Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay
Issued Date
2024-01-01
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ISSN
14779226
eISSN
14779234
Scopus ID
2-s2.0-85187666359
Journal Title
Dalton Transactions
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SCOPUS
Bibliographic Citation
Dalton Transactions (2024)
Suggested Citation
Thammanatpong K., Surawatanawong P. Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay. Dalton Transactions (2024). doi:10.1039/d3dt03960a Retrieved from: https://repository.li.mahidol.ac.th/handle/20.500.14594/97710
Title
Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay
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Abstract
The hydrogen evolution reaction is an important process for energy storage. The six-coordinate cobalt complex [CoIII(L1−)(LH)]2+ (LH = N-(4-amino-6-(pyridin-2-yl)-1,3,5-triazin-2-yl)benzamidine) was found to catalyze photocatalytic hydrogen evolution. In this work, we performed density functional calculations to obtain the reduction potentials and the proton-transfer free energy of possible intermediates to determine the preferred pathways for proton reduction. The mechanism involves the metal-based reduction of Co(iii) to Co(ii) before the protonation at the amidinate N on the pyridinyl-substituted diaminotriazine benzamidinate ligand L1− to form [CoII(LH)(LH)]2+. Essentially, the subsequent electron transfer is not metal-based reduction, but rather ligand-based reduction to form [CoII(LH)(LH˙1−)]1+. Through a proton-coupled electron transfer process, the cobalt hydride [CoIIH(LH)(LH2˙)]1+ is formed as the key intermediate for hydrogen evolution. As the cobalt hydride complex is coordinatively saturated, a structural change is required when the hydride on Co is coupled with the proton on pyridine. Notably, the redox-active nature of the ligand results in the low acidity of the protonated pyridine moiety of LH2˙, which impedes its function as a proton relay. Our findings suggest that separating the proton relay fragment from the electron reservoir fragment of the redox-active ligand is preferred for fully utilizing both features in catalytic H2 evolution.