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(7PPh2NC6H4X)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
(7PPh2NC6H4X, where X = OMe, Me,
Br, Cl, or CF3), have been synthesized and characterized.
X-ray diffraction studies have established that the [Ni­(7PPh2NC6H4X)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(7PPh2NC6H4X)2]2+ family
indicate the catalytic rates reach a maximum when the electron-donating
character of X results in the pKa 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(7PPh2NC6H4X)2]2+ family relative to the analogous [Ni­(PPh2NC6H4X2)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 pKa matching with the acid for
proton delivery to obtain optimal rates of catalysis.