Electrochemistry In Cavea: Endohedral Redox Reactions of Encaged

PERSPECTIVE pubs.acs.org/JPCL

Electrochemistry In Cavea: Endohedral Redox Reactions of Encaged Species in Fullerenes Alexey A. Popov*,†,‡ and Lothar Dunsch†,* †

Department of Electrochemistry and Conducting Polymers, The Center of Spectroelectrochemistry, Leibniz Institute of Solid State and Materials Research (IFW) Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany ‡ Chemistry Department, Moscow State University, RU-19992 Moscow, Russian Federation ABSTRACT: Electron transfer to/from endohedral metallofullerenes usually leads to population/depopulation of the fullerene-based molecular orbitals. However, for certain EMFs, endohedral redox activity was discovered, and the number of such fullerenes is continuously increasing with the developments in EMF synthesis and fulfillment of more dedicated studies of their properties. In this Perspective, the state of the art in the emerging field of endohedral (in cavea) electrochemistry is reviewed. We describe representative examples of EMFs that exhibit endohedral electrochemical activity and discuss the special phenomena (spin
charge separation, spin flow dynamics) accompanying endohedral redox reactions.

EMFs were in the focus of research in the 1990s, during the first decade of the intense fullerene research mainly concerned with the endohedral state of the metals.1 In the past decade, many new types of EMFs have been synthesized, including nitride cluster fullerenes (MIII3N@C2n, hereafter, “M” denotes Sc, Y, and lanthanides),5,6 carbide cluster fullerenes (Sc2C2@C84,7 M2C2@ C2n,8 Sc3C2@C80,9 Sc4C2@C80,10 Lu3C2@C8811), oxide cluster fullerenes (Sc4O2,3@C80,12,13 Sc2O@C8014), sulfide cluster fullerenes (M2S@C82,15 Sc2S@C2n16), as well as Sc3NC@C80,17 Sc3CH@C80,18 and so forth. Starting from dimetallofullerenes, an increasing complexity of the endohedral species opens the way to more peculiar electronic states in EMFs, and it is possible that some of these EMFs can exhibit endohedral redox activity. By this term, we understand that the change of the charge of the EMF molecule (in particular, in electrochemical reaction at electrodes) results in the change of the valence state of the endohedral atoms. In this Perspective, we review the state of the art in the emerging field of electrochemistry, defined hereafter as “electrochemistry in cavea” or “endohedral electrochemistry”. The important question which should be clarified before we turn to the description of the particular reactions is: How do we know that a given redox reaction is endohedral? Currently, the most informative experimental method allowing one to identify the redox mechanism of EMFs is ESR spectroscopy.19
26 When an EMF molecule is reduced or oxidized in the single-electron transfer reaction, the resulting change of its spin state is, in principle, detectable by ESR spectroscopy. If the spin density in a paramagnetic EMF species is, to a large extent, localized on metal

E

ndohedral metallofullerenes (EMFs) are fullerenes encapsulating one or more metal atoms or a cluster in their inner space.1,2 EMFs are characterized by a high degree of electron transfer between the metal and the carbon cage, making the electronic state of such fullerenes very special and stabilizing carbon cages that are not sufficiently stable in their neutral state. The vast majority of EMFs is limited to the group I
group III metals. The frontier molecular orbitals (MOs) of monometallofullerenes are essentially carbon cage MOs, and therefore, electrochemical activity of such EMFs is mainly determined by the properties of the carbon cage, while the electronic state of the endohedral metal atoms remains barely constant irrespective of the charge of the whole EMF molecule. Encapsulation of the nonmetal atoms or molecules into fullerenes also leaves their charge state unchanged.3,4

Increasing the complexity of the endohedral species opens the way to more peculiar electronic states in EMFs, and it is possible that some of these EMFs can exhibit endohedral redox activity. Historically, the first synthesized EMFs contained one encapsulated metal atom (dubbed monometallofullerenes) and were soon followed by dimetallofullerenes. These “classical” r 2011 American Chemical Society

Received: January 13, 2011 Accepted: March 4, 2011 Published: March 15, 2011 786

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atoms (which means that these metal atoms exhibit redox activity), then (i) the g-factor has a characteristic shift from the free-electron value (g = 2.0023) because of the nonquenched angular momentum and (ii) hyperfine structure with large hyperfine coupling constants (hfccs) can be observed (note that the large spin population does not automatically lead to the large hfcc for the atom,27 but usually such an empirical relation holds). Unfortunately, ESR spectroscopy has a limited applicability for lanthanide-based EMFs because the cw-ESR signals are severely broadened and cannot be observed at room temperature, used most in electrochemistry. ESR spectrometers are not so common in standard electrochemical equipment, and therefore, the number of research groups capable of measuring ESR spectra in parallel with electrochemical measurements is limited. UV
vis
NIR spectroscopy, either in situ or ex situ, is more commonly used and readily available for the characterization of electrochemically generated charged states, but the interpretation of the spectra is less straightforward as that for ESR, and it is not a priori clear how the changes in the absorption spectra of EMFs can be attributed to the changes of the valence state of the metal atoms. The use of other techniques that might provide information on the valence state of metal atoms (such as XPS or NMR) in combination with electrochemical measurements of EMFs is not reported yet due to the requirements of these methods. Quantum chemical calculations (in particular, at the DFT level of a theory) can provide a lot of information on the electronic structure of EMFs and their charge states.16,23,26,28
37 In the simplest approximation, MO analysis of the neutral EMFs can already give a hint if endohedral redox activity might be expected. Namely, if the HOMO and/or LUMO is localized on the endohedral cluster, its participation in the electrochemical reactions is anticipated with a high degree of certainty. However, the order of MOs can be changed in the charged states,28 and more reliable predictions require computations of the different charged states of EMFs, either in vacuo or with electrostatic solvation corrections. With the current development of the computer hardware as well as the broad availability of the computational software, DFT computations of EMFs can be performed now in any research group. As a result, computational studies, at least in the form of frontier MO analysis of the neutral molecule, are available for many EMFs. Computations of lanthanide-based EMFs are still challenging due to the multiple degenerate states originating from the partially unfilled f shell. However, in most cases, the f shell is not active in electrochemical reactions (the 4f electron of Ce in CeLu2N@C8028 is an important exception, which will be discussed below), and hence, computations can be performed for analogues with unfilled (La) or complete (Lu) f shells. Because of the close ionic radius, yttrium is especially a convenient surrogate for medium-size lanthanides (such as Gd, Y, and Tm).34,38 In the following review on endohedral redox activity of EMFs, we will preferably rely on the results of ESR spectroscopic studies. However, when such studies are not available, results of DFT computations will be also employed. Nitride Cluster Fullerenes. Nitride cluster fullerenes are the most abundant and stable EMFs synthesized so far.2,6 This family includes a large variety of structures of brutto composition M3N@C2n, where M is Sc, Y, and lanthanides in their trivalent state, while 2n can vary from 68 to 100. Until recently, the majority of NCFs had carbon cage-based frontier orbitals, and hence, the valence state of endohedral metal atoms in NCFs could not be manipulated by redox reactions.27,34 The notable

