Crystal structures of the solvent-free and ethanol disolvate forms of 4,4'-(diazenediyl)bis(2,3,5,6-tetrafluorobenzoic acid) exemplifying self-stabilized azo-benzene 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 azo-benzenes obtained by a common crystallization procedure at room temperature under normal laboratory lighting conditions. The target structure constitutes of two 2,3,5,6-tetra-fluoro-benzoic 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 mol-ecule is characterized by rotational symmetry around a twofold rotation axis bis-ecting 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 mol-ecules are linked through identical hydrogen bonds with a classical R 2 2(8) graph-set ring motif of carb-oxy-lic acids, by generating supra-molecular 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 di-carb-oxy-lic acid mol-ecules.