Guest Controlled Rotational Dynamics of Terephthalate Phenylenes in Metal–Organic Framework MIL-53(Al): Effect of Different Xylene Loadings

MIL-53 is an interesting metal–organic framework (MOF) with a “breathing” framework which is envisioned for a number of potential applications. It is suggested that the processes of hydrocarbon adsorption, diffusion, and separation by this material are strongly influenced by flexibility of the framework and fastly moving terephthalate phenylene fragments, representing the array of molecular rotors. To govern the mentioned processes we need to learn how to monitor the flexibility of the framework and identify the specific effects of particular hydrocarbons on the rotational motion of phenylene fragments. Here we demonstrate that flexibility, i.e., large pore (LP) and narrow pore (NP) crystalline state interconversions of the framework, can be monitored by following the evolution of phenylene fragments dynamics with temperature by ²H solid-state nuclear magnetic resonance. We have established that the dynamics of phenylene fragments is very sensitive to the loading of xylene guests and the MOF structural state. The rotation rate is higher and the activation barrier lower for the LP state of the guest-free or loosely loaded material, whereas the NP and LP states with high loadings and dense guest packing show a decrease of the rotation rates and increase of the rotation energetic barrier

Mobility of the 2‑Methylimidazolate Linkers in ZIF‑8 Probed by ²H NMR: Saloon Doors for the Guests

ZIF-8 is one of the most interesting metal–organic frameworks due to its high stability and unique capabilities for hydrocarbons separation. Its porous network is formed by large almost spherical cavities interconnected by very narrow windows, ∼3.4 Å, which should be too small even for methane. At the same time the direct experimental observations show that ZIF-8 cavities are able to host even such large molecules as benzene. This effect is associated with the flexibility of the cavity widows, arising from dynamical freedom on the 2-methylimidazole (2-mIM) linkers that form the framework. In this work, by means of ²H NMR we show that the 2-mIM linkers of the ZIF-8 are very mobile and their mobility is sensitive to the presence of benzene guest. In contrast with other known MOFs based on linearly bonded carboxylates, in guest-free ZIF-8 the plane of 2-mIM linker exhibits two-site flips within a sector of 2φ_f = 34° with very low activation barrier (1.5 kJ mol^–1) and high rates (∼10¹² s^–1). Above 380 K the linkers begin to demonstrate additional fast librations with gradually increasing amplitudes γ_lib comparable with the two-site flips (γ_lib = ±17° above 560 K). This is direct evidence that the ZIF-8 linkers twist notably, thus increasing the aperture of the windows sufficiently to fit very large molecules. Upon benzene loading, the geometry of the observed motions remains similar but the potential barrier of the linkers flipping rises up to 9 kJ mol^–1

Characterization of Fast Restricted Librations of Terephthalate Linkers in MOF UiO-66(Zr) by ²H NMR Spin–Lattice Relaxation Analysis

²H NMR spin–lattice relaxation was used to probe small-amplitude torsional vibrations (librations) of the organic terephthalate linkers in metal–organic framework (MOF) UiO-66­(Zr) saturated with benzene molecules. In UiO-66 (Zr) the mobile phenylene fragments exhibit a complex rotational dynamics of the phenylene rings with fast librations and much slower π-flips around the <i>C</i>₂ symmetry axis. We show that due to the intrinsic broad distribution of the π-flips rate, the relaxation process for the deuterium in the C–D group of phenylene fragment is multiexponential. Two main modes of <i>T</i>₁ relaxation are clearly detected, corresponding to the fast <i>T</i>₁^fast and the slow <i>T</i>₁^slow relaxation. Based on the experimental observation of two-exponential relaxation, a computational model for this <i>T</i>₁ relaxation behavior capable to reproduce the peculiarities of the MOF linkers dynamics was built. Computational analysis allows to establish that the librational motion affects mostly the <i>T</i>₁^slow, while <i>T</i>₁^fast remains unaffected by this motion. Simulation of the <i>T</i>₁^slow dependence on the libration rate <i>k</i>_lib shows that in the range of the librational frequencies of 10⁶–10⁹ Hz the <i>T</i>₁^slow is not sensitive to the <i>k</i>_lib variation, and therefore a precise correspondence between <i>T</i>₁^slow and <i>k</i>_lib cannot be established. <i>T</i>₁^slow exhibits a specific “peak-like-shape” dependence of <i>k</i>_lib in the range of 10⁹–10¹² Hz. In this range of libration frequencies an unambiguous relation between <i>T</i>₁^slow and <i>k</i>_lib exists only in a very narrow frequency window of 0.1 × 10¹⁰–5 × 10¹⁰ Hz. The best conditions to characterize the librational motion by means of <i>T</i>₁ relaxation analysis are met when the flipping motion is almost frozen (<i>k</i>_flip < 10³ Hz) because <i>T</i>₁^slow becomes extremely sensitive to the variation of <i>k</i>_lib

Mobility of <i>tert-</i>Butyl Alcohol in MFI Framework Type Studied by Deuterium NMR

