Abstract
Hydrogen generation via proton reduction and reduction of CO2 are two very important reactions for attaining a sustainable clean energy cycle. Both processes are catalyzed by low valent transition metals which are highly susceptible to oxidation by molecular oxygen. The sensitivity to molecular oxygen presents a genuine challenge towards practical implementation of these technologies. For example, the instability of [FeFe]-H2ases and their biomimetics towards 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 sub-cluster 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 oxic 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-S(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, and their proton uptake and translocation ability. These complexes show HER at near neutral pH and at low overpotentials (~180 mV). In addition, bridgehead N-protonation, and subsequent H-bonding capability, is established to be effective 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; i.e., O2 tolerant, stable HER activity is achieved. Using a new approach of utilizing spin crossing barrier, oxygen tolerant hydrogen evolution and CO2 reduction is deminstrated.
