Synthesis and solution and solid-state structures of tris(pentafluorophenyl)borane adducts of PhC(O)X (X = H, Me, OEt, NPrⁱ₂)

Reaction of the highly electrophilic borane B(C₆F₅)₃ with the carbonyl Lewis bases benzaldehyde, acetophenone, ethyl benzoate, and N,N-diisopropylbenzamide led to isolation of the crystalline adducts 1-H, 1-Me, 1-OEt, and 1-NPr, respectively, in good to excellent yields (63-89%). Equilibrium measurements and exchange experiments indicated that the order of basicity (from highest to lowest) of these bases toward B(C₆F₅)₃ follows the order N,N-diisopropylbenzamide > benzaldehyde > acetophenone > ethyl benzoate. The solution and solid-state structures were probed to rationalize these observations. In solution, the borane coordinates to the carbonyl lone pair syn to H and Me in the aldehyde and ketone adducts, as indicated by ¹H/¹⁹F NOE difference experiments. The same coordination geometry was observed in the solid state upon X-ray diffraction analysis of the two adducts. The added front strain associated with the ketone adduct (C-O-B=133.6(3)° vs 126.7(5)° for the benzaldehyde complex) accounts for the observed order of basicity with these two bases. For ethyl benzoate and N,N-diisopropylbenzamide, the borane coordinates syn to the phenyl group in both solution and the solid state. In addition to the carbonyl oxygen-boron interaction, the two complexes engage in a π-stacking interaction between one of the borane C₆F₅ rings and the syn phenyl group. In addition to the structural proof of this interaction in the solid state, variable-temperature ¹⁹F NMR experiments suggest it is important in the solution structures of these adducts as well.