Premicellar effects on the reduction of N, N′-phenylenebis(salicylideneiminato)iron(III) by thioglycolic acid: Kinetic study and mechanism

Abstract

The reduction kinetics of N,N′-phenylenebis(salicylideneiminato)iron(III), [Fe(salphen)]+ by thioglycolic acid (TSH) has been carried out in mixed aqueous medium (DMSO:H2O; 1:4) in pseudo-first order condition at 26 ± 1 °C. A CORNING colorimeter 253 was used to monitor the reaction rates by detecting the absorbance reduction of the reaction at 435 nm. The reaction was acid independent, second order overall and demonstrated zero Brønsted-Debye salt effect. Ions such as Mg2+ and CH3COO- catalyzed the reaction. The thermodynamic parameters indicated an activated complex that is much ordered and a highly solvated transition state. Reaction catalysis was observed in the case of sodium dodecylsulphate (SDS) and cetyltrimethylammonium bromide (CTAB) inclusion. The critical micelle concentrations obtained for SDS were 8.0 × 10−3 and 3.3 × 10−3 mol dm−3 whereas that observed for CTAB were 2.0 × 10−3 and 2.8 × 10−3 mol dm−3 in water and [Fe(salphen)]+ reaction media respectively. Piszkiewicz model was used to analyze the results which indicated the existence of pre-micellar aggregates and significant electrostatic and hydrophobic interactions between the metal complex and the pre-micellar aggregates of the surfactants. The cooperativity index and binding constants obtained were n = 1.22, K = 4.78 × 104 dm3 mol−1, n = 1.27 and K = 4.55 × 104 dm3 mol−1 respectively for SDS and CTAB. A plausible mechanism suggesting an outer-sphere mechanism has been postulated.