Metal-organic microporous materials (MOMs) have attracted wide scientific attention owing to their unusual structure and properties, as well as commercial interest due to their potential applications in storage, separation and heterogeneous catalysis. One of the advantages of MOMs compared to other microporous materials, such as activated carbons, is their ability to exhibit a variety of pore surface properties such as hydrophilicity and chirality, as a result of the controlled incorporation of organic functional groups into the pore walls. This capability means that the pore surfaces of MOMs could be designed to adsorb specific molecules; but few design strategies for the adsorption of small molecules have been established so far. Here we report high levels of selective sorption of acetylene molecules as compared to a very similar molecule, carbon dioxide, onto the functionalized surface of a MOM. The acetylene molecules are held at a periodic distance from one another by hydrogen bonding between two non-coordinated oxygen atoms in the nanoscale pore wall of the MOM and the two hydrogen atoms of the acetylene molecule. This permits the stable storage of acetylene at a density 200 times the safe compression limit of free acetylene at room temperature.

Highly controlled acetylene accommodation in a metal–organic microporous material

Accurate charge-density distributions of cubic and tetragonal ${\mathrm{PbTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$ have been obtained by the MEM(maximum entropy method)/Rietveld analysis using synchrotron-radiation powder data. The Pb-O bonds in tetragonal ${\mathrm{PbTiO}}_{3}$ show rather strong covalency, while those in cubic ${\mathrm{PbTiO}}_{3}$ are ionic. This is the clear evidence of the Pb-O hybridization in tetragonal ${\mathrm{PbTiO}}_{3}$, which has been theoretically predicted as a key factor of much larger ferroelectricity of this substance than that of ${\mathrm{BaTiO}}_{3}$. Tetragonal ${\mathrm{PbTiO}}_{3}$ forms a layered structure of a two-dimensional covalent-bonding network consisting of the $\mathrm{Ti}\ensuremath{-}{O}_{5}$ pyramid.

Evidence for Pb-O Covalency in Tetragonal PbTiO 3

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)

Current status and future developments of endohedral metallofullerenes

Single-component molecular metals with extended-TTF dithiolate ligands.

Carceplexes and Hemicarceplexes

THE synthesis of fullerenes encapsulating various metal atoms within the carbon cage (endohedral metallofullerenes) has stimulated wide interest1,2 because of their unusual structural and electronic properties. Most of the metallofullerenes prepared so far have been based on C82, and have incorporated lanthanum1,3a¤-5, yttrium6,7, scandium8a¤-10 and most of the lanthanide elements11,12′. Although there has been some debate about the endohedral nature of these compounds2,13,14, observations using scanning tunnelling microscopy15,16, extended X-ray absorption fine structure17,18, transmission electron microscopy19 and electron spin resonance3,6a¤-8,10 have strongly suggested that the metal atoms are indeed inside the fullerene cages; theoretical calculations20,21 also indicate that this is the case. But until now, no structural model has been derived experimentally to confirm the endohedral nature of the metallofullerenes. Here we report the results of a synchrotron X-ray powder diffraction study of Y@C82 that confirms that the yttrium atom is located within the carbon cage. The yttrium atom is displaced from the centre of the C82 molecule and is strongly bound to the carbon cage.

Confirmation by X-ray diffraction of the endohedral nature of the metallofullerene Y@C82

The endohedral nature of dimetallofullerene Sc{sub 2}{at}C{sub 84} is determined for the first time by a method which is a combination of the maximum entropy method (MEM) and the Rietveld refinement from synchrotron powder diffraction data. The obtained MEM charge density clearly shows the D{sub 2d}-symmetry cage structure, indicating the rotation of Sc{sub 2}{at}C{sub 84} molecules in solid state is almost quenched even at room temperature. From the MEM charge density, the encapsulated Sc-Sc distance and the nearest Sc-C distance are 3.9(1) and 2.4(1)A, respectively. {copyright} {ital 1997} {ital The American Physical Society}

Structure of endohedral dimetallofullerene Sc2@C84

A charge density study of alkali metal doped fullerenes, Rb2CsC60, K2RbC60 and Li2CsC60, was carried out at room temperature by the Maximum Entropy Method (MEM) combined with the Rietveld analysis using synchrotron powder data. The MEM charge densities of the C60 molecules show distinct differences related to the superconducting properties. For both K2RbC60 and Rb2CsC60, which are superconductors at low temperature, the C60 molecules were in a merohedral disorder. On the other hand, the C60 molecules in Li2CsC60; non-superconductor even at very low temperature, were in an orientational disorder. In the interatomic region between the C60 molecules and doped metal atoms, any significant overlapped charge densities related to covalent bond were not found for all three materials. This is the clear evidence for that superconductivity of these materials and covalency of doped metals have no correlation at all. Instead, the charge deficiencies of the doped metal atoms which should be associated with charge transfer from the metal atoms to the C60 molecule seems to have a strong correlation to the superconducting transition temperature, Tc, i.e. the higher the Tc, the more the deficiencies.

The MEM Charge Density Study of Rb2CsC60, K2RbC60, and Li2CsC60by Synchrotron Radiation Powder Method

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Confirmation by X-ray diffraction of the endohedral nature of the metallofullerene Y@C82

Structure of endohedral dimetallofullerene Sc2@C84

The MEM Charge Density Study of Rb2CsC60, K2RbC60, and Li2CsC60by Synchrotron Radiation Powder Method