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

UV–vis spectral
titrations of a manganese­(III) corrolazine complex [MnIII(TBP8Cz)] with HOTf in benzonitrile (PhCN) indicate mono-
and diprotonation of MnIII(TBP8Cz) to give MnIII(OTf)­(TBP8Cz­(H)) and [MnIII(OTf)­(H2O)­(TBP8Cz­(H)2)]­[OTf] with protonation
constants of 9.0 × 106 and 4.7 × 103 M–1, respectively. The protonated sites of MnIII(OTf)­(TBP8Cz­(H)) and [MnIII(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 [MnIII(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 MnIII(OTf)­(TBP8Cz­(H)) and [MnIII(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 MnIII(OTf)­(TBP8Cz­(H)) reacted with O2 with a diffusion-limited
rate constant to produce the putative MnIV(O2•–)­(OTf)­(TBP8Cz­(H)), whereas
the tripseptet excited state of [MnIII(OTf)­(H2O)­(TBP8Cz­(H)2)]­[OTf] exhibited no reactivity
toward O2. In the presence of HOTf, MnV(O)­(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding
alcohols, accompanied by regeneration of MnIII(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, MnIII(OTf)­(TBP8Cz­(H)), acts as an efficient photocatalyst for
the oxidation of HMB by O2 to the alcohol.