Electrochemistry, electrocatalysis, and mechanistic details into hydrogen evolution pathways of hexacoordinated iron scaffolds in hydrogenase mimics

[Display omitted] •Sustainable generation of hydrogen from carbon-neutral sources is fast becoming an active area of research.•Structural and functional mimics of Fe-Fe hydrogenases provide robust platforms for future electrocatalytic designs.•Iron complexes for electrocatalytic H2 generation are discussed in details.•Experimental, spectroscopic and computational methods for probing the make and break mechanisms of Fe-Fe hydrogenases. Iron-sulphur cofactors in the reactive sites of metalloproteins such as Fe-Fe-hydrogenase have played key roles in biological redox reactions such as the generation and activation of hydrogen molecules via proton reduction. In the past few decades, the utilization of structural and functional mimics of the reactive site of Fe-Fe-hydrogenase to produce H2 has attracted special global attention. The generated molecular hydrogen which has been globally considered the next-generation fuel is carbon-neutral and free from contaminants such as carbon monoxide. Coordinatively saturated metal complex systems have been shown to exhibit exceedingly remarkable activity for proton reduction electrocatalysis in aqueous and organic media compared to their unsaturated (tetra- and pentacoordinated) counterparts. Their make-and-break hydride formation mechanisms have resulted in multiple pathways whose catalytic intermediates, transfer sequences, and proton activation transduction procedures can be investigated by experimental, spectroscopic, and computational techniques. Based on the foregoing, this review highlights mimics of the Fe-Fe-hydrogenase systems in the light of the detailed connection between their electrochemistry (potentials/redox couples without the addition of acids) and electrocatalysis (catalytic potentials after the addition of acids) followed by the mechanistic richness of the pathways to hydrogen generation. In conclusion, future catalyst designs in light of structure-catalytic activity relationships have been proposed.