A Lewis Acidic, π‑Conjugated Stibaindole with a Colorimetric Response to Anion Binding at Sb(III)

We describe 1-chloro-2,3-diphenylstibaindole
(1-Cl),
a Sb­(III) heterocycle which exhibits visible-wavelength absorption
due to conjugation of the σ*­(Sb–Cl) orbital with the
π* system of the conjugated backbone. Because the σ*­(Sb–Cl)
orbital is accessible, stibaindole 1-Cl is a Lewis acid
that readily binds halide anions at the site opposite the Sb–Cl
bond. This binding induces a “turn-off” colorimetric
response due to the suppression of the σ*−π* conjugation
in the anion-bound form.