Hydrogen Production Using Nickel Electrocatalysts with Pendant Amines: Ligand Effects on Rates and Overpotentials

Abstract

A Ni-based electrocatalyst for H 2 production, [Ni(8P Ph 2 N C 6 H 4 Br ) 2 ](BF 4 ) 2, featuring eight-membered cyclic diphosphine ligands incorporating a single amine base, 1- para -bromophenyl-3,7-triphenyl-1-aza-3,7-diphosphacycloheptane (8P Ph 2 N C 6 H 4 Br ) has been synthesized and characterized. X-ray diffraction studies reveal that the cation of [Ni(8P Ph 2 N C 6 H 4 Br ) 2 (CH 3 CN)](BF 4 ) 2 has a distorted trigonal bipyramidal geometry. In CH 3 CN, [Ni(8P Ph 2 N C 6 H 4 Br ) 2 ] 2+ is an electrocatalyst for reduction of protons, and it has a maximum turnover frequency for H 2 production of 800 s –1 with a 700 mV overpotential (at E cat/2 ) when using [(DMF)H]OTf as the acid. Addition of H 2 O to acidic CH 3 CN solutions of [Ni(8P Ph 2 N C 6 H 4 Br ) 2 ] 2+ results in an increase in the turnover frequency for H 2 production to a maximum of 3300 s –1 with an overpotential of 760 mV at E cat/2 . Computational studies carried out on [Ni(8P Ph 2 N C 6 H 4 Br ) 2 ] 2+ indicate the observed catalytic rate is limited by formation of nonproductive protonated isomers, diverting active catalyst from the catalytic cycle. The results of this research show that proton delivery from the exogenous acid to the correct position on the proton relay of the metal complex is essential for fast H 2 production.