Dinuclear Co(II)Co(III) Mixed-Valence and Co(III)Co(III) Complexes with N- and O-Donor Ligands: Characterization and Water Oxidation Studies

The tridentate ligand N-methyl-N,N-bis(2-pyridylmethyl)amine (L) has been employed to synthesize a dinuclear Co(II)Co(III) mixed-valence complex containing mu-methoxo and mu-carboxylato bridging ligands, [LCoII(mu-carboxylato)bis(mu-methoxo)(CoL)-L-III](ClO4)(2). In this complex, the two pseudo-octahedral Co centers have an identical ligand environment, yet the average Co-N and Co-O bond distances at the two Co ions differ significantly. Electrochemical, spectroscopic, and magnetic susceptibility measurements confirm that it belongs to a localized Class II mixed-valence system, despite the presence of a short Co center dot center dot center dot Co distance of 3.021 angstrom. Oxidation of this Co(II)Co(III) complex leads to formation of the corresponding Co(III)Co(III) complex that was characterized structurally and spectroscopically. In addition, dinuclear and trinuclear,mu-hydroxo Co(III) complexes have been obtained in the presence of phosphate anions and absence of methanol, respectively, suggesting that an additional bridging ligand is needed to stabilize the Co(III)bis(mu-hydroxo)Co-III fragment. Moreover, the ability of the mixed-valence Co(II)Co(III) complex and the three related Co(III) complexes to electrocatalytically oxidize water was also investigated. The observed limited water oxidation catalytic ability for these systems suggests that a multinuclear Co cluster and/or presence of O-rich ligands may be needed for the generation of efficient molecular Co-based water oxidation catalysts.