Transition-Metal and Rare-Earth-Metal Redox Couples inside Carbon Cages: Fullerenes Acting as Innocent Ligands

Transition-Metal and Rare-Earth-Metal Redox Couples inside CarbonCages: Fullerenes Acting as Innocent LigandsYang 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 carboncage shields the endohedral species from the surroundingenvironment and can stabilize unusual clusters that otherwisewould not exist. This review is focused on the behavior of themetal/π-system interface in EMFs under electron transferconditions. We show that the stabilizing role of the fullerene cagecan be extended from unusual clusters to the peculiar spin andcharge states obtained via endohedral electron transfer. For such redoxprocesses, the role of the fullerene cage can be understood as that ofan innocent ligand. This review is specifically focused on four groupsof 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 havepronounced metal−metal bonding character, and therefore electron transfer affects the metal−metal bonding character in suchmolecules. The LUMO of Sc3N@C80 is equally delocalized over three Sc atoms, resulting in a Sc3N-based reduction, whosemechanism can be modified by exohedral derivatization. Titanium is a rare example of a transition metal that can be encapsulatedwithin fullerenes, and we discuss how its valence state in Ti-EMFs can be tuned via electrochemical reactions. Cerium exhibitsendohedral 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 ofthe redox behavior of these EMFs is accompanied by an analysis of their electronic structure and a discussion of theirspectroelectrochemical studies.

■ INTRODUCTIONThe first evidence of the encapsulation of metal atoms inside afullerene cage was reported in 1985,1 soon after fullerenes werediscovered in molecular beams produced by laser ablation ofgraphite.2 Discovery of the arc-discharge synthesis of fullerenesin 19903 boosted the interest in these carbon clusters, and thesynthesis of “bulk” amounts of the first endohedral metal-lofullerenes (EMFs) was reported in the early 1990s (Figure1a).4−7 It was soon found that fullerene cages can alsoencapsulate two or three metal atoms, yielding respectivelydimetallofullerenes (di-EMFs; Figure 1b,c) and trimetallofuller-enes (tri-EMFs).In EMFs molecules, the endohedral metal atoms donate their

valence electrons to the carbon cage (which is thereforenegatively charged).7 This results in a strong Coulombrepulsion of metal ions if the carbon cage encapsulates morethan one metal atom. The destabilizing Coulomb repulsion canbe leveled off if the endohedral cluster also includes negativelycharged nonmetal atoms. The first EMF containing a nonmetalendohedral 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 enhancedstability, nitride clusterfullerenes (NCFs) M3N@C2n (M = S, Y,lanthanides) have evolved into the largest EMF familyconsisting of carbon cages ranging from C68 to C96.

9,10 Theformal charge distribution in NCFs can be described as(M3+)3N

3−@C2n6−.11−13

The synthesis of NCFs was followed in 200114 by thediscovery 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 manyEMFs thought to be di-EMFs and tri-EMFs were indeedcarbide clusterfullerenes.19,20 During the last 5 years, newclusterfullerene families with oxide (Sc2O@C82,

21 Sc4O2,3@C80,

22,23), sulfide (M2S@C2n24−28), and cyanide (Sc3CN@

C2n,29,30 YCN@C82

31) metal clusters have been synthesized(Figure 1). Very recently, TiLu2C@C80 was discovered as anew type of clusterfullerene with a μ3-carbido ligand and a TiC double bond (Figure 1h,i).32 Several exhaustive reviews ofEMFs have been published recently.33−38

Despite the diversity of endohedral clusters, all EMFs haveone common feature: the interface between metal ions and thesurrounding carbon-based π system. The carbon cage can beconsidered as a special type of ligand, similar to those inorganometallic complexes (e.g., as in ferrocene). Analysis of thechemical bonding in EMFs shows that the large ionic bondingcharacter is actually augmented by considerable covalentcontributions due to d−π orbital overlap.39−41

Special Issue: Organometallic Electrochemistry

Received: January 15, 2014Published: April 25, 2014

Review

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Fullerenes are good electron acceptors and undergo multiplereversible single-electron redox processes in solution. Forinstance, electrochemical studies of C60 showed that it is able toaccept up to six electrons under optimized conditions atreduced temperature,42 whereas four reversible reductions stepsare normally accessible in o-dichlorobenzene (o-DCB) at roomtemperature. Similar reduction behavior has also been describedfor other fullerenes.43−46 Cations of fullerenes are notchemically stable; oxidation of the most abundant C60 andC70 fullerenes occurs at relatively high potentials, making itdifficult to achieve the reversible process in standard electro-chemical studies.47−50 However, oxidation of many otherhigher fullerenes (C76, C78, C82, C84, etc.) occurs at lesspositive potentials and one or two oxidation steps are accessiblestraightforwardly.44,45,51

Encapsulation of metal atoms and clusters in EMFs can resultin redox behavior more complex than that for empty fullerenes.While the carbon cage is the only redox-active center in thelatter, both the cage and the endohedral species can exhibitredox activity in EMFs. Scheme 1 shows two extremes of suchredox behavior. In the first case, only the carbon cage is redox-active, meaning that the metal/π-system interface remains inertduring electrochemical processes. In terms of organometallicelectrochemistry, the fullerene behaves as a noninnocent ligand.All monometallofullerenes (mono-EMFs) exhibit fullerene-based redox properties. The redox behavior of MIII@C2n mono-EMFs (MIII = Y, La, Ce, Pr, Gd, Er, etc.) is neverthelesssubstantially different from that of empty fullerenes becauseMIII@C2n molecules are radicals with an unpaired electrondelocalized over the fullerene cage. As a result, theirelectrochemical (EC) gap is relatively small (less than 1 V),

and they are normally easier to reduce or oxidize than emptyfullerenes (Figure 2).33,52,53 Redox potentials of MII@C2nmono-EMFs (MII = Sm, Tm, Yb), which have a diamagneticcarbon cage, are rather similar to those of empty fullerenes.33,54

