Insights into Formate Oxidation by a Series of Cobalt Piano-Stool Complexes Supported by Bis(phosphino)amine Ligands

A series of (cyclopentadienyl)cobalt(III) half-sandwich complexes (1-4) supported by bidentate bis(phosphino)amine ligands was synthesized and characterized by NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The Co^III-hydride complex 4-H bearing the bis(cyclohexylphosphine) ligand derivative was successfully isolated via protonation of the neutral reduced Co^I complex 5 with a weak acid. Experimental and computational methods were used to determine the thermodynamic hydride accepting ability of these Co^III centers and to evaluate their reactivity toward the oxidation of formate. We find that the hydride accepting ability of 1-4 ranges from 71 to 74 kcal/mol in acetonitrile, which should favor a highly exergonic reaction with formate through direct hydride transfer. Formate oxidation was demonstrated at elevated temperatures in the presence of stoichiometric quantities of 4, generating carbon dioxide and the Co^III-hydride complex 4-H in 72% yield.