Dynamics of Linkers in Metal-Organic Framework Glasses.

Metal-organic framework (MOF) glasses have emerged as a new class of organic-inorganic hybrid glass materials. Considerable efforts have been devoted to unraveling the macroscopic dynamics of MOF glasses by studying their rheological behavior; however, their microscopic dynamics remain unclear. In this work, we studied the effect of vitrification on linker dynamics in ZIF-62 by solid-state 2H nuclear magnetic resonance (NMR) spectroscopy. 2H NMR relaxation analysis provided a detailed picture of the mobility of the ZIF-62 linkers, including local restricted librations and a large-amplitude twist; these details were verified by molecular dynamics. A comparison of ZIF-62 crystals and glasses revealed that vitrification does not drastically affect the fast individual flipping motions with large-amplitude twists, whereas it facilitates slow cooperative large-amplitude twist motions with a decrease in the activation barrier. These observations support the findings of previous studies, indicating that glassy ZIF-62 retains permanent porosity and that short-range disorder exists in the alignment of ligands because of distortion of the coordination angle.

Tuning Proton Conduction by Staggered Arrays of Polar Preyssler-Type Oxoclusters.

Polyoxometalates (POMs), anionic nanosized oxoclusters that can be considered as fragments of metal oxides, have been extensively studied for their diverse composition and structure, showing promise in various fields such as catalysis and electronics. Proton conduction, relevant to catalysis and electronics, has attracted interest in materials chemistry, and POM anions are advantageous in terms of their proton carrier density and mobility. Recently, polar POMs have attracted attention for their unique ferroelectric behaviors, yet they have been little studied with regard to proton conduction, as their polarity has generally been believed to have a negative impact. Here, we propose that polar POMs can be used to align polar proton carriers, such as H2O and polymers, to construct efficient proton-conducting pathways. In this study, we present ionic crystals composed of polar Preyssler-type POMs ([Xn+(H2O)P5W30O110](15-n)-, Xn+ = Ca2+, Eu3+) and K+ exhibiting ultrahigh proton conductivity surpassing 10-2 S cm-1, which is required for practical applications. In contrast, ionic crystals with nonpolar Preyssler-type POMs show an order of magnitude lower proton conductivity. Structural and spectroscopic studies combined with theoretical calculations reveal that proton carriers align with the aid of staggered arrays of polar POMs, forming a hydrogen-bonding network favorable for proton conduction. This study integrates molecular chemistry by the design of POMs and solid-state chemistry by exploring long-range proton conduction mechanisms, offering novel insights for future materials design.

Reduction-Induced Uptake of Cs+ in Metal-Organic Frameworks Loaded with Polyoxometalates.

Materials for Cs+ adsorption continue to be important for the treatment of various solutions. Metal-organic frameworks (MOFs) with large specific surface areas promise adsorption properties for various gases, vapors, and ions. However, the utilization of MOFs for alkali ion capture, specifically, Cs+ capture is still in its infancy. Herein, MOFs are hybridized with polyoxometalates (POMs) to study the effect of i) MOF type, ii) POM type, and iii) POM loading amounts on Cs+ capture. In particular, the composite of ZIF-8 and [α-PMo12O40]3- (PMo12/ZIF-8) adsorbed Cs+ ions effectively when compared to pristine ZIF-8. In addition, the reduction of Mo within the POM from MoVI to MoV by ascorbic acid during the Cs+ uptake process doubled the Cs+ uptake capacity of PMo12/ZIF-8. This observation can be attributed to the increased overall negative charge of the POM facilitating Cs+ uptake to compensate for the charge imbalance. Hybridization with other MOFs (MIL-101 and UiO-66) largely suppresses the Cs+ uptake, highlighting the importance of hydrophobicity in Cs+ capture. Furthermore, PMo12/ZIF-8 led to an outstanding Cs+ uptake (291.5 mg g-1) with high selectivity (79.6%) from quinary mixtures of alkali metal cations even among other representative porous materials (Prussian blue and zeolites).

