obtained by using Lu3N@C80−PCBH as the new acceptor (Figure 61). The higher overall PCE of the P3HT/Lu3N@C80-PCBH devices compared to C60-PCBM is attributed to a better positioned LUMO level that captures more of the energy associated with each absorbed photon.652 In another work published in the same year by the same authors, the influence of the blending ratio of Lu3N@C80−PCBH with P3HT on the active layer morphology and the performance of PSC devices were studied using absorption spectroscopy, grazing incident X-ray diffraction and photocurrent spectra.669 The study revealed that an optimal blend ratio of P3HT and Lu3N@C80-PCBH is 50 wt % fullerene loading, which differs from a molecular equivalent of

an optimized C60−PCBM/P3HT active layer. Such a difference was attributed to the increased size of the Lu3N@C80-PCBH, thereby effectively reducing the amount of fullerene needed for efficient charge transport to occur.669 The authors proposed that, using the improved LUMO level offset of Lu3N@C80PCBH acceptor materials and combining it with the new lowbandgap donor polymers which have been extensively studied recently, PCE greater than 10% may now be feasible for PSCs.652 10.2.2. Endohedral Fullerene-Based Donor−Acceptor Dyads. As already discussed in section 6.5.2, several electron donor−acceptor dyads based on endohedral fullerenes typically M3N@C80 (M = Sc, Y) were synthesized by Prato or Bingel− ́ Torres, and Hirsch reactions. In 2008, Guldi, Martin, Echegoyen et al. synthesized a ferrocenylpyrrolidine adduct of Sc3N@C80-Ih as the first covalent donor−acceptor dyad and studied its photophysical properties by time-resolved transient absorption spectroscopy, confirming photoinduced electron transfer between the ferrocene moiety and the fullerene (see section 6.5. for more details on photophysical studies).671 Compared to an analogous C60-ferrocene conjugate, the authors found that the radical ion pair state was significantly stabilized; thus its application in OPVs as new and promising materials was predicted to be more promising than that of the empty fullerenes.671 In 2009 Echegoyen et al. synthesized two isomeric [5,6]pyrrolidine TPA-Sc3N@C80 (TPA = triphenylamine) donor− acceptor conjugates and found that TPA-Sc3N@C80 with the N-connected TPA donor had significantly better thermal stability and a longer lived photoinduced charge separated state than the corresponding 2-substituted system. Besides, TPA-Sc3N@C80 dyads also have considerably longer lived photoinduced charge separated states and lower first reduction potentials than their C60 analogues, confirming the advantage of using Ih-Sc3N@C80 for replacing C60 as the acceptor moiety for the construction of new donor−acceptor conjugates applicable for OPVs.647 Later on, the same groups carried out the systematic synthesis, electrochemical, theoretical and photophysical studies of a series of donor−acceptor dyads based on M3N@C80 (M = Y, Sc) and different donors (tetrathiafulvalene, 6092


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et al., revealing the interesting difference on the photoinduced electron transfer process compared to the empty fullerene C60.661−664 For instance, by blending Dy@C82 with poly(3hexylthiophene) (P3HT), which was chosen based on its ptype semiconductor character and excellent PV performance together with its good film formation properties, at an optimum ratio of 20:1 (P3HT:Dy@C82), the PEC cell based on the Langmuir−Blodgett (LB) film of P3HT-Dy@C82 exhibited a dramatic enhancement of the cathodic photocurrent (quantum yield ∼3.88%) compared to that of the single component P3HT, which resulted from the facile photoinduced electron transfer between P3HT and [email protected],664 A study of photoelectrochemical applications of micrometer-sized nanorods prepared from Sc3N@C80 was recently reported.966

