Metal-organic Nanocapsules Research Articles

Redox-Controlled Self-Assembly of Vanadium-Seamed Hexameric Pyrogallol[4]Arene Nanocapsules.

Here we report the controlled self-assembly of vanadium-seamed metal-organic nanocapsules with specific metal oxidation state distributions. Three supramolecular assemblies composed of the same numbers of components including 24 metal centers and six pyrogallol[4]arene ligands were constructed: a VIII24L6 capsule, a mixed-valence VIII18VIV6L6 capsule, and a VIV24L6 capsule. Crystallographic studies of the new capsules reveal their remarkable structural complexity and geometries, while marked differences in metal oxidation state distribution greatly affect the photoelectric conversion properties of these assemblies. This work therefore represents a significant step forward in the construction of intricate metal-organic architectures with tailored structure and functionality.

Toward MOF@Polymer Core-Shell Particles: Design Principles and Potential Applications.

ConspectusCompositing MOFs with polymers brings out the best properties of both worlds. The solubility and excellent mechanical properties of polymers endow the brittle, powdery MOFs with enhanced processability, thereby enriching their functions as solid sorbents, filters, membranes, catalysts, drug delivery vehicles, and so forth. While most MOF-polymer composites are random mixtures of two materials with little control over their fine structures, MOF@polymer core-shell particles have recently emerged as a new platform for precise composite design. The well-defined polymer coating can keep the rich pore characteristics of the MOF intact while furnishing the MOF with new properties such as improved dispersibility in various media, tunable surface energy, enhanced chemical stability, and regulated guest diffusion. Nevertheless, the structural and chemical complexity of MOFs poses a grand challenge to the development of a generalizable and feasible strategy for constructing MOF@polymer. Examples in the literature that showcase the presence of a well-defined polymer shell on the MOF with fully reserved porosity are rare. Moreover, methods for coating MOFs with condensation polymers (e.g., polyimide, polysulfone) are severely underexplored, despite their clear potential as membrane materials. In this Account, we present our group's effort over the past 4 years on the synthesis and applications of MOF@polymer composites. We first described a highly generalizable surface polymerization method that utilizes the rapid physisorption of a random copolymer (RCP) to carry initiating groups to the MOF surfaces. Subsequent controlled radical polymerization led to the formation of a uniform methacrylate or styrenic polymer on the MOF with tunable thickness and composition. To utilize the properties of condensation polymers, we pioneered the covalent grafting of polyimide (PI) brushes to UiO-66-NH2 surfaces. In addition, to circumvent the need for a covalent anchoring group, we further developed an MOF surface grafting method based on mechanical linkage. Instead of connecting to the ligand, polyimide (PI) oligomer was linked to a functionalized linear polymer physically entangled within an MOF, thus realizing surface grafting with PI. Alternatively, PIs, polysulfone (PSF), and polycarbonate (PC) can also be grafted to various MOF surfaces through a metal-organic nanocapsule (MONC)-mediated method using a combination of electrostatic interaction and coordination bonds. To find a rapid and low-cost surface coating method suitable for commercialization, a new approach called non-solvent-induced surface-aimed deposition (NISAP) was developed. The action of the solvent phase separation drives dianhydrides and polyamines to the MOF surface, thus realizing accelerated polymerization and the rapid formation of a polymer coating on the MOF. Finally, we provided an overview of the unique properties and potential applications of MOF@polymer composites, including improved stability, MMMs, porous liquids (PLs), and immobilizing homogeneous catalysts. We hope that this Account can inspire more researchers to further develop and optimize the synthetic strategies for MOF@polymer and uncover its full application potential.

Hierarchical 2D honeycomb-like network from barium-seamed nanocapsules.

We report on a two-dimensional hexagonal "honeycomb" network comprising barium-seamed metal-organic nanocapsules involving a hexameric assembly of pyrogallol[4]arene ligands. The incorporated barium ions act as spacers to generate a solvent-accessible void, hierarchical self-assembly having an individual void volume near 13 000 Å3. This work illustrates the surprising chemistry that remains to be discovered by integrating large or classically non-reactive metal ions within supramolecular assemblies, networks, and organic nanocapsules.