Another possibility for the mechanism of a redox process ofan EMF molecule is shown in the lower part of Scheme 1. Inthis second case, the endohedral cluster is the redox-activespecies, whereas the carbon cage merely acts as an inertcontainer, transparent to electrons. In terms of organometallicelectrochemistry, here the fullerene cage is a noninnocentligand, even though the electron transfer occurs across themetal/π-system interface. This type of electron transfer isknown as an endohedral electron transfer process and is themain subject of endohedral electrochemistry, also known aselectrochemistry in cavea.55 It is not known yet if such electrontransfer occurs through the intermediate state with the chargelocalized on the carbon cage or if the electron tunnels throughthe carbon cage wall. An obvious, but not always necessary,prerequisite for endohedral redox activity is a suitable energy ofthe metal-based molecular orbitals, which should be the frontierMOs (HOMO or LUMO) of the EMF molecule. In principle,this condition can be fulfilled for all EMFs whose endohedralspecies are more complex than a single metal atom.

Figure 1. Molecular structures of selected EMFs: (a) monometallo-fullerenes 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 NCFTiSc2N@C80; (h) TiLu2C@C80 with a μ3-carbido ligand; (i) schematicdescription of charge distribution in TiLu2C@C80 and the TiCdouble bond. Color code: La, orange; Sc, magenta; Ti, cyan; Lu, green;N, blue; O, red; S, yellow; endohedral C atom, dark gray. Linesconnecting Sc atoms in Sc2@C82 and Sc4O2@C80 denote Sc−Scbonds.

Scheme 1. Fullerene-Based and Endohedral Redox Processesin Endohedral Fullerenesa

aBlue arrows indicate localization of spin and extraneous charge overthe fullerene cage (upper part) or endohedral cluster (lower part) aftera redox reaction.

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

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Experimentally, the endohedral redox processes can be revealedvia unexpected redox behavior (e.g., shifted potential incomparison to analogous molecules) and/or with the use ofex situ or in situ spectroelectrochemical methods (such aselectron spin resonance or nuclear magnetic resonancespectroelectrochemistry). In the remaining sections of thisreview we will discuss the peculiarities of endohedral redoxprocesses in EMFs with a particular focus on dimetallofuller-enes and the effect of electron transfer on metal−metalbonding, endohedral reduction of Sc3N@C80, and clusterfuller-enes whose endohedral redox activity is caused by single metalatoms changing their valence state, such as Ti and Ce.

■ METAL−METAL BONDING AND ENDOHEDRALREDOX PROCESSES IN EMFS

Whereas mono-EMFs show only cage-based redox properties,the encapsulation of two normally trivalent metal atoms (suchas Sc, Y, and some lanthanides) within an EMF often results indimetallofullerenes with endohedral redox activity. The metal-based redox activity of M2@C2n is due to the metal−metalbonding orbital (Figure 3), which has an energy comparable to

that of the frontier MOs of the carbon cage and can be eitherthe HOMO or the LUMO of the di-EMF molecule. In theformer case, there is an M−M bond already in the neutral stateof the di-EMF, whereas in the latter case the M−M bond canbe formed when the M−M bonding orbital is populated by thereduction process.Whether the M−M bonding MO in a given di-EMF involves

the HOMO or the LUMO depends on the relative energies ofthe cage frontier MO and the energy of the metal−metalbonding orbital. It was shown that the energy of the M−Mbonding MO in EMFs is similar to the lowest energy valenceMO of the free metal dimer, which usually has (ns)σg

2

character.56 The energy of the (ns)σg2 orbital in the M2

dimer correlates with the ns2(n−1)d1 → ns1(n−1)d2 excitationenergy of the free metal atom, and therefore this excitationenergy to a large extent determines the valence state of metalatoms in di-EMFs. Namely, ns2(n−1)d1 → ns1(n−1)d2excitation energies increase in the row La−Sc/Y−Lu as 0.33−1.43/1.36−2.34 eV,57 respectively, and the (ns)σg

2 MOs in thecorresponding M2 dimers are stabilized in the row La2−Sc2/Y2−Lu2.

56 As a result, in di-EMFs, La is always trivalent (the

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 theHOMO or even below the HOMO level in (Lu2+)2@C2n

4−),whereas the valence state of Sc and Y is more sensitive to theenergy of the carbon cage MOs.56

Cluster-Based Reduction in Di-EMFs. Since the La−Labonding MO is the LUMO in La2@C2n di-EMFs (Figure 3),reduction of a La2@C2n molecule should be an endohedralredox process. Electrochemical studies of La2@C2n (2n = 72,78, 80) showed that these EMFs exhibit two to three reversiblesingle-electron-reduction steps and are relatively easy to reduce(Figure 4). For instance, the first reduction of La2@C80-Ih

occurs at −0.31 V (hereafter all redox potentials are givenversus the Fe(Cp)2

+/0 couple),58 whereas nitride clusterfuller-enes M3N@C80 with the same C80-Ih carbon cage are fullerene-based reductions and are reduced at ca. −1.4 V.59,60 The 1.1 Vdifference in the first reduction potentials for the EMFs withthe same carbon cage points to the metal-based reduction inLa2@C80-Ih. Likewise, the first reductions of La2@C72(−0.68),61 La2@C78 (−0.40 V),62 and La2@C80-D5h (−0.36V)63 are also significantly more positive than for EMFs withfullerene-based reduction.Another indication of the cluster-based reduction in La di-

