We
previously reported the preparation and characterization of
a Ni(II) complex capable of electrocatalytic hydrogen generation.
The complex [Ni(LNH2)2Cl]Cl (1) includes a 6-((diphenylphosphino)methyl)pyridin-2-amine
ligand (LNH2), 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(LNH2) complexes with weakly coordinating/noncoordinating
counteranions, [Ni(LNH2)2](OTs)2 (OTs– = p-toluenesulfonate) (2) and [Ni(LNH2)2](BF4)2 (3), were synthesized. Their X-ray crystal structures reveal
that the Ni(II) ion is coordinated with two bidentate LNH2 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 (pKa = 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.