The extra-large calixarene-based MOFs-derived hierarchical composites for photocatalysis of dye: Facile syntheses and contribution of carbon species

The problem of water pollution due to organic dyes has caused serious disturbance and safety to the ecosystem and people's health. In this work, series of Cu/C composites were gained by calcining the synthesized three calkylpyrogallol[4]arenes (PgCs) metal–organic nanocapsules (MONCs) precursors. The photocatalytic optimizations of Cu/C composites were estimated by the disintegration of methyl violet (MV). Incorporated with DRS, Raman, BET, SEM and TEM, the affective factor of calcination temperature and time on catalytic performances of Cu/C-800–4, Cu/C-900–4, Cu/C-1000–4, Cu/C-1100–4, Cu/C-1000–2 and Cu/C-1000–8 composites were discussed. Furthermore, photocatalytic degradation of the Cu@C-1000–4 composites obtained at 1000 °C and 4 h showed highest photodegradation properties close to 96.30% for MV dye. The plausible mechanism displayed by of Cu/C-1000–4 composite for the degradation of dye MV was analyzed by the LC-MS methodology.

Cobalt-seamed C-methylpyrogallol[4]arene nanocapsules-derived magnetic carbon cubes as advanced adsorbent toward drug contaminant removal

Rational designing of carbon-based adsorbent precursors is an important way to enhance adsorption performance. In this work, fascinating Co@C composites have been designed and fabricated using a new cobalt-seamed metal–organic nanocapsule (Co-MONC), which exhibits incredibly high adsorption capacity for chlorpromazine (CPZ) removal from aquatic environments. Benefiting from its interconnected micro- and mesoporous structures, well-dispersed and highly available Co adsorption active sites, the optimal Co@C-900 exhibits ultrahigh CPZ adsorption capacity (1039.53 mg g−1) and excellent recycling ability, outperforming the reported porous materials and commercial activated carbon. Moreover, the adsorption behaviors of CPZ for the composite have been comprehensively investigated and the plausible adsorption mechanism was proposed. Apart from this, the interaction mechanism between exposed Co adsorption active sites and CPZ have been suggested with the aid of density functional theory (DFT) calculations. The presented investigation demonstrated a facile and designable strategy for the fabrication of highly effectively adsorbents for organic contaminant removal.

Coordination Polymers Constructed from Pyrogallol[4]arene-Assembled Metal–Organic Nanocapsules

Coordination polymers, commonly known as infinite crystalline lattices, are versatile networks and have diverse potential applications in the fields of gas storage, molecular separation, catalysis, optics, and drug delivery, among other areas. Secondary building blocks, mainly incorporating rigid polydentate organic linkers and metal ions or clusters, are commonly employed to construct coordination polymers. Recently, novel building blocks such as coordination polyhedra have been utilized as metal nodes to fabricate coordination polymers. Benefiting from the rigid porous structure of the coordination polyhedron, prefabricated designer "pores" can be incorporated in this type of coordinate polymer. In this Account, coordination polymers built by pyrogallol[4]arene-assembled metal-organic nanocapsules are summarized. This class of metal-organic nanocapsule possesses the following advantages that make them excellent candidates in the construction of coordination polymers: (i) Various geometrical shapes with different volumes of the inner cavities can be obtained from these capsules. Among them, the two main categories illustrated are dimeric and hexameric capsules, which comprise two and six pyrogallol[4]arenes units, respectively. (ii) A wide range of possible metal ions ranging from main group metals to transition metals and even lanthanides have been demonstrated to seam the capsules. Therefore, these coordination polymers can be endowed with fascinating functionalities such as magnetism, semiconductivity, luminescence, and radioactivity. (iii) Up to 24 metal ions have been successfully embedded on the surface of the nanocapsule, each a potential reaction site in the construction of coordination polymers, opening up pathways for the formation of multidimensional frameworks.In this Account, we focus primarily on the synthesis and the structural information on pyrogallol[4]arene-derived coordination polymers. Coordination polymers can be formed by introducing linkers with two coordination sites, using pyrogallol[4]arenes with coordination sites on the tail, or even via metal ions cross-linking with each other. Machine learning was recently developed to help us predict and screen the structures of the coordination polymers. With single crystal analysis in hand, detailed structural information provides a molecular-level perspective. Significantly, following the formation of coordination polymers, the overall shape and structure of the discrete metal-organic nanocapsules remains essentially unchanged, with full retention of the prefabricated pores. If a rigid linker is used to connect capsules, more than one lattice void with different volumes can be found within the framework. Thus, molecules with different sizes could potentially be encapsulated within these coordination polymers. In addition, flexible ligands can also be employed as linkers. For example, polymers have been employed as large linkers that transform the crystalline coordination polymers into polymer matrices, paving the way toward the synthesis of advanced functional materials. Overall, coordination polymers constructed with pyrogallol[4]arene-assembled metal-organic nanocapsules show wide diversity and tunability in structure and fascinating properties, as well as the promise of built-in functionality in the future.