EMFs is the difference between the first and the secondreduction potentials. For a cage-based redox process, thedifference between the first and second reduction (oroxidation) steps is usually within the range 0.40−0.45 V ifthe process is based on the same cage MO (Figure 2). Anendohedral redox process results in a much larger potentialdifference for the consequent redox steps, since these steps areeither based on the cluster MO (which has a much higher on-site Coulomb interaction than in the fullerene cage) or affectdifferent MOs (one on the cluster and one on the carbon cage).The difference between the first and second reductionpotentials in all La2@C2n di-EMFs is in the range of 1.23−1.44 V (Figure 4), which unambiguously points to a La2-basedreduction.58,61−63

The M−M bonding orbitals in di-EMFs have hybrid spdcharacter64 and inherit a significant ns-component from the(ns)σg

2 MO of corresponding metal dimers.56 When a metal-based MO is transformed to a singly occupied MO via eitherreduction or oxidation, the resulting SOMO also has a largecontribution of metal ns orbitals. Hence, the presence of M−Mbonds can be verified straightforwardly by ESR spectroscopy ofanion 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.

Figure 4. Cyclic and differential pulse voltammograms of [email protected] with permission from ref 58. Copyright 1995 VCHVerlagsgesellschaft mbH, D-69451 Weinheim, Germany.



SOMO leads to a large metal-based hyperfine coupling (hfc)constant. The ESR spectrum of the La2@C80-Ih radical anionwith a huge 139La coupling constant of 364 G is a veryillustrative example.65 This value can be compared to the 139Lahfc constant of 1.2 G found in La@C82, whose spin density isdelocalized 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−Mbonding MO similar to the anion radicals of La2@C80. In linewith this MO analysis, the ESR study of Y2@C79N revealed anenhanced 89Y hfc constant of 81.2 G66 (in comparison to 0.5 Gin Y@C82 with cage-based spin density68). An electrochemicalstudy of Gd2@C79N showed that its first reduction potential,−0.96 V, is significantly more positive than the cage-based firstreduction potential of M3N@C80-Ih (although not as stronglyshifted as in La2@C80-Ih). The second reduction of Gd2@C79Nis found at −1.98 V: i.e., the difference between the first andsecond reduction potentials is as large as 1.02 V. On the basis ofredox potentials and DFT calculations, which predict a Gd2-localized LUMO, the first reduction of Gd2@C79N can betentatively assigned as an endohedral reduction. Interestingly,the M−M bonding SOMO in M2@C79N is “buried” below thecage-based MOs, and hence oxidation of M2@C79N should be acage-based process.66,67 Indeed, the oxidation potential ofGd2@C79N is +0.51 V, which is close to the values of La2@C80-Ih (+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 di-EMFs is always occupied and is usually the HOMO (e.g., Lu2@C76-Td and Lu2@C82-C3v(8)): i.e., oxidation of Lu2@C2n EMFsis expected to be an endohedral redox process.56,64 To ourknowledge, experimental electrochemical studies of lutetium-based di-EMFs have not yet been reported.Spectroscopic and computational studies of M2@C82 di-

EMFs (M = Sc, Y, Er) show that the formal charge of thecarbon cage (4−), and hence the metal atoms within, are in adivalent state with an M−M bonding HOMO.40,56,69−72

Unfortunately, only a few electrochemical studies of such di-EMFs have been reported. Very illustrative of this is a recentstudy of Sc2@C82-C3v(8) and its comparison to Sc2C2@C82-C3v(8), a carbide clusterfullerene with the same carbon cage.70

UV−vis−NIR absorption spectra of both EMFs are verysimilar, which proves that the C82-C3v(8) carbon cage in bothmolecules has the same formal charge (4−). At the same time,the first oxidation of Sc2@C82 occurs at +0.05 V, whereas theoxidation 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, theoxidation 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 yetbeen reported; however, the chemically generated anion radicalof 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 interestingto see if the cation radical of Y2@C82 also has a large 89Y hfcconstant, since the metal contribution is greater to the HOMOthan to the LUMO.Oxide Clusterfullerene Sc4O2@C80-Ih. In Sc4O2@C80, the

Sc atoms form a tetrahedral cluster, and the two μ3-coordinatedoxygen atoms are located above the centers of two faces of thetetrahedron (Figure 1e).23 The HOMO of the molecule is a

Sc−Sc bonding MO largely localized on two Sc atoms with oneoxygen neighbor (Figure 3c).40,74 The valence state of thesetwo Sc atoms is therefore ScII. The two other Sc atoms (whichhave two oxygen neighbors) are in the ScIII state. The LUMOof Sc4O2@C80 is also localized on the cluster and hasmulticenter Sc bonding character. On the basis of the frontierMO analysis, both the reduction and oxidation of Sc4O2@C80are expected to be endohedral redox processes.Electrochemical studies have shown that Sc4O2@C80-Ih

