Reactivity of substituted copper(II) salicylates with tert -butylperoxyl radical: Structure-reactivity relationships

Abstract

Absolute rate constants (keff) for the chemical reactions of Cu(II)2(3,5-di-iso-propylsalicylate)4(H2O)3, Cu(II)2(3,5-di-tert-butylsalicylate)4, Cu(II)2(3,5-di-tert-butylsalicylate)4(H2O)4, Cu(II)2(3,5-dimethylsalicylate)4(H2O)3, Cu(II)2(3-ethylsalicylate)4(H2O), Cu(II)2(3-phenylsalicylate)4, and Cu(II)(3,5-di-iso-propylsalicylate)2(pyridine)2 with tert-butylperoxyl radical were determined using kinetic electron paramagnetic resonance measurements in 10% toluene in the hexane medium at temperatures ranging from −63°C to 2°C. These antioxidant (AO) chelates were ranked by their reactivity as follows: 2,6-di-tert-butyl-4-methylphenol ≫ Cu(II)2(3,5-di-tert-butylsalicylate)4 ≅ Cu(II)2(3-phenylsalicylate)4 > Cu(II)2(3,5-di-iso-propylsalicylate)4(H2O)3 ≅ Cu(II)2(3,5-di-tert-butylsalicylate)4(H2O)4 ≅ Cu(II)2(3,5-dimethylsalicylate)4(H2O)3 > Cu(II)2(3-ethylsalicylate)4(H2O) ≫ Cu(II)(3,5-di-iso-propylsalicylate)2(pyridine)2 at 20°C. Differential pulse voltammetry was used to determine redox behavior of these chelates in CH2Cl2. Two types of salicylic OH groups were detected in these Cu(II) salicylates, characterized by the presence or absence of AO reactivity. One of them was coordinate covalently bonded to Cu(II) via the oxygen atoms of the salicylic OH groups, displaying oxidation peak potentials in the range from +650 to 970 mV versus Ag/Ag+. The second type was intramolecularly hydrogen bonded to carboxylate oxygens, with an oxidation peak potential in the range from +1100 to 1200 mV versus Ag/Ag+. It was concluded that non–hydrogen-bonded salicylic OH groups are responsible for the antiperoxyl radical reactivity of these chelates, while neither Cu(II) nor salicylate ligands displayed reactivity with peroxyl radical. It has been established in this research that axially bonded electron pair donors such as pyridine and water decrease H-donating reactivity of Cu(II) salicylates by promoting the formation of intramolecular hydrogen bonding between the salicylic OH hydrogen atoms and carboxylate oxygen atoms in the salicylic ligands. Dependences of log keff at 20°C and the anodic oxidation potential (Epa) for the salicylic OH group on the difference between symmetric and asymmetric stretching frequencies of carboxylate groups (in Fourier transform infrared spectra) for the substituted Cu(II) salicylates were determined. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 56–67, 2010