The Reticular Chemist

HP35 Protein in the Mesopore of MIL-101(Cr) MOF: A Model to Study Cotranslocational Unfolding

The immobilization of enzymes in metal-organic framework (MOF) cages is important in biotechnology. In this context, the mechanism of translocation of proteins through the cavities of the MOF and the roles played by confinement and MOF chemistry in giving rise to stable protein intermediates that are otherwise transiently populated in the physiological environment are important questions to be addressed. These unexplored aspects are examined with villin headpiece (HP35) as a model protein confined within a mesopore of MIL-101(Cr) using molecular dynamics simulations. At equilibrium, the protein is located farther from the center of the cavity and closer to the MOF surface. Molecular interactions with the MOF partially unfold helix-1 at its N-terminus. Umbrella sampling simulations inform the range of conformations that HP35 undertakes during translocation from one cavity to another and associated changes in free energy. Relative to its equilibrium state within the cavity, the free energy barrier for the unfolded protein at the cage window is estimated to be 16 kcal/mol. This study of MOF-based protein conformation can also be a general approach to observing intermediates in folding-unfolding pathways.

Structural Chemistry of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) are a subclass of reticular structures based on tetrahedral four-connected networks of zeolites and minerals. They are composed of transition-metal ions and imidazolate-type linkers, and their pore size and shape, surface area, and functionality can be precisely controlled. Despite their potential, two questions remain unanswered: how to synthesize more diverse ZIF structures and how ZIFs differentiate from other crystalline solids. In other words, how can we use our understanding of their unique structures to better design and synthesize ZIFs? In this Review, we first summarize the methods for synthesizing a wide range of ZIFs. We then review the crystal structure of ZIFs and describe the relationship between their structure and properties using an in-depth analysis. We also discuss several important and intrinsic features that make ZIFs stand out from MOFs and discrete molecular cages. Finally, we outline the future direction for this class of porous crystals.

Bipolar Molecular Torque Wrench Modulates the Stacking of Two-Dimensional Metal-Organic Framework Nanosheets

The precise modulation of nanosheet stacking modes introduces unforeseen properties and creates momentous applications but remains a challenge. Herein, we proposed a strategy using bipolar molecules as torque wrenches to control the stacking modes of 2-D Zr-1,3,5-(4-carboxylphenyl)-benzene metal-organic framework (2-D Zr-BTB MOF) nanosheets. The bipolar phenyl-alkanes, phenylmethane (P-C1) and phenyl ethane (P-C2), predominantly instigated the rotational stacking of Zr-BTB-P-C1 and Zr-BTB-P-C2, displaying a wide angular distribution. This included Zr-BTB-P-C1 orientations at 0, 12, 18, and 24° and Zr-BTB-P-C2 orientations at 0, 6, 12, 15, 24, and 30°. With reduced polarity, phenyl propane (P-C3) and phenyl pentane (P-C5) introduced steric hindrance and facilitated alkyl hydrophobic interactions with the nanosheets, primarily resulting in the modulation of eclipsed stacking for Zr-BTB-P-C3 (64.8%) and Zr-BTB-P-C5 (93.3%) nanosheets. The precise angle distributions of four Zr-BTB-P species were in agreement with theoretical calculations. The alkyl induction mechanism was confirmed by the sequential guest replacement and 2-D 13C-1H heteronuclear correlation (HETCOR). In addition, at the single-particle level, we first observed that rotational stacked pores exhibited similar desorption rates for xylene isomers, while eclipsed stacked pores showed significant discrepancy for xylenes. Moreover, the eclipsed nanosheets as stationary phases exhibited high resolution, selectivity, repeatability, and durability for isomer separation. The universality was proven by another series of bipolar acetate-alkanes. This bipolar molecular torque wrench strategy provides an opportunity to precisely control the stacking modes of porous nanosheets.

