Mediated kinetic medium effect in the reaction of bis-3,5-di-iso-propylsalicylatozinc(II) with tert-butylperoxyl radicals

Abstract

Absolute rate constants for the reaction of ZnII(3,5-di-iso-propylsalicylate)2 with tert-butylperoxyl radicals were determined by means of kinetic electron paramagnetic resonance measurements at −31.5°C in 10% (v/v) chlorobenzene in hexane, as well as with addition of approximately equivalent amounts of the Lewis base dimethylsulfoxide (DMSO). Differential pulse voltammetry was used to determine the redox behavior of this chelate with different amounts of DMSO in a CH2Cl2 medium at 25°C. A good correlation was observed between the kinetic effect of the DMSO on the antiperoxyl radical reactivity of the zinc complex, which is due to transfer of a hydrogen atom from the ligand, and the redox behavior. These also correlated with changes in the 1H NMR and FTIR spectra of the chelate in these media. A mediated kinetic medium (solvent) effect (MKME or MKSE) for DMSO has been established. The essence is that electron pair donors such as DMSO at molar equivalent quantities do not directly impede the reaction center of radical scavenging (hydrogen atoms of salicylic OH groups that are not involved in intramolecular hydrogen bonding), but act indirectly by bonding axially to the zinc, which changes the coordination geometry in such a way as to increase intramolecular hydrogen bonding within the coordinated ligand and thus considerably diminish the fraction of OH groups available to transfer a hydrogen atom. As a result, the antiperoxyl radical H-atom donating reactivity of ZnII(3,5-di-iso-propylsalicylate)2 decreases upon coordination of DMSO.