How the Position of Substitution Affects Intermolecular Bonding in Halogen Derivatives of Carboranes: Crystal Structures of 1,2,3- and 8,9,12-Triiodo- and 8,9,12-Tribromo ortho -Carboranes.

The crystal structures of two isomeric triiodo derivatives of ortho -carborane containing substituents in the three most electron-withdrawing positions of the carborane cage, 1,2,3-I 3 -1,2-C 2 B 10 H 9 , and the three most electron-donating positions, 8,9,12-I 3 -1,2-C 2 B 10 H 9 , as well as the crystal structure of 8,9,12-Br 3 -1,2-C 2 B 10 H 9 , were determined by single-crystal X-ray diffraction. In the structure of 1,2,3-I 3 -1,2-C 2 B 10 H 9 , an iodine atom attached to the boron atom (position 3) donates its lone pairs simultaneously to the σ-holes of both iodine atoms attached to the carbon atoms (positions 1 and 2) with the I⋯I distance of 3.554(2) Å and the C-I⋯I and B-I⋯I angles of 169.2(2)° and 92.2(2)°, respectively. The structure is additionally stabilized by a few B-H⋯I-shortened contacts. In the structure of 8,9,12-I 3 -1,2-C 2 B 10 H 9 , the I⋯I contacts of type II are very weak (the I⋯I distance is 4.268(4) Å, the B8-I8⋯I12 and B12-I12⋯I8 angles are 130.2(3)° and 92.2(3)°) and can only be regarded as dihalogen bonds formally. In comparison with the latter, the structure of 8,9,12-Br 3 -1,2-C 2 B 10 H 9 demonstrates both similarities and differences. No Br⋯Br contacts of type II are observed, while there are two Br⋯Br halogen bonds of type I.