Syntheses, structures and properties of cadmium benzenedicarboxylate metal–organic frameworks

Abstract

The products isolated from the reaction between Cd(NO₃)₂·4H₂O and 1,4-benzenedicarboxylic acid (H₂bdc) in DMF are very dependent on the conditions. At 115 °C, the reaction gives [Cd(bdc)(DMF)]_∞1, which has a three-dimensional network structure, whereas at 95 °C, 1 is formed alongside [Cd₃(bdc)₃(DMF)₄]_∞2, which has a two-dimensional network structure. When the reaction is carried out under pressure, it yields [Cd₃(bdc)₃(DMF)₄]_∞3, which is a supramolecular isomer of 2. The structure of 3 differs from that of 2 regarding the way the Cd₃(O₂CR)₆ units are interlinked to form layers. When the reaction was carried out in DMF that had undergone partial hydrolysis, the only isolated product was {(NMe₂H₂)₂[Cd(bdc)₂]·2DMF}_∞4. Compound 4 has a three-dimensional triply-interpenetrated diamondoid structure, with dimethylammonium cations and DMF molecules included within the pores. The reaction between Cd(NO₃)₂·4H₂O and H₂bdc in DEF gave [Cd(bdc)(DEF)]_∞5, regardless of the solvent quality. Compound 5 has a three-dimensional network structure. The reaction of Cd(NO₃)₂·4H₂O and 1,3-benzenedicarboxylic acid (H₂mbdc) in DMF gave [Cd(mbdc)(DMF)]_∞6 which has a bilayer structure. The thermal properties of the new materials have been investigated, and the coordinated DEF molecules from 5 can be removed on heating to 400 °C without any change in the powder X-ray diffraction pattern. The H₂ sorption isotherm for the desolvated material shows marked hysteresis between adsorption and desorption, and less adsorption than predicted by simulations. Kinetic data indicate that the hysteresis is not due to mass transfer limitations, and the most likely explanation for this behaviour lies in partial collapse of the framework to an amorphous phase under the conditions of activation.