Influence of Water in the Synthesis of the Zirconium-Based Metal–Organic Framework UiO-66: Isolation and Reactivity of [ZrCl(OH)2(DMF)2]Cl

Probing oxygen exchange between UiO-66(Zr) MOF and water using 17 O solid-state NMR

The Zr-based Metal Organic Framework (MOF) UiO-66(Zr) is widely employed owing to its good thermal and chemical stabilities. Although the long-range structure of this MOF is preserved in the presence of water during several days, little is known about the formation of defects, which cannot be detected using diffraction techniques. We apply here 17 O solid-state NMR spectroscopy at 18.8 T to investigate the reactivity of UiO-66, through the exchange of oxygen atoms between the different sites of the MOF and water. For that purpose, we have selectively enriched in 17 O isotope the carboxylate groups of UiO-66(Zr) by using it with 17 O-labeled terephthalic acid prepared using mechanochemistry. In the presence of water at 50 °C and a following dehydration at 150 °C, we observe an overall exchange of O atoms between COO- and μ3 -O2- sites. Furthermore, we demonstrate that the three distinct oxygen sites, μ3 -OH, μ3 -O2- and COO- , of UiO-66(Zr) MOF can be enriched in 17 O isotope by post-synthetic hydrothermal treatment in the presence of 17 O-enriched water. These results demonstrate the lability of Zr-O bonds and the reactivity of UiO-66(Zr) with water.

Characterization, Structure, and Reactivity of Hydroxyl Groups on Metal-Oxide Cluster Nodes of Metal-Organic Frameworks: Structural Diversity and Keys to Reactivity and Catalysis

Among the most stable metal-organic frameworks (MOFs) are those incorporating nodes that are metal oxide clusters with frames such as Zr6 O8 . This review is a summary of the structure, bonding, and reactivity of MOF node hydroxyl groups, emphasizing those bonded to nodes containing aluminum and zirconium ions. Hydroxyl groups are often present on these nodes, sometimes balancing the charges of the metal ions. They arise during MOF syntheses in aqueous media or in post-synthesis treatments. They are identified with infrared and 1 H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, paired µ2 -OH, and aqua groups on nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolated µ2 -OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g., µ3 -OH) are mere spectators in catalysis. There are similarities between MOF node hydroxyl groups and those on the surfaces of bulk metal oxides, zeolites, and enzymes, but the comparisons are mostly inexact, and much remains to be understood about MOF node hydroxyl group chemistry. It is posited that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts.

Postsynthetic Photochemical Modification and 2D Structuring of Zr-MOF Thin Films Containing Benzophenone Linker Molecules

For the fabrication of next-generation MOF-based devices the availability of highly adaptable materials in suitable shapes is crucial. Here, we present thin films of a metal-organic framework (MOF) containing photoreactive benzophenone units. Crystalline, oriented and porous films of the zirconium-based bzpdc-MOF (bzpdc=benzophenone-4-4'-dicarboxylate) are prepared by direct growth on silicon or glass substrates. Via a subsequent photochemical modification of the Zr-bzpdc-MOF films, various properties can be tuned postsynthetically by covalent attachment of modifying agents. Apart from the modification with small molecules, also grafting-from polymerization reactions are possible. In a further extension, 2D structuring and photo-writing of defined structures is also possible, for example by using a photolithographic approach, paving the way towards micro-patterned MOF surfaces.

Cathodic deposition of MOF films: mechanism and applications

Metal-organic framework (MOF) thin films could be used for ion/molecular sieving, sensing, catalysis, and energy storage, but thus far no large-scale applications are known. One of the reasons is the lack of convenient and controllable fabrication methods. This work reviews the cathodic deposition of MOF films, which has advantages (e.g., simple operations, mild conditions, and controllable MOF film thickness/morphology) over other reported techniques. Accordingly, we discuss the mechanism of the cathodic deposition of MOF films which consists of the electrochemically triggered deprotonation of organic linkers and the formation of inorganic building blocks. Thereafter, the main applications of cathodically deposited MOF films are introduced with the aim of showing this technique's wide-ranging applications. Finally, we give the remaining issues and outlooks of the cathodic deposition of MOF films to drive its future development.

High-yield halide-assisted synthesis of metal-organic framework UiO-based nanocarriers

The synthesis of nanosized metal-organic frameworks (NMOFs) is requisite for their application as injectable drug delivery systems (DDSs) and other biorelevant purposes. Herein, we have critically examined the role of different synthetic parameters leading to the production of UiO-66 crystals smaller than 100 nm. Of note, we demonstrate the co-modulator role conferred by halide ions, not only to produce NMOFs with precise morphology and size, but also to significantly improve the reaction yield. The resulting NMOFs are highly crystalline and exhibit sustained colloidal stability in different biologically relevant media. As a proof of concept, these NMOFs were loaded with Rhodamine 6G (R6G), which remained trapped in most common biologically relevant media. When incubated with living mammalian cells, the R6G-loaded NMOFs were efficiently internalized and did not impair cell viability even at relatively high doses.

