Cationic Triarylchlorostibonium Lewis Acids

Organopnictogen cations show promise as powerful, tunable
main-group
Lewis acid catalysts. The synthesis, solid-state structures, and reactivity
of a series of weakly coordinated triarylchlorostibonium salts [Ar3SbCl]­[B­(C6F5)4] (Ar = Ph,
3-FC6H4, 4-FC6H4, 3,5-F2C6H3, 2,4,6-F3C6H2) are reported. The cation in each adopts a tetrahedral
coordination environment of antimony, with near complete separation
from the anion. Structural, computational, and reactivity studies
reveal that the Lewis acidity of [Ar3SbCl]+ generally
increases with increased fluorination of the Ar substituents, with
a secondary quenching effect from para fluorination.
[Ar3SbCl]+ is reduced to Ar3Sb in
the presence of Et3SiH, and the mechanism of this reaction
has been modeled computationally. Preliminary studies demonstrate
that they are useful catalysts for the dimerization of 1,1-diphenylethylene
and the Friedel–Crafts alkylation of benzene.