Kinetics and mechanism of the reactions of [FeCl4]− with PhS− or PhSH in MeCN, and the role of cations containing N–H groups

Abstract

The kinetics of the reactions between [FeCl₄]⁻ and an excess of PhS⁻ have been studied using stopped-flow spectophotometry. The associated absorbance–time curves can be fitted to two exponentials, and these first and second phases correspond to the formation of [FeCl₃(SPh)]⁻ and [FeCl₂(SPh)₂]⁻, respectively. It seems likely that the steps involving formation of [FeCl(SPh)₃]⁻ and [Fe(SPh)₄]⁻ are associated with much smaller changes in absorbance and so are not detected. The kinetics of the first phase exhibit a non-linear dependence on the concentration of PhS⁻ indicating an associative mechanism in which PhS⁻ rapidly binds to [FeCl₄]⁻ to form [FeCl₄(SPh)]²⁻ prior to rate-limiting dissociation of chloride and formation of [FeCl₃(SPh)]⁻. The kinetics indicate that at high concentrations of PhS⁻, the five-coordinate intermediate attains stoichiometric concentrations. This is confirmed by the spectroscopic changes. The second phase shows analogous kinetics. The kinetics of the reactions between [FeCl₄]⁻ and an excess of PhSH have also been studied. For the first phase the reaction occurs at a rate independent of the concentration of PhSH, consistent with an associative mechanism in which the solvent (MeCN) is the nucleophile to form [FeCl₃(NCMe)]. Subsequent rapid replacement of the coordinated solvent by PhSH yields [FeCl₃(SHPh)]. The kinetics of the second phase of the reaction with PhSH exhibits a non-linear dependence on the concentration of PhSH, analogous to the kinetics observed with PhS⁻ and consistent with an associative mechanism. The cations [NHEt₃]⁺, [NH₂Et₂]⁺ and [lutH]⁺ (lut = 2,6-dimethylpyridine) form ion pairs with [FeCl₄]⁻ which undergo substitution more rapidly than free [FeCl₄]⁻.