exclusion from this rule known until 2009 was Sc3N@C80, which has a Sc-localized LUMO.27,39 Reduction of Sc3N@C80(Ih) leads to the change of the valence state of the Sc atoms, which can be especially well seen in the ESR spectrum of the anion radical with a large 45Sc hfcc of 56 G and a shift of the g-factor to ∼1.998
1.999.19,24,40 These values can be compared to the results of in situ ESR spectroelectrochemical studies of Sc3N@C68, whose HOMO and LUMO are localized on the carbon cage; 45Sc hfccs for the Sc3N@C68 cation and anion are 1.28 and 1.75 G, respectively, while the g factor values are 2.0010 and 2.0032.23 Exohedral chemical derivatization of EMFs can modify their electronic structure and thus affect their redox properties. Theoretical studies have shown that in simple derivatives of Sc3N@C80 (e.g., in monocycloadducts), the LUMO is still mainly localized on the cluster;34 however, the situation may change with the increase of the number of added groups. Experimental ESR spectroscopic studies have been performed for the anions of Sc3N@C80(CF3)2n derivatives (2n = 2, 10, 12).24,25 Interestingly, addition of only two CF3 groups already drastically changes the dynamics of the Sc3N cluster and the electrochemical properties of the Sc3N@C80 core.24 Sc3N@C80(CF3)2 exhibits two singleelectron oxidation and three single-electron reduction steps, all electrochemically reversible even at low scan rates. Note that reduction of Sc3N@C80 is electrochemically irreversible but chemically reversible; Echegoyen et al. have shown that electrochemical reversibility of the first reduction step can be achieved at the scan rates above 5 V/s.40 High stability of the charge states of Sc3N@C80(CF3)2 in solution enabled us to perform in situ ESR spectroscopic characterization of the electrochemically generated charged paramagnetic states, the cation, anion, and trianion. Sc3N@C80(CF3)23
is the first trianion of any EMF ever characterized spectroscopically.24 Unlike the cation, which has a single-line ESR spectrum, the anion and trianion exhibited a rich 45Sc hyperfine structure in their ESR spectra with a(45Sc) values of 9.3, 9.3, and 10.7 G in Sc3N@C80(CF3)2
and 10.8, 10.8, and 49.2 G in Sc3N@C80(CF3)23
and g-factors of 1.9958 and 2.0006, respectively (Figure 1). Hfcc values show that exohedral addition of two CF3 groups hinders the rotation of the Sc3N cluster, while in Sc3N@C80, the cluster rotates freely. The study also reveals the mixing of the fullerene and cluster states in the frontier orbitals of the derivative and, as a result, the mixed cage
cluster distribution of the spin density in the derivative with the exceptionally high spin population on one Sc atom in the trianion. The cage contribution to the LUMO is further enhanced with the increase of the number of CF3 groups, as can be deduced from the hfcc values and g factors of the radical anions Sc3N@C80(CF3)10
with a(45Sc) = 0.6, 11.1, and 21.5 G and g = 2.0009, and Sc3N@C80(CF3)12
with a(45Sc) = 0.6, 7.4, and 8.1 G and g = 2.0012 (Figure 1).25 In coincidence with experimental data, DFT calculations show a gradual decrease of the net spin population of the Sc3N cluster from 64% in Sc3N@C80
via 42% in Sc3N@C80(CF3)2
to 13 and 11% in Sc3N@C80(CF3)10
and Sc3N@C80(CF3)12
, respectively (Figure 1). In summary, the Sc3N cluster in derivatized Sc3N@C80 still exhibits redox activity, but with the increase of the number of CF3 groups, the main role in redox processes shifts to the carbon cage. In this context, it is worth noting that CF3 is an electronwithdrawing group, and hence, trifluoromethylation tends to stabilize carbon cage LUMOs, at least for some of the addition patterns; E1/2(0/
) values for Sc3N@C80(CF3)x derivatives with respect to the Sc3N@C800/
pair are þ0.10, þ0.20, 787

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Figure 1. (a) DFT-computed spin densities in Sc3N@C80
, Sc3N@C80(CF3)2
, Sc3N@C80(CF3)23
, and Sc3N@C80(CF3)10
; (b
e) ESR spectra (black, experiment; red, simulation) of Sc3N@C80(CF3)2
, Sc3N@C80(CF3)23
, Sc3N@C80(CF3)10
, and Sc3N@C80(CF3)12
. Parts (a
c) are adapted from ref 24 (copyright 2010, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim); parts (d) and (e) are reproduced from the ref 25.

