Oxygen-Tolerant H₂ Production by [FeFe]-H₂ase Active Site Mimics Aided by Second Sphere Proton Shuttle

Abstract

The instability of [FeFe]-H₂ases and their biomimetics toward O₂ renders them inefficient to implement in practical H₂ generation (HER). Previous investigations on synthetic models as well as natural enzymes proved that reactive oxygen species (ROS) generated on O₂ exposure oxidatively degrades the 2Fe subcluster within the H-cluster active site. Recent electrochemical studies, coupled with theoretical investigations on [FeFe]-H₂ase suggested that selective O₂ reduction to H₂O could eliminate the ROS, and hence, tolerance against oxidative degradation could be achieved ( Nat. Chem. 2017, 9, 88-95). We have prepared a series of 2Fe subsite mimics with substituted arenes attached to bridgehead N atoms in the S to S linker, (μ-S₂(CH₂)₂NAr)[Fe(CO)₃]₂. Structural analyses find the nature of the substituent on the arene offers steric control of the orientation of bridgehead N atoms, affecting their proton uptake and translocation ability. The heterogeneous electrochemical studies of these complexes physiadsorbed on edge plane graphite (EPG) electrode show the onset of HER activity at ∼180 mV overpotential in pH 5.5 water. In addition, bridgehead N-protonation and subsequent H-bonding capability are established to facilitate the O-O bond cleavage resulting in selective O₂ reduction to H₂O. This allows a synthetic [FeFe]-H₂ase model to reduce protons to H₂ unabated in the presence of dissolved O₂ in water at nearly neutral pH (pH 5.5); i.e., O₂-tolerant, stable HER activity is achieved.