Coating the Right Polymer: Achieving Ideal Metal-Organic Framework Particle Dispersibility in Polymer Matrixes Using a Coordinative Crosslinking Surface Modification Method.

This work describes the first generalizable method to modify various metal-organic framework (MOF) surfaces with polyimide, polysulfone, polycarbonate, and polymer of intrinsic microporosity-1 (PIM-1). The method first utilizes electrostatic adsorption to rapidly decorate positively charged MOF surfaces with a layer of negatively charged metal-organic nanocapsule, PgC5 Cu. After mixing with the polymer, the copper open metal sites on PgC5 Cu can coordinatively crosslink the polar functional groups on the surface polymer upon thermal activation thereby resulting in the immobilization of a uniform sub-10 nm polymer coating. We quantitatively analyzed the distribution of free path spacing between MOF particles and demonstrated that when the surface polymer matches the matrix polymer, the MOF dispersion was not only visually improved but also found to align perfectly with a theoretically predicted ideal dispersion model where no aggregation driving force was present.

Hierarchical Coordination Frameworks Based on Metal–Organic Dimeric Nanocapsules Comprising Praseodymium and Pyrogallol[4]arene

Hierarchical coordination frameworks based on a novel praseodymium(III)-seamed metal–organic nanocapsule comprising dimerized pyrogallol[4]arene units are synthesized, characterized, and studied. F...

Metal-Organic Nanocapsules with Functionalized s-Heptazine Ligands.

A metalloorganic capsule was synthesized where the ligand is a derivative of heptazine with three carboxylic groups that are coordinated to CuII cations, forming paddle-wheel motifs. Each nanocapsule is neutral, with 12 CuII centers and 8 ligands adopting a rhombicuboctahedron shape. It has almost 3 nm diameter, and the main intermolecular interactions in the solid are π··· π stacking between the C6N7 heptazine moieties. The nanocapsules can form monolayers deposited on graphite as observed by atomic force microscopy, which confirms their stability in solution.

A Barrel-Shaped Metal-Organic Blue-Box Analogue with Photo-/Redox-Switchable Behavior.

Donor-acceptor interactions are ubiquitous in the design and understanding of host-guest complexes. Despite their non-covalent nature, they can readily dictate the self-assembly of complex architectures. Here, a photo-/redox-switchable metal-organic nanocapsule is presented, which was assembled by using lanthanide ions and viologen building blocks, by relying on such donor-acceptor interactions. The potential of this unique barrel-shaped structure is highlighted for the encapsulation of suitable electron donors, akin to the well-investigated "blue-box" macrocycles. The light-triggered reduction of the viologen units has been investigated by single-crystal-to-single-crystal X-ray diffraction experiments, complemented by magnetic, optical, and solid-state electrochemical characterizations. Density functional theory (DFT) calculations were employed to suggest the most likely electron donor in the light-triggered reduction of the viologen-based ligand.

Construction of Polymeric Metal-Organic Nanocapsule Networks via Supramolecular Coordination-Driven Self-Assembly.

Polymeric metal-organic nanocapsule networks (polyMONCs), where metal-organic nanocapsules (MONCs) are connected by functional polymers, could possess the properties of traditional polymers and also retain the structures of MONCs. In this work, we constructed novel polyMONCs based on Mg-seamed pyrogallol[4]arene-containing MONCs through supramolecular coordination-driven self-assembly. The MONCs can be successfully polymerized using poly(ethylene glycol) as the linker, and the prepared polyMONCs can be further made into gels with self-healing properties and stimuli responsiveness. Advantageously, single crystals of MONCs cross-linked by ethylene glycol/diethylene glycol were obtained, giving us direct perspectives to mimic and investigate the self-assembly process of polyMONCs.

