Ligand Coordination Research Articles

Dihydrogen Cleavage by a Zinc-Zinc Bond of a Heteroleptic Dizinc(I) Cation.

Oxidative addition of dihydrogen across a metal-metal bond to form reactive metal hydrides in homogeneous catalysis is known for transition metals but not for zinc(I)-zinc(I) bond as found in Carmona's eponymous dizinconene [Zn2Cp*2] (Cp* = η5-C5Me5). Dihydrogen reacted with the heteroleptic zinc(I)-zinc(I) bonded cation [(L2)Zn-ZnCp*][BAr4F] (L2 = TMEDA, N,N,N',N'-tetramethylethylenediamine, TEEDA, N,N,N',N'-tetraethylethylenediamine; ArF = 3,5-(CF3)2C6H3) under 2 bar at 80 °C to give the zinc(II) hydride cation [(L2)ZnH(thf)][BAr4F] along with zinc metal and Cp*H derived from the intermediate [Cp*ZnH]. DFT calculations show that the cleavage of dihydrogen occurs through a highly unsymmetrical transition state. Mechanistic studies agree with a heterolytic cleavage of dihydrogen as a result of the cationic charge and unsymmetrical ligand coordination. To explore the existence of zinc(I) hydride, thermally unstable hydridotriphenylborate complexes of zinc(I) [(L2)Zn(HBPh3)-ZnCp*] (L2 = TMEDA, TEEDA; TMPDA, N,N,N',N'-tetramethyl-1,3-propylenediamine) have been prepared by salt metathesis and were shown to undergo fast exchange with both BPh3 and [HBPh3]-.

Design and synthesis of different sized polyhedral channel MOF materials for the separation of CO2/N2

A fractal polyhedral pore structural MOFs (Cu-TATB) is synthesized from the coordination of tridentate ligands (H3TATB) and Cu4 clusters. This material is characterized by single crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis. The structure contains two different pore structures, including closed octahedral cages and open tetrakaidecahedron cages. The material has been successfully utilized for the separation of CO2/N2 due to the fractal structure of the channel with different pore sizes. Under optimized temperature, Cu-TATB adsorbed a large amount of CO2 and only negligible amount of N2, and the selectivity reaches 27. Breakthrough experiments demonstrated that this material can efficiently separate N2 and CO2. This structure further demonstrates the infinite potential of 3D MOFs with diverse pore structures in applications such as CO2 adsorption and separation.

A fresh perspective on dissociation mechanism of cellulose in DMAc/LiCl system based on Li bond theory

In this case, various characterization technologies have been employed to probe dissociation mechanism of cellulose in N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) system. These results indicate that coordination of DMAc ligands to the Li+–Cl− ion pair results in the formation of a series of Lix(DMAc)yClz (x = 1, 2; y = 1, 2, 3, 4; z = 1, 2) complexes. Analysis of interaction between DMAc ligand and Li center indicate that Li bond plays a major role for the formation of these Lix(DMAc)yClz complexes. And the saturation and directionality of Li bond in these Lix(DMAc)yClz complexes are found to be a tetrahedral structure. The hydrogen bonds between two cellulose chains could be broken at the nonreduced end of cellulose molecule via combined effects of basicity of Cl− ion and steric hindrance of [Li (DMAc)4]+ unit. The unique feature of Li bond in Lix(DMAc)yClz complexes is a key factor in determination of the dissociation mechanism.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling.

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

Metal-Drug Coordination Nanoparticles and Hydrogels for Enhanced Delivery.

