Planar or Bent? Redox Modulation of Hydrogenase Bimetallic Models by the [Ni₂(μ‐SAr)₂] Core Conformation

Abstract

Five neutral nickel(II) bimetallic models of the active site of [NiFe]-hydrogenase supported by tridentate sulfur-rich RNS 2 ligands, were synthesized and tested as electrocatalysts for proton (H + ) reduction. Complexes were classified according to the À NR substituent (1: 1-methylpyrene; 2: 2-methylthiophene; 3: phenyl) and as type a for those without bulky substituents and type b for the analogues with voluminous groups. Solid state structures were determined for three dimers, revealing [Ni 2 (μ-SAr) 2 ] frameworks, in which the two coordination planes around the Ni centres define a dihedral angle (θ) that is influenced by the substituents on the ligands (2 a:Using CF 3 COOH as H + source, 1 b and 2 b exhibit catalytic activity at À 1.72 V (i cat /i p � 2.40) and À 1.80 V (i cat /i p � 2.89) vs the ferrocenium/ferrocene couple (Fc + / Fc), respectively. In contrast, type a complexes were not viable catalysts. This behaviour suggests a relationship between the dimer conformation and its activity, due to a Ni ••• Ni cooperative effect, which is favoured in angular molecules and appears to assist during electrocatalytic H + reduction.

Electrocatalytic hydrogen evolution by robust square planar nickel complexes in an S2P2coordination environment

“…Significantly, nickel is not only an earth-abundant metal but also has five oxidation states namely Ni IV , Ni III , Ni II , Ni I , and Ni 0 . 8, [19][20][21][22] These abundant redox properties and numerous coordination environments of nickel enable the reasonable design of ligands to endow nickel complexes with excellent electrocatalytic performances, and these nickel complexes can operate at moderate overpotentials (η) with higher catalytic rates for hydrogen production [23][24][25][26] or oxygen evolution. 8,22,[27][28][29][30] The extensive investigations indicate that the electrocatalytic hydrogen evolution reaction by nickel complexes usually involves Ni II reduction to Ni I .…”