Spies Within Metal-Organic Frameworks: Investigating Metal Centers Using Solid-State NMR

Structural characterization of metalorganic frameworks (MOFs) is crucial, since an understanding of the relationship between the macroscopic properties of these industrially relevant materials and their molecular-level structures allows for the development of new applications and improvements in current performance. In many MOFs, the incorporated metal centers dictate the short- and long-range structure and porosity of the material. Here we demonstrate that solid-state NMR (SSNMR) spectroscopy targeting NMR-active metal centers at natural abundance, in concert with ab initio density functional theory (DFT) calculations and X-ray diffraction (XRD), is a powerful tool for elucidating the molecular-level structure of MOFs. Zr-91 SSNMR experiments on MIL-140A are paired with DFT calculations and geometry optimizations in order to detect inaccuracies in the reported powder XRD crystal structure. In-115 and La-139 SSNMR experiments on sets of related MOFs at two different magnetic fields illustrate the sensitivity of the In-115/La-139 electric field gradient tensors to subtle differences in coordination, bond length distribution, and ligand geometry about the metal center. Ti-47/49 SSNMR experiments reflect the presence or absence of guest solvent in MIL-125(Ti), and when combined with DFT calculations, these SSNMR experiments permit the study of local hydroxyl group configurations within the MOF channels. Zn-67 SSNMR experiments and DFT calculations are also used to explore the geometry near Zn within a set of four MOFs as well as local disordering caused by distributions of different linkers around the metal. SSNMR spectroscopy of metal centers offers an impressive addition to the arsenal of techniques for MOF characterization and is particularly useful in cases where XRD information may be ambiguous, incomplete, or unavailable.