Hydrogenase Models

Abstract

Abstract This review will highlight major advances in the synthesis and study of active site analogs for the [NiFe]‐, [FeFe]‐, and [Fe]‐hydrogenases (H 2 ases). In their biological role, the [NiFe]‐ and [FeFe]‐ H 2 ases utilize a combination of redox and acid–base reactions for the activation and formation of H 2 . Both reactions have been demonstrated in models. Relevant to the [NiFe]‐H 2 ases, models have been developed that form hydrides via the biologically relevant pathways of protonation or activation of H 2 . These hydrides serve as catalysts for the reduction of protons or oxidation of H 2 via mixed‐valence intermediates. These studies suggest that the hydride bridges the two metals unsymmetrically, and the Fe– H … Ni center engages in dihydrogen bonding to protic reagents as an intermediate in the formation and activation of H 2 . For the [FeFe]‐H 2 ases, the most informative models feature both an azadithiolate cofactor, which serves as a proton relay, and a terminal hydride ligand. Diiron species undergo oxidation to mixed‐valence Fe(I)Fe( II ) cations, several of which are capable of oxidizing H 2 . The Fe(I)Fe(I) species catalyze hydrogen evolution via the formation of iron hydrides. Catalysis is faster when the hydride is a terminal ligand, but also proceeds for isomeric species with a bridging hydride ligand. Models for [Fe]‐H 2 ases feature biomimetic coordination spheres consisting of acyl, thiolate, and 2‐substituted pyridine ligands bound to a redox‐inactive ferrous center.