Full Exploration of the Diels–Alder Cycloaddition on Metallofullerenes M3N@C80 (M=Sc,Lu, Gd): The D5h versus Ih Isomer and the Influence of the Metal Cluster

In this work a detailed investigation of the exohedral reactivity of the most important and abundant endohedral metallofullerene (EMF) is provided, that is, Sc₃N@I_h‐C₈₀ and its D_5h counterpart Sc₃N@D_5h‐C₈₀, and the (bio)chemically relevant lutetium‐ and gadolinium‐based M₃N@I_h/D_5h‐C₈₀ EMFs (M=Sc, Lu, Gd). In particular, we analyze the thermodynamics and kinetics of the Diels–Alder cycloaddition of s‐cis‐1,3‐butadiene on all the different bonds of the I_h‐C₈₀ and D_5h‐C₈₀ cages and their endohedral derivatives. First, we discuss the thermodynamic and kinetic aspects of the cycloaddition reaction on the hollow fullerenes and the two isomers of Sc₃N@C₈₀. Afterwards, the effect of the nature of the metal nitride is analyzed in detail. In general, our BP86/TZP//BP86/DZP calculations indicate that [5,6] bonds are more reactive than [6,6] bonds for the two isomers. The [5,6] bond D _5h ‐b , which is the most similar to the unique [5,6] bond type in the icosahedral cage, I _h ‐a , is the most reactive bond in M₃N@D_5h‐C₈₀ regardless of M. Sc₃N@C₈₀ and Lu₃N@C₈₀ give similar results; the regioselectivity is, however, significantly reduced for the larger and more electropositive M=Gd, as previously found in similar metallofullerenes. Calculations also show that the D_5h isomer is more reactive from the kinetic point of view than the I_h one in all cases which is in good agreement with experiments.     

Blending Non‐Group‐3 Transition Metal and Rare‐Earth Metal into a C80 Fullerene Cage with D5h Symmetry

Rare‐earth metals have been mostly entrapped into fullerene cages to form endohedral clusterfullerenes, whereas non‐Group‐3 transition metals that can form clusterfullerenes are limited to titanium (Ti) and vanadium (V), and both are exclusively entrapped within an I_h‐C₈₀ cage. Non‐Group‐3 transition‐metal‐containing endohedral fullerenes based on a C₈₀ cage with D_5h symmetry, V_xSc_3?xN@D_5h‐C₈₀ (x=1, 2), have now been synthesized, which exhibit two variable cluster compositions. The molecular structure of VSc₂N@D_5h‐C₈₀ was unambiguously determined by X‐ray crystallography. According to a comparative study with the reported Ti‐ and V‐containing clusterfullerenes based on a I_h‐C₈₀ cage and the analogous D_5h‐C₈₀‐based metal nitride clusterfullerenes containing rare‐earth metals only, the decisive role of the non‐Group‐3 transition metal on the formation of the corresponding D_5h‐C₈₀‐based clusterfullerenes is unraveled.     

Synthesis and Isolation of the Titanium–Scandium Endohedral Fullerenes—Sc2TiC@Ih‐C80, Sc2TiC@D5h‐C80 and Sc2TiC2@Ih‐C80: Metal Size Tuning of the TiIV/TiIII Redox Potentials

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed‐metal Sc–Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH₄ and Ti/CH₄ systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH₄ system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH₄ system are Sc₄C₂@C₈₀ (the most abundant EMF from this synthesis), Sc₃C₂@C₈₀, isomers of Sc₂C₂@C₈₂, and the family Sc₂C_{2 n} (2 n=74, 76, 82, 86, 90, etc.), as well as Sc₃CH@C₈₀. The Sc–Ti/CH₄ system produces the mixed‐metal Sc₂TiC@C_{2 n} (2 n=68, 78, 80) and Sc₂TiC₂@C_{2 n} (2 n=80) clusterfullerene families. The molecular structures of the new, transition‐metal‐containing endohedral fullerenes, Sc₂TiC@I_h‐C₈₀, Sc₂TiC@D_5h‐C₈₀, and Sc₂TiC₂@I_h‐C₈₀, were characterized by NMR spectroscopy. The structure of Sc₂TiC@I_h‐C₈₀ was also determined by single‐crystal X‐ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc₂TiC‐ and Sc₂TiC₂‐containing clusterfullerenes have Ti‐localized LUMOs. Encapsulation of the redox‐active Ti ion inside the fullerene cage enables analysis of the cluster–cage strain in the endohedral fullerenes through electrochemical measurements.     

