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-dihydrocyc lopenta[a]-phenanthren-17-ones are available. These compounds mostly c ontain only one O atom, a carbonyl group at the 17-position, and there fore the only groups available for interactions are C-H groups. In add ition, the crystal structure of a second polymorph of the 11-ethyl der ivative is described. M(r) = 260.33, Pbca, a = 17.012 (2), b = 21.042 (2), c = 7.6465 (6) angstrom, V = 2737.2 (4) angstrom3, Z = 8, D(x) = 1.264 Mg m-3, Cu Kalpha, lambda = 1.5418 angstrom, mu = 0.56 mm-1, F(0 00) = 1104, T = 295K, final R = 0.090 for 1669 reflections above 2sigm a(F). The conformation of the ethyl group is gauche [C(I 2)-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 polym orph is also larger than that in the Pnaa polymorph; this distortion i s evidenced by torsion angles (13-20-degrees) in the bay region and by an out-of-plane displacement (0.8 angstrom) of the C atom of the meth ylene portion of the ethyl group [the C atom attached to C(11)]. Packi ng diagrams and intermolecular distances were analyzed for all the dih ydrocyclopenta[a]phenanthrenes for which structural data are available . There appear to be three types of packing. The first type consists o f a dimer herringbone formed by the interactions of two molecules by w ay 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 struc ture and involves compounds pounds that crystallize with a unit-cell l ength of 7.5-7.6 angstrom (or, in a very planar structure, 13.8 angstr om). The translational stacking (approximately 4 angstrom apart) found in polycyclic aromatic hydrocarbons is not observed in the crystal st ructures of these dihydrocyclopenta[a]-phenanthrenes because of the bu lk of methyl or methylene groups and the dipole moment of the carbonyl group.