Oxidation Efficiencies of High Spin Fe(II)-Azo Amino Acid Complexes by Potassium Peroxydisulfate: Initial State-Transition State Solvation Effects.

Syntheses of four iron(II)-azo amino acid complexes ( 1-4) with the general formula [FeL(HO)]Cl· nHO, where L represents azo amino acid ligands, were carried out by direct mixing of N, N-diethylamino-4-nitrosoaniline, amino acids (alanine, histidine, tryptophan or phenylalanine) and Fe(II) ions in aqueous-methanol media. Azo ligands act as bidentate ligands and coordinate to Fe(II) ions through O-carboxylic and N-azo groups. Complexes 1-4 were characterized by IR, UV-visible spectra, thermogravimetric analyses (TGA) and conductance measurements. Kinetics of 1-4 oxidation by peroxydisulfate ions were studied spectrophotometrically in an aqueous media and in various aqueous-methanol binary mixtures at 25 °C. Kinetics of the oxidation followed pseudo-first-order reaction kinetics, $$ k_{\text{obs}} = (k_{2} [{\text{S}}_{2} {\text{O}}_{8}^{2 - } ])[{\text{complex}}] $$ . Reactivity trends and their rate constants are discussed in terms of polarity, hydrophobicity of 1-4, and solvation effect of methanol. The effect of methanol ratios on the oxidation reaction is analyzed into initial ( is) and transition state ( ts) components. The decrease in the rate constant of the 1-4 oxidation, as the ratio of methanol increases, is predominantly caused by the effect of methanol on the initial state ( ts) or by an increase in the Gibbs energy difference between is and ts. [ABSTRACT FROM AUTHOR]