Oxidation of the thiosulfate ion by some iron(III) complexes with phenolate - amide-amine coordination: a comparative kinetic study

Abstract

The redox reactions of thiosulfate with four iron(III) complexes having phenolate-amide-amine coordination, Fe^III(L){L = 1,2-bis(2-hydroxybenzamido)ethane, L¹; 1,3-bis(2-hydroxybenzamido)propane, L²; 1,5-bis(2-hydroxybenzamido)3-azapentane, L³; and 1,8-bis(2-hydroxybenzamido)3,6-diazaoctane, L⁴} have been investigated in 10% v/v MeOH + H₂O and I = 0.3 mol dm^-3. At constant pH (~ 4.8) and under pseudo-first order conditions of [S₂O₃^2-]_T the reaction obeyed the rate law : - d[Fe^III(L)]/dt = k_obs [Fe^III(L)] + k'_obs' where k'_obs denotes the observed rate constant of thiosulfate decomposition; k_obs = a[S₂O₃^2-] + b[S²O₃ ^2-]²_T is valid for all the complexes, particularly at pH < 6, while k'_obs = [H⁺][S₂O₃^2-]_T ² is consistent with the rate law for thiosulfate decomposition proposed earlier. The rate data (k_obs) were analysed on the basis of the reactivities of various species of Fe^III(L) generated by the equilibrium protonation of the sec-NH of dien and trien spacer units resulting in the ring opening (for [Fe^III(L³/L⁴)]), and acid base equilibrium of the aqua ligand bound to the iron(III) centre ([Fe^III(L)(OH₂)_n]). The redox activities, both for second and third order paths, show the ligand dependencies : L⁴<L³<L¹<L² conforming to the fact that the complexes tend to be less susceptible to electron transfer from S₂O₃^2- with (i) the increase of the number of chelate rings, (ii) the decrease of overall charge, and (iii) the decrease of ring size offered by the amine moiety (from six membered to five membered one as for [Fe^III(L¹/L²)(OH₂)₂]⁺. There was no evidence for the formation of inner sphere thiosulfato complex, the possibility of the formation of the outer sphere ion-pairs, [Fe(L/HL)(OH₂)n ^+/2+, S₂O₃^2- ] with low equilibrium constant value may not be excluded. In view of this, the outer sphere electron transfer (ET) mechanism is the most likely possibility.