Cobalt(III) Corrolato Complexes with Tailored Secondary Spheres: Catalytic Implications for Water Oxidation

Water electrolysis offers a sustainable route to hydrogen and oxygen production, yet its efficiency remains a bottleneck. Cobalt-based molecular complexes are among the most promising catalysts for the oxygen evolution reaction. Here, we report the synthesis and structural, spectroscopic, electrochemical, and computational characterization of two cobalt(III) corrolato complexes differing in their meso-aryl substituents: a pendant 2-(3-(2-formylphenoxy)propoxy)phenyl unit capable of hydrogen bonding and a 2-methoxyphenyl analogue. XRD reveals that the formyl-functionalized aryl ether is laterally displaced from the corrole plane. CV shows a Co(III/II) couple at -0.60 V (vs FeCp₂^+/0) and two corrole-centered oxidations at +0.03 and +0.65 V, corroborated by spectroelectrochemistry. DFT supports a singlet [Co^III(corrole^3-)(py)₂]⁰ ground state, a spin 1/2 intermediate, and a fully oxidized singlet [Co^III(corrole^1-)(py)₂]²⁺ species. The formyl-substituted complex exhibits enhanced OER performance, with a Faradaic efficiency of 74% and a turnover frequency (TOF) of ∼0.89 s^-1, surpassing the methoxy analogue (53%, TOF ∼0.43 s^-1). CPE at 0.95 V demonstrates sustained activity, with oxygen generation verified by fluorometrically and GC. Mechanistic studies indicate that O-O bond formation proceeds via nucleophilic attack on a cobalt-oxo intermediate, with DFT calculations revealing that the aldehydic substituent stabilizes water binding through hydrogen bonding. These results highlight secondary-sphere effects in cobalt corrole-based water oxidation catalysis.