phthalocyanine or ferrocene). On the basis of the photophysical study of the ferrocenyl−Sc3N@C80-fulleropyrrolidine dyad in combination with the spectroelectrochemical study of the IhSc3N@C80 radical anion, they concluded the existence of a photoinduced electron-transfer process that yields a radical ion pair with a lifetime three times longer than that obtained for the analogous C60 dyad.596 In 2011, these authors also synthesized and studied photophysical properties of two Sc3N@C80-ZnP dyads with long linkers and found that very long lifetime of the charge-separated states exceeding 1.0 μs.648 More recently donor−acceptor dyads based on conventional metallofullerenes were also reported by Guldi and Akasaka et al. They first succeeded in synthesizing stable donor−acceptor conjugates of La2@C80-Ih and π-extended tetrathiafulvalene (exTTF) by highly regioselective 1,3-dipolar cycloadditions of exTTF-containing azomethine ylides to [email protected] An absorption spectroscopic study and CV measurements revealed the weak electronic interaction between exTTF and La2@C80 in the ground state, but a transient absorption spectroscopic study confirmed the fast formation of a radical ion-pair state with lifetimes as long as 3.26 ns in the excited state via the reduction of the electron accepting La2@C80 and the oxidation of exTTF.649 In another report they synthesized donor−acceptor conjugate of Ce2@(C80-Ih)-ZnP using a [2 + 1] cycloaddition reaction of diazo precursors and found the unusually strong Ce2@Ih-C80/ZnP (ZnP = zinc tetraphenylporphyrin) interactions.387 A systematic investigation of the charge transfer chemistry by steady-state fluorescence and transient absorption measurements revealed an oxidative electron transfer in polar solvent (with formation of Ce2@C80-based radical-cation) and more common reductive charge-transfer in nonpolar solvents. Considering the oxidative charge transfer pattern, the authors proposed that Ce2@C80-Ih might be used as a promising and unprecedented p-type fullerene material for a solar energy conversion systems.387 10.2.3. Photoelectrochemistry (PEC) Cells Based on Endohedral Fullerenes. Photoelectrochemical (PEC) cells employing a semiconductor/electrolyte junction have also gained popularity in recent years for solar energy conversion. The change in the electrode potential (on open circuit) or in the current flowing in the external circuit (under short-circuit conditions) of an electrode/electrolyte system on irradiation is termed as photoelectrochemical (PEC) effect. A simple PEC cell consists of a photoactive semiconductor electrode (either n- or p-type) and a metal counter electrode, which are all immersed in a suitable redox electrolyte. PEC provides routes to energy conversion devices of efficiencies that are too high to be ignored as an option for future solar energy conversion schemes. The PEC approaches possess several unique opportunities for exploiting the ability to affect chemical control over bulk and interfacial properties of materials and provide improved performance in a relatively simple device implementation. PEC studies are frequently carried out to obtain a better understanding of the nature of the electrode− solution interface. In addition, in the field of electronics and electro-optics, PEC processing has been a subject of considerable recent interests.962 Fullerene-based PEC studies are at present limited to the fundamental issues rather than practical applications. Few PEC studies based on C60 derivatives were reported, indicating comparable efficiency compared to the PV’s solid−solid junction.963−965 PEC studies on endohedral fullerenes are quite limited to the pristine metallofullerenes Dy@C82 by Yang

10.3. Endohedral Fullerene Peapods

The successful synthesis of fullerene peapods, i.e., carbon nanotubes (CNTs) encapsulating fullerene molecules inside the core, has initiated another active branch of fullerene research as well as nanotube. Upon replacing fullerene by endohedral fullerenes, the endohedral fullerene peapods are even more intriguing because of the interplay between the charge transfer within the EMF and the interaction between CNT as the pea and the endohedral fullerene as the pod. Earlier studies on the EMF peapods included (X)m@SWNTs (X = Gd@C82 (Figure 62a), La2@C80, Sc2@C84, and Dy@C82, SWNT = single-wall CNT) and revealed that the band gap modulation in peapods which may generate conceptually new molecular devices with

Figure 62. (a) HRTEM images of the isolated SWNTs containing the Gd@C82 fullerenes. Dark spots seen on most of the fullerene cages correspond to the encapsulated Gd atoms that are oriented randomly in respect to the tube axis (bar = 5 nm). Reproduced with permission from ref.800 2000 Amercian Chemical Society (a) HRTEM images of the isolated SWNTs containing the Gd@C82. (b) HR-TEM image of Dy3N@C80@SWCNT. The inset shows a close-up with one encapsulated Dy3N@C80 marked by a circle. The dark spots can be attributed to Dy and indicate that the intratubular fullerene encages three atoms of Dy. Reproduced with permission from ref 973. Copyright 2007 Wiley. 6093