New Trifluoromethylated Derivatives of Metal Nitride Clusterfullerenes: Sc3N@Ih‐C80(CF3)14 and Sc3N@D5h‐C80(CF3)16

Trifluoromethylated derivatives of Sc₃N@I_h‐C₈₀ and Sc₃N@D_5h‐C₈₀ were synthesized by the reaction with CF₃I at 440 °C. HPLC separation of the mixture of Sc₃N@D_5h‐C₈₀(CF₃)_n derivatives resulted in isolation and X‐ray structure determination of Sc₃N@D_5h‐C₈₀(CF₃)₁₆, which represents a precursor of the known Sc₃N@D_5h‐C₈₀(CF₃)₁₈. Among the CF₃ derivatives of Sc₃N@I_h‐C₈₀, two new isomers of Sc₃N@I_h‐C₈₀(CF₃)₁₄ ( Sc‐14‐VI and Sc‐14‐VII ) were isolated by HPLC, and their molecular structures were determined by X‐ray diffraction, thus enabling a comprehensive comparison of altogether seven isomers. Two types of addition patterns with different orientations of the Sc₃N cluster relative to the I_h‐C₈₀ fullerene cage were established. In particular, Sc‐14‐VII represents a direct precursor of the known Sc₃N@I_h‐C₈₀(CF₃)₁₆‐ II . All molecular structures exhibit an ordered position of a Sc₃N cluster inside the fullerene C₈₀ cage.     

Synthesis and Characterization of the D5h Isomer of the Endohedral Dimetallofullerene Ce2@C80: Two‐Dimensional Circulation of Encapsulated Metal Atoms Inside a Fullerene Cage

Herein we show the synthesis and characterization of the second known Ce₂@C₈₀ isomer. A ¹³C NMR spectroscopic study revealed that the structure of the second isomer has D_5h symmetry. Paramagnetic NMR spectral analysis and theoretical calculation display that the encapsulated Ce atoms circulate two‐dimensionally along a band of ten contiguous hexagons inside a D_5h‐C₈₀ cage, which is in sharp contrast to the three‐dimensional circulation of two Ce atoms in an I_h‐C₈₀ cage. The electronic properties were revealed by means of electrochemical measurements. The D_5h isomer of Ce₂@C₈₀ has a much smaller HOMO–LUMO gap than cluster fullerenes (M₃N@C₈₀, M=Sc, Tm, and Lu) with the same D_5h‐C₈₀ cages. The chemical reactivity was investigated by using disilirane as a chemical probe. The high thermal reactivity toward 1,1,2,2‐tetramesityl‐1,2‐disilirane is consistent with the trends of the redox potentials and the lower LUMO level of the D_5h isomer of Ce₂@C₈₀ compared with that of C₆₀.     

An Efficient Method to Separate Sc3N@C80 Ih and D5h Isomers and Sc3N@C78 by Selective Oxidation with Acetylferrocenium [Fe(COCH3C5H4)Cp]+

Based on the different oxidation potentials of endohedral fullerenes Sc₃N@C₈₀ I_h and D_5h and Sc₃N@C₇₈, an efficient and useful method that avoids HPLC has been developed for their separation. Selective chemical oxidation of the Sc₃N@D_5h‐C₈₀ isomer and Sc₃N@C₇₈ by using an acetylferrocenium salt [Fe(COCH₃C₅H₄)Cp]⁺ followed by column chromatographic separation and reduction with CH₃SNa resulted in the isolation of pure Sc₃N@I_h‐C₈₀, Sc₃N@C₇₈, and a mixture of Sc₃N@D_5h‐C₈₀ and Sc₃N@C₆₈.     

Isolation of CeLu2N@Ih‐C80 through a Non‐Chromatographic,Two‐Step Chemical Process and Crystallographic Characterization of the Pyramidalized CeLu2N within the Icosahedral Cage

By combining two chemical methods of purification, 4 mg of purified CeLu₂N@C₈₀ was readily isolated from 500 mg of carbon soot extract without the use of recycling HPLC, a method which has previously been necessary to obtain pure samples of endohedral fullerenes. In stage 1, CeLu₂N@C₈₀ was selectively precipitated by virtue of its low first oxidation potential (+0.01 V) and the judicious choice of MgCl₂ as the Lewis acid precipitant. For stage 2, we used a stir and filter approach (SAFA), which employed the electron‐rich NH₂ groups immobilized on silica gel to selectively bind residual endohedrals and higher cage fullerenes that were contaminants from stage 1. Crystallographic analysis of CeLu₂N@C₈₀ in the co‐crystal CeLu₂N@I_h‐C₈₀ ? Ni(octaethylporphyrin) ? 2(toluene) reveals that the I_h‐C₈₀ cage is present with a pyramidalized CeLu₂N unit inside.     

