Synthesis and Properties of Triarylhalostibonium Cations

As part of our fundamental interest in the chemistry of main-group Lewis acids, we have decided to target stibonium cations whose Lewis acidity is enhanced by the presence of a halogen substituent directly bound to antimony. Starting from Ph₃Sb(OTf)₂ (1) and Mes₃Sb(OTf)₂ (2), we successfully prepared the triflate derivatives Ph₃SbF(OTf) (3) and Mes₃SbF(OTf) (4). We also synthesized the hexachloroantimonate salt of [Mes₃SbCl]⁺ (6), an analogue of the known [Ph₃SbCl][SbCl₆] (5). The structures of these complexes have been investigated experimentally as well as computationally using density functional theory methods. While direct interaction is observed between the anion and the stibonium center in compounds 3-5, compound 6 exists as an ionic solid with the four-coordinate [Mes₃SbCl]⁺ cation separated from the [SbCl₆]^- anion. The structural difference observed between the two hexachloroantimonate derivatives 5 and 6 is ascribed to the increased steric protection provided by the larger mesityl substituents. To understand how these structural differences affect the properties of these antimony species, we have compared their catalytic activity in two simple reactions, namely, the polymerization of tetrahydrofuran and the Friedel-Crafts dimerization of 1,1-diphenylethylene. These studies show that 5 is the most active catalyst for both reactions, suggesting that the reactivity of these species is controlled by both the coordinating nature of the counteranion and the steric accessibility of the reactive antimony center.