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orientational change or a slight translational motion of each fullerene cage encapsulated in a nanotube by aberrationcorrected TEM, and this observational technique is expected to enable further discussions of the dynamic behavior of individual molecules inside SWNTs in relation to their atomic-level structures and to their interactions with outer graphene walls.805 In a more recent aberration-corrected TEM study of endohedral fullerene peapods, Kaiser and Khlobystov prepared the peapods encapsulating N-methyl-2-(4-(liponyloxy)-benzyl)[5,6]-Sc3N@C80 fulleropyrrolidine as the first report of functionalized endohedral fullerene-based peapod.803 The aberration-corrected TEM study of this peapod revealed the complex dynamic behavior of the functionalized Sc3N@C80 molecules at the atomic level and provided valuable information on the mechanism of their encapsulation into SWNT. For instance, although the endohedral Sc3N cluster exhibits near free rotation in the pristine Sc3N@C80, but when a functional group is attached to the C80 cage, the rotation of Sc3N is more restricted. Their aberration-corrected TEM results revealed that one of the scandium atoms of Sc3N is oriented toward the pyrrolidine group attached to the C80-cage surface, which is a new type of orientation for this type of molecules unseen previously by other techniques.803 In addition to potential applications of fullerene peapods in creating novel low-dimensional materials with useful properties and constructing new molecular devices with different functionalities, endohedral fullerene peapods also show promise in biomedical applications. In 2010 Dorn et al. reported an analogous endohedral fullerene peapod by encapsulating M3N@C80 (M = Gd and Lu) into single walled carbon nanohorns (SWNHs), which was further functionalized with carboxyl groups by a high-speed vibration milling (HSVM) method and conjugated with CdSe/ZnS quantum dots (QDs). Their in vitro and in vivo experiments demonstrated the potential of the M3N@C80@SWNHs as multimodal diagnostic MRI contrast agents.974

different functionalities compared to empty SWNT electronic devices.755,779,797,800,967−969 In 2006 and 2007, Kitaura and Shinohara published two comprehensive reviews on early studies of EMF peapods.970,971 High-resolution transmission electron microscopy studies of peapods are already discussed in section 6.7.2. In the following, we focus on the review of the peapods based on NCFs because the unique electronic properties and outstanding yield of NCFs as discussed above make NCF peapods more practical in preparation and application. In 2005 Lu et al. reported a density functional theory (DFT) study on SWNTs encapsulating Sc3N@C80 in comparison with La@C82 and La2@C80, predicting downshifts of the endohedral fullerene-derived molecular orbitals which increase by increasing the tube-fullerene distance.972 Although DFT predicts that (X)m@SWNTs (X = La@C82, La2@C80) are multicarrier conductors, (Sc3N@C80)m@SWNTs are calculated to be either semiconducting (SWNT = (17,0), (14,7)) or semimetallic (SWNT = (19,0)). Such a difference in their band structures is ascribed to the fact that La@C82 and La2@C80 have larger electron affinities than Sc3N@C80. This calculation also showed local radial deformations of the SWCNTs leading to shifts of the van Hove singularities.972 Several experimental studies on NCF peapods have been reported by different groups. In 2006 and 2007 Shiozawa et al. and Kalbac et al. prepared Dy3N@C80@SWNTs peapods by heating a Dy3N@C80 (I) film with visible thickness deposited onto the SWNT film. The encapsulation of Dy3N@C80 within SWNTs was confirmed by TEM (Figure 62b) while the presence of dysprosium within Dy3N@C80@SWCNT peapods was proved by EELS and in situ Raman spectroelectrochemistry.752,973 The bulk electronic structures Dy3N@C80@SWNTs were also studied by photoemission spectroscopy to get the bulk filling factor (74 ± 10%). Distinct to the spectrum of the SWCNT which shows broad features, the valence-band photoemission spectrum of Dy3N@C80@SWNTs measured with 400 eV photon energy clearly exhibits two prominent peak structures at the binding energies around 7 and 10 eV, respectively, ascribed as the Dy 4f multiplets. A large contribution of the Dy 4f states in the valence-band photoemission spectrum is due to the high atomic photoionization cross section of the Dy 4f state, which is much larger than those of the valence states of C and N with 400 eV photon energy. The Dy 4f multiplet profile extracted from the 4d-4f resonant photoemission spectroscopy of Dy3N@C80@SWNTs and SWNTs was well reproduced with the atomic photoemission multiplets, 4f9 → 4f8 and 4f10→ 4f9, resulting in the estimation of the effective valency of the Dy ions of about 3.0 which is somewhat larger than that for pristine Dy3N@C80 (2.8).750,751 This observed valence change was proposed to be presumably due to the additional charge transfer between the Dy3N@C80 and SWNTs.752 Interestingly, the Dy3N@C80@ SWCNT peapods were successfully transformed into doublewalled carbon nanotubes (DWCNTs) by a pyrolysis carried out at 1200 °C under vacuum for 8 h.973 In 2007 Sato and Suenaga et al. reported the preparation of Er3N@C80@SWCNT peapods and detailed study on timedependent orientational changes of inner Er3N@C80-Ih by aberration-corrected TEM at real atomic-level resolution. The Er−Er distances in Er3N@C80-Ih were determined to be 0.35 ± 0.03 nm, and the C3 symmetry axis of the Er3N cluster was suggested to agree with one of the S6 axes of the spherical IhC80 cage. The authors claimed to be able to detect even a slight

