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π-Stacking Interactions and CH···X (X = O, Aryl) Hydrogen Bonding as Directing Features of the Supramolecular Self-Association in 3-Carboxy and 3-Amido Coumarin Derivatives

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posted on 03.12.2002 by Efrén V. García-Báez, Francisco J. Martínez-Martínez, Herbert Höpfl, Itzia I. Padilla-Martínez
The crystallographic study of 3-carboxy coumarins 14 and 3-amido coumarins 5 and 6 is reported in the context of crystal engineering. The former compounds are described as two fused rings with opposed polarity, which are associated through parallel displaced π-stacking interactions. The benzenoid ring B and the lactone ring L of one molecule interact with the lactone ring L‘ and benzenoid ring B‘ of the partner molecule with mean interplanar and mean intercentroid distances ranging between 3.385(6) and 3.67(2) Å and 3.679(1) and 4.081(3) Å, respectively. Among the six possible arrangements between two pairwise overlapping coumarin molecules, the anti tail-to-tail orientation was the most frequently observed. Pairing through π-stacking interactions is less favored when changing the 3-carboxy for a 3-amido group or even annulled, as in 3-oxalamate 6, because of the increasing H-bonding capability of the 3-amido group. The fused polar rings of coumarins 16 can also associate through π-stacking interactions in the presence of weak interactions such as C−H···X (X = O or aromatic ring), as long as these associations do not slip the molecular planes too far so that subsequent π-stacking interactions are avoided.

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