Synthesis and Characterization of Exohedrally Silylated M@C82 (M = Y and La)

Synthesis and Structural Studies of Two Paramagnetic Metallofullerenes with Isomeric C72 Cage

We synthesized and isolated two paramagnetic metallofullerenes of La@C₇₂ and Y@C₇₂ with different fullerene cages, which were characterized by electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. DFT calculations disclosed two possible isomers of La/Y@C₇₂ with C₇₂- C₂ and C₇₂- C_2v cages, both of which have similar thermodynamic stability and one pair of fused pentagons. Their paramagnetic properties were then studied by EPR spectroscopy, and the obtained EPR signals were analyzed with very different hyperfine coupling constants, revealing distinct electron spin distributions for these two species. Furthermore, the experimental coupling constants were compared with those of calculated coupling constants, and comparison results revealed that the produced La@C₇₂ has a C₇₂- C_2v cage and Y@C₇₂ has a C₇₂- C₂ cage. These studies illustrate that the electron spin can be used as a probe to identify metallofullerene structure due to the susceptibility of spin-metal couplings. The successful isolation and characterizations of La@C₇₂ and Y@C₇₂ with such a small C₇₂ cage reveal their stability that is important for application as paramagnetic molecule materials.

Functionalization of Endohedral Metallofullerenes with Reactive Silicon and Germanium Compounds

Exohedral derivatization of endohedral metallofullerenes (EMFs) has been exploited as a useful method for characterizing the structural and chemical properties of EMFs, and for functionalizing them for potential applications. The introduction of heteroatoms, such as electropositive silicon atoms, to fullerene cages is a novel functionalization method that remarkably affects the electronic characteristics of fullerenes. This review comprehensively describes the results of the reactions of monometallofullerene, dimetallofullerene, and trimetallic nitride template EMFs with disilirane, silirane, silylene, and digermirane, which afforded the corresponding silylated and germylated fullerenes. Several examples emphasize that exohedral functionalization regulates the dynamic behaviors of the encapsulated metal atoms and clusters in the fullerene cages. The electronic effects of silyl and germyl groups are represented by comparing the redox properties of silylated and germylated EMFs with those of other EMFs derivatized with carbon-atom-based functional groups.

Carbide cluster metallofullerenes: structure, properties, and possible origin

Endohedral metallofullerenes (EMFs) are hybrid molecules with different metallic species encapsulated inside the fullerene cages. In addition to conventional EMFs that contain only metal ions, researchers have constructed novel compounds that encapsulate metallic clusters of nitride, carbide, oxide, cyanide, and sulfide. Among these structures, carbide cluster metallofullerenes (CCMFs) are unique because their synthesis requires only graphite and the metal source. As a result the molecular structures of CCMFs are particularly difficult to characterize. Two carbon atoms are encapsulated inside the cage, but they do not participate in constructing the cage framework. Recent X-ray crystallographic studies of EMFs have allowed researchers to unambiguously identify CCMFs (MxC₂@C2n). Previously most of these structures had been described as conventional EMFs Mx@C2n+2. Most of these species are scandium-containing compounds such as Sc3C₂@Ih(7)-C₈₀ [not Sc₃@C3v(7)-C₈₂], Sc₂C₂@C2v(5)-C₈₀ [not Sc₂@C₈₂], Sc₂C₂@Cs(6)-C₈₂ [not Sc₂@Cs(10)-C₈₄], Sc₂C₂@C2v(9)-C₈₂ [not Sc₂@C2v(17)-C₈₄], Sc₂C₂@C3v(8)-C₈₂ [not Sc₂@D2d(23)-C₈₄], and Sc₂C₂@D2d(23)-C₈₄ [not Sc₂@C₈₆]. Additional examples of CCMFs include Gd₂C₂@D₃(85)-C₉₂, Sc₂C₂@C2v(6073)-C₆₈, Ti₂C₂@D3h(5)-C₇₈, M₂C₂@C3v(8)-C₈₂, M₂C₂@Cs(6)-C₈₂ (M = Y, Er, etc.), Y₂C₂@C₈₄, Y₂C₂@D₃(85)-C₉₂, Y₂C₂@D₅(450)-C₁₀₀, and Lu₃C₂@D₂(35)-C₈₈. The existence of so many CCMF species reminds us that the symbol '@' (which denotes the encapsulation status of EMFs) should be used with caution with species whose molecular structures have not been determined unambiguously. This Account presents a detailed summary of all aspects of CCMFs, including historically erroneous assignments and corrected structural characterizations, along with their intrinsic properties such as electrochemical and chemical properties. We emphasize structural issues, features that are fundamental for understanding their intrinsic properties. Finally, we discuss the formation mechanism and possible origin of cluster EMFs, not just CCMFs.

