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.

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.

Solution-Phase Magnetic Mechanistic Study of Ni-Seamed Pyrogallol[4]arene Nanocapsules Reveal Presence of Novel Cylindrical and Spherical Species.

The magnetic properties of nickel-seamed C-pyrogallol[4]arene (PgC3 Ni) hexamers and dimers are studied for the first time in solution. The combination of small-angle neutron scattering and superconducting quantum interference device magnetometer measurements of the solution species reveal their paramagnetic and weakly antiferromagnetic behaviour. Surprisingly, the magnetic results indicated the presence of an unprecedented 13 Å-radius species, larger than both the dimeric and hexameric nanocapsules with both octahedral and square-planar metal centers. To confirm the presence of this novel species, we performed a mechanistic study of PgC3 Ni as a function of temperature and solvent and deduced the presence of two additional new species: a) an 11 Šcylinder with Ni atoms seaming the tubular framework and b) an 8 Å-radius sphere with non-interacting Ni centers located within the internal cavity. Select parameters that shift the equilibrium towards desired species are also identified.

Molecular Entrapment of Polymers by Pyrogallol[4]arenes.

Macromolecular recognition systems are difficult to construct because extremely high recognition ability is required to form a stable host-guest complex toward macromolecules. Herein, we report a novel host-guest recognition motif based on C-propylpyrogallol[4]arene (PgC3) and a commercially available polymer, polyethylene glycol (PEG). The results show that PgC3 can selectively entrap PEG with higher molecular weights to form bilayered host-guest complex structures. Interestingly, this host-guest recognition is strong enough that PgC3 is able to adsorb PEG from an aqueous solution efficiently.

Self-Assembly of a Semiconductive and Photoactive Heterobimetallic Metal-Organic Capsule.

We report the synthesis of a novel metal-organic capsule constructed from six pyrogallol[4]arene macrocycles, which are switched together by 16 FeIII and 16 CoII ions. This supramolecular structure is the first instance of a spheroidal heterometallic nanocage assembled through a one-step metal-ligand coordination approach. This new assembly also demonstrates an important proof of concept through the formation of multiple heterometallic metal-metal interactions within the capsule framework. Photophysical and electrochemical studies of self-assembled capsule films indicate their potential as semiconductors. These materials display unexpected photoelectric conversion properties, thus representing an emergent phenomenon in discrete metal-organic supramolecular assemblies.

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.

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.

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.

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.

A Supramolecular Coordination-Polymer-Derived Electrocatalyst for the Oxygen Evolution Reaction.

An iron oxide decorated nickel iron alloy nanoparticle/porous graphene hybrid exhibits high electrocatalytic activity and excellent durability toward oxygen evolution reaction (OER). It displays a low overpotential of 274 mV at 10 mA cm-2 , and low Tafel slope of 37 mV dec-1 , showing a superior performance to the state-of-the-art RuO2 OER electrocatalyst.

Oxidation State Distributions Provide Insight into Parameters Directing the Assembly of Metal-Organic Nanocapsules.

Two structurally analogous Mn-seamed C-alkylpyrogallol[4]arene (PgC n)-based metal-organic nanocapsules (MONCs) have been synthesized under similar reaction conditions and characterized by crystallographic, electrochemical, and magnetic susceptibility techniques. Both MONCs contain 24 Mn centers, but, somewhat surprisingly, marked differences in oxidation state distribution are observed upon analysis. One MONC contains exclusively MnII ions, while the other is a mixed-valence MnII/ MnIII assembly. We propose that these disparate oxidation state distributions arise from slight differences in pH achieved during synthesis, a factor that may lead to many spectacular new MONCs (and associated host-guest chemistries).

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.

Hierarchical Self-Assembly of Supramolecular Coordination Polymers Using Giant Metal-Organic Nanocapsules as Building Blocks.

Controlling the self-assembly of giant molecular building blocks into complex architectures with similar hierarchy to biological species remains a major challenge in supramolecular chemistry. Akin to protein structure, here we present the self-assembly of giant molecular nanocapsules into supramolecular coordination polymers with controlled hierarchy from primary to secondary and tertiary structures. First, we successfully prepared discrete nanocapsules (secondary structures) consisting of multicomponents, such as organic macrocycles and metal-based secondary building units (primary structures). Second, these nanocapsules can self-organize into various 2D and 3D supramolecular coordination polymers (tertiary structures) through coordination-driven assembly. The periphery 24 flexible alkyl chains and 24 metal ions available for potential coordination make these nanocapsules comparable to functionalized solid nanoparticles with non-specific binding sites at the surface and allow the nanocapsules to self-adjust their orientations and coordination modes to facilitate the self-assembly process. This study sheds light on the self-assembly of giant building units with complex molecular structures and opens up possibilities for the design of new hierarchical architectures with innovative properties and functions in many applications such as biomimics, biomedicine, and molecular devices.

Formation of Water Channels in the Crystalline Hydrates of Macrocyclic Compounds.

