PNNL

Abstract

Four new cyclic 1,5-diaza-3,7-diphosphacyclooctane ligands have been prepared and used to synthesize [Ni(PPh2NR2)2]2+ complexes in which R is a mono- or dipeptide. These complexes represent a first step in developing 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(PPh2NPh2)2]2+ complex in acetonitrile solution with turnover frequencies (TOF) for hydrogen production ranging from 14 to 25 s-1 in the presence of p-cyanoanilinium trifluoromethanesulphonic acid and 135-1000 s-1 in the presence of triflic acid salt of protonated dimethylformamide, with moderately low overpotentials, ~0.3 V. The addition of small amounts of water result in rate increases of 5-7 times. Unlike the parent complex, these complexes demonstrate dynamic structural transformations in solution whereby the dipeptide tail interacts with the nickel center. These results establish a building block from which larger peptide scaffolding can be added to allow the [Ni(PR2NR’2)2]2+ molecular catalytic core to begin to mimic the multifunctional outer coordination sphere of enzymes. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.