The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as Co-I and Co-III-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)](n+) (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)](n+) where L is the pentadentate DPA-Bpy ligand or [Co(OH2)](n+) as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a Co-II species results in weakening the Co-O bond. The further reduction to a Co-I species leads to the loss of the aqua ligand and the formation of [Co-I-VS)](+) (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [Co-I(kappa(4)-L)(OH2)](+) species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the Co-I species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [Co-III-H](2+). We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [Co-I(kappa(4))-L)(OH2)](+) in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation.
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