Reactions of [S2M(µ-S)2FeCl2]2- (M = Mo or W) with arenethiolate ions: influence of neighbouring metal sites on the rates and mechanisms of substitution at iron

Abstract

The kinetics of substitution of the chloro-groups in [S ₂ Mo(µ-S) ₂ FeCl ₂ ] ^2- by 4-RC ₆ H ₄ S ^- (R = Cl, H, Me or MeO) to form [S ₂ Mo(µ-S) ₂ Fe(SC ₆ H ₄ R-4) ₂ ) ^2- has been studied in MeCN using stopped-flow spectrophotometry. The mechanism involves initial binding of thiolate to Mo to generate [S ₂ (4-RC ₆ H ₄ S)Mo(µ-S) ₂ FeCl ₂ ] ^3- , which has been detected by spectrophotometry and ¹ H NMR spectroscopy. Subsequently, this species undergoes substitution of the chloro-groups on the neighbouring Fe by associative and dissociative pathways. The Mo–SC ₆ H ₄ R-4 residue affects these mechanisms in two ways: electron-withdrawing substituents facilitate the associative pathway (Hammett ρ = +0.63) whilst electron-releasing substituents facilitate the dissociative pathway (ρ = -1.4). Studies on the analogous clusters [S ₂ W(µ-S) ₂ FeCl ₂ ] ^2- and [O ₂ Mo(µ-S) ₂ FeCl ₂ ] ^2- indicated similar mechanisms.