Synthesis and Structure of LaSc2N@Cs(hept)‐C80 with One Heptagon and Thirteen Pentagons

The synthesis and single‐crystal X‐ray structural characterization of the first endohedral metallofullerene to contain a heptagon in the carbon cage are reported. The carbon framework surrounding the planar LaSc₂N unit in LaSc₂N@C_s(hept)‐C₈₀ consists of one heptagon, 13 pentagons, and 28 hexagons. This cage is related to the most abundant I_h‐C₈₀ isomer by one Stone–Wales‐like, heptagon/pentagon to hexagon/hexagon realignment. DFT computations predict that LaSc₂N@C_s(hept)‐C₈₀ is more stable than LaSc₂N@D_5h‐C₈₀, and suggests that the low yield of the heptagon‐containing endohedral fullerene may be caused by kinetic factors.     

Mapping the Metal Positions inside Spherical C80 Cages: Crystallographic and Theoretical Studies of Ce2@D5h‐C80 and Ce2@Ih‐C80

The dynamic positions of the dimetallic cluster inside the mid‐sized spherical cages of C₈₀–C₈₂ have been seldom studied, despite the high abundance of M₂@C_2n (2n=80, 82) species among various endohedral metallofullerenes. Herein, using crystallographic methods, we first unambiguously map the metal positions for both Ce₂@D_5h‐C₈₀ and Ce₂@I_h‐C₈₀, showing how the symmetry or geometrical change in cage structure can influence the motional behavior of the cluster. Inside the D_5h cage, the primary cerium sites have been identified along a cage belt of the contiguous hexagons, which suggests the significant influence of such a cage motif on endohedral cluster motion. Further analysis revealed a distorted D_5h cage owing to the “punch‐out” effect of cerium atoms. The consequence is the presence of two localized electrostatic potential minima inside the cage of (D_5h‐C₈₀)^6?, thus reflecting the primary ionic cerium–cage interaction. In contrast, a different motional behavior of Ce₂ cluster was observed inside the I_h cage. With the major cerium sites, the molecule of Ce₂@I_h‐C₈₀ presented an approximate D_2h configuration. With the combined theoretical study, we propose that the additional unidentified influence of Ni^II(OEP) (OEP=octaethylporphyrin) might be also relevant for the location of cerium sites inside the I_h cage.     

An Expanded Family of Dysprosium–Scandium Mixed‐Metal Nitride Clusterfullerenes: The Role of the Lanthanide Metal on the Carbon Cage Size Distribution

A large family of dysprosium–scandium (Dy‐Sc) mixed‐metal nitride clusterfullerenes (MMNCFs), Dy_xSc_3?xN@C_2n (x=1, 2, 2n=68, 70, 76–86) have been successfully synthesized and isolated. Among these, the C₇₀ and C₈₂‐based MMNCFs are two new cages that have never been isolated for MMNCFs. Synthesis of Dy_xSc_3?xN@C_2n was accomplished by the “selective organic solid” route using guanidinium thiocyanate as the nitrogen source, and their isolation was fulfilled by recycling HPLC. UV/Vis‐NIR spectroscopic study indicates that almost all Dy_xSc_3?xN@C_2n MMNCFs are kinetically stable fullerenes with optical band gaps beyond 1 eV. This feature is distinctly different to their counterparts Dy₃N@C_2n (78≤2n≤88), whose for optical band‐gaps are below 1 eV for relatively large cages such as C₈₄ and C₈₆. An FTIR spectroscopic study in combination with DFT calculations enables reasonable assignments of the cage isomeric structures of all isolated Dy_xSc_3?xN@C_2n (x=1, 2, 2n=68, 70, 76–86) MMNCFs. The carbon cage size distribution of Dy_xSc_3?xN@C_2n (2n=68, 70, 76–86) is compared to the reported Dy₃N@C_2n (78≤2n≤8) homogeneous NCF and Dy_xSc_3?xN@C_2n (78≤2n≤88) MMNCF families, revealing that the medium‐sized Dy metal plays a crucial role on the expanded cage size distribution of MMNCFs. As a result, Dy_xSc_3?xN@C_2n MMNCFs are the largest MMNCF family reported to date.     