Drug delivery is a key component of nanomedicine, and conventional delivery relies on the adsorption or encapsulation of drug molecules to a nanomaterial. Many delivery vehicles contain metal ions, such as metal-organic frameworks, metal oxides, transition metal dichalcogenides, MXene, and noble metal nanoparticles. These materials have a high metal content and pose potential long-term toxicity concerns leading to difficulties for clinical approval. In this review, recent developments are summarized in the use of drug molecules as ligands for metal coordination forming various nanomaterials and soft materials. In these cases, the drug-to-metal ratio is much higher than conventional adsorption-based strategies. The drug molecules are divided into small-molecule drugs, nucleic acids, and proteins. The formed hybrid materials mainly include nanoparticles and hydrogels, upon which targeting ligands can be grafted to improve efficacy and further decrease toxicity. The application of these materials for addressing cancer, viral infection, bacterial infection inflammatory bowel disease, and bone diseases is reviewed. In the end, some future directions are discussed from fundamental research, materials science, and medicine.

Improving the hydrostability of zeolitic imidazolate framework coatings using a facile silk fibroin protein modification method

Zeolitic imidazolate frameworks (ZIFs) are an important subclass of metal-organic frameworks (MOFs) with zeolite-type topology, which can be fabricated under ambient synthesis conditions. However, the applications of ZIFs are commonly limited due to the weak hydrostability of their metal–ligand coordination bonds, particularly under humid and aqueous conditions. In this work, as an example, the hydrolysis behaviours of ZIF-L with a special focus on ZIF-L coatings were tested at aqueous conditions with a wide range of pHs to systematically study and fundamentally understand their structural stability and degradation mechanism. Pristine ZIF-L powder and ZIF-L coatings were severely damaged after only 24 h in aqueous media. Interestingly, the ZIF-L coatings showed two distinct hydrolyzation pathways regardless of pH conditions, exhibiting either a ring-shaped etching or unfolding behaviours. While the ZIF-L powders were hydrolyzed almost identically across all pH conditions. With this new understanding, a facile silk fibroin (SF) protein modification method was developed to enhance the hydrostability of ZIF-L coatings in aqueous media. The effect of protein concentration on surface coating was systemically studied. ZIF-L coating retained its surface morphology after soaking in water and demonstrated switchable super wetting properties and superior separation performance for oil/water mixture. As a result, the quick SF protein modification significantly enhanced the stability of ZIF-L coatings under various pHs, while retaining their switchable wetting property and excellent separation performance.

A New Dy(III) Complex: Fluorescence Performances, Loading with Resveratrol-Hydrogels Against Skin Aging and Molecular Docking.

In the present research, novel lanthanide coordination compounds [DyL(PhCOO)(CH3OH)](ClO4)2·(CH3OH)2 (1) were characterized by the compression of 2,6-diformyl-4-methyl-phenol (dmp) and 1,3-diamino-2-propanol using benzoate as the secondary ligand, where L indicates the deprotonated macrocyclic ligand. Through the high structural rigidity driven by the coordination of the macrocyclic ligand formed by condensation in methanol solution and sodium benzoate with Dy(ClO4)3·6H2O, compound 1 exhibits outstanding cyan-emitting fluorescence performance and potential applications as a fluorescent material. Additionally, hyaluronic acid (HA)/ carboxymethyl chitosan (CMCS) hydrogels were prepared with loaded resveratrol metal-organic complexes according to the synthetic chemical approach. In biological study, we evaluated the effect of hydrogels on oxidative stress on human dermal fibroblasts. Examined by molecular docking simulation, the results showed that the binding interactions were from the phenol group, the carboxyl group and also the "-N=" group, indicating Dy metal complex has excellent biological capability.

Immobilized transition metal (II) bis–imine chelate on ZnO@TiO2 nanoparticles as sufficient catalysts for alcohol oxidation and for α,β‐cinnamic acid decarboxylative bromination

