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 [Fe II L 2 (H 2 O) 2 ]Cl 2 · n H 2 O, 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_obs = (k_2 [S_2O_8^2 - ])[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 .