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

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.