exhibits two reversible reduction and two reversible oxidationsteps.36,75 The redox potentials are consistent with cluster-based redox processes. Namely, the first reduction potential, at−1.10 V, is more positive than that in M3N@C80-Ih (ca. −1.4V) and the gap between the first and second reductions is 0.63V. The oxidation potential of Sc4O2@C80-Ih is 0.0 V, which iswell below the +0.5−0.6 V that is expected for a fullerene-basedoxidation process in EMFs with a C80-Ih cage. The differencebetween the first and second oxidation steps is 0.79 V. Thus,the electrochemical data as well as the DFT computationsindicate that the redox activity of Sc4O2@C80 is dominated bycluster-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 stepsshowed rich 45Sc hyperfine structure due to two pairs ofequivalent Sc atoms in the Sc4O2 cluster. In the cation radical,the 45Sc hfc constants are 2 × 154.4 and 2 × 18.0 G. The largestconstant is assigned to the ScII atoms, and the large value isconsistent with the nature of the ScII−ScII bonding with a large4s contribution.56,75 The 45Sc hfc constants in the anion radicalare considerably smaller, 2 × 2.6 and 2 × 27.4 G, but still pointto a large Sc contribution to the spin density. The smallera(45Sc) values are due to the different nature of the Sc−Scbonding LUMO of Sc4O2@C80, with a larger 3d and a smaller4s contribution.

■ Sc3N@C80 AND ITS DERIVATIVESSc3N@C80 versus Other Nitride Clusterfullerenes.

Nitride clusterfullerenes (NCFs), including Sc3N@C80-Ih,usually exhibit two or three irreversible reduction steps andone or two oxidations, the first of which is usually reversible(Figure 5a). Due to their higher abundance, M3N@C80 NCFswith Ih cages have been the most studied to date. When M isyttrium or a lanthanide, the first reduction and oxidationpotentials 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 potentialis shifted anodically to −1.25 V, and its electrochemicalreversibility can be achieved at scan rate of a few volts persecond.76,77 ESR spectroscopic studies of the Sc3N@C80anion77−79 as well as computational studies80,81 indicated thatthe reduction of Sc3N@C80 proceeds via the occupation of thecluster-based LUMO (Figure 6). In particular, 45Sc hfcconstants 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, asdiscussed in the previous section. For comparison, the spindensity in the cation and anion radicals of Sc3N@C68 islocalized on the carbon cage, and the 45Sc hfc constants aresmaller than 2 G.82,83 Thus, reduction of Sc3N@C80-Ih is anendohedral redox process.



The reason for the special cathodic behavior of Sc3N@C80 isthe higher electronegativity of Sc, whose 3d orbitals have lowerenergies than the 4d orbitals of Y and the 5d orbitals of thelanthanides. It leads to the enhanced contribution of Sc to theLUMO of Sc3N@C80, whereas the cage contribution to theLUMO is dominating for non-Sc M3N@C80. For instance, DFTcomputations show that the net spin population of the Y3Ncluster in Y3N@C80

− is less than 29% (in comparison to 65% inSc3N@C80

−; see Figure 6).80,81 The ESR studies of the anionradicals of pristine M3N@C80-Ih NCFs have not been reported,but in the anion radical of the Y3N@C80 pyrrolidinecycloadduct the 89Y hfc constants are 1.4 and 2 × 6.3 G,84

which is substantially smaller than the 81.2 G that is found inY2@C79N with Y-localized spin density.66

When a mixture of two metals is used in the synthesis, theresulting NCFs have mixed-metal nitride clusters. It might beexpected that the difference in the electronegativity of Sc andother metals should affect localization of the LUMO in Sc-lanthanide mixed-metal NCFs. Indeed, reduction potentials ofSc3M3−xN@C2n are usually more positive than those of M3N@C2n, but the effect is not very large.

86,87 DFT computations alsoshow that Sc has an enhanced contribution to the LUMOs ofSc3M3−xN@C2n NCFs in comparison to other metal atoms.86

ESR spectroelectrochemical studies could give more detailedinformation on the spin density distribution in anion radicals ofmixed-metal NCFs but have yet to be reported.

Exohedral Derivatives of Sc3N@C80. Chemical derivatiza-tion changes the π system of the fullerene and hence cansubstantially affect electrochemical properties. For instance,derivatization of Sc3N@C80 shifts its redox potentials and oftenmakes reduction behavior more electrochemically reversiblethan that of nonderivatized Sc3N@C80 (Figure 5).59,79,85,88

Although predominant localization of the LUMO on the Sc3Ncluster is preserved, at least at lower degrees of addition,redistribution between the cluster and the carbon cage usuallytakes place. In addition, the dynamics of the cluster are altered:in pristine Sc3N@C80-Ih, the Sc3N cluster is known to rotatefreely inside the fullerene cage, whereas its rotation is frozen inthe derivatives.One of the first synthesized derivatives of Sc3N@C80 was its

pyrrolidine cycloadduct, denoted hereafter as [5,6]-pyrrolidino-Sc3N@C80 ([5,6] means that the cycle is added across thepentagon/hexagon edge). This cycloaddition makes the firstreduction reversible and shifts its potential anodically by +0.11V.59 The ESR spectrum of the anion radical of [5,6]-pyrrolidino-Sc3N@C80 exhibits a hyperfine structure with 45Schfc constants of 9.6 and 2 × 33.4 G (Figure 7a).89 The Scatoms in the anion radical are no longer equivalent, whichmeans that the cluster is not rotating on the ESR time scale andthat the spin density is redistributed so that only two of the Scatoms have considerable contribution to the spin density(Figure 7b,c). Thus, reduction of [5,6]-pyrrolidino-Sc3N@C80is still a cluster-based process, but only two of the Sc atoms inthe 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 studiedderivatives are reversible and are more positive than that of theparent 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 byelectrochemical means or via reaction with cobaltocene showeda systematic decrease of a(45Sc) values with an increase in thenumber of CF3 groups. DFT calculations and a decrease of the45Sc coupling constants in anion radicals of Sc3N@C80(CF3)xshows that the spin density in anions is gradually shifting fromthe Sc3N cluster to the carbon cage in higher trifluoromethy-lated derivatives.88 Thus, the “innocent” C80-Ih cage in Sc3N@C80 becomes “non-innocent” after the addition of 12 CF3groups. An interesting situation was observed for Sc3N@C80(CF3)2 at the third electron reduction step: the trianionradical exhibited a large 45Sc coupling constant for one of the Scatoms (49.2 G), which agreed with DFT-predicted localizationof the spin density on one of the three Sc atoms.79 Hence, the

