The amino terminal copper and nickel binding (ATCUN) motif consists of an N-terminal X-X-His tripeptide that coordinates metals through the N-terminal nitrogen, two peptide backbone nitrogens, and the histidine Nδ. We have developed cobalt derivatives of ATCUN tripeptides (CoXXH) as catalysts for proton reduction to hydrogen and nitrite reduction to hydroxylamine and ammonium in water. As an electrocatalyst, CoGGH evolves hydrogen at a rate that is strongly dependent on the nature of the proton donor. While CoGGH is active in water alone, adding protonated acids (buffer acids) enhances the catalytic rate constant kobs by an amount proportional to the pKa of the acid; this Brønsted relationship is consistent with a proton transfer from the acid concerted with an electron transfer in the transition state. Modification of the CoXXH motif to append internal proton shuttles enhances activity in water, further demonstrating the importance of the pKa of the proton-donating group in catalysis. These cobalt peptides furthermore are shown to be active in systems for photocatalytic hydrogen production from neutral water when paired with [Ru(bpy)3]2+ as a photosensitizer and ascorbate as an electron donor; notably, the activity of the system is limited by the photosensitizer rather than the catalyst. Finally, the activity of CoGGH and related derivatives toward the multielectron reduction of nitrite will be presented. Notably, CoGGH reduces nitrite as well as nitric oxide and hydroxylamine to ammonium, mimicking the activity of cytochrome c nitrite reductase. As with its activity toward proton reduction, the nature of the proton donor in this multi-proton reaction is shown to be critical in determining activity.

Cobalt metallopeptide catalysts for hydrogen evolution and nitrite reduction

ACS Spring 2022

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