Tuning catalytic activity with steric and electron-withdrawing effects of a porphyrin substituent

Abstract

Tuning of the electron-withdrawing effect of a ligand is an essential process to develop an efficient metal-complex catalyst. We studied here the steric and electron-withdrawing effects on the catalytic activity of an iron(III) porphyrin catalyst for the hydroxylation reaction of a nonactivated alkane. Although previous studies reported that the catalytic activity increases with an increase in the electron-withdrawing effect of the porphyrin substituent, this study shows that an excessive electron-withdrawing effect of the porphyrin substituent leads to a decrease of the catalytic activity. Kinetic analysis of the catalytic reaction reveals that the decrease in the catalytic activity can be rationalized with the change of the rate limiting step, which is controlled by the formation and reaction rates of a reactive intermediate: compound I. The increase in the electron-withdrawing effect increases the reaction rate, but decreases the formation rate, shifting the rate limiting step from the reaction step to the formation step. Because of the low formation rate, compound I having an excessive electron-withdrawing effect is hardly generated in the catalytic reaction, resulting in a decrease in the catalytic activity. This study also shows that the steric effect of the o-position of the meso-aryl group of the iron porphyrin is an essential factor for it to be an efficient catalyst. The results presented in this study provide us insight on how to tune the catalytic activity of a metal-catalyst with the electron-withdrawing and steric effect of the ligand.