Tetracarbaboranes: nido structures without bridging hydrogens.

Abstract

The structures and energetics of the tetracarbaboranes C4Bn-4Hn (n = 6 to 13) have been investigated by density functional theory and coupled cluster calculations. In general, the lowest energy structures of the tetracarbaboranes C4Bn-4Hn minimize the number of C-C polyhedral edges as well as the degrees of the carbon vertices. For the C4B2H6 and C4B3H7 systems the lowest energy structures are pyramidal structures having all four carbon atoms located on the base of the pyramid. The lowest energy structure for the 9-vertex C4B5H9 system is a capped square antiprism. The frameworks of the lowest energy C4B4H8 and C4B6H10 structures resemble those of the isoelectronic experimentally known B8H12 and B10H14 structures. However, an experimentally known S4 adamantane-like 10-vertex structure found in Me4C4B6Et6 based on a tetracapped octahedron lies only ∼7 kcal mol-1 in energy above the lowest energy structure. The lowest energy structures for the 11- to 13-vertex C4Bn-4Hn (n = 11, 12, 13) systems can be derived from an (n + 1)-vertex closo deltahedron by removing a high-degree vertex. At least three of the four carbon atoms are located on edges of the resulting pentagonal or hexagonal open face in the low-energy structures. However, the structures of the experimentally known R4C4B8H8 (R = Me, Et) obtained from the dimerization of R2C2B4H42- differ from these low-energy structures. The Me4C4B8H8 polyhedron has a C4 chain and two tetragonal faces whereas the Et4C4B8H8 polyhedron has a hexagonal face with two C2 units. These structures lie within 2 kcal mol-1 of each other thereby accounting for the fluxional properties of these systems observed by NMR spectroscopy.