Thermodynamic Properties of Hydrogen-Producing Cobaloxime Catalysts: A Density Functional Theory Analysis

Density functional theory calculations were carried out to study the electrochemical properties including reduction potentials, pK _a values, and thermodynamic hydricities of three prototypical cobaloxime complexes, Co(dmgBF₂)₂ (dmgBF₂ = difluoroboryl-dimethylglyoxime), Co(dmgH)₂ (dmgH = dimethylglyoxime), and Co(dmgH)₂(py)(Cl) (py = pyridine) in the acetonitrile (AN)-water solvent mixture. The electrochemical properties of Co(dmgBF₂)₂ in pure AN and pure water were also considered for comparison to reveal the key roles of the solvent on the catalytic reaction. In agreement with previous studies, hydrogen production pathways starting from reduction of the resting state of Co^II and involving formation of the Co^IIIH and Co^IIH intermediates are the favorable ones for both bimetallic and monometallic pathways. However, we found that in pure AN, both the Co^IIIH and Co^IIH intermediates can react with a proton to produce H₂. In the presence of water in the solvent, the reduction of Co^IIIH to Co^IIH is necessary for the reaction with a proton to occur to form H₂. This suggests that it is possible to design catalytic systems by suitably tuning the composition of the AN-water mixture. We also identified the key role of axial coordination of the solvent molecules in affecting the catalytic reaction, which allows further catalyst design strategy. The highest hydride donor ability of Co(dmgH)₂(py)(Cl) indicates that this complex displays the best catalytic hydrogen-producing performance among the three cobaloximes studied in this work.