Monitoring and Understanding the Paraelectric-Ferroelectric Phase Transition in the Metal-Organic Framework [NH4][M(HCOO)3] by Solid-State NMR Spectroscopy

Understanding Cu(i) local environments in MOFs via63/65Cu NMR spectroscopy

The field of metal-organic frameworks (MOFs) includes a vast number of hybrid organic and inorganic porous materials with wide-ranging applications. In particular, the Cu(i) ion exhibits rich coordination chemistry in MOFs and can exist in two-, three-, and four-coordinate environments, which gives rise to many structural motifs and potential applications. Direct characterization of the structurally and chemically important Cu(i) local environments is essential for understanding the sources of specific MOF properties. For the first time, 63/65Cu solid-state NMR has been used to investigate a variety of Cu(i) sites and local coordination geometries in Cu MOFs. This approach is a sensitive probe of the local Cu environment, particularly when combined with density functional theory calculations. A wide range of structurally-dependent 63/65Cu NMR parameters have been observed, including 65Cu quadrupolar coupling constants ranging from 18.8 to 74.8 MHz. Using the data from this and prior studies, a correlation between Cu quadrupolar coupling constants, Cu coordination number, and local Cu coordination geometry has been established. Links between DFT-calculated and experimental Cu NMR parameters are also presented. Several case studies illustrate the feasibility of 63/65Cu NMR for investigating and resolving inequivalent Cu sites, monitoring MOF phase changes, interrogating the Cu oxidation number, and characterizing the product of a MOF chemical reaction involving Cu(ii) reduction to Cu(i). A convenient avenue to acquire accurate 65Cu NMR spectra and NMR parameters from Cu(i) MOFs at a widely accessible magnetic field of 9.4 T is described, with a demonstrated practical application for tracking Cu(i) coordination evolution during MOF anion exchange. This work showcases the power of 63/65Cu solid-state NMR spectroscopy and DFT calculations for molecular-level characterization of Cu(i) centers in MOFs, along with the potential of this protocol for investigating a wide variety of MOF structural changes and processes important for practical applications. This approach has broad applications for examining Cu(i) centers in other weight-dilute systems.

Application of solid-state NMR techniques for structural characterization of metal-organic frameworks

Solid-state NMR can afford the structural information about the chemical composition, local environment, and spatial coordination at the atomic level, which has been extensively applied to characterize the detailed structure and host-guest interactions in metal-organic frameworks (MOFs). In this review, recent advances for the structural characterizations of MOFs using versatile solid-state NMR techniques were briefly introduced. High-field sensitivity-enhanced solid-state NMR method enabled the direct observation of metal centers in MOFs containing low-γ nuclei. Two-dimensional (2D) homo- and hetero-nuclear correlation MAS NMR experiments provided the spatial proximity among linkers, metal clusters and the introduced guest molecules. Moreover, quantitative measurement of inter-nuclear distances using solid-state NMR provided valuable structural information about the connectivity geometry as well as the host-guest interactions within MOFs. Furthermore, solid-state NMR has exhibited great potential for unraveling the structure property of MOFs containing paramagnetic metal centers.

Origin of Ferroelectricity in Two Prototypical Hybrid Organic-Inorganic Perovskites

Hybrid organic-inorganic perovskite (HOIP) ferroelectrics are attracting considerable interest because of their high performance, ease of synthesis, and lightweight. However, the intrinsic thermodynamic origins of their ferroelectric transitions remain insufficiently understood. Here, we identify the nature of the ferroelectric phase transitions in displacive [(CH₃)₂NH₂][Mn(N₃)₃] and order-disorder type [(CH₃)₂NH₂][Mn(HCOO)₃] via spatially resolved structural analysis and ab initio lattice dynamics calculations. Our results demonstrate that the vibrational entropy change of the extended perovskite lattice drives the ferroelectric transition in the former and also contributes importantly to that of the latter along with the rotational entropy change of the A-site. This finding not only reveals the delicate atomic dynamics in ferroelectric HOIPs but also highlights that both the local and extended fluctuation of the hybrid perovskite lattice can be manipulated for creating ferroelectricity by taking advantages of their abundant atomic, electronic, and phononic degrees of freedom.