þ0.42, and þ0.31 V for x = 2, 4, 10, and 12, respectively.25 If Sc3N@C80 is, on the contrary, derivatized by electron-donating groups, which should destabilize carbon cage MOs, it is then possible that the LUMO of such derivatives preserves its localization on the cluster. The reduction activity of the nitride cluster in M3N@C80 appears to be a unique property of Sc3N@C80 and is not found for NCFs with Y and lanthanides in the cluster. The reason is the higher electronegativity of Sc, whose 3d orbitals have lower energies than the 4d orbitals of Y and the 5d orbitals of lanthanides. In comparison to other M3N@C80, the first reduction of Sc3N@C80 is shifted anodically by ∼0.2 V. As a result, the carbon cage contribution to the LUMO is dominating for non-Sc M3N@C80. DFT computations also show that the net spin population of the Y3N cluster in Y3N@C80
is less than 16%.27,34 While the experimental ESR spectrum of Y3N@C80
is not reported yet, Echegoyen et al. succeeded in the ESR characterization of the anion of the pyrrolidine cycloadduct Y3N@C80(C4H9N).21 Measured hfcc values with a(89Y) = 6.26, 6.26, and 1.35 G, a(14N) = 0.51 G, and a g factor of 1.9989 point to the noticeable but not pronounced localization of the spin density on the Y3N cluster. DFT computations have shown that the net spin population of the nitride cluster in Y3N@C80(C4H9N)
is 18%.34 Development and implementation of the procedures for the synthesis and separation of mixed-metal NCFs (i.e., NCFs with two different metal atoms in one M3N cluster) opened the way to EMFs with unprecedented electronic properties.41,42 By this approach, it is now possible to synthesize NCFs with such metal atoms that do not form homometallic M3N clusters. For instance, Ti3N@C2n NCFs cannot be synthesized,43 but the NCF with a mixed Sc
Ti cluster, TiSc2N@C80, is readily available. Besides, in the mixed-metal NCFs, the metal atoms can exhibit specific properties that they do not show in the homometallic clusters (e.g., CeLu2N@C80; see below). Importantly, the valence state of the metal can be tuned electrochemically in certain mixed-metal NCFs.

In the mixed-metal NCFs, the metal atoms can exhibit specific properties that they do not show in the homometallic clusters. A particular example of the mixed-metal NCF with unexpected properties of the encapsulated metal atoms is CeLu2N@C80(Ih).28 Spectroscopic studies have shown that similar to all other Ce-based EMFs, in CeLu2N@C80, the Ce atom is in the 3þ valence state with one localized 4f electron (its spin can be followed by a characteristic temperature-dependent shift in 13C NMR spectra). All previous electrochemical studies of Ce(III)-containing EMFs in comparison to their La and lanthanide analogues (such as M@C82, M2@C72,78,80, and M3N@C88,92,96) have not revealed the special behavior of Ce; that is, redox potentials of Ce-based endohedral metallofullerenes were always close to those of the non-Ce analogues. However, for CeLu2N@C80, it was found that its oxidation potential is shifted by 0.6 V to lower values than those of all other M3N@C80 NCFs (Figure 2a).28 Such a strong shift indicates that its origin lies in the change of the valence state of the Ce. Indeed, DFT calculations have shown that the 4f orbital in CeLu2N@C80 is 0.37 eV below the HOMO, but removal of the 4f electron from Ce atom in CeLu2N@C80 is by 0.43 eV more favorable than oxidation of the carbon cage. Thus, CeLu2N@C80 is the first Ce-based endohedral metallofullerene exhibiting a redox activity of the endohedral Ce(III), which can be reversibly oxidized into the Ce(IV) state (Figure 2b). Moreover, the CeIVLu2N@C80þ cation is the first EMF with the tetravalent cerium atom. Note that the reduction behavior of CeLu2N@C80 is not different from that of other M3N@C80 NCFs. Unprecedented electrochemical properties were also revealed for the TiSc2N@C80(Ih).26 This NCF is paramagnetic in its virgin state and isoelectronic to Sc3N@C80
. In accordance with 788

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Figure 3. Cyclic voltammetry of TiSc2N@C80 (in 0.1 M TBABF4/o-DCB solution, 20 mV/s). The charge of the TiSc2N@C80 molecule and valence states of Ti are also indicated for certain potential ranges. The inset shows the spin density distribution in TiSc2N@C80 (note the preferential localization of the spin density on the Ti atom). Adapted from ref 26.

anion radical and obtained refined values of the hfc tensor (axx = 337 G, ayy = 333 G, azz = 423 G) and g-tensor (gxx = 1.850, gyy = 1.865, gzz = 1.995).45 Very similar ESR spectra at 7K were also reported by Akasaka et al. for the anions of 5,6 and 6,6 isomers of pyrrolidino-adducts of [email protected] Although ESR studies of the anions of other dilanthano EMFs (La2@C72, La2@C78, La2@C80(D5h)) have not been reported yet, quantum chemical calculations show that in all of these molecules, the LUMO is the same in nature as La2@C80(Ih), and thus, the valence state of La is changed upon reduction.27,46,47 The same can be said about isostructural Ce2@C2n EMFs (note that redox potentials of Ce and La analogues are almost identical).48
50 A special case of dimetallofullerenes was reported by Dorn et al.22 As a byproduct of the NCF synthesis, the authors have isolated M2@C79N for several metal atoms (nitrogen substitutes one of the carbon atoms of the fullerene cage). A large 89Y hfcc value (81.23 G) and a shift of the g factor (1.9740) observed in the ESR spectrum of Y2@C79N show that the spin density is localized on Y atoms; the same conclusion is obtained by DFT computations.22 Electrochemical studies and computations of the charge states of M2@C79N are not reported yet, but it is likely that reduction or oxidation of M2@C79N should change the valence state of metal atoms. As there are no other reports on the experimental ESR spectroscopic studies of dimetallofullerenes in the charged states, we have to focus on theoretical studies which show that endohedral redox activity can be expected for Y2@C82 because its HOMO is localized on the Y atoms (Y is thus in the divalent state).29 Likewise, the divalent state of Y is found in Y3@C80 (in this molecule, a non-nuclear maximum of the electron density, that is, a pseudoatom, is found, which mimics the nitrogen atom of Y3N@C80), and it is also predicted that its oxidation should occur via removal of electrons from Y atoms.51 Other Cluster Fullerenes. Although carbide cluster fullerenes were isolated among the first EMFs, their nature was first recognized only in 2000, when Wang et al. reported that “Sc2@C86” was actually [email protected] Soon after that, it was shown that several classical EMFs, such as Sc3@C82, M2@C84 (M = Sc, Y, Er, etc.), or Ti2@C80, were actually carbide cluster fullerenes Sc3C2@C80, M2C2@C82, or Ti2C2@C78 , respectively.8,9,52

