Role of Anation on the Mechanism of Proton Reduction Involving a Pentapyridine Cobalt Complex: A Theoretical Study

Abstract

Kinetic and thermodynamic aspects of proton reduction involving pentapyridine cobalt(II) complex were investigated with the help of quantum chemical calculations. Free energy profile of all possible mechanistic routes for proton reduction was constructed with the consideration of both anation and solvent bound pathways. The computed free energy profile shows that acetate ion plays a significant role in modulating the kinetic aspects of Co(III)-hydride formation which is found to be the key intermediate for proton reduction. Upon replacing solvent by acetate ion, one electron reduction and protonation of Co species become more rapid along with slow displacement reaction. Most favorable pathways for hydrogen evolution from Co(III)-hydride species is also investigated. Among the four possible pathways, reduction followed by protonation of Co(III)-hydride (RPP) is found to be the most feasible pathway. On the basis of QTAIM and NBO analyses, the electronic origin of most favorable pathway is explained. The basicity of cobalt center along with thermodynamic stability of putative Co-H species is essentially a prime factor in deciding the most favorable pathway for hydrogen evolution. Our computed results are in good agreement with experimental observations and also provided adequate information to design cobalt-based molecular electrocatalysts for proton reduction in future.

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

“…Specially, the presence of solvent molecules or anions from the acid serving as the proton source can play a crucial role in modulating the kinetic and thermodynamic aspects of the reaction. 317 For instance, the proton source can affect the thermodynamics of catalytic event and a theoretical description of these effects on the pK a value, metal redox states or protonation states are necessary to inspect the feasibility of different pathways for the catalytic cycle of cobalt catalysts. 293,318 The DFT investigation of a nickel catalyst reported the effect of the solvent on redox properties and how it can affect the stability of the metal oxidation state by changing its coordination.…”

“…17). 317 Thus, accounting for these effects and the possible direct involvement of the solvent are critical considerations in computational studies.…”

Molecular Electrocatalysts for the Hydrogen Evolution Reaction: Input from Quantum Chemistry

“…Artero and co-workers arrived at similar conclusions upon directlyc alculating the redox potentials for a cobalt diimine-dioxime catalyst in the presence of different acids. [55] An example of such an investigation was performed by Panneerselvam and Jaccob, [56] based on experimental studies by Chang and co-workers. [57] They used DFT (B3LYP/6-31G(d)) cal-Scheme6.Proposed mechanism for hydrogen evolutionw ith at hiosemicarbazone copper catalyst.…”

Two routes to hydrogen evolution for a Co-polypyridyl complex with two open sites

“…They could not rule out contributions from a bimolecular pathway, but established an upper limit of 5% with regards to hydrogen production. Bimolecular reactions between two polypyridyl complexes were modeled in reference [76], for both Co III -H and Co II -H. These reactions had unfavorable reaction energies compared to reactions involving a single Co-hydride, and were therefore ruled out. Although these calculations were performed for pentapyridyl complexes, the clear energy differences means that the bimolecular reactions should be unfavorable also for the Co(bpma) complex.…”