Mechanism of the Reactions of Synthetic Fe−S-Based Clusters with PhCOCl:  Parallel Pathways Involving Free and Coordinated Thiolate as Nucleophiles

Abstract

Terminal thiolate ligands on the synthetic Fe-S-based clusters [Fe4S4(SR)4]2- (R = Et or SPh) or [{MoFe3S4(SPh)3}2(mu-SPh)3]3- are replaced by chloride in a reaction with PhCOCl to produce [Fe4S4Cl4]2- and [{MoFe3S4Cl3}2(mu-SPh)3]3-, respectively. Kinetic studies using stopped-flow spectrophotometry show that, in general, the mechanisms of these reactions in MeCN occur by two pathways. One pathway is independent of the concentration of PhCOCl and involves rate-limiting dissociation of the thiolate ligand. The free thiolate subsequently reacts with PhCOCl to produce PhCOSR and the Cl- which binds to the vacant site on the cluster. The second pathway exhibits a nonlinear dependence on the concentration of PhCOCl and involves initial, rapid binding of PhCOCl to the cluster followed by intramolecular thiolate ligand attack on the coordinated acid chloride. The intermediate in which PhCOCl is bound to the cluster has been detected spectrophotometrically. The ways in which the rates of the reactions between PhCOCl and Fe-S-based clusters are affected by changes of the terminal thiolate, the metal composition of the cluster core, and the protonation state of the cluster have been investigated and are compared with the effect these same changes have on the rates of nucleophilic substitution.

Structural insight into halide-coordinated [Fe4S4XnY4−n]2− clusters (X, Y = Cl, Br, I) by XRD and Mössbauer spectroscopy

“…Homoleptic halide clusters have thus been studied previously and a couple of preparation methods has been published (Scheme 1). 12,14,15,[32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] 37 (2) Kanatzidis et al, 33 (3) Pohl et al, [38][39][40] (4) Holm et al, 36,41 Henderson et al for X = Cl, [43][44][45] Tuczek et al for X = Cl or Br, 46 (5) Holm et al for X = Cl, 41 (6) Holm et. al.…”

“…Homoleptic halide clusters have thus been studied previously and a couple of preparation methods have been published (Scheme 1). 12,14,15,[32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] Scheme 1 Reported preparation pathways for the [Fe 4 S 4 X 4 ] 2− clusters.…”

Involvement of thiolate ligands in binding substrates to Fe–S clusters

“…using stopped-flow spectrophotometry. 8 The reaction between [Fe 4 S 4 (SEt) 4 ] 2and C 6 H 5 COCl exhibits a simple first order dependence on the concentration of the cluster but a complicated, non-linear dependence on the concentration of the acid chloride. ‡ These kinetics are described by the rate law in eqn (2…”

“…The reasons for this have been discussed in earlier papers. 6,8 In this paper we only discuss the kinetics of the first phase corresponding to the replacement of the first thiolate, thus avoiding complications in the interpretation due to the effect of a mixture of chloro and thiolate terminal ligands on the cluster. The kinetic data and plots of k obs versus [acid chloride] for both phases are presented in the ESI.…”

“…However, in these reactions the absorbance differences are too small to permit an accurate analysis except in the reaction of [Fe 4 S 4 (SEt) 4 ] 2with C 6 H 5 COCl, where the agreement between the values of Et K 0 H determined by kinetics and spectrophotometry are excellent. 8 In systems exhibiting the limiting rate law eqn (3) the initial absorbance of the absorbance-time curves is that of the parent cluster at all concentrations of acid chloride. For example, in the reaction between [Fe 4 S 4 (SBu t ) 4 ] 2-(0.1 mmol dm -3 ) and 4-MeOC 6 H 4 COCl, the initial absorbance of the absorbance-time curve when [MeOC 6 H 4 COCl] = 1 or 40 mmol dm -3 is 0.20.…”