A comparative kinetic study for the oxidation of 2-mercaptoethanol by di-μoxo-bis(1,4,7,10-tetraazacyclododecane)-dimanganese(III,IV) and di-μ-oxo-bis(1,4,8,11-tetraazacyclotetradecane)dimanganese(III,IV) complexes:: Influence of copper(II)

A comparative kinetic study of the reactions of two mixed valence manganese(III,IV) complexes with macrocyclic ligands, [(LMnIV)-Mn-1(O)(2)(MnL1)-L-III], 1 (L-1 = 1,4,7,10-tetraazacycloclodecane) and [(LMnIV)-Mn-2(O)(2)(MnL2)-L-III], 2 (L-2 = 1,4,8,11-tetraazacyclotetradecane) with 2-mercaptoethanol (RSH) has been carried out by spectrophotometry in aqueous buffer at (30 +/- 0.1)degreesC. Rate of the reactions between the oxidants and the reductant was found to be negligibly slow with no systematic dependence on either redox partners. Externally added copper(II) (usually 5 x 10(-7) mol dm(-3)), however, increases the rate of the reduction of 1 and 2 significantly. In the presence of catalytic amount of copper(II), the rate of the reaction is nearly proportional to [RSH] at lower concentration of the reductant but follows a saturation kinetics at higher concentration of the latter for the reaction between I and the thiol. Reaction rate was found to be strongly influenced by the variation of acidity of the medium and the observed kinetics suggests that the two reductant species ([Cu(RSH)](2+) and [Cu(RS)](+)) are significant for the reaction between 1 and the thiol. The dependence of the rate on I RSH I for the reduction of 2 by the thiol was complex and rationalized considering two equilibria involving the catalyst (Cu 2+) and the reductant. The pH rate profile suggests that both the mu-O protonated [Mn-III(O)(OH)Mn-IV] and the deprotonated [Mn-III(O)(2)Mn-IV] forms of the oxidant 2 become important. The kinetic results presented in this study indicate the domination of outer-sphere path. (C) 2003 Wiley Periodicals, Inc.