Carborane anions in superacidity and the stabilization of reactive cations.

摘要

Carborane anions based on the CB11H12 - framework have grown as a class of weakly coordinating anions. These anions are extremely stable, very weakly nucleophilic and nonoxidizing, and their properties can be finely tuned through controlled substitution of external substituents on the anion framework. Because of their inertness, they are ideal counterions for a variety of highly reactive cations, and allow for the preparation of extremely strong Bronsted acids when paired with a proton.;The synthesis and characterization of H(CHB11Me5Br 6), a new Caborane based Bronsted acid, is reported. Unlike all Carborane acids isolated previously, H(CHB11Me5Br 6) is not sublimable, and it appears not to dissociate when dissolved in liquid sulfur dioxide. Carborane anions were also used to isolate two beta-disilyl-substituted vinyl cation salts. Under inert atmosphere, these salts were stable for weeks at room temperature. The x-ray crystal structure obtained for one of the salts is the first direct structural characterization for vinyl cations.;The strength of carborane acids in dilute solution was examined by a 13C NMR study in liquid sulfur dioxide using a variety of weakly basic alpha,beta-unsaturated ketones as indicators. As a class, the acidity of carborane acids is shown to greatly exceed that of all Bronsted oxyacids in dilute SO2 solution. In addition to providing a comparison of dilute acid strength, the results are consistent with prior claims that the carborane acids protonate sulfur dioxide.;Gas phase acidities calculated at the B3LYP/6-311+G(d,p) level also indicate that the carborane acids are much stronger as a class than the oxyacids. In the gas phase, the acidity of the carborane acids falls in the order predicted by the electronegativity and polarizablilty of their substituents, giving a range of DeltaG for deprotonation from 213.0 kcal/mol for H(CHB11 F12) to 265.5 kcal/mol for H(CB11H12). Of the acids that have been prepared, H(CHB11Cl11) is the strongest by all experimental and theoretical criteria.