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The selected derivatives of the 2- and 3-benzo[b]furancarboxylic acids were synthesized and their structures were studied using the X-ray crystallography and the computational methods. The monocarboxylic acids (1–3) crystallize as dimers stabilized by the O–H···O intermolecular hydrogen bonds. Moreover, intramolecular hydrogen bonds are formed between the OH and C(=O)CH3 groups, substituted to the aromatic ring (2–4). In the crystal structures of 1–4, weak C–H···O, C–H···π, and C–H···Br interactions stabilize the three-dimensional packing of molecules. The crystalline sodium complex of 1 has the stoichiometry [Na  + ·1A  − ·1B]·1C, thus, the asymmetric unit contains three different moieties of 1. In this complex, the Na+ cation is hexacoordinated having a strongly distorted tetragonal bipyramidal polyhedron. For each molecule 1–4, several conformers were obtained in the gas phase. It was achieved by the rotations of substituents [COOR and/or C(=O)CH3, where R = H, CH3] with respect to the rigid benzo[b]furan system. As indicated by the quantum-chemical calculations, the solid-state conformers for 3 and 4 (3-benzo[b]furancarboxylic acid derivatives) are the most stable ones. In contrast, the solid-state conformers of the 2-benzo[b]furancarboxylic acid derivatives (1, 2) have the energies higher than the lowest energy conformer by 1.23 and 0.69 kcal/mol, respectively. It seems that intermolecular contacts in the crystal influence on the orientation of substituents, and the conformers observed in the sodium complex of 1 provide evidence of such flexibility.

Derivatives of benzo[b]furan. Part II. Structural studies of derivatives of 2- and 3-benzo[b]furancarboxylic acids