Sc2 dimer in IPR-violated C66 fullerene: a covalent bonded metallofullerene

Different Factors Govern Chlorination and Encapsulation in Fullerenes: The Case of C66

C66 is one of the smallest fullerenes that is able to encapsulate more than one metal atom, as in Sc2@C66, as well as to get chlorinated at a low level, C66Cl10 or C66Cl6. We show here, with the help of computations at density functional theory level, that these two means of obtaining derivatives of non-isolated pentagon rule fullerenes are dictated by different factors. Chlorination takes place at temperatures lower than 2000 K, once the neutral fullerenes are formed. Encapsulation is, however, mainly governed by the charge transfer, although the Sc···Sc distance is also playing a role in the stability of Sc2@C66.

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).

ELECTRON TRANSFER MAKES D3h (78:5) CAGE EASY TO FORM M2@C78(M = La, Ce): A RELATIVISTIC DENSITY-FUNCTIONAL THEORY STUDY

Applying relativistic density functional theory to isomers of C 78 and M 2@ C 78( M = La , Ce ), we calculate and analyze the relative energies and HOMO–LUMO gaps of neutral and hexaanion ( -6 charged) C 78 isomers. Our results indicate that the [Formula: see text] (5) isomer is the most stable, and it illustrate that electron transfer plays an important role in controlling the stability of endohedral metallofullerenes. We also calculate the electronic structures of there neutral isomers, and based on their LUMO + 2 and LUMO + 3 gaps, we explain why it is easier to encage two metal atoms in D3h′ (78:5). To further elucidate this issue, we theoretically characterize M 2@ C 78( M = La , Ce ) and compare the relative energies and the HOMO–LUMO gap of the two isomers M 2@ C 78 (4) and the M 2@ C 78 (5) ( M = La , Ce ). The results indicate that M 2@ C 78 (5) is more stable than M 2@ C 78 (4). Furthermore, the good agreement between the experimental and computed 13C NMR chemical shift of the isomer M 2@ C 78 (5) provided strong evidence that M 2@ C 78 forms a D3h′ (78:5) cage.

The way of stabilizing non-IPR fullerenes and structural elucidation of C54Cl8

Recently, a new non-IPR chlorofullerene C(54)Cl(8) was isolated experimentally (Science 2004, 304, 699). To explore the ways to stabilize non-IPR fullerenes, the authors studied all of the possible isomers of C(54) fullerene and some of the C(54)Cl(8) isomers at PM3, B3LYP/3-21G, and B3LYP/6-31G* levels. Combined with analysis of pentagon distributions, bond resonance energies, and steric strains, C(54):540 with the least number of 5/5 bonds was determined to be the thermodynamically best isomer for the C(54)Cl(8). Based on C(54):540, the most probable structure of the experimental C(54)Cl(8) was elucidated. The results suggested one of the necessary conditions of stabilizing non-IPR fullerenes: chemical derivatizations of either endohedral complexation or exohedral addition need to sufficiently stabilize all of the kinetically unstable 5/5 bonds of the cages.

Dimetalloendofullerene U2@C60 Has a U−U Multiple Bond Consisting of Sixfold One-Electron-Two-Center Bonds

Endohedral metallofullerenes (EMFs) have been extensively studied since their discovery in 1985. Metal-metal bonds, nevertheless, have never been explicitly observed in EMFs synthesized so far. In this contribution, we show by means of all-electron relativistic density functional computations that the dimetalloendofullerene, U(2)@C(60), has an unprecedented U-U multiple bond consisting solely of sixfold ferromagnetically coupled one-electron-two-center bonds with the electronic configuration (5fpi(u))(2)(5fsigma(g))(1)(5fdelta(g))(2)(5fphi(u))(1), which are dominated by the uranium 5f atomic orbitals. This bonding scheme is completely distinct from the metal-metal bonds discovered thus far in the d- and f-block polynuclear metal complexes. This finding initiates a connection of the metal-metal multiple bonding chemistry and the fullerene chemistry.

Tb3N@C84: An Improbable, Egg-Shaped Endohedral Fullerene that Violates the Isolated Pentagon Rule

The structure of isomer 2 of Tb3N@C84 has been determined through single-crystal X-ray diffraction on Tb3N@C84.NiII(OEP).2(C6H6). The carbon cage has a distinct egg shape due to the presence of a single pair of fused pentagons at one apex of the molecule. Thus, although 24 IPR structures are available to the C84 cage, Nature utilizes one of the 51 568 isomeric structures that do not conform to the IPR for this unusual molecule. The Tb3N portion of isomer 2 of Tb3N@C84 is strictly planar. One Tb atom is nestled within the fold of the fused pentagons, while the other Tb atoms are disordered over four pairs of sites.