Solvent-induced reversible high-temperature phase transition in crown ether clathrates

The phase transitions of crown ether complexes with molecular motor motion triggered by the solvent-induced effect are reported.

13 C chemical shift tensors in MOF α-Mg3 (HCOO)6 : Which component is more sensitive to host-guest interaction?

Metal-organic frameworks (MOFs) are a class of important porous materials with many current and potential applications. Their applications almost always involve the interaction between host framework and guest species. Therefore, understanding of host-guest interaction in MOF systems is fundamentally important. Solid-state NMR spectroscopy is an excellent technique for investigating host-guest interaction as it provides information complementary to that obtained from X-ray diffraction. In this work, using MOF α-Mg₃ (HCOO)₆ as an example, we demonstrated that ¹³ C chemical shift tensor of organic linker can be utilized to probe the host-guest interaction in MOFs. Obtaining ¹³ C chemical shift tensor components (δ₁₁ , δ₂₂ , and δ₃₃ , where δ₁₁ ≥ δ₂₂ ≥ δ₃₃ ) in this MOF is particularly challenging as there are six coordinatively equivalent but crystallographically non-equivalent carbons in the unit cell with very similar local coordination environment. Two-dimensional magic-angle-turning experiments were employed to measure the ¹³ C chemical shift tensors of each individual crystallographically non-equivalent carbon in three microporous α-Mg₃ (HCOO)₆ samples with different guest species. The results indicate that the δ₂₂ component (with its direction approximately being co-planar with the formate anion and perpendicular to the C-H bond) is more sensitive to the adsorbate molecules inside the MOF channel due to the weak C-H···O hydrogen bonding or the ring current effect of benzene. The ¹³ C isotropic chemical shift, on the other hand, seems much less sensitive to the subtle changes in the local environment around formate linker induced by adsorption. The approach described in this study may be used in future studies on host-guest interaction within MOFs.

Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn2+ doped [NH4][Zn(HCOO)3] hybrid formate framework

We present an X- and Q-band continuous wave (CW) and pulse electron paramagnetic resonance (EPR) study of a manganese doped [NH4][Zn(HCOO)3] hybrid framework, which exhibits a ferroelectric structural phase transition at 190 K. The CW EPR spectra obtained at different temperatures exhibit clear changes at the phase transition temperature. This suggests a successful substitution of the Zn2+ ions by the paramagnetic Mn2+ centers, which is further confirmed by the pulse EPR and 1H ENDOR experiments. Spectral simulations of the CW EPR spectra are used to obtain the temperature dependence of the Mn2+ zero-field splitting, which indicates a gradual deformation of the MnO6 octahedra indicating a continuous character of the transition. The determined data allow us to extract the critical exponent of the order parameter (β = 0.12), which suggests a quasi two-dimensional ordering in [NH4][Zn(HCOO)3]. The experimental EPR results are supported by the density functional theory calculations of the zero-field splitting parameters. Relaxation time measurements of the Mn2+ centers indicate that the longitudinal relaxation is mainly driven by the optical phonons, which correspond to the vibrations of the metal-oxygen octahedra. The temperature behavior of the transverse relaxation indicates a dynamic process in the ordered ferroelectric phase.

Chemical exchange at the ferroelectric phase transition of lead germanate revealed by solid state 207Pb nuclear magnetic resonance

Multi-dimensional NMR is used to quantitatively identify a mixed order–disorder and displacive mechanism for the ferroelectric phase transition of lead germanate.

Recent advances in solid-state nuclear magnetic resonance spectroscopy of exotic nuclei

We present a review of recent advances in solid-state nuclear magnetic resonance (SSNMR) studies of exotic nuclei. Exotic nuclei may be spin-1/2 or quadrupolar, and typically have low gyromagnetic ratios, low natural abundances, large quadrupole moments (when I > 1/2), or some combination of these properties, generally resulting in low receptivities and/or prohibitively broad line widths. Some nuclides are little studied for other reasons, also rendering them somewhat exotic. We first discuss some of the recent progress in pulse sequences and hardware development which continues to enable researchers to study new kinds of materials as well as previously unfeasible nuclei. This is followed by a survey of applications to a wide range of exotic nuclei (including e.g., ⁹Be, ²⁵Mg, ³³S, ³⁹K, ⁴³Ca, ^47/49Ti, ⁵³Cr, ⁵⁹Co, ⁶¹Ni, ⁶⁷Zn, ⁷³Ge, ⁷⁵As, ⁸⁷Sr, ¹¹⁵In, ¹¹⁹Sn, ^121/123Sb, ^135/137Ba, ^185/187Re, ²⁰⁹Bi), most of them quadrupolar. The scope of the review is the past ten years, i.e., 2007-2017.

