Electrocatalytic Hydrogen Production by [Ni(7PPh2NH)2]2+: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni(II) complex, [N-i(7P(2)(Ph)N(H))(2)H](3+) (7P(2)(Ph)N(HH) = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P(2)(Ph)N(HH) ligand features an NH, ensuring properly positioned protonated amine groups (NH+) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H-2 production in the presence of organic acids (pK(a) range 1013 in CH3CN), with turnover frequencies ranging from 160 to 780 s(1) at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni((P2N2R')-N-Ph)(2)](2+) ((P2N2R')-N-Ph = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni(7P(2)(Ph)N(R'))(2)](2+) complexes, catalytic turnover frequencies for H-2 production by [Ni(7P(2)(Ph)N(H))(2)](2+) do not show catalytic rate enhancement upon the addition of H2O. This finding supports the assertion that [Ni(7P(2)(Ph)N(H'))(2)](2+) eliminates the distinction between the endo- and exo-protonation isomers.