Four new supramolecular compounds, (2-mimH)[Ca(pydcH)3][Ca(pydcH2)(pydc)(H2O)2]∙ 4H2O 1, (1-mimH)2[Zr(pydc)3] 2, (2-mimH)2[Cd(pydc)2]∙8H2O 3, (2-mimH)2[Hg(pydc)2]∙8H2O 4, [where pydcH2 = pyridine-2,6-dicarboxylic acid (dipicolinic acid), 1-mim = 1-methylimidazole and 2-mim = 2-methylimidazole] have been synthesized and characterized by elemental analysis, spectroscopic techniques (IR, UV-Vis), 1H NMR and 13C NMR) thermal (TG/DTG/DTA) analysis as well as single crystal X-ray diffraction. All four compounds are proton transfer salts of the methylimidazolium cations and metal complex anions that crystallized from a solution of pyridine-2,6-dicarboxylic acid, methylimidazole, metal nitrates or chloride as starting materials. The coordinating dicarboxylic acid is deprotonated at the carboxyl group, and methylimidazole is protonated to balance the charge. In the crystal structures of 1-4 complexes hydrogen bonding and π–π stacking play important roles. Water clusters are formed in compounds 1, 3 and 4. The equilibrium constants of dipicolinic acid (pydc) and methylimidazole derivatives (1-mim and 2-mim), pydc-2-mim, pydc-1-mim proton transfer systems as well as those of their complexes, were investigated by a potentiometric pH titration method. The stoichiometries of most of the complexes species in solution were found to be very similar to the cited crystalline metal ion complexes.
