Purification of endohedral trimetallic nitride fullerenes in a single, facile step

A major hurdle hampering the development of fullerenes, endohedral metallofullerenes, and nanotubes has been the difficulty of obtaining high purity samples. Soots prepared in the usual manner via a Kr?tschmer-Huffman electric-arc generator consist of mixtures of insoluble carbonaceous materials and soluble fullerenes: C60, C70, C76, C78, C84, etc. When metals are introduced as endohedral species the complexity of the resultant soot is even greater because of the presence of multiple isomers of both the empty fullerenes and the endohedral metallofullerenes. Here, for the first time, we report that lanthanide trimetallic nitride endohedral metallofullerenes, A3N@C80 (A = lanthanide atom, e.g., Er, Gd, Ho, Lu, Sc, Tb, Tm, Y), can be obtained in pure form directly from as-prepared soots in a single facile step by taking advantage of their extraordinary kinetic chemical stability with respect to the other fullerenes in Diels-Alder reactions with a cyclopentadiene-functionalized resin. We show that careful control of conditions (stoichiometry, time, temperature) allows separation of fullerenes with different cage sizes, as well as isomeric species. Furthermore, the Diels-Alder reaction is thermally reversible, and we demonstrated that the bound empty-cage fullerenes and classical endohedral metallofullerenes can be recovered by displacement with maleic anhydride.     

M4 @Si28 (M=Al,Ga): metal-encapsulated tetrahedral silicon fullerene

It is known that silicon fullerenes cannot maintain perfect cage structures like carbon fullerenes. Previous density-functional theory calculations have shown that even with encapsulated species, nearly all endohedral silicon fullerenes exhibit highly puckered cage structures in comparison with their carbon counterparts. In this work, we present theoretical evidences that the tetrahedral fullerene cage Si(28) can be fully stabilized by encapsulating a tetrahedral metallic cluster (Al(4) or Ga(4)). To our knowledge, this is the first predicted endohedral silicon fullerene that can retain perfectly the same cage structure (without puckering) as the carbon fullerene counterpart (T(d)-C(28) fullerene). Density-functional theory calculations also suggest that the two endohedral metallosilicon fullerenes T(d)-M(4)@Si(28) (M=Al and Ga) can be chemically stable because both clusters have a large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap ( approximately 0.9 eV), strong spherical aromaticity (nucleus-independent chemical shift value of -36 and -44), and large binding and embedding energies.     

The smallest stable fullerene, M@C28 (m = Ti, Zr, U): stabilization and growth from carbon vapor

The smallest fullerene to form in condensing carbon vapor has received considerable interest since the discovery of Buckminsterfullerene, C(60). Smaller fullerenes remain a largely unexplored class of all-carbon molecules that are predicted to exhibit fascinating properties due to the large degree of curvature and resulting highly pyramidalized carbon atoms in their structures. However, that curvature also renders the smallest fullerenes highly reactive, making them difficult to detect experimentally. Gas-phase attempts to investigate the smallest fullerene by stabilization through cage encapsulation of a metal have been hindered by the complexity of mass spectra that result from vaporization experiments which include non-fullerene clusters, empty cages, and metallofullerenes. We use high-resolution FT-ICR mass spectrometry to overcome that problem and investigate formation of the smallest fullerene by use of a pulsed laser vaporization cluster source. Here, we report that the C(28) fullerene stabilized by encapsulation with an appropriate metal forms directly from carbon vapor as the smallest fullerene under our conditions. Its stabilization is investigated, and we show that M@C(28) is formed by a bottom-up growth mechanism and is a precursor to larger metallofullerenes. In fact, it appears that the encapsulating metal species may catalyze or nucleate endohedral fullerene formation.     

Endohedral metallofullerenes: a unique host-guest association

In this tutorial review taking X-ray crystallographically characterized compounds as a starting point a walk is taken through the electronic and structural properties of endohedral metallofullerenes. After classification of the fullerenes according to the encapsulated guest, particular attention is given to identifying factors that determine the selection of a particular carbon cage network by the internal metal cluster. Some of the physical rules that determine which particular fullerene cage is formed will be discussed. Concepts such as charge transfer between the cage and the guest metal ions, the topology of the cage, the separations between the 12 pentagons on the fullerene surface, and the effect of entropic factors are used to rationalize the selection of a particular cage. The roles of electrochemistry and vibrational spectroscopy in combination with theoretical calculations are considered in understanding the structures of the endohedral fullerenes.     

