Texas A&M University, USA
Nitric Oxide Assisted First Row Transition Metals for Hydrogenase-Inspired HER Electrocatalysis
The versatile organometallic type active sites in biology that harbor intact metallo-sulfur units are inspirations for biomimetic studies. Experimental and computational studies address key questions in a structure-function analysis of bioinspired electrocatalysts for the HER. Combinations of NiN2S2 or (NO)FeN2S2 as redox-active donors to (η5-C5H5)Fe(CO)+ or [Fe(NO)2]+/0 receivers, generate a series of bimetallics, gradually “softened” by increasing
nitrosylation, from 0 to 3, by the non-innocent NO ligands (1). The NiN2S2•Fe(NO)2 and (NO)FeN2S2•Fe(NO)2 complexes are isolated and structurally characterized in two redox levels, yielding information regarding key steps of electrocatalytic cycle (2).
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. Localized electron density that features in relatively harder donor-receiver adducts, [NiN2S2•Fe(NO)2]+/0 and [MN2S2•(η5-C5H5)Fe(CO)]+, allow dihydrogen production via proton/hydride coupling by internal S-H+•••−H-Fe units. However, more delocalized electron density as observed in soft-soft donor-receiver adducts, (NO)FeN2S2•Fe(NO)2 complex, produce dihydrogen by converting H−•••H− via reductive elimination from two Fe-H, derived from the highly delocalized, doubly reduced [Fe2(NO)3]− derivative. The Ni2Fe complex, featuring built in pendant bases, resulted in a pinched – Sδ−•••H+•••δ−S− arrangement, that accounts for their inactivity in proton reduction