Figure 2. (a) Cyclic voltammetry curves of CeLu2N@C80 in comparison to that of Y3N@C80 (in 0.1 M TBABF4/o-DCB solution, 20 mV/s); the large shift of the oxidation potential of CeLu2N@C80 is indicated; (b) scheme of the oxidation of CeLu2N@C80 (note the spin density localized on the Ce atom in the neutral state). Adapted from ref 28.

the g-factor of 1.9454 revealed by a room-temperature ESR study and the a(45Sc) value estimated as ∼7 G (more precise measurement was not possible because of the signal broadening), DFT has shown that a single-occupied MO (SOMO) and hence the spin density in TiSc2N@C80 are, to a large extent, localized on the Ti atom (Figure 3). Unlike all other M3N@C80 NCFs with either homometallic or mixed-metal nitride clusters, which exhibit electrochemically irreversible but chemically reversible reductions, TiSc2N@C80 has three electrochemically reversible reductions and one reversible oxidation step (Figure 3). DFT computations show that both reduction and oxidation of TiSc2N@C80 proceed through the change of the valence state of the Ti atom, from Ti(II) in the TiSc2N@C80
anion through Ti(III) in the neutral state to Ti(IV) in the TiSc2N@C80þ cation. TiSc2N@C80 is the first example of an EMF with a metal-only redox system in which the endohedral metal of the cluster is electrochemically active in both reduction and oxidation.26 Dimetallofullerenes. La2@C80(Ih) was one of the first synthesized EMFs,44 and its electrochemical study accompanied by Hartree
Fock computations was reported in 1995.35 Computations have shown that the LUMO of La2@C80 is localized on the metal atoms, and hence, the authors suggested that the electron transfer to the endohedral metal atoms occurs at the first two reduction steps.35 To our knowledge, this may be considered as the first report of endohedral redox activity in EMFs. Experimental proof by ESR spectroscopy was delayed until 2006, when Dinse and Kato published the result of the ESR studies of the chemically and electrochemically generated La2@C80
anion with an isotropic 139La hfcc value of 396 G and a g factor of 1.984.20 One year later, Kato also reported W- and X-band ESR spectra of the frozen solution of the electrochemically generated 789

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the mixed valence state, that is, two Sc atoms are formally divalent with a bond between them. Both the LUMO and HOMO in Sc4O2@C80 are localized on the Sc4O2 cluster;29,33 likewise, DFT computations show that the spin density of the anion and cations are also localized on the cluster.33 Thus, Sc4O2@C80 is a promising candidate for endohedral electrochemistry. In Sc4O3@C80, all Sc atoms are in the Sc(III) state, and the HOMO is of fullerene cage nature. However, the LUMO is still localized on the cluster, and thus, reduction of Sc4O3@C80 is expected to proceed via a change of the valence state of Sc atoms.13,29 Spin
Charge Separation. An important question still to be addressed in more detail in the future is the mechanism of the electron transfer to and from endohedral species through the formally inert carbon cage. An interesting phenomenon of the spin
charge separation was found in computational studies of the electron density distribution of EMFs with in cavea redox activity and their charged states.26,27 Occupation of the metalbased LUMO and localization of the spin density in the thusformed 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, 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.27,29 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 physical background. Clear visualization of this effect can be obtained by plotting the difference of the electron densities of the neutral and charged states.27 As an example, Figure 4 shows the difference in densities of La2@C80
/0, Sc3C2@C800/þ, and Ti2C2@C78
/0 pairs in comparison to the spin densities of anion radicals or neutral Sc3C2@C80. As seen, addition of the electron to EMF molecules results in a complex redistribution of the electron density on metal atoms, including both regions with an increase and a depletion of the electron density that balance each other upon integration, giving small changes of the atomic charges. On the contrary, the lobes of the difference density for the carbon cage atoms, even though they are visually smaller than the lobes of the difference density for metal atoms, sum up to values approaching one electron for the whole carbon cage. 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. Theoretical studies of Petek and co-workers

Figure 4. DFT-computed spin density (a
c) and difference density [F(EMF
)
F(EMF)] (a0
c0 ) of La2@C80
(a,a0 ), Sc3C2@C80 (b,b0 ), and Ti2C2@C78
(c,c0 ). For Sc3C2@C80, the difference of electron densities of the neutral and cationic forms [F(Sc3C2@C80)
F(Sc3C2@C80þ)] is plotted in (b0 ). Adapted from ref 27.

Sc3C2@C80 is a radical with a(45Sc) = 6.27 G and a g factor of 1.9987.53 These parameters indicate that the spin density is, to a large extent, localized on the Sc3C2 cluster. Detailed computations of the neutral and charged states of Sc3C2@C80 performed by Lu et al. proved that the SOMO of Sc3C2@C80 is indeed localized on the cluster with approximately equal contributions from Sc atoms and the C2 unit (Figure 4).37 Moreover, their computations show that the charging of Sc3C2@C80 changes the formal charge of the C2 unit; thus, Sc3C2@C80 exhibits endohedral redox activity in both the cathodic and anodic ranges. Recently reported Sc3NC@C80 is isoelectronic to the anion Sc3C2@C80
. DFT shows that the LUMO is localized on Sc atoms of the Sc3NC cluster, and hence, it is also likely an EMF with endohedral redox activity.17 Localization of the SOMO on the endohedral cluster was also reported for Lu3C2@C88, that is, upon oxidation, one electron is likely to be removed from the cluster.11 Finally, the HOMO and HOMO
1 of Sc4C2@C80 are shown to be localized on the Sc4C2 cluster, and thus, oxidation of Sc4C2@C80 is expected to proceed in cavea.36 The data on other carbide cluster fullerenes are inconclusive. For Sc2C2@C82-C3v(7), DFT shows that the LUMO has a large contribution from the Sc2C2 cluster.15,54 DFT computations of the anion with the BP GGA functional show that the spin density is also localized on the cluster;54 however, the use of the hybrid functional B3LYP shows that the anion with the cage-localized spin density is preferable.27 Experimental ESR spectroscopic studies should be performed to clarify whether Sc2C2@C82 indeed exhibits an endohedral redox activity. DFT computations also predict that the net spin density population of two Ti's in the Ti2C2@C78
anion is 54% (Figure 4).27 Experimental electrochemical studies of Ti2C2@C78 are not reported yet. A new interesting type of EMFs are oxide cluster fullerenes such as Sc4O2@C80 and [email protected],13 In Sc4O2@C80, Sc has

