Facilitated Hydride Binding in an Fe−Fe Hydrogenase Active−Site Biomimic Revealed by X-ray Absorption Spectroscopy and DFT Calculations

Abstract

Iron-only hydrogenases are high-efficiency biocatalysts for the synthesis and cleavage of molecular hydrogen. Their active site is a diiron center, which carries CO and CN ligands. Remarkably, the two iron atoms likely are connected by a non-protein azadithiolate (adt = S−CH₂−NH−CH₂−S). To dwell on the role of the adt in H₂ catalysis, a specific biomimetic diiron compound, 1 = [Fe₂(μ-adt-CH₂−Ph)(CO)₄(PMe₃)₂], with unprecedented positive reduction potential, has been synthesized and crystallized previously. It comprises two protonation sites, the N-benzyl-adt nitrogen that can hold a proton (H) and the Fe−Fe bond that will formally carry a hydride (Hy). We investigated changes in the solution structure of 1 in its four different protonation states (1‘, [1H]⁺, [1HHy]²⁺, and [1Hy]⁺) by X-ray absorption spectroscopy at the iron K-edge. EXAFS reveals that already protonation at the adt nitrogen atom causes a change of the ligand geometry involving a significant lengthening of the Fe−Fe distance and CO and PMe₃ repositioning, respectively, thereby facilitating the subsequent binding of a bridging hydride. Hydride binding clearly is discernible in the XANES spectra of [1HHy]²⁺ and [1Hy]⁺. DFT calculations are in excellent agreement with the experimentally derived structural parameters and provide complementary insights into the electronic structure of the four protonation states. In the iron-only hydrogenases, protonation of the putative adt ligand may cause the bridging CO to move to a terminal position, thereby preparing the active site for hydride binding en route to H₂ formation.