X-ray structures of Sc2C2@C2n (n = 40-42): in-depth understanding of the core-shell interplay in carbide cluster metallofullerenes

X-ray analyses of the cocrystals of a series of carbide cluster metallofullerenes Sc(2)C(2)@C(2n) (n = 40-42) with cobalt(II) octaethylporphyrin present new insights into the molecular structures and cluster-cage interactions of these less-explored species. Along with the unambiguous identification of the cage structures for the three isomers of Sc(2)C(2)@C(2v)(5)-C(80), Sc(2)C(2)@C(3v)(8)-C(82), and Sc(2)C(2)@D(2d)(23)-C(84), a clear correlation between the cluster strain and cage size is observed in this series: Sc-Sc distances and dihedral angles of the bent cluster increase along with cage expansion, indicating that the bending strain within the cluster makes it pursue a planar structure to the greatest degree possible. However, the C-C distances within Sc(2)C(2) remain unchanged when the cage expands, perhaps because of the unusual bent structure of the cluster, preventing contact between the cage and the C(2) unit. Moreover, analyses revealed that larger cages provide more space for the cluster to rotate. The preferential formation of cluster endohedral metallofullerenes for scandium might be associated with its small ionic radius and the strong coordination ability as well.     

Structural elucidation and regioselective functionalization of an unexplored carbide cluster metallofullerene Sc2C2@C(s)(6)-C82

A Sc(2)C(84) isomer, previously assumed to be Sc(2)@C(84), is unambiguously identified as a new carbide cluster metallofullerene Sc(2)C(2)@C(s)(6)-C(82) using both NMR spectroscopy and X-ray crystallography. The (13)C-nuclei signal of the internal C(2)-unit was observed at 244.4 ppm with a 15% (13)C-enriched sample. Temperature-dependent dynamic motion of the internal Sc(2)C(2) cluster is also revealed with NMR spectrometry. Moreover, the chemical property of Sc(2)C(2)@C(s)(6)-C(82) is investigated for the first time using 3-triphenylmethyl-5-oxazolidinone (1) which provides a 1,3-dipolar reagent under heating. Regarding the low cage symmetry of this endohedral which contains 44 types of nonequivalent cage carbons, it is surprising to find that only one monoadduct isomer is formed in the reaction. Single-crystal X-ray results of the isolated pyrrolidino derivative Sc(2)C(2)@C(s)(6)-C(82)N(CH(2))(2)Trt (2) reveal that the addition takes place at a [6,6]-bond junction, which is far from either of the two Sc atoms. Such a highly regioselective addition pattern can be reasonably interpreted by analyzing the frontier molecular orbitals of the endohedral. Electronic and electrochemical investigations reveal that adduct 2 has a larger highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap than pristine Sc(2)C(2)@C(s)(6)-C(82); accordingly, it is more stable.     

Tunable charge-transport properties of I(h)-C80 endohedral metallofullerenes: investigation of La2@C80, Sc3N@C80, and Sc3C2@C80

Fullerene crystals or films have drawn much interest because they are good candidates for use in the construction of electronic devices. The results of theoretical calculations revealed that the conductivity properties of I(h)-C(80) endohedral metallofullerenes (EMFs) vary depending on the encapsulated metal species. We experimentally investigated the solid-state structures and charge-carrier mobilities of I(h)-C(80) EMFs La(2)@C(80), Sc(3)N@C(80), and Sc(3)C(2)@C(80). The thin film of Sc(3)C(2)@C(80) exhibits a high electron mobility μ = 0.13 cm(2) V(-1) s(-1) under normal temperature and atmospheric pressure, as determined using flash-photolysis time-resolved microwave conductivity measurements. This electron mobility is 2 orders of magnitude higher than the mobility of La(2)@C(80) or Sc(3)N@C(80).     

Large fullerenes stabilized by encapsulation of metallic clusters

Structures are proposed for six endohedral metallofullerenes with large carbon cages (from C(92) to C(100)) on the basis of sizeable (LUMO-4)-(LUMO-3) gap and the formal transfer of six electrons to the cages.     

