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.

1H/17O Chemical Shift Waves in Carboxyl-Bridged Hydrogen Bond Networks in Organic Solids.

We report solid-state 1H and 17O NMR results for four 17O-labeled organic compounds each containing an extensive carboxyl-bridged hydrogen bond (CBHB) network in the crystal lattice: tetrabutylammonium hydrogen di-[17O2]salicylate (1), [17O4]quinolinic acid (2), [17O4]dinicotinic acid (3), and [17O2]Gly/[17O2]Gly·HCl cocrystal (4). The 1H isotropic chemical shifts found for protons involved in different CBHB networks are between 8.2 and 20.5 ppm, which reflect very different hydrogen-bonding environments. Similarly, the 17O isotropic chemical shifts found for the carboxylate oxygen atoms in CBHB networks, spanning a large range between 166 and 341 ppm, are also remarkably sensitive to the hydrogen-bonding environments. We introduced a simple graphical representation in which 1H and 17O chemical shifts are displayed along the H and O atomic chains that form the CBHB network. In such a depiction, because wavy patterns are often observed, we refer to these wavy patterns as 1H/17O chemical shift waves. Typical patterns of 1H/17O chemical shift waves in CBHB networks are discussed. The reported 1H and 17O NMR parameters for the CBHB network models examined in this study can serve as benchmarks to aid in spectral interpretation for CBHB networks in proteins.

Experimental Evidence for Non-Fermi-Contact J Coupling Across Chalcogen Bonds in Ionic Salt Cocrystal Polymorphs.

A pair of novel polymorphic ionic cocrystals of 3,4-dicyanotelluradiazole and tetraphenylphosphonium bromide are synthesized and are characterized by single-crystal XRD. Strong and directional non-covalent chalcogen bonds (ChB) between Te and Br are analyzed via solid-state NMR to reveal large and anisotropic J(125Te,79/81Br) coupling tensors, providing unequivocal evidence for non-Fermi contact contributions across ChBs. Along with large 79/81Br quadrupolar couplings for the Br- anions, these data provide new tools to characterize chalcogen bonds and to differentiate between ChB polymorphs.

Cold, Hot, Dry, and Wet: Locations and Dynamics of CO2 and H2O Co-Adsorbed in an Ultramicroporous MOF.

Climate change from anthropogenic carbon dioxide (CO2) emissions poses a severe threat to society. A variety of mitigation strategies currently include some form of CO2 capture. Metal-organic frameworks (MOFs) have shown great promise for carbon capture and storage, but several issues must be solved before feasible widespread adoption is possible. MOFs often exhibit reduced chemical stabilities and CO2 adsorption capacities in the presence of water, which is ubiquitous in nature and many practical settings. A comprehensive understanding of water influence on CO2 adsorption in MOFs is necessary. We have used multinuclear nuclear magnetic resonance (NMR) experiments at temperatures ranging from 173 to 373 K, along with complementary computational techniques, to investigate the co-adsorption of CO2 and water across various loading levels in the ultra-microporous ZnAtzOx MOF. This approach yields detailed information regarding the number of CO2 and water adsorption sites along with their locations, guest dynamics, and host-guest interactions. Guest adsorption and motional models proposed from NMR data are supported by computational results, including visualizations of adsorption locations and the spatial distribution of guests in different loading scenarios. The wide variety and depth of information presented demonstrates how this experimental methodology can be used to investigate humid carbon capture and storage applications in other MOFs.

Overcoming challenges in 67Zn NMR: a new strategy of signal enhancement for MOF characterization.

67 Zn solid-state NMR suffers from low sensitivity, limiting its ability to probe the Zn2+ surroundings in MOFs. We report a breakthrough in overcoming challenges in 67Zn NMR. Combining new cryogenic MAS probe technology and performing NMR experiments at a high magnetic field results in remarkable signal enhancement, yielding enhanced information for MOF characterization.

Multinuclear solid-state NMR: Unveiling the local structure of defective MOF MIL-120.

