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

The kinetics of the non-catalysed and acid-catalysed substitution of the first two chloride ligands of [Cl2FeS2VS2FeCl2](3-), by 4-RC(6)H(4)S(-) to form [(4-RC(6)H(4)S)ClFeS2VS2FeCl(SC(6)H(4)R-4)](3-) (R = Cl; H, Me or MeO), has been studied in MeCN at 25.0 degrees C using stopped-flow spectrophotometry. Whereas the non-catalysed substitution occurs by a dissociative mechanism, the acid-catalysed substitution (using [NHEt(3)](+) 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),{mu-Fe(SEt)(6)}](3-).