A Combined 25 Mg Solid-State NMR and Ab Initio DFT Approach to Probe the Local Structural Differences in Magnesium Acetate Phases Mg(CH3 COO)2 ⋅ nH2 O (n=0, 1, 4)

Multinuclear (¹ H, ¹³ C, ²⁵ Mg) solid-state NMR data is reported for a series of magnesium acetate phases Mg(CH₃ COO)₂  ⋅ nH₂ O (n=0 (two polymorphs), 1, 4). The central focus here is ²⁵ Mg as this set of compounds provides an expanded range of local magnesium coordinations compared to what has previously been reported in the literature using NMR. These four compounds provide 10 distinct magnesium sites with varying NMR interaction parameters. One of the anhydrous crystal structures (α) has an MgO₇ site which is reported, to the best of our knowledge, for the first time. For those phases with a single crystal structure, a combination of magic angle spinning (MAS) NMR at high magnetic field (20 T) and first principles density functional theory (DFT) calculations demonstrates the value of including ²⁵ Mg in NMR crystallography approaches. For the second anhydrate phase (β), where no single crystal structure exists, the multinuclear NMR data clearly show the multiplicity of sites for the different elements, with ²⁵ Mg satellite transition (ST) MAS NMR revealing four inequivalent magnesium environments, which is new information constraining future refinement of the structure. This study highlights the sensitivity of ²⁵ Mg NMR to the local environment, an observation important for several sub-disciplines of chemistry where the structural chemistry of magnesium is likely to be crucial.

Characterization of Metal-Organic Frameworks: Unlocking the Potential of Solid-State NMR

An exciting advance in materials science is the discovery of hybrid organic-inorganic solids known as metal-organic frameworks (MOFs). Although they have numerous important applications, the local structures, specific molecular-level features, and guest behaviors underpinning desirable properties and applications are often unknown. Solid-state nuclear magnetic resonance (SSNMR) is a powerful tool for MOF characterization as it provides information complementary to that from X-ray diffraction (XRD). We describe our novel pursuits in the three primary applications of SSNMR for MOF characterization: interrogating the metal center, targeting linker molecules, and probing guests. MOFs have relatively low densities, and the incorporated metals are often quadrupolar nuclei, making SSNMR detection difficult. Recently, we examined the local structures of metal centers (i.e., ²⁵Mg, ^47/49Ti, ^63/65Cu, ⁶⁷Zn, ^69/71Ga, ⁹¹Zr, ¹¹⁵In, ^135/137Ba, ¹³⁹La, ²⁷Al) in representative MOFs by SSNMR at a high magnetic field of 21.1 T, addressing several important issues: (1) resolving chemically and crystallographically nonequivalent metal sites; (2) exploring the origin of disorder around metals; (3) refining local metal geometry; (4) probing the effects of activation and adsorption on the metal local environment; and (5) monitoring in situ phase changes in MOFs. Organic linkers can be characterized by ¹H, ¹³C, and ¹⁷O SSNMR. Although the framework structure can be determined by X-ray diffraction, hydrogen atoms cannot be accurately located, and thus the local structure about hydrogen is poorly characterized. Our work demonstrates that magic-angle spinning (MAS) experiments at very high magnetic field along with ultrafast spinning rates and isotope dilution enables one to obtain ultrahigh resolution ¹H MAS spectra of MOFs, yielding structural information truly complementary to that obtained from single-crystal XRD. Oxygen is a key constituent of many important MOFs but ¹⁷O SSNMR work on MOFs is scarce due to the low natural abundance of ¹⁷O. ¹⁷O enriched MOFs can now be prepared in an efficient and economically feasible manner using solvothermal approaches involving labeled H₂¹⁷O water; the resulting ¹⁷O SSNMR spectra provide distinct spectral signatures of various key oxygen species in representative MOFs. MOFs are suitable for the capture of the greenhouse gas CO₂ and the storage of energy carrier gases such as H₂ and CH₄. A better understanding of gas adsorption obtained using ¹³C, ²H, and ¹⁷O SSNMR will enable researchers to improve performance and realize practical applications for MOFs as gas adsorbents and carriers. The combination of SSNMR with XRD allows us to determine the number of adsorption sites in the framework, identify the location of binding sites, gain physical insight into the nature and strength of host-guest interactions, and understand guest dynamics.