The molecular mobility of deuterated <i>tert-</i>butyl alcohol (TBA) adsorbed in MFI framework type (silicalite-1 and ZSM-5 zeolite) has been studied by use of ²H NMR spectroscopy in the range of 106–453 K. In H-ZSM-5, the reorientation of the molecule as a whole is strongly restricted (τ_C ≪ <i>Q</i>₀^–1 ≈ 10^–6 s) by hydrogen bonding to Brønsted acid sites (BAS). Being adsorbed to BAS, the motion of TBA molecules is described by intramolecular rotations around two successive <i>C</i>₃ and <i>C</i>₃′ axes (CD₃–C and C–O bonds). The activation energy for the methyl groups rotation around the C–O bond (<i>E</i>_a = 8.0 ± 1.6 kJ mol^–1) is two times lower compared to that in solid TBA (<i>J. Phys. Chem. A</i> <b>2011</b>, <i>115</i>, 7428). This shows that the motion of the butyl fragment of TBA in MFI framework is less restricted compared to the case of solid TBA. In silicalite-1, the TBA molecule is additionally involved into reorientational motions as a whole: one of the motions represents an exchange among orientations provided by the directions of four framework channels driven by the translational jump diffusion. The activation barriers for the jump diffusion between two zigzag channels (<i>E</i>_a = 7 ± 2 kJ mol^–1) and between zigzag and straight channels (<i>E</i>_a = 5 ± 1 kJ mol^–1) are of the similar values. The other motion represents a large-amplitude wobbling of the TBA molecule localized at a channel intersection site cavity. This motion is described by a fast restricted wobbling of the molecular axis in a sphere sector. The wobbling boundaries become gradually broader as temperature increases, allowing thus a larger accessible space for the TBA molecule to explore. The anisotropy of this motion persists even at 453 K, revealing the presence of a strong intracavity barrier that blocks the TBA from free rotational diffusion inside the cavity. This study demonstrates that the ²H NMR analysis of both line shape and spin–lattice relaxation represents a powerful tool to investigate the particular pore confinement effect on the molecular mobility of TBA adsorbed in the MFI framework

Characterization and Dynamics of the Different Protonic Species in Hydrated 12-Tungstophosphoric Acid Studied by ²H NMR

Solid heteropolyacids (HPAs) are promising nonpolluting superacids used as catalysts and solid proton conductors. The catalytic and conducting properties of HPAs are very sensitive to the amount of hydration water present in the system, as water molecules tend to interact with the HPA acid protons to form [H₃O]⁺ and [H₅O₂]⁺ ions. These ions constitute active species that govern the catalytic reaction pathways and the proton migration mechanism. Establishing the structure and mobility of protonic species could yield important information concerning the functions of material based on HPA hydrates. In this work, we have performed the analysis of both ²H NMR line shape and the evolution of <i>T</i>₁, <i>T</i>₂ relaxation with temperature for the deuterated analogue of the solid 12-tungstophosphoric acid (TPA × <i>n</i>H₂O) at different hydration levels (0 < <i>n</i> < 6) in the wide temperature range of 103–503 K. This allowed us to characterize in detail the mobility of different protonic species, including acidic OH groups, water molecules, and hydroxonium ions. Kinetic parameters of internal and diffusional motions for different protonic species at different hydration levels <i>n</i> of TPA × <i>n</i>H₂O were derived

Probing the Guest-Mediated Structural Mobility in the UiO-66(Zr) Framework by ²H NMR Spectroscopy

The solid-state ²H NMR technique (analysis of both the spectrum line shape and the spin–lattice relaxation) was used to probe both slow and fast dynamical modes of the phenylene fragments of terephthalate linkers of the UiO-66­(Zr) framework affected by the presence of benzene guest in the pores of the material. Such approach allowed us to probe different motions within a broad range of time scale, 10^–3–10^–11 s. The internal dynamics in the UiO-66­(Zr) framework is represented by torsional motions of the phenylene fragment of the linker including 2-site 180° flips (π-flips) of the plane of the phenylene ring and its restricted librations. In the presence of benzene loaded in the MOF pores the rate of π-flips decreases essentially and the activation barrier for this motion increases. The activation barrier has been found to increase almost in a linear fashion on benzene loading. Such observation is surprisingly unique among other MOFs with mobile linkers, like MIL-53­(Al) or MOF-5. The fast librational motion occurs on a scale of ∼10¹⁰ Hz and shows no notable dependence on the guest loading. It has been established that anisotropy of <i>T</i>₁ relaxation of the ²H NMR powder pattern of the phenylene fragments is especially sensitive to the librational motion when this motion is in a range of 10⁷–10¹¹ Hz. Within this range of libration frequencies, analysis of the anisotropic spin–lattice (<i>T</i>₁) relaxation allows quantitative estimation of the rate of librational motion

Structural Dynamics in a “Breathing” Metal–Organic Framework Studied by Electron Paramagnetic Resonance of Nitroxide Spin Probes

Reversible structural rearrangements (“breathing”) of metal–organic frameworks (MOFs) are interesting and complex phenomena with many potential applications. They are often triggered by small amounts of adsorbed guest molecules; therefore, the guest–host interactions in breathing MOFs are intensively investigated. Due to the sensitivity limitations, most analytical methods require relatively high concentrations of guests in these studies. However, because guest molecules are not “innocent”, breathing behavior may become suppressed and unperturbed structural states inaccessible. We propose here the use of guest nitroxide molecules in tiny concentrations (such as 1 molecule per 1000 unit cells), which serve as spin probes for electron paramagnetic resonance (EPR), for effective study of breathing phenomena in MOFs. Using a perspective MIL-53­(Al) framework as an example, we demonstrate the great advantage of this general approach, which avoids perturbation of the framework structure and allows in-depth investigation of guest–host interactions in the breathing mode