Figure 5. Cyclic voltammograms of (a) Sc3N@C80-Ih, (b) N-tritylpyrrolidino-[5,6]-Sc3N@C80-Ih, and (c) N-tritylpyrrolidino-[6,6]-Sc3N@C80-Ih. The scan rate is 100 mV s−1. Reproduced withpermission 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 ChemicalSociety. (c) ESR spectrum of Sc3N@C80

−, obtained by the reaction ofSc3N@C80 and cobaltocene in o-dichlorobenzene at room temper-ature. The asterisk marks an impurity signal (less than 1% pf the totalsignal intensity). Adapted from ref 79.



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

■ Ti AS A REDOX-ACTIVE CENTER IN ENDOHEDRALMETALLOFULLERENES

The majority of clusterfullerenes are synthesized using groupIII metals such as Sc, Y, and lanthanides. Ti is a rare example ofa transition metal that is capable of forming EMFs. Until 2009,Ti@C28, Ti2C2@C78, and Ti2C84 were the only Ti-EMFsknown.91−94 Although Ti@C28 has been detected by massspectrometry, 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 twodimetallofullerene isomers, Ti2@C80-Ih and Ti2@C80-D5h,

92

but later computational studies showed that the moleculeTi2C2@C78-D3h is much more stable and was also moreconsistent with the 13C NMR spectrum.95,96 A detailedcharacterization of three isomers of Ti2C84 has not beenreported, and their structures remain unknown (e.g., it is notclear if Ti2C84 is actually Ti2C2@C82). None of these Ti-EMFshave been studied by electrochemical techniques. DFTcalculations show that the LUMO of Ti2C2@C78 has a largecontribution from the metal atoms,80 and hence its reduction isexpected 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 mixtureaffords the mixed-metal NCF TiSc2N@C80-Ih (Figure 1f).97

Likewise, the synthesis of TiY2N@C80-Ih was reported in2012.98 Both of these Ti-based mixed-metal NCFs exhibit

specific electrochemical properties that are drastically differentfrom those of all other M3N@C80-Ih NCFs.

97−99 First, their firstreduction is reversible at moderate scan rates (for TiSc2N@C80,three reversible reductions are observed even at a small scanrate of 20 mV/s; see Figure 8). Second, their redox potentials

are also substantially different from those of NCFs. The firstreduction steps are more positive (−0.94 V in TiSc2N@C80-Ihand −1.11 V in TiY2N@C80-Ih versus −1.25 V in Sc3N@C80-Ihand ca. −1.4 V in other M3N@C80-Ih). Their oxidation stepsare shifted cathodically to +0.16 V for TiSc2N@C80 and 0.00 Vfor TiY2N@C80 (compare to ca. +0.6 V in M3N@C80-Ih). As aresult, the electrochemical gaps of Ti-based NCFs are around1.1 V, which is nearly half of those for all other [email protected] reason for the unique redox behavior of TiM2N@C80-Ih

NCFs can be understood by taking into account that Ti is thenext element after Sc in the periodic table. As a result, TiSc2N@C80 is isoelectronic with the anion radical of Sc3N@C80 and istherefore paramagnetic. ESR studies of TiM2N@C80 (M = Sc,Y) at room temperature showed broad signals; however, theline width decreased at lower temperatures.97−99 Below liquid

Figure 7. (a) Experimental (top) and simulated (bottom) X-band ESRspectra of the chemically reduced monoanion of [5,6]-pyrrolidino-Sc3N@C80. The asterisk denotes an impurity. (b, c) Spin densityisosurfaces 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 withpermission from ref 89. Copyright 2013 American Chemical Society.

Figure 8. (a) Cyclic voltammograms of TiSc2N@C80-Ih in o-DCBsolution. 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 [email protected] with permission from ref 98. Copyright 2012 AmericanChemical Society.



nitrogen temperature, TiSc2N@C80 exhibited a fine structure,although it was not sufficiently resolved for a precisedetermination of hyperfine coupling constants.99 A roughestimation showed that 45Sc hfc constants are less than 7 G. Animportant feature common to the ESR spectra of TiM2N@C80NCFs is the strong shift of the g factor (1.9454 in TiSc2N@C80and 1.9579 in TiY2N@C80) from the free electron value of2.0023. Such a shift shows that the orbital momentum is notquenched and indicates localization of unpaired spin on Ti.DFT calculations also show that in both TiM2N@C80molecules the spin density is predominantly localized on Ti(Figure 8a inset). Thus, the formal valence state of titanium isTiIII with a localized 3d1 electron. The SOMO of TiM2N@C80is thus largely a Ti-based orbital; therefore, oxidation andreduction steps proceed via a change in the valence state of theendohedral Ti, which becomes TiII in TiM2N@C80

