Probing the generation, reactivity, and kinetics of high-valent manganese-oxo phthalocyanines: Insights into oxidation mechanisms

Abstract

In this study, manganese(IV)-oxo phthalocyanines [Mn^IV(Pc)(O)] (3) (Pc = phthalocyanine) were produced either through visible light photolysis of [Mn^III(Pc)(ClO₃)] or by chemical oxidation of [Mn^III(Pc)Cl] (1) with iodobenzene diacetate. The manganese(IV)-oxo species under study include tetra-tert-butylphthalocyaine‑manganese(IV)-oxo (3a) and phthalocyanine‑manganese(IV)-oxo (3b). As anticipated, the generated 3 reacted with various organic substrates to yield the oxidized products and further proved to be a competent oxidant via an H₂¹⁸O isotope labeling experiment. The kinetics of oxygen atom transfer (OAT) reactions for these generated 3 species with a range of substrates were examined in CH₃CN solutions unless other specified. Overall, the second-order rate constants under pseudo-first-order conditions for 3a and 3b with the same substrates display similar modest reactivity, with the nature of the substrate playing a major role in influencing the reactivity of species 3. The phenol substrates, in particular, reacted the fastest. Of note, second-order rate constants determined for thioanisoles are comparable to those of alkene epoxidations and activated CH bond oxidations. Conventional Hammett analyses of rate constants for substituted styrenes revealed a linear correlation with the σ constant, indicating minimal charge developed at the transition state during the oxidation process. Additionally, the competition product studies and the Hammett correlation analysis further suggested that the manganese(IV)-oxo species observed in those kinetic studies are unlikely to serve as the primary oxidant for the epoxidation reactions catalyzed by manganese(III) phthalocyanine with PhI(OAc)₂.