New EPR signals of endohedral metallofullerenes

The EPR spectra of endohedral metallofullerenes (EMF), La-EMF and Y-EMF, which were free of admixtures of C₆₀ and of other empty fullerenes, were examined. Endohedral metallofullerenes were prepared by extraction of fullerene-containing soots with DMF. New signals withg factors close to those of fullerene radical anions were observed in the EPR spectra of solutions of EMF in DMF and DMSO. At −20 °C, these signals are observed as a doublet (ΔH _pp ≈0.04 mT) and singlet (ΔH _pp ≈0.01 mT) in solutions of La-EMF and Y-EMF, respectively. These EPR signals belong to solvated La@C₈₂ and Y@C₈₂ molecules and are characterized by small hyperfine interaction constantsa _M due to a substantial decrease in the spin density of the unpaired electron at the metal atom. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1765–1769, October, 2000.     

Spectral study of reactions of La@C82 and Y@C82 with amino-containing solvents

A solvent effect on the electronic absorption UV-VIS and ESR spectra of La@C₈₂ and Y@C₈₂ was found. The UV-VIS spectra of La@C₈₂ in pyridine, dimethylformamide, and hexamethylphosphorotriamide are identical with that of the La@C₈₂ ^– anion, which is evidence for La@C₈₂ reduction in these solvents. In amino-containing solvents, the shape of the ESR spectra depends on the temperature and the time of preparation of the solutions. Changes in the ESR spectra of La@C₈₂ and Y@C₈₂ in dimethylformamide are due to functionalization of these compounds upon reduction.     

Structural and electronic properties of endohedral metallofullerenes

This account presents an overview of our achievements in structural and chemical understanding of endohedral metallofullerenes (EMFs), a new class of metal‐carbon hybrid materials formed by encapsulation of metals inside fullerene cavities. Structural determination of EMFs is of fundamental importance for understanding their intrinsic properties and the formation mechanism, and for broadening their applications. We have developed an effective method for determining the structures of paramagnetic EMFs, and also succeeded in observing the motion of cluster in a di‐metal EMF for the first time. Recently, we unambiguously established the structures of some carbide EMFs which had been wrongly assumed as conventional EMFs previously. More importantly, we have obtained some insoluble EMF species which had never been explored or even expected before. Meanwhile, the chemical properties of various EMFs with different cage structures or different metallic cores have been systematically investigated by means of both covalent and supramolecular considerations, yielding many fascinating results relating to the dictating effect of internal metals. It is noteworthy that all these achievements are based on unambiguous X‐ray results of pristine or functionalized EMFs. DOI 10.1002/tcr.201100038     

Robust metal-pentagon interactions in the Th-based endohedral metallofullerenes revealed by DFT calculations

Endohedral metallofullerenes (EMFs) all feature obvious charge transfer from the metallic core to the carbon shell with the donated electrons largely accepted by the cage pentagons. In this work, a series of Th@C_2n (2n = 64-88) were thoroughly investigated by means of density functional theory calculations. Interestingly, we found that the tetravalent thorium atom mainly coordinates to three pentagonal rings with the metal–pentagon interactions independent on the distribution and distance among these pentagons. This coordination pattern is not only in sharp contrast to that of common organometallic complexes, where four pentagons are indispensable for stabilizing Th(IV), but also different from that of Ti-containing fullerenes, whose valence state highly depends on the pentagon distribution. The specificity of Th-based EMFs was rationalized by the synergetic effect of small metal ion size, low electronegativity, strong metal-cage electrostatic attractions and effective orbital overlap between the metal and cage orbitals. Our work highlights the role of cage pentagons in the Th-cage interactions, and points out the fundamental difference between EMFs and common organometallic complexes.     

ESR spectra of endohedral metallofullerene Ce@C82 radical anions in dimethylformamide and pyridine

Endohedral metallofullerene Ce@C₈₂ dissolved in dimethylformamide or pyridine is reduced to the radical anion. Analysis of hyperfine coupling with the topologically different ¹³C nuclei indicates the electronic structure with bivalent cerium and the paramagnetic carbon framework Ce²⁺@C₈₂ ^·3–. The ESR spectra of the radical anions of the functionalized Ce@C₈₂ derivatives are detected.     

内嵌富勒烯的研究进展

内嵌富勒烯由于其结构新颖以及独特而优异的性质在国际上引起持续而广泛的关注,成为近年来的研究热点之一.目前已经研究发现的内嵌富勒烯多达近百种,从惰性气体到碱土金属再到稀土元素都已被成功地嵌入到不同尺寸的碳笼中.其中金属离子或含金属的离子簇内嵌入富勒烯碳笼形成的内嵌金属富勒烯,以其种类丰富、结构多样成为内嵌富勒烯的主要研究对象.本文就近年来研究报道的种类繁多的内嵌富勒烯按其内嵌物类型进行归纳阐述,为今后开发更多新型的内嵌富勒烯提供一定的参考.     

