Transition-Metal and Rare-Earth-Metal Redox Couples inside Carbon

Apr 25, 2014 - Transition-Metal and Rare-Earth-Metal Redox Couples inside Carbon. Cages: Fullerenes Acting as Innocent Ligands. Yang Zhang and Alexey ...
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Transition-Metal and Rare-Earth-Metal Redox Couples inside Carbon Cages: Fullerenes Acting as Innocent Ligands Yang Zhang and Alexey A. Popov* Leibniz-Institute for Solid State and Materials Research (IFW Dresden), D-01171 Dresden, Germany ABSTRACT: In endohedral metallofullerenes (EMFs), the carbon cage shields the endohedral species from the surrounding environment and can stabilize unusual clusters that otherwise would not exist. This review is focused on the behavior of the metal/π-system interface in EMFs under electron transfer conditions. We show that the stabilizing role of the fullerene cage can be extended from unusual clusters to the peculiar spin and charge states obtained via endohedral electron transfer. For such redox processes, the role of the fullerene cage can be understood as that of an innocent ligand. This review is specifically focused on four groups of EMFs with different kinds of endohedral redox activity: (i) dimetallofullerenes, (ii) Sc3N@C80 and its derivatives, (iii) titanium-based EMFs, and (iv) cerium-based nitride clusterfullerenes. Frontier orbitals of dimetallofullerenes usually have pronounced metal−metal bonding character, and therefore electron transfer affects the metal−metal bonding character in such molecules. The LUMO of Sc3N@C80 is equally delocalized over three Sc atoms, resulting in a Sc3N-based reduction, whose mechanism can be modified by exohedral derivatization. Titanium is a rare example of a transition metal that can be encapsulated within fullerenes, and we discuss how its valence state in Ti-EMFs can be tuned via electrochemical reactions. Cerium exhibits endohedral redox activity in many nitride clusterfullerenes, allowing for the redox potential of the strain-driven Ce(IV)/Ce(III) redox couple to be tuned by varying the composition of the endohedral cluster and the size of the carbon cage. A discussion of the redox behavior of these EMFs is accompanied by an analysis of their electronic structure and a discussion of their spectroelectrochemical studies.

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The synthesis of NCFs was followed in 200114 by the discovery of carbide clusterfullerenes M2C2@C2n (M = Sc, Y, lanthanides), M3C2@C2n,15,16 and Sc4C2@C8017 with the negatively charged acetylide unit C2q− (its formal charge changes from 2− to 6−18). It was realized then that many EMFs thought to be di-EMFs and tri-EMFs were indeed carbide clusterfullerenes.19,20 During the last 5 years, new clusterfullerene families with oxide (Sc2O@C82,21 Sc4O2,3@ C80,22,23), sulfide (M2S@C2n24−28), and cyanide (Sc3CN@ C2n,29,30 YCN@C8231) metal clusters have been synthesized (Figure 1). Very recently, TiLu2C@C80 was discovered as a new type of clusterfullerene with a μ3-carbido ligand and a Ti C double bond (Figure 1h,i).32 Several exhaustive reviews of EMFs have been published recently.33−38 Despite the diversity of endohedral clusters, all EMFs have one common feature: the interface between metal ions and the surrounding carbon-based π system. The carbon cage can be considered as a special type of ligand, similar to those in organometallic complexes (e.g., as in ferrocene). Analysis of the chemical bonding in EMFs shows that the large ionic bonding character is actually augmented by considerable covalent contributions due to d−π orbital overlap.39−41

INTRODUCTION The first evidence of the encapsulation of metal atoms inside a fullerene cage was reported in 1985,1 soon after fullerenes were discovered in molecular beams produced by laser ablation of graphite.2 Discovery of the arc-discharge synthesis of fullerenes in 19903 boosted the interest in these carbon clusters, and the synthesis of “bulk” amounts of the first endohedral metallofullerenes (EMFs) was reported in the early 1990s (Figure 1a).4−7 It was soon found that fullerene cages can also encapsulate two or three metal atoms, yielding respectively dimetallofullerenes (di-EMFs; Figure 1b,c) and trimetallofullerenes (tri-EMFs). In EMFs molecules, the endohedral metal atoms donate their valence electrons to the carbon cage (which is therefore negatively charged).7 This results in a strong Coulomb repulsion of metal ions if the carbon cage encapsulates more than one metal atom. The destabilizing Coulomb repulsion can be leveled off if the endohedral cluster also includes negatively charged nonmetal atoms. The first EMF containing a nonmetal endohedral atom, the nitride clusterfullerene Sc3N@C80 (Figure 1d), was discovered in 1999 by Dorn and co-workers.8 Since then, thanks to their relatively high yields and enhanced stability, nitride clusterfullerenes (NCFs) M3N@C2n (M = S, Y, lanthanides) have evolved into the largest EMF family consisting of carbon cages ranging from C68 to C96.9,10 The formal charge distribution in NCFs can be described as (M3+)3N3−@C2n6−.11−13 © 2014 American Chemical Society

Special Issue: Organometallic Electrochemistry Received: January 15, 2014 Published: April 25, 2014 4537

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Scheme 1. Fullerene-Based and Endohedral Redox Processes in Endohedral Fullerenesa

a

Blue arrows indicate localization of spin and extraneous charge over the fullerene cage (upper part) or endohedral cluster (lower part) after a redox reaction.

and they are normally easier to reduce or oxidize than empty fullerenes (Figure 2).33,52,53 Redox potentials of MII@C2n mono-EMFs (MII = Sm, Tm, Yb), which have a diamagnetic carbon cage, are rather similar to those of empty fullerenes.33,54

Figure 1. Molecular structures of selected EMFs: (a) monometallofullerenes La@C82; (b) di-EMF La2@C80; (c) di-EMF Sc2@C82; (d) nitride clusterfullerene Sc3N@C80; (e) oxide clusterfullerene Sc4O2@ C80; (f) sulfide clusterfullerene Ti2S@C78; (g) Ti-based NCF TiSc2N@C80; (h) TiLu2C@C80 with a μ3-carbido ligand; (i) schematic description of charge distribution in TiLu2C@C80 and the TiC double bond. Color code: La, orange; Sc, magenta; Ti, cyan; Lu, green; N, blue; O, red; S, yellow; endohedral C atom, dark gray. Lines connecting Sc atoms in Sc2@C82 and Sc4O2@C80 denote Sc−Sc bonds.

