Structure and Dynamics of the Functionalized MOF Type UiO-66(Zr): NMR and Dielectric Relaxation Spectroscopies Coupled with DFT Calculations

Abstract

Advanced one- and two-dimensional high-field and ultrafast MAS NMR measurements have been conducted in tandem with DFT calculations for the NMR parameters to deeply characterize the local environment and the long-range structure order of the porous metal–organic framework (MOF) type UiO-66(Zr) (UiO for University of Oslo) functionalized by a series of polar −Br, −2OH, and −NH2 groups. Such an innovative combining approach applied to the complex architecture of MOFs has been revealed successful not only to unambiguously assign all the NMR signals to the corresponding crystallographic sites but also to validate the crystal structures for each functionalized material that were only predicted so far. A further step consisted of probing the impact of the grafted functions on the ligand dynamics of these MOFs by means of dielectric relaxation spectroscopy measurements. It has been evidenced that the rotational motion of the organic linker requires overpassing an energy barrier that strongly depends on the functional groups, the −NH2 functionalized version implying the highest activation energy. Such a finding was further explained by the relatively strong intraframework interactions which take place between the grafted function and the inorganic node as suggested by the analysis of the corresponding simulated crystal structure.

Selective Glucose‐to‐Fructose Isomerization over Modified Zirconium UiO‐66 in Alcohol Media

“…Next, we investigated by 13 C solid‐state NMR spectroscopy whether the sugars adsorption occurs through ring opening (Figure b). The spectrum of the UiO‐66‐Fm sample shows three main resonances at 171, 132, and 128.9 ppm, assigned to the carbonyl, quaternary, and aromatic carbons of terephthalic acid on the framework . Resonances in the regions between 200–180 and 85–65 ppm, and at 112 ppm and 58 ppm were identified as sidebands, confirmed by experiments at different spinning rates (Figure S16).…”

Statistical mechanical model of gas adsorption in porous crystals with dynamic moieties

“…For example, spin crossover transitions of a metal in a MOF (68) have been shown to modulate the rotational freedom of coordinating ligands (69), opening an avenue for switching the ligand state with an electric field, temperature, or light (70). For asymmetric ligands with polar groups, an electric field could modulate the rotational conformations (71,72).…”

Dynamic Motion of Organic Ligands in Polar Layered Cobalt Phosphonates

“…When the frequency of the applied electric field is increased, the maximum of the peak shifts to a higher temperature. This kind of frequency‐dependent behavior indicates a thermally activated single relaxation process . Thus, the relaxation rate (1/ τ ), characterizing the dielectric relaxation process, should follow the Arrhenius law [Eq.…”

“…A number of new architectures with various pore sizes, shapes, and chemical functions have been obtained, which show promising potential applications in gas storage and separation, catalysis, proton conduction, and magnetism, etc. MOFs can also be good candidates for studying the molecular motor dynamics because of their porosity, thermal stability, and flexibility, which allow the free rotation of organic ligands . However, the rotation frequency of the ligands can be affected or controlled by the structure, such as the shape and volume of the pores, the temperature, and other external stimuli.…”