Current status and future developments of endohedral metallofullerenes

Endohedral metallofullerenes (EMFs), a new class of hybrid molecules formed by encapsulation of metallic species inside fullerene cages, exhibit unique properties that differ distinctly from those of empty fullerenes because of the presence of metals and their hybridization effects via electron transfer. This critical review provides a balanced but not an exhaustive summary regarding almost all aspects of EMFs, including the history, the classification, current progress in the synthesis, extraction, isolation, and characterization of EMFs, as well as their physiochemical properties and applications in fields such as electronics, photovoltaics, biomedicine, and materials science. Emphasis is assigned to experimentally obtained results, especially the X-ray crystallographic characterizations of EMFs and their derivatives, rather than theoretical calculations, although the latter has indeed enhanced our knowledge of metal-cage interactions. Finally, perspectives related to future developments and challenges in the research of EMFs are proposed. (381 references).

Chemistry of endohedral metallofullerenes: the role of metals

Recent breakthroughs achieved in the chemical functionalization of endohedral metallofullerenes (EMFs), especially single crystallographic X-ray characterizations of their derivatives, have presented fundamentally new insights into the structures and properties of these metal-carbon hybrid molecules, and have also brought immense potential applications. In particular, the interplay between the encapsulated metallic species and the fullerene cage has been well investigated. On one hand, the position and motion of the encapsulated metals can be effectively controlled by exohedral modification. On the other hand, the cage structures, the chemical behaviours of cage carbons and thus the chemical reactivity of the whole molecule are also apparently influenced by the electronic configuration and geometrical conformation of the internal metals via strong metal-cage interactions. In this article, we contribute a systematic review of the important chemical transformations of EMFs reported to date, including disilylation, 1,3-dipolar cycloaddition with ylides, cyclopropanation with carbenes and carbanions, cycloaddition with dienes and benzyne, radical reactions, and other miscellaneous reactions, in addition to noncovalent interactions such as supramolecular complexation. The roles that internal metals play in controlling the reactivity of cage carbons are particularly emphasized. Finally, some applicable materials based on EMFs and their derivatives are summarized and practical perspectives are proposed.

Current progress on the chemical functionalization and supramolecular chemistry of M@C82

Since the first discovery of fullerenes in 1985, the insertion of one or more atoms into a hollow fullerene cage has been attempted. Furthermore, synthesis and extraction of metallofullerene, La@C(n), were reported in 1991. Recent successful isolation and purification of metallofullerenes have facilitated the investigation of their chemical properties. This mini-review presents a summary of the recent progress of chemical functionalization and supramolecular chemistry of M@C(82). Selective functionalization and successful structural analysis of derivatives have revealed their chemical features arising from endohedral metal doping.

Regioselective exohedral functionalization of La@C82 and its 1,2,3,4,5-pentamethylcyclopentadiene and adamantylidene adducts

The first regioselective functionalization of La@C(82) by two different groups has been performed. Bis-adducts of La@C(82) with Cp* and adamantylidene were synthesized by using two different routes and characterized. Spectroscopic analysis and theoretical calculations reveal that the addition position is controlled by the charge density and p-orbital axis vector value of the fullerene cage.

Endohedral metal atoms in pristine and functionalized fullerene cages

Fullerene, an allotropic form of carbon made up of spherical molecules formed from pentagonal and hexagonal rings, was first discovered in 1985. Because fullerenes have spacious inner cavities, atoms and clusters can be encapsulated inside the fullerene cages to form endohedral fullerenes. In particular, the unique structural and electronic properties of endohedral metallofullerenes (EMFs), where metal atoms are encapsulated within the fullerene, have attracted wide interest from physicists and chemists as well as materials scientists and biologists. The remarkable characteristics of these molecules originate in the electron transfer from the encapsulated metal atoms to the carbon cage. The positions and movements of the encapsulated metal atoms are important determinants of the chemical and physical properties of EMFs. In this Account, we specifically describe the positions and dynamic behavior of the metal atoms encapsulated in pristine and functionalized fullerene cages. First, we examined whether the metal atoms are attached rigidly to cage carbons or move around. Our systematic investigations of EMFs, including M@C(2v)-C(82), M(2)@D(2)(10611)-C(72), M(2)@D(3h)(5)-C(78), M(2)@I(h)-C(80), and M(2)@D(5h)-C(80), revealed that the metal positions and movements vary widely with different cage structures and numbers of metal atoms. Second, we wanted to understand whether we could control the positions and movements of the untouchable metal atoms in EMFs. One possible way to modulate this behavior was through attachment of a molecule to the outer surface of the cage. We developed synthetic methods to modify EMFs and have examined the metal positions and movements in the functionalized carbon cages. Remarkably, we could alter the dynamic behavior of the encaged metal atoms in M(2)@I(h)-C(80) drastically through chemical modification of the outer cage. We anticipate that the control of metal atom structures and dynamics within a cage could be valuable for designing functional molecular devices with new electronic or magnetic properties.

