Versatile packing modes of 9‐substituted 9‐fluorenols. X‐ray crystal structures of 9‐methyl‐, 9‐phenyl‐, 9‐(1‐naphthyl)‐ and 9‐(2‐biphenylyl)fluoren‐9‐ol

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Abstract
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 [P1for the 9‐methyl derivative (1) andP21/nfor the 9‐(1‐naphthyl) (3) and 9‐(2‐biphenylyl) (4) derivatives]. Crystal data: 1,a= 8·779(1),b= 11·570(1),c= 11·747(1), α = 91·197(5), β = 104·345(4), γ = 105·848(6)° 2,a= 8·425(1),b= 13·819(1),c= 23·902(2)Å, β = 95·87(1)° 3,a= 15·679(4),b= 8·567(2);c= 23·884(17)Å, β = 92·60(6)° 4,a= 7·770(6),b= 33·766(10),c= 13·649(4)Å, β = 95·30(1)°. 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 π‐electron cloud of an aromatic ring in the neighbouring molecule the O…centroid distances are 3·182(3) and 3·248(4)Å, and in the 9‐naphthyl derivative the OH groups are involved in some short (˜3·3Å) intermolecular approach, possibly indicating an electrostatically favourable packing for the compound.

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