The title compounds, C(12)H(9)ClO(4), (I), and C(12)H(9)BrO(4), (II), are isomorphous and crystallize in the monoclinic space group P2(1)/c. Both compounds present an anti conformation between the 3-carboxy and the lactone carbonyl groups. Both carbonyl groups are out of the plane defined by the remaining chromene atoms, by 8.37 (6) and 17.57 (6) degrees for (I), and by 9.07 (8) and 18.96 (18) degrees for (II), owing to their involvement in intermolecular interactions. In both compounds, layers of centrosymmetric hydrogen-bonded dimers are developed in the [-5 -2 22] plane through C-H...O interactions, involving both carbonyl groups as acceptors. Two families of dimers stack through C=O...C=O, C=O...pi and C-X...C=O (X = Cl and Br) dipolar interactions, as well as a C-H...pi interaction, developing the three-dimensional structure along the c axis.

The crystallographic study of 3-carboxy coumarins 1-4 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 1-6 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.

An NMR titration method has been used to simultaneously measure the acid dissociation constant (pK a ) and the intramolecular NHO prototropic constant ΔK NHO on a set of Schiff bases. The model compounds were synthesized from benzylamine and substituted ortho-hydroxyaldehydes, appropriately substituted with electron-donating and electron-withdrawing groups to modulate the acidity of the intramolecular NHO hydrogen bond. The structure in solution was established by

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The mechanochemically induced complexation between diethyl N,N′-1,3-phenyldioxalamate tweezers and resorcinol, orcinol, 4,6-di-tert-butyl-1,3-benzenediol, and 4-hexyl-1,3-benzenediol is described. IR-spectroscopy, X-ray powder diffraction, 13C CPMAS, and single X-ray diffraction allowed establishing the structures of the complexes as hydrogen-bonded heterodimers and heterotetramers. Complexation occurs through O–H···OC hydrogen-bonding interactions with the participation of phenolic OH and amide carbonyl groups. The initial conformation and steric factors coming from the 1,3-benzenediols exert a strong influence on the final structure of the complex formed. Complexation twists both oxalyl arms by 180°, strengthens the intramolecular (amide)CO···H(Csp 2)···OC(amide) three-centered hydrogen bond, and moves apart the oxalyl arms to allow the accommodation of the 1,3-benzenediol inside the cavity. The supramolecular architectures of the complexes in 1-D are directed by R 1 2(6), R 2 2(10), and R 1 2(6) adjacent hydrogen-bonding ring motifs; meanwhile, the 2-D and 3-D arrays are driven by multipolar interactions. Theoretical DFT calculations at the B3LYP/6-31G(d,p) level of theory were performed to support the experimental findings. The complexes herein reported constitute the first examples of molecular complexes with phenyldioxalamate.