Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

The instability of [FeFe]-Hases and their biomimetics toward O renders them inefficient to implement in practical H generation (HER). Previous investigations on synthetic models as well as natural enzymes proved that reactive oxygen species (ROS) generated on O exposure oxidatively degrades the 2Fe subcluster within the H-cluster active site. Recent electrochemical studies, coupled with theoretical investigations on [FeFe]-Hase suggested that selective O reduction to HO 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-S(CH)NAr)[Fe(CO)]. 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 O reduction to HO. This allows a synthetic [FeFe]-Hase model to reduce protons to H unabated in the presence of dissolved O in water at nearly neutral pH (pH 5.5); i.e., O-tolerant, stable HER activity is achieved.

Reduction of CO holds the key to solving two major challenges taunting the society-clean energy and clean environment. There is an urgent need for the development of efficient non-noble metal-based catalysts that can reduce CO selectively and efficiently. Unfortunately, activation and reduction of CO can only be achieved by highly reduced metal centers jeopardizing the energy efficiency of the process. A carbon monoxide dehydrogenase inspired Co complex bearing a dithiolato ligand can reduce CO, in wet acetonitrile, to CO with ∼95% selectivity over a wide potential range and 1559 s rate with a remarkably low overpotential of 70 mV. Unlike most of the transition-metal-based systems that require reduction of the metal to its formal zerovalent state for CO reduction, this catalyst can reduce CO in its formal +1 state making it substantially more energy efficient than any system known to show similar reactivity. While covalent donation from one thiolate increases electron density at the Co(I) center enabling it to activate CO, protonation of the bound thiolate, in the presence of HO as a proton source, plays a crucial role in lowering overpotential (thermodynamics) and ensuring facile proton transfer to the bound CO ensuring facile (kinetics) reactivity. A very covalent Co(III)-C bond in a Co(III)-COOH intermediate is at the heart of selective protonation of the oxygen atoms to result in CO as the exclusive product of the reduction.

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The chemical and electrochemical reduction of CO2 to value added chemicals entails the development of efficient and selective catalysts. Synthesis, characterization and electrochemical CO2 reduction activity of a air-stable cobalt­(III) diphenylphos­phenethano-bis­(2-pyridinethiolate)­chloride [{Co­(dppe)­(2-PyS)2}­Cl, 1-Cl] complex is divulged. The complex reduces CO2 under homogeneous electrocatalytic conditions to produce CO with high Faradaic efficiency (FE > 92%) and selectivity in the presence of water. Through detailed electrochemical investigations, product analysis, and mechanistic investigations supported by theoretical calculations, it is established that complex 1-Cl reduces CO2 in its Co­(I) state. A reductive cleavage leads to a dangling protonated pyridine arm which enables facile CO2 binding through a H-bond donation and facilitates the C–O bond cleavage via a directed protonation. A systematic benchmarking of this catalyst indicates that it has a modest overpotential (∼180 mV) and a TOF of ∼20 s–1 for selective reduction of CO2 to CO with H2O as a proton source.