Based on the strong coordination ability of substituted imines towards transition metals of different oxidation states, the coordination of the HL imine ligand with the Cu2+ ion to form the Cu–bis–imine complex was achieved in 2:1 molar ratios. Their structures were confirmed with various spectroscopic tools. Heterogeneously, CuL2 was successfully appended on ZnO@TiO2 nanoparticles, as the less harmful materials, within the deprotonated p‐hydroxy chain of the bonded ligand in CuL2. The surface and internal structural morphologies of ZnO@TiO2@CuL2 were analyzed using X‐ray diffraction (XRD), field‐emission scanning electron microscopy (FESEM), high‐resolution transmission electron microscopy (HRTEM), energy‐dispersive X‐ray spectroscopy (EDS), and infrared (IR) spectroscopy, in addition to thermogravimetric analysis (TGA). The catalytic behavior of both catalysts, CuL2 (the homogeneous catalyst) and ZnO@TiO2@CuL2 (the heterogeneous catalyst), was examined in the benzyl alcohol redox protocols using H2O2 for their optimization. α,β‐Cinnamic acid decarboxylative bromination in the presence of potassium bromide and H2O2 in water was studied with both catalysts, in which (2‐bromovinyl)benzene was the chemoselective product. Both Cu catalysts displayed appreciable catalytic effectiveness for the oxidation protocols and the decarboxylative bromination reactions. CuL2 recorded less required time for optimization than that of the heterogeneous catalyst. The catalytic effectiveness of CuL2 was less promoted than that of ZnO@TiO2@CuL2. Despite the less performance in yield of benzaldehyde for the oxidation protocols with ZnO@TiO2@CuL2, ZnO@TiO2@CuL2 had more recycling than its homogeneous counterpart, with seven to three times, respectively.

“Turn-on” Eu(III) chemosensory polymer materials for ammonia and amines detection: Experimental and molecular modeling study

Polymer films doped by Eu(III) complexes are promising candidates for the development of luminescent chemosensors. Here, luminescent properties of Eu(III) carboxylate dibenzoylmethanates in interaction with ammonia and amine vapors have been studied. Quantitative measurements of the optical response showed that with an increase of the analyte concentration in the range of 3–330 ppm, an increase in the luminescence intensity of europium(III) is observed. The reversibility of the luminescent response is established. Quantum chemical calculations showed that ammonia and amine molecules enter not in the Eu(III) coordination sphere, but forms hydrogen bonds between analyte molecules and carbonyl groups of the dibenzoylmethanate ion. Moreover, analyte changes ligand coordination mode: there occur the transformation from tridentate-bridging-cyclic coordination of carboxyl groups to bidentate-bridging which leads to decreasing in the efficiency of cross-relaxation of lanthanide ions. The obtained “turn-on” chemosensory polymer materials are promising for environmental monitoring and food safety testing.

Characterization of the Coordination and Solvation Dynamics of Solvated Systems─Implications for the Analysis of Molecular Interactions in Solutions and Pure H2O.

The characterization of solvation shells of atoms, ions, and molecules in solution is essential to relate solvation properties to chemical phenomena such as complex formation and reactivity. Different definitions of the first-shell coordination sphere from simulation data can lead to potentially conflicting data on the structural properties and associated ligand exchange dynamics. The definition of a solvation shell is typically based on a given threshold distance determined from the respective solute-solvent pair distribution function g(r) (i.e., GC). Alternatively, a nearest neighbor (NN) assignment based on geometric properties of the coordination complex without the need for a predetermined cutoff criterion, such as the relative angular distance (RAD) or the modified Voronoi (MV) tessellation, can be applied. In this study, the effect of different NN algorithms on the coordination number and ligand exchange dynamics evaluated for a series of monatomic ions in aqueous solution, carbon dioxide in aqueous and dichloromethane solutions, and pure liquid water has been investigated. In the case of the monatomic ions, the RAD approach is superior in achieving a well separated definition of the first solvation layer. In contrast, the MV algorithm provides a better separation of the NNs from a molecular point of view, leading to better results in the case of solvated CO2. When analyzing the coordination environment in pure water, the cutoff-based GC framework was found to be the most reliable approach. By comparison of the number of ligand exchange reactions and the associated mean ligand residence times (MRTs) with the properties of the coordination number autocorrelation functions, it is shown that although the average coordination numbers are sensitive to the different definitions of the first solvation shell, highly consistent estimates for the associated MRT of the solvated system are obtained in the majority of cases.

