Aluminum‐poor hexacarbalane structures: The transition from localized organoaluminum structures to delocalized polyhedra.

Abstract: The series of hexacarbalanes C₆Al_{<italic>n</italic>–6}Me_{<italic>n</italic>} (<italic>n</italic> = 7–11) represent a progression from localized organoaluminum structures to delocalized polyhedral structures en route to experimentally known 13‐ and 14‐vertex hexacarbalanes such as (AlMe)₈(CCH₂Ph)₅(µ₄H), (AlMe)₈(CCH₂Ph)₅(CCPh), [R₄N⁺]₂[(AlH)₈(CR)₆], and (AlNMe₃)₂(AlR)₆(CR)₆. In this connection, the lowest energy seven‐vertex C₆AlMe₇ structure has a tetrahapto benzene ring with the four AlC(cage) bonding interactions required to give the aluminum the favored octet configuration. Related eight‐vertex C₆Al₂Me₈ structures are found with a benzene ring bound to an Al₂ unit with a short AlAl distance of ∼2.55 Å suggesting a formal double bond. However, the lowest energy C₆Al₂Me₈ structure has a dialuminacyclobutene unit fused to a tricyclohexane unit through an Al₂ edge. Other relatively low‐energy C₆AlMe₇ and C₆Al₂Me₈ structures consist of a six‐carbon hexatriene chain either forming a seven‐membered C₆Al ring in the seven‐vertex structure or acting as a “flyover” between an Al₂ unit. The lowest energy nine‐vertex hexacarbalane C₆Al₃Me₉ has two separate C₃ units bridged by both an Al₂ pair and a single aluminum atom. Higher energy C₆Al₃Me₉ hexacarbalanes contain a pentadienyl chain and an isolated carbon atom with an imbedded bonded Al₃ triangle. The low‐energy 10‐vertex C₆Al₄Me₁₀ structures have a central Al₄ butterfly with nonbonding distances between the wingtips ranging from 3.35 to 3.91 Å. The lowest energy 11‐vertex C₆Al₅Me₁₁ structure has a central Al₄ quadrilateral with a diagonal bridged by the fifth aluminum atom. Higher energy C₆Al₅Me₁₁ structures have an edge rather than a diagonal of the central Al₄ quadrilateral bridged by the fifth aluminum atom. [ABSTRACT FROM AUTHOR]