Two different products from the reaction between 2-formylphenylboronic acid and benzhydrazide: benzoyl hydrazone <i>versus</i> dimer

The reaction of benzhydrazide and 2-formylphenylboronic in H₂O at ambient temperature afforded the benzoyl hydrazone (E)-{2-[(2-benzoylhydrazinylidene)methyl]phenyl}boronic acid, C₁₄H₁₃BN₂O₃ (5). However, when the reaction was performed in EtOH at ambient temperature, a dimer product, namely, {oxybis[benzo[d][2,3,1]diazaborinine-1,2(1H)-diyl]}bis(phenylmethanone), C₂₈H₂₀B₂N₄O₃ (6), is the sole product isolated. Both compounds were characterized using ¹H, ¹³C and ¹¹B NMR spectroscopy, high-resolution mass spectrometry (HRMS) and single-crystal X-ray diffraction following crystallization from hot DMF/H₂O for 5 and from hot CH₃CN for 6. Benzoyl hydrazone 5 crystallized in the orthorhombic space group Pbca and revealed a strong intramolecular hydrogen-bonding interaction between the boranol O atom and hydrazonoyl N atom [H...N = 1.734 (18) Å, O...N = 2.6139 (12) Å and O-H...N = 157.4 (15)°], ensuring a near-planar benzhydrazonoyl moiety. Benzoyl hydrazone 5 packs in the solid state in dimeric assemblies through intermolecular boranol hydrogen bonds, that are further extended to two-dimensional sheets by intermolecular hydrazonoyl nitrogen to hydrazonoyl oxygen hydrogen bonds. Dimer 6 crystallized in the monoclinic space group C2/c with the borinine O atom lying on the twofold rotational axis. In contrast to the structure of benzoyl hydrazone 5, dimer 6 lacks hydrogen-bond donors and packs with an absence of intermolecular hydrogen bonding. A dative bond between the B centre and an adjacent carbonyl O atom is observed for dimer 6 [B-O = 1.574 (2) Å], enabling the formation of a [3.3.1]bicyclic system at its core. This article provides unambiguous evidence that, depending on the conditions employed, the reaction between 2-formylphenylboronic acid and benzhydrazide produces benzoyl hydrazone 5 or dimer 6 rather than 2-benzoyl-1-hydroxybenzo[d][2,3,1]diazaborine (2), despite literature reports suggesting that the latter is the case.