NMR Crystallography: A tool for the characterization of microporous hybrid solids

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.

Computational NMR investigation of mixed-metal (Al,Sc)-MIL-53 and its phase transitions

Compositionally complex metal-organic frameworks (MOFs) have properties that depend on local structure that is often difficult to characterise. In this paper a density functional theory (DFT) computational study of mixed-metal (Al,Sc)-MIL-53, a flexible MOF with several different forms, was used to calculate the relative energetics of these forms and to predict NMR parameters that can be used to evaluate whether solid-state NMR spectroscopy can be used to differentiate, identify and characterise the forms adopted by mixed-metal MOFs of different composition. The NMR parameters can also be correlated with structural features in the different forms, giving fundamental insight into the nature and origin of the interactions that affect nuclear spins. Given the complexity of advanced NMR experiments required, and the potential need for expensive and difficult isotopic enrichment, the computational work is invaluable in predicting which experiments and approaches are likely to give the most information on the disorder, local structure and pore forms of these mixed-metal MOFs.

Complete resonance assignment of a pharmaceutical drug at natural isotopic abundance from DNP-Enhanced solid-state NMR

Solid-state dynamic nuclear polarization enhanced magic angle spinning (DNP-MAS) NMR measurements coupled with density functional theory (DFT) calculations enable the full resonance assignment of a complex pharmaceutical drug molecule without the need for isotopic enrichment. DNP dramatically enhances the NMR signals, thereby making possible previously intractable two-dimensional correlation NMR spectra at natural abundance. Using inputs from DFT calculations, herein we describe a significant improvement to the structure elucidation process for complex organic molecules. Further, we demonstrate that a series of two-dimensional correlation experiments, including ¹⁵N-¹³C TEDOR, ¹³C-¹³C INADEQUATE/SARCOSY, ¹⁹F-¹³C HETCOR, and ¹H-¹³C HETCOR, can be obtained at natural isotopic abundance within reasonable experiment times, thus enabling a complete resonance assignment of sitagliptin, a pharmaceutical used for the treatment of type 2 diabetes.

Monitoring Dynamics, Structure, and Magnetism of Switchable Metal-Organic Frameworks via 1 H-Detected MAS NMR

We present a toolbox for the rapid characterisation of powdered samples of paramagnetic metal-organic frameworks at natural abundance by 1 H-detected solid-state NMR. Very fast MAS rates at room and cryogenic temperatures and a set of tailored radiofrequency irradiation schemes help overcome the sensitivity and resolution limits often associated with the characterisation of MOF materials. We demonstrate the approach on DUT-8(Ni), a framework containing Ni2+ paddle-wheel units which can exist in two markedly different architectures. Resolved 1 H and 13 C resonances of organic linkers are detected and assigned in few hours with only 1-2 mg of sample at natural isotopic abundance, and used to rapidly extract information on structure and local internal dynamics of the assemblies, as well as to elucidate the metal electronic properties over an extended temperature range. The experiments disclose new possibilities for describing local and global structural changes and correlating them to electronic and magnetic properties of the assemblies.

Doxorubicin-Loaded Metal-Organic Frameworks Nanoparticles with Engineered Cyclodextrin Coatings: Insights on Drug Location by Solid State NMR Spectroscopy

Recently developed, nanoscale metal-organic frameworks (nanoMOFs) functionalized with versatile coatings are drawing special attention in the nanomedicine field. Here we show the preparation of core-shell MIL-100(Al) nanoMOFs for the delivery of the anticancer drug doxorubicin (DOX). DOX was efficiently incorporated in the MOFs and was released in a progressive manner, depending on the initial loading. Besides, the coatings were made of biodegradable γ-cyclodextrin-citrate oligomers (CD-CO) with affinity for both DOX and the MOF cores. DOX was incorporated and released faster due to its affinity for the coating material. A set of complementary solid state nuclear magnetic resonance (ssNMR) experiments including 1H-1H and 13C-27Al two-dimensional NMR, was used to gain a deep understanding on the multiple interactions involved in the MIL-100(Al) core-shell system. To do so, 13C-labelled shells were synthesized. This study paves the way towards a methodology to assess the nanoMOF component localization at a molecular scale and to investigate the nanoMOF physicochemical properties, which play a main role on their biological applications.

Application of NMR Crystallography to Highly Disordered Templated Materials: Extensive Local Structural Disorder in the Gallophosphate GaPO-34A

We present an NMR crystallographic investigation of two as-made forms of the recently characterized gallophosphate GaPO-34A, which has an unusual framework composition with a Ga:P ratio of 7:6 and contains both hydroxide and fluoride anions and either 1-methylimidazolium or pyridinium as the structure-directing agent. We combine previously reported X-ray crystallographic data with solid-state NMR spectroscopy and periodic density functional theory (DFT) calculations to show that the structure contains at least three distinct types of disorder (occupational, compositional, and dynamic). The occupational disorder arises from the presence of six anion sites per unit cell, but a total occupancy of five of these, leading to full occupancy of four sites and partial occupancy of the fifth and sixth (which are related by symmetry). The mixture of OH and F present leads to compositional disorder on the occupied anion sites, although the occupancy of some sites by F is calculated to be energetically unfavorable and signals relating to F on these sites are not observed by NMR spectroscopy, confirming that the compositional disorder is not random. Finally, a combination of high-field ⁷¹Ga NMR spectroscopy and variable-temperature ¹³C and ³¹P NMR experiments shows that the structure directing agents are dynamic on the microsecond time scale, which can be supported by averaging the ³¹P chemical shifts calculated with the SDA in different orientations. This demonstrates the value of an NMR crystallographic approach, particularly in the case of highly disordered crystalline materials, where the growth of large single crystals for conventional structure determination may not be possible owing to the extent of disorder present.

