A novel nickel(II) complex [Ni(L)2 Cl]Cl with a bidentate phosphinopyridyl ligand 6-((diphenylphosphino)methyl)pyridin-2-amine (L) was synthesized as a metal-complex catalyst for hydrogen production from protons. The ligand can stabilize a low Ni oxidation state and has an amine base as a proton transfer site. The X-ray structure analysis revealed a distorted square-pyramidal Ni(II)  complex with two bidentate L ligands in a trans arrangement in the equatorial plane and a chloride anion at the apex. Electrochemical measurements with the Ni(II) complex in MeCN indicate a higher rate of hydrogen production under weak acid conditions using acetic acid as the proton source. The catalytic current increases with the stepwise addition of protons, and the turnover frequency is 8400 s(-1) in 0.1 m [NBu4 ][ClO4 ]/MeCN in the presence of acetic acid (290 equiv) at an overpotential of circa 590 mV.

A novel nickel(II) complex [Ni(L)2Cl]Cl with a bidentate phosphinopyridyl ligand 6‐((diphenylphosphino)methyl)pyridin‐2‐amine (L) was synthesized as a metal‐complex catalyst for hydrogen production from protons. The ligand can stabilize a low Ni oxidation state and has an amine base as a proton transfer site. The X‐ray structure analysis revealed a distorted square‐pyramidal NiII complex with two bidentate L ligands in a trans arrangement in the equatorial plane and a chloride anion at the apex. Electrochemical measurements with the NiII complex in MeCN indicate a higher rate of hydrogen production under weak acid conditions using acetic acid as the proton source. The catalytic current increases with the stepwise addition of protons, and the turnover frequency is 8400 s−1 in 0.1 m [NBu4][ClO4]/MeCN in the presence of acetic acid (290 equiv) at an overpotential of circa 590 mV.

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.

Die elektrokatalytische H2‐Erzeugung durch einen molekularen Katalysator mit einem billigen Metall kann zur Lösung von Umweltproblemen beitragen. H. Masuda et al. beschreiben in der Zuschrift auf S. 5333 ff. die Synthese eines NiII‐Komplexes mit einem zweizähnigen Phosphanylpyridylliganden und einer Aminbase als Protonentransferzentrum, der die H2‐Bildung katalysiert. Elektrochemische Messungen ergaben, dass die H2‐Erzeugung mit Essigsäure als Protonenquelle unter schwach sauren Bedingungen beschleunigt werden kann.

Electrocatalytic H2 production by a molecular catalyst containing an inexpensive metal can contribute to the resolution of environmental problems. In their Communication on page 5247 ff., H. Masuda et al. describe the synthesis of a novel NiII complex, containing a bidentate phosphinopyridyl ligand with an amine base as a proton‐transfer site, as a H2 production catalyst. Electrochemical measurements with the complex show a higher rate of H2 production under weak‐acid conditions using acetic acid as the proton source.