C~4~0, C~4~0^+, Nb@C~4~0^+, NbC~3~9^+, Nb@C~4~0H~4^+的 量子化学研究

用量子化学从头计算方法研究了C~4~0,C~4~0^+,Nb@C~4~0^+,NbC~3~9^+,Nb@C~4~0H~4^+的几何构型、电子结构和C~2~8一样,C~4~0(T~d)基态也为^5A~2态,笼骨架上具有四个悬挂键。计算结果表明C~4~0和C~4~0^+比NbC~3~9^+和Nb@C~4~0^+稳定,与实验结果一致。     

内嵌金属碳氮化物团簇富勒烯的稳定性与生成机理

Fullerene molecules have nano-scale cavities in which various metal or metal clusters of different sizes can be embedded to form metallofullerenes with unique core-shell structures. The physical and chemical properties of metallofullerenes can be modified through the interaction between the encapsulated metals and the fullerene cages. As such, the investigation of metallofullerenes with novel structures has been a principal research focus in the field of fullerenes. In this study, we investigated the size matching effect between encapsulated clusters and fullerene cages for the endohedral metal carbonitride clusterfullerenes in order to discover new metallofullerenes. The stability and electronic structure of the metallofullerenes formed by encapsulating M₃NC clusters (M = Y, La, Gd) into D₂(186)-C₉₆ and D₂(35)-C₈₈ fullerenes were studied using quantum chemical calculations. It was found that the fullerene cages formed stable structures by accepting six electrons transferred from the encapsulated clusters. The change in configuration of the encapsulated clusters was clarified by a comparison with the corresponding M₃N@C_2n metal nitride clusterfullerenes; the size matching effect between M₃NC cluster and fullerene cage was elucidated on the basis of the calculated results and previous studies on Sc₃NC@I_h(7)-C₈₀. For the D₂(186)-C₉₆ fullerene, the Gd₃NC cluster was found to have smaller changes in the configuration as compared with the La₃NC cluster, proving that Gd₃NC is more suitable than La₃NC for encapsulation in the D₂(186)-C₉₆ fullerene cage. In addition, it was determined that the La₃NC cluster requires a large structural change to maintain its planar configuration. For the D₂(35)-C₈₈ fullerene cage, the Y₃NC cluster is more suitable than Gd₃NC for encapsulation owing to the smaller size of the Y₃NC cluster. The spatial distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of Gd₃NC@D₂(186)-C₉₆ were found to be similar to those of Gd₃N@D₂(186)-C₉₆. However, a unique endohedral cluster-based occupied molecular orbital was found for Gd₃NC@D₂(186)-C₉₆. This orbital is derived from the interaction between the NC unit and the Gd atoms. The spatial distribution of the HOMO of Y₃NC@D₂(35)-C₈₈ is similar to that of Y₃N@D₂(35)-C₈₈, while the LUMO of Y₃NC@D₂(35)-C₈₈ has a much larger contribution from the endohedral cluster as compared to Y₃N@D₂(35)-C₈₈. Thus, the addition of a carbon atom in the cluster has a remarkable impact on the electronic structure of the metallofullerenes. With respect to structural characteristics, we found that the three fullerene cages, D₂(186)-C₉₆, D₂(35)-C₈₈, and I_h(7)-C₈₀, have a uniform distribution of five-membered carbon atom rings; these fullerenes can be greatly stabilized in the form of C_2n^6- anions. However, the formation mechanism of fullerenes and metallofullerenes, at present, is poorly understood. Based on the structural analysis, we propose a direct mechanism for the formation of fullerenes without the Stone-Wales isomerization, i.e., the rearrangement of five-membered rings through the addition of carbon atoms and the transformation into larger carbon cages while maintaining stable structural units.     

内嵌金属富勒烯的笼外化学修饰

内嵌金属富勒烯以其独特的结构和新奇的性质吸引了众多科学家的目光,对它们进行笼外化学修饰是最近十年来新兴的研究热点,这对于考察内嵌金属富勒烯的结构及化学物理性质并拓宽其应用范围具有重要意义。本文将内嵌金属富勒烯与各种底物的不同作用分类,以反应类型为线索,详细概括了已发表的内嵌金属富勒烯的各种笼外化学反应,包括各种环化反应、内嵌金属富勒烯与杯芳烃及冠醚的自组装、单键相连的衍生物、水溶性衍生物以及用内嵌金属富勒烯填充碳纳米管等。在对各种化学反应阐述的同时,对内嵌金属富勒烯的可能应用也进行了总结,并提出了自己的看法。     

The Cage and Metal Effect: Spectroscopy and Electrochemical Survey of a Series of Sm‐Containing High Metallofullerenes