Size-Controlled Synthesis of Luminescent Few-Atom Silver Clusters via Electron Transfer in Isostructural Redox-Active Porous Ionic Crystals.

Ag clusters with a controlled number of atoms have received significant interest because they show size-dependent catalytic, optical, electronic, or magnetic properties. However, the synthesis of size-controlled, ligand-free, and air-stable Ag clusters with high yields has not been well-established. Herein, it is shown that isostructural porous ionic crystals (PICs) with redox-active polyoxometalates (POMs) can be used to synthesize Ag clusters via electron transfer from POMs to Ag+ . Ag clusters with average numbers of three, four, or six atoms emitting blue, green, or red colors, respectively, are formed and stabilized in the PICs under ambient conditions without any protecting ligands. The cluster size solely correlates with the degree of electron transfer, which is controlled by the reduction time and types of ions or elements of the PICs. Thus, advantages have been taken of POMs as electron sources and PICs as scaffolds to demonstrate a convenient method to obtain few-atom Ag clusters.

Oxygen Evolution Reaction Driven by Charge Transfer from a Cr Complex to Co-Containing Polyoxometalate in a Porous Ionic Crystal.

Considerable efforts have been devoted to developing oxygen evolution reaction (OER) catalysts based on transition metal oxides. Polyoxometalates (POMs) can be regarded as model compounds of transition metal oxides, and cobalt-containing POMs (Co-POMs) have received significant interest as candidates. Nanocomposites based on Co-POMs have been reported to show high OER activities due to synergistic effects among the components; however, the role of each component is unclear due to its complex structure. Herein, we utilize porous ionic crystals (PICs) based on Co-POMs, which enable a composition-structure-function relationship to be established to understand the origin of the synergistic catalysis. Specifically, a Keggin-type POM [α-CoW12O40]6- and a Cr complex [Cr3O(OOCCH2CN)6(H2O)3]+ are implemented as PIC building blocks for the OER under nonbasic conditions. The potentially OER-active but highly soluble [α-CoW12O40]6- was successfully anchored in the crystalline PIC matrix via Coulomb interactions and hydrogen bonding induced by polar cyano groups of the Cr complex. The PIC exhibits efficient and sustained OER catalytic activity, while each building block is inactive. The Tafel slope of the linear sweep voltammetry curve and the relatively large kinetic isotope effect value suggest that elementary steps closely related to the OER rate involve single-electron and proton transfer reactions. Electrochemical and spectroscopic studies clearly show that the synergistic catalysis originates from the charge transfer from the Cr complex to [α-CoW12O40]6-; the increased electron density of [α-CoW12O40]6- may increase its basicity and accelerate proton abstraction as well as enhance electron transfer to stabilize the reaction intermediates adsorbed on [α-CoW12O40]6-.

Ligand-Functionalization-Controlled Activity of Metal-Organic Framework-Encapsulated Pt Nanocatalyst toward Activation of Water.

We first report the systematic control of the reactivity of H2O vapor in metal-organic frameworks (MOFs) with Pt nanocrystals (NCs) through ligand functionalization. We successfully synthesized Pt NCs covered with a water-stable MOF, UiO-66 (Pt@UiO-66), having different metal ions or functionalized ligands. The ligand functionalization of UiO-66 significantly affected the catalytic performance of the water-gas shift reaction, and the replacement of Zr4+ ions with Hf4+ ions in UiO-66 had no impact on the catalytic activity. The introduction of a -Br group lowered the reactivity of Pt@UiO-66 by nearly half, whereas the substitution of -Br with a -Me2 group triply enhanced the activity. The origin of the enhanced catalytic activity was found to be the change in H2O activity in the UiO-66 pores by the ligand functionalization, which was investigated using H2O sorption, solid-state NMR, X-ray photoelectron spectroscopy, and in situ IR measurements. This work opens a new prospect to develop MOFs as a platform to activate H2O.

