A highly Lewis acidic triarylborane bearing peripheral o-carborane cages

Abstract

A triarylborane (2) bearing three o-carborane cages at peripheral positions on the aryl groups was prepared and its crystal structure was determined from X-ray diffraction study. Treatment of 2 with KF in the presence of 18-crown-6 led to the potassium salt, [2F]⁻. A UV-vis titration experiment carried out in THF/H₂O (9/1 v/v) showed that 2 binds fluoride ions with a binding constant (K) of 4.8 × 10⁴ M⁻¹, which is an order-of-magnitude greater than K for the mono-carborane substituted triarylborane. The enhanced fluoride ion affinity of 2 indicates an apparent additive effect of multiple carborane substitutions on the Lewis acidity enhancement of the triarylborane. The highly Lewis acidic nature of 2 was further utilized in evaluating the fluoride ion affinity of tris(pentafluorophenyl)borane (B(C₆F₅)₃). A fluoride exchange reaction between [2F]⁻ and B(C₆F₅)₃ resulted in 15 times higher fluorophilicity for B(C₆F₅)₃ than for 2. The lower Lewis acidity of 2 compared with B(C₆F₅)₃ was confirmed from its greater cathodic reduction potential.