Supramolecular Open-Framework of a Bipyridinium-Carboxylate Based Copper Complex with High and Reversible Water Uptake

The rigid zwitterionic ligand 1,1'-bis(4-carboxylatphenyl)-(4,4'-bipyridinium) (pc1) and copper(II) ions give rise to a linear complex [Cu(pc1)(2)(H2O)(4)](2+) which self assembles in a pseudo tetragonal supramolecular arrangement leading to the supramolecular open-framework [Cu(pc1)(2)(H2O)(4)](Cl)(2)8H(2)O exhibiting an open structure including water molecules and chlorides in pores. The dehydration of this material occurs at relatively low temperature (70 C) and results in structure modification accompanied by shrinking of the crystals. Characterization of the obtained material by FT-IR spectroscopy reveals appearance of coordinated carboxylates groups, which may indicate the formation of coordination polymer after dehydration. Water adsorption (maximum uptake 0.23 g H2O per g) on dehydrated material allows to restore the initial structure. A large hysteresis in water adsorption-desorption isotherm is characteristic of a significant modification of the structure during the hydration-dehydration cycle which is in line with the structural transition determined from thermodiffractometry and FT-IR spectroscopy.