Tetracarbaboranes: nido structures without bridging hydrogens

Abstract

The structures and energetics of the tetracarbaboranes C₄B_n−4H_n (n = 6 to 13) have been investigated by density functional theory and coupled cluster calculations. In general, the lowest energy structures of the tetracarbaboranes C₄B_n−4H_n 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 C₄B_n−4H_n (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.