Preparation of Zirconium Hydrogen Phosphate Coatings on Sandblasted/Acid-Etched Titanium for Enhancing Its Osteoinductivity and Friction/Corrosion Resistance

Background

Sandblasted/acid-etched titanium (SLA-Ti) implants are widely used for dental implant restoration in edentulous patients. However, the poor osteoinductivity and the large amount of Ti particles/ions released due to friction or corrosion will affect its long-term success rate.

Purpose

Various zirconium hydrogen phosphate (ZrP) coatings were prepared on SLA-Ti surface to enhance its friction/corrosion resistance and osteoinduction.

Methods

The mixture of ZrCl₄ and H₃PO₄ was first coated on SLA-Ti and then calcined at 450°C for 5 min to form ZrP coatings. In addition to a series of physiochemical characterization such as morphology, roughness, wettability, and chemical composition, their capability of anti-friction and anti-corrosion were further evaluated by friction-wear test and by potential scanning. The viability and osteogenic differentiation of MC3T3-E1 cells on different substrates were investigated via MTT, mineralization and PCR assays.

Results

The characterization results showed that there were no significant changes in the morphology, roughness and wettability of ZrP-modified samples (SLA-ZrP0.5 and SLA-ZrP0.7) compared with SLA group. The results of electrochemical corrosion displayed that both SLA-ZrP0.5 and SLA-ZrP0.7 (especially the latter) had better corrosion resistance than SLA in normal saline and serum-containing medium. SLA-ZrP0.7 also exhibited the best friction resistance and great potential to enhance the spreading, proliferation and osteogenic differentiation of MC3T3-E1 cells.

Conclusion

We determined that SLA-ZrP0.7 had excellent comprehensive properties including anti-corrosion, anti-friction and osteoinduction, which made it have a promising clinical application in dental implant restoration.

Exploring the Role of Cluster Formation in UiO Family Hf Metal-Organic Frameworks with in Situ X-ray Pair Distribution Function Analysis

The structures of Zr and Hf metal-organic frameworks (MOFs) are very sensitive to small changes in synthetic conditions. One key difference affecting the structure of UiO MOF phases is the shape and nuclearity of Zr or Hf metal clusters acting as nodes in the framework; although these clusters are crucial, their evolution during MOF synthesis is not fully understood. In this paper, we explore the nature of Hf metal clusters that form in different reaction solutions, including in a mixture of DMF, formic acid, and water. We show that the choice of solvent and reaction temperature in UiO MOF syntheses determines the cluster identity and hence the MOF structure. Using in situ X-ray pair distribution function measurements, we demonstrate that the evolution of different Hf cluster species can be tracked during UiO MOF synthesis, from solution stages to the full crystalline framework, and use our understanding to propose a formation mechanism for the hcp UiO-66(Hf) MOF, in which first the metal clusters aggregate from the M₆ cluster (as in fcu UiO-66) to the hcp-characteristic M₁₂ double cluster and, following this, the crystalline hcp framework forms. These insights pave the way toward rationally designing syntheses of as-yet unknown MOF structures, via tuning the synthesis conditions to select different cluster species.

In Situ Coupling of Catalytic Centers into Artificial Substrate Mesochannels as Super-Active Metalloenzyme Mimics

Highly evolved substrate channels in natural enzymes facilitate the rapid capture of substrates and direct transfer of intermediates between cascaded catalytic units, thus rationalizing their efficient catalysis. In this study, a nanoscale ordered mesoporous Ce-based metal-organic framework (OMUiO-66(Ce)) is designed as an artificial substrate channel, where MnO₂ is coupled to Ce-O clusters as a super-active catalase (CAT). An in situ soft template reduction strategy is developed to deposit well-dispersed and exposed MnO₂ in the mesochannels of OMUiO-66(Ce). Several synthesis parameters are optimized to minimize the particle size to ≈150 nm for efficient intracellular endocytosis. The mesochannels provide interaction guidance that not only rapidly drove H₂ O₂ substrates to CAT-like catalytic centers, but also seamlessly transfer H₂ O₂ intermediates between superoxide dismutase-like and CAT-like biocatalytic cascades. As a result, the biomimetic system exhibits high efficiency, low dosage, and long-lasting intracellular antioxidant function. Under disease-related oxidative stress, the artificial substrate channels promote the rate of the reactions catalyzed by MnO₂ , which exceeds that of the reactions catalyzed by natural CAT. Based on this observation, a set of design rules for substrate channels are proposed to guide the rational design of super-active biomimetic systems.

