Competition between Hydrogen Bonds and Lewis Acid-Base Interactions in the Equilibria between Bis(pentafluorophenyl)borinic Acid and Pyridine : Insights from NMR, Diffractometric and Computational Studies

1H and 19F NMR spectroscopy, X-ray diffractometry and DFT computations have been used for investigating the interaction between the Lewis base pyridine and bis(pentafluorophenyl)borinic acid Ar2BOH (1, Ar=C6F5). Previous studies showed that the latter species in solution exists as an equilibrium mixture of the monomer (1m) and a cyclic trimer (1t), in variable ratios, and that the trimer is stabilized in the presence of Lewis bases, by the formation of strong 1 t⋯ L hydrogen bonds. In the present case, upon addition of 0.33 equivalents of pyridine, low-temperature NMR spectra showed the formation of deprotonated 1t (anion 3), strongly hydrogen-bonded to the pyridinium cation Hpy+. The presence of this cation was confirmed by the high value of 1JHN (88 Hz). DFT computations confirmed the higher stability of the O-⋯ H-N+ limit form over the neutral O-H⋯N one. Variable temperature NMR spectra showed that the Hpy+·3 ion pair has high conformational freedom. At temperatures higher than 260 K, 3 underwent reversible partial fragmentation to give 1m and the Lewis acid-base covalent adducts between pyridine and either 1m (2py), or its anhydride (5py). The fragmentation was perfectly reversible on lowering the temperature. The adduct 2py became the only species in solution in the presence of 1 equivalent of pyridine, showing that the Lewis basicity of pyridine plays the major role, at variance with what previously observed with other bases. Hydrogen bond interactions, however, promote the supramolecular organization of 2 py in the form of a cyclic tetramer, as revealed by X-ray single crystal diffractometric analysis. The formation of the deprotonated trimer 3 was previously observed in the reaction of 1 with 1,8-bis(dimethylamino)naphtalene (DMAN). However the stepwise dearylation processes, which were dominant in the reactions with DMAN, have a very marginal role in the reaction with pyridine, due to its lower Brønsted basicity.