Abstract
Construction of a water-soluble, oxygen-tolerant, and acid-stable synthetic H-2 production catalyst is vital for the development of an economic and user-friendly H-2-based renewable energy infrastructure. Natural enzyme hydrogenases exhibit excellent energy-efficient H-2 production activity. However, fragility of the overall protein structure has restricted their sustainable and practical application. Among the synthetic functional models of hydrogenase, cobaloxime-based complexes offer O-2-insensitivity. However, they are only active near neutral conditions with moderate rates and poor aqueous solubility properties. Here, in this work, we have specifically stationed a series of enzyme-inspired, multicomponent outer coordination sphere components around the cobaloxime core to simultaneously improve its catalytic rate, aqueous solubility, and activity even under acidic conditions. We have also established that cobaloximes display catalytic H-2 production via two independent mechanisms: (i) Co(II)-centric and (ii) Co(I)-centric. Initial Co(II)-centric H-2 evolution occurred at a relatively less reducing potential following the substitution of the axial Cl- ligand with solvent water. Dominant Co(I)-centric H-2 production reactivity was observed in further cathodic potential. Incorporation of dynamic peripheral basic functionalities enhanced H+ trafficking around the cobaloxime core to significantly improve (similar to 2.0-9.5 times) Co(I)-centric H-2 production reactivity. Complementary NMR and electrochemical results suggest that formation of an intricately interactive water-assisted proton relay neighboring the metal core is the prime reason for this improved activity. Additionally, these peripheral basic functionalities, blended with proton relay, provided an alternative protonation site during the catalysis to induce unprecedented H-2 production for cobaloximes under acidic aqueous conditions (pH < 5). Thus, this work provides a prime example of catalytic upgradation of an already existing, moderately active synthetic complex core by encompassing it with precisely positioned enzyme-inspired basic functionalities and water molecules.