Competition between pi-pi stacking and hydrogen bonding in (1 : 2) picrates of 1,5-diamino-3-oxapentane, 1,8-diamino-3,6-dioxaoctane and 1,5-diamino-3-azapentane - solid state studies

Crystallographic studies of (2:1) salts of picric acid with 1,5-diamino-3-o xapentane (IOPICR), 1,8-diamino-3,6-dioxaoctane (20PICR) and 1,5-diamino-3- azapentane (1NPICR) showed significant conformational change of the picrate ion due to numerous electrostatic, H-bonding and pi-pi stacking interactio ns present in the crystal lattice. In particular, intermolecular N-H ... O H-bonds were found to cause significant twisting of the o-NO2 groups from t he plane of the benzene ring, whereas overlapping of the picrate ions due t o electrostatic interactions and pi-pi stacking caused flattening of the mo lecule. Analysis of the geometry of 74 picrate ions found in the Cambridge Crystallographic Database, in their various crystallochemical environments, showed that competition between essentially weak but numerous intermolecul ar interactions of different types led to systematic changes in geometric p arameters within the picrate ion. In particular, relations found between th e C1-C2-N-O (C1-C6-N-O) torsion angle and the endocyclic C1-C2-C3 (C1-C6-C5 ) valence angle can be explained on the basis of competition between resona nce effects of the o-NO2 group and pi-pi stacking. (C) 2000 Elsevier Scienc e B.V. All rights reserved.