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 (1) bearing a pyrazine moiety as the active site in tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz) as a ligand was employed as a photocatalyst. The active species for the photocatalysis was revealed to be not complex 1 itself but the protonated form, 1-H⁺, protonated at the vacant diimine site of tpphz. Upon photoexcitation in the presence of an organic substrate, 1-H⁺ was converted to the corresponding dihydro-intermediate (2-H⁺), where the pyrazine moiety of the ligand received 2e^– and 2H⁺ from the substrate. 2-H⁺ was facilely oxidized by O₂ to recover 1-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 2-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 1-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 1-H⁺ can be categorized into unusual anti-Kasha photocatalysis.