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

Abstract

AbstractFive neutral nickel(II) bimetallic models of the active site of [NiFe]‐hydrogenase supported by tridentate sulfur‐rich RNS2 ligands, were synthesized and tested as electrocatalysts for proton (H+) reduction. Complexes were classified according to the −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 [Ni2(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: θ=180.0°, Ni ⋅⋅⋅ Ni=3.356 Å; 2 b: θ=98.55°, Ni ⋅⋅⋅ Ni=2.760 Å; 3 a: θ=107.32°, Ni ⋅⋅⋅ Ni=2.825 Å). Using CF3COOH as H+ source, 1 b and 2 b exhibit catalytic activity at −1.72 V (icat/ip≈2.40) and −1.80 V (icat/ip≈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.