Catalytic Properties of Cobalt(III)−Oxo Cubanes in the TBHP Oxidation of Benzylic Alcohols

Two series of oxo-bridged Co(III) complexes of the type Co4O4(O2CC6H4-X)4(py)4 (1) and Co4O4(O2CC6H4-X)4(4-Mepy)4 (2), where X = H (a), Me (b), MeO (c), Cl (d), NO2 (e), have been synthesized and characterized in detail. The molecular structures of the complexes consist of a cubelike Co4(μ3-O)4 core having Co and O atoms at alternate vertices with carboxylato ligands bridging the Co3+ ions along four face diagonals of the approximate cube. Nitrogen atoms from pyridyl ligands complete the distorted-octahedral coordination around each Co(III). These neutral Co(III) complexes undergo a nearly reversible one-electron oxidation involving the redox couple [Co4III(μ3-O)4]4+ ↔ [Co3IIICoIV(μ3-O)4]5+ at potentials (∼0.7−1.0 V) that linearly depend on the electronic influence of X. The cobalt(III) clusters of types 1 and 2 have been found to effectively promote the TBHP oxidation of benzylic alcohols under homogeneous conditions to produce the corresponding carbonyl compounds.