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

Abstract

We previously reported the preparation and characterization of a Ni­(II) complex capable of electrocatalytic hydrogen generation. The complex [Ni­(L NH2 )2Cl]Cl (1) includes a 6-((diphenylphosphino)­methyl)­pyridin-2-amine ligand (L NH2 ), 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 NiII(L NH2 ) complexes with weakly coordinating/noncoordinating counteranions, [Ni­(L NH2 )2]­(OTs)2 (OTs– = p-toluenesulfonate) (2) and [Ni­(L NH2 )2]­(BF4)2 (3), were synthesized. Their X-ray crystal structures reveal that the Ni­(II) ion is coordinated with two bidentate L NH2 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.

Rational development of molecular earth-abundant metal complexes for electrocatalytic hydrogen production

“…The combination of the square-planar geometry and counteranion change from coordinating Clto noncoordinating BF4led to an overall increase in proton reduction efficiency. Additionally, increasing the amount of acetic acid increased TOF to a point when the catalyst began to degrade under such harsh conditions [152].…”