Chemical, electrochemical, and structural properties of endohedral metallofullerenes

Ever since the first experimental evidence of the existence of endohedral metallofullerenes (EMFs) was obtained, the search for carbon cages with encapsulated metals and small molecules has become a very active field of research. EMFs exhibit unique electronic and structural features, with potential applications in many fields. Furthermore, functionalized EMFs offer additional potential applications because of their higher solubility and their ease of characterization by X-ray crystallography and other techniques. Herein we review the general field of EMFs, particularly of functionalized EMFs. We also address their structures and their (electrochemical) properties, as well as applications of these fascinating compounds.

Radical coupling reaction of paramagnetic endohedral metallofullerene La@C82

The thermal reaction of La@C(82)(C(2v)) with 3-triphenylmethyl-5-oxazolidinone (1) in toluene affords benzyl monoadducts La@C(82)(C(2v))(CH(2)C(6)H(5)) (2a-2d). The same monoadducts are also obtained by the photoirradiation of La@C(82)(C(2v)) in toluene without the existence of 1. These reactions are applicable to paramagnetic metallofullerenes, such as La@C(82)(C(s)) and Ce@C(82)(C(2v)). The photoirradiation of La@C(82)(C(2v)) in 1,2-dichlorobenzene in the presence of alpha,alpha,2,4-tetrachlorotoluene also affords the monoadducts La@C(82)(C(2v))(CHClC(6)H(3)Cl(2)) (3a-3d). The monoadducts are fully characterized by spectroscopic analyses. Single-crystal X-ray structure analysis for 3d reveals the unique structure. Theoretical calculations show that the cage carbons having high spin densities are selectively attacked by radical species to form the monoadducts linked by a carbon-carbon single bond. The thermal reaction of La@C(82)(C(2v)) with 1 in benzene affords metallofulleropyrrolidine La@C(82)(C(2v))(C(2)H(4)NCPh(3)) (5), unlike the reaction in toluene.

Chemical understanding of a non-IPR metallofullerene: stabilization of encaged metals on fused-pentagon bonds in La2@C72

Fullerenes violating the isolated pentagon rule (IPR) are only obtained in the form of their derivatives. Since the [5,5]-bond carbons are highly reactive, they are easily attacked by reagents to release the bond strains. Non-IPR endohedral metallofullerenes, however, still have unsaturated sp (2) carbons at the [5,5] bond junctions, which allow their chemical properties to be probed. In this work, La 2@C 72 was chosen as a representative non-IPR metallofullerene, since it has been experimentally proposed to have either the #10611 or #10958 non-IPR cage structure ( J. Am. Chem. Soc. 2003, 125, 7782 ), while theoretical calculations have suggested that the #10611 cage is more stable ( J. Phys. Chem. A 2006, 110, 2231 ). La 2@C 72 was modified by photolytic reaction with the carbene reagent 2-adamantane-2,3-[3H]-diazirine. Six isomers of adamantylidene monoadducts were isolated and characterized using various kinds of measurements, including high-performance liquid chromatography, matrix-assisted laser desorption ionization mass spectrometry, UV-vis-near-infrared spectroscopy, cyclic voltammetry, differential-pulse voltammetry, (13)C NMR spectroscopy, and single-crystal X-ray diffraction. Electronic spectra and electrochemical studies revealed that the essential electronic structures of La 2@C 72 are retained in the six isomers and the adamantylidene group acts as a weak electron-donating group toward La 2@C 72. X-ray structural results unambiguously elucidated that La 2@C 72 has the #10611 chiral cage (i.e., D 2 symmetry) with two pairs of fused pentagons at each pole of the cage and that the two La atoms reside close to the two fused-pentagon pairs. On the basis of these results and theoretical calculations, it is concluded that the fused-pentagon sites are very reactive toward carbene but that the carbons forming the [5,5] junctions are less reactive than the adjacent ones; this confirms that these carbons interact strongly with the encaged metals and thus are stabilized by them.

The Bingel Monoadducts of La@C82: Synthesis, Characterization, and Electrochemistry

The reaction of La@C82 with diethyl bromomalonate in the presence of base (the Bingel reaction) generated five monoadducts which have been fully characterized. It was found that four of them (mono-A, -B, -C, and -D) are ESR-inactive, suggesting singly bonded regioisomers. In contrast, the fifth product (mono-E) is ESR-active, indicating that it possesses a cyclic moiety between the appended malonate group and the fullerene cage, analogous to conventional Bingel adducts. The differences in the molecular structures of mono-A, -B, -C, and -E result in varying thermal stabilities and electrochemical behavior. In particular, the singly bonded monoadducts undergo the retro-Bingel reaction either under thermal treatment or during electron transfer on the cyclic voltammetric timescale. However, mono-E shows remarkable thermal stability and perfect reversibility under the same experimental conditions.