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

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 −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 : 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 CF 3 COOH as H + source, 1 b and 2 b exhibit catalytic activity at −1.72 V ( i cat / i p ≈2.40) and −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.