Series of Cadmium-Organic Frameworks Based on Mixed Flexible and Rigid Ligands: Single-Crystal-to-Single-Crystal Transformations, Sorption, and Luminescence Properties

Here, we present a series of Cd(II) coordination polymers
containing
two types of ligands: sterically rigid terephthalate derivatives (bdc-NO22– and bdc-Br2–) and flexible bis(2-methylimidazolyl)propane (bmip).
The combination of two types of ligands is used to obtain and characterize
compounds by single crystal and powder X-ray diffraction, FT-IR, elemental
analysis, and TGA. Guest exchange results in structural transformations.
2-fold interpenetrated 1·DMF and 2·DMF
rapidly undergo to 4-fold interpenetrated 1·Et2O, 1·EtOH, and 1·H2O, or 2·Et2O, respectively. Also,
changes in the coordinating numbers and length of the N,N′-donor bmip ligand were observed according
to single crystal X-ray analysis. Activated guest-free compounds [Cd(bdc-NO2)(bmip)] (1) and [Cd(bdc-Br)(bmip)] (2) are shown to be porous with a BET surface area of 103 and 283 m2·g–1, respectively. Moreover, both
compounds demonstrate gate-opening behavior of ethylene adsorption
isotherms at low pressures (<1 bar) and highly selective adsorption
of benzene over cyclohexane or lower alcohols. Also, both compounds
demonstrate a strong dependence of the maximum of the photoluminescence
emission on an excitation wavelength. As a result, the photoluminescence
color changes from white to red and from blue to red through green
and yellow for compounds 1 and 2, respectively,
with excitation wavelength changing from 360 to 540 nm.