C-H...O packing motifs in some cyclopenta[a]phenanthrenes

An analysis has been made of the C-H...O interactions in cyclopenta[a]phenanthrenes, for which structural data on fifteen 15,16-dihydrocyclopenta[a]-phenanthren-17-ones are available. These compounds mostly contain only one O atom, a carbonyl group at the 17-position, and therefore the only groups available for interactions are C-H groups. In addition, the crystal structure of a second polymorph of the 11-ethyl derivative is described. M(r) = 260.33, Pbca, a = 17.012 (2), b = 21.042 (2), c = 7.6465 (6) A, V = 2737.2 (4) A, Z = 8, Dx = 1.264 Mg m-3, Cu K alpha, lambda = 1.5418 A, mu = 0.56 mm-1, F(000) = 1104, T = 295 K, final R = 0.090 for 1669 reflections above 2 sigma (F). The conformation of the ethyl group is gauche [C(12)-C(11)-C(18)-C(19) = 75.8 (7) degrees], differing from the cis value of -1.3 (5) degrees for the Pnaa form. The molecular distortion in the Pbca polymorph is also larger than that in the Pnaa polymorph; this distortion is evidenced by torsion angles (13-20 degrees) in the bay region and by an out-of-plane displacement (0.8 A) of the C atom of the methylene portion of the ethyl group [the C atom attached to C(11)]. Packing diagrams and intermolecular distances were analyzed for all the dihydrocyclopenta[a]phenanthrenes for which structural data are available. There appear to be three types of packing. The first type consists of a dimer herringbone formed by the interactions of two molecules by way of the ketone group and the C-H of C(12) of the adjacent ring. The second type of packing also involves a dimer but involves C-H and O-C groups at either ends of the molecule. The third type is a layer structure and involves compounds that crystallize with a unit-cell length of 7.5-7.6 A (or, in a very planar structure, 13.8 A). The translational stacking (approximately 4 A apart) found in polycyclic aromatic hydrocarbons is not observed in the crystal structures of these dihydrocyclopenta[a]-phenanthrenes because of the bulk of methyl or methylene groups and the dipole moment of the carbonyl group.