An Indium-Seamed Hexameric Metal-Organic Cage as an Example of a Hexameric Pyrogallol[4]arene Capsule Conjoined Exclusively by Trivalent Metal Ions.

A hexameric metal-organic nanocapsule is assembled from pyrogallol[4]arene units, which are stitched together with indium ions. This indium-seamed capsule is the first instance of a M24 L6 type hexameric coordination cage held together exclusively by trivalent metal ions. Explicitly, unlike previously reported pyrogallol[4]arene-based metal-seamed capsules, the current In3+ seamed capsule is entirely supported by O→In coordinate bonds. This work demonstrates the important proof of concept of the ability of pyrogallol[4]arene to react with metals in higher oxidation states to assemble into atomically-precise hexameric coordination cages. As such, these results open up exciting avenues toward the assembly of previously unanticipated metal-organic capsules, for example offering inspiration for tackling metals exhibiting high valence states such as in the lanthanide and actinide series.

Resveratrol-loaded core-shell nanostructured delivery systems: Cyclodextrin-based metal-organic nanocapsules prepared by ionic gelation

In this study, cyclodextrin metal-organic framework/chitosan (CD-MOF/CS) nanocapsules, which have a hydrophobic core and a hydrophilic shell, were fabricated as delivery systems for bioactive agents. The nanocapsules were prepared by electrostatic deposition of cationic chitosan onto the anionic CD-MOF core. The presence of the CS coating reduced the mean diameter and polydispersity index of the nanocapsules, which was attributed to their ability to inhibit particle aggregation. Moreover, the encapsulation efficiency of resveratrol within the nanocapsules increased appreciably after coating them with chitosan (from 66.5 to 91.3%). The chitosan coating was also shown to increase the antioxidant activity and photostability of the encapsulated resveratrol. Information about the binding interactions of the resveratrol with the nanocapsules was obtained using fluorescence spectroscopy, infrared spectroscopy, and thermal analysis. The new nanocapsules created in this study may have applications for the encapsulation, protection, and delivery of bioactive agents in food, supplement, and pharmaceutical applications.

Guiding solvothermal synthesis

Machine Learning Machine learning is rapidly revolutionizing computer-aided synthesis design, occasionally producing vivid use cases when the reaction parameters important for the synthesis are hidden in a complex chemical space. Xie et al. report a machine learning–assisted framework for the synthesis of metal-organic nanocapsules (MONCs), giant molecular building units potentially useful in different fields, based on predicting the crystallization propensity using experimental attempts as a training dataset. Machine-learning algorithms achieve prediction accuracies of more than 90%, considerably outperforming trained chemists, and the generated synthesis parameters direct solvothermal crystallization to new structures of MONCs. The proposed strategy for the discovery of new materials can be applied more broadly beyond MONCs. J. Am. Chem. Soc. 10.1021/jacs.9b11569 (2019).

Engineering plasticization resistant gas separation membranes using metal-organic nanocapsules.

Membrane technologies hold great potential for industrial gas separation. Nevertheless, plasticization, a common phenomenon that is responsible for the loss of gas pair selectivity and the decrease of membrane lifespan, is one of the top challenges withholding the deployment of advanced membrane materials in realistic applications. Here, we report a highly generalizable approach, that utilizes PgC5Cu, a copper metal–organic nanocapsule (MONC) containing 24 open metal sites (OMSs) as a multi-dentate node to coordinatively crosslink polymers. By adding merely 1–3 wt% of PgC5Cu, a wide range of carbonyl group-containing polymers can be effectively crosslinked. Through rigorous dissolution tests, molecular dynamic simulations, and in situ FT-IR spectroscopy, we qualitatively and quantitatively unveiled the coordinative binding nature at the polymer–MONC interface. As a result, we produced a series of composite membranes showing near complete plasticization resistance to CO2, C2H4, and C2H6 under high pressure with no loss of mechanical and gas transport properties.

