Spectral, electrochemical, and catalytic properties of a homologous series of manganese porphyrins as cytochrome P450 model: The effect of the degree of β-bromination

A homologous series of β-brominated porphyrins derived from meso-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride, i.e., Mn(III)(BrxTCMPP)Cl (x = 0, 2, 4, 6, and 8), was prepared and investigated as cytochrome P450 models. Hydroxylations of cyclohexane by iodosylbenzene (PhIO) and iodobenzene diacetate (PhI(OAc)2) in the presence or absence of water were carried out as P450 model reactions. The influence of the degree of β-bromination of the macrocycle on the UV–vis spectra, the Mn(III)/Mn(II) reduction potential, and the catalytic properties of the Mn(III)(BrxTCMPP)Cl (x = 0, 2, 4, 6, and 8) series were examined. The catalytic efficiency does not correlate with the Mn(III)/Mn(II) reduction potential and shows a bell-shaped behavior, where the best results are achieved with the hexabrominated complex. Better hydroxylation yields were achieved by using PhI(OAc)2 as oxygen donor, but at expenses of catalyst recovery; addition of water to this system resulted in a increase in the reaction rate. Recycling of the more oxidatively robust complexes Mn(III)(Br6TCMPP)Cl and Mn(III)(Br8TCMPP)Cl is feasible when using PhIO as oxygen donor. Selectivity and UV–vis data suggested that hydroxylation by both PhIO and PhI(OAc)2 share closely related active species and mechanism. We also show that the Mn(III)/Mn(II) reduction potentials are inappropriate predictors of P450-type activity of Mn porphyrin-catalyzed oxidations.