The crystal structure of the diethyl 2-benzimidazol-1ylsuccinate-picric acid (1 / 1) molecular complex has been determined by X-ray diffraction analysis. Diethyl 2benzimidazol-l-ylsuccinate molecules form channels along the a axis, in which the picric acid molecules are located. The benzimidazole moiety and the phenol group are held together by hydrogen bonding between the hydrogen of the phenol and the N3 atom of benzimidazole. Additionally, this hydrogen forms an intramolecular hydrogen bond with one O atom of the ortho-nitro group, thus producing a bifurcated hydrogen bond. I H NMR spectra in DMSO-d6 solution and CP/MAS solid 13C NMR studies of this 2-benzimidazol-1-ylsuccinate-picric acid (1/1) molecular complex, as well as those of dimethyl, diethyl, di-n-butyl and 1-n-butyl-4-ethyl 2imidazol-1-ylsuccinates, diethyl 2-pyrazol-1-ylsuccinate, ethyl imidazol-1-ylacetate, ethyl pyrazol-1-ylacetate and ethyl pyrazol-l-ylsuccinate, suggest that the picric acid linkage depends on the nature of the azole. Actual proton transfer is deduced for the imidazole derivatives, but only weak hydrogen bonding could be inferred for pyrazole derivatives.

A self-assembled supramolecular dimeric entity via an exceptionally short (2.404 Å) and strong (22.9 kcal mol(-1)) [O-H-O](-) hydrogen bond is the key to the special reactivity of vasarenes with fluoride salts. Vasarene is a self-assembled, vase-shaped compound, obtained by the reaction between ninhydrin and phloroglucinol. Analogous compounds are prepared by replacing the phloroglucinol with other polyhydroxy aromatics. Vasarenes show special affinity towards compounds of the type M(+)F(-), where M being a large monovalent cation, producing ion-pair-vasarene adducts. The first step in the proposed mechanism is the dissociation of the M(+)F(-) salt releasing F(-) to the solution, which may provide an explanation as to why only MF salts, which include large monovalent cations, undergo this reaction. From a practical point of view, the ease of their preparation and their special affinity towards fluoride salts make vasarenes potential means for salt separation. The readily formed dimeric structure with the very short [O-H-O](-) negative charge-assisted H-bond (-CAHB) can also be further used as a model in theoretical studies of such systems and understanding their role in biological processes.

The title compound, C24H14O9·2CH3OH, displays a chair-shaped form. The two di­hydro­indenone ring systems are located above and below the central fused-ring system, the dihedral angles between the mean planes of di­hydro­indenone ring systems and the mean plane of central fused-ring system are 67.91 (5) and 73.52 (4)°, respectively. In the crystal, extensive O—H⋯O hydrogen bonds, weak C—H⋯O hydrogen bonds and C—H⋯π inter­actions link the mol­ecules into a three-dimensional supra­molecular architecture.

The title compound forms a supra­molecular dimeric entity via [O—H—O]− negative charge-assisted hydrogen bonds (–CAHB) following a reaction with benzyl­tri­methyl­ammonium fluoride salt.

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