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

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.