Structure Elucidation of Mixed-Linker Zeolitic Imidazolate Frameworks by Solid-State ¹H CRAMPS NMR Spectroscopy and Computational Modeling

Abstract

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are nanoporous materials that exhibit continuous and controllable tunability of properties like effective pore size, hydrophobicity, and organophilicity. The structure of mixed-linker ZIFs has been studied on macroscopic scales using gravimetric and spectroscopic techniques. However, it has so far not been possible to obtain information on unit-cell-level linker distribution, an understanding of which is key to predicting and controlling their adsorption and diffusion properties. We demonstrate the use of (1)H combined rotation and multiple pulse spectroscopy (CRAMPS) NMR spin exchange measurements in combination with computational modeling to elucidate potential structures of mixed-linker ZIFs, particularly the ZIF 8-90 series. All of the compositions studied have structures that have linkers mixed at a unit-cell-level as opposed to separated or highly clustered phases within the same crystal. Direct experimental observations of linker mixing were accomplished by measuring the proton spin exchange behavior between functional groups on the linkers. The data were then fitted to a kinetic spin exchange model using proton positions from candidate mixed-linker ZIF structures that were generated computationally using the short-range order (SRO) parameter as a measure of the ordering, clustering, or randomization of the linkers. The present method offers the advantages of sensitivity without requiring isotope enrichment, a straightforward NMR pulse sequence, and an analysis framework that allows one to relate spin diffusion behavior to proposed atomic positions. We find that structures close to equimolar composition of the two linkers show a greater tendency for linker clustering than what would be predicted based on random models. Using computational modeling we have also shown how the window-type distribution in experimentally synthesized mixed-linker ZIF-8-90 materials varies as a function of their composition. The structural information thus obtained can be further used for predicting, screening, or understanding the tunable adsorption and diffusion behavior of mixed-linker ZIFs, for which the knowledge of linker distributions in the framework is expected to be important.

Unexpected Crystallization Patterns of Zinc Boron Imidazolate Framework ZBIF‐1: NMR Crystallography of Integrated Metal–Organic Frameworks

“…5 ppm) units, were the slowest to evolve; however, these signals were possible to detect at longer SD times (in the microsecond range). Jayachandrababu et al . used 1 H combined rotation and multiple pulse spectroscopy (CRAMPS)/MAS NMR spectroscopy in combination with SD to investigate the interconnections within ZIF‐8–90 hybrid materials.…”

Synthetic Strategies and Structural Arrangements of Isoreticular Mixed‐Component Metal–Organic Frameworks

“…The structure B can be seen as a transition between a random distribution of linkers ( A ) and an ordered core–shell structure ( C ). The different linker molecules are neither distributed statistically nor well ordered, but grouped in domains within the framework . This circumstance makes the arrangement B the most challenging one regarding a reproducible synthesis and a reliable characterization.…”

“…No information about the spatial distribution can be obtained, though. On the other hand, solid‐state NMR measurements in combination with DFT or Monte Carlo calculations can be used to determine the linker distribution in the nanometer range . Yeung et al.…”

Vapor-Phase Linker Exchange of the Metal-Organic Framework ZIF-8: A Solvent-Free Approach to Post-Synthetic Modification

“…Likely,t he vacuum conditions enhance the vaporization rate and gas phase diffusion of the incoming linker through an increased mean free path length. [27] Thes ignal highlighted in Figure 5a corresponds to the polarization transfer between the aldehyde and methyl group of CHO-Im and mIm, respectively.This result confirms the close proximity of both linkers in ZIF DS .I nc ontrast, the absence of this signal in ZIF VPLE (Figure 5b)i ndicates the formation of segregated domains.T his observation is in accordance with the spectrum of physical mixtures of ZIF-8 and ZIF-90 crystals,t hat is,w hen both domains are completely segregated, and is further supported by the spin diffusion curves (Figures S24-25). We hypothesized that moisture in the ambient air aids VPLE, compared to dry argon and nitrogen.…”

“…Te sts with ambient air in the reaction vessel led to VPLE rates similar as under vacuum. [27] To further understand the linker segregation in ZIF VPLE , the same crystals were imaged using stimulated Raman scattering (SRS). [25] The catalyzing effect of moisture was confirmed by adding water vapor after evacuating the reaction vessel, to achieve 5% relative humidity at 298 K. VPLE under these conditions led to the incorporation of 63 %C HO-Im after 24 h. Since neither ZIF-8 nor ZIF-90 have significant water uptakes at such al ow relative humidity (Figure S23), the effect is likely due to co-adsorption of water with the incoming linker.…”