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.

Two nickel complexes supported by tridentate NS2 ligands, [Ni2(κ‐N,S,S,S′‐NPh{CH2(MeC6H2R′)S}2)2] (1; R′=3,5‐(CF3)2C6H3) and [Ni2(κ‐N,S,S,S′‐NiBu{CH2C6H4S}2)2] (2), were prepared as bioinspired models of the active site of [NiFe] hydrogenases. The solid‐state structure of 1 reveals that the [Ni2(μ‐ArS)2] core is bent, with the planes of the nickel centers at a hinge angle of 81.3(5)°, whereas 2 shows a coplanar arrangement between both nickel(II) ions in the dimeric structure. Complex 1 electrocatalyzes proton reduction from CF3COOH at −1.93 (overpotential of 1.04 V, with icat/ip≈21.8) and −1.47 V (overpotential of 580 mV, with icat/ip≈5.9) versus the ferrocene/ferrocenium redox couple. The electrochemical behavior of 1 relative to that of 2 may be related to the bent [Ni2(μ‐ArS)2] core, which allows proximity of the two Ni⋅⋅⋅Ni centers at 2.730(8) Å; thus possibly favoring H+ reduction. In contrast, the planar [Ni2(μ‐ArS)2] core of 2 results in a Ni⋅⋅⋅Ni distance of 3.364(4) Å and is unstable in the presence of acid.