Direct studies on 5-coordinate intermediates formed during substitution at tetrahedral Fe sites: role of bound nucleophile in labilisation of leaving group

Abstract

The substitution reactions of the tetrahedral Fe sites in [FeCl₄]⁻, [Fe₂S₂Cl₄]²⁻, [Fe₄S₄Cl₄]²⁻ and [{MoFe₃S₄Cl₃}₂(µ-SEt)₃]³⁻ with 4-RC₆H₄S⁻ (R = MeO, Me, H, Cl or NO₂) all involve rapid binding of the thiolate to a Fe site and formation of a kinetically and spectroscopically detectable intermediate. Kinetic studies allow calculation of the rate of Fe–Cl dissociation from the 5-coordinate site of the intermediate (k₂^R). The rate of Fe–Cl dissociation from the intermediate exhibits a marked dependence on the nature of the bound thiolate with log₁₀(k₂^R) increasing in a linear manner with the calculated NBO charge on the sulfur atom of the coordinated thiolate. This behaviour indicates that Fe–Cl bond dissociation at the 5-coordinate intermediate involves a process in which Fe–thiolate bond shortening occurs prior to movement of the Fe–Cl bond.