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)] (H 2 papy = 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 (H 2 pppy = 2-[bis(2-hydroxybenzyl)aminomethyl]-pyridine; Hdpm = dipivaloylmethane). It is proposed that in a reaction of [Mn(pppy)(dpm)] with t-butyl hydroperoxide ( t BuOOH), the species [Mn(III)(pppy)(OH)( t BuOO · )] 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)] (H 2 tbpppy = 2-[bis(2-hydroxy-3,5-di-tert-butylbenzyl) aminomethyl]pyridine) is expected to be the best potential alkyd paint drier.

Four‐Coordinate Trispyrazolylboratomanganese and ‐iron Complexes with a Pyrazolato Co‐ligand: Syntheses and Properties as Oxidation Catalysts

“…Cyclohexene was chosen as the substrate as it is often used representatively for activity tests. [17][18][19][20][21][22][23][24] Two possible pathways are known for the reaction of complex metal(II) fragments with tBuOOH, [21,25] which differ from each other in the way the OÀO bond of the alkyl hydroperoxide is cleaved. Homolytic bond cleavage results in a metalA C H T U N G T R E N N U N G (III) hydroxido complex and an alkoxo radical that is able to abstract a hydrogen atom from the substrate, thus triggering the radical chain reaction described by Equations (1)- (10).…”

A monocarboxylate-bridged diiron(iii) μ-oxido complex that catalyzes alkane oxidation by hydrogen peroxide

“…A manganese complex with a similar coordination environment has been shown to exhibit good autooxidation activity. 26 It is possible, however, that both of these mechanisms may work simultaneously, as suggested earlier for a similar biomimetic system, viz. oxidation by H 2 O 2 catalyzed by [Fe 2 O(bpy) 4 ](ClO 4 ) 4 .…”

Low temperature oxidation of linseed oil: a review

“…Likewise, another polyamine 1,4,7-trimethyl-1,4,7-triazacyclononane (MeTACN) forms a complex with manganese(IV) (MnMeTACN) that catalyses the oxidation of ethyl linoleate (Oyman et al 2004). Working with ethyl linoleate as a surrogate for linseed oil, van Gorkum has reported [Mn(III)(tbpppy)(dpm)] (where H 2 tbpppy is 2-[bis(2-hydroxy-3,5-di-tert-butylbenzyl)aminomethyl] pyridine and Hdpm is dipivaloylmethane) to be the best potential alkyd paint dryer, acting through a reduction of Mn(III) to Mn(II) (van Gorkum et al 2007). Cobalt and copper salts have been reported to enhance the catalytic activity of the manganese salt in the decomposition of hydroperoxides (Minisci et al 2003).…”