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]-.

Kinetics of the Reactions between [(RS)2Fe(µ-S)2M(µ-S)2Fe(SR)2]n− (M = Mo, R = Ph, n = 2; M = V, R = Et, n = 3) and [Fe(SR)4]2−: Key Steps in the Assembly of Cuboidal Fe—S-based Clusters

“…The studies reported in this paper, together with the studies on: (i) the substitution of ½FeCl 4 À by PhS ) to produce ½Fe(SPh) 4 À (prior to reduction and aggregation to form ½Fe 4 (SPh) 10 2À ) [5], and (ii) reaction [6] of ½Fe 4 ðSPhÞ 10 2À with ½MoS 4 2À to produce ½S 2 Moðl-SÞ 2 Fe(SPh) 2 2À , allow us to sketch out the pathway by which cuboidal clusters containing the fMoFe 3 S 4 g 3þ core framework are constructed. The pathway, which is presumably also followed by other clusters containing fMFe 3 S 4 g nþ cores, is shown in Figure 8.…”

“…We have already published kinetic and mechanistic studies on systems related to the initial stages in the formation of these cuboidal clusters; namely the substitution reactions of [FeCl 4 À with PhS ) or PhSH [5], and the reaction of ½Fe 4 ðSPhÞ 10 2À with ½MS 4 2À (M=Mo, W or V) [6] to form [S 2 Mðl-SÞ 2 Fe(SPh) 2 2À . In this paper we present the kinetics of the subsequent reactions of ½S 2 Moðl-SÞ 2 Fe(SPh) 2 2À with ½Fe(SPh) 4 2À to form the dicuboidal ½fMoFe 3 S 4 ðSPhÞ 3 g 2 ðl-SPhÞ 3 3À and the reaction of ½ðEtSÞ 2 Feðl-SÞ 2 Vðl-SÞ 2 Fe(SEt) 2 3À with ½Fe(SEt) 4 2À to form the dicuboidal ½fVFe 3 S 4 ðSEtÞ 3 g 2 ðl-SEtÞ 3 3À .…”

Mechanisms of reactions in solution

“…Reaction of PhS À with [FeCl 4 ] À in MeCN, occurs in at least two steps, the first an associative process involving [FeCl 3 SPh] À formation, significant amounts of which may be present at high concentrations of PhS À , followed by rate-determining loss of Cl À . 115 Reactions with PhSH involve initial formation of a solvento complex, [FeCl 3 (CH 3 CN)] followed by rapid substitution by PhSH.…”