Fullerenes are good electron acceptors and undergo multiple reversible single-electron redox processes in solution. For instance, electrochemical studies of C60 showed that it is able to accept up to six electrons under optimized conditions at reduced temperature,42 whereas four reversible reductions steps are normally accessible in o-dichlorobenzene (o-DCB) at room temperature. Similar reduction behavior has also been described for other fullerenes.43−46 Cations of fullerenes are not chemically stable; oxidation of the most abundant C60 and C70 fullerenes occurs at relatively high potentials, making it difficult to achieve the reversible process in standard electrochemical studies.47−50 However, oxidation of many other higher fullerenes (C76, C78, C82, C84, etc.) occurs at less positive potentials and one or two oxidation steps are accessible straightforwardly.44,45,51 Encapsulation of metal atoms and clusters in EMFs can result in redox behavior more complex than that for empty fullerenes. While the carbon cage is the only redox-active center in the latter, both the cage and the endohedral species can exhibit redox activity in EMFs. Scheme 1 shows two extremes of such redox behavior. In the first case, only the carbon cage is redoxactive, meaning that the metal/π-system interface remains inert during electrochemical processes. In terms of organometallic electrochemistry, the fullerene behaves as a noninnocent ligand. All monometallofullerenes (mono-EMFs) exhibit fullerenebased redox properties. The redox behavior of MIII@C2n monoEMFs (MIII = Y, La, Ce, Pr, Gd, Er, etc.) is nevertheless substantially different from that of empty fullerenes because MIII@C2n molecules are radicals with an unpaired electron delocalized over the fullerene cage. As a result, their electrochemical (EC) gap is relatively small (less than 1 V),

Figure 2. Cyclic and differential pulse voltammograms of La@C82 in comparison to that of C60. Reproduced with permission from ref 53. Copyright 1993 American Chemical Society.

Another possibility for the mechanism of a redox process of an EMF molecule is shown in the lower part of Scheme 1. In this second case, the endohedral cluster is the redox-active species, whereas the carbon cage merely acts as an inert container, transparent to electrons. In terms of organometallic electrochemistry, here the fullerene cage is a noninnocent ligand, even though the electron transfer occurs across the metal/π-system interface. This type of electron transfer is known as an endohedral electron transfer process and is the main subject of endohedral electrochemistry, also known as electrochemistry in cavea.55 It is not known yet if such electron transfer occurs through the intermediate state with the charge localized on the carbon cage or if the electron tunnels through the carbon cage wall. An obvious, but not always necessary, prerequisite for endohedral redox activity is a suitable energy of the metal-based molecular orbitals, which should be the frontier MOs (HOMO or LUMO) of the EMF molecule. In principle, this condition can be fulfilled for all EMFs whose endohedral species are more complex than a single metal atom. 4538

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La−La bonding MO is the LUMO in (La3+)2@C2n6−), while Lu tends to adopt a divalent state (the Lu−Lu bonding MO is the HOMO or even below the HOMO level in (Lu2+)2@C2n4−), whereas the valence state of Sc and Y is more sensitive to the energy of the carbon cage MOs.56 Cluster-Based Reduction in Di-EMFs. Since the La−La bonding MO is the LUMO in La2@C2n di-EMFs (Figure 3), reduction of a La2@C2n molecule should be an endohedral redox process. Electrochemical studies of La2@C2n (2n = 72, 78, 80) showed that these EMFs exhibit two to three reversible single-electron-reduction steps and are relatively easy to reduce (Figure 4). For instance, the first reduction of La2@C80-Ih

Experimentally, the endohedral redox processes can be revealed via unexpected redox behavior (e.g., shifted potential in comparison to analogous molecules) and/or with the use of ex situ or in situ spectroelectrochemical methods (such as electron spin resonance or nuclear magnetic resonance spectroelectrochemistry). In the remaining sections of this review we will discuss the peculiarities of endohedral redox processes in EMFs with a particular focus on dimetallofullerenes and the effect of electron transfer on metal−metal bonding, endohedral reduction of Sc3N@C80, and clusterfullerenes whose endohedral redox activity is caused by single metal atoms changing their valence state, such as Ti and Ce.

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METAL−METAL BONDING AND ENDOHEDRAL REDOX PROCESSES IN EMFS Whereas mono-EMFs show only cage-based redox properties, the encapsulation of two normally trivalent metal atoms (such as Sc, Y, and some lanthanides) within an EMF often results in dimetallofullerenes with endohedral redox activity. The metalbased redox activity of M2@C2n is due to the metal−metal bonding orbital (Figure 3), which has an energy comparable to

Figure 4. Cyclic and differential pulse voltammograms of La2@C80. Reproduced with permission from ref 58. Copyright 1995 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, Germany.

occurs at −0.31 V (hereafter all redox potentials are given versus the Fe(Cp)2+/0 couple),58 whereas nitride clusterfullerenes M3N@C80 with the same C80-Ih carbon cage are fullerenebased reductions and are reduced at ca. −1.4 V.59,60 The 1.1 V difference in the first reduction potentials for the EMFs with the same carbon cage points to the metal-based reduction in La 2 @C 80 -I h . Likewise, the first reductions of La 2 @C 72 (−0.68),61 La2@C78 (−0.40 V),62 and La2@C80-D5h (−0.36 V)63 are also significantly more positive than for EMFs with fullerene-based reduction. Another indication of the cluster-based reduction in La diEMFs is the difference between the first and the second reduction potentials. For a cage-based redox process, the difference between the first and second reduction (or oxidation) steps is usually within the range 0.40−0.45 V if the process is based on the same cage MO (Figure 2). An endohedral redox process results in a much larger potential difference for the consequent redox steps, since these steps are either based on the cluster MO (which has a much higher onsite Coulomb interaction than in the fullerene cage) or affect different MOs (one on the cluster and one on the carbon cage). The difference between the first and second reduction potentials in all La2@C2n di-EMFs is in the range of 1.23− 1.44 V (Figure 4), which unambiguously points to a La2-based reduction.58,61−63 The M−M bonding orbitals in di-EMFs have hybrid spd character64 and inherit a significant ns-component from the (ns)σg2 MO of corresponding metal dimers.56 When a metalbased MO is transformed to a singly occupied MO via either reduction or oxidation, the resulting SOMO also has a large contribution of metal ns orbitals. Hence, the presence of M−M bonds can be verified straightforwardly by ESR spectroscopy of anion or cation radicals, since a large ns contribution in the

Figure 3. Metal−metal bonding molecular orbitals in selected EMFs: (a) HOMO of Sc2@C82-C3v(8); (b) HOMO of Y2@C82-C3v(8); (c) HOMO of Sc4O2@C80-Ih(7); (d) LUMO of La2@C80-Ih(7); (e) LUMO of La2@C100-D5(450). Adapted from ref 56.

