Multi-shelled Hollow Metal–Organic Frameworks

Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.     

Alkali Phosphonate Metal–Organic Frameworks

A new family of porous metal–organic frameworks (MOFs), namely alkali phosphonate MOFs, is reported. [Na₂Cu(H₄TPPA)] ⋅ (NH₂(CH₃)₂)₂ ( GTUB-1 ) was synthesized using the tetratopic 5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin ( H₈-TPPA ) linker with planar X-shaped geometrical core. GTUB-1 is composed of rectangular void channels with BET surface area of 697 m² g⁻¹. GTUB-1 exhibits exceptional thermal stability. The toxicity analysis of the ( H₈-TPPA ) linker indicates that it is well tolerated by an intestinal cell line, suggesting its suitability for creating phosphonate MOFs for biological applications.     

Metal–Organic Frameworks as Electrocatalysts

Transition metal complexes are well-known homogeneous electrocatalysts. In this regard, metal–organic frameworks (MOFs) can be considered as an ensemble of transition metal complexes ordered in a periodic arrangement. In addition, MOFs have several additional positive structural features that make them suitable for electrocatalysis, including large surface area, high porosity, and high content of accessible transition metal with exchangeable coordination positions. The present review describes the current state in the use of MOFs as electrocatalysts, both as host of electroactive guests and their direct electrocatalytic activity, particularly in the case of bimetallic MOFs. The field of MOF-derived materials is purposely not covered, focusing on the direct use of MOFs or its composites as electrocatalysts. Special attention has been paid to present strategies to overcome their poor electrical conductivity and limited stability.     

Covalent Organic Frameworks(COFs) for Sequestration of99TcO4–

Covalent organic frameworks(COFs), as a class of crystalline porous materials with periodic lattices and porous structures, have received extensive attention in the fields of gas storage and separation, energy storage, catalysis and optoelectronics and so on. However, COFs are still in their infancy in the field of nuclear waste treatment, especially for sequestration of long-live problematic radionuclides, such as ⁹⁹Tc. Battle of decontamination of pertechnetate(TcO₄^–), a main existence of ⁹⁹Tc under aerobic environments, is far from finished. In this review, recent progresses of COFs and some relative materials in the sequestration of pertechnetate, and perspective on surmounting the unmet issues are elucidated.     

Solvent-Assisted Metal Metathesis: A Highly Efficient and Versatile Route towards Synthetically Demanding Chromium Metal–Organic Frameworks

Chromium(III)-based metal–organic frameworks (Cr-MOFs) are very attractive in a wide range of investigations because of their robustness and high porosity. However, reports on Cr-MOFs are scarce owing to the difficulties in their direct synthesis. Recently developed postsynthetic routes to obtain Cr-MOFs suffered from complicated procedures and a lack of general applicability. Herein, we report a highly efficient and versatile strategy, namely solvent-assisted metal metathesis, to obtain Cr-MOFs from a variety of Fe^III-MOFs, including several well-known MOFs and a newly synthesized one, through judicious selection of a coordinating solvent. The versatility of this strategy was demonstrated by producing Cr-MIL-100, Cr-MIL-142A/C, Cr-PCN-333, and Cr-PCN-600 from their Fe^III analogues and Cr-SXU-1 from a newly synthesized MOF precursor, Fe-SXU-1, in acetone as the solvent under very mild conditions. We have thus developed a general approach for the preparation of robust Cr-MOFs, which are difficult to synthesize by direct methods.     

Microscopic and Mesoscopic Dual Postsynthetic Modifications of Metal–Organic Frameworks

We report the dual postsynthetic modification (PSM) of a metal–organic framework (MOF) involving the microscopic conversion of C−H bonds into C−C bonds and the mesoscopic introduction of hierarchical porosity. MOF crystals underwent single-crystal-to-single-crystal transformations during the electrophilic aromatic substitution of Co₂(m-DOBDC) (m-DOBDC⁴⁻=4,6-dioxo-1,3-benzenedicarboxylate) with alkyl halides and formaldehyde. The steric hindrance caused by the proximity of the introduced functional groups to the coordination bonds reduced bond stability and facilitated the transformation into hierarchically porous mesostructures by etching with in situ generated protons (hydroniums) and halides. The numerous defect sites in the mesostructural MOFs are potential water-sorption sites. However, since the introduced functional groups are close to the main adsorption sites, even methyl groups are able to considerably decrease water adsorption, whereas hydroxy groups increase adsorption at low vapor pressures.     