− and TiIV inTiM2N@C80

+. So far, TiM2N@C80 is the only example of anEMF with an endohedral redox activity in both the reductionand 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 centralcarbon atom and a TiC double bond.32 Unlike paramagneticTiM2N@C80 NCFs, TiLu2C@C80 is diamagnetic and itselectronic structure has a certain similarity to that of the Sc-based nitride clusterfullerene Lu2ScN@C80 as a result of theTi3+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 (thepotential and reversibility) for the first reduction of TiLu2C@C80 is similar to that of TiSc2N@C80 and TiY2N@C80. This canbe compared to the first reduction step of Lu2ScN@C80, whichis irreversible and occurs at a potential 0.51 V more negative(Figure 9a). However, the first oxidation of Lu2ScN@C80 isreversible and occurs at +0.66 V, which is very close to theoxidation potential of TiLu2C@C80 (+0.63 V). These valuescan be compared to the much more negative oxidationpotentials of TiSc2N@C80 (+0.16 V) and TiY2N@C80 (0.00V). Thus, electrochemical measurements show that theelectronic structure of TiLu2C@C80 is intermediate between

that of conventional M3N@C80 NCFs with group III metalsand NCFs with one Ti atom, [email protected] behavior can be rationalized by the DFT-based frontier

orbital analysis. Figure 9 compares the MO energy levels inTiLu2C@C80, TiLu2N@C80, and Lu2ScN@C80 as well as theisosurfaces of their HOMOs and LUMOs. The LUMO ofTiLu2C@C80 is largely localized on the Ti atom and is 0.27 eVbelow the LUMO energy of Lu2ScN@C80, which has a muchlarger carbon cage contribution. Hence, the first reduction ofLu2ScN@C80 is much more negative. In contrast, the Ti-localized LUMO of TiLu2N@C80 is almost isoenergetic withthat of TiLu2C@C80, and hence their reduction potentialsshould be similar. The shapes and the energies of the HOMOsof TiLu2C@C80 and Lu2ScN@C80 are almost identical, andboth MOs are essentially carbon cage orbitals. Hence, theoxidation potentials of TiLu2C@C80 and Lu2ScN@C80 aresimilar and are more positive in comparison to those ofTiM2N@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 Ti-containing clusterfullerene with a sulfide cluster (Figure 1e).100

The molecule is isostructural with Ti2C2@C78 and likewise hastitanium in a TiIV state with a metal-localized LUMO (Figure10). Hence, an endohedral reduction of Ti2S@C78 can beanticipated.Ti2S@C78 exhibits three reversible reduction and two

reversible oxidation steps (Figure 10a). The first reductionpotential 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 V

99) and TiLu2C@C80 (−0.91 V). The difference between the first and the secondreduction potentials is 0.61 V. Thus, electrochemical dataconfirm that the reduction of Ti2S@C78 is an endohedral redoxprocess. Its oxidation, however, is a cage-based process, inaccordance with the HOMO shape and the difference in thefirst and second oxidation potentials (0.42 V).

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-shellTiLu2N@C80, spin-up and spin-down levels are shown separately). (c) Isosurfaces of the frontier MOs of TiLu2C@C80-Ih and [email protected] with permission from ref 32. Copyright 2014 Macmillan Publishers Limited.



■ STRAIN-DRIVEN ENDOHEDRAL CeIV/CeIII REDOXCOUPLE

Ce is different from all other lanthanides in that its CeIV valencestate is accessible and is known for a plethora of inorganic andorganometallic compounds, and the redox potential of theCeIV/CeIII couple can vary over a broad range.102,103 However,in all known cerium EMFs, including mono- and di-EMFs aswell as nitride clusterfullerenes, this metal always adopts a CeIII

valence state and therefore Ce-based EMFs are isostructuralwith La-EMFs. The redox behaviors of Ce and La EMFs areusually very similar, which shows that the CeIII state in Ce2@C72, 78, 80,

63,104−106 Ce@C82,52 and Ce3N@C92,96

107,108 remainsunaffected by the electrochemical oxidation of the EMFmolecules, similar to the LaIII state in La-EMFs (however, thereduction of Ce di-EMFs is expected to form a Ce−Ce bondsimilar 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 andelectrochemical studies were reported in 2010.109 Thetemperature-dependent paramagnetic shifts in the 13C NMRspectra of CeLu2N@C80-Ih confirmed the CeIII valence statewith a Ce-localized 4f1 electron. The electrochemical measure-ments revealed an unprecedented negative shift of the oxidationpotential of CeLu2N@C80 (+0.01 V), which compares to thestandard values of ca. +0.60−0.65 V in other M3N@C80-IhNCFs. DFT calculations showed that the oxidation potential ofthe cage-based process in CeLu2N@C80-Ih would be similarthan those of other NCFs. However, removal of the 4f1 electronfrom Ce, i.e. its endohedral oxidation, was predicted to be moreenergetically favorable by 0.45 eV at the PBE0 level. Thus,endohedral oxidation of CeIII to CeIV was proposed as aplausible explanation of the unusual electrochemical behavior of

[email protected] However, it remained unclear what was so

special about CeLu2N@C80 that forced the CeIII redoxbehavior, whereas all other Ce-EMFs do not show suchbehavior.This question was clarified in a more recent study of other

CeM2N@C80-Ih NCFs (M = Sc, Y).110 In this work, Sc and Ywere chosen to vary the size of the endohedral cluster in theCeM2N@C80 series (Shannon’s radii of Sc

3+, Lu3+, and Y3+ ionsare 0.745, 0.86, and 0.90 Å, respectively111). Figure 11 shows

cyclic voltammograms of all three CeM2N@C80-Ih NCFs incomparison to that of PrSc2N@C80-Ih (the latter was chosen asa 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 (peakpotentials Ep are listed), close to the values of all other M3N@C80-Ih NCFs. The variation of Ep values with different clustercompositions does not exceed 0.1 V. The anodic behavior ofCeM2N@C80 NCFs is more peculiar. All compounds exhibitone reversible oxidation step whose potential varies from −0.07V for CeY2M@C80 to +0.33 V for CeSc2N@C80. The oxidationpotential of PrSc2N@C80 measured under the same conditionsis +0.64 V.