that of the frontier MOs of the carbon cage and can be either the HOMO or the LUMO of the di-EMF molecule. In the former case, there is an M−M bond already in the neutral state of the di-EMF, whereas in the latter case the M−M bond can be formed when the M−M bonding orbital is populated by the reduction process. Whether the M−M bonding MO in a given di-EMF involves the HOMO or the LUMO depends on the relative energies of the cage frontier MO and the energy of the metal−metal bonding orbital. It was shown that the energy of the M−M bonding MO in EMFs is similar to the lowest energy valence MO of the free metal dimer, which usually has (ns)σg2 character.56 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, and therefore this excitation energy to a large extent determines the valence state of metal atoms in di-EMFs. Namely, ns 2 (n−1)d 1 → ns 1(n−1)d2 excitation energies increase in the row La−Sc/Y−Lu as 0.33− 1.43/1.36−2.34 eV,57 respectively, and the (ns)σg2 MOs in the corresponding M2 dimers are stabilized in the row La2−Sc2/ Y2−Lu2.56 As a result, in di-EMFs, La is always trivalent (the 4539

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Sc−Sc bonding MO largely localized on two Sc atoms with one oxygen neighbor (Figure 3c).40,74 The valence state of these two Sc atoms is therefore ScII. The two other Sc atoms (which have two oxygen neighbors) are in the ScIII state. The LUMO of Sc4O2@C80 is also localized on the cluster and has multicenter Sc bonding character. On the basis of the frontier MO analysis, both the reduction and oxidation of Sc4O2@C80 are expected to be endohedral redox processes. Electrochemical studies have shown that Sc4O2@C80-Ih exhibits two reversible reduction and two reversible oxidation steps.36,75 The redox potentials are consistent with clusterbased redox processes. Namely, the first reduction potential, at −1.10 V, is more positive than that in M3N@C80-Ih (ca. −1.4 V) and the gap between the first and second reductions is 0.63 V. The oxidation potential of Sc4O2@C80-Ih is 0.0 V, which is well below the +0.5−0.6 V that is expected for a fullerene-based oxidation process in EMFs with a C80-Ih cage. The difference between the first and second oxidation steps is 0.79 V. Thus, the electrochemical data as well as the DFT computations indicate that the redox activity of Sc4O2@C80 is dominated by cluster-based processes. Unambiguous proof of cluster-based reduction and oxidation in Sc4O2@C80 was provided by ESR spectroelectrochemistry.75 The ESR spectra measured during the electrolysis of Sc4O2@ C80 at the potentials of the first reduction and oxidation steps showed rich 45Sc hyperfine structure due to two pairs of equivalent Sc atoms in the Sc4O2 cluster. In the cation radical, the 45Sc hfc constants are 2 × 154.4 and 2 × 18.0 G. The largest constant is assigned to the ScII atoms, and the large value is consistent with the nature of the ScII−ScII bonding with a large 4s contribution.56,75 The 45Sc hfc constants in the anion radical are considerably smaller, 2 × 2.6 and 2 × 27.4 G, but still point to a large Sc contribution to the spin density. The smaller a(45Sc) values are due to the different nature of the Sc−Sc bonding LUMO of Sc4O2@C80, with a larger 3d and a smaller 4s contribution.

SOMO leads to a large metal-based hyperfine coupling (hfc) constant. The ESR spectrum of the La2@C80-Ih radical anion with a huge 139La coupling constant of 364 G is a very illustrative example.65 This value can be compared to the 139La hfc constant of 1.2 G found in La@C82, whose spin density is delocalized over the C82 cage.7 Substituting one carbon atom for a nitrogen atom in M2@ C80-Ih results in the paramagnetic azafullerenes M2@C79N (M = Y, Tb, Gd).66,67 The SOMO of these molecules is an M−M bonding MO similar to the anion radicals of La2@C80. In line with this MO analysis, the ESR study of Y2@C79N revealed an enhanced 89Y hfc constant of 81.2 G66 (in comparison to 0.5 G in Y@C82 with cage-based spin density68). An electrochemical study of Gd2@C79N showed that its first reduction potential, −0.96 V, is significantly more positive than the cage-based first reduction potential of M3N@C80-Ih (although not as strongly shifted as in La2@C80-Ih). The second reduction of Gd2@C79N is found at −1.98 V: i.e., the difference between the first and second reduction potentials is as large as 1.02 V. On the basis of redox potentials and DFT calculations, which predict a Gd2localized LUMO, the first reduction of Gd2@C79N can be tentatively assigned as an endohedral reduction. Interestingly, the M−M bonding SOMO in M2@C79N is “buried” below the cage-based MOs, and hence oxidation of M2@C79N should be a cage-based process.66,67 Indeed, the oxidation potential of Gd2@C79N is +0.51 V, which is close to the values of La2@C80Ih (+0.56)58 and M3N@C80 (ca. +0.6 V). Cluster-Based Oxidation in Di-EMFs. Computational studies predict that the Lu−Lu bonding MO in Lu2@C2n diEMFs is always occupied and is usually the HOMO (e.g., Lu2@ C76-Td and Lu2@C82-C3v(8)): i.e., oxidation of Lu2@C2n EMFs is expected to be an endohedral redox process.56,64 To our knowledge, experimental electrochemical studies of lutetiumbased di-EMFs have not yet been reported. Spectroscopic and computational studies of M2@C82 diEMFs (M = Sc, Y, Er) show that the formal charge of the carbon cage (4−), and hence the metal atoms within, are in a divalent state with an M−M bonding HOMO.40,56,69−72 Unfortunately, only a few electrochemical studies of such diEMFs have been reported. Very illustrative of this is a recent study of Sc2@C82-C3v(8) and its comparison to Sc2C2@C82C3v(8), a carbide clusterfullerene with the same carbon cage.70 UV−vis−NIR absorption spectra of both EMFs are very similar, which proves that the C82-C3v(8) carbon cage in both molecules has the same formal charge (4−). At the same time, the first oxidation of Sc2@C82 occurs at +0.05 V, whereas the oxidation potential of Sc2C2@C82 is ca. +0.50 V. In the latter, the HOMO is a cage-based MO, whereas the HOMO of Sc2@ C82 is a Sc−Sc bonding orbital (Figure 3a). Therefore, the oxidation of Sc2@C82 is an endohedral redox process. DFT calculations have shown that both the HOMO and the LUMO of Y2@C82-Cs(6) have considerable metal contribution. Unfortunately, electrochemical studies of Y2@C82 have not yet been reported; however, the chemically generated anion radical of Y2@C82-Cs(6) has a relatively large 89Y hfc value of 34.3 G,73 which is smaller than that in Y2@C79N. It would be interesting to see if the cation radical of Y2@C82 also has a large 89Y hfc constant, since the metal contribution is greater to the HOMO than to the LUMO. Oxide Clusterfullerene Sc4O2@C80-Ih. In Sc4O2@C80, the Sc atoms form a tetrahedral cluster, and the two μ3-coordinated oxygen atoms are located above the centers of two faces of the tetrahedron (Figure 1e).23 The HOMO of the molecule is a