Mesoporous Metal–Organic Frameworks for Catalytic RNA Deprotection and Activation

A metal–organic framework (MOF) with mespores (2 to 50 nm) allows the inclusion of large biomolecules, such as nucleic acids. However, chemical reaction on the nucleic acids, to further regulate their bioactivity, is yet to be demonstrated within MOF pores. Here, we report the deprotection of carbonate protected RNA molecules (21 to 102 nt) to restore their original activity using a MOF as a heterogeneous catalyst. Two MOFs, MOF-626 and MOF-636 are designed and synthesized, with mesopores of 2.2 and 2.8 nm, respectively, carrying isolated metal sites (Ni, Co, Cu, Pd, Rh and Ru). The pores favor the entrance of RNA, while the metal sites catalyze C−O bond cleavage at the carbonate group. Complete conversion of RNA is achieved by Pd-MOF-626, 90 times more efficiently than Pd(NO₃)₂. MOF crystals are also removable from the aqueous reaction media, leaving a negligible metal footprint, 3.9 ppb, only 1/55 of that using homogeneous Pd catalysts. These features make MOF potentially suited for bioorthogonal chemistry.     

Effect of the Metal within Regioisomeric Paddle-Wheel-Type Metal–Organic Frameworks

The effect of metal on the degree of flexibility upon evacuation of metal–organic frameworks (MOFs) has been revealed with positional control of the organic functionalities. Although Co-, Cu-, and Zn-based DMOFs (DMOF = DABCO MOF, DABCO = 1,4-diazabicyclo[2.2.2]octane) with ortho-ligands (2,3-NH₂Cl) have frameworks that are inflexible upon evacuation, MOFs with para-ligands (2,5-NH₂Cl) showed different N₂ uptake amounts after evacuation by metal exchange. Considering that the structural analyses were not fully sufficiently different to explain the drastic changes in N₂ adsorption after evacuation, quantum chemical simulation was explored. A new index (η) was defined to quantify the regularity around the metal based on differences in the oxygen-metal-oxygen angles. Within 2,5-NH₂Cl, the η value becomes larger as the metal are varied from Co to Zn. A large η value means that the structures around the metal center are less ordered. These results can be used to explain flexibility changes upon evacuation by altering the metal cation in this regioisomeric system.     

Rabies Virus-Inspired Metal–Organic Frameworks (MOFs) for Targeted Imaging and Chemotherapy of Glioma

The blood–brain barrier (BBB) restricts access to the brain of more than 98 % of therapeutic agents and is largely responsible for treatment failure of glioblastoma multiforme (GBM). Therefore, it is of great importance to develop a safe and efficient strategy for more effective drug delivery across the BBB into the brain. Inspired by the extraordinary capability of rabies virus (RABV) to enter the central nervous system, we report the development and evaluation of the metal–organic framework-based nanocarrier MILB@LR, which closely mimicked both the bullet-shape structure and surface functions of natural RABV. MILB@LR benefited from a more comprehensive RABV-mimic strategy than mimicking individual features of RABV and exhibited significantly enhanced BBB penetration and brain tumor targeting. MILB@LR also displayed superior inhibition of tumor growth when loaded with oxaliplatin. The results demonstrated that MILB@LR may be valuable for GBM targeting and treatment.     

Conversion from Heterometallic to Homometallic Metal–Organic Frameworks

Two new heterometallic metal–organic frameworks (MOFs), LnZnTPO 1 and 2 , and two homometallic MOFs, LnTPO 3 and 4 (Ln=Eu for 1 and 3 , and Tb for 2 and 4 ; H₃TPO=tris(4-carboxyphenyl)phosphine oxide) were synthesized, and their structures and properties were analyzed. They were prepared by solvothermal reaction of the C₃-symmetric ligand H₃TPO with the corresponding metal ion(s) (a mixture of Ln³⁺ and Zn²⁺ for 1 and 2 , and Ln³⁺ alone for 3 and 4 ). Single-crystal XRD (SXRD) analysis revealed that 1 and 3 are isostructural to 2 and 4 , respectively. TGA showed that the framework is thermally stable up to about 400 °C for 1 and 2 , and about 450 °C for 3 and 4 . PXRD analysis showed their pore-structure distortions without noticeable framework–structure changes during drying processes. The shapes of gas sorption isotherms for 1 and 3 are almost identical to those for 2 and 4 , respectively. Solvothermal immersion of 1 and 2 in Tb³⁺ and Eu³⁺ solutions resulted in the framework metal-ion exchange affording 4 and 3 , respectively, as confirmed by photoluminescence (PL), PXRD, IR, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and energy-dispersive X-ray (EDX) analyses.     

