Octamethylcuneane and Octamethylcubane: the First X-ray Crystal Structure of a Cuneane

In the crystal of octamethylcuneane (1,2,3,4,5,6,7,8-octamethylpentacyclo[3.3.0.0^2,4.0^3,7.0^6,8]octane), C₁₆H₂₄, there are two independent molecules in the asymmetric unit. Octamethylcubane (1,2,3,4,5,6,7,8-octamethylpentacyclo[4.2.0.0^2,5.0^3,8.0^4,7]octane), C₁₆H₂₄, exhibits crystallographic m symmetry. Due to the eclipsed orientation of the methyl groups in octamethylcubane, the average bond length is extended to 1.563 (3) Å compared with cubane itself. In the cuneane derivative, the cage bond lengths vary between 1.517 (2) and 1.573 (2) Å in accordance with their bicyclic character and the repulsive forces between the methyl groups. The exocyclic bonds to the methyl C atoms in both molecules are shortened due to the increased s character of the cage C atoms.

-1