Structural, thermal, dielectric and phonon properties of perovskite-like imidazolium magnesium formate

We report the synthesis and characterisation of a magnesium formate framework templated by protonated imidazole. Single-crystal X-ray diffraction data showed that this compound crystallizes in the monoclinic structure in the P21/n space group with lattice parameters a = 12.1246(4) Å, b = 12.2087(5) Å, c = 12.4991(4) Å and β = 91.39(1)°. The antiparallel arrangement of the dipole moments associated with imidazolium cations suggests the antiferroelectric character of the room-temperature phase. The studied compound undergoes a structural phase transition at 451 K associated with a halving of the c lattice parameter and the disappearance of the antiferroelectric order. The monoclinic symmetry is preserved and the new metrics are a = 12.261(7) Å, b = 12.290(4) Å, c = 6.280(4) Å, and β = 90.62(5)°. Raman and IR data are consistent with the X-ray diffraction data. They also indicate that the disorder of imidazolium cations plays a significant role in the mechanism of the phase transition. Dielectric data show that the phase transition is associated with a relaxor nature of electric ordering. We also report high-pressure Raman scattering studies of this compound that revealed the presence of two pressure-induced phase transitions near 3 and 7 GPa. The first transition is most likely associated with a rearrangement of the imidazolium cations without any significant distortion of these cations and the magnesium formate framework, whereas the second transition leads to strong distortion of both the framework and imidazolium cations. High-pressure data also show that imidazolium magnesium formate does not show any signs of amorphization up to 11.4 GPa.

Deducing CO2 motion, adsorption locations and binding strengths in a flexible metal-organic framework without open metal sites

Microporous metal-organic frameworks (MOFs) have high surface areas and porosities, and are well-suited for CO2 capture. MIL-53 features corner-sharing MO4(OH)2 (M = Al, Ga, Cr, etc.) octahedra interconnected by benzenedicarboxylate linkers that form one-dimensional rhombic tunnels, and exhibits an excellent adsorption ability for guest molecules such as CO2. Studying the behavior of adsorbed CO2 in MIL-53 via solid-state NMR (SSNMR) provides rich information on the dynamic motion of guest molecules as well as their binding strengths to the MOF host, and sheds light on the specific guest adsorption mechanisms. Variable-temperature (13)C SSNMR spectra of (13)CO2 adsorbed within various forms of MIL-53 are acquired and analyzed. CO2 undergoes a combination of two motions within MIL-53; we report the types of motion present, their rates, and rotational angles. (1)H-(13)C CP SSNMR experiments are used to examine the proximity of (1)H atoms in the MOF to (13)C atoms in CO2 guests. By replacing (1)H with (2)H in MIL-53, the location of the CO2 adsorption site in MIL-53 is experimentally confirmed by (1)H-(13)C CP SSNMR. The binding strength of CO2 within these MIL-53 MOFs follows the order MIL-53-NH2 (Al) > MIL-53-NH2 (Ga) > MIL-53 (Al) > MIL-53 (Ga).

Structural, electronic and magnetic properties of metal–organic-framework perovskites [AmH][Mn(HCOO)3]: a first-principles study

Novel multiferroic Metal–Organic-Frameworks (MOFs) [AmH][M(HCOO)₃] are investigated in structural, electronic and magnetic properties using density functional theory.

The role of solid-state nuclear magnetic resonance in crystal engineering

This Highlight article discusses the role of solid-state NMR spectroscopy in crystal engineering with the aid of several examples from the literature.