Ideal Octahedral Geometry Leads to Poor H2 Evolution Performance: Theoretical Insights on Reaction Intermediates of Cobalt-Pentapyridyl Molecular Catalyst

The type and coordination of ligands surrounding water reduction photo-catalysts affect the H2 production reaction pathway. In this respect, we examined two successive reductions followed by two protonations (EECC) mechanism of H2 evolution reaction catalysed by Cobalt-based pentapyridyl molecular catalyst in an aqueous solution employing Ab-initio Molecular Dynamics (AIMD), Density Functional Theory (DFT), and Free Energy Perturbation Theory. Each intermediate step of the EECC mechanism was simulated to determine the allowable spin states and solvent response surrounding the reaction center of the catalyst. A single electron transfer to the catalyst, leading to a singlet spin state, induces a conformational change in the water molecule in the first solvation shell, facilitating the formation of a Co-proton bond. Subsequent to the second electron transfer, the Co center acquires the proton from this water molecule, while the remaining OH− ion diffuses swiftly through the solvent. The first protonation step was determined to continue adopting a quartet spin state. Due to the catalyst’s cage-like structure around the Co center, following the second protonation, H2 forms; however, it fails to diffuse from the reaction center into the solvent, suggesting the importance of maintaining an open structure around the Co center when designing ligand structures for water-splitting catalysts. The first and the second reduction free energies were calculated as -2 eV and -1.3 eV, respectively.