Harnessing Electron Transfer from the Perchlorotriphenylmethide Anion to Y@C82(C2v) to Engineer an Endometallofullerene‐Based Salt

We show that electron transfer from the perchlorotriphenylmethide anion (PTM^?) to Y@C₈₂(C_2v) is an instantaneous process, suggesting potential applications for using PTM^? to perform redox titrations of numerous endohedral metallofullerenes. The first representative of a Y@C₈₂‐based salt containing the complex cation was prepared by treating Y@C₈₂(C_2v) with the [K⁺([18]crown‐6)]PTM^? salt. The synthesis developed involves the use of the [K⁺([18]crown‐6)]PTM^? salt as a provider of both a complex cation and an electron‐donating anion that is able to reduce Y@C₈₂(C_2v). For the first time, the molar absorption coefficients for neutral and anionic forms of the pure isomer of Y@C₈₂(C_2v) were determined in organic solvents with significantly different polarities.     

The Unanticipated Dimerization of Ce@C2v(9)‐C82 upon Co‐crystallization with Ni(octaethylporphyrin) and Comparison with Monomeric M@C2v(9)‐C82 (M = La,Sc, and Y)

We report that Ce@C_2v(9)‐C₈₂ forms a centrosymmetric dimer when co‐crystallized with Ni(OEP) (OEP = octaethylporphyrin dianion). The crystal structure of {Ce@C_2v(9)‐C₈₂}₂?2[Ni(OEP)]?4 C₆H₆ shows that a new C?C bond with a bond length of 1.605(5) Å connects the two cages. The high spin density of the singly occupied molecular orbital (SOMO) on the cage and the pyramidalization of the cage are factors that favor dimerization. In contrast, the treatment of Ni(OEP) with M@C_2v(9)‐C₈₂ (M = La, Sc, and Y) results in crystallization of monomeric endohedral fullerenes. A systematic comparison of the X‐ray structures of M@C_2v(9)‐C₈₂ (M = Sc, Y, La, Ce, Gd, Yb, and Sm) reveals that the major metal site in each case is located at an off‐center position adjacent to a hexagonal ring along the C₂ axis of the C_2v(9)‐C₈₂ cage. DFT calculations at the M06‐2X level revealed that the positions of the metal centers in these metallofullerenes M@C_2v(9)‐C₈₂ (M = Sc, Y, and Ce), as determined by single‐crystal X‐ray structure studies, correspond to an energy minimum for each compound.     

Regioselective Synthesis,Crystallographic Characterization,and Electrochemical Properties of Pyrazole- and Pyrrole-Ring-Fused Derivatives of Y2@C3v(8)-C82

Chemical modification of endohedral metallofullerenes (EMFs) is an efficient strategy to realize their ultimate applications in many fields. Herein, we report the highly regioselective and quantitative mono-formation of pyrazole- and pyrrole-ring-fused derivatives of the prototypical di-EMF Y₂@C_3v(8)-C₈₂, that is, Y₂@C_3v(8)-C₈₂(C₁₃N₂H₁₀) and Y₂@C_3v(8)-C₈₂(C₉NH₁₁), from the respective 1,3-dipolar reactions with either diphenylnitrilimine or N-benzylazomethine ylide, without the formation of any bis- or multi-adducts. Crystallographic results unambiguously reveal that only one [6,6]-bond out of the twenty-five different types of nonequivalent C−C bonds of Y₂@C_3v(8)-C₈₂ is involved in the 1,3-dipolar reactions. Our theoretical results rationalize that the remarkably high regioselectivity and the quantitative formation of mono-adducts are direct results from the anisotropic distribution of π-electron density on the C_3v(8)-C₈₂ cage and the local strain of the cage carbon atoms as well. Interestingly, electrochemical and theoretical studies demonstrate that the reversibility of the redox processes, in particular the reversibility of the reductive processes of Y₂@C_3v(8)-C₈₂, has been markedly altered upon exohedral functionalization, but the oxidative process was less influenced, indicating that the oxidation is mainly influenced by the internal Y₂ cluster, whereas the reduction is primarily associated with the fullerene cage. The pyrazole and pyrrole-fused derivatives may find potential applications as organic photovoltaic materials and biological reagents.     

