Large structural changes upon protonation of Fe4S4 clusters: the consequences for reactivity

Density functional calculations reveal that protonation of a μ3-S in [Fe4S4X4](2-) clusters (X = halide, thiolate, phenoxide) results in the breaking of one S-Fe bond (to >3 Å, from 2.3 Å). This creates a doubly-bridging SH ligand (μ3-SH is not stable), and a unique three-coordinated planar Fe atom. The under-coordination of this unique Fe atom is the basis of revised mechanisms for the acid-catalysed ligand substitution reactions in which substitution of X by PhS occurs at the unique Fe site by an indirect pathway involving initial displacement of X by acetonitrile (solvent), followed by displacement of coordinated acetonitrile by PhSH. When X = Cl or Br the rate of attack by PhSH is slower than the dissociation of X(-), and is the rate-determining step; in contrast, when X = SEt, SBu(t) or OPh the rate of dissociation of XH is slower than attack by PhSH and is rate-determining for these clusters. A full and consistent interpretation of all kinetic data is presented including new explanations of many of the kinetic observations on the acid-catalysed substitution reactions of [Fe4S4X4](2-) clusters. The proposed mechanisms are supported by density functional calculations of the structures of intermediates, and simulations of some of the steps. These findings are expected to have widespread ramifications for the reaction chemistry of both natural and synthetic clusters with the {Fe4S4} core.