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

Figure 11. (a) Cyclic voltammograms of CeM2N@C80 (M = Y, Lu,Sc) and PrSc2N@C80 measured in o-DCB solution with TBABF4 assupporting electrolyte and scan rate 100 mV/s. (b) Schematicdescription of endohedral oxidation of CeIII in [email protected] with permission from ref 110. Copyright 2013 AmericanChemical Society.



A considerable negative shift of the first anodic process inCeM2N@C80-Ih in comparison to PrSc2N@C80-Ih and otherM3N@C80-Ih NCFs served as a first indication of a Ce-basedredox process. Compelling evidence of the endohedraloxidation of CeIII in CeM2N@C80-Ih NCFs was obtained by13C NMR spectroscopy. If the oxidation of CeM2N@C80 is afullerene-based process, their radical cations are expected togive no detectable signals in 13C NMR spectra because of thespin density on the fullerene cage. In contrst, an endohedralCeIII → CeIV oxidation produces diamagnetic cations, whichshould be accessible by 13C NMR spectroscopy. Figure 12

shows 13C NMR spectra of pristine CeM2N@C80-Ih NCFs andthe spectra of the same compounds after addition of oxidationagents 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 ofCeM2N@C80 results in diamagnetic CeM2N@C80

+ cations withthe same two-line 13C NMR pattern for the C80-Ih(7) cage as inthe initial compounds, but with the position of the signalsshifted. Furthermore, the peak at δ 280 ppm in the 45Sc NMR

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, butit could not clarify why the oxidation potential of CeIII in theCeM2N@C80 NCF depends so strongly on the second clustermetal, M, which is not involved in the redox process. Thisquestion was answered with the help of DFT calculations,which showed that in the CeIVM2N@C80

+ cations the Ce−Nbond lengths are shortened in comparison to those in theneutral CeM2N@C80, whereas the M−N bonds are elongated.These structural changes are caused by the smaller ionic radiusof Ce4+ (0.87 Å) in comparison to Ce3+ (1.01 Å). In short,oxidation reduces the size of the endohedral cluster. It shouldbe noted that encapsulation of relatively large CeIIIM2N clustersin a C80-Ih(7) cage within the limited interior space results in asignificant strain. For CeY2N@C80, it even forces pyramidaliza-tion of the CeY2N cluster. M3N clusters are usually planar inNCFs, and pyramidalization is an indication of the strain causedby insufficient space for the large cluster inside the cage. Adecrease in the cluster size for CeIVM2N@C80

+ cationstherefore leads to a decrease of the cluster-induced strain. Inparticular, 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 makesthe corresponding CeM2N@C80 more prone to oxidation as away to release the strain. Therefore, the oxidation potential ofCeM2N@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 whyendohedral CeIII → CeIV was not observed in many otherpreviously studied Ce-based EMFs. In these molecules, eitherthe number of Ce atoms is too small (mono- and diceriumfullerenes) or the cage size is too large (Ce3N@C92,96),resulting in a relatively low inner strain in comparison toCeM2N@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 weredetermined for 12 Ce-containing NCFs and compared to thoseof the non-Ce analogues. On the basis of the shift of theoxidation potential and an increased difference between the firstand second oxidation potentials, an endohedral CeIII → CeIV

redox process at the first oxidation step was proven forCeSc2N@C78, CeY2N@C84, and Ce2YN@C86. Less confidently,an endohedral oxidation of Ce was also proposed for Ce2ScN@C86, CeY2N@C86, and [email protected] cages larger than C80, the cluster-induced strain is weaker

than for the C80-Ih cage, whereas cage oxidation potentials arebelow those for M3N@C80-Ih. Thus, the preference of anendohedral Ce or a fullerene-based oxidation at the first anodicstep depends on whether the inner strain is high enough torender a Ce-based oxidation below the oxidation potential ofthe fullerene cage. It is thus possible that varying the clustercomposition can switch the oxidation mechanism. As anexample, Figure 13 compares cyclic voltammograms of Y3N@C86 and CexM3−xN@C86 (x = 1, 2; M = Sc, Y). The oxidationpotentials of Y3N@C86 (+0.36 and +0.77 V) and CeSc2N@C86(+0.34 and +0.80 V) are virtually identical, showing that theoxidation 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 ofCeSc2N@C80 and [CeSc2N@C80]

+. (c) Change in the clustergeometry in CeY2N@C80 after oxidation (the pyramidal CeIIIY2Ncluster becomes planar CeIVY2N; Y atoms are green, nitrogen is blue,and Ce is pink). Parts (a) and (b) are reproduced with permissionfrom ref 110. Copyright 2013 American Chemical Society.



