Tetracarbalane structures: nido polyhedra and non-spherical deltahedra

Abstract

Uhl and coworkers have synthesized and characterized structurally the 11-vertex tetracarbalanes (AlMe)₇(CEt)₄(μ-H)₂ and (AlEt)₇(CCH₂Ph)₄(CCPh)(μ–H). In order to understand the nature of the unusual C₄Al₇ polyhedra in such systems, the complete series of permethylated tetracarbalanes C₄Al_n−4Me_n (n = 6 to 14) as model compounds have been investigated using density functional theory. An overriding factor in determining the polyhedra for the lowest energy structures of a wide range of tetracarbalanes C₄Al_n−4Me_n (n = 6 to 14) is the availability of vertices of degrees 3 and 4 for all four carbon atoms. For the 11-vertex C₄Al₇Me₁₁ system the experimentally observed C₄Al₇ deltahedron with carbon atoms at the four degree 4 vertices is found in the lowest energy structure but severely distorted from ideal C_2h to C_s symmetry. This distortion is removed by adding two bridging hydrogens to the C₄Al₇Me₁₁ structure in the same locations as in the experimental structures. The lowest energy structures of the tetracarbalanes C₄Al_n−4Me_n (n = 12, 13, 14) with more than 11 vertices are deltahedra having the carbon atoms at degree 4 vertices similar to the lowest energy C₄Al₇Me₁₁ structures. The lowest energy structures for the smaller tetracarbalanes C₄Al_n−4Me_n (n = 6, 7, 8) are nido structures with pentagonal or hexagonal open faces related to nido borane structures. The intermediate 9- and 10-vertex tetracarbalanes C₄Al_n−4Me_n (n = 9, 10) have relatively complicated energy surfaces.