Substitution reactions of [Fe(4)S(4)Cl(4)](2-) with Bu(t)NC or Et(2)NCS(2)(-): trapping intermediates and detecting new pathways

Kinetic studies on the substitution reaction between Fe(4)S(4)Cl(4)](2-) and Bu(t)NC or Et(2)NCS(2)(-) are reported. The binding of small molecules and ions to Fe-S clusters is a fundamental step in substitution reactions but can be difficult to follow directly because these reactions are rapid and often associated with small spectroscopic changes. A novel kinetic method is reported which allows the time course of molecule and ion binding to Fe-S clusters to be followed by monitoring the lability of the cluster. Using a stopped-flow, sequential-mix apparatus, Fe(4)S(4)Cl(4)](2-) and L (L = Et(2)NCS(2)(-) or Bu(t)NC) are rapidly mixed, and after a known time (delta) the resulting solution is mixed with a solution of PhS(-). The thiolate substitutes for the chloro ligands on the cluster, in a reaction which is easy to follow because of the large change in the visible absorption spectrum. The rate of this substitution is extremely sensitive to whether L is bound to the cluster or not. By correlation of delta with the rate of the reaction with PhS(-), the time course of the reaction between Fe(4)S(4)Cl(4)](2-) and L can be mapped out. In studies where L = Bu(t)NC this technique has allowed the detection of an intermediate (Fe(4)S(4)Cl(4)(CNBu(t))](2-)) which cannot be detected spectrophotometrically. In further studies, the substitution reactions of Fe(4)S(4)Cl(4)](2-) with PhS(-), Et(2)NCS(2)(-), or Bu(t)NC are all perturbed by the addition of Cl(-). In all cases a common pathway for substitution is evident, but with Et(2)NCS(2)(-) an additional, slower pathway becomes apparent under conditions where the common pathway is completely inhibited by Cl(-).