C86, the first oxidation potential is shifted to +0.29 V, whereasthe difference between the first and the second oxidation stepsis increased to 0.53 V. The oxidation potential of CeY2N@C86is further shifted to +0.27 V and the difference of the oxidationpotentials is increased to 0.55 V. Thus, an endohedral oxidationof Ce can be tentatively proposed for Ce2ScN@C86 andCeY2N@C86. Finally, the first oxidation potential of Ce2YN@C86 is further shifted down to +0.17 V, allowing anunambiguous conclusion for endohedral oxidation of Ce.DFT computations of cage- and Ce-based ionization

potentials (IPcage and IPCe, respectively) in CexM3−xN@C2n

NCFs were performed using the GGA PBE functional mixedwith 15% of exact exchange.112 Computed values areschematically presented in Figure 14. The border betweenblue and yellow fields corresponds to the IPcage value. When theIPCe value appears in the blue field, oxidation of the cage ispreferred, whereas appearance of the point in the yellow fieldsignifies 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 orderCeSc2N > (CeLu2N, Ce2ScN) > CeY2N > Ce2YN > Ce3N,corresponding to an increase in cluster size; (ii) the IPCe valuesfor a given cluster increase with an increase in carbon cage size(the only exclusion is CeSc2N, whose IPCe value reaches itsmaximum within a C84 cage and then decreases when housedwithin C86 and C88 cages). Both trends are manifestations of thesame factor, the steric strain experienced by the nitride clusterinside the carbon cage of a limited size (“cage pressure”). Whenthe steric strain in the Ce-NCF is high, it is more easily oxidizedthan Ce-NCFs with lower strain (i.e., with smaller cluster and/or larger cage). In other words, IPCe values drop down whenthe cage pressure increases. Therefore, variations in the redoxpotentials of endohedral CeIV/CeIII redox couples can berationalized using simple geometrical arguments.

■ CONCLUDING REMARKS

The encapsulation of metal clusters within a carbon cage givesrise to numerous possibilities for unconventional redoxprocesses within the fullerene. Under certain conditions, thecarbon cage can be transparent to electrons, so that the valenceand spin state of the encapsulated metal atoms is changed,while the state of the carbon cage remains intact. Theseendohedral (or in cavea) redox processes can populate ordepopulate metal−metal bonding orbitals of EMFs and thuscan be used to study the metal−metal bonding within. The useof ESR spectroelectrochemistry is especially convenient for theanalysis of these phenomena.The family of nitride clusterfullerenes (NCFs) provides a

unique platform for the establishment and tuning of endohedralredox-active species. While in general the M3N cluster remainsredox-inert in NCFs (Sc3N@C80 is a notable exception), it is

Figure 13. Cyclic voltammograms of Y3N@C86, CeSc2N@C86,Ce2ScN@C86, CeY2N@C86, and a mixture of Ce2YN@C86 andY3N@C86. The scan rate was 100 mV/s. As a guide, the verticalbars denote oxidation (E1/2) and reduction (Ep) potentials of Y3N@C86. Reproduced with permission from ref 112. Copyright 2014 RoyalSociety of Chemistry.

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 blueand yellow fields correspond to IPcage values (they are different for different carbon cages). When the IPCe value is predicted to be above theborderline (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-basedoxidation takes place. Reproduced with permission from ref 112. Copyright 2014 Royal Society of Chemistry.



possible to use the nitride cluster as a matrix to introduce anelectroactive metal in the form of the mixed-metal NCFs. Ti-and Ce-based NCFs are well-established examples that arediscussed in this review. In TiM2N@C80 NCFs, titanium adoptsa TiIII valence state, which can be changed to TiII or TiIV viaendohedral reduction or oxidation. In Ce-based NCFs such asCeM2N@C80, the large size of the nitride cluster and thelimited inner space of the carbon cage result in an inherentstrain. The driving force for endohedral oxidation of CeIII inCe-NCFs is the release of this strain by forming CeIV, which hasa smaller ionic radius. Since the increase of the ionic radius ofM3+ (Sc → Lu → Y) or the decrease of the fullerene cage sizeincreases the cluster-induced strain, the oxidation potential ofCexM3−xN@C2n NCFs shifts to more negative values for largerM3+ ions and smaller cages. To our knowledge, there are noother reports showing that the geometric strain of Cecompounds can be used to tune the redox potential of theCeIV/CeIII couple.The ongoing progress in the synthesis and characterization of

endohedral metallofullerenes promises that new and uniqueredox properties of these molecules will be discovered in thenear future.

■ AUTHOR INFORMATION

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

Notes

The authors declare no competing financial interest.

Biographies

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.

Alexey A. Popov received his M.S. (1999) and Ph.D. (2003) degrees inphysical chemistry from Moscow State University (MSU), Russia. In2003−2008 he worked at the Chemistry Department of MSU as asenior researcher. In 2008 he received a Humboldt fellowship for aresearch stay at the Leibniz Institute of Solid State and MaterialsResearch (IFW Dresden, Germany). Now he is a head of the fullerenegroup at IFW Dresden. His current interests include chemical andphysical properties of empty and endohedral metallofullerenes andtheir derivatives and their synthesis, spectroelectrochemistry, opticalspectroscopy, magnetic properties, and quantum-chemical computa-tions.

■ ACKNOWLEDGMENTS

The manuscript is devoted to the memory of Lothar Dunsch(1948−2013), who introduced the authors into the field ofendohedral fullerenes and their electrochemistry. The authorsacknowledge Sandra Shiemenz, Marco Rosenkranz, and FrankZiegs (all at IFW Dresden) for continuous technical supportand Dr. Bryon W. Larson (NREL) for his careful reading of themanuscript. The authors acknowledge the Deutsche For-schungsgemeinschaft (DFG) project PO 1602/1-1 for financialsupport.

■ ABBREVIATIONS

EMF, endohedral metallofullerenes; NCF, nitride clusterfuller-enes; ESR, electron spin resonance; IP, ionization potential

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