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.

New insights on the supramolecular structure of highly porous core-shell drug nanocarriers using solid-state NMR spectroscopy

Nano-sized metal-organic frameworks (nanoMOFs), with engineered surfaces to enhance the targeting of the drug delivery, have proven efficient as drug nanocarriers. To improve their performances a step further, it is essential to understand at the molecular level the interactions between the nanoMOF interfaces and both the surface covering groups and the drug loaded inside the micropores. Here we show how solid-state NMR spectroscopy allows us to address these issues in an aluminum-based nanoMOF coated and loaded with phosphorus-containing species.

Solid-state NMR tools for the structural characterization of POSiSils: 29 Si sensitivity improvement with MC-CP and 2D 29 Si-29 Si DQ-SQ at natural abundance

The ¹ H-²⁹ Si multiple-contact cross polarization (MC-CP) MAS NMR experiment is evaluated for the class of silicate-siloxane copolymers called POSiSils, that is, polyoligosiloxysilicones. It proves a reasonably good solution to tackle the challenge of recording quantitative ²⁹ Si NMR data in experimental time much reduced compared with single pulse acquisition. In a second time, we report ²⁹ Si-²⁹ Si MC-CP double-quantum single-quantum (MC-CP-DQ-SQ) NMR experiment, which provides information about the through-space proximities between all silicon species despite the high degree of heterogeneity of this material. This work furthers the NMR tools for NMR crystallography for inorganic polymers, as it covers flexible polymers with different dimensionalities and long or heterogeneous relaxation characteristics at low ²⁹ Si natural abundance.

Enhanced 1H-X D-HMQC performance through improved 1H homonuclear decoupling

The sensitivity of solid-state NMR experiments that utilize ¹H zero-quantum heteronuclear dipolar recoupling, such as D-HMQC, is compromised by poor homonuclear decoupling. This leads to a rapid decay of recoupled magnetization and an inefficient recoupling of long-range dipolar interactions, especially for nuclides with low gyromagnetic ratios. We investigated the use, in symmetry-based ¹H heteronuclear recoupling sequences, of a basic R element that was principally designed for efficient homonuclear decoupling. By shortening the time required to suppress the effects of homonuclear dipolar interactions to the duration of a single inversion pulse, spin diffusion was effectively quenched and long-lived recoupled coherence lifetimes could be obtained. We show, both theoretically and experimentally, that these modified sequences can yield considerable sensitivity improvements over the current state-of-the-art methods and applied them to the indirect detection of ⁸⁹Y in a metal-organic framework.

Overview of Current and Future Perspectives of Saudi Arabian Natural Clinoptilolite Zeolite: A Case Review

After a thorough review of existing studies of clinoptilolite zeolites, three areas for potential investigation of the Saudi Arabian zeolites were found. They are the characterizations, the catalytic activity, active sites, and uses of natural clinoptilolite zeolites. First, no analysis is available worldwide to compare the percentage weight of local zeolites with those sourced from other countries, nor does one exist for the establishment on the zeolite conversion of MBOH with water on acidic catalysts at lower temperatures. Secondly, a review of current literature on the topic revealed that basic and active sites of Saudi Arabian zeolites have yet to be examined. Future investigation of zeolite catalytic activity can be achieved by methyl butynol test reaction (MBOH) and absorption-desorption of ammonia. In the characterization of a range of international materials, the methyl butynol test reaction was utilized, including on natural zeolites, natural clays, and synthesized hydrotalcites. However, the catalytic performance of natural Saudi Arabian clinoptilolite zeolites by test reaction of MBOH conversion has not been yet investigated. Therefore, this article also includes an outline of the general testing conditions and parameters required to execute the accurate characterization of local Saudi clinoptilolite under optimal test conditions. Likewise, knowledge of the important active acidic centers of local materials is prescribed. This can be ascertained by determining the conditions together with the test parameters for the application of the “temperature-programmed desorption of ammonia” method in order to obtain an accurate determination of local Saudi clinoptilolite acidic centers. Additionally, an outline of the catalytic activity of worldwide clinoptilolite is given in this article together with kinetic investigations of other sources for the clinoptilolite zeolite in order to form the basis for the testing of local Saudi clinoptilolite. The percentage average of chemical composition (Wt.%) of natural clinoptilolite from various countries is also included. Finally, a future research plan is proposed here. This will form the basis for a complete study or survey to be compiled detailing the modifications needed to increase the surface areas for Saudi natural clinoptilolite zeolites using different methods of modifications. This could enhance its application as acid catalysts for use in the retardation of coke formation and for membrane separation on cationic exchange.

Solid-State NMR Spectroscopy: A Powerful Technique to Directly Study Small Gas Molecules Adsorbed in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have shown great potential in gas separation and storage, and the design of MOFs for these purposes is an on-going field of research. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is a valuable technique for characterizing these functional materials. It can provide a wide range of structural and motional insights that are complementary to and/or difficult to access with alternative methods. In this Concept article, the recent advances made in SSNMR investigations of small gas molecules (i.e., carbon dioxide, carbon monoxide, hydrogen gas and light hydrocarbons) adsorbed in MOFs are discussed. These studies demonstrate the breadth of information that can be obtained by SSNMR spectroscopy, such as the number and location of guest adsorption sites, host-guest binding strengths and guest mobility. The knowledge acquired from these experiments yields a powerful tool for progress in MOF development.