Crystal engineering of o-carboranyl alcohols: Syntheses, crystal structures and thermal properties

The syntheses of new o-carboranyl alcohols 1-[R(hydroxy)methyl]-2-methyl-1, 2-dicarba-closo-dodecaborane (R = 9-anthracenyl 2, 4-pyridyl 4), 1-[R(hydroxy)methyl]-2-phenyl-1,2-dicarba-closo-dodecaborane (R = 3-phenyl-2-propen-1-ol 6, 2-furanyl 7) are reported. Their crystal structures, along with those for the related 1-[R(hydroxy)methyl]-2-methyl-1,2-dicarba- closo-dodecaborane (R = phenyl 1, 2-pyridyl 3) and 1-[phenyl(hydroxy)methyl]-2- phenyl-1,2-dicarba-closo-dodecaborane (5) derivatives are reported. 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 packing for these alcohols can be described in terms of three major supramolecular synthons based on hydrogen bonding of OH groups with themselves (O-H⋯O homosynthons: 1, 6 and 7), with the pyridine nitrogen atom (O-H⋯N heterosynthons: 3 and 4) or with aromatic rings (O-H⋯π: 2). The latter shows an unusual combination of O-H⋯π and π-π interactions. The supramolecular heterosynthons are clearly favoured over their respective supramolecular homosynthons. In the absence of a nitrogen atom, the bulkiness around the OH groups seems to play a key role in the formation or not of full O-H⋯O hydrogen bonding. The supramolecular arrays are finally supported by both, weak dihydrogen or hydrophobic interactions, due to the presence of o-carboranyl fragments in these molecules. These compounds display good thermal stabilities, with compound 4 showing an unexpectedly high melting temperature of 256 °C probably arising from an infinite O-H⋯N hydrogen bonding network. Gas phase calculations demonstrate that O-H⋯N hydrogen bonded dimers are 3.5 kcal mol^-1 more stable than analogous for O-H⋯O hydrogen bonded ones.