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, [Cd2(btc)2(H2O)2]n·n(H2bmib)·6n(H2O) (1) and [Cd4(cpip)2(Hcpip)2]n·n(H2bmib)·n(H2O) (2), have been prepared by
hydrothermal reactions of Cd­(NO3)2·4H2O, 1,4-bis­(2-methylimidazol-1′-yl)­butane (bmib) with
1,3,5-benzenetricarboxylic acid (H3btc) and 5-(4-carboxyphenoxy)­isophthalic
acid (H3cpip), respectively. Complexes 1 and 2 are 3-D anionic frameworks containing 1-D channels and consisting
of tetranuclear Cd­(II)-carboxylate units, respectively. H2bmib and lattice water molecules are located in their void spaces
and form extensive hydrogen bonds and C–H···π
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 H3btc and H3cpip,
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 × 10–5 and
2.2 × 10–5 S cm–1, respectively.