1H, 19F, and 15N NMR study of the interaction between bis(pentafluorophenyl)borinic acid and nitrogen bases

Bis(pentafluorophenyl)borinic acid ((ArF)2BOH, ArF = C6F5, 1) is a molecule that, due to its multiple reactivity, can behave in different and not always straightforward ways in the presence of nucleophiles. It has been already shown its behaviour in dicloromethane solution in the presence of water, THF and methanol. Briefly, it exists as monomer and trimer (1m and 1t, see Chart 1), and the presence of nucleophiles strongly influence the thermodynamic and the kinetic of the monomer-trimer interconversion. Moreover, according to the nature of the nucleophile, 1 shows a camaleonic nature by forming several, sometimes unexpected, species.2,3 We have studied now the reaction of 1 in the presence of nitrogen bases, that can act as Lewis and Brønsted bases. Two nitrogen bases were studied, namely 1,8-bis(dimethylamino)naphthalene (DMAN), which can act as Brønsted base only, and pyridine, which is a good Lewis base but weak Brønsted base. First of all, the Brønsted base DMAN has been proved to be more efficient than oxygenated Lewis bases in catalyzing the trimerization process. Indeed it is enough a catalytic amount of base to cause the complete trimerization of 1. The so obtained trimeric deprotonated anion 2 (Chart 2) is unstable with respect to dearylation reactions, leading eventually (in the presence of more than 0.33 equiv of DMAN and at higher temperatures) to the tetrarylic species 3 (Chart 2). On the other hand, the behaviour of pyridine is quite different, since 0.33 equiv of base are requested to complete the trimerization of 1, and also in this case the obtained trimer is anionic. Moreover, the presence of 1 equivalent of pyridine does not cause the dearylation process but gives rise to the monomeric neutral 1:1 adduct 4 (Chart 2). Both the identification and the characterization of the involved species were performed by multiparametric and multinuclear low temperature NMR spectroscopy.