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 O₂ as a terminal oxidant to afford only 2e^--oxidized products without the reductive activation of O₂ in acidic water under visible-light irradiation. A Ru^II complex (<b>1</b>) bearing a pyrazine moiety as the active site 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. The active species for the photocatalysis was revealed to be not complex <b>1</b> itself but the protonated form, <b>1</b>-H⁺, protonated at the vacant diimine site of tpphz. Upon photoexcitation in the presence of an organic substrate, <b>1</b>-H⁺ was converted to the corresponding dihydro-intermediate (<b>2</b>-H⁺), where the pyrazine moiety of the ligand received 2e^- and 2H⁺ from the substrate. <b>2</b>-H⁺ was facilely oxidized by O₂ to recover <b>1</b>-H⁺. Consequently, an oxidation product of the substrate and H₂O₂ derived from dioxygen reduction were obtained; however, the H₂O₂ formed was also used for oxidation of <b>2</b>-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 <b>1</b>-H⁺ was achieved in neither the lowest singlet excited state nor triplet excited state (S₁ or T₁) but in the second lowest singlet excited state (S₂), i.e., ¹(π-π*)* of the tpphz ligand. Thus, the photocatalytic substrate oxidation by <b>1</b>-H⁺ can be categorized into unusual anti-Kasha photocatalysis.