Most Stable Conformation of the Cyclopropane Ring Attached at a Carbon Atom in a 1,2-Dicarba-<i>c</i><i>loso</i>-dodecaborane(12) System

We obtained two crystal structures of electronically interesting dicarba-closo-dodecaborane(12)s (hereafter, “carboranes”) substituted with a cyclopropyl group at a caged carbon atom, i.e., C-cyclopropyl-o-carborane (4) and C-cyclopropyl-o-carboranylphenylmethanol (9), at 123 K. In these C-cyclopropyl-o-carboranes, the cyclopropyl group adopted a slightly twisted perpendicular conformation with respect to the electron-deficient carbon−carbon (C−C) bond axis in the o-carborane cage. In contrast, it has previously been shown that a phenyl group substituted at the caged carbon atom, i.e., C-phenyl-o-carborane (3), is almost parallel to this axis at both 150 and 199 K. In other words, the π system of the phenyl ring adopted an almost bisected conformation in 3. The preferred conformation of the cyclopropane ring in these C-cyclopropyl-o-carboranes was compared among the solid, the solution, and the gaseous states and was retained under the present conditions. Moreover, we found that the C−C bond length in the o-carborane cage not only varied along the rotation of the cyclopropyl group in 4 but also was longer than that in 3, which bears a phenyl group at the dominant conformation. These phenomena may be related to homoconjugations between the caged o-carborane system that serves as an electron acceptor and the homoconjugative substituent that serves as an electron donor. In these C-cyclopropyl-o-carboranes, two types of homoconjugations would result in a slightly twisted perpendicular conformation and 4 would be more strongly stabilized than would the phenyl derivative 3. On the basis of these studies, we propose the existence of a third type of strongly stabilizing interactive geometry for a cyclopropane ring in an o-carborane system.