Journal of Molecular Catalysis A-Chemical, Vol.266, No.1-2, 274-283, 2007
Cyclohexane hydroxylation by iodosylbenzene and iodobenzene, diacetate catalyzed by a new beta-octahalogenated Mn-porphyrin complex: The effect of meso-3-pyridyl substituents
The synthesis of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(3-pyridyl)porphyrinato manganese(II) (Mn(H)Br(8)T3PyP), anew compound, is described along with some of its electrochemical and catalytic properties. Relative to 5,10,15,20-tetrakis(3-pyridyl)porphyrinatomanganese(RI) chloride (Mn(III)T3PyPCl, E-1/2 = -0.07 V versus SCE, N,N-dimethylformamide) the Mn(III)/Mn(II) reduction potential of Mn(II)Br(8)T3PyP shows anodic shift of 0.66 V. These compounds have been studied as catalysts in the hydroxylation of cyclohexane by iodosylbenzene (PhIO) and iodobenzene diacetate (PhI(OAc)(2)). The roles of water and an exogenous axial ligand (imidazole) were also examined. The results show that whereas Mn(HI)T3PyPCl performs much better with PhI(OAc)(2), Mn(II)Br(8)T3PyP performs better with PhIO; no significant differences in selectivity were observed on changing the oxygen donor. Moderate to good catalytic efficiency may be achieved with the use of a low oxidation-state Mn-porphyrin catalyst, i.e. Mn(II)Br(8)T3PyP, but the beta-octabromination in this particular case neither increased overall catalyst performance (as compared to the non-brominated analogue), nor yielded a more oxidatively robust catalyst. Catalytic and electrochemical properties of these 3-pyridyl systems are in sharp contrast with their isomeric, 2-pyridyl counter-parts. (c) 2006 Elsevier B.V. All rights reserved.