Metal-organic frameworks (MOFs) are emerging materials with many current and potential applications due to their unique properties. One critical feature is that the physical and chemical properties of MOFs are tunable. One of the methods for tuning MOF properties is to introduce defects by design for desired applications. Characterization of MOF defects is important, but very challenging due to the local nature and short-range ordering. In this work, we have introduced the ordered vacancies (the defects) in the form of the coordinatively unsaturated sites (CUSs) into the framework of MOF MIL-120(Al). The creation of ordered vacancies is achieved by replacing one quarter of the BTEC (1,2,4,5-benzenetetracarboxylate) with BDC (benzene-1,4-dicarboxylate) linkers. Both parent and defective MOFs were characterized by multinuclear solid-state NMR spectroscopy. 1H MAS NMR is used to characterize the hydrogen bonding in these MOFs, whereas 13C CP MAS NMR confirms unambiguously that the BDC is incorporated into the framework. One-dimensional 27Al MAS NMR provides direct evidence of the coordinatively unsaturated Al sites (the defects). Furthermore, 27Al 3QMAS experiments at 21.1 â€‹T allow direct identification of one penta-coordinated and three chemically inequivalent octahedral Al sites in the defective MIL-120(Al). Two of the above-mentioned octahedral Al sites are in the domain which appears defect-free. The third octahedral Al site is near the defective site. This work clearly demonstrates the power of solid-state NMR spectroscopy for characterization of defective MOFs.

Solid-state 17O NMR study of α-d-glucose: exploring new frontiers in isotopic labeling, sensitivity enhancement, and NMR crystallography.

We report synthesis and solid-state 17O NMR characterization of α-d-glucose for which all six oxygen atoms are site-specifically 17O-labeled. Solid-state 17O NMR spectra were recorded for α-d-glucose/NaCl/H2O (2/1/1) cocrystals under static and magic-angle-spinning (MAS) conditions at five moderate, high, and ultrahigh magnetic fields: 14.1, 16.4, 18.8, 21.1, and 35.2 T. Complete 17O chemical shift (CS) and quadrupolar coupling (QC) tensors were determined for each of the six oxygen-containing functional groups in α-d-glucose. Paramagnetic Cu(ii) doping was found to significantly shorten the spin-lattice relaxation times for both 1H and 17O nuclei in these compounds. A combination of the paramagnetic Cu(ii) doping, new CPMAS CryoProbe technology, and apodization weighted sampling led to a sensitivity boost for solid-state 17O NMR by a factor of 6-8, which made it possible to acquire high-quality 2D 17O multiple-quantum (MQ) MAS spectra for carbohydrate compounds. The unprecedented spectral resolution offered by 2D 17O MQMAS spectra permitted detection of a key structural difference for a single hydrogen bond between two types of crystallographically distinct α-d-glucose molecules. This work represents the first case where all oxygen-containing functional groups in a carbohydrate molecule are site-specifically 17O-labeled and fully characterized by solid-state 17O NMR. Gauge Including Projector Augmented Waves (GIPAW) DFT calculations were performed to aid 17O and 13C NMR signal assignments for a complex crystal structure where there are six crystallographically distinct α-d-glucose molecules in the asymmetric unit.

Exploring Structural Nuances in Germanium Halide Perovskites Using Solid-State 73Ge and 133Cs NMR Spectroscopy.

Metal halide perovskites remain top candidates for higher-performance photovoltaic devices, but concerns about leading lead-based materials remain. Ge perovskites remain understudied for use in solar cells compared to their Sn-based counterparts. In this work, we undertake a combined 73Ge and 133Cs solid-state Nuclear Magnetic Resonance (NMR) spectroscopy and density functional theory (DFT) study of the bulk CsGeX3 (X = Cl, Br, or I) series. We show how seemingly small structural variations within germanium halide perovskites have major effects on their 73Ge and 133Cs NMR signatures and reveal a near-cubic phase at room temperature for CsGeCl3 with severe local Ge polyhedral distortion. Quantum chemical computations are effective at predicting the structural impact on NMR parameters for 73Ge and 133Cs. This study demonstrates the value of a combined solid-state NMR and DFT approach for investigating promising materials for energy applications, providing information that is out of reach with conventional characterization methods, and adds the challenging 73Ge nucleus to the NMR toolkit.

Perspectives of fast magic-angle spinning 87 Rb NMR of organic solids at high magnetic fields.

