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

• Published in: Journal of the American Chemical Society Vol. 146; no. 48; pp. 33022 - 33034
• Main Authors: Ishizuka, Tomoya, Nishi, Taichiro, Namura, Nanase, Kotani, Hiroaki, Osakada, Yasuko, Fujitsuka, Mamoru, Shiota, Yoshihito, Yoshizawa, Kazunari, Kojima, Takahiko
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.12.2024
• Subjects: active sites; benzaldehyde; benzyl alcohol; hydrides; irradiation; ligands; light; moieties; oxidants; oxidation; oxygen; photocatalysis; photocatalysts; pyrazines; ruthenium; species
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c09962

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 RuII 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.