Organic ammonium ion-occluded flexible coordination polymers: Thermal activation, structure transformation and proton transfer

Solvothermal reactions of 1,3,5-benzenetricarboxylic acid (H₃btc) with cadmium acetate or zinc acetate yielded two compounds formulated as (Me₂NH₂)Cd(btc)]·DMA (1) (btc = 1,3,5-benzenetricarboxylate, DMA = N,N-dimethylacetamide) and (Me₂NH₂)Zn(btc)]·DMF (2) (DMF = N,N-dimethylformamide). Both are 3-D frameworks with the rutile topology, which are constructed from six-connected dimeric metal cores and three-connected btc linkers. The solvent molecules and counter cations are located in the 1-D channels of the frameworks. A slight difference between the two compounds is the different connectivity modes of the metal atoms with the carboxylate groups of the ligands. However, this slight difference results in distinct flexibilities of the two frameworks. Variable-temperature powder X-ray diffraction studies revealed that the framework of 1 collapses when heated at 180 °C with loss of the guest species, but compound 2 undergoes two structural transformations below 380 °C. Thermogravimetry-infrared spectroscopy analysis for 2 showed that the two structural transformations are induced by separate losses of solvent molecules and counter cations, and that the dimethylammonium cations are eliminated as neutral dimethylamine molecules. IR spectroscopy demonstrated that the protons are transferred from the counter cations onto the uncoordinated carboxylate oxygen atoms on the channel walls. Sorption and proton conduction studies have also been performed for the compounds.