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

Five 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(mu-SAr)2] frameworks, in which the two coordination planes around the Ni centres define a dihedral angle (.) that is influenced by the substituents on the ligands (2a:.= 180.0 degrees, Ni center dot center dot center dot Ni= 3.356 A; 2b:.= 98.55 degrees, Ni center dot center dot center dot Ni= 2.760 A; 3a:.= 107.32 degrees, Ni center dot center dot center dot Ni= 2.825 A). Using CF3COOH as H+ source, 1b and 2b 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 center dot center dot center dot Ni cooperative effect, which is favoured in angular molecules and appears to assist during electrocatalytic H+ reduction.

Automated summary

Five 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 reduction. The b-type complexes with bulky substituents exhibit better catalytic activity compared to the a-type complexes without bulky substituents. This suggests a relationship between the dimer conformation and its catalytic activity, with the Ni center dot center dot center dot Ni cooperative effect playing a crucial role.