PNNL

Abstract

A series of mononuclear nickel(II) bis(diphosphine) complexes [Ni(PPh2NPhX2)2](BF4)2 (PPh2NPhX2 = 1,5-di(para¬-X-phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; X = OMe, Me, CH2P(O)(OEt)2, Br, and CF3) have been synthesized and characterized. X-ray diffraction studies reveal that [Ni(PPh2NPhMe2)2](BF4)2 and [Ni(PPh2NPhOMe2)2](BF4)2 are tetracoordinate with distorted square planar geometries. The Ni(II/I) and Ni(I/0) redox couples of each complex are electrochemically reversible in acetonitrile (0.2 M tetraethylammonium tetrafluoroborate) with potentials that are increasingly cathodic as the electron-donating character of X is increased. All of these complexes are efficient electrocatalysts for hydrogen production, with rates generally increasing as the electron-donating character of X is decreased. Catalytic studies using 2,6-dichloroanilinium triflate (2,6-Cl2AnH+OTf , pKaMeCN = 5.0) 4-cyanoanilinium tetrafluoroborate (4-CNAnH+OTf , pKaMeCN = 7.0) and protonated dimethylformamide ([(DMF)H]+OTf , pKaMeCN = 6.1) reveal that turnover frequencies do not correlate with substrate acid pKa values, but are highly dependent on the acid structure, with this effect being related to substrate size. Addition of water is shown to dramatically increase catalytic rates for all catalysts. With [Ni(PPh2NPhCH2P(O)(OEt)22)2](BF4)2 using [(DMF)H]+OTf as acid and with added water, a turnover frequency of 1850 s-1 was obtained. 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.