Kinetics of self-exchange of bis(1,4,7-trithiacyclononane)iron(III)/(II) in acetonitrile and in acidic aqueous solution

Abstract

The rate constant kex of the [Formula: see text] self-exchange reaction cannot be measured in most common solvents because of spontaneous reduction of the [Formula: see text] ion, which is also sensitive to photolysis by visible light. However, in CD3CN at −41 to −19 °C, reproducible proton-decoupled 13C NMR line broadening measurements are possible, and give kex = (5.3 ± 0.3) × 104 kg mol−1s−1 at 0 °C, ΔH* = 10.3 ± 1.8 kJ mol−1, and ΔS* = −116 ± 7 J K−1 mol−1, at ionic strength I = 0.1 mol kg−1. Proton NMR line broadening experiments are marginally practicable in very acidic water (2.0 mol kg−1 D2SO4/D2O) near 0 °C, and give kex = 3.2 × 106 kg mol−1 s−1 at 1 °C. The relative kex values of these and other low-spin/low-spin FeIII/II self-exchange reactions follow the predictions of the Marcus–Hush theory at least qualitatively. The effect of ionic strength, however, is less than predicted, probably because of the formation of less reactive anion–cation pairs. Keywords: electron transfer kinetics, crown thioether complexes.