VERSATILE PACKING MODES OF 9-SUBSTITUTED 9-FLUORENOLS - X-RAY CRYSTAL-STRUCTURES OF 9-METHYLFLUOREN-9-OL, 9-PHENYLFLUOREN-9-OL, 9-(1-NAPHTHYL)FLUOREN-9-OL AND 9-(2-BIPHENYLYL)FLUOREN-9-OL

The four studied semi-rigid fluorenol derivatives differ only in the size of the 9-substituents. All four compounds crystallize with two molecules per asymmetric unit and with relatively low crystallographic symmetry [P1BAR for the 9-methyl derivative (1) and P2(1)/n for the 9-(1-phenyl) (2), 9-(1-naphthyl) (3) and 9-(2-bipbenylyl) (4) derivatives 1. Crystal data: 1, a = 8.779(1), b = 11.570(1), c = 11-747(1), alpha = 9.197(5), beta = 104.345(4), gamma = 105.848(6)degrees; 2, a = 8.425(1), b = 13.819(1), c = 23.902(2)angstrom, beta = 95.87(1)degrees, 3, a = 15.679(4), b = 8.567(2), c = 23-884(17) angstrom, beta = 92.60(6)degrees; 4, a = 7.770(6), b = 33.766(10), c = 13.649(4) angstrom, beta = 95.30(1)degrees. Nevertheless, the packing modes are different. The 9-methylfluoren-9-ol molecules, forming H-bonded tetramers with full H-bond saturation, give rise to the lowest packing coefficient among the four compounds. The crystals of the 9-phenyl derivative are built up of H-bonded dimers, whereas those of the bulky 9-naphthyl and 9-diphenylyl derivatives consist of single molecules only. The latter two compounds, however, have a denser packing than the two that contain H-bonded aggregates. The crystal structures are held together mainly by ordinary Van der Waals forces. In the case of 9-biphenylyfluoren-9-ol, the structure seems to be stabilized also by weak H-bond-type interactions from the alcoholic hydroxyl group to the pi-electron cloud of an aromatic ring in the neighbouring molecule [the O...centroid distances are 3.182(3) and 3.248(4) angstrom, and in the 9-naphthyl derivative the OH groups are involved in some short (approximately 3.3 angstrom) intermolecular approach, possibly indicating an electrostatically favourable packing for that compound.