Anomalously slow rates of reduction of iron(III) polypyridine complexes by triphenyl verdazyl radicals in acetonitrile

The kinetics and mechanisms of the electron transfer reactions of [Fe(N-N)₃](ClO₄)₃ [N-N = 2,2′-bipyridine, 1,10-phenanthroline] and [Fe(terpy)₂](ClO₄)₃ [terpy = 2,2′:6,2″-terpyridine] with a range of triphenyl verdazyl radicals have been investigated in acetonitrile at 25°C and I = 0.05 mol dm^-3 (C₄H₉)₄NPF₆ using stopped-flow spectrophotometry. It was found that all three iron(III) complexes underwent extensive dissociation in acetonitrile and it was necessary to carry out the reactions in the presence of an excess of the relevant ligand. Under these conditions, reaction of the oxidant with the verdazyl radical resulted in an absorbance change that could be described by a single exponential. This was confirmed by global analysis of time-dependent spectra. A mechanism is proposed that accounts for the kinetic data. The rate constants for the electron transfer reaction are many orders of magnitudes less than would be predicted on the basis of the redox potentials and the self exchange rate constants of the reacting species.