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Sc3N@C80 AND ITS DERIVATIVES Sc3N@C80 versus Other Nitride Clusterfullerenes. Nitride clusterfullerenes (NCFs), including Sc3N@C80-Ih, usually exhibit two or three irreversible reduction steps and one or two oxidations, the first of which is usually reversible (Figure 5a). Due to their higher abundance, M3N@C80 NCFs with Ih cages have been the most studied to date. When M is yttrium or a lanthanide, the first reduction and oxidation potentials of M3N@C80-Ih are near ca. −1.4 and +0.65 V, respectively, and the values are almost independent of the metal (see ref 33 for a complete list of redox potentials). However, Sc3N@C80-Ih behaves differently in that its reduction potential is shifted anodically to −1.25 V, and its electrochemical reversibility can be achieved at scan rate of a few volts per second.76,77 ESR spectroscopic studies of the Sc3N@C80 anion77−79 as well as computational studies80,81 indicated that the reduction of Sc3N@C80 proceeds via the occupation of the cluster-based LUMO (Figure 6). In particular, 45Sc hfc constants in the Sc3N@C80− anion radical are 3 × 55.6 G (all Sc atoms are equivalent), which is the second highest value; the cation radical Sc4O2@C80+ exhibits the highest value, as discussed in the previous section. For comparison, the spin density in the cation and anion radicals of Sc3N@C68 is localized on the carbon cage, and the 45Sc hfc constants are smaller than 2 G.82,83 Thus, reduction of Sc3N@C80-Ih is an endohedral redox process. 4540

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which is substantially smaller than the 81.2 G that is found in Y2@C79N with Y-localized spin density.66 When a mixture of two metals is used in the synthesis, the resulting NCFs have mixed-metal nitride clusters. It might be expected that the difference in the electronegativity of Sc and other metals should affect localization of the LUMO in Sclanthanide mixed-metal NCFs. Indeed, reduction potentials of Sc3M3−xN@C2n are usually more positive than those of M3N@ C2n, but the effect is not very large.86,87 DFT computations also show that Sc has an enhanced contribution to the LUMOs of Sc3M3−xN@C2n NCFs in comparison to other metal atoms.86 ESR spectroelectrochemical studies could give more detailed information on the spin density distribution in anion radicals of mixed-metal NCFs but have yet to be reported. Exohedral Derivatives of Sc3N@C80. Chemical derivatization changes the π system of the fullerene and hence can substantially affect electrochemical properties. For instance, derivatization of Sc3N@C80 shifts its redox potentials and often makes reduction behavior more electrochemically reversible than that of nonderivatized Sc3N@C80 (Figure 5).59,79,85,88 Although predominant localization of the LUMO on the Sc3N cluster is preserved, at least at lower degrees of addition, redistribution between the cluster and the carbon cage usually takes place. In addition, the dynamics of the cluster are altered: in pristine Sc3N@C80-Ih, the Sc3N cluster is known to rotate freely inside the fullerene cage, whereas its rotation is frozen in the derivatives. One of the first synthesized derivatives of Sc3N@C80 was its pyrrolidine cycloadduct, denoted hereafter as [5,6]-pyrrolidinoSc3N@C80 ([5,6] means that the cycle is added across the pentagon/hexagon edge). This cycloaddition makes the first reduction reversible and shifts its potential anodically by +0.11 V.59 The ESR spectrum of the anion radical of [5,6]pyrrolidino-Sc3N@C80 exhibits a hyperfine structure with 45Sc hfc constants of 9.6 and 2 × 33.4 G (Figure 7a).89 The Sc atoms in the anion radical are no longer equivalent, which means that the cluster is not rotating on the ESR time scale and that the spin density is redistributed so that only two of the Sc atoms have considerable contribution to the spin density (Figure 7b,c). Thus, reduction of [5,6]-pyrrolidino-Sc3N@C80 is still a cluster-based process, but only two of the Sc atoms in the cluster are redox active. Detailed electrochemical and ESR spectroscopic studies were reported for a series of trifluoromethylated derivatives Sc3N@ C80(CF3)x (x = 2−12).79,88,90 The first reductions of all studied derivatives are reversible and are more positive than that of the parent molecule Sc3N@C80-Ih. The positive shift ranges from +0.10 V in Sc3N@C80(CF3)2 to +0.42 V in Sc3N@C80(CF3)10. The Sc-based hyperfine structure in the ESR spectra of Sc3N@C80(CF3)2,10,12 anion radicals generated either by electrochemical means or via reaction with cobaltocene showed a systematic decrease of a(45Sc) values with an increase in the number of CF3 groups. DFT calculations and a decrease of the 45 Sc coupling constants in anion radicals of Sc3N@C80(CF3)x shows that the spin density in anions is gradually shifting from the Sc3N cluster to the carbon cage in higher trifluoromethylated derivatives.88 Thus, the “innocent” C80-Ih cage in Sc3N@ C80 becomes “non-innocent” after the addition of 12 CF3 groups. An interesting situation was observed for Sc3N@ C80(CF3)2 at the third electron reduction step: the trianion radical exhibited a large 45Sc coupling constant for one of the Sc atoms (49.2 G), which agreed with DFT-predicted localization of the spin density on one of the three Sc atoms.79 Hence, the

Figure 5. Cyclic voltammograms of (a) Sc3N@C80-Ih, (b) Ntritylpyrrolidino-[5,6]-Sc3N@C80-Ih, and (c) N-tritylpyrrolidino[6,6]-Sc3N@C80-Ih. The scan rate is 100 mV s−1. Reproduced with permission from ref 85. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.

Figure 6. (a) Spin density distribution in Sc3N@C80−. (b) Spin density distribution in Y3N@C80−. Note that the lowest energy orientations of the M3N cluster in M3N@C80− are different for Sc and Y. Reproduced with permission from ref 80. Copyright 2008 American Chemical Society. (c) ESR spectrum of Sc3N@C80−, obtained by the reaction of Sc3N@C80 and cobaltocene in o-dichlorobenzene at room temperature. The asterisk marks an impurity signal (less than 1% pf the total signal intensity). Adapted from ref 79.