Synthesis and characterization of the Cu(II)-quinoxaline, 3,3-thiodipropionate mixed ligand coordination polymer for the removal of methylene blue from aqueous solution

Herein, we report the removal of methylene blue (MB) dye from water by Cu(II)-quinoxaline, 3,3-thiodipropionate mixed ligand coordination polymer synthesized as [Cu2(TDPH)4(QNX)]·DMF. Characterization of the synthesized material was carried out by elemental analysis, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), BET surface area analysis, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and UV–visible spectroscopy. ν(C = O) and ν(C–O) absorption bands were detected at 1735 and 1093 cm−1, respectively, while ν(C = N) was observed at 1608 cm−1 in the synthesized compound. The synthesized material was used as adsorbent for the removal of MB dye from aqueous solution using batch adsorption technique. The maximum adsorption capacity was found to be 7.9745 mg/g, while the maximum decolorization efficiency for the adsorbent was observed to be 99.04%. Adsorption data from the study fitted the Langmuir isotherm with an R 2 value of 0.9815, while the adsorption kinetics followed the pseudo-second-order kinetic model with an R 2 value of 0.9953. The results obtained from the study, therefore showed that the synthesized materials could be used as adsorbents in wastewater treatment.

Zn(II) and Cd(II) complexes with 3-furoic acid. Exploring the role of C[sbnd]H···O interactions

Hydrogen bond interactions implying CH groups and O atoms either from carboxylate, carbonyl, or ether functionalities have been demonstrated to play a pivotal role in the packing of crystal structures. To analyze and compare their significance in metal carboxylate complexes with pyridine derivatives (dPy), we have synthesized four Zn(II) and Cd(II) compounds containing 3-furoic acid (3-FA) and two different pyridine ligands (isonicotinamide, isn; and 4-acetylpyridine, 4-acpy) affording two isostructural dimeric complexes [Zn(μ-3-FA)(3-FA)(isn)2]2 (1) and [Cd(μ-3-FA)(3-FA)(isn)2]2 (2), one dimeric paddle-wheel [Zn(μ-3-FA)2(4-acpy)]2 (3), and one monomer [Cd(3-FA)2(4-acpy)2(OH2)] (4). Their crystal structures have been elucidated and studied in detail, showing variable coordination modes of the 3-FA ligand and different coordination numbers. The CH···O interactions analysis of the four complexes in combination with their previously reported analogs with 2-furoic (2-FA) and piperonylic (HPip) acids, highlights the importance of such associations in the final crystal arrangement benefitting from cooperative effects. Finally, the photophysical properties in MeOH solution of 1–4 have been analyzed and their quantum yields calculated and compared.

Influences of coordination structures on the electronic properties of Zn ion and the adsorption of O-containing and S-containing molecules

Ligands in minerals have an important effect on the chemical properties of metal ions. The electronic properties of Zn ions formed by O and S ligands have been studied using density functional theory (DFT), and the interaction strength between O and S-containing molecules and Zn ions has been analyzed. The results show that the electronic properties of Zn ions may be influenced by the type of ligands, the number of ligands, and the distance between ligands and Zn ions. The adsorption capacity of zinc ions decreased with an the in increase ligand coordination number, but increased with an increase in the distance between the ligand and zinc ion. The adsorption of O- and S-containing molecules on sphalerite smithsonite and hemimorphite was then studied. It was indicated that O-containing molecules had a strong collecting ability for sphalerite, smithsonite and hemimorphite, but the adsorption capacity of S-containing molecules was weaker than that of O-containing molecules. The influence of water molecules on the adsorption behavior of O- and S-containing molecules was studied. The results of the calculations show that the relationship between H2O and O-containing sulfur-containing molecules is competitive adsorption on the surface of sphalerite smithsonite and hemimorphite. Adsorption of water molecules can reduce the adsorption energy of O-containing and S-containing molecules on mineral surfaces.