Fabrication of Integrated Copper-Based Nanoparticles/Amorphous Metal-Organic Framework by a Facile Spray-Drying Method: Highly Enhanced CO2 Hydrogenation Activity for Methanol Synthesis.

We report on Cu/amUiO-66, a composite made of Cu nanoparticles (NPs) and amorphous [Zr6 O4 (OH)4 (BDC)6 ] (amUiO-66, BDC=1,4-benzenedicarboxylate), and Cu-ZnO/amUiO-66 made of Cu-ZnO nanocomposites and amUiO-66. Both structures were obtained via a spray-drying method and characterized using high-resolution transmission electron microscopy, energy dispersive spectra, powder X-ray diffraction and extended X-ray absorption fine structure. The catalytic activity of Cu/amUiO-66 for CO2 hydrogenation to methanol was 3-fold that of Cu/crystalline UiO-66. Moreover, Cu-ZnO/amUiO-66 enhanced the methanol production rate by 1.5-fold compared with Cu/amUiO-66 and 2.5-fold compared with γ-Al2 O3 -supported Cu-ZnO nanocomposites (Cu-ZnO/γ-Al2 O3 ) as the representative hydrogenation catalyst. The high catalytic performance was investigated using in situ Fourier transform IR spectra. This is a first report of a catalyst comprising metal NPs and an amorphous metal-organic framework in a gas-phase reaction.

Probing dynamics of carbon dioxide in a metal-organic framework under high pressure by high-resolution solid-state NMR.

The application of high-resolution NMR analysis for CO2 adsorbed in a MOF under high pressure is reported for the first time. The results showed that CO2 adsorbed in MOF-74 had an unusually slow mobility (τ ∼ 10-8 s). CO2-CO2 interactions suppressed the mobility of CO2 under high pressure, which, in turn, would have contributed to the stability of CO2 at the adsorption sites.

The First Study on the Reactivity of Water Vapor in Metal-Organic Frameworks with Platinum Nanocrystals.

We first studied the reactivity of H2 O vapor in metal-organic frameworks (MOFs) with Pt nanocrystals (NCs) through the water-gas shift (WGS) reaction. A water-stable MOF, UiO-66, serves as a highly effective support material for the WGS reaction compared with ZrO2 . The origin of the high catalytic performance was investigated using in situ IR spectroscopy. In addition, from a comparison of the catalytic activities of Pt on UiO-66, where Pt NCs are located on the surface of UiO-66 and Pt@UiO-66, where Pt NCs are coated with UiO-66, we found that the competitive effects of H2 O condensation and diffusion in the UiO-66 play important roles in the catalytic activity of Pt NCs. A thinner UiO-66 coating further enhanced the WGS reaction activity of Pt NCs by minimizing the negative effect of slow H2 O diffusion in UiO-66.

Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect.

Two lanthanide-containing structurally analogous porous coordination polymers (PCPs) have been isolated with the general molecular formula [Ln2(L1)2(H2O)4(ox)] n.4 nH2O (where L1 = fumarate, ox = oxalate; Ln = Dy (1), Gd (2)). Thermogravimetric analysis (TGA) and TG-MS measurements performed on 1 and 2 suggest that not only the solvated water molecules in the crystal lattice but also the four coordinated water molecules on the respective lanthanides in 1 and 2 are removed upon activation. Due to the removal of the waters, 1 and 2 lost their crystallinity and became amorphous, as confirmed by powder X-ray diffraction (PXRD). We propose the molecular formula [Ln2(L1)2(ox)] n for the amorphous phase of 1 and 2 (where Ln = Dy (1'), Gd (2')) on the basis of XANES, EXAFS, and other experimental investigations. Magnetization relaxation dynamics probed on 1 and 1' reveal two different relaxation processes with effective energy barriers of 53.5 and 7.0 cm-1 for 1 and 45.1 and 6.4 cm-1 for 1', which have been rationalized by detailed ab initio calculations. For the isotropic lanthanide complexes 2 and 2', magnetocaloric effect (MCE) efficiency was estimated through detailed magnetization measurements. We have estimated -Δ S m values of 52.48 and 41.62 J kg1- K-1 for 2' and 2, respectively, which are one of the largest values reported for an extended structure. In addition, a 26% increase in -Δ Sm value in 2' in comparison to 2 is achieved by simply removing the passively contributing (for MCE) solvated water molecule in the lattice and coordinated water molecules.

