Abstract:
Metal-organic frameworks (MOFs) are three-dimensional coordination polymers comprised of organic and inorganic subunits connected by self-assembled metal-ligand bonds. A chromium-based MOF was synthesized by reaction of Cr(CO)6 and 1,4-di(2H-tetrazole-5yl)benzene (H2BDT) under a nitrogen atmosphere. This chromium azolate MOF, Cr(BDT)–DMF, is comprised of chains of chromium atoms bridged by dimethylformamide (DMF) solvent molecules and connected by tetrazolate linker molecules. This material forms rhombus-shaped channels and exhibits framework flexibility, or breathing effects, when external stimuli are applied. The inclusion or exclusion of solvent molecules or oxidation of the chromium centers are two external parameters that can contribute to framework changes. The effects of oxidation state on the framework flexibility of Cr(BDT)–DMF were studied by oxidizing the as synthesized form of Cr(BDT)–DMF with both atmospheric O2 and elemental S8. These materials were then analyzed via powder X-ray diffraction (PXRD), infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, and N2 adsorption. Oxidation of the Cr2+ centers with atmospheric O2 resulted in a loss of framework flexibility as shown by N2 adsorption isotherms. In order to study the solvent effects on framework flexibility, suspensions of Cr(BDT)–DMF were soaked in ethanol to form Cr(BDT)–EtOH. This solvent exchange process resulted in changes in framework structure as determined by PXRD. These changes are also readily observed through differences in the shape of the N2 adsorption isotherms measured for the DMF and EtOH-exchanged MOFs and suggest that they experience larger changes in framework structure.
