Preparation, structural diversity and characterization of a family of Cd(II)-organic frameworks

Two Cd(II)-organic frameworks based on 5-iodoisophthalate (IIP(2-)), {[Cd(IIP)(bte)(H2O)]·H2O}n (1) and [Cd(IIP)(bpp)(H2O)]n (2), were obtained either at an ambient temperature or under solvothermal conditions at 120 °C in the presence of 1,4-bis(1,2,4-triazol-1-yl)ethane (bte) and 1,3-bis(4-pyridyl)propane (bpp) as auxiliary ligands, respectively. 1 is a novel discrete single-walled Cd(II)-organic tube (SWCOT) which further extends into a 3D supramolecular interdigitated microporous columnar architecture supported by C-I···I halogen bonds and hydrogen bonds, while 2 exhibits an interesting two-fold interpenetrated 3D diamond network architecture. When the auxiliary ligands bte or bpp were replaced by a longer spacer ligand with more flexibility, 1,4-bis(triazol-1-ylmethyl)benzene (bbtz), the unique discrete single-walled Cd(II)-organic nanotube (SWCONT), {[Cd(IIP)(bbtz)(H2O)]·H2O}n (3), which further extends into a 3D supramolecular microporous framework supported by face-to-face π···π stacking interactions and hydrogen bonds, was generated at room temperature. Under solvothermal conditions at 120 °C, an interesting two-fold 2D "embracing" (4,4) topological network, [Cd(IIP)(bbtz)(H2O)] (4), which further extends into a two-fold 3D "embracing" supramolecular framework through O-H···O hydrogen bonds, is obtained. 4 loses crystallinity in air, leading to the formation of [Cd(IIP)(bbtz)]·0.5H2O (4A) evidenced by elemental analysis, thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD). Remarkably, in situ rapid and reversible dehydration-rehydration in static air occurs in 1-3, indicating their potential applications as water absorbent and sensing materials. Dehydrated 1 and 3 show selective gas adsorption of CO2 over N2 and dehydrated 3 can adsorb methanol and ethanol vapors strongly. These compounds exhibit blue fluorescence in the solid state.