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

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Heterocubane‐type complexes containing a central [CoIII4O4] 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 [CoIII3(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.