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

摘要

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](-).