Kinetics and Mechanism of Electron Transfer Reaction of an Adipato Bridged Iron(III)-Salen Complex with Dithionite Ion in Perchloric Acid Medium

Redox kinetics of the reaction of an adipato bridged iron(III)-salen complex, [(Fe(salen))(2)adi] with dithionite ion, S2O42-, was investigated in perchloric acid at I = 0.05 mol dm(-3) (NaClO4) and T = 29 +/- 1 degrees C. Spectrophotometric titrations indicated consumption of one mole of S2O42- per mole of [(Fe(salen)) 2adi] reduced. Under pseudo-first order conditions of [S2O42-] above ten-fold excess of concentration of [(Fe(salen))(2)adi], observed rates increased with increase in [S2O42-] and second order rate constants were fairly constant (0.285 +/- 0.01 dm(3) mol(-1) s(-1)) indicating first order dependence of the rate on [(Fe(salen))(2)adi]. A plot of logkobs versus log[S2O42-] was linear and gave a slope of 1.0 indicating first order dependence of the rate on [S2O42-]. The reaction rate increased with increase in [H+] within 3 x 10(-3) mol dm(-3) <= [H+] <= 14 x 10(-3) mol dm(-3). The reaction was unaffected by variation of ionic strength and dielectric constant of the medium. Addition of anion and cation did not catalyze the reaction. The reaction has been analyzed on the basis of an inner-sphere mechanism mediated by proton transfer.