Bivariate Metal–Organic Frameworks with Tunable Spin-Crossover Properties

In this work, pyrazine ( A ), aminopyrazine ( B ), quinoxaline ( C ), and 5,6,7,8-tetrahydroquinoxaline ( D ) have been screened out among a large number of pyrazine derivatives to construct Hofmann-type metal–organic frameworks (MOFs) Fe(L)[M(CN)₄] (M=Pt, Pd) with similar 3D pillared-layer structures. X-ray single-crystal diffraction reveals that the alternate linkage between M and Fe^II ions through cyano bridges forms the 2D extended metal cyanide sheets, and ligands A – D acted as vertical columns to connect the 2D sheets to give 3D pillared-layer structures. Subsequently, a series of bivariate MOFs were constructed by pairwise combination of the four ligands A–D , which were confirmed by ¹H NMR, PXRD, FTIR, and Raman spectroscopy. The results demonstrated that ligand size and crystallization rate play a dominant role in constructing bivariate Hofmann-type MOFs. More importantly, the spin-crossover (SCO) properties of the bivariate MOFs can be finely tuned by adjusting the proportion of the two pillared ligands in the 3D Hofmann-type structures. Remarkably, the spin transition temperatures, T_c↑ and T_c↓ of Fe( A )_x( B )_1−x[Pt(CN)₄] (x=0 to 1) can be adjusted from 239 to 254 K and from 248 to 284 K, respectively. Meanwhile, the width of the hysteresis loops can be widened from 9 to 30 K. Changing Pt to Pd, the hysteresis loops of Fe( A )_x( B )_1−x[Pd(CN)₄] can be tuned from 9 (T_c↑=215 K, T_c↓=206 K) to 24 K (T_c↑=300 K, T_c↓=276 K). This research provides wider implications in the development of advanced bistable materials, especially in precisely regulating SCO properties.     

Photoinduced Nonlinear Contraction Behavior in Metal–Organic Frameworks

The photoinduced dynamic behavior of flexible materials has received considerable attention for potential applications, such as in data storage or as smart optical devices and molecular mechanical actuators. Until now, precisely controlling expansion and contraction with light has remained a challenge. Unraveling the detailed mechanisms of photoinduced structural transformations remains a critical step necessary to understand the molecular architecture necessary for the design of sensitive photomechanical actuators. Herein, a two-dimensional flexible metal–organic framework [Zn₂(bdc)₂(3-CH₃-spy)₂]⋅H₂O ( Zn₂-1 ; H₂bdc=1,4-benzenedicaboxylic acid; 3-CH₃-spy=3-methylstyrylpyridine) with a positive volumetric thermal expansion coefficient of +78.78×10⁻⁶ K⁻¹ is reported. Upon light irradiation at different wavelengths, the MOF underwent a [2+2] cycloaddition, which afforded a family of isomeric, three-dimensional MOFs ( Zn₂-2 n , n=a–d) in a single-crystal-to-single-crystal (SCSC) manner. An unprecedented phenomenon, that is, photoinduced nonlinear contraction (PINC), was observed during this conversion. The PINC is caused by conformational changes in the 3-CH₃-spy and bdc²⁻ ligands, the bending of metal–ligand bonds, and the local distortion of the paddle-wheel SBUs. The formation of a “wrinkle morphology” on the crystal surface after the photoreaction was observed by AFM. This PINC behavior can broaden the studies on materials expansion and offer a photodriven approach for the future design of supersensitive photomechanical actuators.     

Metal-Mediated Directional Capping of Rod-Packing Metal–Organic Frameworks

Two new rod-packing metal–organic frameworks (RPMOF) are constructed by regulating the in situ formation of the capping agent. In CPM-s7, carboxylate linkers extend 1D manganese-oxide chains in four additional directions, forming 3D RPMOF. The substitution of Mn²⁺ with a stronger Lewis acidic Co²⁺, leads to an acceleration of the hydrolysis-prone sulfonate linker, resulting in presence of sulfate ions to reduce two out of the four carboxylate-extending directions, and thus forming a new 2D rod-packing CPM-s8. Density functional theory calculations and magnetization measurements reveal ferrimagnetic ordering of CPM-s8, signifying the potential of exploring 2D RPMOF for effective low-dimensional magnetic materials.     