Bonding between strongly repulsive metal atoms: an oxymoron made real in a confined space of endohedral metallofullerenes

Endohedral metallofullerenes (EMFs) are able to encapsulate up to four metal atoms. In EMFs, metal atoms are positively charged because of the electron transfer from the endohedral metal atoms to the carbon cage. It results in the strong Coulomb repulsion between the positively charged ions trapped in the confined inner space of the fullerene. At the same time, in many EMFs, such as Lu(2)@C(76), Y(2)@C(79)N, M(2)@C(82) (M = Sc, Y, Lu, etc.), Y(3)@C(80), or Sc(4)O(2)@C(80), metals do not adopt their highest oxidation states, thus yielding a possibility of the covalent metal-metal bonding. In some other EMFs (e.g., La(2)@C(80)), metal-metal bonding evolves as the result of the electrochemical or chemical reduction, which leads to the population of the metal-based LUMO with pronounced metal-metal bonding character. This article highlights different aspects of the metal-metal bonding in EMFs. It is concluded that the valence state of the metal atoms in dimetallofullerenes is not dependent on their third ionization potential, but is determined by their ns(2)(n- 1)d(1)→ns(1)(n- 1)d(2) excitation energies. Peculiarities of the metal-metal bonding in EMFs are described in terms of molecular orbital analysis as well as topological approaches such as Quantum Theory of Atoms in Molecules and Electron Localization Function. Interplay of Coulomb repulsion and covalent bonding is analyzed in the framework of the Interacting Quantum Atom approach.     

Sc2@C70 rather than Sc2C2@C68: density functional theory characterization of metallofullerene Sc2C70

Detailed study on Sc(2)C(70) series has been performed based on fully screening for C(70) tetra- and hexa- anions. With a combined methodology of quantum chemistry and statistical mechanics, our calculation results reveal that the Sc(2)C(70), which was proposed as the first metal-carbide endohedral metallofullerene with a non-isolated pentagon rule (non-IPR) cage (Sc(2)C(2)@C(68):6073_C(2v)), is in fact a C(70) non-IPR metallofullerene structure (Sc(2)@C(70):7854_C(2v)) with three pair of pentagon adjacency thanks to its significant thermodynamic and kinetic stability. According to the natural bond analysis and orbital interaction diagram, each scandium atom should only transfer two 4s electrons to the carbon cages and the valence state of Sc(2)@C(70) is (Sc(2+))(2)@C(70) (4-). In addition, the simulation of UV-Vis-NIR spectrum for Sc(2)@C(70):7854_C(2v) shows good accordance to the experimental spectrum.     

Sc2@C3v(8)-C82 vs. Sc2C2@C3v(8)-C82: drastic effect of C2 capture on the redox properties of scandium metallofullerenes

We describe the first example of scandium dimetallofullerenes, Sc(2)@C(3v)(8)-C(82), which has the same cage as the previously assigned scandium carbide cluster fullerene Sc(2)C(2)@C(3v)(8)-C(82) but they exhibit distinctly different electronic configurations and electronic behaviours, confirming the drastic influence of the internal C(2) unit.     

Chemical understanding of carbide cluster metallofullerenes: a case study on Sc2C2@C2v(5)-C80 with complete X-ray crystallographic characterizations

Little is known about the chemical properties of carbide cluster metallofullerenes (CCMFs). Here we report the photochemical reaction of a newly assigned CCMF Sc(2)C(2)@C(2v)(5)-C(80) with 2-adamantane-2,3-[3H]-diazirine (AdN(2), 1), which provides a carbene reagent under irradiation. Five monoadduct isomers (2a-2e), with respective abundances of 20%, 40%, 25%, 5%, and 10%, were isolated and characterized with a combination of experimental techniques including unambiguous single-crystal X-ray crystallography. Results show that the two Sc atoms of the bent Sc(2)C(2) cluster tend to move in most cases, whereas the C(2)-unit is almost fixed. Accordingly, it is difficult to explain the addition patterns by considering the strain and charge density on the cage with a fixed cluster, and thus a moving cluster may account for the addition patterns. These results show that the situation of CCMFs is more complicated than those in other metallofullerenes. Furthermore, a thermal isomerization process from 2b to 2c was observed, confirming that the most abundant isomer 2b is a kinetically favored adduct. Finally, it is revealed that the electronic and electrochemical properties of pristine Sc(2)C(2)@C(2v)(5)-C(80) have been markedly altered by exohedral modification.     