Synthesis and characterization of carbene derivatives of Th@C3v(8)-C82 and U@C2v(9)-C82: exceptional chemical properties induced by strong actinide–carbon cage interaction

Chemical functionalization of endohedral metallofullerenes (EMFs) is essential for the application of these novel carbon materials. Actinide EMFs, a new EMF family member, have presented unique molecular and electronic structures but their chemical properties remain unexplored. Here, for the first time, we report the chemical functionalization of actinide EMFs, in which the photochemical reaction of Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ with 2-adamantane-2,3′-[3H]-diazirine (AdN₂, 1) was systematically investigated. The combined HPLC and MALDI-TOF analyses show that carbene addition by photochemical reaction afforded three isomers of Th@C_3v(8)-C₈₂Ad and four isomers of U@C_2v(9)-C₈₂Ad (Ad = adamantylidene), presenting notably higher reactivity than their lanthanide analogs. Among these novel EMF derivatives, Th@C_3v(8)-C₈₂Ad(I, II, III) and U@C_2v(9)-C₈₂Ad(I, II, III) were successfully isolated and were characterized by UV-vis-NIR spectroscopy. In particular, the molecular structures of first actinide fullerene derivatives, Th@C_3v(8)-C₈₂Ad(I) and U@C_2v(9)-C₈₂Ad(I), were unambiguously determined by single crystal X-ray crystallography, both of which show a [6,6]-open cage structure. In addition, isomerization of Th@C_3v(8)-C₈₂Ad(II), Th@C_3v(8)-C₈₂Ad(III), U@C_2v(9)-C₈₂Ad(II) and U@C_2v(9)-C₈₂Ad(III) was observed at room temperature. Computational studies suggest that the attached carbon atoms on the cages of both Th@C_3v(8)-C₈₂Ad(I) and U@C_2v(9)-C₈₂Ad(I) have the largest negative charges, thus facilitating the electrophilic attack. Furthermore, it reveals that, compared to their lanthanide analogs, Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ have much closer metal–cage distance, increased metal-to-cage charge transfer, and strong metal–cage interactions stemming from the significant contribution of extended Th-5f and U-5f orbitals to the occupied molecular orbitals, all of which give rise to their unusual high reactivity. This study provides first insights into the exceptional chemical properties of actinide endohedral fullerenes, which pave ways for the future functionalization and application of these novel EMF compounds.

Photochemical reaction of Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ with 2-adamantane-2,3′-[3H]-diazirine (AdN₂, 1) afforded three isomers of Th@C_3v(8)-C₈₂Ad and four isomers of U@C_2v(9)-C₈₂Ad (Ad = adamantylidene), respectively.     

In‐Depth Understanding of the Chemical Properties of Rarely Explored Carbide Cluster Metallofullerenes: A Case Study of Sc2C2@C3v(8)‐C82 that Reveals a General Rule

The chemical properties of carbide‐cluster metallofullerenes (CCMFs) remain largely unexplored, although several new members of CCMFs have been discovered recently. Herein, we report the reaction between Sc₂C₂@C_3v(8)‐C₈₂, which is viewed as a prototypical CCMF because of its high abundance, and 3‐triphenylmethyl‐5‐oxazolidinone ( 1 ) to afford the corresponding pyrrolidino derivative Sc₂C₂@C_3v(8)‐C₈₂(CH₂)₂NTrt ( 2 ; Trt=triphenylmethyl). Single‐crystal X‐ray crystallography studies of 2 revealed that the reaction takes place at a [6,6]‐bond junction, which is directly over the encapsulated C₂ unit and is far from either of the two scandium atoms. On the basis of theoretical calculations and by considering previously reports, we have found that a hexagonal carbon ring on the cage of Sc₂C₂@C_3v(8)‐C₈₂ is highly reactive toward different reagents due to the overlap of high p‐orbital axis vector (POAV) angles and large LUMO coefficients. We propose that this highly concentrated area of reactivity is generated by the encapsulation of the Sc₂C₂ cluster because this region is absent from the empty fullerene C_3v(8)‐C₈₂. Moreover, the absorption and electrochemical results confirm that derivative 2 is more stable than pristine Sc₂C₂@C_3v(8)‐C₈₂, thus illuminating its potential applications.     

