Intramolecular Resonance Assisted Hydrogen Bond and Aggregation of 2-Quinolones: A Neutron Diffraction Study

G. PUNTE; G. POZZI; A.E. GOETA; C. WILSON; G. ROMANELLI; J.C. AUTINO

Ginebra, Suiza

Congreso; XIX IUCr Congress; 2002

The nature of pi-pi stacking interactions and their capability to be used as recognition elements are still open questions. The strength of these interactions seems to be influenced by the extension of the delocalization and the presence of heteroatoms. 4-Hydroxy-2-quinolones derivatives with appropriate substituents at C3 have shown extended electron delocalization through resonance assisted hydrogen bonds (RAHB). Furthermore, various derivatives of N-alkyl-4-hydroxy-3-ethoxycarbonyl-2-quinolones present diverse biological activities. For all these reasons, this family of compounds is interesting not only to disclose the competitive influence of the intramolecular interactions and the different cohesive forces at play but also to help towards the understanding of the mechanisms involved in their biological action. To this end, and as part of a more general project, we have performed a single crystal neutron diffraction study of 3-ethoxycarbonyl-4-hydroxy-6-methoxymethyleneoxy-1-methyl-2-quinolone at 100K. It yields a molecular model that shows an intramolecular RAHB and delocalization beyond the fusioned rings and the resonant fragment. Packing analysis indicates that stacking interactions and C-H…O hydrogen bonds (HBs) rule the three dimensional structure. Stacking originates centrosymmetric dimers, but bulky substituents at 3 and 6 prevent an infinite stacking arrangement. Dimers form ribbons along a through C-H…O HB interactions. These ribbons are assembled in layers perpendicular to c. Dimers in adjacent layers, and so the layers themselves, are connected by C-H ...O HBs and a C(aryl)-H.. interaction which induce dimers belonging to adjacent layers to have their mean planes perpendicular to [011] and [0-11] respectively. Possible cooperativity of the interactions is discussed.