Syntheses, Structures, and Surface Aromaticity of the New Carbaalane [(AlH)6(AlNMe3)2(CCH2R)6] (R = Ph, CH2SiMe3) and a Stepwise Functionalization of the Inner and Outer Sphere of the Cluster†

The reaction of the acetylene RC⋮CH (R = Ph, CH2SiMe3) with an excess of AlH3·NMe3 in boiling toluene leads to the carbaalane [(AlH)6(AlNMe3)2(CCH2R)6] (R = Ph 1, CH2SiMe3 2) in good yield. Treatment of 2 with BCl3 under varying conditions gives the chlorinated products [(AlCl)6(AlNMe3)2(CCH2CH2SiMe3)6] 3 and [(AlCl)6(AlNMe3)2(CCH2CH2SiMe2Cl)6] 4, respectively. The latter clearly demonstrates that the cluster can be stepwise functionalized within the inner and outer sphere. The X-ray single-crystal structures of 1, 2, and 4 have been determined. All compounds have in common that the central core consists of a cluster having eight aluminum and six carbon atoms. The bonding properties in this cluster are described as a new manifestation of three-dimensional surface aromaticity. Each Al4C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. A phenomenological modeling using a three-dimensional Huckel scheme with fitted parameters to...