The reason for the special cathodic behavior of Sc3N@C80 is the higher electronegativity of Sc, whose 3d orbitals have lower energies than the 4d orbitals of Y and the 5d orbitals of the lanthanides. It leads to the enhanced contribution of Sc to the LUMO of Sc3N@C80, whereas the cage contribution to the LUMO is dominating for non-Sc M3N@C80. For instance, DFT computations show that the net spin population of the Y3N cluster in Y3N@C80− is less than 29% (in comparison to 65% in Sc3N@C80−; see Figure 6).80,81 The ESR studies of the anion radicals of pristine M3N@C80-Ih NCFs have not been reported, but in the anion radical of the Y3N@C80 pyrrolidine cycloadduct the 89Y hfc constants are 1.4 and 2 × 6.3 G,84 4541

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specific electrochemical properties that are drastically different from those of all other M3N@C80-Ih NCFs.97−99 First, their first reduction is reversible at moderate scan rates (for TiSc2N@C80, three reversible reductions are observed even at a small scan rate of 20 mV/s; see Figure 8). Second, their redox potentials

Figure 7. (a) Experimental (top) and simulated (bottom) X-band ESR spectra of the chemically reduced monoanion of [5,6]-pyrrolidinoSc3N@C80. The asterisk denotes an impurity. (b, c) Spin density isosurfaces for the radical anion of [5,6]-pyrrolidino-Sc3N@C80. (b) and (c) show two conformations of the cluster. For the sake of clarity, the conformer in (b) is shown in two projections. Reproduced with permission from ref 89. Copyright 2013 American Chemical Society.

third reduction of Sc3N@C80(CF3)2 can be described as endohedral reduction of one particular Sc ion.

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Ti AS A REDOX-ACTIVE CENTER IN ENDOHEDRAL METALLOFULLERENES The majority of clusterfullerenes are synthesized using group III metals such as Sc, Y, and lanthanides. Ti is a rare example of a transition metal that is capable of forming EMFs. Until 2009, Ti@C28, Ti2C2@C78, and Ti2C84 were the only Ti-EMFs known.91−94 Although Ti@C28 has been detected by mass spectrometry, bulk amounts have never been isolated. Ti2C2@ C78 was a matter of controversy after its first synthesis. At first, the compound was believed to be a mixture of two dimetallofullerene isomers, Ti2@C80-Ih and Ti2@C80-D5h,92 but later computational studies showed that the molecule Ti2C2@C78-D3h is much more stable and was also more consistent with the 13C NMR spectrum.95,96 A detailed characterization of three isomers of Ti2C84 has not been reported, and their structures remain unknown (e.g., it is not clear if Ti2C84 is actually Ti2C2@C82). None of these Ti-EMFs have been studied by electrochemical techniques. DFT calculations show that the LUMO of Ti2C2@C78 has a large contribution from the metal atoms,80 and hence its reduction is expected to be an endohedral process. In 2009 Yang et al. discovered that while Ti alone does not form nitride clusterfullerenes, the use of a Ti and Sc mixture affords the mixed-metal NCF TiSc2N@C80-Ih (Figure 1f).97 Likewise, the synthesis of TiY2N@C80-Ih was reported in 2012.98 Both of these Ti-based mixed-metal NCFs exhibit

Figure 8. (a) Cyclic voltammograms of TiSc2N@C80-Ih in o-DCB solution. Dotted vertical bars denote redox potentials of Sc3N@C80, and the inset shows the spin density distribution in the molecule. Reproduced from ref 55. Copyright 2011 American Chemical Society. (b) Cyclic voltammograms of TiY2N@C80-Ih in o-DCB solution. Dotted vertical bars denote redox potentials of TiSc2N@C80. Reproduced with permission from ref 98. Copyright 2012 American Chemical Society.

are also substantially different from those of NCFs. The first reduction steps are more positive (−0.94 V in TiSc2N@C80-Ih and −1.11 V in TiY2N@C80-Ih versus −1.25 V in Sc3N@C80-Ih and ca. −1.4 V in other M3N@C80-Ih). Their oxidation steps are shifted cathodically to +0.16 V for TiSc2N@C80 and 0.00 V for TiY2N@C80 (compare to ca. +0.6 V in M3N@C80-Ih). As a result, the electrochemical gaps of Ti-based NCFs are around 1.1 V, which is nearly half of those for all other M3N@C80 NCFs. The reason for the unique redox behavior of TiM2N@C80-Ih NCFs can be understood by taking into account that Ti is the next element after Sc in the periodic table. As a result, TiSc2N@ C80 is isoelectronic with the anion radical of Sc3N@C80 and is therefore paramagnetic. ESR studies of TiM2N@C80 (M = Sc, Y) at room temperature showed broad signals; however, the line width decreased at lower temperatures.97−99 Below liquid 4542

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Figure 9. (a) Cyclic voltammetry and square wave voltammetry (SWV) curves of TiLu2C@C80-Ih (black lines) and Lu2ScN@C80-Ih (blue lines). (b) Kohn−Sham MO energy levels (occupied, black; unoccupied, pink) of TiLu2C@C80-Ih, TiLu2N@C80-Ih, and Lu2ScN@C80-Ih (for open-shell TiLu2N@C80, spin-up and spin-down levels are shown separately). (c) Isosurfaces of the frontier MOs of TiLu2C@C80-Ih and Lu2ScN@C80-Ih. Reproduced with permission from ref 32. Copyright 2014 Macmillan Publishers Limited.

nitrogen temperature, TiSc2N@C80 exhibited a fine structure, although it was not sufficiently resolved for a precise determination of hyperfine coupling constants.99 A rough estimation showed that 45Sc hfc constants are less than 7 G. An important feature common to the ESR spectra of TiM2N@C80 NCFs is the strong shift of the g factor (1.9454 in TiSc2N@C80 and 1.9579 in TiY2N@C80) from the free electron value of 2.0023. Such a shift shows that the orbital momentum is not quenched and indicates localization of unpaired spin on Ti. DFT calculations also show that in both TiM 2 N@C 80 molecules the spin density is predominantly localized on Ti (Figure 8a inset). Thus, the formal valence state of titanium is TiIII with a localized 3d1 electron. The SOMO of TiM2N@C80 is thus largely a Ti-based orbital; therefore, oxidation and reduction steps proceed via a change in the valence state of the endohedral Ti, which becomes TiII in TiM2N@C80− and TiIV in TiM2N@C80+. So far, TiM2N@C80 is the only example of an EMF with an endohedral redox activity in both the reduction and oxidation caused by a single metal atom. Ti-based redox activity was also found in the recently discovered molecule TiLu2C@C80-Ih, which has a central carbon atom and a TiC double bond.32 Unlike paramagnetic TiM2N@C80 NCFs, TiLu2C@C80 is diamagnetic and its electronic structure has a certain similarity to that of the Scbased nitride clusterfullerene Lu2ScN@C80 as a result of the Ti3+C3− fragment being isoelectronic with Sc3+−N3−. At room temperature in o-DCB solution, TiLu2C@C80 exhibits reversible reduction and oxidation steps at −0.91 and +0.63 V, respectively (Figure 9a). The redox behavior (the potential and reversibility) for the first reduction of TiLu2C@ C80 is similar to that of TiSc2N@C80 and TiY2N@C80. This can be compared to the first reduction step of Lu2ScN@C80, which is irreversible and occurs at a potential 0.51 V more negative (Figure 9a). However, the first oxidation of Lu2ScN@C80 is reversible and occurs at +0.66 V, which is very close to the oxidation potential of TiLu2C@C80 (+0.63 V). These values can be compared to the much more negative oxidation potentials of TiSc2N@C80 (+0.16 V) and TiY2N@C80 (0.00 V). Thus, electrochemical measurements show that the electronic structure of TiLu2C@C80 is intermediate between

