Carboxylic Acids as Lewis Bases: Structures and Properties of Strongly Acidic Carboxylic Acid Adducts of B(C₆F₅)₃

Carboxylic acids react with B(C6F5)3 in CH2Cl2 to form 1:1 adducts, which have been characterized by room-temperature IR and low-temperature (−30 °C) 1H, 19F, 13C, and 11B NMR spectroscopy. Coordination appears to occur via the carbonyl oxygen atom, and exchange between an adduct and its components is generally sufficiently slow that 1H, 19F, 13C, and 11B resonances of the adduct are readily distinguished from the 1H and 13C resonances of the corresponding free acid and the 19F and 11B resonances of free B(C6F5)3. The electron-withdrawing power of the highly electrophilic B(C6F5)3 increases the Brønsted acidity of the coordinated carboxylic acids sufficiently that they are able to protonate isobutene and thus initiate its carbocationic polymerization. The enhanced acidity also results in slow, partial cleavage of B−C6F5 bonds, and with benzoic acid the compound [(μ-C6H5CO2)(μ-OH){B(C6F5)2}2] has been isolated and characterized crystallographically. Because of slow B−C6F5 cleavage, crystallographically useful crystals of a 1:1 adduct of a carboxylic acid have not been obtained, but C2H5CO2Me·B(C6F5)3, the 1:1 adduct of methyl propionate, has for purposes of comparison been prepared and characterized spectroscopically and crystallographically. In this case coordination occurs unequivocally via the carbonyl oxygen atom.