We report solid-state 87 Rb NMR spectra from two Rb-ionophore complexes obtained with fast magic-angle spinning (MAS) (up to 60 kHz) at 21.1 T. These Rb-ionophore complexes containing macrocycles such as benzo-15-crown-5 and cryptand [2.2.2] are typical of organic Rb salts that exhibit very large 87 Rb quadrupole coupling constants (close to 20 MHz). We have also obtained static 87 Rb NMR spectra for these two compounds and determined both 87 Rb quadrupole coupling and chemical shift tensors. The experimental 87 Rb NMR tensor parameters are compared with those obtained by quantum chemical computations. Our results demonstrate that the combination of fast MAS (60 kHz or higher) and a high magnetic field (21.1 T or higher) is sufficient to produce high-quality solid-state 87 Rb NMR spectra for organic Rb solids at the natural abundance level. We anticipate that, with additional 87 Rb isotope enrichment (up to 99%), the sensitivity of solid-state 87 Rb NMR will be 400 times higher than 39 K NMR, which makes the former an attractive surrogate probe for studying K+ ion binding in biological systems.

Chlorine-35 Solid-State Nuclear Magnetic Resonance Spectroscopy as an Indirect Probe of the Oxidation Number of Tin in Tin Chlorides.

Ultrawideline 35Cl solid-state nuclear magnetic resonance (SSNMR) spectra of a series of 12 tin chlorides were recorded. The magnitude of the 35Cl quadrupolar coupling constant (CQ) was shown to consistently indicate the chemical state (oxidation number) of the bound Sn center. The chemical state of the Sn center was independently verified by tin Mössbauer spectroscopy. CQ(35Cl) values of >30 MHz correspond to Sn(IV), while CQ(35Cl) readings of <30 MHz indicate that Sn(II) is present. Tin-119 SSNMR experiments would seem to be the most direct and effective route to interrogating tin in these systems, yet we show that ambiguous results can emerge from this method, which may lead to an incorrect interpretation of the Sn oxidation number. The accumulated 35Cl NMR data are used as a guide to assign the Sn oxidation number in the mixed-valent metal complex Ph3PPdImSnCl2. The synthesis and crystal structure of the related Ph3PPtImSnCl2 are reported, and 195Pt and 35Cl SSNMR experiments were also used to investigate its Pt-Sn bonding. Plane-wave DFT calculations of 35Cl, 119Sn, and 195Pt NMR parameters are used to model and interpret experimental data, supported by computed 119Sn and 195Pt chemical shift tensor orientations. Given the ubiquity of directly bound Cl centers in organometallic and inorganic systems, there is tremendous potential for widespread usage of 35Cl SSNMR parameters to provide a reliable indication of the chemical state in metal chlorides.

Ultramicropore Engineering by Dehydration to Enable Molecular Sieving of H2 by Calcium Trimesate.

The high energy footprint of commodity gas purification and increasing demand for gases require new approaches to gas separation. Kinetic separation of gas mixtures through molecular sieving can enable separation by molecular size or shape exclusion. Physisorbents must exhibit the right pore diameter to enable separation, but the 0.3-0.4 nm range relevant to small gas molecules is hard to control. Herein, dehydration of the ultramicroporous metal-organic framework Ca-trimesate, Ca(HBTC)⋅H2 O (H3 BTC=trimesic acid), bnn-1-Ca-H2 O, affords a narrow pore variant, Ca(HBTC), bnn-1-Ca. Whereas bnn-1-Ca-H2 O (pore diameter 0.34 nm) exhibits ultra-high CO2 /N2 , CO2 /CH4 , and C2 H2 /C2 H4 binary selectivity, bnn-1-Ca (pore diameter 0.31 nm) offers ideal selectivity for H2 /CO2 and H2 /N2 under cryogenic conditions. Ca-trimesate, the first physisorbent to exhibit H2 sieving under cryogenic conditions, could be a prototype for a general approach to exert precise control over pore diameter in physisorbents.

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.

Solid-State 17O NMR Study of Carboxylic Acid Dimers: Simultaneously Accessing Spectral Properties of Low- and High-Energy Tautomers.

