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

The crystallographic study of 3-carboxy coumarins 1-4 and 11-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 a-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) (A) over circle and 3.679(1) and 4.081(3) (A) over circle, 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 T-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 1-6 can also associate through pi-stacking interactions in the presence of weak interactions such as C-H...X (X = 0 or aromatic ring), as long as these associations do not slip the molecular planes too far so that subsequent pi-stacking interactions are avoided.