Recent advances in the structural determination of endohedral metallofullerenes

The structural determination of endohedral metallofullerenes has attracted special attention in disclosing the formation mechanism and developing new routes to bulk production. Recent advances in the theoretical and experimental studies are summarized with representative mono- and dimetallofullerenes such as M@C₈₂ (M=Ca, Sc, Y, and La), Sc₂@C₈₄, and La₂@C₈₀. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 232–239, 1998     

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.     

Th@C76. Computational characterization of larger actinide endohedral fullerenes

The actinide endohedral fullerene Th@C₇₆ was successfully prepared in a very recent experiment (Wang et al., J. Am. Chem. Soc. 2017 , 139, 5110) yet without any structural information. In this work, density functional theory calculations revealed that this novel molecule bears a T_d(19151)‐C₇₆ cage obeying the isolated pentagon rule. The internal Th atom is off‐centered and resides under a sumanene‐type hexagonal ring, formally donating 4e to the carbon cage. The metal position was rationalized by the structure and charge distribution of the negatively charged cage. Interestingly, an octahedron‐like dynamic trajectory of metal inside the C₇₆ cage at high temperature was found based on the cage symmetry and located transition state structures. Furthermore, the infrared, NMR, and UV/vis spectra of Th@C₇₆ were simulated to assist future experimental characterization.     

Lanthanum endohedral metallofulleropyrrolidines: synthesis, isolation, and EPR characterization

Lanthanum endohedral metallofulleropyrrolidines have been synthesized for the first time through addition of an azomethine ylide to La@C(82)-A in toluene. It was found that the addition reaction is very efficient and, to some extent, regioselective. Two major endohedral metallofulleropyrrolidines, a monoadduct and a bisadduct of La@C(82)-A with abundance ratio of approximately 1:0.4, have been isolated by HPLC chromatography and characterized by mass spectrometry, UV/Vis-NIR absorption, and EPR spectroscopy. The electronic structure of La@C(82)-A has been modified slightly upon monoaddition and significantly upon bisaddition of the pyrrolidines.     

Dimetallic sulfide endohedral metallofullerene Sc2S@C76: Density functional theory characterization

In terms of density functional theory combined with statistic mechanics computations, we investigated a dimetallic sulfide endohedral fullerene Sc₂S@C₇₆ which has been synthesized without any characterization in experiments. Our theoretical study reveals that Sc₂S@T_d(19151)‐C₇₆ which satisfies the isolated‐pentagon rule (IPR) possesses the lowest energy, followed by three non‐IPR structures (Sc₂S@C_2v(19138)‐C₇₆, Sc₂S@C_s (17490)‐C₇₆, and Sc₂S@C₁(17459)‐C₇₆). To clarify the relative stabilities of those isomers at high temperatures, enthalpy–entropy interplay has been taken into consideration. Calculation results indicate that three species Sc₂S@T_d(19151)‐C₇₆, Sc₂S@C_2v(19138)‐C₇₆, and Sc₂S@C₁(17459)‐C₇₆ have noticeable molar fractions at the fullerene‐formation temperature region (500–3000K), and the Sc₂S@C₁(17459)‐C₇₆ with one pentagon pair becomes the most predominant isomer above 1800 K, suggesting that the unexpected non‐IPR structure is thermodynamically favorable at elevated temperatures. In addition, the structural characteristics, electron features, UV‐vis‐NIR adsorptions, and ¹³C NMR spectra of those three stable structures are introduced to assist experimental identification and characterization in future. © 2014 Wiley Periodicals, Inc.     

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.     

The problem of fullerenes. The chemical aspect

The preparation, structure, and reactivity of the family of polyhedral carbon clusters, novel allotropes of carbon, are reviewed.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 10–19, January, 1993.     

ESR spectra of products of oxidation of endometallofullerenes M@C82 (M = Y,La, Ce,Gd)

The ESR spectra of the products of oxidation of solutions and powders of Y@C₈₂ and La@C₈₂ with fuming sulfuric acid were studied. Based on the oxidation conditions and the sequence of spectral patterns, the spectra were attributed to the radical cations M@C₈₂ ⁿ⁺ (n = 2, 4), dimers M³⁺ ₂@C₁₆₄ ⁺, and polyendometallofullerenes.