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

Abstract

AbstractThe series of hexacarbalanes C6Aln–6Men (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(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.