Photocatalytic Substrate Oxidation Catalyzed by a Ruthenium(II) Complex with a Phenazine Moiety as the Active Site Using Dioxygen as a Terminal Oxidant.

We have developed photocatalytic oxidation of aromatic substrates using O as a terminal oxidant to afford only 2e-oxidized products without the reductive activation of O in acidic water under visible-light irradiation. A Ru complex () bearing a pyrazine moiety as the active site in tetrapyrido[3,2-:2',3'-:3″,2″-:2‴,3‴-]phenazine (tpphz) as a ligand was employed as a photocatalyst. The active species for the photocatalysis was revealed to be not complex itself but the protonated form, -H, protonated at the vacant diimine site of tpphz. Upon photoexcitation in the presence of an organic substrate, -H was converted to the corresponding dihydro-intermediate (-H), where the pyrazine moiety of the ligand received 2e and 2H from the substrate. -H was facilely oxidized by O to recover -H. Consequently, an oxidation product of the substrate and HO derived from dioxygen reduction were obtained; however, the HO formed was also used for oxidation of -H. In the oxidation of benzyl alcohol to benzaldehyde, the turnover number reached 240 for 10 h, and the quantum yield was determined to be 4.0%. The absence of ring-opening products in the oxidation of cyclobutanol suggests that the catalytic reaction proceeds through a mechanism involving formal hydride transfer. Mechanistic studies revealed that the photocatalytic substrate oxidation by -H was achieved in neither the lowest singlet excited state nor triplet excited state (S or T) but in the second lowest singlet excited state (S), i.e., (π-π*)* of the tpphz ligand. Thus, the photocatalytic substrate oxidation by -H can be categorized into unusual anti-Kasha photocatalysis.