We report a solid-state 17O NMR study of several crystalline carboxylic acids. We found that, while each of these compounds forms discrete hydrogen-bonded dimers in the crystal lattice, their solid-state 17O magic-angle spinning (MAS) NMR spectra display quite different features and different temperature dependencies. We showed that experimentally observed 17O NMR spectral behaviors can be explained as being due to thermal averaging between the two tautomers that are produced as a result of concerted double-hydrogen hopping dynamics within each dimer. In general, the two tautomers have different energies due to intramolecular interactions and crystal packing. From an analysis of variable-temperature 17O MAS NMR spectra, energy asymmetry between the two tautomers was experimentally determined for each of the carboxylic acid compounds studied. The same data analysis also offers an opportunity to simultaneously assess 17O NMR parameters in both low- and high-energy tautomers. We concluded that the periodic plane-wave density functional theory (DFT) calculations can produce reliable 17O NMR parameters (chemical shift and quadrupolar coupling tensors) for both tautomers. The same periodic DFT calculations have also produced reasonable energy asymmetry values for the studied carboxylic acid dimers. We have also observed substantial H/D isotope shifts in solid-state 17O NMR.

Cleaving Carboxyls: Understanding Thermally Triggered Hierarchical Pores in the Metal-Organic Framework MIL-121.

Carboxylic acid linker ligands are known to form strong metal-carboxylate bonds to afford many different variations of permanently microporous metal-organic frameworks (MOFs). A controlled approach to decarboxylation of the ligands in carboxylate-based MOFs could result in structural modifications, offering scope to improve existing properties or to unlock entirely new properties. In this work, we demonstrate that the microporous MOF MIL-121 is transformed to a hierarchically porous MOF via thermally triggered decarboxylation of its linker. Decarboxylation and the introduction of hierarchical porosity increases the surface area of this material from 13 to 908 m2/g and enhances gas adsorption uptake for industrially relevant gases (i.e., CO2, C2H2, C2H4, and CH4). For example, CO2 uptake in hierarchically porous MIL-121 is improved 8.5 times over MIL-121, reaching 215.7 cm3/g at 195 K and 1 bar; CH4 uptake is 132.3 cm3/g at 298 K and 80 bar in hierarchically porous MIL-121 versus zero in unmodified MIL-121. The approach taken was validated using a related aluminum-based MOF, ISOMIL-53. However, many specifics of the decarboxylation procedure in MOFs have yet to be unraveled and demand prompt examination. Decarboxylation, the formation of heterogeneous hierarchical pores, gas uptakes, and host-guest interactions are comprehensively investigated using variable-temperature multinuclear solid-state NMR spectroscopy, X-ray diffraction, electron microscopy, and gas adsorption; we propose a mechanism for how decarboxylation proceeds and which local structural features are involved. Understanding the complex relationship among the molecular-level MOF structure, thermal stability, and the decarboxylation process is essential to fine-tune MOF porosity, thus offering a systematic approach to the design of hierarchically porous, custom-built MOFs suited for targeted applications.

A Quadrupole-Central-Transition 59 Co NMR Study of Cobalamins in Solution.

We report the first observation of quadrupole-central-transition (QCT) 59 Co (I=7/2) NMR signals from three cobalamin (Cbl) compounds (CNCbl, MeCbl, and AdoCbl) dissolved in glycerol/water. Measurements were performed at four magnetic fields ranging from 11.7 to 21.1 T. We found that the 59 Co QCT signals observed for cobalamin compounds in the slow motion regime (ω0 τC ≫1) are significantly narrower than those observed from their aqueous solutions where the molecular tumbling is near the condition of ω0 τC ≈1. We demonstrated that an analysis of 59 Co QCT signals recorded over different temperatures and at multiple magnetic fields allowed determination of both the 59 Co quadrupole coupling constant and chemical shift anisotropy for each of the three cobalamins. We successfully applied the 59 Co QCT NMR approach to monitor in situ the transformation of CNCbl to its "base off" form in the presence of KCN. We further discovered that, to obtain the maximum QCT signal intensity with the Hahn-echo sequence, a strong B1 field should be used for the first 90°â€…pulse, but a weak B1 field for the second 180°â€…pulse. The reported 59 Co QCT NMR methodology opens up a new direction for studying structure and function of cobalamin compounds and their roles in biological processes.

Pyrolysis of the Cellulose Fraction of Biomass in the Presence of Solid Acid Catalysts: An Operando Spectroscopy and Theoretical Investigation.