Addition of the electron to EMF molecules results in a complex redistribution of the electron density on metal atoms, including both regions with an increase and a depletion of the electron density that balance each other upon integration. have recently shown that a similar effect can be expected for graphene due to polarization of the carbon π-system by an external charge.55 790

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Figure 5. Spin population as a function of time in Sc3N@C80
(a) and TiSc2N@C80 (b) obtained in the course of DFT-based Born
Oppenheimer molecular dynamics (BOMD) at 300 K; (c) BOMD trajectories of TiSc2N@C80 and Sc3N@C80
(Ti, cyan; Sc, magenta and violet; N, blue; C, gray; displacements of carbon atoms are not shown); note the frustrated rotation of the cluster in TiSc2N@C80 in comparison to the free rotation in Sc3N@C80
; (d) spin flow vibrational spectrum of TiSc2N@C80 (the scale for Ti and TiSc2N is ∼10 times higher than that for Sc atoms; because the total spin of TiSc2N@C80 is constant, variations of the net population of the TiSc2N cluster are equal to those of the carbon cage). Adapted from ref 26.

Spin Flow. Localization of the spin density on the endohedral cluster in certain EMFs and fast dynamics of the endohedral clusters raise the question of their mutual influence. Detailed DFT studies have shown that the spin density distribution in such EMFs is quite flexible. Even slight reorientation of the cluster results in considerable changes of the spin populations of the metal atoms.26,27 This conclusion was further corroborated by DFT-based Born
Oppenheimer molecular dynamics, which provided details of the spin density dynamics (dubbed as the spin flow).26 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 (Figure 5). Fourier transformation of the time dependencies of the spin populations resulted in the spin flow vibrational spectra, which reveal the major spin flow channels. In particular, for TiSc2N@C80, it is shown that the cluster T cage spin flow is selectively coupled to one vibrational mode originating from the frustrated rotation of the cluster (Figure 5d).26 A more complex scenario was recently found in Sc3N@C80(CF3)2
; in this radical anion, two Sc atoms (which are located close to the CF3-bearing carbon atoms of the cage) exchange spin density between each other but do not contribute to the cluster T cage spin flow.57 That is, the cluster T cage spin flow in Sc3N@C80(CF3)2
is totally dominated by the third Sc atom (note that the net spin

Localization of the spin density on the endohedral cluster in certain EMFs and fast dynamics of the endohedral clusters raises the question of their mutual influence.

population of the cage varies in time within the range of 10
60%). Future Prospects. Localization of the spin density on the endohedral cluster in certain EMFs and fast dynamics of the endohedral clusters raises the question of their mutual influence. Manipulation and tuning of the valence states of thus stabilized endohedral species is an interesting and promising task. Up to now, there have been several EMFs for which an endohedral redox activity has been convincingly proved by both experimental and theoretical studies. Some theoretically predicted endohedral redox reactions are still waiting for the experimental verification. Recent examples with mixed NCFs show how a judicious choice of synthetic techniques and target EMFs opens the way to compounds with chemically and electrochemically tunable valence states of the endohedral species. Endohedral 791

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redox activity also raises fundamental question on the physics of the electron transfer through the carbon cage. Such phenomena as spin
charge separation and spin flow already discovered for EMFs require further detailed analysis. Thus, the matter of reversible charge storage in endohedral fullerenes raises questions of fundamental importance. In short, electrochemistry in cavea is a new and rapidly developing field of electrochemistry which has a bright future.

