Interplay of hemilability and redox activity in models of hydrogenase active sites.

The hydrogen evolution reaction, as catalyzed by two electrocatalysts [M(NS)·Fe(NO)], [-Fe] (M = Fe(NO)) and [Ni-Fe] (M = Ni) was investigated by computational chemistry. As nominal models of hydrogenase active sites, these bimetallics feature two kinds of actor ligands: Hemilabile, MNS ligands and redox-active, nitrosyl ligands, whose interplay guides the H production mechanism. The requisite base and metal open site are masked in the resting state but revealed within the catalytic cycle by cleavage of the MS-Fe(NO) bond from the hemilabile metallodithiolate ligand. Introducing two electrons and two protons to [Ni-Fe] produces H from coupling a hydride temporarily stored on Fe(NO) (Lewis acid) and a proton accommodated on the exposed sulfur of the MNS thiolate (Lewis base). This Lewis acid-base pair is initiated and preserved by disrupting the dative donation through protonation on the thiolate or reduction on the thiolate-bound metal. Either manipulation modulates the electron density of the pair to prevent it from reestablishing the dative bond. The electron-buffering nitrosyl's role is subtler as a bifunctional electron reservoir. With more nitrosyls as in [-Fe], accumulated electronic space in the nitrosyls' π*-orbitals makes reductions easier, but redirects the protonation and reduction to sites that postpone the actuation of the hemilability. Additionally, two electrons donated from two nitrosyl-buffered irons, along with two external electrons, reduce two protons into two hydrides, from which reductive elimination generates H.