that of conventional M3N@C80 NCFs with group III metals and NCFs with one Ti atom, TiM2N@C80. This behavior can be rationalized by the DFT-based frontier orbital analysis. Figure 9 compares the MO energy levels in TiLu2C@C80, TiLu2N@C80, and Lu2ScN@C80 as well as the isosurfaces of their HOMOs and LUMOs. The LUMO of TiLu2C@C80 is largely localized on the Ti atom and is 0.27 eV below the LUMO energy of Lu2ScN@C80, which has a much larger carbon cage contribution. Hence, the first reduction of Lu2ScN@C80 is much more negative. In contrast, the Tilocalized LUMO of TiLu2N@C80 is almost isoenergetic with that of TiLu2C@C80, and hence their reduction potentials should be similar. The shapes and the energies of the HOMOs of TiLu2C@C80 and Lu2ScN@C80 are almost identical, and both MOs are essentially carbon cage orbitals. Hence, the oxidation potentials of TiLu2C@C80 and Lu2ScN@C80 are similar and are more positive in comparison to those of TiM2N@C80 NCFs with a Ti-localized SOMO. In 2013, Echegoyen et al. reported the synthesis and electrochemical study of Ti2S@C78-D3h, a new type of Ticontaining clusterfullerene with a sulfide cluster (Figure 1e).100 The molecule is isostructural with Ti2C2@C78 and likewise has titanium in a TiIV state with a metal-localized LUMO (Figure 10). Hence, an endohedral reduction of Ti2S@C78 can be anticipated. Ti2S@C78 exhibits three reversible reduction and two reversible oxidation steps (Figure 10a). The first reduction potential of −0.92 V is much more positive than that of Sc3N@ C78-D3h, which has the same carbon cage (−1.54 V101). Instead, it is closer to those of TiSc2N@C80 (−0.94 V99) and TiLu2C@ C80 (−0.91 V). The difference between the first and the second reduction potentials is 0.61 V. Thus, electrochemical data confirm that the reduction of Ti2S@C78 is an endohedral redox process. Its oxidation, however, is a cage-based process, in accordance with the HOMO shape and the difference in the first and second oxidation potentials (0.42 V). 4543

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[email protected] However, it remained unclear what was so special about CeLu2N@C80 that forced the CeIII redox behavior, whereas all other Ce-EMFs do not show such behavior. This question was clarified in a more recent study of other CeM2N@C80-Ih NCFs (M = Sc, Y).110 In this work, Sc and Y were chosen to vary the size of the endohedral cluster in the CeM2N@C80 series (Shannon’s radii of Sc3+, Lu3+, and Y3+ ions are 0.745, 0.86, and 0.90 Å, respectively111). Figure 11 shows

Figure 10. (a) Cyclic voltammetry of Ti2S@C78-D3h in o-DCB solution. (b) HOMO and LUMO of Ti2S@C78-D3h. Reproduced with permission from ref 100. Copyright 2013 Royal Society of Chemistry.

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STRAIN-DRIVEN ENDOHEDRAL CeIV/CeIII REDOX COUPLE Ce is different from all other lanthanides in that its CeIV valence state is accessible and is known for a plethora of inorganic and organometallic compounds, and the redox potential of the CeIV/CeIII couple can vary over a broad range.102,103 However, in all known cerium EMFs, including mono- and di-EMFs as well as nitride clusterfullerenes, this metal always adopts a CeIII valence state and therefore Ce-based EMFs are isostructural with La-EMFs. The redox behaviors of Ce and La EMFs are usually very similar, which shows that the CeIII state in Ce2@ C72, 78, 80,63,104−106 Ce@C82,52 and Ce3N@C92,96107,108 remains unaffected by the electrochemical oxidation of the EMF molecules, similar to the LaIII state in La-EMFs (however, the reduction of Ce di-EMFs is expected to form a Ce−Ce bond similar to that in La di-EMFs, as discussed above). The first Ce-EMF that showed the endohedral Ce atom can be redox active was CeLu2N@C80-Ih, whose synthesis and electrochemical studies were reported in 2010.109 The temperature-dependent paramagnetic shifts in the 13C NMR spectra of CeLu2N@C80-Ih confirmed the CeIII valence state with a Ce-localized 4f1 electron. The electrochemical measurements revealed an unprecedented negative shift of the oxidation potential of CeLu2N@C80 (+0.01 V), which compares to the standard values of ca. +0.60−0.65 V in other M3N@C80-Ih NCFs. DFT calculations showed that the oxidation potential of the cage-based process in CeLu2N@C80-Ih would be similar than those of other NCFs. However, removal of the 4f1 electron from Ce, i.e. its endohedral oxidation, was predicted to be more energetically favorable by 0.45 eV at the PBE0 level. Thus, endohedral oxidation of CeIII to CeIV was proposed as a plausible explanation of the unusual electrochemical behavior of

Figure 11. (a) Cyclic voltammograms of CeM2N@C80 (M = Y, Lu, Sc) and PrSc2N@C80 measured in o-DCB solution with TBABF4 as supporting electrolyte and scan rate 100 mV/s. (b) Schematic description of endohedral oxidation of CeIII in CeM2N@C80. Reproduced with permission from ref 110. Copyright 2013 American Chemical Society.

cyclic voltammograms of all three CeM2N@C80-Ih NCFs in comparison to that of PrSc2N@C80-Ih (the latter was chosen as a reference because of the close ionic radii of Ce3+ at 1.01 Å, and Pr3+ at 0.99 Å). All four compounds exhibit similar cathodic behavior with two irreversible reductions near −1.36 and −1.92 V (peak potentials Ep are listed), close to the values of all other M3N@ C80-Ih NCFs. The variation of Ep values with different cluster compositions does not exceed 0.1 V. The anodic behavior of CeM2N@C80 NCFs is more peculiar. All compounds exhibit one reversible oxidation step whose potential varies from −0.07 V for CeY2M@C80 to +0.33 V for CeSc2N@C80. The oxidation potential of PrSc2N@C80 measured under the same conditions is +0.64 V. 4544

