Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

摘要

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn^III(TBP8Cz): TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato³⁻] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn^III(TBP8Cz) to give Mn^III(OTf)(TBP8Cz(H)) and [Mn^III(OTf)(H2O)(TBP8Cz(H)2)][OTf] with the protonation constants of 9.0 × 10⁶ M⁻¹ and 4.7 × 10³ M⁻¹, respectively. The protonated site of Mn^III(OTf)(TBP8Cz(H)) and [Mn^III(OTf)(H2O)-(TBP8Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn^III(OTf)(TBP8Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn^III(OTf)(TBP8Cz(H)) and [Mn^III(OTf)(H2O)(TBP8Cz(H)₂)][OTf] in the presence of O₂ revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn^III(OTf)(TBP₈Cz(H)) reacted with O₂ with a diffusion-limited rate constant to produce the putative Mn^IV(O₂^•−)(OTf)(TBP₈Cz(H)), whereas the tripseptet excited state of [Mn^III(OTf)(H₂O)(TBP₈Cz(H)₂)][OTf] exhibited no reactivity toward O₂. In the presence of HOTf, Mn^V(O)(TBP₈Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn^III(OTf)(TBP₈Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d₁₂, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn^III(OTf)(TBP₈Cz(H)), acts as an efficient photocatalytic for the oxidation of HMB by O₂ to the alcohol.