NMR study of 1-aryl-1,2-dicarba-closo-dodecaboranes: intramolecular C–H⋯O hydrogen bonding in solution

Dicarba-closo-dodecaborane(12) (carborane) has recently received much attention as a building block for supramolecular assemblies and bioactive compounds. Among carborane isomers, 1,2-dicarba-closo-dodecaborane(12) (o-carborane) has unique chemical properties, including the ability of the o-carborane C–H hydrogens to form hydrogen bonds. To evaluate intramolecular hydrogen bond formation between the o-carborane C–H hydrogen and various hydrogen bond acceptors in solution, we have designed and synthesized 1-aryl-o-carboranes 2. Intramolecular hydrogen bonding ability was evaluated by means of 1H NMR measurement of the o-carborane C–H hydrogen signal of 2. The 1-(2-methoxyphenyl)-o-carborane derivative 2m appeared to form an intramolecular hydrogen bond between o-carborane C–H hydrogen and the oxygen atom acting as a hydrogen bonding acceptor. In this study, we present evidence for hydrogen bond formation in solution between the o-carborane C–H and hydrogen bond acceptors positioned with appropriate geometry.