10.4. Other Potential Applications

Other potential applications of endodedral fullerenes were also predicted on the basis of their peculiar electronic, physical and chemical properties discussed in sections 6−9, including superconductors,62,376,975 metallofullerene laser,62 ferroelectric materials,976,977 nanomemory device,62 and quantum computer etc.978−983 Several works on the preparation of EMF-based nanomaterials were published.966,984−986 Although so far only few detailed results have been reported on these topics, it will be of high interest in which way these predictions might be fulfilled.

11. CONCLUSIONS AND OUTLOOK The present review is a clear indication that endohedral fullerenes are a well established class of carbon nanostructures. Here, the interplay of carbon cages and encaged species results in new nanomaterials with exciting electronic states and properties. The background of this type of nanostructures is well understood independent of further strong activities needed for a more precise and detailed description of the endohedral structure and the electronic properties of endohedral fullerenes. Furthermore, prospective applications of endohedral fullerenes in biomedicine and photoinduced electron transfer are a strong motivation for further active work in the field. Therefore a main topic in endohedral fullerene research is the search for further and more efficient routes in synthesis to make larger amounts 6094


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of these structures available for different studies in fundamental and applied research. Besides arc synthesis, further procedures especially at lower temperatures are required to get new types of metals and clusters encaged in a carbon cage. There is a need for synthetic routes resulting in endohedral structures as the main product what is currently rarely the case. With the help of new synthetic routes, it will be possible to have endohedral structures at hand to use the interior of the fullerene cage as a nanolab. Besides the chemical aspects in the interior of the fullerene cage, the electron transfer to endohedral species as the topic of endohedral electrochemistry will play an increasing role in endohedral fullerene research as well as fundamental electrochemistry, thus getting new insights in the stabilization and storage of charges inside a fullerene cage. This research work will initiate further activities in the research on photochemical charge separation of endohedral fullerene dyads with a broad variation of the chemical structure of both the fullerene and the side group(s). Because of the material gathered with magnetic species in endohedral fullerenes, it will be a further main goal to play with magnetic interactions inside and among endohedral fullerene with special emphasis on applications. The inclusion of radioactive isotopes may contribute to applications in nuclear medicine and in deposition studies of such active structures. These studies include the transfer of stable ions into radioactive ones by irradiation for different kinds of bioimaging. The final conclusion is: There is no limit to creative work in the field of endohedral fullerenes!

