Mechanism of H2 Evolution from a Photogenerated Hydridocobaloxime

Proton transfer from the triplet excited state of brominated naphthol to a difluoroboryl bridged CoI-diglyoxime complex, forming CoIIIH, was monitored via transient absorption. The second-order rate constant for CoIIIH formation is in the range (3.5−4.7) × 109 M−1 s−1, with proton transfer coupled to excited-state deactivation of the photoacid. CoIIIH is subsequently reduced by excess CoI-diglyoxime in solution to produce CoIIH (k red = 9.2 × 106 M−1 s−1), which is then protonated to yield CoII-diglyoxime and H2.