H2@C60 Derivatives Covalently Linked to a Nitroxide Radical. J. Phys. Chem. Lett. 2010, 1, 2135–2138. (5) Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; et al. SmallBandgap Endohedral Metallofullerenes in High Yield and Purity. Nature 1999, 401, 55–57. (6) Dunsch, L.; Yang, S. Metal Nitride Cluster Fullerenes: Their Current State and Future Prospects. Small 2007, 3, 1298–1320. (7) Wang, C. R.; Kai, T.; Tomiyama, T.; Yoshida, T.; Kobayashi, Y.; Nishibori, E.; Takata, M.; Sakata, M.; Shinohara, H. A Scandium Carbide Endohedral Metallofullerene: (Sc2C2)@C84. Angew. Chem., Int. Ed. 2001, 40, 397–399. (8) Iiduka, Y.; Wakahara, T.; Nakajima, K.; Tsuchiya, T.; Nakahodo, T.; Maeda, Y.; Akasaka, T.; Mizorogi, N.; Nagase, S. 13C NMR Spectroscopic Study of Scandium Dimetallofullerene, Sc2@C84 vs. Sc2C2@C82. Chem. Commun. 2006, 2057–2059. (9) Iiduka, Y.; Wakahara, T.; Nakahodo, T.; Tsuchiya, T.; Sakuraba, A.; Maeda, Y.; Akasaka, T.; Yoza, K.; Horn, E.; Kato, T.; et al. Structural Determination of Metallofullerene Sc3C82 Revisited: A Surprising Finding. J. Am. Chem. Soc. 2005, 127, 12500–12501. (10) Wang, T.-S.; Chen, N.; Xiang, J.-F.; Li, B.; Wu, J.-Y.; Xu, W.; Jiang, L.; Tan, K.; Shu, C.-Y.; Lu, X.; et al. Russian-Doll-Type Metal Carbide Endofullerene: Synthesis, Isolation, and Characterization of Sc4C2@C80. J. Am. Chem. Soc. 2009, 131, 16646–16647. (11) Xu, W.; Wang, T.-S.; Wu, J.-Y.; Ma, Y.-H.; Zheng, J.-P.; Li, H.; Wang, B.; Jiang, L.; Shu, C.-Y.; Wang, C.-R. Entrapped Planar Trimetallic Carbide in a Fullerene Cage: Synthesis, Isolation, and Spectroscopic Studies of Lu3C2@C88. J. Phys. Chem. C 2011, 115, 402–405. (12) Stevenson, S.; Mackey, M. A.; Stuart, M. A.; Phillips, J. P.; Easterling, M. L.; Chancellor, C. J.; Olmstead, M. M.; Balch, A. L. A Distorted Tetrahedral Metal Oxide Cluster inside an Icosahedral Carbon Cage. Synthesis, Isolation, and Structural Characterization of Sc4(μ3-O)2@Ih-C80. J. Am. Chem. Soc. 2008, 130, 11844–11845. (13) Mercado, B. Q.; Olmstead, M. M.; Beavers, C. M.; Easterling, M. L.; Stevenson, S.; Mackey, M. A.; Coumbe, C. E.; Phillips, J. D.; Phillips, J. P.; Poblet, J. M.; et al. A Seven Atom Cluster in a Carbon Cage, the Crystallographically Determined Structure of Sc4(μ3-O)3@IhC80. Chem. Commun. 2010, 46, 279–281. (14) Mercado, B. Q.; Sruart, M. A.; Mackey, M. A.; Pickens, J. E.; Confait, B. S.; Stevenson, S.; Easterling, M. L.; Valencia, R.; RodriguezFortea, A.; Poblet, J. M.; et al. Sc2(μ2-O) Trapped in a Fullerene Cage: The Isolation and Structural Characterization of Sc2(μ2-O)@Cs(6)-C82 and the Relevance of the Thermal and Entropic Effects in Fullerene Isomer Selection. J. Am. Chem. Soc. 2010, 132, 12098–12105. (15) Dunsch, L.; Yang, S.; Zhang, L.; Svitova, A.; Oswald, S.; Popov, A. A. Metal Sulfide in a C82 Fullerene Cage: A New Form of Endohedral Clusterfullerenes. J. Am. Chem. Soc. 2010, 132, 5413–5421. (16) Chen, N.; Chaur, M. N.; Moore, C.; Pinzon, J. R.; Valencia, R.; Rodriguez-Fortea, A.; Poblet, J. M.; Echegoyen, L. Synthesis of a New Endohedral Fullerene Family, Sc2S@C2n (n = 40
50) by the Introduction of SO2. Chem. Commun. 2010, 46, 4818–4820. (17) Wang, T.-S.; Feng, L.; Wu, J.-Y.; Xu, W.; Xiang, J.-F.; Tan, K.; Ma, Y.-H.; Zheng, J.-P.; Jiang, L.; Lu, X.; Shu, C.-Y.; Wang, C.-R. Planar Quinary Cluster inside a Fullerene Cage: Synthesis and Structural Characterizations of Sc3NC@C80-Ih. J. Am. Chem. Soc. 2010, 132, 16362–16364. (18) Krause, M.; Ziegs, F.; Popov, A. A.; Dunsch, L. Entrapped Bonded Hydrogen in a Fullerene: The Five-Atom Cluster Sc3CH in C80. ChemPhysChem 2007, 8, 537–540. (19) Jakes, P.; Dinse, K. P. Chemically Induced Spin Transfer to an Encased Molecular Cluster: An EPR Study of Sc3N@C80 Radical Anions. J. Am. Chem. Soc. 2001, 123, 8854–8855. (20) Dinse, K. P.; Kato, T., Multi-Frequency EPR Study of MetalloEndofullerenes. In Novel NMR and EPR Techniques, Lect. Notes Phys.  umer, S., Eds.; Springer: Berlin, Heidel684, Dolinsek, J., Vilfan, M., Z berg, Germany, 2006; pp 185
207. (21) Echegoyen, L.; Chancellor, C. J.; Cardona, C. M.; Elliott, B.; Rivera, J.; Olmstead, M. M.; Balch, A. L. X-Ray Crystallographic and

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ49-351-4659660. Fax: þ49-351-4659811. E-mail: l.dunsch@ ifw-dresden.de (L.D.); Tel: þ49-351-4659658. Fax: þ49-3514659745. E-mail: [email protected] (A.P.).

’ BIOGRAPHIES Alexey A. Popov received his M.S. (1999) and Ph.D. (2003) degrees in physical chemistry from Moscow State University (MSU), Russia. Starting from 2008, he worked with L. Dunsch in IFW Dresden, first as a Humboldt Fellow and then as a leader of the group “Electronic Structure of Molecular Materials”. His current interests include endohedral metallofullerenes, spectroelectrochemistry, vibrational spectroscopy, and quantum chemical computations. See: http://www.ifw-dresden.de/institutes/ iff/org/members/ap5. Lothar Dunsch studied chemistry at the TU Bergakademie Freiberg, Germany, and received there his diploma in chemistry (1972) and his Ph.D. (1973) in electrochemistry. In 1974, he turned to the Institutes of Solid State Research and then that of Polymer Technology of the Academy of Sciences in Dresden before he joined the IFW Dresden in 1992, heading the Department of Electrochemistry and Conducting Polymers. Since his habilitation in 1996, he has also taught at the TU Dresden. His current interests are focused on endohedral fullerenes, conducting polymers and oligomers, as well as the different methods in spectroelectrochemistry. See: http://www.ifw-dresden.de/institutes/ iff/org/Dep/14. ’ ACKNOWLEDGMENT A.A.P. acknowledges the Humboldt foundation for financial support and the Research Computing Center of Moscow State University for a computer time on the “Chebyshev SKIF-MSU” supercomputer. The authors acknowledge L. Zhang, S. Yang, N. Shustova, O. Boltalina, S. Strauss, and S. Stevenson for the fruitful collaborations on the synthesis of EMFs and their derivatives. Technical assistance of U. Nitzsche with local computer resources in IFW is highly appreciated. ’ REFERENCES (1) Shinohara, H. Endohedral Metallofullerenes. Rep. Prog. Phys. 2000, 63, 843–892. (2) Chaur, M. N.; Melin, F.; Ortiz, A. L.; Echegoyen, L. Chemical, Electrochemical, and Structural Properties of Endohedral Metallofullerenes. Angew. Chem., Int. Ed. 2009, 48, 7514–7538. (3) Frunzi, M.; Lei, X.; Murata, Y.; Komatsu, K.; Iwamatsu, S.-I.; Murata, S.; Lawler, R. G.; Turro, N. J. Magnetic Interaction of SolutionState Paramagnets with Encapsulated H2O and H2. J. Phys. Chem. Lett. 2010, 1, 1420–1422. (4) Li, Y.; Lei, X.; Lawler, R. G.; Murata, Y.; Komatsu, K.; Turro, N. J. Distance-Dependent Paramagnet-Enhanced Nuclear Spin Relaxation of 792

