Abstract
Experimental and computational studies address key questions in a structure-function analysis of bioinspired electrocatalysts for the HER. Combinations of NiN(2)S(2) or [(NO)Fe]N(2)S(2) as donors to (η(5)-C(5)H(5))Fe(CO)(+) or [Fe(NO)(2)](+/0) generate a series of four bimetallics, gradually "softened" by increasing nitrosylation, from 0 to 3, by the non-innocent NO ligands. The nitrosylated NiFe complexes are isolated and structurally characterized in two redox levels, demonstrating required features of electrocatalysis. Computational modeling of experimental structures and likely transient intermediates that connect the electrochemical events find roles for electron delocalization by NO, as well as Fe-S bond dissociation that produce a terminal thiolate as pendant base well positioned to facilitate proton uptake and transfer. Dihydrogen formation is via proton/hydride coupling by internal S-H(+)···(-)H-Fe units of the "harder" bimetallic arrangements with more localized electron density, while softer units convert H(-)···H(-)via reductive elimination from two Fe-H deriving from the highly delocalized, doubly reduced [Fe(2)(NO)(3)](-) derivative. Computational studies also account for the inactivity of a Ni(2)Fe complex resulting from entanglement of added H(+) in a pinched -S (δ-)···H(+)··· (δ-)S- arrangement.