Catalytic Mechanism of Competing Proton Transfer Events from Water and Acetic Acid by [Co II (bpbH 2 )Cl 2 ] for Water Splitting Processes.

We performed first principles simulations to explore the water reduction process of the cobalt complex [Co II (bpbH 2 )Cl 2 ], where bpbH 2 = N , N '-bis(2'-pyridine carboxamide)-1,2-benzene. We considered the sequence steps of electron reduction followed by the proton addition process to observe the hydrogen evolution process. An experimental study of the catalyst showed that the increase in the acetic acid concentration triggers catalytic current and reduction of Co(II) to Co(I), and protonation occurred, yielding a Co(III)-H intermediate. Therefore, we used water and acetic acid as the proton sources. We compare the proton transfer kinetics from both the water and acetic acid. The reduction potentials and proton transfer kinetics from water or acetic acid to the reaction center were studied in a DMF solvent through the implicit solvent model. The first proton transfer from the acetic acid is more favorable, forming a Co III -H complex and further reducing to Co II -H. The second proton transfer from water to the Co II -H moiety requires less free energy than acetic acid and is the rate-limiting step. The nature of the reduction process is also examined through the charge analysis, which reveals that the ligand becomes softer due to the C═O groups.