Electronic structure and redox properties of the open-shell metal-carbide endofullerene Sc3C2@C80: a density functional theory investigation

Density functional theory calculations have shown that the open-shell metal-carbide endofullerene Sc3C2@C80 has the valence state (Sc3+)3(C2)(3-)@C80(6-). A lot of low-lying isomers differing in geometries and locations of the endohedral [(Sc3+)3(C2)(3-)] cluster have been located, indicating unusual dual intramolecular dynamic behaviors of this endofullerene at room temperature. The electrochemical redox properties of this endofullerene have been elucidated in terms of electronic structure theory. Its redox states are found to follow the general charge-state formula (Sc3+)3C2(3-q)-@C80(6-) (q is the charge of the whole molecule ranging from +1 to -3), demonstrating the high charge flexibility of the endohedral metal-carbide cluster. The structure of the endohedral [(Sc3+)3C2(3-q)-)] cluster varies with the redox processes, shifting from a planar structure (for q = 0 and -1) to a trifoliate structure (for q = +1, -2, -3).     

The shape of the Sc2(μ2-S) unit trapped in C82: crystallographic, computational, and electrochemical studies of the isomers, Sc2(μ2-S)@C(s)(6)-C82 and Sc2(μ2-S)@C(3v)(8)-C82

Single-crystal X-ray diffraction studies of Sc(2)(μ(2)-S)@C(s)(6)-C(82)·Ni(II)(OEP)·2C(6)H(6) and Sc(2)(μ(2)-S)@C(3v)(8)-C(82)·Ni(II)(OEP)·2C(6)H(6) reveal that both contain fully ordered fullerene cages. The crystallographic data for Sc(2)(μ(2)-S)@C(s)(6)-C(82)·Ni(II)(OEP)·2C(6)H(6) show two remarkable features: the presence of two slightly different cage sites and a fully ordered molecule Sc(2)(μ(2)-S)@C(s)(6)-C(82) in one of these sites. The Sc-S-Sc angles in Sc(2)(μ(2)-S)@C(s)(6)-C(82) (113.84(3)°) and Sc(2)(μ(2)-S)@C(3v)(8)-C(82) differ (97.34(13)°). This is the first case where the nature and structure of the fullerene cage isomer exerts a demonstrable effect on the geometry of the cluster contained within. Computational studies have shown that, among the nine isomers that follow the isolated pentagon rule for C(82), the cage stability changes markedly between 0 and 250 K, but the C(s)(6)-C(82) cage is preferred at temperatures ≥250 °C when using the energies obtained with the free encapsulated model (FEM). However, the C(3v)(8)-C(82) cage is preferred at temperatures ≥250 °C using the energies obtained by rigid rotor-harmonic oscillator (RRHO) approximation. These results corroborate the fact that both cages are observed and likely to trap the Sc(2)(μ(2)-S) cluster, whereas earlier FEM and RRHO calculations predicted only the C(s)(6)-C(82) cage is likely to trap the Sc(2)(μ(2)-O) cluster. We also compare the recently published electrochemistry of the sulfide-containing Sc(2)(μ(2)-S)@C(s)(6)-C(82) to that of corresponding oxide-containing Sc(2)(μ(2)-O)@C(s)(6)-C(82).     

