Cd-113 Nuclear Magnetic Resonance as a Probe of Structural Dynamics in a Flexible Porous Framework Showing Selective O-2/N-2 and CO2/N-2 Adsorption

Abstract

Two new isomorphous three-dimensional porous coordination polymers, {[Cd(bpe)(0.5)(bdc)(H2O)]center dot EtOH}n (1) and {[Cd(bpe)(0.5)(bdc)(H2O)]center dot 2H(2)O}n (2) [bpe = 1,2-bis(4-pyridyl)ethane, and H2bdc = 1,4-benzenedicarboxylic acid], have been synthesized by altering the solvent media. Both structures contain one-dimensional channels filled with metal-bound water and guest solvent molecules, and desolvated frameworks show significant changes in structure. However, exposure to the solvent vapors (water and methanol) reverts the structure back to the as-synthesized structure, and thus, the reversible flexible nature of the structure was elucidated. The flexibility and permanent porosity were further reinforced from the CO2 adsorption profiles (195 and 273 K) that show stepwise uptake. Moreover, a high selectivity for O-2 over N-2 at 77 K was realized. The framework exhibits interesting solvent vapor adsorption behavior with dynamic structural transformation depending upon the size, polarity, and coordination ability of the solvent molecules. Further investigation was conducted by solid state Cd-113 nuclear magnetic resonance (NMR) spectroscopy that unambiguously advocates the reversible transformation pentagonal-bipyramidal CdO6N -> octahedral CdO5N geometry in the desolvated state. For the first time, Cd-113 NMR has been used as a probe of structural flexibility in a porous coordination polymer system.