Supramolecular architectures in o-carboranyl alcohols bearing N-aromatic rings: syntheses, crystal structures and melting points correlation

The syntheses of new o-carboranyl alcohols bearing N-aromatic rings, 1-[R(hydroxy)methyl]-2-methyl-1,2-dicarba-closo-dodecaborane (R = 6-methyl-2-pyridyl 1b, 3-pyridyl 1c, 2-quinolyl 1e, 4-quinolyl 1f), 1-[R(hydroxy)methyl]-1,2-dicarba-closo-dodecaborane (R = 2-pyridyl 2a, 6-methyl-2-pyridyl 2b, 3-pyridyl 2c, 4-pyridyl 2d, 2-quinolyl 2e, 4-quinolyl 2f)), are reported. The crystal structures of all compounds, except that for 1b, are reported and compared with those related compounds previously synthesized by us (1-[R(hydroxy) methyl]-2-methyl-1,2-dicarba-closo-dodecaborane (R = 2-pyridyl 1a, 4-pyridyl 1d)). We provide an analysis of these compounds by means of X-ray crystallography, NMR/IR spectroscopies, thermal analyses and gas phase calculations in the context of crystal engineering. The results show that the crystal packings of these alcohols are dominated by the supramolecular O-H center dot center dot center dot N heterosynthon, but also include other weaker interactions such as C-H center dot center dot center dot O hydrogen bonds, H center dot center dot center dot H contacts and some degree of (C center dot center dot center dot B)-H center dot center dot center dot pi interactions. There are four types of O-H center dot center dot center dot N hydrogen bonded chains, two giving rise to high melting point compounds (1c, 1d, 1f, 2c, 2d, and 2f) and two affording lower melting point compounds (1a, 2a and 2b). The lowest melting compounds (1e and 2e) show no such infinite O-H center dot center dot center dot N hydrogen bonding networks and only intramolecular O-H center dot center dot center dot N hydrogen bonds. We correlate the presence of the infinite O-H center dot center dot center dot N hydrogen bonding network in the crystal structure for the pyridine derivatives with their melting points. Gas phase calculations show that the energy for the O-H center dot center dot center dot N interactions in hydrogen bonded dimers is in the range 4-8 kcal mol(-1).