Kinetics of reduction of hexakis(acetamide)- and hexakis(dimethylacetamide)-iron(III) by tris(3,4,5,6,7,8-hexamethyl-1,10-phenanthroline)iron(II) in acetonitrile

Abstract

The kinetics of outer-sphere electron transfer between [Fe(hmphen)₃]²⁺(hmphen = 3,4,5,6,7,8-hexamethyl-1,10-phenanthroline) and substitutionally labile Fe³⁺ introduced as [FeL₆]³⁺[L = dma (dimethylacetamide) or aa (acetamide)] in acetonitrile has been studied at 25 °C with the salts added as the perchlorates. Both reactions can be described by one mechanism implying that the acidic hydrogens of aa do not appreciably complicate the issue. To accommodate the variations in the redox rate constants, three rapid solvation pre-equilibria [FeL₆]³⁺⇌[FeL₃]³⁺+ 3L (co-ordinated MeCN is omitted) have to be invoked where, for driving-force reasons, only [FeL₄]³⁺ and [FeL₃]³⁺ are redox-active. Further, the acceleration with salt is well described by ion pairing and ion tripling of the trivalent ions with perchlorate, treated in terms of the reduction in Coulombic repulsion in forming the precursor complexes. For the solvate speciation, the overall reaction rates are controlled by both the reactivities of the reacting species and their availabilities. In the context of previous work, the differences between the two reactions are as follows: the redox rate constants of the aa species are larger than those of the corresponding dma species, which is primarily a driving-force effect with dma being the stronger Lewis base; on the other hand, the availabilities of the reacting species are higher in the case of dma, contrary to expectation. On the balance of the experimental evidence available, it does appear that the stabilities of the [FeL₆]³⁺ solvate species are not a function of the donor strength of L but rather of its size tentatively expressed by the effective molecular hard-sphere diameter.