Manganese dihydrogen complex

The oxidation of saturated hydrocarbons in the presence of iron- or manganese-containing catalysts can be achieved by using a variety of oxidants including alkyl hydroperoxides, peroxycarboxylic acids, iodosyl-benzene, dihydrogen peroxide, and dioxygen (9-11). It has been shown that chiral iron- and manganese-porphyrin complexes catalyze the asymmetric epoxidation of unfunctionalized alkenes (75). Except for a number of experiments in which up to 96 % enantiomeric excess (ee) has been reported (16,17), in most epoxidation reactions with chiral porphyrins only a low to moderate enantiomeric excess of the product is obtained (18,19). In association with these catalysts, alkyl hydroperoxides and iodosylbenzene are often used as primary oxidants (18,19). 

UV irradiation of [cp Re (CO)3] (M = Mn, Re cp = rj -CsHs) in the presence of hydrogen yielded the non-classical dihydrogen complex [cp Re (CO)2 (H2)], but the classical dihydride [cpRe (CO)2 ra 5-H2]. The manganese complex possessed sufficient thermodynamic stability to arrest further oxidative addition at the metal centre. Formation of a rhenium trans-dihydhde complex must have occurred via a c/.s-intermediate. The latter is thermodynamically unstable and the researchers deduced the role of the hydrogen-rich environment in kinetically sustaining the mechanism [12]. 

Many manganese complexes decompose dihydrogen peroxide, but we limit our discussion to the functional dinuclear ones the catalase activity of bis-dinuclear (tetranuclear) photosystem II (PSII) models is discussed later. Furthermore, mainly model complexes reported in the last 5 years are discussed in detail since previous work is covered in several excellent reviews [1,2b,3a,5,8-12],... 

The present review has outlined the efforts to develop biomimetic non-heme iron and manganese catalysts for alkane hydroxylation, olefin epoxidation, and cis-dihydroxylation reactions. However, the examples reviewed here are mostly presented as reported in the literature, since the various reaction conditions involved in the catalytic oxidations hamper a direct comparison and critical evaluation of the data. The survey has not only illustrated a rich variety of iron and manganese complexes that lead to the successful structural modeling of important non-heme iron and manganese enzymes, but also significant features of the oxidation reactions catalyzed by these complexes in combination with dihydrogen peroxide. 

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