Reductions of hexakis(urea)- and hexakis(dimethyl sulfoxide)-manganese(III) by tris(3,4,7,8-tetramethyl-1,10-phenanthroline)iron(II) in acetonitrile

Abstract

The kinetics of outer-sphere electron transfer between [MnL₆]³⁺[L = urea or dimethyl sulfoxide (dmso)] and [Fe(tmphen)₃]²⁺(tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) has been studied at 25 °C in acetonitrile with the salts added as the perchlorates. Contrary to the literature, the manganese complexes are substitutionally labile as recognised by the considerable decrease in redox rates when extra L is added. Slight ligand displacement on [Mn(dmso)₆]³⁺ in CD₃CN could also be detected by ¹H NMR spectroscopy. To accommodate the variations in rate constant, three rapid solvation equilibria, [MnL₆]³⁺⇌[MnL₃]³⁺+3L (co-ordinated MeCN is omitted), have to be invoked with all solvate species available reacting. Further, the strong acceleration with salt is well described by ion pairing and ion tripling of the trivalent ions, reducing the Coulombic repulsion in forming the precursor complexes. Perchlorate association with [Mn(dmso)₆]³⁺ is somewhat stronger than with [Mn{OC(NH₂)₂}₆]³⁺ despite the presence of acidic hydrogens in the latter. This unexpected result is in keeping with conductance data analysed by the Lee-Wheaton equation.