Nickel is a Different Pickle; Trends in Water Oxidation Catalysis for Molecular Nickel Complexes.

The development of novel water oxidation catalysts is important in the context of renewable fuels production. Ligand design is one of the key tools to improve the activity and stability of molecular catalysts. The establishment of ligand design rules can facilitate the development of improved molecular catalysts. In this paper we show that chemical oxidants can be used to probe oxygen evolution activity for nickel-based systems and report trends that can improve future ligand design. Interestingly, we observe different ligand effects in comparison to other first-row transition metal complexes. For example, nickel complexes with secondary amine donors are more active than with tertiary amine donors, which is the opposite for iron complexes. The incorporation of imine donor groups in a cyclam ligand results in the fastest and most durable nickel catalyst of our series, achieving oxygen evolution turnover numbers up to 380 and turnover frequencies up to 68 min-1 in a pH 5.0 acetate buffer using Oxone as oxidant. Initial kinetic experiments with this catalyst reveal a first order in chemical oxidant and a half order in catalyst. This implies a rate determining oxidation step from a dimeric species that needs to break up to generate the active catalyst. These findings lay the foundation for the rational design of molecular nickel catalysts for water oxidation, and highlight that catalyst design rules are not generally applicable for different metals.