The role of halogen bonding in crystal structures of 3-halogeno cytisine derivatives

Abstract

The crystal structures of three 3-halogeno derivatives of 13N-substituted cytisine have been determined by X-ray diffraction. The two 13-acetyl substituted compounds, 3-bromo (1) and 3-iodo (2) are isostructural, with the isostructurality index as high as 99%. They both crystallize in monoclinic P2 1 space group, with unit cell parameters of a = 8.4709(10) Å, b = 9.2266(12) Å , c = 8.6051(10) Å , β = 98.528(11)8 (1) and a = 8.2322(6) Å , b = 9.1724(7) Å , c = 8.5494(6) Å , b = 98.181(7)8 (2). In turn, 3-bromo-13-t-butyl-carbonate derivative (3) crystallizes in orthorhombic P2 12 12 1 space group with a = 6.8171(3) Å , b = 7.8994(4) Å , c = 31.4657(15) Å . Conformation of the cytisine skeleton is similar in all three molecules, with almost planar A ring, sofa conformation of B-ring and C ring being an almost ideal chair. However, the orientations of the double C=O bonds in 13N-substituents are completely different: in 1 and 2 it is cis with respect to C12 (C12-N13-C14-O15 torsion angle is 3.3(4)8 in 1 and 1.3(6)8 in 2) while in 3 it is trans (-175.4(3)8). In the structures of 1 and 2 the driving force of the crystal architecture are quite strong C-H⋯X halogen bonds with C⋯X distances far shorter than the sums of van der Waals radii (C⋯Br in 1 is 2.9430(19) Å and C⋯I in 2 2.974(3)). In contrast in 3-partially as the consequence of different orientation of the substituent-there are no halogen bondings but instead some weak C-H⋯O contacts organize the molecules into two-dimensional patterns. © Springer Science+Business Media, LLC 2012.