Solid state transformations in stoichiometric hydrogen bonded molecular salts: ionic interconversion and dehydration processes3

We report structural aspects of three new stoichiometric 5-sulfosalicylate benzimidazolium (A1B1), 5-sulfonatosalicylate bis(benzimidazolium) monohydrate (A1B2·H2O) and 5-sulfonatosalicylate bis(benzimidazolium) (A1B2) molecular salts. Using single crystal and powder X-ray diffraction, we describe the reversible solid state ionic interconversion between A1B1 and A1B2·H2O upon neat grinding by adding one equivalent of benzimidazole or 5-sulfosalicilic acid dihydrate (B or A·2H2O), respectively. In the process water molecules are included/excluded upon grinding suggesting that water could mediate the proton transfer. We have also studied the hydrogen bond network rearrangement after the dehydration of A1B2·H2O by growing single crystals of the anhydrous salt using the seeding method. X-ray powder diffraction analysis confirmed that the product obtained directly upon heating corresponds to the structure obtained by the seeding method. In order to rationalize the experimental results and to gain insights into the role of water molecules in the formation of crystalline salts, the relative stabilities of the crystalline phases were compared by quantum mechanics calculations which are specific for solid state systems.