A series of Sm‐containing high metallofullerenes, namely, Sm@C₈₂ (I, II, III, IV), Sm@C₈₄ (I, II, III), Sm@C₈₆, Sm@C₈₈ (I, II, III), Sm@C₉₀ (I, II, III), Sm@C₉₂ (I, II), Sm@C₉₄ (I, II, III), and Sm@C₉₆, is successfully synthesized and characterized by UV/Vis/NIR absorption spectroscopy, and cyclic and differential pulse voltammetry. Sm‐containing high metallofullerenes have a relatively larger number of isomers compared with other divalent ones. The highest boiling point of Sm among Group II metals may be responsible for this phenomenon. Comparing the spectroscopic and electrochemical behaviors of Sm‐containing metallofullerenes with those of other divalent ones, it is seen that when the size of the carbon cage enlarges, different structures form stable molecules with different metals. Furthermore, there are also some important differences in the electrochemistry properties. The cage effect on the electronic structures of high metallofullerenes is also estimated from the differences in reduction potentials between metallofullerenes and their corresponding fullerenes. It is believed that the influence of transferred electrons from the metal to the carbon cage becomes much weaker for high fullerenes. The redox property of high metallofullerene is more dependent on the carbon‐cage structure than the effect of electron transfer.     

Exohedral reactivity of trimetallic nitride template (TNT) endohedral metallofullerenes

[structures: see text] Fullerenes containing a trimetallic nitride template (TNT) within the cage are a particularly interesting class of endohedral metallofullerenes. Recently two exohedral derivatives of the Sc3N@C80 fullerene have been synthesized: a Diels-Alder and a fulleropyrrolidine cycloadduct. The successful isolation, purification, and structural elucidation of these metallofullerenes derivatives have encouraged us to understand how the chemical reactivity is affected by TNT encapsulation. First of all, we predicted the most reactive exohedral sites, taking into account the double bond character and the pyramidalization angle of the C-C bonds. For this purpose, a full characterization of all different types of C-C bonds of the following fullerenes was carried out: I(h)-C60:1, D3-C68:6140, D3-Sc3N@C68, D(5h)-C70:1, D(3h')-C78:5, D(3h)-Sc3N@C78, I(h)-C80:7 and several isomers of Sc3N@C80. Finally the exohedral reactivity of these TNT endohedral metallofullerenes, via [4 + 2] cycloaddition reactions of 1,3-butadiene, was corroborated by means of DFT calculations.     

Stable Non‐IPR C60 and C70 Fullerenes Containing a Uniform Distribution of Pyrenes and Adjacent Pentagons

The most abundant fullerenes, C₆₀ and C₇₀, and all the pure carbon fullerenes larger than C₇₀, follow the isolated‐pentagon rule (IPR). Non‐IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Euler’s theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non‐IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well‐separated from one other, stabilization of non‐IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non‐IPR derivatives might be even more common than IPR ones.     

On the formation of smaller p‐block endohedral fullerenes: Bonding analysis in the E@C20 (E = Si,Ge, Sn,Pb) series from relativistic DFT calculations

Experimentally characterized endohedral metallofullerenes are of current interest in expanding the range of viable fullerenic structures and their applications. Smaller metallofullerenes, such as M@C₂₈, show that several d‐ and f‐block elements can be efficiently confined in relatively small carbon cages. This article explores the potential capabilities of the smallest fullerene cage, that is, C₂₀, to encapsulate p‐block elements from group 14, that is, E = Si, Ge, Sn, and Pb. Our interest relates to the bonding features and optical properties related to E@C₂₀. The results indicate both s‐ and p‐type concentric bonds, in contrast to the well explored endohedral structures encapsulating f‐block elements. Our results suggest the E@C₂₀ series to be a new family of viable endohedral fullerenes. In addition spectroscopic properties related to electron affinity, optical, and vibrational were modeled to gain further information useful for characterization. Characteristic optical patterns were studied predicting a distinctive first peak located between 400 and 250 nm, which is red‐shifted going to the heavier encapsulated Group 14 atoms. Electron affinity properties expose different patterns useful to differentiate the hollow C₂₀ fullerene to the proposed p‐block endohedral counterparts. © 2017 Wiley Periodicals, Inc.     

Selective extraction and purification of endohedral metallofullerene from carbon soot

A preferential extraction of endohedral metallofullerenes (EMFs) from carbon soot through the use of reduction in the extraction process and a convenient isolation of endohedral metallofullerene anions (EMFs(-)) and empty fullerenes utilizing their difference in solubility are accomplished. EMFs are easily isolated by one-stage high-performance liquid chromatography after chemical oxidation of the extracted endohedral EMFs(-).     

