Modulating Effect of Ligand Charge on the Electronic Properties of 2Ni–2S Structures and Implications for Biological 2M–2S Sites

Abstract

Sulfur-bridged bimetallic 2M−2S type structures are essential cofactors that participate in biological long-range electron transport and metabolism. Metal−sulfur bond covalency is a decisive property for inner sphere (through-bond) type electron transfer that dominates in buried or hydrophobic protein environments. This work reports on a combined experimental and computational study of the effect of ligand charge on the electronic structure of a 2Ni−2S model site that adopts the biologically relevant S = 1 / 2 redox state. Starting out from an isostructural dinickel(1.5+)-dithiophenolate platform with sulfur-bridged tetrahedral Ni sites, η 2 :η 2 -μ-coordination of the S = 1 / 2 [2Ni-2S] + core to either a neutral π-system or strongly σ-donating cyclohexadienido renders its electronic structure substantially different. Density functional theory analysis corroborates pulse and continuous wave electron paramagnetic resonance data that associate co-ligand charge with the significant change in the mechanism and size of electron− 31 P nuclear spin hyperfine coupling to a phosphine reporter ligand at each nickel center. An increasing level of charge donation attenuates direct and through-bridge electronic coupling of the metal sites, resulting in a stronger electronic coupling of the 2Ni−2S core to its terminal phosphine donors. Drawing a connection to biological 2M−2S sites, our 2Ni−2S system indicates that a fine balance of intracore and core−protein electronic coupling is key to biological function for which the degree of charge donation by peripheral donors appears to be a significant parameter.