New M(3)N@C(2n) endohedral metallofullerene families (M=Nd, Pr, Ce; n=40-53): expanding the preferential templating of the C(88) cage and approaching the C(96) cage

Three new families of trimetallic nitride template endohedral metallofullerenes (TNT EMFs), based on cerium, praseodymium, and neodymium clusters, were synthesized by vaporizing packed graphite rods in a conventional Kr?tschmer-Huffman arc reactor. Each of these families of metallofullerenes was identified and characterized by mass spectroscopy, HPLC, UV/Vis-NIR spectroscopy, and cyclic voltammetry. The mass spectra and HPLC chromatograms show that these larger metallic clusters are preferentially encapsulated by a C(88) cage. When the size of the cluster is increased, the C(96) cage is progressively favored over the predominant C(88) cage. It is also observed that the smaller cages (C(80)-C(86)) almost disappear on going from neodymium to cerium endohedral metallofullerenes. The UV/Vis-NIR spectra and cyclic voltammograms confirm the low HOMO-LUMO gap and reversible electrochemistry of these M(3)N@C(88) metallofullerenes.     

Large endohedral fullerenes containing two metal ions, Sm2@D2(35)-C88, Sm2@C1(21)-C90, and Sm2@D3(85)-C92, and their relationship to endohedral fullerenes containing two gadolinium ions

The carbon soot obtained by electric arc vaporization of carbon rods doped with Sm(2)O(3) contains a series of monometallic endohedral fullerenes, Sm@C(2n), along with smaller quantities of the dimetallic endohedrals Sm(2)@C(2n) with n = 44, 45, 46, and the previously described Sm(2)@D(3d)(822)-C(104). The compounds Sm(2)@C(2n) with n = 44, 45, 46 were purified by high pressure liquid chromatography on several different columns. For endohedral fullerenes that contain two metal atoms, there are two structural possibilities: a normal dimetallofullerene, M(2)@C(2n), or a metal carbide, M(2)(μ-C(2))@C(2n-2). For structural analysis, the individual Sm(2)@C(2n) endohedral fullerenes were cocrystallized with Ni(octaethylporphyrin), and the products were examined by single-crystal X-ray diffraction. These data identified the three new endohedrals as normal dimetallofullerenes and not as carbides: Sm(2)@D(2)(35)-C(88), Sm(2)@C(1)(21)-C(90), and Sm(2)@D(3)(85)-C(92). All four of the known Sm(2)@C(2n) endohedral fullerenes have cages that obey the isolated pentagon rule (IPR). As the cage size expands in this series, so do the distances between the variously disordered samarium atoms. Since the UV/vis/NIR spectra of Sm(2)@D(2)(35)-C(88) and Sm(2)@C(1)(21)-C(90) are very similar to those of Gd(2)C(90) and Gd(2)C(92), we conclude that Gd(2)C(90) and Gd(2)C(92) are the carbides Gd(2)(μ-C(2))@D(2)(35)-C(88) and Gd(2)(μ-C(2))@C(1)(21)-C(90), respectively.     

Violating the isolated pentagon rule (IPR): endohedral non-IPR C98 cages of Gd2@C98

The geometric, electronic structure, and thermodynamic stability of large gadolinium-containing endohedral metallofullerenes, Gd(2)@C(98), have been systematically investigated by comprehensive density functional theory calculations combined with statistical mechanics treatments. The Gd(2)@C(2)(230924)-C(98) structure, which satisfies the isolated-pentagon rule (IPR), is determined to possess the lowest energy followed with some stable non-IPR isomers. In order to clarify the relative stabilities at elevated temperatures, entropy contributions are taken into account on the basis of the Gibbs energy at the B3LYP level for the first time. Interestingly, a novel non-IPR Gd(2)@C(1)(168785)-C(98) isomer which has one pair of pentagon adjacency is more thermodynamically stable than the lowest energy IPR species within a wide temperature interval related to fullerene formation. Therefore, the Gd(2)@C(1)(168785)-C(98) is predicted to be the most proper isomer obtained experimentally, which is the largest non-IPR carbon cage found so far. Our findings demonstrate that interaction between metals and carbon cages could stabilize the fused pentagons effectively, and thus, the non-IPR isomers should not be ignored in some cases of endohedral metallofullerenes. The IR features of Gd(2)@C(98) are simulated to assist its future experimental characterization.     

