Manganese molecular complexes

Selective Oxidations by Heterogeneous Manganese Molecular Complexes... 

Another area of active research is the development of stable low molecular weight metal complexes, which could serve as SOD mimics. Fridovich has described a complex of mangsmese (III) with desferral, which can catalyse the dismutation of superoxide anion in vitro and can protect green algae against paraquat toxicity (Beyer and Fridovich, 1989). This manganese-desferral complex was evaluated in models of circulatory shock and also found to improve survival rate (de Garavilla etal., 1992). 

A new stereoselective epoxidation catalyst based on a novel chiral sulfonato-salen manganese(III) complex intercalated in Zn/Al LDH was used successfully by Bhattacharjee et al. [125]. The catalyst gave high conversion, selectivity, and enantiomeric excess in the oxidation of (i )-limonene using elevated pressures of molecular oxygen. Details of the catalytic activities with other alkenes using both molecular oxygen and other oxidants have also been reported [126]. 

Asymmetric imidations of aryl alkyl sulfides with [(tosylimino)iodo]ben-zene, catalyzed by various chiral (salen)manganese(III) complexes, have been investigated in some detail [31,32]. The influence of catalyst structure, solvent, temperature, 3°-amine AT-oxides, and the presence of molecular sieves on product yields and the enantioselectivity of imidation with 17 was evaluated. Enan-tioselectivities as high as 90 % ee and 97 % ee with methyl 2-nitrophenyl sulfide and methyl 2,4-dinitrophenyl sulfide, respectively, were achieved. 

The reactions of the several manganese gluconate complexes with molecular oxygen and hydrogen peroxide have been studied in terms of stoichiometry and reaction kinetics. Reaction mechanisms are proposed on the basis of the kinetic data. In addition, the thermodynamic and mechanistic characteristics of an ideal model system for photosystem-II are analyzed and evaluated. 

On the basis of these prior electrochemical (62, 63) and kinetic (64) studies and the present results, the reaction stoichiometries for the manganese gluconate complexes with molecular oxygen and hydrogen peroxide can be expressed by a series of reactions. The primary reaction between the Mn(II) complex and molecular oxygen is rapid (Reaction... 

One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. 

Figure 4. Molecular structure of 1,3- cyclohexadienyl)manganese tricarbonyl complexed with three units of thiourea.

Luneau, D. and Rey, P. (1995) New manganese(II) complexes of nitronyl nitroxide radicals. Synthesis, structure and magnetic properties. Molecular Crystals and Liquid Crystals Science and Technology Section A, 273, 81-87. 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

Z.H. Luan, J. Xu, and L. Kevan, Manganese-bipyridine Complex Incorporated into Mesoporous MCM-41 Molecular Sieves. Chem. Mater., 1998, 10, 3699-3706. 

Series Pd-Mn / SiOa catalysts have been prepared from molecular complexes. The precursors, palladium (II) bis-acetylacetonate and manganese (II) bis-acetylacetonate, were purchased fi om STREM Chemicals Inc. 

Taken together, the various reactions and interconversions of these manganese porphyrin complexes have allowed the examination of each step in the activation of molecular oxygen by the mechanism suggested for P-450. Detailed mechanistic studies of the 0-0 bond cleavage event in 29 by kinetics, substituent effects, and product analysis showed that the reaction proceeds via heterolysis to produce 27 when acid is present, whereas homolysis is predominant in the absence of acid but in the presence of hydroxide ion (95). Under basic conditions, homo-lytic cleavage of the 0-0 bond of 29 forms Mn (=0)TMP (28) and an acyl-oxyl radical. Thus, when an alkyl peroxy acid is employed, decarboxylation competes with electron transfer, as shown in Scheme IX, to afford a mixture of 27 and 28. Yuan and Bruice have proposed a similar heterolysis mechanism based on the kinetic analysis for the reaction of mCPBA with catalytic amounts ofMn TPP(/(W). 

The preparation and characterization of novel manganese (III) complexes of various porphyrin and porphyrin-likes macrocycles have continued to attract strong attention especially because of their importance in catalytical oxidation processes through the formation of a Mn(V)0 intermediate (see Section 6) and as model for metalloenzymes. In this line, an artificial enzyme formed through a directed assembly of a molecular square that encapsulated a Mn porphyrin has been prepared and investigated as a catalyst. In contrast to symmetrical binuclear bis(phenoxo) bridged macrocyclic Mn(III)Mn(III) complexes, unsymmetrical ones are rare. A new series of these kinds of carboxylate-free complexes has been described and their redox properties investigated. " ... 

In plant photosynthesis molecular oxygen is produced from water. In this process tyrosine molecules carry electron holes from oxidized chlorophyll molecules to manganese-protein complexes. Short-lived tyrosine radicals keep the photoexcited chlorophyll molecules apart from reactive oxygen intermediates, such as peroxides and oxygen atoms (Calvin, 1978 Tommos et al, 1995, 1998)... 

Figure 7.2.7 A salicylaldimine-manganese(III) complex dimerizes and is protonated. The visible spectrum shows a small charge-transfer band close to 600 nm belonging to the dimer. Irradiation with visible light produces some molecular oxygen. Its formation may occur in the way indicated by the given stoichiometric equation. (From Ashmawy et al., 1985, and Awad and Anderson, 1989.)...

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