Porous Cadmium(II) Anionic Metal-Organic Frameworks Based on Aromatic Tricarboxylate Ligands: Encapsulation of Protonated Flexible Bis(2-methylimidazolyl) Ligands and Proton Conductivity

Two porous 3-D anionic metal-organic frameworks (MOFs) containing protonated bmib, [Cd-2(btc)(2)(H2O)(2)](n)center dot n(H(2)bmib)center dot 6n(H2O) (1) and [Cd-4(cpip)(2)(Hcpip)(2)](n)center dot n(H(2)bmib)center dot n(H2O) (2), have been prepared by hydrothermal reactions of Cd(NO3)(2)center dot 4H(2)O, 1,4-bis(2-methylimidazol-1'-yl)butane (bmib) with 1,3,5-benzenetricarboxylic acid (H(3)btc) and 5-(4-carboxyphenoxy)isophthalic acid (H(3)cpip), respectively. Complexes 1 and 2 are 3-D anionic frameworks containing 1-D channels and consisting of tetranudear Cd(II)-carboxylate units, respectively. H2bmib and lattice water molecules are located in their void spaces and form extensive hydrogen bonds and C-H center dot center dot center dot pi interaction with the anionic frameworks. TGA studies and XRD patterns show the anionic frameworks of 1 and 2 are intact after the removal of lattice water molecules. The luminescent emission of 1 and 2 shows an obvious red shift in comparison with free H(3)btc and H(3)cpip, respectively. Complexes 1 and 2 possess proton conduction owing to the presence of the extensive hydrogen bonds and protonation of bmib; their proton conductivity at 333 K and 95% relative humidity are 5.4 x 10(-5) and 2.2 x 10(-5) S cm(-1), respectively.