富勒烯合成化学研究进展

富勒烯是一类由12个五元环和若干六元环组成的笼状分子, 自20世纪80年代中期被发现以来就以其独特的结构和新奇的性质而成为科学界研究的热点, 25年来, 无论在基础研究还是在实际应用领域都有了长足的进步, 人们在发展富勒烯合成新方法和寻找富勒烯新结构方面做了大量的工作。本文对富勒烯的各种宏量合成方法进行了回顾, 并概述了迄今已发表的60余种富勒烯新结构,包括各种富勒烯空笼、内嵌富勒烯、富勒烯笼外修饰衍生物及氮杂富勒烯等结构。     

笼内金属富勒烯的合成与分析

自1985年,Smalleyr等人首次在飞行时间质谱仪中检测到C60以来,富勒烯碳原子族的研究便不断深入,最初发现的激光蒸发含稀土化合物的石墨只能得到微量的金属富勒烯,仅限于质谱检测,直到1990年,Kratschmer等人采用石墨电子弧放电首次得到宏观量的富勒烯,才使得富勒烯研究进入了实质性的研究阶段。富勒烯及其化合物的分子结构确定对于研究这类化合物无疑是非常重要的,红外光谱、核磁共振、X射线衍射、质谱以及电化学方法都是非常重要的手段。在富勒烯研究中,质谱是相当重要的,在80年代中期的富勒烯以及富勒烯衍生物如金属富勒烯多面体都在质谱中首次发现的。电弧法成功地得到富勒烯后,质谱因其灵敏度高、用量少、简便、快速等优点,成为富勒烯化合物物理化学性能研究以及结构表征工具,其中激光解吸电离为软电离技术只给出化合物分子质量而不产生碎片离子,已成为研究富勒烯首选的重要工具。     

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.     

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.     

Understanding the stabilization of metal carbide endohedral fullerenes m(2)c(2)@c(82) and related systems

We analyze the electronic structure of carbide endohedral metallofullerenes of the type Sc(2)C(2)@C(82) and study the possibility of rotation of the encapsulated Sc(2)C(2) moiety in the interior of the cage. Moreover, we rationalize the higher abundance of M(2)C(2)@C(82) (M = Sc, Y) in which the metal-carbide cluster is encapsulated in the C(3v)-C(82):8 carbon cage with respect to other carbides of the same family on the basis of the formal transfer of four electrons from the cluster to the cage and sizeable (LUMO-3)-(LUMO-2) gap in the empty cages. This rule also applies to all those endohedral metallofullerenes in which the encapsulated cluster transfers four electrons to the carbon cage as, for example, the reduced [M@C(82)](-) systems (M = group 3 or lanthanide metal ion).     

Endohedral Metallofullerenes—Filled Fullerene Derivatives towards Multifunctional Reaction Center Mimics

In recent years, endohedral metallofullerenes have attracted tremendous interest not only in physics and chemistry, but also in interdisciplinary areas, such as materials and biological sciences. In this concept article we highlight recent results on different endohedral metallofullerenes based on lanthanides and their derivatives. The chemical and excited state reactivities of endohedral metallofullerenes are discussed for various endohedral clusters. Most important is the part that covers spectroscopic and kinetic assays of reductive and oxidative charge transfer evolving from photoexcited electron donors and electron acceptors, respectively, in a variety of electron donor–acceptor conjugates. Towards this end, we refer to the applications of endohedral metallofullerenes in photovoltaic devices that feature greater efficiency than devices fabricated with empty fullerenes. Herein, we focus mainly on results obtained in the groups of Akasaka, Echegoyen, and Guldi.     

Endohedral metallofullerenes-filled fullerene derivatives towards multifunctional reaction center mimics

In recent years, endohedral metallofullerenes have attracted tremendous interest not only in physics and chemistry, but also in interdisciplinary areas, such as materials and biological sciences. In this concept article we highlight recent results on different endohedral metallofullerenes based on lanthanides and their derivatives. The chemical and excited state reactivities of endohedral metallofullerenes are discussed for various endohedral clusters. Most important is the part that covers spectroscopic and kinetic assays of reductive and oxidative charge transfer evolving from photoexcited electron donors and electron acceptors, respectively, in a variety of electron donor-acceptor conjugates. Towards this end, we refer to the applications of endohedral metallofullerenes in photovoltaic devices that feature greater efficiency than devices fabricated with empty fullerenes. Herein, we focus mainly on results obtained in the groups of Akasaka, Echegoyen, and Guldi.     

Electronic structure of IPR and non-IPR endohedral metallofullerenes: Connecting orbital and topological rules

The electronic structure of endohedral metallofullerenes is rationalized by connecting the apparently independent orbital and topological rules that explain the stability of this family of fullerenes. The separation of the 12 pentagons of the fullerene, which is maximized in order to minimize the Coulomb repulsion, is found to be correlated with the orbital energies of the cage that accepts the electron transfer from the internal cluster. An explanation for the absence of non-IPR cages in large-size EMFs is also provided.