Manganese(II) complexes with Bn-tpen as powerful catalysts of cyclohexene oxidation

Manganese(II) complex [(Bn-tpen)MnII]2+ activated dioxygen for oxidation of cyclohexene in acetonitrile (MeCN) and methanol (MeOH). In MeCN, ketone (2-cyclohexen-1-one), alcohol (2-cyclohexen-1-ol) and small amounts of epoxide (cyclohexene oxide) were produced in this reaction, while in MeOH only ketone was formed. In the most efficient experiment, the combination of 2.5 × 10-4 mol% [(Bn-tpen)MnII]2+ and 4 M cyclohexene under dioxygen atmosphere (pO2 = 1 atm) in MeCN after 24 h of reaction, gave the TON equal to 716, and the main oxidation products were ketone (196 mM) and alcohol (147 mM), whereas epoxide was formed in insignificant amounts (15 mM). The formation of [(Bn-tpen)MnIV=O]2+ and [(Bn-tpen)MnIII-OH]2+ species was confirmed. The novelty of this work is the observation, that in both solvents, [(Bn-tpen)MnII]2+ complex is initially oxidized by t-BuOOH to produce Mn(III)-complex, which is reduced back by cyclohexene to [(Bn-tpen)MnII]2+, and the latter species is an active catalyst of c-C6H10 oxidation. Knowledge of the electrochemical properties of the system components may contribute to understanding the mechanisms involving participation of the active agents created in the system.