Characteristics of acid-catalysed substitution mechanisms and sites of protonation in iron–sulfur-based clusters, as revealed by studies on [Cl2FeS2VS2FeCl2]3–

Abstract

The kinetics of the non-catalysed and acid-catalysed substitution of the first two chloride ligands of [Cl₂FeS₂VS₂FeCl₂]^3–, by 4-RC₆H₄S^– to form [(4-RC₆H₄S)ClFeS₂VS₂FeCl(SC₆H₄R-4)]^3–(R = Cl, H, Me or MeO), has been studied in MeCN at 25.0 °C using stopped-flow spectrophotometry. Whereas the non-catalysed substitution occurs by a dissociative mechanism, the acid-catalysed substitution (using [NHEt₃]⁺ as the acid) is associative. In the acid-catalysed mechanism the rate of substitution is maximised when protonation and binding of the nucleophile occur at adjacent sites. Two features of this study appear to be general for a range of structurally different iron–sulfur-based clusters: (1) the proton affinities of the clusters and (2) the acid-catalysed substitution mechanisms. These aspects are discussed for a variety of iron–sulfur-based clusters, including [(MoFe₃S₄Cl₃)₂{µ-Fe(SEt)₆}]^3–.