Observation of hydrogen in deuterated methane hydrate by maximum entropy method with neutron powder diffraction

The crystal structure of deuterated methane hydrate (structure I, space group: Pm(-)3n) was investigated by neutron powder diffraction at temperatures of 7.7-185 K. The scattering amplitude density distribution was examined by a combination of Rietveld method and maximum entropy method (MEM). The distribution of the D atoms in both D(2)O and CD(4) molecules was clarified from three-dimensional graphic images of the scattering amplitude density. The MEM results showed that there were low-density sites for the D atom of D(2)O in a particular location within the D(2)O cage at low temperatures. The MEM provided more reasonable results because of the decrease in the R factor that is attainable by this method. Accordingly, the low-density sites for the D atom of D(2)O probably exist within the D(2)O cage. This suggests that a spatial disorder of the D atom of D(2)O occurs at these sites and that hydrogen bonds between D(2)O molecules become partially weakened. With regard to the CD(4) molecules, there were high-density sites for the D atom of CD(4), and the density distribution of the C and D atoms was observed separately in the scattering amplitude density image. Consequently, the C-D bonds of CD(4) were not observed clearly because the CD(4) molecules had an orientational disorder. The D atoms of CD(4) were displaced from the line between the C and O atoms, and were located near the face center of the polygon in the cage. Accordingly, the D atoms of CD(4) were not bonded to specific O atoms. This result is consistent with the hydrophobicity of the CD(4) molecule. We also report the difference between the small and the large cages in the density distribution map and the temperature dependence of the scattering amplitude density.

Structure and electronic properties of fullerenes C(52)q+: is C(52)2+ an exception to the pentagon adjacency penalty rule?

The structure, vibrational spectra and electronic properties of the neutral, singly and doubly charged C52 fullerenes were studied by means of the Hartree-Fock method and density functional theory. Different isomers were considered, in particular those with the lowest possible number (five or six) of adjacent pentagons, and an isomer with a four-atom ring. For neutral and singly charged species, the most stable isomer is that with the lowest number of adjacent pentagons, namely five. However, for C(52)2+, the most stable structure has six adjacent pentagons. This finding, which contradicts the pentagon adjacency penalty rule, is a consequence of complete filling of the HOMO pi shell and the near-perfect sphericity of the most stable isomer. The simulated vibrational spectra show important differences in the positions and intensities of the vibrations for the different isomers.

La2@C72 and Sc2@C72: Computational Characterizations

The La2@C72 and Sc2@C72 metallofullerenes have been characterized by systematic density functional computations. On the basis of the most stable geometry of 39 C72 hexaanions and the computed energies of the best endofullerene candidates, the experimentally isolated La2@C72 species was assigned the structure coded #10611. The good agreement between the computed and the experimental 13C chemical shifts for La2@C72 further supports the literature assignment (Kato, H.; Taninaka, A.; Sugai, T.; Shinohara, H. J. Am. Chem. Soc. 2003, 125, 7782). The geometry, IR vibrational frequencies, and 13C chemical shifts of Sc2@C72 were predicted to assist its future experimental characterization.

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).

Ti2C80 is more likely a titanium carbide endohedral metallofullerene (Ti2C2)@C78

We show by means of density functional calculations that the previously synthesized metallofullerene Ti2C80 does not take the form of Ti2@C80, but is a titanium carbide endohedral metallofullerene, Ti2C2@C78, that has a C78(6-)(D3h) cage which follows faithfully the stable closed-shell electronic rule.

The Structure of Ba@C74

The title compound has been produced by using the radio frequency (RF) method. Barium and carbon were evaporated simultaneously under dynamic flow of helium at different temperatures. About 0.5 mg of pure Ba@C(74) was isolated via a three-step high-pressure liquid chromatography separation. For the first time, the structure of a monometallofullerene has been analyzed by means of single-crystal synchrotron diffraction on microcrystals of Ba@C(74).Co(OEP).2C(6)H(6) (Co(II)(OEP): cobalt(II) octaethylporphyrin) at 100 K. The monometallofullerene exhibits a high degree of localization of the endohedral metal ion, with just two split positions for Ba and two orientations of the C(74)-cage. The barium atom is localized inside the C(74)-cage and displaced off-center, toward the Co(OEP) molecule (d approximately 127 pm). The shortest Ba-C distance is 265 pm. The Co(OEP) molecules form dimers in which the coordination of the cobalt is (4 + 1). Due to the all-syn conformation of the ethyl groups, each Co(OEP) molecule of the dimer coordinates one C(74)-fullerene. The units (Ba@C(74))[Co(OEP)](2)(Ba@C(74)) are arranged in a distorted primitive hexagonal packing. The free space between these complex units is filled by benzene molecules of crystallization. The Ba L(III) XANES spectrum of a thin film sample of Ba@C(74) exhibits a pronounced double maximum structure at about E = 5275 eV. The comparison of the shape resonances of the experimental data with simulated XANES spectra, based on different exo- and endohedral structure models, confirm that the Ba atom is located inside the C(74)-cage (D(3)(h)()) in an off-center position. The Ba atom is shifted by about 130-150 pm from the geometric center of the C(74)-cage. This is in good agreement with quantum chemical results. Thus, despite the disorder still present, a consistent and conclusive structure model for the title compound has been derived by employing a combination of X-ray diffraction, XANES spectroscopy, and quantum chemical calculations.