Oxidations by the system ‘hydrogen peroxide–[Mn2L2O3]2+ (L = 1,4,7-trimethyl-1,4,7-triazacyclononane)–oxalic acid’. Part 11. Degradation of dye Rhodamine 6G and oxygenation of cyclohexene

Two dinuclear manganese(IV) compounds, soluble [LMn(O)3MnL](PF6)2 (1a) and insoluble [LMn(O)3MnL]2[SiW12O40] (1b) (L is 1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN) are efficient catalysts in oxidative decoloration of dye Rhodamine 6G and oxygenation of cyclohexene with hydrogen peroxide in acetonitrile or water in the presence of oxalic acid which is an obligatory co-catalyst. It has been concluded on the basis of the detailed kinetic study that both oxidation processes occur with the formation of transient species D (which exhibits the properties of a relatively weak radical and is able to abstract the hydrogen atom from C–H bonds of hydrocarbons) and E (which is responsible for the cyclohexene epoxidation). Species D is probably an oxygen-centered radical containing also manganese ions. Species E is apparently an oxoderivative of high-valent manganese. Catalyst 1a generates predominantly species E. When heterogenized catalyst 1b is used species D prevail. As a very intensive dye degradation occurs also in aqueous solution the studied systems can be used for decoloration of pollutants in waste streams.

Graphical abstractSoluble [LMn(O)3MnL](PF6)2 and insoluble [LMn(O)3MnL]2[SiW12O40] (L is 1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN) manganese complexes efficiently catalyze decoloration of dye Rhodamine 6G and oxygenation of cyclohexene with hydrogen peroxide in the presence of oxalic acid which is an obligatory co-catalyst. The kinetics of the reaction have been studied and a mechanism for the generation of two different oxidizing species has been proposed.Figure optionsDownload full-size imageDownload as PowerPoint slide