A Density Functional Theory Study on Comparing the Reactivity of [Mn(13-TMC)(OOH)]2+ and [Mn(13-TMC)(O2)]+ for the Sulfoxidation of Thioanisole: Elucidation of Substrate and Non-Redox Metal Ion Effects

The reactivities of [Mn(13-TMC)(OOH)](2+) (1) and [Mn(13-TMC)-(O-2)](+) (2) in the sulfoxidation of thioanisole have been compared using density functional theory methods. The orientation of the 13-TMC ligand and substrate and non-redox metal ion effects have been considered to improve the oxidation efficiency of 1 and 2. In 1, the syn- and anti-orientation of the 13-TMC ligand do not change the coordination of the Mn ion. In contrast, the orientation of the 13-TMC ligand regulates the geometry of 2, wherein the syn-13-TMC ligand exhibits the Mn-III-peroxo (2(hs) and (2l)s) species, while the anti-13-TMC shows the MnII-superoxo (2' hs and 2'ls) species. However, the MnII-superoxo species are found to be less stable than the MnIII-peroxo complexes by around +26.6 kcal/mol. The ground state geometries of 1 and 2 with the syn-13-TMC ligand are found to be more stable in the high- (S = 2) spin states (1hs and 2hs) than the low- (S = 1) spin complexes (1(ls) and 2(ls)), by +15.6 and +25.5 kcal/mol, respectively. The computed mechanistic pathways clearly indicate that the sulfoxidation of thioanisole by 1(hs) is kinetically (by +16.6 to +46.1 kcal/mol) and thermodynamically (+14.4 to +56.1 kcal/mol) more preferred than 1(ls), 2(hs), and 2(ls) species. This is mainly due to the feasible heterolytic O1-O2 bond cleavage followed by the proton transfer step. In addition, the molecular electrostatic potential analysis indicates that the higher oxidation efficacy of 1hs than 2hs is due to the -OOH moiety. The reactivity of 1hs is further enhanced by incorporating electron donating substituents in thioanisole, wherein the p-NH2 thioanisole decreases the.Go of 1hs by 28%. Interestingly, the incorporation of non-redox metal ions (Mn+ = Sc3+, Y3+, Mg2+, and Zn2+) improves the reactivity of 2hs, wherein the non-redox metal ions tend to bind with the oxygen atoms of 2(hs) and subsequently shift the one-electron reduction potential (E0 (red) vs SCE) toward the positive side. The positive shift in the E0 (red) is more evident in 2hs-Y3+ that significantly decreases the.Go of 2hs by 58.7%, which is in fact lower than the.GO of 1hs by +2.0 kcal/mol. Hence, in the presence of Y3+, the reactivity of 2hs is comparable with 1hs in the sulfoxidation of thioanisole.

Automated summary

The study compared the reactivities of [Mn(13-TMC)(OOH)](2+) and [Mn(13-TMC)-(O-2)](+) in the sulfoxidation of thioanisole, showing that the former is kinetically and thermodynamically preferred due to feasible heterolytic O1-O2 bond cleavage and proton transfer steps in the reaction mechanism. Incorporating electron donating substituents in thioanisole and non-redox metal ions such as Y3+ can enhance the reactivity of [Mn(13-TMC)-(O-2)](+) species, making it comparable to [Mn(13-TMC)(OOH)](2+) in the sulfoxidation process, especially with Y3+ present.