Optimizing hexafluoro-2,4-pentanedione based Eu(III) complexes: A comprehensive study on the synthesis, spectroscopic characterization with Judd-Ofelt calculation

Utilizing Europium(III), compounds having eight-coordination geometry of the type [Eu(hfpd)3L2] and [Eu(hfpd)3Lʹ] were synthesized [hfpd = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, L = tri-octyl-phosphine oxide (TOPO) and Lʹ = 1,10-phenanthroline (phen), Neocuproine (neoc) and bathocuproine (bathoc)] and characterized through elemental, FTIR, CV and Proton-NMR techniques. Photoluminescent studies and quantum yield helped to understand the role of coordination environment and ligands on the emission intensity. Luminescence decay curves served as the basis for calculating the excited-state lifetimes of the synthesized complexes. JO parameters (Ω2, Ω4) and asymmetry ratio were also evaluated to study the coordination environment around the central metal ion. Solid-state complexes exhibited an intense, monochromatic bright red emission, as characterized by its Commission International de l’Eclairage (CIE) color coordinates. Owing to red luminescence and semi-conducting properties, these Eu(III) complexes can be employed as red-emitting material for fabricating lighting devices.

Phosphonate-based iron complex for a cost-effective and long cycling aqueous iron redox flow battery

A promising metal-organic complex, iron (Fe)-NTMPA2, consisting of Fe(III) chloride and nitrilotri-(methylphosphonic acid) (NTMPA), is designed for use in aqueous iron redox flow batteries. A full-cell testing, where a concentrated Fe-NTMPA2 anolyte (0.67 M) is paired with a Fe-CN catholyte, demonstrates exceptional cycling stability over 1000 charge/discharge cycles, and noteworthy performances, including 96% capacity utilization, a minimal capacity fade rate of 0.0013% per cycle (1.3% over 1,000 cycles), high Coulombic efficiency and energy efficiency near 100% and 87%, respectively, all achieved under a current density of 20 mA·cm-². Furthermore, density functional theory unveils two potential coordination structures for Fe-NTMPA2 complexes, improving the understanding between the ligand coordination environment and electron transfer kinetics. When paired with a high redox potential Fe-Dcbpy/CN catholyte, 2,2′-bipyridine-4,4′-dicarboxylic (Dcbpy) acid and cyanide (CN) ligands, Fe-NTMPA2 demonstrates a notably elevated cell voltage of 1 V, enabling a practical energy density of up to 9 Wh/L.

Синтез и характеристика четырех новых карбоксилатных комплексов меди(II) с фрагментом адамантана

The paper considers the results of synthesis, elemental analysis, and spectroscopic methods (IR, UV-vis) of four new copper(II) complexes with anions of 1-adamantanecarboxylic acid, 4-(1-adamantyl)benzoic acid and their L-valine derivatives as ligands. Based on the obtained spectral data, the authors assumed the biyaderic structure of these complexes with bidentate coordination of carboxylate ligands of the type [Cu2+2(RCOO-)4(H2O)2], with RCOO-as carboxylate ligands. The synthesised compounds are of interest as low toxic potential therapeutic agents with anti-inflammatory and anti-cancer activity.

The tightest self-assembled ruthenium metal-organic framework combined with proximity hybridization for ultrasensitive electrochemiluminescence analysis of paraquat.

