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

International Journal of Quantum Chemistry

5 years ago

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

Robert Bruce King, Alexandru Lupan, Amr A. A. Attia

The 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(µ4H), (AlMe)8(CCH2Ph)5(CCPh), [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 AlC(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 AlAl 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. The series of hexacarbalanes C6Aln–6Men (n = 7–11), representing a progression from localized organoaluminum structures to delocalized polyhedral structures, have been studied by density functional theory. The C6 subunits remain intact in low-energy 7- and 8- vertex structures but split into two smaller subunits in the larger structures containing three or more aluminum atoms.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/qua.25506