Site specific ligand substitution in cubane-type Mo3FeS44+clusters: Kinetics and mechanism of reaction and isolation of mixed ligand Cl/SPh complexes

Abstract

The synthesis, crystal structure and solution characterization of the cubane-type [Mo₃(FeCl)S₄(dmpe)₃Cl₃] (1) (dmpe = 1,2-bis(dimethylphophane-ethane)) cluster are reported and the ligand substitution processes of chloride by thiophenolate investigated. The kinetics and the intimate mechanism of these substitutions reveal that compound 1 undergoes a number of Fe and Mo site specific ligand substitution reactions in acetonitrile solutions. In particular, PhS⁻ coordination at the tetrahedral Fe site proceeds in a single resolved kinetic step whereas such substitutions at the Mo sites proceed more slowly. The effect of the presence of acids in the reaction media is also investigated and reveals that an acid excess hinders substitution reactions both at the Fe and Mo sites; however, an acid-promoted solvolysis of the Fe–Cl bonds is observed. Electrospray ionization (ESI) and tandem (ESI-MS/MS) mass spectrometry allow the identification of all the reaction intermediates proposed on the basis of stopped-flow measurements. The distinctive site specific reactivity made it possible to isolate two new clusters of the Mo₃FeS₄⁴⁺ family featuring mixed chlorine/thiophenolate ligands, namely Mo₃S₄(FeSPh)(dmpe)₃Cl₃ (2) and [Mo₃S₄(FeSPh)(dmpe)₃(SPh)₃] (3). A detailed computational study has also been carried out to understand the details of the mechanism of substitution at the M–Cl (M = Mo and Fe) bonds as well as the solvolysis at the Fe–Cl sites, with particular emphasis on the role of acids on the substitution process. The results of the calculations are in agreement with the experimental observations, thus justifying the non-existence of an accelerating effect of acids on the thiophenolate substitution reaction, which differs from previous proposals for the Fe₄S₄ and MoFe₃S₄ clusters and some related compounds.