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.