Exploring Short-Range Order in a Cadmium-Based Zeolitic Imidazolate Framework Glass by Solid State NMR Spectroscopy

Metal-Organic Frameworks (MOFs) have garnered significant scientific interest due to their chemical flexibility, high porosity, and thermal stability. While early research primarily focused on their crystalline phases, recent studies have revealed the potential of amorphous MOFs, particularly through their vitrification, establishing them as a fourth class of glass-forming materials alongside conventional organic, inorganic, and metallic glasses. The vitrification of Metal-Organic Frameworks (MOFs) has opened new avenues in glass science, with Zeolitic Imidazolate Frameworks (ZIFs) serving as model systems due to their melt-quench ability.^(1,2) ZIF glasses, such as ZIF-62, have shown promise in gas separation membranes, photoelectrocatalysis, and molecular sieving, owing to their retained porosity, chemical tunability, and ease of processing in the amorphous state. ⁽³⁾ Additionally, metal-ion substitution (e.g., Zn²⁺ → Co²⁺) expands their application scope by enhancing catalytic activity, electronic conductivity, and enabling functionalization for environmental or energy-related uses.⁽⁴⁾ While the organic linker environment in ZIF glasses has been extensively studied by ¹³C/¹H NMR and pair distribution function (PDF) analysis, the metal coordination sphere—critical to understanding their amorphous structure—remains poorly characterized. This gap arises from the challenges of studying quadrupolar metal nuclei (e.g., ⁶⁷Zn) by NMR, as demonstrated by recent ultra-high-field studies revealing Zn-site disorder in vitrified ZIFs.⁽⁵⁾ Here, we developed a fundamental study, employing ¹¹³Cd solid-state NMR (SSNMR) to investigate the first cadmium-based MOF glass, Cd(Im)₂₋ₓ(bim)ₓ (Im = imidazole, bim = benzimidazole), using the more favorable NMR properties of  ¹¹³Cd (I = ½, 12.22% abundance, high γ) to resolve local structural changes during vitrification. By combining ¹¹³Cd, ¹H, and ¹³C NMR, we correlate the evolution of organic linker order (retained post-melting) with significant disorder in the Cd²⁺ coordination environment. Thermal analysis revealed re-crystallization behavior for this glass system, being the first imidazole-based MOF-glass to show this behavior. We also observed that a higher amount of benzimidazole impacted the glasses by decreasing the glass transition temperature (Tg) and increasing the crystallization temperature. This work demonstrates how multinuclear NMR can unravel structural dynamics in MOF glasses, complementing the information from diffraction methods. There are still many questions to be elucidated about the structure of glassy MOFs and their amorphization dynamics, especially with studies that focus on the metallic environment of ZIFs. This work therefore emerges as one of the precursors for studies in different metallic environments.

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Acknowledgements: Acknowledgement to FAPESP for support under project number 2023/07712-8.