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

The series of hexacarbalanes C 6 Al n –6 Me n ( n  = 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 (CCH 2 Ph) 5 (µ 4 H), (AlMe) 8 (CCH 2 Ph) 5 (CCPh), [R 4 N + ] 2 [(AlH) 8 (CR) 6 ], and (AlNMe 3 ) 2 (AlR) 6 (CR) 6 . In this connection, the lowest energy seven‐vertex C 6 AlMe 7 structure has a tetrahapto benzene ring with the four AlC(cage) bonding interactions required to give the aluminum the favored octet configuration. Related eight‐vertex C 6 Al 2 Me 8 structures are found with a benzene ring bound to an Al 2 unit with a short AlAl distance of ∼2.55 Å suggesting a formal double bond. However, the lowest energy C 6 Al 2 Me 8 structure has a dialuminacyclobutene unit fused to a tricyclohexane unit through an Al 2 edge. Other relatively low‐energy C 6 AlMe 7 and C 6 Al 2 Me 8 structures consist of a six‐carbon hexatriene chain either forming a seven‐membered C 6 Al ring in the seven‐vertex structure or acting as a “flyover” between an Al 2 unit. The lowest energy nine‐vertex hexacarbalane C 6 Al 3 Me 9 has two separate C 3 units bridged by both an Al 2 pair and a single aluminum atom. Higher energy C 6 Al 3 Me 9 hexacarbalanes contain a pentadienyl chain and an isolated carbon atom with an imbedded bonded Al 3 triangle. The low‐energy 10‐vertex C 6 Al 4 Me 10 structures have a central Al 4 butterfly with nonbonding distances between the wingtips ranging from 3.35 to 3.91 Å. The lowest energy 11‐vertex C 6 Al 5 Me 11 structure has a central Al 4 quadrilateral with a diagonal bridged by the fifth aluminum atom. Higher energy C 6 Al 5 Me 11 structures have an edge rather than a diagonal of the central Al 4 quadrilateral bridged by the fifth aluminum atom.