Biomass pyrolysis by solid acid catalysts is one of many promising technologies for sustainable production of hydrocarbon liquid fuels and value-added chemicals, but these complex chemical transformations are still poorly understood. A series of well-defined model SiO2 -supported alumina catalysts were synthesized and molecularly characterized, under dehydrated conditions and during biomass pyrolysis, with the aim of establishing fundamental catalyst structure-activity/selectivity relationships. The nature and corresponding acidity of the supported AlOx nanostructures on SiO2 were determined with 27 Al/1 H NMR and IR spectroscopy of chemisorbed CO, and DFT calculations. Operando time-resolved IR-Raman-MS spectroscopy studies revealed the molecular transformations taking place during biomass pyrolysis. The molecular transformations during biomass pyrolysis depended on both the domain size of the AlOx cluster and molecular nature of the biomass feedstock. These new insights allowed the establishment of fundamental structure-activity/selectivity relationships during biomass pyrolysis.

Loading across the Periodic Table: Introducing 14 Different Metal Ions To Enhance Metal-Organic Framework Performance.

Loading metal guests within metal-organic frameworks (MOFs) via secondary functional groups is a promising route for introducing or enhancing MOF performance in various applications. In this work, 14 metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Zn2+, Co2+, Mn2+, Ag+, Cd2+, La3+, In3+, and Pb2+) have been successfully introduced within the MIL-121 MOF using a cost-efficient route involving free carboxylic groups on the linker. The local and long-range structure of the metal-loaded MOFs is characterized using multinuclear solid-state NMR and X-ray diffraction methods. Li/Mg/Ca-loaded MIL-121 and Ag nanoparticle-loaded MIL-121 exhibit enhanced H2 and CO2 adsorption; Ag nanoparticle-loaded MIL-121 also demonstrates remarkable catalytic activity in the reduction of 4-nitrophenol.

Welcoming Gallium- and Indium-Fumarate MOFs to the Family: Synthesis, Comprehensive Characterization, Observation of Porous Hydrophobicity, and CO2 Dynamics.

The properties and applications of metal-organic frameworks (MOFs) are strongly dependent on the nature of the metals and linkers, along with the specific conditions employed during synthesis. Al-fumarate, trademarked as Basolite A520, is a porous MOF that incorporates aluminum centers along with fumarate linkers and is a promising material for applications involving adsorption of gases such as CO2. In this work, the solvothermal synthesis and detailed characterization of the gallium- and indium-fumarate MOFs (Ga-fumarate, In-fumarate) are described. Using a combination of powder X-ray diffraction, Rietveld refinements, solid-state NMR spectroscopy, IR spectroscopy, and thermogravimetric analysis, the topologies of Ga-fumarate and In-fumarate are revealed to be analogous to Al-fumarate. Ultra-wideline 69Ga, 71Ga, and 115In NMR experiments at 21.1 T strongly support our refined structure. Adsorption isotherms show that the Al-, Ga-, and In-fumarate MOFs all exhibit an affinity for CO2, with Al-fumarate being the superior adsorbent at 1 bar and 273 K. Static direct excitation and cross-polarized 13C NMR experiments permit investigation of CO2 adsorption locations, binding strengths, motional rates, and motional angles that are critical to increasing adsorption capacity and selectivity in these materials. Conducting the synthesis of the indium-based framework in methanol demonstrates a simple route to introduce porous hydrophobicity into a MIL-53-type framework by incorporation of metal-bridging -OCH3 groups in the MOF pores.

Probing Calcium-Based Metal-Organic Frameworks via Natural Abundance 43 Ca Solid-State NMR Spectroscopy.

Calcium-based metal-organic frameworks (MOFs) are of high importance due to their low cost and bio-compatible metal centers. Understanding the local environment of calcium in these materials is critical for unraveling the origins of specific MOF properties. 43 Ca solid-state NMR spectroscopy is one of the very few techniques that can directly characterize calcium metal centers, however, the 43 Ca nucleus is a very challenging target for solid-state NMR spectroscopy due to its extremely low natural abundance and resonant frequency. In this work, natural abundance 43 Ca solid-state NMR spectroscopy, at a high magnetic field of 21.1 T, has been employed to characterize several calcium-based MOFs. We demonstrate that 43 Ca NMR spectra and quantum chemical calculations can probe the local structure of calcium metal centers within MOFs, investigate the presence of guests, and monitor phase changes.