Most Stable Conformation of the Cyclopropane Ring Attached at a Carbon Atom in a 1,2-Dicarba-closo-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 pi 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.