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

Article abstract of DOI:10.1021/om060601k

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 Bronsted 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(C ₆F₅)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 C₂H₅CO ₂Me·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.