Oxidation of 1,1′-bis(methoxycarbonyl)ferrocene by hexakis(N,N-dimethylformamide)iron(III) perchlorate in acetonitrile

Abstract

The kinetics of the outer-sphere oxidation of 1,1′-bis(methoxycarbonyl)ferrocene, [Fe(η⁵-C₅H₄CO₂Me)₂], by substitutionally labile Fe³⁺ introduced as [Fe(dmf)₆][ClO₄]₃(dmf =N,N-dimethylformamide) in MeCN as a solvent has been measured at 25 °C. The reactive species are [Fe(dmf)₃(MeCN)₃]³⁺ and [Fe(dmf)₄(MeCN)₂]³⁺ produced in fast pre-equilibria. This reaction system is compared to that of the oxidation of [Fe(tmphen)₃]²⁺(tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) by the same oxidant reported previously. Since [Fe(η⁵-C₅H₄CO₂Me)₂] and [Fe(tmphen)₃]²⁺ have nearly identical oxidation potentials, reactivity differences between them arise mainly from the different charge type of 0/3+ and 2+/3+. The rate constants for the reaction between [Fe(tmphen)₃]²⁺ and Fe³⁺ extrapolated to infinite ionic strength are about three times greater than the corresponding values for the [Fe(η⁵-C₅H₄CO₂Me)₂]-Fe³⁺ reaction. This small difference is attributable to slightly higher bond distortional barriers towards oxidation of [Fe(η⁵-C₅H₄CO₂Me)₂] compared to [Fe(tmphen)₃]²⁺. An interesting effect of adding tetrabutylammonium salts to the [Fe(η⁵-C₅H₄CO₂Me)₂]–Fe³⁺ reaction is observed. The anions BF₄^– and CF₃SO₃^– slow down the reaction appreciably, whereas ClO₄^– and PF₆^– virtually do not affect the experimenatal rate constants.