2 H Solid‐State NMR Spectroscopy Reveals the Dynamics of a Pyridine Probe Interacting with Coordinatively Unsaturated Metal Sites of MIL‐100(Al) Metal–Organic Frameworks

Properties of Aliphatic Ligand-Based Metal-Organic Frameworks

Ligands with a purely aliphatic backbone are receiving rising attention in the chemistry of coordination polymers and metal-organic frameworks. Such unique features inherent to the aliphatic bridges as increased conformational freedom, non-polarizable core, and low light absorption provide rare and valuable properties for their derived MOFs. Applications of such compounds in stimuli-responsive materials, gas, and vapor adsorbents with high and unusual selectivity, light-emitting, and optical materials have extensively emerged in recent years. These properties, as well as other specific features of aliphatic-based metal-organic frameworks are summarized and analyzed in this short critical review. Advanced characterization techniques, which have been applied in the reported works to obtain important data on the crystal and molecular structures, dynamics, and functionalities, are also reviewed within a general discussion. In total, 132 references are included.

On phosphorus chemical shift anisotropy in metal (IV) phosphates and phosphonates: A 31 P NMR experimental study

The phosphorus chemical shift anisotropies, ³¹ PΔcs, and asymmetry parameters η were measured by the ³¹ P{¹ H} NMR experiments in static and low-frequency spinning samples of the zirconium phosphates and phosphonates and also in the mixed Zr (IV)/Sn (IV) phosphate/phosphonate material. The data obtained have shown a 111 connectivity in the HPO₄ and PO₃ groups, which does not change at modification and intercalation of the materials. The ³¹ PΔcs values of the phosphonate groups (43-49 ppm) significantly surpass the values characterizing the HPO₄ groups (23-37 ppm). The ³¹ P Δcs values obtained for the metal (IV) phosphates were discussed in terms of P-O distances. The ³¹ P chemical shift anisotropy parameters can help at elucidation of local structures in phosphate and phosphonate materials.

Elucidating structure and dynamics of crystalline α-zirconium phosphates intercalated with water and methanol by multinuclear solid-state MAS NMR: A comprehensive NMR approach

Solid-state NMR experiments on ² H, ³¹ P, ¹³ C, and ¹ H nuclei, including ³¹ P T₁ , ¹ H T₁ , and ¹ H T_1ρ measurements, as well as on the kinetics of proton-phosphorus cross-polarization have been performed to characterize the crystalline and amorphous α-zirconium phosphates, which were intercalated with D₂ O and/or CD₃ OD. The ¹³ C{¹ H} CP MAS NMR experiment performed for compound 1-CD₃ OD (Zr (HPO₄ )₂^. 0.2CD₃ OD) with carbon cross-polarization via protons of phosphate groups has provided a prove that the methanol was intercalated into the interlayer spaces of this compound. The variable-temperature ² H solid-echo MAS NMR spectra of intercalated compounds demonstrated that the methanol molecules, in contrast to the mobile water, were immobile, keeping, however, free CD₃ rotations around the C₃ -axis. It has been demonstrated that the intercalated species, D₂ O and CD₃ OD, do not affect the high-frequency motions of the phosphate groups. By utilizing local structural models that satisfy the constraints of the experimental data, it has been suggested that the immobile methanol molecules are located in the cavity between two neighboring layers of the zirconium phosphates. Thus, the present work illustrates the reliable criteria in a comprehensive NMR approach to structural and dynamic studies of such systems.

Correlated motion and mechanical gearing in amphidynamic crystalline molecular machines

In this review we highlight the recent efforts towards the development of molecular gears with an emphasis on building molecular gears in the solid state and the role that molecular gearing and correlated motions may play in the function of crystalline molecular machines. We discuss current molecular and crystal engineering strategies, challenges associated with engineering correlated motion in crystals, and outline experimental and theoretical tools to explore gearing dynamics while highlighting key advances made to date.

Molecular Mobility of Tert-butyl Alcohol Confined in a Breathing MIL-53 (Al) Metal-Organic Framework

We present a detailed solid-state NMR characterization of the molecular dynamics of tert-butyl alcohol (TBA) confined inside breathing metal-organic framework (MOF) MIL-53(Al). ²⁷ Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL-53 material with partially shrunk channels. ² H solid-state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two C 3 axes and librations of the molecule as a whole about the axis passing through the TBA C-O bond. These librations are realized by two distinct ways: fast molecule orientation change during the translational jump diffusion along the channel with characteristic time τ_D of about 10^-9  s at 300 K; slow local librations at a single coordination site, representing framework hydroxyl groups, with τ_l ≈10^-6  s at 300 K. Self-diffusion coefficient of the alcohol in the MOF has been estimated: D=3.4×10^-10  m²  s^-1 at 300 K. It has been inferred that both the framework flexibility and the interaction with framework hydroxyl groups define the dynamics of TBA confined in the channels of MIL-53 (Al).

Dynamics of propene and propane in ZIF-8 probed by solid-state 2H NMR

We present a detailed ²H NMR characterization of molecular mobility of propene and propane propagating though the microporous ZIF-8, a zeolitic imidazolate framework renowned for its outstandingly high separation selectivity for industrially relevant propene/propane mixtures. Experimental characterization of both propene and propane diffusivity in ZIF-8 has been provided. Using ²H NMR spin relaxation analysis, the motional mechanisms for propene and propane guests trapped within the ZIF-8 framework have been elucidated. Kinetic parameters for each type of motion were derived. The characteristic times for microscopic translational diffusion and activation barriers (E_C3H8 = 38 kJ mol^-1, E_C3H6 = 13.5 kJ mol^-1) for propane and propene diffusivities have been estimated. A notable difference in the observed activation barriers emphasizes that the ZIF-8 window crossing is associated with the "gate-opening" and represents an extremely shape selective process. Finally, we show that the ²H NMR technique is capable of providing reliable information on microscopic diffusivity in the ZIF-8 MOF even for molecules with slow diffusivity (<10^-14 m² s^-1).

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

A comprehensive overview of characterization tools for the analysis of well-known metal–organic frameworks and physico-chemical phenomena associated to their applications.

Pyridine-d5 as a 2H NMR probe for investigation of macrostructure and pore shapes in a layered Sn(iv) phosphonate–phosphate material

Isotropic and anisotropic motions and molecular states of pyridine-d₅, adsorbed on the surface within the pores of a layered Sn(iv) phosphonate–phosphate material (1) have been characterized thermodynamically and kinetically by solid-state NMR.

Dynamics of isobutane is a sensitive probe for framework breathing in MIL-53 (Al) MOF

²H solid-state NMR shows that the dynamics of adsorbed isobutane is very sensitive to MIL-53 framework breathing detected by ²⁷Al MAS NMR.