Why the Perfectly Symmetric Cobalt-Pentapyridyl Loses the H₂ Production Challenge: Theoretical Insight into Reaction Mechanism and Reduction Free Energies

Researchers have extensively investigated photo-catalytic water reduction utilizing Cobalt-based catalysts with polypyridyl ligands. While catalysts exhibiting distorted polypyridyl ligand demonstrate higher H-2 production yields, those with ideal octahedral coordination display poor performance. This outcome suggests the crucial role of ligand framework in catalytic activity, yet reasons behind the disparity in H-2 production rates for catalysts with octahedral geometries remain unclear. We theoretically examined the water reduction mechanism of Co-based poly-pyridyl catalyst, CoPy5, having perfect octahedral coordination. We clarified the effect of octahedral coordination by utilizing each intermediate step of ECEC mechanism. We determined spin states, solvent response, electronic structures, and reduction free energies. CoPy5 with perfect octahedral coordination, alongside its distorted counterparts, exhibit similar spin states as the reaction progresses through each intermediate step. However, the first reduction free energy obtained for the CoPy5 is slightly higher than that of its distorted counterparts. Following the second protonation, resulting H-2 molecule experiences limited diffusion from the Co center due to the compact structure of the CoPy5, which blocks the Co center for the next H-2 production cycle. Catalysts having distorted octahedral geometries facilitate fast removal of H-2 into the solvent. Thus, the reaction center becomes immediately available for subsequent H-2 production.