Photocatalytic\nSubstrate Oxidation Catalyzed by a\nRuthenium(II) Complex with a Phenazine Moiety as the Active Site Using\nDioxygen as a Terminal Oxidant

We have developed\nphotocatalytic oxidation of aromatic substrates\nusing O<sub>2</sub> as a terminal oxidant to afford only 2e<sup>–</sup>-oxidized products without the reductive activation of O<sub>2</sub> in acidic water under visible-light irradiation. A Ru<sup>II</sup> complex (<b>1</b>) bearing a pyrazine moiety as the active\nsite in tetrapyrido[3,2-<i>a</i>:2′,3′-<i>c</i>:3″,2″-<i>h</i>:2‴,3‴-<i>j</i>]phenazine (tpphz) as a ligand was employed as a photocatalyst.\nThe active species for the photocatalysis was revealed to be not complex <b>1</b> itself but the protonated form, <b>1</b>-H<sup>+</sup>, protonated at the vacant diimine site of tpphz. Upon photoexcitation\nin the presence of an organic substrate, <b>1</b>-H<sup>+</sup> was converted to the corresponding dihydro-intermediate (<b>2</b>-H<sup>+</sup>), where the pyrazine moiety of the ligand received\n2e<sup>–</sup> and 2H<sup>+</sup> from the substrate. <b>2</b>-H<sup>+</sup> was facilely oxidized by O<sub>2</sub> to\nrecover <b>1</b>-H<sup>+</sup>. Consequently, an oxidation product\nof the substrate and H<sub>2</sub>O<sub>2</sub> derived from dioxygen\nreduction were obtained; however, the H<sub>2</sub>O<sub>2</sub> formed\nwas also used for oxidation of <b>2</b>-H<sup>+</sup>. In the\noxidation of benzyl alcohol to benzaldehyde, the turnover number reached\n240 for 10 h, and the quantum yield was determined to be 4.0%. The\nabsence of ring-opening products in the oxidation of cyclobutanol\nsuggests that the catalytic reaction proceeds through a mechanism\ninvolving formal hydride transfer. Mechanistic studies revealed that\nthe photocatalytic substrate oxidation by <b>1</b>-H<sup>+</sup> was achieved in neither the lowest singlet excited state nor triplet\nexcited state (S<sub>1</sub> or T<sub>1</sub>) but in the second lowest\nsinglet excited state (S<sub>2</sub>), i.e., <sup>1</sup>(π–π*)*\nof the tpphz ligand. Thus, the photocatalytic substrate oxidation\nby <b>1</b>-H<sup>+</sup> can be categorized into unusual anti-Kasha\nphotocatalysis.