[Cr(8)(PhCO(2))(16)O(4)].4CH(3)CN.2H(2)O: structural origin of magnetic anisotropy in a molecular spin cluster

The Cr(4)O(4) hetero-cubane-centered octachromium(III) cluster [Cr(8)(PhCO(2))(16)O(4)] crystallizes from fluorobenzene-acetonitrile as dodeca-mu(2)-benzoato-tetrabenzoatotetra-mu(4)-oxido-octachromium(III) acetonitrile tetrasolvate dihydrate, [Cr(8)(C(7)H(5)O(2))(16)O(4)].4C(2)H(3)N.2H(2)O, (I). Crystals produced by this method are significantly more stable than the originally published dichloromethane pentasolvate, [Cr(8)(PhCO(2))(16)O(4)].5CH(2)Cl(2) [Atkinson et al. (1999). Chem. Commun. pp. 285-286], leading to a significantly higher quality structure and allowing the production of large quantities of high-quality nondeuterated and deuterated material suitable for inelastic neutron scattering (INS) measurements. Compound (I) reveals a higher symmetry structure in which the cluster sits on a twofold rotation axis, and is based on an asymmetric unit containing four crystallographically independent Cr positions, two oxide ligands, eight benzoate ligands, two acetonitrile solvent molecules and one disordered water molecule. All the Cr atoms are six-coordinate, with an octahedral geometry for the inner cubane and a more highly distorted coordination environment in the outer positions. Despite the higher symmetry, the coordination geometries observed in (I) are largely similar to the dichloromethane pentasolvate structure, indicating that crystal-packing effects have little influence on the molecular structure of [Cr(8)(PhCO(2))(16)O(4)]. Close structural analysis reveals that the high magnetic anisotropy observed in the INS measurements is a consequence of the distorted coordination geometry of the four outer Cr atoms.