Maneuvering Applications of Covalent Organic Frameworks via Framework-Morphology Modulation

Translating the performance of covalent organic frameworks (COFs) from laboratory to macroscopic reality demands specific morphologies. Thus, the advancement in morphological modulation has recently gained some momentum. A clear understanding of nano- to macroscopic architecture is critical to determine, optimize, and improve performances of this atomically precise porous material. Along with their chemical compositions and molecular frameworks, the prospect of morphology in various applications should be discussed and highlighted. A thorough insight into morphology versus application will help produce better-engineered COFs for practical implications. 2D and 3D frameworks can be transformed into various solids such as nanospheres, thin films, membranes, monoliths, foams, etc., for numerous applications in adsorption, separation photocatalysis, the carbon dioxide reduction, supercapacitors, and fuel cells. However, the research on COF chemistry mainly focuses on correlating structure to property, structure to morphology, and structure to applications. Here, critical insights on various morphological evolution and associated applications are provided. In each case, the underlying role of morphology is unveiled. Toward the end, a correlation between morphology and application is provided for the future development of COFs.

Three-Dimensional Covalent Organic Framework with Topology for Drug Delivery

Three-dimensional (3D) covalent organic frameworks (COFs) exemplify a new generation of crystalline extended solids with intriguing structures and unprecedented porosity. Notwithstanding substantial scope, the reticular synthesis of 3D COFs from pre-designed building units leading to new network topologies yet remains a demanding task owing to the shortage of 3D building units and inadequate reversibility of the linkages between the building units. In this work, by linking a tetragonal prism (8-connected) node with a square planar (4-connected) node, we report the first 3D COF with scu-c topology. The new COF, namely, TUS-84, features a two-fold interpenetrated structure with well-defined porosity and a Brunauer-Emmett-Teller surface area of 679 m² g^-1. In drug delivery applications, TUS-84 shows efficient drug loading and sustained release profile.

Gate Opening without Volume Change Triggers Cooperative Gas Interactions, Underpins an Isotherm Step in Metal-Organic Frameworks

Three halogenated metal-organic frameworks (MOFs) reported recently exhibited a second step in their CO₂ gas adsorption isotherms. The emergence of halogen-bonding interactions beyond a threshold gas pressure between the framework halogen and the CO₂ guest was conjectured to be the underlying reason for the additional step in the isotherm. Our investigation employing periodic density functional theory calculations did not show significant interactions between the halogen and CO₂ molecules. Further, using a combination of DFT-based ab initio molecular dynamics and grand canonical Monte Carlo simulations, we find that the increased separation of framework nitrate pairs facing each other across the pore channel enables the accommodation of an additional CO₂ molecule which is further stabilized by cooperative interactions─an observation that facilely explains the second isotherm step. The increased separation between the nitrate groups can occur without any lattice expansion, consistent with experiments. The results point to a structural feature to achieve this isotherm step in MOFs that neither possess large pores nor exhibit large-scale structural changes such as breathing.

Accessing 14-Connected Nets: Continuous Breathing, Hydrophobic Rare-Earth Metal Organic Frameworks Based on 14-c Hexanuclear Clusters with High Affinity for Non-Polar Vapors

Highly connected metal organic frameworks (MOFs) in which at least one building block has connectivity higher than twelve are very rare and much desirable. We report here the first examples of isostructural 14-connected MOFs, RE-frt-MOF-1, constructed from the assembly of 14-c hexanuclear rare-earth clusters, [RE₆(μ₃-X)₈(COO)₁₂]^2- (RE: Y³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Yb³⁺ and X: OH^-/F^-) with a tritopic carboxylate-based organic linker. This linker serves as a 3-c and 4-c organic node resulting in the formation of a unique, trinodal (3,4,14)-c framework. RE-frt-MOF-1 are stable in air and alkaline aqueous solutions and show an intriguingly continuous, reversible breathing behavior, between a wide and a narrow-pore phase, upon guest removal. Crystallinity is retained during breathing, and single-crystal X-ray diffraction shed light into the associated structural transformation. Vapor sorption studies performed on Y-frt-MOF-1 revealed a high affinity for non-polar vapors such as n-hexane, cyclohexane, and benzene, displaying type I isotherms with high uptake at low relative pressures (<10^-3 p/p₀), associated with the hydrophobic nature of the 1D channels and also with their rhombic shape. In contrast, polar vapors such as acetonitrile and ethanol show type V isotherms due to favorable vapor-vapor interactions. Notably these vapors, except cyclohexane, trigger the transition from the narrow to the wide pore phase, accompanied by a remarkable increase in uptake, reaching 70.6, 109, 100.4, and 87.7% for n-hexane, benzene, acetonitrile, and ethanol, respectively.

