Syntheses, characterization and reactivity of Lewis acid–base adducts based on B–N dative bonds

Three Lewis acid–base adducts [(PhBO)3·2hmt] (1), [(PhBO)3–hmt–(PhBO)3] (2) and [{(F-Ph)BO}3·hmt] (3) (hmt = hexamethylenetetramine) were prepared by the treatment of phenylboronic acid [PhB(OH)2] or 4-fluorophenylboroxine [{(F-Ph)BO}3] with hexamethylenetetramine (hmt) in methanol or acetone. The solid state structures of all the compounds were established by single crystal X-ray diffraction analysis. Crystal structure determination indicates the formation of boroxine ring through ligand-mediated dehydration of arylboronic acids. Triphenylboroxine acts as a bridge between two hmt molecules in 1, whereas hmt plays the role of a bridging ligand between two triphenylboroxine rings in 2. Compound 1 is rare 1:2 adduct of triphenylboroxine and hmt whereas compound 2 and 3 are 1:2 and 1:1 adducts of triphenylboroxine or 4-fluorophenylboroxine and hmt respectively. The lability of the B–N bond in solution was confirmed by NMR spectroscopy as well as by chemical reaction of 1 with [Co(H2O)6](ClO4)2, resulting in the formation of [Co(H2O)6](ClO4)2·(hmt)2·2H2O (4), a boroxine free hydrogen bonded framework structure.