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A considerable negative shift of the first anodic process in CeM2N@C80-Ih in comparison to PrSc2N@C80-Ih and other M3N@C80-Ih NCFs served as a first indication of a Ce-based redox process. Compelling evidence of the endohedral oxidation of CeIII in CeM2N@C80-Ih NCFs was obtained by 13 C NMR spectroscopy. If the oxidation of CeM2N@C80 is a fullerene-based process, their radical cations are expected to give no detectable signals in 13C NMR spectra because of the spin density on the fullerene cage. In contrst, an endohedral CeIII → CeIV oxidation produces diamagnetic cations, which should be accessible by 13C NMR spectroscopy. Figure 12

spectrum of CeSc2N@C80 is shifted to 175 ppm for [CeSc2N@ C80]+[PF6]− (Figure 12b), which is close to the value δ(45Sc) 190 ppm measured for Sc3N@C80 in o-DCB. NMR spectroscopy clearly proved that the oxidation of CeM2N@C80 is an endohedral CeIII → CeIV redox process, but it could not clarify why the oxidation potential of CeIII in the CeM2N@C80 NCF depends so strongly on the second cluster metal, M, which is not involved in the redox process. This question was answered with the help of DFT calculations, which showed that in the CeIVM2N@C80+ cations the Ce−N bond lengths are shortened in comparison to those in the neutral CeM2N@C80, whereas the M−N bonds are elongated. These structural changes are caused by the smaller ionic radius of Ce4+ (0.87 Å) in comparison to Ce3+ (1.01 Å). In short, oxidation reduces the size of the endohedral cluster. It should be noted that encapsulation of relatively large CeIIIM2N clusters in a C80-Ih(7) cage within the limited interior space results in a significant strain. For CeY2N@C80, it even forces pyramidalization of the CeY2N cluster. M3N clusters are usually planar in NCFs, and pyramidalization is an indication of the strain caused by insufficient space for the large cluster inside the cage. A decrease in the cluster size for CeIVM2N@C80+ cations therefore leads to a decrease of the cluster-induced strain. In particular, the CeY2N cluster becomes planar in CeIVY2N@C80+ (Figure 12c). The increase of the ionic radius of M3+ in the Sc → Lu → Y series increases the cluster-induced strain and makes the corresponding CeM2N@C80 more prone to oxidation as a way to release the strain. Therefore, the oxidation potential of CeM2N@C80-Ih shifts to more negative values for larger M3+ ions. The influence of the strain on the oxidation potential of the endohedral CeIII atom in CeM2N@C80 also explains why endohedral CeIII → CeIV was not observed in many other previously studied Ce-based EMFs. In these molecules, either the number of Ce atoms is too small (mono- and dicerium fullerenes) or the cage size is too large (Ce3N@C92,96), resulting in a relatively low inner strain in comparison to CeM2N@C80 NCFs. The concept of the strain-driven CeIV/CeIII endohedral redox couple was further developed in a recent study of CexM3−xN@ C2n NCFs with different cages and cluster compositions (x = 1, 2; M = Sc, Y; 2n = 78, 84, 86, 88).112 Redox potentials were determined for 12 Ce-containing NCFs and compared to those of the non-Ce analogues. On the basis of the shift of the oxidation potential and an increased difference between the first and second oxidation potentials, an endohedral CeIII → CeIV redox process at the first oxidation step was proven for CeSc2N@C78, CeY2N@C84, and Ce2YN@C86. Less confidently, an endohedral oxidation of Ce was also proposed for Ce2ScN@ C86, CeY2N@C86, and Ce3N@C88. For cages larger than C80, the cluster-induced strain is weaker than for the C80-Ih cage, whereas cage oxidation potentials are below those for M3N@C80-Ih. Thus, the preference of an endohedral Ce or a fullerene-based oxidation at the first anodic step depends on whether the inner strain is high enough to render a Ce-based oxidation below the oxidation potential of the fullerene cage. It is thus possible that varying the cluster composition can switch the oxidation mechanism. As an example, Figure 13 compares cyclic voltammograms of Y3N@ C86 and CexM3−xN@C86 (x = 1, 2; M = Sc, Y). The oxidation potentials of Y3N@C86 (+0.36 and +0.77 V) and CeSc2N@C86 (+0.34 and +0.80 V) are virtually identical, showing that the oxidation of CeSc2N@C86 is a cage-based process. In Ce2ScN@

Figure 12. (a) 13C NMR spectra of paramagnetic CeIIIM2N@C80 (M = Sc, Y, Lu) and their oxidized diamagnetic counterparts [CeIVM2N@ C80]+ measured in o-d4-DCB at 288 K. (b) 45Sc NMR spectra of CeSc2N@C80 and [CeSc2N@C80]+. (c) Change in the cluster geometry in CeY2N@C80 after oxidation (the pyramidal CeIIIY2N cluster becomes planar CeIVY2N; Y atoms are green, nitrogen is blue, and Ce is pink). Parts (a) and (b) are reproduced with permission from ref 110. Copyright 2013 American Chemical Society.

shows 13C NMR spectra of pristine CeM2N@C80-Ih NCFs and the spectra of the same compounds after addition of oxidation agents chosen to singly oxidize the compounds ([Fe(Cp)2]+[BF4]− for CeY2N@C80 and Ag+[PF6]− for CeSc2N@ C80 and CeLu2N@C80). The spectra show that the oxidation of CeM2N@C80 results in diamagnetic CeM2N@C80+ cations with the same two-line 13C NMR pattern for the C80-Ih(7) cage as in the initial compounds, but with the position of the signals shifted. Furthermore, the peak at δ 280 ppm in the 45Sc NMR 4545

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NCFs were performed using the GGA PBE functional mixed with 15% of exact exchange. 112 Computed values are schematically presented in Figure 14. The border between blue and yellow fields corresponds to the IPcage value. When the IPCe value appears in the blue field, oxidation of the cage is preferred, whereas appearance of the point in the yellow field signifies that a Ce-based oxidation takes place. Two trends in the IPCe values in Figure 14 are noted: (i) within the same cage, the IPCe values decrease in the order CeSc2N > (CeLu2N, Ce2ScN) > CeY2N > Ce2YN > Ce3N, corresponding to an increase in cluster size; (ii) the IPCe values for a given cluster increase with an increase in carbon cage size (the only exclusion is CeSc2N, whose IPCe value reaches its maximum within a C84 cage and then decreases when housed within C86 and C88 cages). Both trends are manifestations of the same factor, the steric strain experienced by the nitride cluster inside the carbon cage of a limited size (“cage pressure”). When the steric strain in the Ce-NCF is high, it is more easily oxidized than Ce-NCFs with lower strain (i.e., with smaller cluster and/ or larger cage). In other words, IPCe values drop down when the cage pressure increases. Therefore, variations in the redox potentials of endohedral CeIV/CeIII redox couples can be rationalized using simple geometrical arguments.

