Crystal structures of the solvent-free and ethanol disolvate forms of 4,4′-(diazenediyl)bis(2,3,5,6-tetrafluorobenzoic acid) exemplifying self-stabilized azobenzene cis-configurations

cis-4,4′-(Diazenediyl)bis(2,3,5,6-tetrafluorobenzoic acid), C14H2F8N2O4, and its ethanol disolvate, C14H2F8N2O4·2C2H5OH, represent new examples of self-stabilized cis-configured azobenzenes obtained by a common crystallization procedure at room temperature under normal laboratory lighting conditions. The target structure constitutes of two 2,3,5,6-tetrafluorobenzoic acid residues linked to each other by a cis-configured azo group and was confirmed for two isolated specimens extracted from the same sample, corresponding to a solvent-free form and an ethanol disolvate. In the solvent-free form, the molecule is characterized by rotational symmetry around a twofold rotation axis bisecting its central N=N bond while this symmetry is not present in the solvated form. The values of the inclination angles of the terminal carboxyl groups towards the corresponding benzene rings vary from 5.2 (4) to 45.7 (2)°, depending on the crystal composition. In the unsolvated form, the molecules are linked through identical hydrogen bonds with a classical R 2 2(8) graph-set ring motif of carboxylic acids, by generating supramolecular chains running approximately parallel to [101]. The presence of ethanol in the solvated form also leads to changes in the short-contact pattern to produce both the R 4 4(12) ring and open-chain motifs with alternating alcohol and dicarboxylic acid molecules.

An Efficient Synthesis of 3,5-Dimethoxy-2,4-Difluorobenzoic Acid

3,5-Dimethoxy-2,4-difluorobenzoic acid, as a key intermediate for preparing an impurity of moxifloxacin, was synthesised from 2,3,4,5-tetrafluorobenzoic acid in moderate yield by nitration, methoxyl substitution, reduction of NO2, diazotisation and reduction. The structures of the intermediates and the target compound were identified and determined with IR, NMR and HRMS (ESI).