2D Covalent Organic Frameworks: From Synthetic Strategies to Advanced Optical-Electrical-Magnetic Functionalities

Covalent organic frameworks (COFs), an emerging class of organic crystalline polymers with highly oriented structures and permanent porosity, can adopt 2D or 3D architectures depending on the different topological diagrams of the monomers. Notably, 2D COFs have particularly gained much attention due to the extraordinary merits of their extended in-plane π-conjugation and topologically ordered columnar π-arrays. These properties together with high crystallinity, large surface area, and tunable porosity distinguish 2D COFs as an ideal candidate for the fabrication of functional materials. Herein, this review surveys the recent research advances in 2D COFs with special emphasis on the preparation of 2D COF powders, single crystals, and thin films, as well as their advanced optical, electrical, and magnetic functionalities. Some challenging issues and potential research outlook for 2D COFs are also provided for promoting their development in terms of structure, synthesis, and functionalities.

Synergetic Dielectric and Magnetic Losses of a Core-Shell Co/MnO/C Nanocomplex toward Highly Efficient Microwave Absorption

High-performance microwave-absorbing materials (MAMs) derived from metal-organic frameworks (MOFs) have attracted considerable attention due to their tunable chemical composition and microstructure. In this contribution, a core-shell-structured Co/MnO/C nanocomplex was prepared using a CoMn-MIL MOF by a facile hydrothermal synthesis and subsequent pyrolysis process. The optimal microwave absorption (MA) property of the as-prepared Co/MnO/C nanocomplex was achieved by the regulation of the Co²⁺/Mn²⁺ molar ratio. The minimum reflection loss (RL_min) of the Co/MnO/C-31 nanocomplex was low to -55.0 dB at 16.2 GHz with a thickness of 1.49 mm, and the effective absorption bandwidth (EAB) was high to 5.95 GHz (12.05-18 GHz) at a thickness of 1.8 mm. The mixed-metal nanocomplex with the core-shell structure exhibited outstanding MA performance, corresponding to the synergetic effect of the magnetic and dielectric loss. It provides a high efficiency strategy for rendering low reflection loss and broad EAB to high-performance MAMs.

Hierarchical Co/CoO/FeO/C nanocomplex derived from Co(OH)2@NH2-MIL-88 to aid highly efficient microwave absorption

Facile preparation of a flake-like Co/CoO/FeO/C nanocomplex employing Co(OH)2 on NH2-MIL-88B through one-step pyrolysis. The nanocomplex can efficiently scatter microwaves.

De Novo Fabrication of Large-Area and Self-Standing Covalent Organic Framework Films for Efficient Separation

Covalent organic frameworks (COFs) have aroused extensive attention from various fields owing to their numerous advantages, including permanent porosity, high crystallinity, strong robustness, and well-ordered channels. However, the poor processability of the crystallite powder has greatly impeded their further utilization in many advanced devices and frontier areas. In this work, we fabricate a series of COF films using an interfacial polymerization strategy at a liquid-liquid interface under ambient conditions. The as-synthesized freestanding films are continuous, flexible, and defect-free and have large areas of up to 4 × 6 cm². In addition, the pore sizes of these COF films can be well controlled based on the principle of reticular chemistry. These films exhibit high chemical stability even in acidic and basic aqueous solutions. More significantly, the highly robust COF films can serve as a nanofiltration membrane for efficient separation of pollutant molecules with different dimensions. These films show high selectivity for the separation of mixed molecule feed and excellent recyclability without a significant loss in the rejection rate.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

Continuous Breathing Rare-Earth MOFs Based on Hexanuclear Clusters with Gas Trapping Properties

