High catalytic rates for hydrogen production using nickel electrocatalysts with seven-membered cyclic diphosphine ligands containing one pendant amine

A series of Ni-based electrocatalysts, [Ni(7P^Ph ₂N^C6H4X)₂](BF₄)₂, featuring seven-membered cyclic diphosphine ligands incorporating a single amine base, 1-para-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane (7P ^Ph₂N^C6H4X, where X = OMe, Me, Br, Cl, or CF₃), have been synthesized and characterized. X-ray diffraction studies have established that the [Ni(7P^Ph₂N ^C6H4X)₂]²⁺ complexes have a square planar geometry, with bonds to four phosphorus atoms of the two bidentate diphosphine ligands. Each of the complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni(II/I) couple, with turnover frequencies ranging from 2400 to 27 000 s^-1 with [(DMF)H]⁺ in acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis, giving turnover frequencies ranging from 4100 to 96 000 s^-1. Computational studies carried out on the [Ni(7P^Ph₂N^C6H4X) ₂]²⁺ family indicate the catalytic rates reach a maximum when the electron-donating character of X results in the pK_a of the Ni(I) protonated pendant amine matching that of the acid used for proton delivery. Additionally, the fast catalytic rates for hydrogen production by the [Ni(7P^Ph₂N^C6H4X)₂]²⁺ family relative to the analogous [Ni(P^Ph₂N ^C6H4X₂)₂]²⁺ family are attributed to preferred formation of endo protonated isomers with respect to the metal center in the former, which is essential to attain suitable proximity to the reduced metal center to generate H₂. The results of this work highlight the importance of precise pK_a matching with the acid for proton delivery to obtain optimal rates of catalysis.