Solid-state supramolecular array through cooperative π–π interactions of 1-(2-methoxyphenyl)-o-carborane

Dicarba-closo-dodecaborane (carborane) has received much attention as a building block for supramolecular assemblies and bioactive compounds. Among the carborane isomers, 1,2-dicarba-closo-dodecaborane (o-carborane) has unique chemical properties, including the ability of the o-carborane C–H hydrogens to form H-bonds. We have designed and synthesized 1-(2-methoxyphenyl)-o-carborane 1a to study its ability to form an intramolecular H-bond between the o-carborane C–H hydrogen and various H-bond acceptors both in solution and in the solid state. Intramolecular H-bonding ability in solution was evaluated by means of 1H NMR spectroscopic measurements of the C–H hydrogen signal. The signal of the C–H hydrogen of 1a showed a remarkable downfield shift in CDCl3 and various other solvents, i.e., the shift was almost solvent-independent. We suggest that 1a forms an intramolecular H-bond in these solvents. Crystal structure analysis of 1a showed a C–H⋯O distance of 2.05 Å and a nearly planar torsion angle C(2)–C(1)–C(7)–C(8) of 6.5°, indicating intramolecular C–H⋯O H-bond formation in the solid state. The crystal packing of 1a indicates that a supramolecular array is stabilized by cooperative π–π stacking interactions among the methoxyphenyl groups and by hydrophobic interactions of the o-carborane cages. DFT calculations indicate that the strength of the intramolecular H-bond of 1a is about 3.53 kcal/mol. These observations indicate the potential value of o-carborane in supramolecular chemistry and materials chemistry; it should be possible to design novel materials by utilizing both the H-bonding ability of the o-carborane C–H hydrogen and the high hydrophobicity of the o-carborane cage.