Triangulation of guest species in porous frameworks using solid-state NMR spectroscopy and quantum chemical calculations

<p dir="ltr">Porous functional materials such as zeolites and metal-organic frameworks (MOFs) have applications across a range of chemical industries for chemical conversion and gas separation and storage owing to their high surface areas and chemical tunability. Characterising how the active sites of porous frameworks interact with adsorbates is an important but challenging task. Solid-state (ss)NMR spectroscopy is a powerful tool in this regard but in isolation often lacks the capability to resolve the precise locations of adsorbed species; complementary neutron or synchrotron-based diffraction techniques are thus required. Herein, fluorinated pyridine is used as a probe molecule for ssNMR analysis to facilitate the identification of adsorption modes and locations in the prototypical UiO-66(Zr). This highly stable MOF material provides a methodology and benchmark for binding-site analysis. The high NMR receptivity of ¹⁹F and its sensitivity to its surrounding environment enable measurement of internuclear distances, and, when combined with density functional theory calculations and numerical simulations, aid in the triangulation of the guest species within the pores of MOFs. These measurements confirm that fluorinated pyridine binds to both the bridging hydroxyl Brønsted acid sites as well as to Lewis acid sites at defects, both with specific preferred orientations. These orientations are stabilised by additional intermolecular host-guest interactions and the formation of H⋯O and F⋯H hydrogen bonds and pi⋯pi interactions. This work establishes a proof-of-concept for an experimental-computational pipeline, which can be employed generally for the study of porous frameworks and nanomaterials. This affords insights into adsorption structures that can be related to material properties and will aid the design of materials with improved and tailored functions.</p>