Alcohol composition effects on the kinetics of manganese(V)(acetato)(oxo)(tetrakispentafluorophenylporphyrinato) generation

Abstract

Reaction rates of manganese(III)(acetato)(tetrakispentafluorophenylporphyrinato), Mn(III)(OAc)(TPFPP), with tetra-n-butylammonium hydrogen monopersulfate, nBu4NHSO5, in the presence of excess tetra-n-butylammonium acetate, nBu4NOAc, which results in the formation of manganese(V)(acetato)(oxo)(tetrakispentafluorophenylporphyrinato), Mn(V)(OAc)(O)(TPFPP), were studied in the presence and absence of four alcohols in CH2Cl2 solution, in order to determine the composition effects of alcohols on the kinetics of the reactions. A modified mechanistic model is employed for analysis of the obtained results. Addition of excess amounts of CH3OH to the reaction system reduced the formation rate of Mn(V)(OAc)(O)(TPFPP). The formation rate of Mn(V)(OAc)(O)(TPFPP) in the absence and presence of excess alcohols decreased in the order neat CH2Cl2 > CH3OH > t-C4H9OH > i-C3H7OH. The observed order for the alcohols complies with the order of their acidities, excepting t-C4H9OH. On the contrary, in the presence of only small amounts of either CH3OH or t-C4H9OH, k obs values were larger than that of neat CH2Cl2. Furthermore, k obs values initially increased and then decreased as CH3OH or t-C4H9OH/Mn(III)(OAc)(TPFPP) mole ratio increased from 0 to 1600. The apparent second-order rate constants for CH3OH and t-C4H9OH were different, under similar conditions. These results are primarily reflective of different acidities and molecular sizes of alcohols, as they form hydrogen bonds to HSO5 − anions in the reaction solution, prior or after the formation of Mn(III)(HSO5)(OAc)(TPFPP)−. Attempts are made to describe these observations and the obtained order of activation parameters in the presence of the alcohols in terms of their hydrogen bonding effects during formation of the activated complex.