Theoretical and Quantitative Structural Relationship Studies of Electrochemical Properties of the Nanostructures of Cis-Unsaturated Thiocrown Ethers and Their Supramolecular Complexes [X-UT-Y][M@C82] (M˭Ce,Gd)

The endohedral lanthanidofullerenes, an important type of organolanthanides, are stabilized by the delocalization of the negative charges on the cages of fullerenes. Since the discovery of these classes of carbon compounds and their unusual structures and properties of these molecules, many potential applications have been suggested. Unsaturated thiocrown ethers with cis-geometry are a group of crown ethers that, in light of the size of their cavities and their conformational restriction compared to a corresponding saturated system (1–9), demonstrate interesting properties for physicochemical studies. Endohedral lanthanidofullerenes M@C_x (x = 82 and M = Ce, Gd) were introduced as a new class of the spherical fullerene group with unique properties. Formation of endohedral metallofullerenes is thought to involve the transfer of electrons from the encapsulated metal atom(s) to the surrounding fullerene cage. Two of these molecules are the Ce@C₈₂ (10) and Gd@C₈₂ (11). The supramolecular complexes of 1–9 with Ce@C₈₂ (10) and Gd@C₈₂ (11) have been shown to possess a host–guest interaction for electron transfer processes, and these behaviors have previously been reported. Topological indices have been successfully used to construct effective and useful mathematical methods for finding good relationships between structural data and the various chemical and physical properties. To establish a good structural relationship between the structures of 1–9 and M@C_x that were introduced here, an index that is represented as μ_cs was utilized. This index is the ratio of summation of the number of carbon atoms (n_c ) and the number of sulfur atoms (n_s ) to the product of these two numbers for 1–9. In this study, the relationship between this index and oxidation potential ( ^oxE₁ ) of 1–9, as well as the free energy of electron transfer (ΔG_et , by the Rehm-Weller equation) between 1–9 and 10 and 11 as [X-UT-Y][Ce@C₈₂] (12) and [X-UT-Y][Gd@C₈₂] (13) complexes, is presented.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.     

Determination of molar extinction coefficients for endohedral metallofullerene Dy@C<Subscript>82</Subscript>(C<Subscript>2v</Subscript>)

Isomerically pure endohedral metallofullerene Dy@C₈₂(C _2v) was synthesized by the electric arc method, extracted from the soot with o-dichlorobenzene, isolated from the extract by HPLC, and characterized by mass spectrometry and spectrophotometry. The spectrophotometric titration of a solution of endohedral metallofullerene Dy@C₈₂(C _2v) was conducted with potassium perchlorotriphenylmethide. The concentration of Dy@C82(C _2v) in o-dichlorobenzene was determined, and the molar absorption coefficients for its neutral and anionic forms were calculated (3.0?10³ (at 927 nm) and 4.0?10³ mol^–1 L cm^–1 (at 884 nm), respectively.     

Endohedral gadolinium-containing metallofullerenes in the trifluoromethylation reaction

An efficient method for the synthesis of trifluoromethyl derivatives of endohedral gadolinium-containing metallofullerenes was proposed. High-purity (98–99%) trifluoromethyl derivatives Gd@C₈₂(CF₃)₅ (two isomers) and Gd₂@C₈₀(CF₃) have been synthesized for the first time. They were isolated and characterized by HPLC, MALDI-TOF mass spectrometry, and UV-Vis spectroscopy. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1457–1462, July, 2008.     

Preferential Formation of Mono‐Metallofullerenes Governed by the Encapsulation Energy of the Metal Elements: A Case Study on Eu@C2n (2n=74–84) Revealing a General Rule

Encapsulating one to three metal atoms or a metallic cluster inside fullerene cages affords endohedral metallofullerenes (EMFs) classified as mono‐, di‐, tri‐, and cluster‐EMFs, respectively. Although the coexistence of various EMF species in soot is common for rare‐earth metals, we herein report that europium tends to prefer the formation of mono‐EMFs. Mass spectroscopy reveals that mono‐EMFs (Eu@C_2n) prevail in the Eu‐containing soot. Theoretical calculations demonstrate that the encapsulation energy of the endohedral metal accounts for the selective formation of mono‐EMFs and rationalize similar observations for EMFs containing other metals like Ca, Sr, Ba, or Yb. Consistently, all isolated Eu‐EMFs are mono‐EMFs, including Eu@D_3h(1)‐C₇₄, Eu@C_2v(19138)‐C₇₆, Eu@C_2v(3)‐C₇₈, Eu@C_2v(3)‐C₈₀, and Eu@D_3d(19)‐C₈₄, which are identified by crystallography. Remarkably, Eu@C_2v(19138)‐C₇₆ represents the first Eu‐containing EMF with a cage that violates the isolated‐pentagon‐rule, and Eu@C_2v(3)‐C₇₈ is the first C₇₈‐based EMF stabilized by merely one metal atom.     

