Efficient homogeneous electrochemical water oxidation catalysed by macrocyclic nickel complexes with redox non-innocent pyridine coordination structures†

Efficient electrocatalysts are anticipated to mitigate the high overpotential and slow kinetics of water splitting, which is a feasible way to produce hydrogen as an environmentally friendly renewable fuel. However, the development of low-cost catalysts with high activity and stability is still challenging. Herein, a Ni complex, [Ni(Me₃pyclen)(CH₃CN)₂](ClO₄)₂ (1), with a macrocyclic pyridine-triamine ligand was developed as an efficient molecular catalyst for electrocatalytic water oxidation, which occurred at an onset overpotential of only 520 mV and attained a high faradaic efficiency of 93% under neutral conditions. The single-site catalytic mechanism involving proton-coupled electron transfer (PCET) processes of both the Ni center and the ligand was proposed based on the electrochemical test results. Furthermore, comparative studies on the catalytic behaviors of 1 and its derivative all-amine coordinated Ni complex [Ni(12-TMC)(OAc)]PF₆ (2) illustrated that the stability of 1 was dependent on the hybridization form of the coordinated nitrogen atom, avoiding the decomposition of 2 into nickel-hydroxides during oxygen evolution. Therefore, the pyridine-triamine ligand shows its superiority in constructing a homogeneous electrochemical water oxidation system over the all-amine-based ligand.