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(L)₂Cl]Cl (1) includes a 6-((diphenylphosphino)methyl)pyridin-2-amine ligand (L), 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^II(L) complexes with weakly coordinating/noncoordinating counteranions, [Ni(L)₂](OTs)₂ (OTs^- = p-toluenesulfonate) (2) and [Ni(L)₂](BF₄)₂ (3), were synthesized. Their X-ray crystal structures reveal that the Ni(II) ion is coordinated with two bidentate L ligands in both complexes. Complex 2 contains both trans and cis isomers in the unit cell. The former is in an axially elongated square-pyramidal geometry (τ₅ = 0.17), and the latter is in a nearly square planar geometry (τ₄ = 0.11) with two weakly interacting OTs^- anions at the axial sites. Complex 3 has only the cis isomer in the solid state, which is in a nearly square planar geometry (τ₄ = 0.10). These complexes are slightly different from 1, which has a distorted-square-pyramidal geometry (τ₅ = 0.25) with a coordinated chloride anion. UV-vis spectra of 2 and 3 in MeCN show a spectral pattern characteristic of a square-planar Ni(II) complex. These spectra are slightly different from the unique spectrum of 1, 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 (pK_a = 22.3 in MeCN) as a proton source. The complexes demonstrate turnover frequencies (TOF) of 3800 s^-1 for 1, 5400 s^-1 for 2, and 8800 s^-1 for 3 in MeCN (3 mL) containing 0.1 M [n-Bu₄N](ClO₄) in the presence of HOAc (145 equiv) at overpotentials of ca. 530, 490, and 430 mV, respectively.