Synthesis, Characterization, and Electrochemical Properties of Tri- and Tetranuclear CoIII-Oxo Complexes

Heterocubane-type complexes containing a central [Co^III₄O₄] core have received increasing interest as surface cutouts of active spinel-type Co₃O₄ water splitting catalysts. The synthesis of a series of tetranuclear ([Co₄O₄(O₂CR)₄(py)₄] (R = Me 1, Et 2, n-Pr 3)) and trinuclear Co(III) carboxylate complexes [Co^III₃(μ₃–O)(μ₂–OH)(O₂CR)₃(py)₅](NO₃)₂ (R = i-Pr 4, t-Bu 5) and their structural (single-crystal X-ray diffraction) and spectroscopic (except 5, ¹H NMR, UV/vis, IR) characterization is reported. Quantum chemical calculations using density functional theory show that the relative stability of the tetranuclear complexes versus the trinuclear complexes decreases with increasing steric demand of the carboxylate ligand. In addition, the UV/vis and IR spectra of the tetranuclear complexes are calculated to deconvolute contributions from the carboxylate ligands and the Co-oxo core. Furthermore, the electrochemical properties of the complexes 1–4 are studied by cyclic voltammetry, demonstrating that the redox potential of the complexes correlates with the steric demand of the carboxylate ligand.