dx.doi.org/10.1021/jz200063k |J. Phys. Chem. Lett. 2011, 2, 786–794

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PERSPECTIVE

(38) Popov, A. A. Metal-Cage Bonding, Molecular Structures and Vibrational Spectra of Endohedral Fullerenes: Bridging Experiment and Theory. J. Comput. Theor. Nanosci. 2009, 6, 292–317. (39) Campanera, J. M.; Bo, C.; Olmstead, M. M.; Balch, A. L.; Poblet, J. M. Bonding within the Endohedral Fullerenes Sc3N@C78 and Sc3N@C80 as Determined by Density Functional Calculations and Reexamination of the Crystal Structure of Sc3N@C78 3 Co(OEP) 3 1.5(C6H6) 3 0.3(CHCl3). J. Phys. Chem. A 2002, 106, 12356–12364. (40) Elliott, B.; Yu, L.; Echegoyen, L. A Simple Isomeric Separation of D5h and Ih Sc3N@C80 by Selective Chemical Oxidation. J. Am. Chem. Soc. 2005, 127, 10885–10888. (41) Yang, S. F.; Kalbac, M.; Popov, A.; Dunsch, L. GadoliniumBased Mixed Metal Nitride cluster fullerenes GdxSc3
xN@C80 (x = 1, 2). ChemPhysChem 2006, 7, 1990–1995. (42) Yang, S.; Popov, A. A.; Chen, C.; Dunsch, L. Mixed Metal Nitride Clusterfullerenes in Cage Isomers: LuxSc3
xN@C80 (x = 1, 2) as Compared with MxSc3
xN@C80 (M = Er, Dy, Gd, Nd). J. Phys. Chem. C 2009, 113, 7616–7623. (43) Yang, S.; Chen, C.; Popov, A.; Zhang, W.; Liu, F.; Dunsch, L. An Endohedral Titanium(III) in a cluster fullerene: Putting a Non-GroupIII Metal Nitride into the C80-Ih Fullerene Cage. Chem. Commun. 2009, 6391–6393. (44) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K. S.; Whetten, R. L. La2C80 — A Soluble Dimetallofullerene. J. Phys. Chem. 1991, 95, 10561–10563. (45) Kato, T. Metal Dimer and Trimer within Spherical Carbon Cage. J. Mol. Struct. 2007, 838, 84–88. (46) Cao, B. P.; Wakahara, T.; Tsuchiya, T.; Kondo, M.; Maeda, Y.; Rahman, G. M. A.; Akasaka, T.; Kobayashi, K.; Nagase, S.; Yamamoto, K. Isolation, Characterization, and Theoretical Study of La2@C78. J. Am. Chem. Soc. 2004, 126, 9164–9165. (47) Lu, X.; Nikawa, H.; Tsuchiya, T.; Maeda, Y.; Ishitsuka, M. O.; Akasaka, T.; Toki, M.; Sawa, H.; Slanina, Z.; Mizorogi, N.; Nagase, S. BisCarbene Adducts of Non-IPR La2@C72: Localization of High Reactivity around Fused-Pentagons and toward Controllable Tuning of Electrochemical Properties of Metallofullerenes. Angew. Chem., Int. Ed. 2008, 47, 8642–8645. (48) Muthukumar, K.; Larsson, J. A. Explanation of the Different Preferential Binding Sites for Ce and La in M2@C80 (M = Ce, La) — A Density Functional Theory Prediction. J. Mater. Chem. 2008, 18, 3347–3351. (49) Yamada, M.; Wakahara, T.; Tsuchiya, T.; Maeda, Y.; Kako, M.; Akasaka, T.; Yoza, K.; Horn, E.; Mizorogi, N.; Nagase, S. Location of the Metal Atoms in Ce2@C78 and Its Bis-Silylated Derivative. Chem. Commun. 2008, 558–560. (50) Yamada, M.; Mizorogi, N.; Tsuchiya, T.; Akasaka, T.; Nagase, S. Synthesis and Characterization of the D5h Isomer of the Endohedral Dimetallofullerene Ce2@C80: Two-Dimensional Circulation of Encapsulated Metal Atoms inside a Fullerene Cage. Chem.—Eur. J. 2009, 15, 9486–9493. (51) Popov, A. A.; Zhang, L.; Dunsch, L. A Pseudoatom in a Cage: Trimetallofullerene Y3@C80 Mimics Y3N@C80 with Nitrogen Substituted by a Pseudoatom. ACS Nano 2010, 4, 795–802. (52) Tan, K.; Lu, X. Ti2C80 Is More Likely a Titanium Carbide Endohedral Metallofullerene (Ti2C2)@C78. Chem. Commun. 2005, 4444–4446. (53) Rahmer, J.; Dunsch, L.; Dorn, H.; Mende, J.; Mehring, M. Wand X-Band EPR Investigation of Sc3@C82. Magn. Reson. Chem. 2005, 43, S192–S198. (54) Valencia, R.; Rodríguez-Fortea, A.; Poblet, J. M. Understanding the Stabilization of Metal Carbide Endohedral Fullerenes M2C2@C82 and Related Systems. J. Phys. Chem. A 2008, 112, 4550–4555. (55) Silkin, V. M.; Zhao, J.; Guinea, F.; Chulkov, E. V.; Echenique, P. M.; Petek, H. Image Potential States in Graphene. Phys. Rev. B 2009, 80, 121408. (56) Yamada, M.; Okamura, M.; Sato, S.; Someya, C. I.; Mizorogi, N.; Tsuchiya, T.; Akasaka, T.; Kato, T.; Nagase, S. Two Regioisomers

