Incorporating peptides in the outer-coordination sphere of bioinspired electrocatalysts for hydrogen production

Four new cyclic 1,5-diaza-3,7-diphosphacyclooctane ligands have been prepared and used to synthesize [Ni(P^Ph₂N^R ₂)₂]²⁺ complexes in which R is a mono- or dipeptide. These complexes represent a first step in the development of an outer-coordination sphere for this class of complexes that can mimic the outer-coordination sphere of the active sites of hydrogenase enzymes. Importantly, these complexes retain the electrocatalytic activity of the parent [Ni(P^Ph₂N^Ph₂)₂] ²⁺ complex in an acetonitrile solution with turnover frequencies for hydrogen production ranging from 14 to 25 s^-1 in the presence of p-cyanoanilinium trifluoromethanesulfonate and from 135 to 1000 s^-1 in the presence of protonated dimethylformamide, with moderately low overpotentials, ∼0.3 V. The addition of small amounts of water results in rate increases of 2-7 times. Unlike the parent complex, these complexes demonstrate dynamic structural transformations in solution. These results establish a building block from which larger peptide scaffolding can be added to allow the [Ni(P^R₂N^R′₂) ₂]²⁺ molecular catalytic core to begin to mimic the multifunctional outer-coordination sphere of enzymes.