Mechanical Alloying of Metal-Organic Frameworks.

The solvent-free mechanical milling process for two distinct metal-organic framework (MOF) crystals induced the formation of a solid solution, which is not feasible by conventional solution-based syntheses. X-ray and STEM-EDX studies revealed that performing mechanical milling under an Ar atmosphere promotes the high diffusivity of each metal ion in an amorphous solid matrix; the amorphous state turns into the porous crystalline structure by vapor exposure treatment to form a new phase of a MOF solid solution.

Encapsulating Mobile Proton Carriers into Structural Defects in Coordination Polymer Crystals: High Anhydrous Proton Conduction and Fuel Cell Application.

We describe the encapsulation of mobile proton carriers into defect sites in nonporous coordination polymers (CPs). The proton carriers were encapsulated with high mobility and provided high proton conductivity at 150 °C under anhydrous conditions. The high proton conductivity and nonporous nature of the CP allowed its application as an electrolyte in a fuel cell. The defects and mobile proton carriers were investigated using solid-state NMR, XAFS, XRD, and ICP-AES/EA. On the basis of these analyses, we concluded that the defect sites provide space for mobile uncoordinated H3PO4, H2PO4(-), and H2O. These mobile carriers play a key role in expanding the proton-hopping path and promoting the mobility of protons in the coordination framework, leading to high proton conductivity and fuel cell power generation.

Glass Formation of a Coordination Polymer Crystal for Enhanced Proton Conductivity and Material Flexibility.

The glassy state of a two-dimensional (2D) Cd(2+) coordination polymer crystal was prepared by a solvent-free mechanical milling process. The glassy state retains the 2D structure of the crystalline material, albeit with significant distortion, as characterized by synchrotron X-ray analyses and solid-state multinuclear NMR spectroscopy. It transforms to its original crystal structure upon heating. Thus, reversible crystal-to-glass transformation is possible using our new processes. The glass state displays superior properties compared to the crystalline state; specifically, it shows anhydrous proton conductivity and a dielectric constant two orders of magnitude greater than the crystalline material. It also shows material flexibility and transparency.

Control of molecular rotor rotational frequencies in porous coordination polymers using a solid-solution approach.

Rational design to control the dynamics of molecular rotors in crystalline solids is of interest because it offers advanced materials with precisely tuned functionality. Herein, we describe the control of the rotational frequency of rotors in flexible porous coordination polymers (PCPs) using a solid-solution approach. Solid-solutions of the flexible PCPs [{Zn(5-nitroisophthalate)x(5-methoxyisophthalate)1-x(deuterated 4,4'-bipyridyl)}(DMF·MeOH)]n allow continuous modulation of cell volume by changing the solid-solution ratio x. Variation of the isostructures provides continuous changes in the local environment around the molecular rotors (pyridyl rings of the 4,4'-bipyridyl group), leading to the control of the rotational frequency without the need to vary the temperature.

Formation of Foam-like Microstructural Carbon Material by Carbonization of Porous Coordination Polymers through a Ligand-Assisted Foaming Process.

Porous carbon material with a foam-like microstructure has been synthesized by direct carbonization of porous coordination polymer (PCP). In situ generation of foaming agents by chemical reactions of ligands in PCP during carbonization provides a simple way to create lightweight carbon material with a foam-like microstructure. Among several substituents investigated, the nitro group has been shown to be the key to obtain the unique foam-like microstructure, which is due to the fast kinetics of gas evolution during carbonization. Foam-like microstructural carbon materials showed higher pore volume and specific capacitance compared to a microporous carbon.

