Piridilporfirinas de manganês: estudo comparativo da eficiência catalítica de isômeros de 2ª e 3ª gerações para a oxidação de cicloexano

Abstract

In this work are described the synthesis and characterization o fmanganese porphyrin (MnP) derivedof 4-pyridyilporphyrin: chlorideof 5,10,15,20-tetrakis(4-pyridyl)manganeseporphyrin(III), MnIII(T4PyP)Cl a 2nd generation metalloporphyrin, and 5,10,15,20-tetrakis(4-pyridyl)-2,3,7,8,12,13,17,18-octabromoporphyrin-manganese(II), MnII(Br8T4PyP) a 3rd generation unpublished metalloporphyrin. Cyclicvoltammetry studies for these metalloporphyrins showed that MnII(Br8T4PyP) presented an anodic shift of its half-wave potential (centered on the metal ion, MnIIIP MnIIP) of 300 mV compared to the analogous non-brominated. Furthermore, the redox process (MnIII/MnII) for metalloporphyrins MnIII(T4PyP)Cl and MnII(Br8T4PyP) is considered reversible. The Cyclic voltammetry studies also were performed for 2nd generation isomers, MnIII(TXPyP)Cl, and 3rd generation, MnII(Br8TXPyP) (with X = 2, 3) and similar results were obtained. Regarding the characterization of isomers MnII(Br8TXPyP), with X = 2, 3 e 4, by electron paramagnetic resonance (EPR), the results show the six lines concerning the hyperfine structure of the coupling of the nucleus f spin (55Mn , S=5/2) with the electron spin, characteristics of manganese ion +2.The 2nd generation of manganese porphyrins, MnIII(TXPyP)Cl, and 3rd generation, MnII(Br8TXPyP) (with X = 2, 3and 4) were used as catalysts in the oxidation reaction of cyclohexane by PhIO and PhI(OAc)2. For systems containing any of the oxidants, the position of the nitrogen atom in the pyridine ring of meso positions, does not significant influence on the yield of products, for the isomers MnIII(T2PyP)Cl and MnIII(T3PyP)Cl. Systems with manganeseporphyrin 3rd generation À-octabrominated, MnII(Br8T2PyP) and MnIII(Br8T3PyP) were slightly higher yields for cyclohexanol, in relation to manganese porphyrins 2nd generation, MnIII(T2PyP)Cl and MnIII(T3PyP)Cl,when the oxygen donor used was PhI(OAc)2. However, for systems with oxygen donor PhIO, the À-octabrominated did not significantly influence the yields for oxygenates. The addition of water or the nitrogenous base imidazole to systems with any of the catalysts, did not cause significant changes in income for oxygenates, cyclohexanol and cyclohexanone. In general, the oxidation reactions of cyclohexane using PhIO as oxidant led to a lower total yield (C-ol + C-one) than those using PhI(OAc)2, however the degree of destruction for systems with PhIO were much lower. Lastly, systems with manganese porphyrins MnIII(T4PyP)Cl and MnII(Br8T4PyP)had lower incomes for oxygenatescompared to other isomers.