Shangfeng Yang received his Ph.D. from Hong Kong University of Science and Technology (HKUST) in 2003. He then joined LeibnizInstitute for Solid State and Materials Research (IFW) Dresden, Germany, as an Alexander von Humboldt (AvH) Fellow (2004−2005) and a Guest Scientist (2005−2007). In December 2007, he joined University of Science and Technology of China (USTC) as a full professor of Hefei National Laboratory for Physical Sciences at Microscale & Department of Materials Science and Engineering. He has published over 100 peer-reviewed papers, which have received more than 1200 citations, and contributed to 3 book chapters. He was awarded “Hundreds of Talents Programme” of Chinese Academy of Sciences (2008), “Young Faculty Award” of USTC Alumni Foundation (2010), and “the first Anhui Provincial Hundreds of Talents” (2011). His current research interests include the synthesis and characterization of novel endohedral fullerenes as well as their potential applications in organic solar cells.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (L.D.); [email protected] (A.A.P.); [email protected] (S.Y.). Notes

The authors declare no competing financial interest. Biographies

Lothar Dunsch studied chemistry at the TU Bergakademie Freiberg, Germany, and received his diploma there in chemistry (1972) and his Ph.D. (1973) in electrochemistry. In 1974, he turned to the Institute of Solid State Research of the Academy of Sciences in Dresden and moved in 1978 to the Institute of Polymer Technology before he joined the IFW Dresden in 1992, heading the Department of Electrochemistry and Conducting Polymers and the Center of Spectroelectrochemistry. Since his habilitation in 1996, he has taught at the TU Dresden, where he is Professor in Electrochemistry of Organic Systems. His current interests are focused on endohedral fullerenes, conducting polymers and oligomers as well as the different methods of in situ spectroelectrochemistry. See http://www.ifwdresden.de/institutes/iff/org/Dep/14.

Alexey A. Popov received his M.S. (1999) and Ph.D. (2003) degrees in physical chemistry from Moscow State University (MSU), Russia. In 2003−2008, he worked at the Chemistry Department of MSU as a senior researcher. From 2008, he has worked at Leibniz Institute of Solid State and Materials Research (IFW Dresden, Germany), first as a Humboldt Fellow, and now as a leader of the group “Electronic Structure of Molecular Materials”. His current interests include chemical and physical properties of empty and endohedral metallofullerenes and their derivatives, spectroelectrochemistry, vibrational spectroscopy, magnetic properties, and quantum-chemical computations.

ACKNOWLEDGMENTS The authors thank the current and former members of their groups in Dresden and Hefei for the productive work in the 6095


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field of fullerenes and all of those colleagues who contributed by their cooperation to the work in our groups as visible in the published works cited here. The research work on fullerenes by the authors was supported by Humboldt Foundation, Deutsche Forschungsgemeinschaft (PO 1602/1-1), National Natural Science Foundation of China (20801052, 90921013, 21132007), “100 Talents Programme” of CAS, and the National Basic Research Program of China (2010CB923300, 2011CB921400). Computational studies of EMF were supported by the Supercomputing Center of Lomonosov Moscow State University (time on supercomputer “Chebyshev”), Forschungszentrum Jülich (grant for the use of supercomputer JUROPA) and the Center for Information Services and High Performance Computing (ZIH) at TU Dresden.

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NOTE ADDED IN PROOF During the production of this manuscript, several important contributions to the field of endohedral fullerenes were published. The first unambiguous structural characterization of a trimetallofullerene, Sm3@C80, by single-crystal X-ray diffraction was reported by Feng, Zerbetto et al.988 Yang et al. isolated the first cyanide clusterfullerene based on a single metal atom, YCN@C82-Cs(6).989 Murata et al. succeeded in a 6112


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single-crystal X-ray diffraction study of He@C60 and ESR measurements of [email protected] Extended study of mixed-metal nitride clusterfullerenes GdxSc3−xN@C2n (2n = 78−88) revealed an interplay between the size of the cluster and a preferable cage size.991 Akasaka and coworkers synthesized the first chemical derivative of a divalent metallofullerene by regioselective addition of adamantylidene carbene to Yb@C80C2v(3).992

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Endohedral Fullerenes - ACS Publications - American Chemical Society

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