The smallest member of the family consisting of 4 fused cyclopropane rings is [3,3,3,3]fenestrane or pyramidane a molecule related to tetrahedrane with an extensive history on its own. In the next member 4 cyclobutane rings are fused to the archetypical window motif. It is called in its own chemical nomenclature [4,4,4,4]fenestrane simply by counting the number of carbon atoms in each ring. The formal name for this compound is less catchy: tetracyclo[3.3.1.03,9.07,9]nonane.
In an extreme case the central carbon atom ordinarily with a tetrahedral molecular geometry gets completely flattened. In the molecular orbital picture for square planar methane two of a total of 3 sp2 hybridized carbon atomic orbitals form regular bonds with two of the hydrogen atoms as in a planar alkene. The third sp2 orbital interacts in a three-center two-electron bond with the two remaining hydrogen atoms utilizing only the hydrogen electrons. Two additional carbon valence electrons are situated in a p-orbital perpendicular to the plane of the molecule. The four C-H bonds are equal because they resonate. In silico calculations show that it takes 95 to 250 kcal/mol (400 to 1,050 kJ/mol) for this process.
One of the highest strained fenestranes actually isolated is a [4,4,4,5]fenestrane with bond angles through the central carbon atom of around 130° based on X-ray diffraction. In this molecule the bonds extending from the central carbon atom are shortened with bond lengths of 149 picometer while those at the perimeter are extended (159 pm.)
The first ever synthesized fenestrane is a [4,5,5,6]fenestrane :
In a recent effort a [4,5,5,5]fenestrane was synthesized with one carbon atom replaced by nitrogen because aza compounds and their salts are more likely to form crystalline compounds suitable for x-ray analysis :
In the borane salt the N-C-C angle is now 126°.