EPR Spectroscopic Characterization of a Pyrrolidine Adduct of Y3N@C80. Chem. Commun. 2006, 2653–2655. (22) Zuo, T.; Xu, L.; Beavers, C. M.; Olmstead, M. M.; Fu, W.; Crawford, T. D.; Balch, A. L.; Dorn, H. C. M2@C79N (M = Y, Tb): Isolation and Characterization of Stable Endohedral Metallofullerenes Exhibiting M 3 3 3 M Bonding Interactions inside Aza[80]Fullerene Cages. J. Am. Chem. Soc. 2008, 130, 12992–12997. (23) Rapta, P.; Popov, A. A.; Yang, S. F.; Dunsch, L. The Charged States of Sc3N@C68: An In Situ Spectroelectrochemical Study of the Radical Cation and Radical Anion of a Non-IPR Fullerene. J. Phys. Chem. A 2008, 112, 5858–5865. (24) Popov, A. A.; Shustova, N. B.; Svitova, A. L.; Mackey, M. A.; Coumbe, C. E.; Phillips, J. P.; Stevenson, S.; Strauss, S. H.; Boltalina, O. V.; Dunsch, L. Redox-Tuning Endohedral Fullerene Spin States: From the Dication to the Trianion Radical of Sc3N@C80(CF3)2 in Five Reversible Single-Electron Steps. Chem.—Eur. J. 2010, 16, 4721–4724. (25) Shustova, N. B.; Peryshkov, D. V.; Kuvychko, I. V.; Chen, Y.-S.; Mackey, M. A.; Coumbe, C. E.; Heaps, D. T.; Confait, B. S.; Heine, T.; Phillips, J. P. Poly(Perfluoroalkylation) of Metallic Nitride Fullerenes Reveals Addition-Pattern Guidelines: Synthesis and Characterization of a Family of Sc3N@C80(CF3)n (n = 2
16) and Their Radical Anions. J. Am. Chem. Soc. 2011, 133, 2672–2690. (26) Popov, A. A.; Chen, C.; Yang, S.; Lipps, F.; Dunsch, L. SpinFlow Vibrational Spectroscopy of Molecules with Flexible Spin Density: Electrochemistry, ESR, Cluster and Spin Dynamics, and Bonding in TiSc2N@C80. ACS Nano 2010, 4, 4857–4871. (27) Popov, A. A.; Dunsch, L. Hindered Cluster Rotation and 45Sc Hyperfine Splitting Constant in Distonoid Anion Radical Sc3N@C80, and Spatial Spin Charge Separation as a General Principle for Anions of Endohedral Fullerenes with Metal-Localized Lowest Unoccupied Molecular Orbitals. J. Am. Chem. Soc. 2008, 130, 17726–17742. (28) Zhang, L.; Popov, A. A.; Yang, S.; Klod, S.; Rapta, P.; Dunsch, L. An Endohedral Redox Sytem in a Fullerene Cage: The Ce Based Mixed Cluster Fullerene Lu2CeN@C80. Phys. Chem. Chem. Phys. 2010, 12, 7840–7847. (29) Popov, A. A.; Dunsch, L. The Bonding Situation in Endohedral Metallofullerenes as Studied by Quantum Theory of Atoms in Molecules (QTAIM). Chem.—Eur. J. 2009, 15, 9707–9729. (30) Zhao, J.; Feng, M.; Yang, J.; Petek, H. The Superatom States of Fullerenes and Their Hybridization into the Nearly Free Electron Bands of Fullerites. ACS Nano 2009, 3, 853–864. (31) Chaur, M. N.; Aparicio-Angles, X.; Mercado, B. Q.; Elliott, B.; Rodriguez-Fortea, A.; Clotet, A.; Olmstead, M. M.; Balch, A. L.; Poblet, J. M.; Echegoyen, L. Structural and Electrochemical Property Correlations of Metallic Nitride Endohedral Metallofullerenes. J. Phys. Chem. C 2010, 114, 13003–13009. (32) Chaur, M. N.; Valencia, R.; Rodriguez-Fortea, A.; Poblet, J. M.; Echegoyen, L. Trimetallic Nitride Endohedral Fullerenes: Experimental and Theoretical Evidence for the M3N6þ@C2n6
Model. Angew. Chem., Int. Ed. 2009, 48, 1425–1428. (33) Valencia, R.; Rodriguez-Fortea, A.; Stevenson, S.; Balch, A. L.; Poblet, J. M. Electronic Structures of Scandium Oxide Endohedral Metallofullerenes, Sc4(μ3-O)n@Ih-C80 (n = 2, 3). Inorg. Chem. 2009, 48, 5957–5961. (34) Valencia, R.; Rodriguez-Fortea, A.; Clotet, A.; de Graaf, C.; Chaur, M. N.; Echegoyen, L.; Poblet, J. M. Electronic Structure and Redox Properties of Metal Nitride Endohedral Fullerenes M3N@C2n (M=Sc, Y, La, and Gd; 2n=80, 84, 88, 92, 96). Chem.—Eur. J. 2009, 15, 10997–11009. (35) Suzuki, T.; Maruyama, Y.; Kato, T.; Kikuchi, K.; Nakao, Y.; Achiba, Y.; Kobayashi, K.; Nagase, S. Electrochemistry and Ab-Initio Study of the Dimetallofullerene La2@C80. Angew. Chem., Int. Ed. Engl. 1995, 34, 1094–1096. (36) Tan, K.; Lu, X.; Wang, C. R. Unprecedented μ4-C26
Anion in Sc4C2@C80. J. Phys. Chem. B 2006, 110, 11098–11102. (37) Tan, K.; Lu, X. Electronic Structure and Redox Properties of the Open-Shell Metal-Carbide Endofullerene Sc3C2@C80: A Density Functional Theory Investigation. J. Phys. Chem. A 2006, 110, 1171–1176. 793

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of Endohedral Pyrrolidinodimetallofullerenes M2@Ih-C80(CH2)2 NTrt (M = La, Ce; Trt = Trityl): Control of Metal Atom Positions by Addition Positions. Chem.—Eur. J. 2009, 15, 10533–10542. (57) Popov, A. A.; Dunsch, L. Charge Controlled Changes in the Cluster and Spin Dynamics of Sc3N@C80(CF3)2: The Flexible Spin Density Distribution and Its Impact on ESR Spectra. Phys. Chem. Chem. Phys. 2011, DOI:10.1039/C0CP02070B.

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