The autoxidation activity of new mixed-ligand manganese and iron complexes with tripodal ligands

Abstract

The activity of new manganese and iron complexes of dianionic tripodal ligands in the autoxidation of ethyl linoleate (EL) is reported. EL consumption rates were monitored using time-resolved FTIR and the degree of oligomerisation was determined by SEC. Almost all complexes showed the same trend in the autoxidation of EL. After a short induction time, the reaction started at a relatively high constant rate; later, this rate changes to a lower rate, which was again constant and on average was the same for all complexes studied. The observation that the autoxidation rate eventually became the same for all complexes can be explained with the assumption that a catalytic species was formed that was structurally similar for each complex. The crystal structure of a representative new complex, [Mn(papy)(acac)] (H2papy = N-(2-hydroxybenzyl)-N-(2-picolyl)-glycine; Hacac = acetylacetone), was elucidated, demonstrating a Jahn–Teller distorted octahedral geometry of the Mn(III) ion. To gain insight into the reaction mechanisms by which these manganese complexes react with peroxides, the reaction of [Mn(pppy)(dpm)] with alkyl hydroperoxides was studied using UV–vis and EPR spectroscopy (H2pppy = 2-[bis(2-hydroxybenzyl)aminomethyl]-pyridine; Hdpm = dipivaloylmethane). It is proposed that in a reaction of [Mn(pppy)(dpm)] with t-butyl hydroperoxide (tBuOOH), the species [Mn(III)(pppy)(OH)(tBuOO•)] is formed. In analogy with the formation of that species, which is postulated to occur through a reduction to Mn(II), it is proposed that in the direct initiation reaction of EL with a manganese(III) complex, a 1 electron–1 proton transfer reaction leads to the formation of Mn(II) and an EL radical. Considering parameters such as autoxidation activity, complex solubility and ease of complex preparation, the compound [Mn(III)(tbpppy)(dpm)] (H2tbpppy = 2-[bis(2-hydroxy-3,5-di-tert-butylbenzyl) aminomethyl]pyridine) is expected to be the best potential alkyd paint drier.