Dinuclear Cobalt Complexes for Homogeneous Water Oxidation: Tuning Rate and Overpotential through the Non-Innocent Ligand

In this study, dinuclear cobalt complexes (1 and 2) featuring bis(benzimidazole)pyrazolide-type ligands (H2L and Me2L) were prepared and evaluated as molecular electrocatalysts for water oxidation. Notably, 1 bearing a non-innocent ligand (H2L) displayed faster catalytic turnover than 2 under alkaline conditions, and the base dependence of water oxidation and kinetic isotope effect analysis indicated that the reaction mediated by 1 proceeded by a different mechanism relative to 2. Spectroelectrochemical, cold-spray ionization mass spectrometric and computational studies found that double deprotonation of 1 under alkaline conditions cathodically shifted the catalysis-initiating potential and further altered the turnover-limiting step from nucleophilic water attack on (H2L)Co-2(III)(superoxo) to deprotonation of (L)Co-2(III)(OH)(2). The rate-overpotential analysis and catalytic Tafel plots showed that 1 exhibited a significantly higher rate than previously reported Ru-based dinuclear electrocatalysts at similar overpotentials. These observations suggest that using non-innocent ligands is a valuable strategy for designing effective metal-based molecular water oxidation catalysts.

Automated summary

This study synthesized dinuclear cobalt complexes with bis(benzimidazole)pyrazolide-type ligands and evaluated their performance as molecular electrocatalysts for water oxidation. The results showed that the complex with a non-innocent ligand exhibited faster catalytic turnover than the other complex under alkaline conditions, suggesting a different mechanism. This study highlights the use of non-innocent ligands as a valuable strategy for designing effective metal-based molecular water oxidation catalysts.