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Figure 13. Cyclic voltammograms of Y3N@C86, CeSc2N@C86, Ce2ScN@C86, CeY2N@C86, and a mixture of Ce2YN@C86 and Y3N@C86. The scan rate was 100 mV/s. As a guide, the vertical bars denote oxidation (E1/2) and reduction (Ep) potentials of Y3N@ C86. Reproduced with permission from ref 112. Copyright 2014 Royal Society of Chemistry.

CONCLUDING REMARKS

The encapsulation of metal clusters within a carbon cage gives rise to numerous possibilities for unconventional redox processes within the fullerene. Under certain conditions, the carbon cage can be transparent to electrons, so that the valence and spin state of the encapsulated metal atoms is changed, while the state of the carbon cage remains intact. These endohedral (or in cavea) redox processes can populate or depopulate metal−metal bonding orbitals of EMFs and thus can be used to study the metal−metal bonding within. The use of ESR spectroelectrochemistry is especially convenient for the analysis of these phenomena. The family of nitride clusterfullerenes (NCFs) provides a unique platform for the establishment and tuning of endohedral redox-active species. While in general the M3N cluster remains redox-inert in NCFs (Sc3N@C80 is a notable exception), it is

C86, the first oxidation potential is shifted to +0.29 V, whereas the difference between the first and the second oxidation steps is increased to 0.53 V. The oxidation potential of CeY2N@C86 is further shifted to +0.27 V and the difference of the oxidation potentials is increased to 0.55 V. Thus, an endohedral oxidation of Ce can be tentatively proposed for Ce2ScN@C86 and CeY2N@C86. Finally, the first oxidation potential of Ce2YN@ C86 is further shifted down to +0.17 V, allowing an unambiguous conclusion for endohedral oxidation of Ce. DFT computations of cage- and Ce-based ionization potentials (IPcage and IPCe, respectively) in CexM3−xN@C2n

Figure 14. DFT-computed IPCe values for different CexM3−xN@C2n (x = 1, 2; M = Sc, Lu, Y) mapped on IPcage ranges. The borders between blue and yellow fields correspond to IPcage values (they are different for different carbon cages). When the IPCe value is predicted to be above the borderline (i.e., in the blue field), oxidation of the cage is more energetically preferable. When the IPCe is in the yellow field, it means that a Ce-based oxidation takes place. Reproduced with permission from ref 112. Copyright 2014 Royal Society of Chemistry. 4546

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possible to use the nitride cluster as a matrix to introduce an electroactive metal in the form of the mixed-metal NCFs. Tiand Ce-based NCFs are well-established examples that are discussed in this review. In TiM2N@C80 NCFs, titanium adopts a TiIII valence state, which can be changed to TiII or TiIV via endohedral reduction or oxidation. In Ce-based NCFs such as CeM2N@C80, the large size of the nitride cluster and the limited inner space of the carbon cage result in an inherent strain. The driving force for endohedral oxidation of CeIII in Ce-NCFs is the release of this strain by forming CeIV, which has a smaller ionic radius. Since the increase of the ionic radius of M3+ (Sc → Lu → Y) or the decrease of the fullerene cage size increases the cluster-induced strain, the oxidation potential of CexM3−xN@C2n NCFs shifts to more negative values for larger M3+ ions and smaller cages. To our knowledge, there are no other reports showing that the geometric strain of Ce compounds can be used to tune the redox potential of the CeIV/CeIII couple. The ongoing progress in the synthesis and characterization of endohedral metallofullerenes promises that new and unique redox properties of these molecules will be discovered in the near future.

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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. In 2008 he received a Humboldt fellowship for a research stay at the Leibniz Institute of Solid State and Materials Research (IFW Dresden, Germany). Now he is a head of the fullerene group at IFW Dresden. His current interests include chemical and physical properties of empty and endohedral metallofullerenes and their derivatives and their synthesis, spectroelectrochemistry, optical spectroscopy, magnetic properties, and quantum-chemical computations.

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ACKNOWLEDGMENTS

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ABBREVIATIONS

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REFERENCES

The manuscript is devoted to the memory of Lothar Dunsch (1948−2013), who introduced the authors into the field of endohedral fullerenes and their electrochemistry. The authors acknowledge Sandra Shiemenz, Marco Rosenkranz, and Frank Ziegs (all at IFW Dresden) for continuous technical support and Dr. Bryon W. Larson (NREL) for his careful reading of the manuscript. The authors acknowledge the Deutsche Forschungsgemeinschaft (DFG) project PO 1602/1-1 for financial support.

AUTHOR INFORMATION

Corresponding Author

*E-mail for A.A.P.: [email protected]. Notes

The authors declare no competing financial interest. Biographies

EMF, endohedral metallofullerenes; NCF, nitride clusterfullerenes; ESR, electron spin resonance; IP, ionization potential

(1) Heath, J. R.; O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Tittel, F. K.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 7779−7780. (2) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162−163. (3) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354−358. (4) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843−892. (5) Chai, Y.; Guo, T.; Jin, C. M.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L. H.; Alford, J. M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564−7568. (6) Yannoni, C. S.; Hoinkis, M.; Devries, M. S.; Bethune, D. S.; Salem, J. R.; Crowder, M. S.; Johnson, R. D. Science 1992, 256, 1191− 1192. (7) Johnson, R. D.; Devries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992, 355, 239−240. (8) Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L.; Dorn, H. C. Nature 1999, 401, 55−57. (9) Dunsch, L.; Yang, S. Small 2007, 3, 1298−1320. (10) Zhang, J.; Stevenson, S.; Dorn, H. C. Acc. Chem. Res. 2013, 46, 1548−1557.

Yang Zhang received his Master’s degree (2009) in physical chemistry from Beijing Normal University (BNU, People’s Republic of China), under the supervision of Prof. Louzhen Fan. Subsequently, he obtained his Ph.D. in physical chemistry (2013) from the Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden, Germany), under the supervision of Prof. Lothar Dunsch and Dr. Alexey A. Popov. His main research interests are the synthesis and spectroscopic, paramagnetic, and electrochemical properties of endohedral metallofullerenes, as well as the synthesis and applications of fullerene-based nanomaterials. 4547

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Organometallics

Review

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Organometallics

Review

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