Bidirectional association of branched noncovalent complexes of tetrazoles and 1,3,5-tris(4,5-dihydroimidazol-2-yl)benzene in solution

Noncovalent 3:1 complexes were obtained by combining acidic tetrazoles with the tribasic 1,3,5-tris(4,5-dihydroimidazol-2-yl)benzene (1). A branched structure and the use of solubilizing groups ensured that the resulting complexes dissolved in a range of nonpolar organic solvents. An X-ray crystal structure analysis of a model complex with tetrazole showed a completely planar, C₃-symmetrical, hydrogen-bonded molecule that salt-packed along the crystallographic c axis with an interplanar spacing of 3.31 Å. Model binding studies between a tetrazolate and a protonated 1,3-bis(4,5-dihydroimidazol-2- yl)benzene allowed an association constant of 2470 ± 400 M^-1 to be measured in the competitive solvent mixture CDCl₃/CD₃OD (97:3). The ionic nature and the extended planarity of the tetrazole complexes' core favored the formation of supramolecular stacks not only in the solid, but also in (nonpolar) solution. Self-association was evidenced by NMR and CD spectroscopy as well as by vapor-pressure osmometry.