Machine Learning Assisted Synthesis of Metal-Organic Nanocapsules.

Herein, we report machine learning algorithms by training data sets from a set of both successful and failed experiments for studying the crystallization propensity of metal-organic nanocapsules (MONCs). Among a variety of studied machine learning algorithms, XGBoost affords the highest prediction accuracy of >90%. The derived chemical feature scores that determine importance of reaction parameters from the XGBoost model assist to identify synthesis parameters for successfully synthesizing new hierarchical structures of MONCs, showing superior performance to a well-trained chemist. This work demonstrates that the machine learning algorithms can assist the chemists to faster search for the optimal reaction parameters from many experimental variables, whose features are usually hidden in the high-dimensional space.

Control of random self-assembly of pyrogallol[4]arene-based nanocapsule or framework

The controlled self-assembly of discrete metal-organic nanocapsules (MONCs), and metal-organic frameworks (MOFs) based on the MONCs are achieved. Specifically, the solvothermal reaction of nickel nitrate hexahydrate and C-methylpyrogallol[4]arene in mixed DMF/MeOH solution leads to the unexpected form of discrete nickel-seamed hexameric pyrogallol[4]arene MONCs, and MONC-based three-dimensional (3D) MOF. Notably, the latter MOF is constructed from the aforementioned nickel-seamed MONC building blocks and formate linkers in-situ generated from the hydrolysis of DMF solvent. Interestingly, introducing pyridine and formic acid in the reaction conditions leads to the controlled assemblies of the discrete MONC and MONC-based 3D MOF structures. Moreover, the variable-temperature magnetic susceptibilities of both the abovementioned compounds have been investigated, indicating typical antiferromagnetic interactions between the metal centers.

Biomimetic Self-Assembly of CoII-Seamed Hexameric Metal-Organic Nanocapsules.

A CoII18L6 hexameric metal-organic nanocapsule (MONC) has been prepared and characterized using biomimetic self-assembly as the synthetic methodology. Akin to the biological behavior of zinc-finger proteins' release, uptake, and electrophilic substitution of Zn2+ ions, the assembly of this novel MONC has been accomplished by employing three sequential processes: assembly of the framework, metal ion insertion, and metal exchange, resulting in the formation of the CoII18L6 hexameric MONC. In this work, inspired by the biological behavior of metalloproteins, rational control of multiple complex supramolecular self-assembly has been achieved.

Irreversible solid-state metal ion exchange in cobalt(II) metal–organic nanocapsules synthesized by green mechanochemical process

Irreversible solid-state conversion of Co(II) metal–organic nanocapsule [Co2(bda)2(bpy)2] (1), [bda2− = 2,2′-biphenyl dicarboxylate and bpy = 2,2′-bipyridine], prepared by mechanochemical procedure with empty pore and lozenge-shaped microcube morphology similar to [Cu2(bda)2(bpy)2]∙4H2O (2) analogue with agglomerated nanostructure morphology was studied. The structure of 1 is exactly the same as 2 and the metal ion has distorted octahedral coordination in both of them. In addition to the same structures of 1 and 2, the similar crystal data of them also approved that this reaction can occur in solid state by cation exchange process. Solid-state cation exchange processes of these metal–organic nanocapsules were studied by IR spectroscopy, X-ray powder diffraction (XRD) and scanning electron microscopy (SEM).

Site-Specific Metal Chelation Facilitates the Unveiling of Hidden Coordination Sites in an FeII/FeIII-Seamed Pyrogallol[4]arene Nanocapsule.

Under suitable conditions, C-alkylpyrogallol[4]arenes (PgCs) arrange into spherical metal-organic nanocapsules (MONCs) upon coordination to appropriate metal ions. Herein we present the synthesis and structural characterization of a novel FeII/FeIII-seamed MONC, as well as studies related to its electrochemical and magnetic behaviors. Unlike other MONCs that are assembled through 24 metal ions, this nanocapsule comprises 32 Fe ions, uncovering 8 additional coordination sites situated between the constituent PgC subunits. The FeII ions are likely formed by the reducing ability of DMF used in the synthesis, representing a novel synthetic route toward polynuclear mixed-valence MONCs.