Effect of Counteranions in Electrocatalytic Hydrogen Generation Promoted by Bis(phosphinopyridyl) Ni(II) Complexes.

We previously reported the preparation and characterization of a Ni(II) complex capable of electrocatalytic hydrogen generation. The complex [Ni()Cl]Cl () includes a 6-((diphenylphosphino)methyl)pyridin-2-amine ligand (), which has an amino group as a base that acts as a proton transfer site by virtue of its location near the metal center. In order to study the effect of counteranions in hydrogen generation, two additional Ni() complexes with weakly coordinating/noncoordinating counteranions, [Ni()](OTs) (OTs = -toluenesulfonate) () and [Ni()](BF) (), were synthesized. Their X-ray crystal structures reveal that the Ni(II) ion is coordinated with two bidentate ligands in both complexes. Complex contains both and isomers in the unit cell. The former is in an axially elongated square-pyramidal geometry (τ = 0.17), and the latter is in a nearly square planar geometry (τ = 0.11) with two weakly interacting OTs anions at the axial sites. Complex has only the isomer in the solid state, which is in a nearly square planar geometry (τ = 0.10). These complexes are slightly different from , which has a distorted-square-pyramidal geometry (τ = 0.25) with a coordinated chloride anion. UV-vis spectra of and in MeCN show a spectral pattern characteristic of a square-planar Ni(II) complex. These spectra are slightly different from the unique spectrum of , which is typical of an axially coordinating Ni(II) species as a result of having a Cl anion at the apical position. Electrocatalytic hydrogen generation promoted by these three Ni(II) complexes (1.0 mmol) demonstrates an increase in the catalytic current induced by stepwise addition of HOAc (p = 22.3 in MeCN) as a proton source. The complexes demonstrate turnover frequencies (TOF) of 3800 s for , 5400 s for , and 8800 s for in MeCN (3 mL) containing 0.1 M [-BuN](ClO) in the presence of HOAc (145 equiv) at overpotentials of ca. 530, 490, and 430 mV, respectively.