Guest responsive porous materials represent an important and fascinating class of multifunctional solids that have attracted considerable attention in recent years. An understanding of how these structures form is essential toward their rational design, which is a prerequisite for the development of tailor-made materials for advanced applications. We herein report a novel series of stable rare-earth (RE) MOFs that show a rare continuous breathing behavior and an unprecedented gas-trapping property. We used an asymmetric 4-c tetratopic carboxylate-based organic ligand that is capable of affording highly crystalline materials upon controlled reaction with RE cations. These MOFs, denoted as RE-thc-MOF-1 (RE: Y³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, and Er³⁺), feature hexanuclear RE₆ clusters that display a highly unusual connectivity and serve as unique 8-c hemi-cuboctahedral secondary building block, resulting in a new (3,3,8)-c thc topology. Extensive single-crystal to single-crystal structural analyses coupled with detailed gas (N₂, Ar, Kr, CO₂, CH₄, and Xe) and vapor (EtOH, CH₃CN, C₆H₆, and C₆H₁₄) sorption studies, supported by accurate theoretical calculations, shed light onto the unique swelling behavior. The results reveal a synergistic action involving steric effects, associated with coordinated solvent molecules and 2-fluorobenzoate (2-FBA) nonbridging ligands, as well as cation-framework electrostatic interactions. We were able to probe the individual role of the coordinated solvent molecules and 2-FBA ligands and found that both cooperatively control the gas-breathing and -trapping properties, while 2-FBA controls the vapor adsorption selectivity. These findings provide unique opportunities toward the design and development of tunable RE-based flexible MOFs with tailor-made properties.

Conductive Metallophthalocyanine Framework Films with High Carrier Mobility as Efficient Chemiresistors

The poor electrical conductivity of two-dimensional (2D) crystalline frameworks greatly limits their utilization in optoelectronics and sensor technology. Herein, we describe a conductive metallophthalocyanine-based NiPc-CoTAA framework with cobalt(II) tetraaza[14]annulene linkages. The high conjugation across the whole network combined with densely stacked metallophthalocyanine units endows this material with high electrical conductivity, which can be greatly enhanced by doping with iodine. The NiPc-CoTAA framework was also fabricated as thin films with different thicknesses from 100 to 1000 nm by the steam-assisted conversion method. These films enabled the detection of low-concentration gases and exhibited remarkable sensitivity and stability. This study indicates the enormous potential of metallophthalocyanine-based conductive frameworks in advanced stand-off chemical sensors and provides a general strategy through tailor-make molecular design to develop sensitive and stable chemical sensors for the detection of low-concentration gases.

Tuning the Structural Flexibility for Multi-Responsive Gas Sorption in Isonicotinate-Based Metal-Organic Frameworks

Flexible metal-organic frameworks (MOFs) are of high interest as smart programmable materials for gas sorption due to their unique structural changes triggered by external stimuli. Owing to this property, which leads to opportunities such as maximizing deliverable gas capacity, flexible MOFs sometimes offer more advantages in sorption applications compared to their more rigid counterparts. Herein, we elucidate the effect of transition metal identity of a series of isonicotinate-based flexible MOFs, M(4-PyC)₂ [M═Mg, Mn, and Cu; 4-PyC = 4-pyridine carboxylic acid] on the structural dynamic response to different gases (C₂H₄, C₂H₆, Xe, Kr, and SO₂). Isotherms at different temperatures show that C₂H₄, C₂H₆, and Xe can form sufficiently strong interactions with both Mg(4-PyC)₂ and Mn(4-PyC)₂ frameworks resulting in gate-opening behavior due to the rotation of the linker's pyridine ring, while Kr cannot induce this phenomenon for the two MOFs under the measured conditions. In contrast, the gate-opening behavior occurs for Cu(4-PyC)₂ solely in the presence of C₂H₄, and no other measured gas, due to the open metal sites of Cu centers. In comparison, SO₂, a strong polar molecule, triggers the gate-opening effect in all three MOFs. Interestingly, a shape memory effect is observed for Cu(4-PyC)₂ during the second SO₂ sorption cycle. When comparing the different gate-opening pressures of each gas, we observed that the structural flexibility of the three frameworks is highly sensitive to the chemical hardness of the Lewis acidic metal ions (Mg²⁺ > Mn²⁺ > Cu²⁺). As a result, the gate opening behavior is observed at lower pressures for the MOFs containing weaker M-N bonds (harder metal ions), with the exception of Cu(4-PyC)₂ toward C₂H₄. These observations reveal that different transition metals can be used to finely control the structural flexibility of the frameworks.