Metastable Zr/Hf-MOFs: the hexagonal family of EHU-30 and their water-sorption induced structural transformation

Four new EHU-30 isoreticular compounds, based on amino-functionalized linkers and Zr and Hf metal centres are reported, in which H2O adsorption isotherms show an anomalous behaviour due to a localized structural transformation from EHU-30 to UiO-66.

Improved continuous synthesis of UiO-66 enabling outstanding production rates

Optimization of the continuous synthesis of UiO-66 and derivatives with extremely high space time yield using a tubular reactor.

Synthesis, Structural, and Physicochemical Characterization of a Ti6 and a Unique Type of Zr6 Oxo Clusters Bearing an Electron-Rich Unsymmetrical {OON} Catecholate/Oxime Ligand and Exhibiting Metalloaromaticity

The chelating catechol/oxime ligand 2,3-dihydroxybenzaldehyde oxime (H₃dihybo) has been used to synthesize one titanium(IV) and two zirconium(IV) compounds that have been characterized by single-crystal X-ray diffraction and ¹H and ¹³C NMR, solid-state UV-vis, and ESI-MS spectroscopy. The reaction of TiCl₄ with H₃dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium(IV) compound K₂[Ti^IV₆(μ₃-O)₂(μ-O)₃(OCH₃)₄(CH₃OH)₂(μ-Hdihybo)₆]·CH₃OH (1), while the reaction of ZrCl₄ with H₃dihybo and either ⁿBu₄NOH or KOH also gave the hexanuclear zirconium(IV) compounds 2 and 3, respectively. Compounds 1-3 have the same structural motif [M^IV₆(μ₃-Ο)₂(μ-Ο)₃] (M = Ti, Zr), which constitutes a unique example with a trigonal-prismatic arrangement of the six zirconium atoms, in marked contrast to the octahedral arrangement of the six zirconium atoms in all the Zr₆ clusters reported thus far, and a unique Zr₆ core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1 and 3 retain their integrity. The marriage of the catechol moiety with the oxime group in the ligand H₃dihybo proved to be quite efficient in substantially reducing the band gaps of TiO₂ and ZrO₂ to 1.48 and 2.34 eV for the titanium and zirconium compounds 1 and 3, respectively. The application of 1 and 3 in photocurrent responses was investigated. ESI-MS measurements of the clusters 1 and 3 revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M₃ (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M₆ (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICS_zz scan curves of these systems revealed that the triangular M₃ (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The latter depends upon the nature of the metallic center with NICS_zz(1) values equal to -30 and -42 ppm for the Ti (compound 1) and Zr (compound 2) systems, respectively, comparable to the NICS_zz(1) value of the benzene ring of -29.7 ppm calculated at the same level of theory.

Zirconium Metal-Organic Frameworks Containing a Biselenophene Linker: Synthesis, Characterization, and Luminescent Properties

The bicyclic ditopic linker 2,2'-biselenophene-5,5'-dicarboxylic acid (H₂SpSp), specifically designed for metal-organic framework (MOF) construction, has been synthesized in good yield and fully characterized. The corresponding zirconium MOF (Zr-MOF) [Zr₆O₄(OH)₄(SpSp)_3.8Cl_4.4] (1; where missing linkers are replaced by chloride anions as shown by X-ray fluorescence and elemental analysis) is isostructural with its bithiophene and bithiazole analogues. Starting from 1, an extension of the biselenophene-based Zr-MOF family has been successfully achieved, exploiting the structural analogy of the five-membered heterocycles selenophene, thiophene, and thiazole. Thus, three mixed-linker MOFs containing variable amounts of different bis(heterocyclic) dicarboxylic acids have been prepared and fully characterized: the two double-mixed [Zr₆O₄(OH)₄(SpSp)_2.6(TpTp)_1.3Cl_4.2] (2; H₂TpTp = 2,2'-bithiophene-5,5'-dicarboxylic acid) and [Zr₆O₄(OH)₄(SpSp)₂(TzTz)_1.8Cl_4.4] (3; H₂TzTz = 2,2'-bithiazole-5,5'-dicarboxylic acid) materials, as well as the triple-mixed [Zr₆O₄(OH)₄(SpSp)_1.6(TpTp)_1.2(TzTz)_1.4Cl_3.6] (4) compound. The four MOFs are luminescent under UV irradiation, exhibiting emission wavelengths falling in the blue-green visible region, as observed for their constitutive linkers. These materials open new horizons in the preparation of porous luminescent sensors or multicolor emitters for light-emitting diodes.