Voltammetric Behavior of Water‐Soluble Endohedral Metallofullerene Derivatives

Voltammetric behavior of water‐soluble endohedral metallofullerene derivatives Gd@C₈₂(OH)₅(NHCH₂COOH)₉ (GN) and Gd@C₈₂(OH)₆(NHCH₂CH₂SO₃H)₈ (GS) was characterized in 0.1 M KCl solution by CV and DPV. They showed similar redox behavior, that is, a reversible electroreduction process on HMDE was found; in the mean time, an irreversible oxidation process and an irreversible reduction process on GC electrodes were also observed. The results reveal that these two water‐soluble endohedral metallofullerene derivatives have good electron donating ability and poor electron accepting ability due to hydroxy groups, aminoacetic acid and aminoethyl sulfonic acid connected to the C₈₂ cage in comparison with Gd@C₈₂.     

Study of multihydroxylated processes of Gd@C<Subscript>82</Subscript> by ICP-MASS

The water-solubilization of metallofullerenes is important for their potential applications, but their formation processes are still not clear, and the formation yield is uncontrollable. In this paper, we quantitatively studied the water-solubilizing process of Gd@C₈₂ with hydroxylation reaction using ICP-MASS techniques. For the first time, it was found that the formation yield of the multihydroxylated Gd@C₈₂ is declined quickly with the break up of carbon cage of Gd@C₈₂ in the hydroxylated processes. The observation revealed a way to control the hydroxylation processes and increase the formation yield.     

Isolation and Crystallographic Characterization of the Labile Isomer of Y@C82 Cocrystallized with Ni(OEP): Unprecedented Dimerization of Pristine Metallofullerenes

Although the major isomers of M@C₈₂ (namely M@C_2v(9)‐C₈₂, where M is a trivalent rare‐earth metal) have been intensively investigated, the lability of the minor isomers has meant that they have been little studied. Herein, the first isolation and crystallographic characterization of the minor Y@C₈₂ isomer, unambiguously assigned as Y@C_s(6)‐C₈₂ by cocrystallization with Ni(octaethylporphyrin), is reported. Unexpectedly, a regioselective dimerization is observed in the crystalline state of Y@C_s(6)‐C₈₂. In sharp contrast, no dimerization occurs for the major isomer Y@C_2v(9)‐C₈₂ under the same conditions, indicating a cage‐symmetry‐induced dimerization process. Further experimental and theoretical results disclose that the regioselective dimer formation is a consequence of the localization of high spin density on a special cage‐carbon atom of Y@C_s(6)‐C₈₂ which is caused by the steady displacement of the Y atom inside the C_s(6)‐C₈₂ cage.     

Crystallographic and Theoretical Investigations of Er2@C2 n (2 n=82, 84, 86): Indication of Distance-Dependent Metal–Metal Bonding Nature

Successful isolation and characterization of a series of Er-based dimetallofullerenes present valuable insights into the realm of metal–metal bonding. These species are crystallographically identified as Er₂@C_s(6)-C₈₂, Er₂@C_3v(8)-C₈₂, Er₂@C₁(12)-C₈₄, and Er₂@C_2v(9)-C₈₆, in which the structure of the C₁(12)-C₈₄ cage is unambiguously characterized for the first time by single-crystal X-ray diffraction. Interestingly, natural bond orbital analysis demonstrates that the two Er atoms in Er₂@C_s(6)-C₈₂, Er₂@C_3v(8)-C₈₂, and Er₂@C_2v(9)-C₈₆ form a two-electron-two-center Er−Er bond. However, for Er₂@C₁(12)-C₈₄, with the longest Er⋅⋅⋅Er distance, a one-electron-two-center Er−Er bond may exist. Thus, the difference in the Er⋅⋅⋅Er separation indicates distinct metal bonding natures, suggesting a distance-dependent bonding behavior for the internal dimetallic cluster. Additionally, electrochemical studies suggest that Er₂@C_82–86 are good electron donors instead of electron acceptors. Hence, this finding initiates a connection between metal–metal bonding chemistry and fullerene chemistry.     

Calculated relative yields for Sc2S@C82 and Y2S@C82

The paper reports computations for a newly observed class of the mixed, sulfur-containing X₂S@C₈₂ metallofullerenes, namely for X = Sc and Y, based on encapsulation into the C _s and C_3v ?C₈₂ isolated pentagon rule cages. Their structural, vibrational, and energetic characteristics from the density-functional-theory calculations with the standard 6-31G* and LanL2DZ basis sets are used for the evaluations of the relative production yields. The encapsulation Gibbs energy terms from the partition functions combined with the observed saturated metal pressures are employed. A model scheme is used dealing with the XS template gas-phase presence. The computations predict that Sc₂S@C₈₂ should be produced in considerably larger amounts than Y₂S@C₈₂ which is in agreement with available observations. The yield order originates in the fact that both energetics and saturated metal pressure favor Sc over Y.     

