Insight into the Mechanism of Water Adsorption/Desorption in Hydrophilic Viologen-Carboxylate Based PCP

A water stable and highly hydrophilic porous coordination polymer based on viologen-carboxylate type ligand, the 4,4'-bipyridinium,1,1-bis(3-carboxyphenyl) (pc2), is obtained by the solvothermal method: [Cd-3(pc2) (BTC)(2)(H2O)(2)]center dot 6H(2)O ([1(H2O)(2)]center dot 6H(2)O; BTC3- = 1,3,5-carboxybenzene). Its crystal structure and the ones of two partially dehydrated phases have been determined, allowing insight into the mechanism of water adsorption/desorption of this PCP material. It is shown that the dehydrated compound [1] first adsorbs two water molecules which fill the pores, leading to [1]center dot 2H(2)O. On the other hand, the partial dehydration of the as-synthesized compound leads to the intermediate phase [1(H2O)]center dot 3H(2)O in which one H2O molecule is bound to Cd2+ ions of trinudear building units, and three others are localized in the pores. The structural analysis also reveals that the pyridiniumN+ Lewis sites, which interact with water molecules in [1(H2O)(2)]center dot 6H(2)O, interact with carboxylate groups after structural reorganization in [1]center dot 2H(2)O. The water adsorption isotherm clearly shows that the dehydrated compound is highly hydrophilic and adsorbs water in three steps: first and second at P/P-s < 0.01 and third at P/P-s similar to 0.1 with the overall water adsorption reaching 0.13 g H2O/g. This material is also able to reversibly adsorb ammonia (up to 0.3 g/g or 17.6 mmol/g).