PNNL

Abstract

Hydrogenase enzymes are reversible catalysts for H2 production and oxidation. The catalytic bias, which is the directional preference for either H2 oxidation or H2 production, can be controlled by the enzyme outer coordination sphere, as demonstrated by different strains of [NiFe]-hydrogenase. The structures of the active site in all of those enzymes are similar so the surrounding protein scaffold must hold the key for controlling their catalytic preference. Reversibility and the resultant ability to bias the catalyst are seldom achieved for synthetic catalysts, which may be due to the absence of an outer coordination sphere. In this work, three [Ni(PCy2NR´2)2]2+ H2 oxidation catalysts, with identical first and second coordination sphere but different outer coordination spheres were investigated to study their ability to influence catalytic preference: [Ni(PCy2NBn2)2]2+ (CyBn), [Ni(PCy2N(3-pyridazyl)methyl2)2]2+ (CyPyz), and [Ni(PCy2NCH2CH2OCH32)2]2+ (CyEtOMe). These complexes demonstrated different degrees of homogeneous bidirectional (but irreversible) H2 production/oxidation under acidic conditions in acetonitrile with a significant impact of the identity of the outer coordination sphere. To monitor catalytic preference during the observed irreversible bidirectional catalysis, catalytic directionality, based on the ratio of the turnover frequencies in either direction, is evaluated. The catalytic preference of the complexes for H2 production over H2 oxidation was CyPyz > CyEtOMe > CyBn, in contrast to the expected behavior based on the calculated free energy of hydrogen addition (?GH2: CyEtOMe > CyBn > CyPyz), which considers only the first and second coordination spheres. Complementary 31P{1H} NMR spectra for the three complexes under these conditions demonstrate a preference for the species catalytically active for hydrogen production only for CyPyz and CyEtOMe, but not CyBn, a preference which may facilitate activity in the H2 production direction. Collectively, this data suggests a strong correlation between catalytic directionality and outer coordination sphere functionality, and provides insight into how enzymes may achieve regulation and control. This work was funded by the Office of Science Early Career Research Program through the US DOE, BES (AD, SL, WJS), and the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US DOE, BES (PD, RS, JASR). PNNL is operated by Battelle for the US DOE.