Reaction Mechanism and Regioselectivity of the Bingel–Hirsch Addition of Dimethyl Bromomalonate to La@C2v‐C82

We quantum chemically explore the thermodynamics and kinetics of all 65 possible mechanistic pathways of the Bingel–Hirsch addition of dimethyl bromomalonate to the endohedral metallofullerene La@C_2v‐C₈₂ that result from the combination of 24 nonequivalent carbon atoms and 35 different bonds present in La@C_2v‐C₈₂ by using dispersion‐corrected DFT calculations. Experimentally, this reaction leads to four singly bonded derivatives and one fulleroid adduct. Of these five products, only the singly bonded derivative on C23 could be experimentally identified unambiguously. Our calculations show that La@C_2v‐C₈₂ is not particularly regioselective under Bingel–Hirsch conditions. From the obtained results, however, it is possible to make a tentative assignment of the products observed experimentally. We propose that the observed fulleroid adduct results from the attack at bond 19 and that the singly bonded derivatives correspond to the C2, C19, C21, and C23 initial attacks. However, other possibilities cannot be ruled out completely.     

C74(BN)2的UV和IR 光谱研究

Equilibrium geometries of 16 possible isomers for C74（BN）2 were studied by INDO series of methods, to indicate that the most stable three geometries are those where boron and nitrogen atoms substitute carbon atoms located at the same hexagon near the longest axis of C78 （C2v） to form B-N-B-N unit. Electronic spectra of C74（BN）2 were investigated with INDO/CIS method. The reason for the red shift of UV absorptions for C74（BN）2 compared with those of C78 （C2v） was discussed. IR spectra for 9,8,28,29-C74（BN）2 and 28,29,30,31-C74（BN）2 were calculated on the basis of AM1 geometries.     

Theoretical study on the smallest endohedral metallofullerenes: TM@C20 (TM = Ce and Gd)

The structure, electronic property, and infrared spectroscopy of endohedral metallofullerenes TM@C₂₀ (TM = Ce and Gd) have been systematically investigated with the aid of the hybrid DFT‐B3LYP functional. It is found that in the endohedral metallofullerenes the average C? C bond lengths are obvious longer than those of empty cage. The frontier orbital analyses show that the endohedral metallofullerene Gd@C₂₀ has the high‐thermodynamic stability. Natural population analysis also tells us that only in the Ce@C₂₀, the Ce atom acts as an electron acceptor with the negative charges, and the 4f orbitals of Ce and Gd atoms have a significant contribution in the formation of chemical bonding. Additionally, the analyses of harmonic vibrational frequencies reveal that when the TM atoms are encapsulated into the C₂₀ cage, the strongest absorption peaks are characterized by a mixture of TM?C bending and C? C stretching vibrations. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Facile Separation and Crystal Structure Determination of C2‐C82(3) Fullerene

A report of the facile separation of C₂‐C₈₂(3) fullerene is given, which has resulted in the isolation of isomerically pure C₈₂(3) on a preparative scale, avoiding time consuming recycling procedures. By single‐crystal X‐ray analysis of C₈₂(3)·Ni(OEP) (Nickel octaethylporphyrine) the first crystal structure containing pristine C₈₂(3) has been revealed.     

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.     

Synchrotron Radiation for Structural Chemistry—Endohedral Natures of Metallofullerenes Found by Synchrotron Radiation Powder Method

The recent progress of the structural studies of endohedral metallofullerenes by the synchrotron radiation (SR) powder diffraction utilizing the maximum entropy method (MEM) is reviewed. Results of the endohedral metallofullerenes (Y@C₈₂, La@C₈₂, Sc@C₈₂, Sc₂@C₈₄, Sc₃@C₈₂, Sc₂@C₆₆, La₂@C₈₀ and Sc₂C₂@C₈₄) are given. The precise MEM charge densities of metallofullerenes presents the direct image of endohedral nature of metallofullerenes indicating the charge transfer from metal atoms to carbon cage, which governs the stability of the unique endohedral structures. The MEM/Rietveld method and SR powder method using imaging plate (IP), which are the crucial methods for data analysis and measurement in order to determine structure of fulleride, are also mentioned in some detail.