Hydrogen Production Using Nickel Electrocatalysts
with Pendant Amines: Ligand Effects on Rates and Overpotentials
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Abstract
A Ni-based electrocatalyst for H<sub>2</sub> production, [Ni(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub>,
featuring eight-membered cyclic
diphosphine ligands incorporating a single amine base, 1-<i>para</i>-bromophenyl-3,7-triphenyl-1-aza-3,7-diphosphacycloheptane (8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>) has
been synthesized and characterized. X-ray diffraction studies reveal
that the cation of [Ni(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>(CH<sub>3</sub>CN)](BF<sub>4</sub>)<sub>2</sub> has a distorted trigonal bipyramidal geometry. In CH<sub>3</sub>CN, [Ni(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> is an electrocatalyst
for reduction of protons, and it has a maximum turnover frequency
for H<sub>2</sub> production of 800 s<sup>–1</sup> with a 700
mV overpotential (at <i>E</i><sub>cat/2</sub>) when using
[(DMF)H]OTf as the acid. Addition of H<sub>2</sub>O to acidic CH<sub>3</sub>CN solutions of [Ni(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> results in
an increase in the turnover frequency for H<sub>2</sub> production
to a maximum of 3300 s<sup>–1</sup> with an overpotential of
760 mV at <i>E</i><sub>cat/2</sub>. Computational studies
carried out on [Ni(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> 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<sub>2</sub> production
