KINETIC STUDIES AND MECHANISMS OF REDUCTION OF IRON(III) AND MANGANESE(III) COMPLEXES OF BIS(SALICYLIDENE)PHENYLENEDIAMINE BY SOME REDUCTANTS IN MIXED AQUEOUS AND SURFACTANT MEDIUM

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Date
2021
Authors
IBRAHIM, Ismaila
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Abstract
The kinetics of reduction of N,N′-phenylenebis(salicylideneiminato)iron(III) ([Fe(salphen)]+ ) at 435 nm by thioglycolic acid, oxalic acid, hypophosphorous acid and hydrogen peroxide and reduction of N,N′-phenylenebis(salicylideneiminato)manganese(III) ([Mn(salphen)]+ ) at 450 nm by hydrogen peroxide, thioglycolic acid, thioglycol, thiocyanate ion, thiosemicarbazide and thiodiglycol were studied in mixed aqueous and surfactant medium under pseudo-first order conditions. A stoichiometry of 1:1 was obtained for [Fe(salphen)]+ – TSH reaction while 2:1 (complex:reductant) was obtained for [Fe(salphen)]+ – H3PO2 and [Fe(salphen)]+ – H2O2 reactions and 2:3 (complex:reductant) for [Fe(salphen)]+ – H2C2O4 reaction. Also, a stoichiometry of 1:1 was obtained for [Mn(salphen)]+ – TSH, [Mn(salphen)] + – RSH, and [Mn(salphen)]+ – SCNreactions while 2:1 (complex:reductant) was obtained for [Mn(salphen)]+ – H2O2, [Mn(salphen)]+ – H2NNHCSNH2 and [Mn(salphen)]+ – S(CH2CH2OH)2 reactions. The reactions were first order in [oxidant] and [reductant] in all the systems. The [Fe(salphen)]+ -TSH, [Fe(salphen)]+ – H2C2O4, [Fe(salphen)]+ – H3PO2, [Mn(salphen)]+ – TSH, [Mn(salphen)]+ – SCNand [Mn(salphen)]+ – H2NNHCSNH2 reactions were independent of [H+ ] while [Fe(salphen)]+ – H2O2, [Mn(salphen)]+ – H2O2 and [Mn(salphen)]+ – S(CH2CH2OH)2 reactions showed an inverse [H+ ] dependence with non-zero intercept but [Mn(salphen)]+ – RSH reaction showed a positive [H+ ] dependence. The reactions conform to the following rate equations: = k2[M(salphen)+ ][A] = k2[Fe(salphen)+ ][H2C2O4] = k2[M(salphen)+ ][B] = (a + )[M(salphen)+ ][C] = (e + f[H+ ])[Mn(salphen)+ ][RSH] where M = Fe or Mn, A = TSH or SCN- , B = H3PO2 or H2NNHCSNH2 and C = H2O2 or S(CH2CH2OH)2 The rates of reactions displayed a negative salt effect in [Fe(salphen)]+ – H2O2, [Mn(salphen)]+ – H2O2, [Mn(salphen)]+ – SCNand [Mn(salphen)]+ – S(CH2CH2OH)2 reactions respectively, a positive salt effect in [Mn(salphen)]+ – RSH reaction and a zero salt effect in [Fe(salphen)]+ – TSH, [Fe(salphen)]+ – H3PO2, [Fe(salphen)]+ – H2C2O4, [Mn(salphen)]+ – TSH and [Mn(salphen)]+ – H2NNHCSNH2 reactions respectively. Added ion catalysis/ or inhibition was observed in all the reactions studied except the reaction of [Fe(salphen)]+ with H2C2O4 where neither catalysis nor reaction inhibition was observed. There was no evidence of an intermediate complex of significant stability for all the reactions except the reactions of H2O2 with [Fe(salphen)]+ and [Mn(salphen)]+ where it was observed from Michaelis-Menten plot. Free radicals were detected in all the reactions except the reaction of H2C2O4 with [Fe(salphen)]+ . The reaction rates were catalysed/ or inhibited in the presence of surfactants (SDS and CTAB) and Piszkiewicz model was used to analyze the results which indicated the existence of significant electrostatic and hydrophobic interactions between the metal complexes and the pre-micellar aggregates of the surfactants. Based on the results obtained, outersphere mechanistic pathway is proposed for all the reactions under investigation.
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