Mechanism of H2 Evolution from a Photogenerated Hydridocobaloxime

Proton transfer from the triplet excited state of brominated naphthol to a difluoroboryl bridged Co-I-diglyoxime complex, forming (CoH)-H-III, was monitored via transient absorption. The second-order rate constant for (CoH)-H-III formation is in the range (3.5-4.7) x 10(9) M-1 s(-1), with proton transfer coupled to excited-state deactivation of the photoacid. (CoH)-H-III is subsequently reduced by excess Co-I-diglyoxime in solution to produce (CoH)-H-II (k(red) = 9.2 x 10(6) M-1 s(-1)), which is then protonated to yield Co-II-diglyoxime and H-2.