Metal Complexes as Antioxidants. I. The Reaction of Zinc Dialkyldithiophosphates and Related Compounds with Peroxy Radicals

Abstract

The kinetics of the inhibition of the autoxidation of several hydrocarbons by a number of zinc dialkyldithiophosphates and by zinc isopropylxanthate and zinc diethyldithiocarbamate have been studied at 30' and at 50'. The oxidations were generally auto-retarding but initial rate measurements showed that these compounds trapped peroxy radicals and allowed rate constants for this process to be calculated. Rate constants for the reaction of t-butylperoxy radicals with these compounds have been measured by a kinetic e.p.r. method in the temperature range 0' to -90'. Extrapolation of the e.p.r. data to the temperatures of the inhibition studies showed that the various experimental procedures yielded results in satisfactory agreement with one another.It is suggested that the reaction of peroxy radicals with zinc complexes involves reaction at the metal center either by an electron transfer or an SH2 process.Les cinetiques d'inhibition de I'auto-oxydation de plusieurs hydrocarbures par un certain nombre de phosphates de dialkyldithio zinc, d'isopropylxanthate de zinc et de diethyldithiocarbamate de zinc, ont etC CtudiCes h 30'et a 50'. Les oxydations sont generalement auto-retardees rnais les mesures de vitesse initiale montrent que ces composes piegent les radicaux peroxy et permettent ainsi le calcul des constantes de vitesse pour ce processus. Les constantes de vitesse pour la reaction des radicaux I-butylperoxy sur ces composes ont kt6 mesurees par une methode cinetique r.p.e. dans I'intervalle de temperature de 0' a -90'.L'extrapolation des donnees r.p.e. aux temperatures d'etude de I'inhibition montre que tous les divers procedes experimentaux donnent des resultats en accord satisfaisant.On a suggCrC que la reaction des radicaux peroxy sur les co~nplexes de zinc implique une reaction sur le centre metallique soit par un transfert d'electron soit par un processus SH2.

The Quenching of Singlet Oxygen (1Δg) by Transition Metal Chelates

“…lrradiation of either a sensitizer-free solution of 3-methyl-2-pentene in chlorobenzene or an olefin-free solution of tetraphenylporphine in chlorobenzene yielded no detectable hydroperoxide ( 4 x M ) . Many metal chelates are rapid decoinposers of hydroperoxides (6,16). However, separate dark experiments with oxidized 3-methyl-2-pentene solutions showed that the hydroperoxide concentrations did not decrease either in the presence of nickel(I1) [2,2'-thiobis-4-(I, l,3,3-tetramethylbuty1)phenolate]-12-butylam~ne or of the sensitizer.…”

Decomposition reactions of cumyl hydroperoxide with sulfur-containing metal complexes.

“…Now, the formation of the phenoxy radical is well interpreted by the proposal that a complex can decompose cumyl hydroperoxide homolytically by the sulfur-containing chelate ring as reported by Burn et al2), (1) and the resulting cumyloxy (or cumyl peroxy) radical reacts immediately with the phenolic moiety contained in the complex to give the phenoxy The g-values and splitting constants (a) of ESR signals for phenoxy radicals are shown in Table 3. The triplet-triplet splitting is ascribable to coupling with protons of -CH2-on para position and mhydrogen.…”

Metal complexes as antioxidants. 9. Application of the spin scavenging technique to the reaction of transition metal dialkyldithiophosphates and dialkyldithiocarbamates with tertiary alkyl hydroperoxides

“…Zinc diisopropyldithiophosphate, nickel diisopropylor stable free-radical (2, 3). Moreover, these radi-dithiophosphate, zinc diethyldithiocarbamate, nickel diethylcals may initiate hydrocarbon autoxidation, induce dithiocarbamate, and nickel di-n-butyldithiocarbamate had been hydroperoxide decomposition, and give rise to the prepared previously in our laboratory (4)(5)(6). tert-Butyl hydroperoxide, cumene hydroperoxide, diisopropyldithiophosphoryl chemiluminescence with autoxi-disulphide.…”

“…Application of the usual steady-state assumptions indicates that the rate of disappearance of 2 should be given by [4].…”

“…dimethyldithiuram disulphide, diethyldithiophosdation (2). phoric acid, tert-butyl hyponitrite, and chlorobenzene were In addition to qualitatively demonstrating the prepared and purified by the methods described previously in production of free-radicals, the spin scavenging this series (4)(5)(6). technique can be used to provide quantitative Procedure estimates of rates of free-radical initiation.…”