Carboxylic acids as Lewis bases:: Structures and properties of strongly acidic carboxylic acid adducts of B(C6F5)3

Carboxylic acids react with B(C(6)F(5))(3) in CH(2)Cl(2) to form 1:1 adducts, which have been characterized by room-temperature IR and low-temperature (-30 degrees C) (1)H, (19)F, (13)C, and (11)B NMR spectroscopy. Coordination appears to occur via the carbonyl oxygen atom, and exchange between an adduct and its components is generally sufficiently slow that (1)H, (19)F, (13)C, and (11)B resonances of the adduct are readily distinguished from the (1)H and (13)C resonances of the corresponding free acid and the (19)F and (11)B resonances of free B(C(6)F(5))(3). The electron-withdrawing power of the highly electrophilic B(C(6)F(5))(3) increases the Brlnsted 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-C(6)F(5) bonds, and with benzoic acid the compound [(mu-C(6)H(5)CO(2))(mu-OH){B(C(6)F(5))(2)}(2)] has been isolated and characterized crystallographically. Because of slow B-C(6)F(5) cleavage, crystallographically useful crystals of a 1:1 adduct of a carboxylic acid have not been obtained, but C(2)H(5)CO(2)Me center dot B(C(6)F(5))(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.