Preparation and Characterization of New Ruthenium and Osmium Containing Polyoxometalates, [M (DMSO)3Mo7O24]4- (M = Ru(II), Os(II)), and Their Use as Catalysts for the Aerobic Oxidation of Alcohols

A new heptamolybdate polyoxometalate structure containing ruthenium(ll) or osmium(ll) metal centers, [M(11)(DMSO)3Mo7O24] (M = Ru, Os), was synthesized by reaction between (NH4)6Mo7O24 and cis-M(DMSO)4Cl2. X-ray structure analysis revealed the complexes to contain a ruthenium/osmium center in a trigonal antiprismatic coordination mode bound to three DMSO moieties via the sulfur atom of DMSO and three oxygen atoms of the new heptamolybdate species. The heptamolybdate consists of seven condensed edge-sharing MoO6 octahedra with C symmetry. Three Mo atoms are in classic type II octahedra with a cis dioxo configuration. Two Mo atoms are also type-ll-like, but one of the short Mo-O bonds is associated with bridging oxygen atoms rather than terminal oxygen atoms. Two molybdenum atoms are unique in that they are in a trigonally distorted octahedral configuration with three short Mo-O bonds and two intermediate-long M-O bonds and one long Mo-O bond. The [M(ll)(DMSO)3Mo7024 polyoxometalates were effective and in some cases highly selective catalysts for the aerobic oxidation of alcohols to ketones/aldehydes. The integrity of the polyoxometalate was apparently retained at high turnover numbers and throughout the reaction, and a variation of an oxometal type mechanism was proposed to explain the results.