Sc2S@C(s)(10528)-C72: a dimetallic sulfide endohedral fullerene with a non isolated pentagon rule cage

A non isolated pentagon rule metallic sulfide clusterfullerene, Sc(2)S@C(s)(10528)-C(72), has been isolated from a raw mixture of Sc(2)S@C(2n) (n = 35-50) obtained by arc-discharging graphite rods packed with Sc(2)O(3) and graphite powder under an atmosphere of SO(2) and helium. Multistage HPLC methods were utilized to isolate and purify the Sc(2)S@C(72). The purified Sc(2)S@C(s)(10528)-C(72) was characterized by mass spectrometry, UV-vis-NIR absorption spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic analysis unambiguously elucidated that the C(72) fullerene cage violates the isolated pentagon rule, and the cage symmetry was assigned to C(s)(10528)-C(72). The electrochemical behavior of Sc(2)S@C(s)(10528)-C(72) shows a major difference from those of Sc(2)S@C(s)(6)-C(82) and Sc(2)S@C(3v)(8)-C(82) as well as the other metallic clusterfullerenes. Computational studies show that the Sc(2)S cluster transfers four electrons to the C(72) cage and C(s)(10528)-C(72) is the most stable cage isomer for both empty C(72)(4-) and Sc(2)S@C(72), among the many possibilities. The structural differences between the reported fullerenes with C(72) cages are discussed, and it is concluded that both the transfer of four electrons to the cage and the geometrical requirements of the encaged Sc(2)S cluster play important roles in the stabilization of the C(s)(10528)-C(72) cage.     

Sc2C2@C80 rather than Sc2@C82: templated formation of unexpected C2v(5)-C80 and temperature-dependent dynamic motion of internal Sc2C2 cluster

Unambiguous X-ray crystallographic results of the carbene adduct of Sc(2)C(82) reveal a new carbide cluster metallofullerene with the unexpected C(2v)(5)-C(80) cage, that is, Sc(2)C(2)@C(2v)(5)-C(80). More interestingly, DFT calculations and NMR results disclose that the dynamic motion of the internal Sc(2)C(2) cluster depends strongly on temperature. At 293 K, the cluster is fixed inside the cage with two nonequivalent Sc atoms on the mirror plane, thereby leading to C(s) symmetry of the whole molecule. However, when the temperature increases to 413 K, the (13)C and (45)Sc NMR spectra show that the cluster rotates rapidly inside the C(2v)(5)-C(80) cage, featuring two equivalent Sc atoms and weaker metal-cage interactions.     

Structure, stability, and cluster-cage interactions in nitride clusterfullerenes M3N@C2n (M = Sc, Y; 2n = 68-98): a density functional theory study

Extensive semiempirical calculations of the hexaanions of IPR (isolated pentagon rule) and non-IPR isomers of C(68)-C(88) and IPR isomers of C(90)-C(98) followed by DFT calculations of the lowest energy structures were performed to find the carbon cages that can provide the most stable isomers of M(3)N@C(2n) clusterfullerenes (M = Sc, Y) with Y as a model for rare earth ions. DFT calculations of isomers of M(3)N@C(2n) (M = Sc, Y; 2n = 68-98) based on the most stable C(2n)(6-) cages were also performed. The lowest energy isomers found by this methodology for Sc(3)N@C(68), Sc(3)N@C(78), Sc(3)N@C(80), Y(3)N@C(78), Y(3)N@C(80), Y(3)N@C(84), Y(3)N@C(86), and Y(3)N@C(88) are those that have been shown to exist by single-crystal X-ray studies as Sc(3)N@C(2n) (2n = 68, 78, 80), Dy(3)N@C(80), and Tb(3)N@C(2n) (2n = 80, 84, 86, 88) clusterfullerenes. Reassignment of the carbon cage of Sc(2)@C(76) to the non-IPR Cs: 17490 isomer is also proposed. The stability of nitride clusterfullerenes was found to correlate well with the stability of the empty 6-fold charged cages. However, the dimensions of the cage in terms of its ability to encapsulate M(3)N clusters were also found to be an important factor, especially for the medium size cages and the large Y(3)N cluster. In some cases the most stable structures are based on the different cage isomers for Sc(3)N and Y(3)N clusters. Up to the cage size of C(84), non-IPR isomers of C(2n)(6-) and M(3)N@C(2n) were found to compete with or to be even more stable than IPR isomers. However, the number of adjacent pentagon pairs in the most stable non-IPR isomers decreases as cage size increases: the most stable M(3)N@C(2n) isomers have three such pairs for 2n = 68-72, two pairs for n = 74-80, and only one pair for n = 82, 84. For C(86) and C(88) the lowest energy IPR isomers are much more stable than any non-IPR isomer. The trends in the stability of the fullerene isomers and the cluster-cage binding energies are discussed, and general rules for stability of clusterfullerenes are established. Finally, the high yield of M(3)N@C(80) (Ih) clusterfullerenes for any metal is explained by the exceptional stability of the C(80)(6-) (Ih: 31924) cage, rationalized by the optimum distribution of the pentagons leading to the minimization of the steric strain, and structural similarities of C(80) (Ih: 31924) with the lowest energy non-IPR isomers of C(760(6-), C(78)(6-), C(82)(6-), and C(84)(6-) pointed out.     

