Issue 1, 2016

Exploring the acid-catalyzed substitution mechanism of [Fe4S4Cl4]2−

Abstract

Kinetic studies on the acid-catalyzed substitution reactions of the teminal chloro-ligands in [Fe4S4Cl4]2− by PhS in the presence of the acids NHR3+ (R = Me, Prn or Bun) are reported. Although these acids have very similar pKas (17.6–18.4) the reactions show a variety of different kinetics, some of which are inconsistent with a mechanism involving simple protonation of the cluster followed by substitution of a terminal ligand. The observed behaviour is more consistent with the recently proposed mechanism in which Fe–(μ3-SH) bond elongation/cleavage occurs upon protonation of a μ3-S, and suggests that both the acidity and bulk of the acid is important in the protonation step. Other studies have determined the activation parameters (ΔH and ΔS) for both the protonation and substitution steps of the acid-catalyzed substitution reactions of [Fe4S4X4]2− (X = Cl or SEt). A significantly negative ΔS is observed for the substitution steps of both clusters indicating associative pathways. This is inconsistent with earlier interpretation of the kinetics of these reactions (based exclusively on the dependence of the rate on the concentration of nucleophile) and indicates that there is no dissociative substitution mechanism and the pathway associated with a zero order dependence on the concentration of PhS involves associative substitution with the solvent (MeCN) being the nucleophile.

Graphical abstract: Exploring the acid-catalyzed substitution mechanism of [Fe4S4Cl4]2−

Supplementary files

Article information

Article type
Paper
Submitted
13 Oct 2015
Accepted
18 Nov 2015
First published
20 Nov 2015

Dalton Trans., 2016,45, 307-314

Author version available

Exploring the acid-catalyzed substitution mechanism of [Fe4S4Cl4]2−

T. M. M. Al-Rammahi and R. A. Henderson, Dalton Trans., 2016, 45, 307 DOI: 10.1039/C5DT04008F

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