Metal-organic frameworks (MOFs), as porous materials, have great potential for exploring high-performance electrochemiluminescence (ECL) probes. However, the constrained applicability of MOFs in the realm of ECL biosensing is primarily attributed to their inadequate water stability, which consequently impairs the overall ECL efficiency. Herein, we developed a competitive ECL biosensor based on a novel tightest structural ruthenium-based organic framework emitter combining the proximity hybridization-induced catalytic hairpin assembly (CHA) strategy and the quenching effect between the Ru-MOF and ferrocene for detecting paraquat (PQ). Through a simple hydrothermal synthesis strategy, ruthenium and 2,2'-bipyrimidine (bpm) are head-to-head self-assembled to obtain a novel tightest structural Ru-MOF. Due to the metal-ligand charge-transfer (MLCT) effect between ruthenium and the bpm ligand and the connectivity between the internal chromophore units, the Ru-MOF exhibits strong ECL emissions. Meanwhile, the coordination-driven Ru-MOF utilizes strong metal-organic coordination bonds as building blocks, which effectively solves the problem of serious leakage of chromophores caused by water solubility. The sensitive analysis of PQ is realized in the range of 1 pg/mL to 1 ng/mL with a detection limit of 0.352 pg/mL. The tightest structural Ru-MOF driven by the coordination of ruthenium and bridging ligands (2,2'-bipyrimidine, bpm) provides new horizons for exploring high-performance MOF-based ECL probes for quantitative analysis of biomarkers.

Synthesis and characteristics of four new copper(II) carboxylate complexes with adamantane fragment

The paper considers the results of synthesis, elemental analysis, and spectroscopic methods (IR, UV-vis) of four new copper(II) complexes with anions of 1-adamantanecarboxylic acid, 4-(1-adamantyl)benzoic acid and their L-valine derivatives as ligands. Based on the obtained spectral data, the authors assumed the biyaderic structure of these complexes with bidentate coordination of carboxylate ligands of the type [Cu2+2(RCOO-)4(H2O)2], with RCOO-as carboxylate ligands. The synthesised compounds are of interest as low toxic potential therapeutic agents with anti-inflammatory and anti-cancer activity.

Fabrication of Self‐Expanding Metal–Organic Cages Using a Ring‐Openable Ligand

AbstractMetal–organic cages (MOCs), which are formed via coordination‐driven assembly, are being extensively developed for various applications owing to the utility of their accessible molecular‐sized cavity. While MOC structures are uniquely and precisely predetermined by the metal coordination number and ligand configuration, tailoring MOCs to further modulate the size, shape, and chemical environment of the cavities has become intensively studied for a more efficient and adaptive molecular binding. Herein, we report self‐expanding MOCs that exhibit remarkable structural variations in cage size and flexibility while maintaining their topology. A cyclic ligand with an oligomeric chain tethering the two benzene rings of stilbene was designed and mixed with RhII ions to obtain the parent MOCs. These MOCs were successfully transformed into expanded MOCs via the selective cleavage of the double bond in stilbene. The expanded MOCs could effectively trap multidentate N‐donor molecules in their enlarged cavity, in contrast to the original MOCs with a narrow cavity. As the direct synthesis of expanded MOCs is impractical because of the entropically disfavored structures, self‐expansion using ring‐openable ligands is a promising approach that allows precision engineering and the production of functional MOCs that would otherwise be inaccessible.

Fabrication of Self-Expanding Metal-Organic Cages Using a Ring-Openable Ligand.

Metal-organic cages (MOCs), which are formed via coordination-driven assembly, are being extensively developed for various applications owing to the utility of their accessible molecular-sized cavity. While MOC structures are uniquely and precisely predetermined by the metal coordination number and ligand configuration, tailoring MOCs to further modulate the size, shape, and chemical environment of the cavities has become intensively studied for a more efficient and adaptive molecular binding. Herein, we report self-expanding MOCs that exhibit remarkable structural variations in cage size and flexibility while maintaining their topology. A cyclic ligand with an oligomeric chain tethering the two benzene rings of stilbene was designed and mixed with RhII ions to obtain the parent MOCs. These MOCs were successfully transformed into expanded MOCs via the selective cleavage of the double bond in stilbene. The expanded MOCs could effectively trap multidentate N-donor molecules in their enlarged cavity, in contrast to the original MOCs with a narrow cavity. As the direct synthesis of expanded MOCs is impractical because of the entropically disfavored structures, self-expansion using ring-openable ligands is a promising approach that allows precision engineering and the production of functional MOCs that would otherwise be inaccessible.