Stimuli-Responsive Luminescent Properties of Tetraphenylethene-Based Strontium and Cobalt Metal–Organic Frameworks

Herein we report two new TPE-based 3D MOFs, that is, Sr-ETTB and Co-ETTB (TPE=Tetraphenylethylene, H₈ETTB=4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1′-biphenyl]-3,5-dicarboxylic acid))). Through tailoring outer shell electron configurations of Sr^II and Co^II cations, the fluorescence intensity of the MOFs is tuned from high emission to complete non-emission. Sr-ETTB with strong blue fluorescence shows reversible fluorescence variations in response to pressure and temperature, which is directly related to the reversible deformation of the crystal structure. In addition, non-emissive Co-ETTB counterpart exhibits a turn-on fluorescent enhancement under the stimulation of analyte histidine. In the process, TPE-cored linkers in the MOFs are released through competitive coordination substitution and subsequently reassembled to perform aggregation-induced luminescence behavior originated from the organic linkers.     

Atypical Hybrid Metal–Organic Frameworks (MOFs): A Combinative Process for MOF-on-MOF Growth,Etching, and Structure Transformation

The structural, compositional, and morphological features of metal–organic frameworks (MOFs) govern their properties and applications. Construction of hybrid MOFs with complicated structures, components, or morphologies is significant for the development of well-organized MOFs. An advanced route is reported for construction of atypical hybrid MOFs with unique morphologies and complicated components: 1) MOF-on-MOF growth of a 3D zeolitic imidazolate framework (ZIF) on a ZIF-L template, 2) etching of a part of the 2D ZIF-L template, and 3) structural transformation of 2D ZIF-L into 3D ZIF. The formation of core–shell-type MOF rings and plates is controlled by regulating the three processes. The formation route for the core–shell-type MOF rings and plates was monitored by tracking changes in morphology, structure, and composition. Carbon materials prepared from the pyrolysis of the core–shell-type hybrid MOFs displayed enhanced oxygen reduction reaction activities compared to their monomeric counterparts.     

Cooperative Carbon Dioxide Adsorption in Alcoholamine- and Alkoxyalkylamine-Functionalized Metal–Organic Frameworks

A series of structurally diverse alcoholamine- and alkoxyalkylamine-functionalized variants of the metal–organic framework Mg₂(dobpdc) are shown to adsorb CO₂ selectively via cooperative chain-forming mechanisms. Solid-state NMR spectra and optimized structures obtained from van der Waals-corrected density functional theory calculations indicate that the adsorption profiles can be attributed to the formation of carbamic acid or ammonium carbamate chains that are stabilized by hydrogen bonding interactions within the framework pores. These findings significantly expand the scope of chemical functionalities that can be utilized to design cooperative CO₂ adsorbents, providing further means of optimizing these powerful materials for energy-efficient CO₂ separations.     

Intrinsic Apyrase-Like Activity of Cerium-Based Metal–Organic Frameworks (MOFs): Dephosphorylation of Adenosine Tri- and Diphosphate

Apyrase is an important family of extracellular enzymes that catalyse the hydrolysis of high-energy phosphate bonds (HEPBs) in ATP and ADP, thereby modulating many physiological processes and driving life activities. Herein, we report an unexpected discovery that cerium-based metal–organic frameworks (Ce-MOFs) of UiO-66(Ce) have intrinsic apyrase-like activity for ATP/ADP-related physiological processes. The abundant Ce^III/Ce^IV couple sites of Ce-MOFs endow them with the ability to selectively catalyse the hydrolysis of HEPBs of ATP and ADP under physiological conditions. Compared to natural enzymes, they could resist extreme pH and temperature, and present a broad range of working conditions. Based on this finding, a significant inhibitory effect on ADP-induced platelet aggregation was observed upon exposing the platelet-rich plasma (PRP) to the biomimetic UiO-66(Ce) films, prefiguring their wide application potentials in medicine and biotechnology.     

Synthetic Strategies and Structural Arrangements of Isoreticular Mixed-Component Metal–Organic Frameworks

In recent years, the synthesis of mixed-metal and mixed-linker metal–organic frameworks with multiple metals and/or linker molecules combined in one framework has become a growing field of interest. These mixed-component or multivariate metal–organic framework materials provide the possibility to introduce multiple functionalities inside one framework. The interaction of guest molecules with different functionalities in the same material is a promising approach in the fields of gas storage, separation, catalysis and drug delivery. Furthermore, the combination of different components may lead to synergistic effects that cannot be achieved otherwise. These mixed-component approaches open up new pathways to an even larger range of possible customizations in the field of metal–organic frameworks.