Expanding the number of stable isomeric structures of the C80 cage: a new fullerene Dy3N@C80

The production, isolation, and spectroscopic characterization of a new Dy3N@C80 cluster fullerene that exhibits three isomers (1-3) is reported for the first time. In addition, the third isomer (3) forms a completely new C80 cage structure that has not been reported in any endohedral fullerenes so far. The isomeric structures of the Dy3N@C80 cluster fullerene were analyzed by studying HPLC retention behavior, laser desorption time-of-flight (LD-TOF) mass spectrometry, and UV-Vis-NIR and FTIR spectroscopy. The three isomers of Dy3N@C80 were all large band-gap (1.51, 1.33, and 1.31 eV for 1-3, respectively) materials, and could be classified as very stable fullerenes. According to results of FTIR spectroscopy, the Dy3N@C80 (I) (1) was assigned to the fullerene cage C80:7 (I(h)), whereas Dy3N@C80 (II) (2) had the cage structure of C80:6 (D(5h)). The most probable cage structure of Dy3N@C80 (III) (3) was proposed to be C80:1 (D(5d)). The significant differences between Dy3N@C80 and other reported M3N@C80 (M = Sc, Y, Gd, Tb, Ho, Er, Tm) cluster fullerenes are discussed in detail, and the strong influence of the metal on the nitride cluster fullerene formation is concluded.     

Effect of cage size on the third‐order optical properties of endohedral metallofullerenes Sc3N@C2n (2n = 68, 70, 78, and 80): A theoretical study

Geometrical structures of the investigated endohedral metallofullerenes Sc₃N@C_2n (2n = 68, 70, 78, and 80) were optimized at the B3LYP/6‐31G* level. The analyses of electronic structures display that the contribution of fullerene cage to the lowest unoccupied molecular orbital decreases as the cage size increases. Based on the optimized structures, the time‐dependent density functional theory combined with the sum‐over‐states method was used to investigate their nonlinear optical properties. Calculated third‐order polarizabilities γ and two‐photon absorption (TPA) cross‐section δ do not present the monotone variation with the size of fullerene cage, with largest γ of 0.48 × 10^?34 esu for Sc₃N@C₇₈ in static state, and largest δ of 12.374 GM for Sc₃N@C₇₀ in the wavelength of 902.5 nm. However, the obtained TPA resonant peaks shift red with the size of fullerene cage. By analyzing the electronic origin of the third‐order optical properties, it is found that the charge transfers from the fullerene cage to the encapsulated Sc₃N cluster make important contributions to the studied properties. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Regiospecific Coordination of Re3 Clusters with the Sumanene‐Type Hexagons on Endohedral Metallofullerenes and Higher Fullerenes That Provides an Efficient Separation Method

The reactions of [(μ‐H)₃Re₃(CO)₁₁(NCMe)] with Sc₂@C₈₂‐C_3v(8), Sc₂C₂@C₈₀‐C_2v(5), Sc₂O@C₈₂‐C_s(6), C₈₆‐C₂(17), and C₈₆‐C_s(16) have been carried out to produce face‐capping cluster complexes. The Re₃ triangles are found to bind to the sumanene‐type hexagons on the fullerene surface regiospecifically. In contrast, Sc₃N@C₇₈‐D_3h(5) and Sc₃N@C₈₀‐I_h show no reactivity toward [(μ‐H)₃Re₃(CO)₁₁(NCMe)], probably due to electronic and steric factors. These complexes can be easily purified by using HPLC. Carbonylation of each complex releases the corresponding higher fullerene or endohedral metallofullerene in pure form. Remarkably, the C₈₆‐C₂(17) and C₈₆‐C_s(16) isomers were successively separated through Re₃ cluster complexation/decomplexation. This unique bonding feature may provide an attractive general strategy to purify as yet unresolved fullerene mixtures.     

