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.

Abstract

The reactivities of [Mn(13-TMC)(OOH)]2+ (1) and [Mn(13-TMC)(O2)]+ (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 MnIII-peroxo (2hs and 2ls) 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 (1ls and 2ls), by +15.6 and +25.5 kcal/mol, respectively. The computed mechanistic pathways clearly indicate that the sulfoxidation of thioanisole by 1hs is kinetically (by +16.6 to +46.1 kcal/mol) and thermodynamically (+14.4 to +56.1 kcal/mol) more preferred than 1ls, 2hs, and 2ls 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 ΔG‡ 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 2hs 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 ΔG‡ of 2hs by 58.7%, which is in fact lower than the ΔG‡ 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.