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。     

Unprecedented mu4-C2(6-) anion in Sc4C2@C80

Metal carbide compound containing highly charged C2(q-) (q = 5, 6) moiety is rather scarce. We show by means of density functional calculations that an unprecedented mu4-C2(6-) anion can viably exist as an endohedral [Sc4C2]6+ cluster in the endofullerene Sc4C2@C80. The electronic structure, ionization energy, electron affinity, 13C NMR chemical shifts, vibrational frequencies, and electrochemical redox potentials of this unique endofullerene have been predicted to assist future experimental characterization.     

Non-HPLC rapid separation of metallofullerenes and empty cages with TiCl4 Lewis acid

Rapid and efficient separation/purification of pure metallofullerenes M(x)@C(n) (M = metal; x = 1, 2; n > 70) and carbide metallofullerenes of the type M(y)C(2)@C(n-2) (y = 2, 3, 4; n - 2 > 68) has been reported. The present method utilizes rapid and almost perfect preferential formation of TiCl(4) (generally known as a Lewis acid)-metallofullerene complexes, which easily decompose to provide pure metallofullerene powders by a simple water treatment. The present method enables one to separate the metallofullerenes up to >99% purity within 10 min without using any type of high-performance liquid chromatography (HPLC). It is found that the oxidation potentials of the metallofullerenes are crucial factors for efficient purification. The current separation/purification technique may open a brand-new era for inducing further applications and commercialization of endohedral metallofullerenes.     

Nanoscale fullerene compression of an yttrium carbide cluster

The nanoscale parameters of metal clusters and lattices have a crucial influence on the macroscopic properties of materials. Herein, we provide a detailed study on the size and shape of isolated yttrium carbide clusters in different fullerene cages. A family of diyttrium endohedral metallofullerenes with the general formula of Y(2)C(2n) (n = 40-59) are reported. The high field (13)C nuclear magnetic resonance (NMR) and density functional theory (DFT) methods are employed to examine this yttrium carbide cluster in certain family members, Y(2)C(2)@D(5)(450)-C(100), Y(2)C(2)@D(3)(85)-C(92), Y(2)C(2)@C(84), Y(2)C(2)@C(3v)(8)-C(82), and Y(2)C(2)@C(s)(6)-C(82). The results of this study suggest that decreasing the size of a fullerene cage with the same (Y(2)C(2))(4+) cluster results in nanoscale fullerene compression (NFC) from a nearly linear stretched geometry to a constrained "butterfly" structure. The (13)C NMR chemical shift and scalar (1)J(YC) coupling parameters provide a very sensitive measure of this NFC effect for the (Y(2)C(2))(4+) cluster. The crystal structural parameters of a previously reported metal carbide, Y(2)C(3) are directly compared to the (Y(2)C(2))(4+) cluster in the current metallofullerene study.