Theoretical study on the one‐, two‐, and three‐photon absorption properties of exohedral functionalized derivative of Sc3N@C80

Geometrical structures of three investigated molecules Sc₃N@C₈₀, Sc₃N@C₈₀‐Fc, and C₆₀‐Fc were optimized by density functional theory (DFT) at the B3LYP/6‐31G* level. Then the time‐dependent DFT was employed to investigate the excited states of these molecules. After exohedral functionalization by ferrocene (Fc‐) group as the electron donor or replacing C₆₀ with Sc₃N@C₈₀ as the electron acceptor, the wavelengths of the first one‐photon absorption peak and the strongest two‐photon absorption (2PA) and three‐photon absorption (3PA) peaks shift red. The corresponding cross sections of Sc₃N@C₈₀‐Fc in the 2PA and 3PA processes increase as compared with those of Sc₃N@C₈₀, which originate from the contributions of charge transfers from Fc‐ group to C₈₀ cage and simultaneously the transfers from the C₈₀ cage to the encapsulated Sc₃N cluster. When compared with C₆₀‐Fc, the 2PA and 3PA cross sections of Sc₃N@C₈₀‐Fc decrease, which may result from the more negative charge surface of C₈₀ cage in Sc₃N@C₈₀‐Fc molecule which blocks the charge transfers from Fc‐ moiety to the C₈₀ cage in the excitation processes by compared with C₆₀‐Fc. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

2‐Aminoethanol Extraction as a Method for Purifying Sc3N@C80 and for Differentiating Classes of Endohedral Fullerenes on the Basis of Reactivity

Extraction with 2‐aminoethanol is an inexpensive method for removing empty cage fullerenes from the soluble extract from electric‐arc‐generated fullerene soot that contains endohedral metallofullerenes of the type Sc₃N@C_2n (n=34, 39, 40). Our method of separation exploits the fact that C₆₀, C₇₀, and other larger, empty cage fullerenes are more susceptible to nucleophilic attack than endohedral fullerenes and that these adducts can be readily extracted into 2‐aminoethanol. This methodology has also been employed to examine the reactivity of the mixture of soluble endohedral fullerenes that result from doping graphite rods used in the Krätschmer–Huffman electric‐arc generator with the oxides of Y, Lu, Dy, Tb, and Gd. For example, with Y₂O₃, we were able to detect by mass spectrometry several new families of endohedral fullerenes, namely Y₃C₁₀₈ to Y₃C₁₂₆, Y₃C₁₀₇ to Y₃C₁₂₅, Y₄C₁₂₈ to Y₄C₁₄₆, that resisted reactivity with 2‐aminoethanol more than the empty cage fullerenes and the mono‐ and dimetallo fullerenes. The discovery of the family Y₃C₁₀₇ to Y₃C₁₂₅ with odd numbers of carbon atoms is remarkable, since fullerene cages must involve even numbers of carbon atoms. The newly discovered families of endohedral fullerenes with the composition M₄C_2n (M=Y, Lu, Dy, Tb, and Gd) are unusually resistant to reaction with 2‐aminoethanol. Additionally, the individual endohedrals, Y₃C₁₁₂ and M₃C₁₀₂ (M=Lu, Dy, Tb and Gd), were remarkably less reactive toward 2‐aminoethanol.     

Bingel–Hirsch Addition on Endohedral Metallofullerenes: Kinetic Versus Thermodynamic Control

An extensive theoretical study of the Bingel–Hirsch addition of bromomalonate on scandium nitride endohedral fullerenes has been carried out. The prototypical and highly symmetrical Sc₃N@I_h‐C₈₀, with a structure that satisfies the isolated pentagon rule (IPR), and the non‐IPR Sc₃N@D₃(6140)‐C₆₈ fullerene show analogous reaction paths despite the distinct topology of the carbon networks and different rotation freedom of the internal nitride cluster. For the two metallofullerenes, our results predict that the reaction takes place under kinetic control yielding open‐cage fulleroids on [6,6] bonds, which is in good agreement with experimental data. The theoretical studies also show that predicting the reactivity of endohedral metallofullerenes is not straightforward and often an accurate analysis of the potential energy surface is required.     