A redox-active inorganic crown ether based on a polyoxometalate capsule.

Cation-uptake has been long researched as an important topic in materials science. Herein we focus on a molecular crystal composed of a charge-neutral polyoxometalate (POM) capsule [MoVI72FeIII30O252(H2O)102(CH3CO2)15]3+ encapsulating a Keggin-type phosphododecamolybdate anion [α-PMoVI12O40]3-. Cation-coupled electron-transfer reaction occurs by treating the molecular crystal in an aqueous solution containing CsCl and ascorbic acid as a reducing reagent. Specifically, multiple Cs+ ions and electrons are captured in crown-ether-like pores {MoVI3FeIII3O6}, which exist on the surface of the POM capsule, and Mo atoms, respectively. The locations of Cs+ ions and electrons are revealed by single-crystal X-ray diffraction and density functional theory studies. Highly selective Cs+ ion uptake is observed from an aqueous solution containing various alkali metal ions. Cs+ ions can be released from the crown-ether-like pores by the addition of aqueous chlorine as an oxidizing reagent. These results show that the POM capsule functions as an unprecedented "redox-active inorganic crown ether", clearly distinguished from the non-redox-active organic counterpart.

Evaluation of the nutrient foramen as a suitable landmark in spinal surgery.

BACKGROUND CONTEXT: The posterior cervical approach is a very popular surgical procedure. Because of the slope-shaped laminae, a suitable landmark for laminectomy, laminoplasty, and pedicle screw placement does not exist. PURPOSE: We investigated the association between the nutrient foramen and spinal canal, and the safety of bone gutter placement using nutrient foramina for safe and effective spinal surgery. STUDY DESIGN: Observational SETTING: University Hospital PATIENT SAMPLES: Two hundred and sixty-six consecutive patients treated for cervical pathologies in our university hospital between January 2005 and December 2019. OUTCOME MEASURES: We identified the location of nutrient foramina and their anatomical association with the spinal canal using preoperative three-dimensional CT scanning. METHODS: We studied the distance between each foramen and the spinal canal, and the angle of the nutrient foramen at each vertebral level. RESULTS: The nutrient foramina were always located outside the spinal canal, with an average distance of +3.06±1.74 mm. The lower the spinal level, the closer the nutrient foramen to the spinal canal. CONCLUSIONS: The nutrient foramen can be considered a reliable landmark when using the posterior cervical approach.

Vanadium-substituted polycationic Al-oxo cluster in a porous ionic crystal exhibiting Lewis acidity.

Herein we present the first example of a vanadium-substituted δ-Keggin Al13-based (Al28V4) porous Ionic crystal (PIC) (Al28V4-PW9V3). Structural and theoretical studies suggested that Al28V4 may exhibit Lewis acidity, which was confirmed by acetalization reaction with Al28V4-PW9V3 as a solid catalyst and investigations with pyridine or 2,6-lutidine as basic probe molecules. A comparative study with the δ-Keggin Al13 monomer based PIC indicated that the vanadium-substitution gave rise to the Lewis acidity.

Basicity of isostructural porous ionic crystals composed of Nb/Ta-substituted Keggin-type polyoxotungstates.

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5- (BW12), [SiW11NbO40]5- (SiW11Nb), and [SiW11TaO40]5- (SiW11Ta)) are synthesized. Experimental and theoretical characterizations of the three PICs (1_BW12, 1_SiW11Nb, and 1_SiW11Ta) show that the substitution of Nb/Ta for W in the POMs enhances the basicity of PICs, which increases in the order of 1_BW12 < 1_SiW11Nb < 1_SiW11Ta. These findings clearly show that the increase in basicity is due not only to the increase in negative charge of the POM molecule as is often explained, but also to the character of the substituted element itself.