C68 Fullerene Isomers,Anions, and their Metallofullerenes: Charge‐Stabilizing Different Isomers

The complete set of 6332 classical isomers of the fullerene C₆₈ as well as several non‐classical isomers is investigated by PM3, and the data for some of the more stable isomers are refined by the DFT‐based methods HCTH and B3LYP. C₂:0112 possesses the lowest energy of all the neutral isomers and it prevails in a wide range of temperatures. Among the fullerene ions modeled, C₆₈^2?, C₆₈^4? and C₆₈^6?, the isomers C₆₈^2?(C_s:0064), C₆₈^4?(C_2v:0008), and C₆₈^6?(D₃:0009) respectively, are predicted to be the most stable. This reveals that the pentagon adjacency penalty rule (PAPR) does not necessarily apply to the charged fullerene cages. The vertical electron affinities of the neutral C_s:0064, C_2v:0008, and D₃:0009 isomers are 3.41, 3.29, and 3.10 eV, respectively, suggesting that they are good electron acceptors. The predicted complexation energy, that is, the adiabatic binding energy between the cage and encapsulated cluster, of Sc₂C₂@C₆₈(C_2v:0008) is ?6.95 eV, thus greatly releasing the strain of its parent fullerene (C_2v:0008). Essentially, C₆₈ fullerene isomers are charge‐stabilized. Thus, inducing charge facilitates the isolation of the different isomers. Further investigations show that the steric effect of the encaged cluster should also be an important factor to stabilize the C₆₈ fullerenes effectively.     

Substrate‐Independent Magnetic Bistability in Monolayers of the Single‐Molecule Magnet Dy2ScN@C80 on Metals and Insulators

Magnetic hysteresis is demonstrated for monolayers of the single‐molecule magnet (SMM) Dy₂ScN@C₈₀ deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy₂ScN@C₈₀ adsorbed on Au(111) were studied by STM. X‐ray magnetic CD studies show that the Dy₂ScN@C₈₀ monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy₂ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene‐surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene‐SMMs on conducting substrates and facilitates their application in molecular spintronics.     

Size effect of fullerene cages and encaged clusters in M<Subscript>3</Subscript>N@C<Subscript>2<Emphasis Type="Italic">n</Emphasis></Subscript>

Based on the calculated findings that the sizes of encaged clusters determine the structures and the stability of C₈₀-based trimetallic nitride fullerenes (TNFs), more extensive density functional theory calculations were performed on M₃N@C₆₈, M₃N@C₇₈ and M₃N@C₈₀ (M=Sc, Y and La). The calculated results demonstrated that the structures and stability undergo a transition with the increasing of the sizes of the cages and clusters. Sc₃N is planar inside the three considered cages, Y₃N is slightly pyramidal inside C₆₈-6140 and C₇₈-5 and planar inside I_h C₈₀-7, however, La₃N is pyramidal inside all the three cages. Those cages with pyramidal clusters inside deformed considerably, compared with their parent cages. In these cases, the bonding of metallic atoms toward the cages does not play an important role, and the encaged cluster tends to be located inside the cages with the largest M-M and M-C distances so that the strain energy can be released mostly. These calculations revealed the size effect of fullerene cages and encaged clusters, and can explain the position priority of M3N inside fullerene cages and the differences in yield of M₃N@C_2n . Supported by the Southwest University, China (Grant No. SWNUB2005002)     

Electronic Structure and Redox Properties of Metal Nitride Endohedral Fullerenes M3N@C2n (M=Sc,Y, La,and Gd; 2n=80, 84, 88, 92, 96)

An extensive study of the redox properties of metal nitride endohedral fullerenes (MNEFs) based on DFT computational calculations has been performed. The electronic structure of the singly oxidized and reduced MNEFs has been thoroughly analyzed and the first anodic and cathodic potentials, as well as the electrochemical gaps, have been predicted for a large number of M₃N@C_2n systems (M=Sc, Y, La, and Gd; 2n=80, 84, 88, 92, and 96). In particular, calculations that include thermal and entropic effects correctly predict the different anodic behavior of the two isomers (I_h and D_5h) of Sc₃N@C₈₀, which is the basis for their electrochemical separation. Important differences were found in the electronic structure of reduced M₃N@C₈₀ when M=Sc or when M is a more electropositive metal, such as Y or Gd. Moreover, the changes in the electrochemical gaps within the Gd₃N@C_2n series (2n=80, 84, and 88) have been rationalized and the use of Y‐based computational models to study the Gd‐based systems has been justified. The redox properties of the largest MNEFs characterized so far, La₃N@C_2n (2n=92 and 96), were also correctly predicted. Finally, the quality of these predictions and their usefulness in distinguishing the carbon cages for MNEFs with unknown structures is discussed.