Crystalline hydrates of macrocyclic compounds such as pyroagllol[4]arenes (PgCs) and resorcin[4]arenes (RsCs) are rare owing to their lower water solubility. Functionalization of these macrocyclic compounds is an affordable way to enhance water solubility. However, functionalization also encounters the formation of multiple conformers and subsequent difficulty in purification of the product. Herein, four novel crystalline hydrates of functionalized PgCs and RsCs were synthesized. Formation of water channels and the effects of intermolecular interactions on the physical properties of these novel hydrates are discussed.

Controlled 2D Assembly of Nickel-Seamed Hexameric Pyrogallol[4]arene Nanocapsules.

The two-dimensional framework of nickel-seamed hexameric metal-organic nanocapsules has been synthesized by connecting the tailed hydroxyl groups of C-propan-3-ol pyrogallol[4]arene with adjacent hexameric capsules via nickel-hydroxyl coordination. In addition, functionalization of nanocapsules with multiple pyridine molecules at the capsule surface prevents them from assembling into hierarchical structures and leads to the formation of discrete nickel-seamed pyrogallol[4]arene nanocapsules. This work shows that surface functionalization of nanocapsules is an effective and innovative method of controlling the assembly of these nanometric building blocks.

Preparation of Magnesium-Seamed C-Alkylpyrogallol[4]arene Nanocapsules with Varying Chain Lengths.

Novel supramolecular nanocapsules based on metal-directed assembly have captured tremendous interest due to their applications in fields such as catalysis, selective gas adsorption, and biomedicine. Functionalization of metal-organic nanocapsules (MONCs) by using organic ligands with different pendant groups affords more complexity to the structure and may lead to novel properties. In this work, we report the solvothermal synthesis of a group of magnesium-based MONCs using C-alkylpyrogallol[4]arenes with varying alkyl chain lengths. The structures of these nanocapsules are characterized by single-crystal X-ray diffraction analysis. As expected, a progression in size of the nanocapsules is observed as the alkyl chain length increases. The effect of the chain length on the solubility of MONCs in water has been determined. This work shows the generality of the solvothermal approach for the synthesis of MONCs with different organic ligands and demonstrates that surface functionalization of MONCs may serve as an effective way to tailor their properties. The unique biocompatible nature and inherent large cavity of these magnesium-based MONCs make these nanocapsules promising for potential applications in biomedicine.

Process Development for Separation of Conformers from Derivatives of Resorcin[4]arenes and Pyrogallol[4]arenes.

Macrocyclic compounds, such as resorcin[4]arenes and pyrogallol[4]arenes, have proven to be useful building blocks in the construction of supramolecular organic frameworks (SOFs) because of their unique bowl-like shape and ability to interact through variety of intermolecular interactions. Herein, we report the synthesis and crystal structures of two functionalized resorcin[4]arenes and pyroagllol[4]arenes, 4-hydroxyphenylresorcin[4]arenes, and 4-hydroxyphenylpyrogallol[4]arenes. These phenyl-functionalized macrocycles usually have different conformers, such as cone, boat, chair, saddle, and diamond. The successful separation of predominant conformers from the crude product was carried out with solvent-extraction technique. The shape and molecular arrangement of these conformers in the individual crystal structure was verified with single-crystal X-ray diffraction studies.

Noria: A Highly Xe-Selective Nanoporous Organic Solid.

Separation of xenon and krypton is of industrial and environmental concern; the existing technologies use cryogenic distillation. Thus, a cost-effective, alternative technology for the separation of Xe and Kr and their capture from air is of significant importance. Herein, we report the selective Xe uptake in a crystalline porous organic oligomeric molecule, noria, and its structural analogue, PgC-noria, under ambient conditions. The selectivity of noria towards Xe arises from its tailored pore size and small cavities, which allows a directed non-bonding interaction of Xe atoms with a large number of carbon atoms of the noria molecular wheel in a confined space.

Selective CO2 Adsorption in a Supramolecular Organic Framework.

Considering the rapidly rising CO2 level, there is a constant need for versatile materials which can selectively adsorb CO2 at low cost. The quest for efficient sorptive materials is still on since the practical applications of conventional porous materials possess certain limitations. In that context, we designed, synthesized, and characterized two novel supramolecular organic frameworks based on C-pentylpyrogallol[4]arene (PgC5 ) with spacer molecules, such as 4,4'-bipyridine (bpy). Highly optimized and symmetric intermolecular hydrogen-bonding interactions between the main building blocks and comparatively weak van der Waals interactions between solvent molecules and PgC5 leads to the formation of robust extended frameworks, which withstand solvent evacuation from the crystal lattice. The evacuated framework shows excellent affinity for carbon dioxide over nitrogen and adsorbs ca. 3 wt % of CO2 at ambient temperature and pressure.

Solution structures of nanoassemblies based on pyrogallol[4]arenes.

Nanoassemblies of hydrogen-bonded and metal-seamed pyrogallol[4]arenes have been shown to possess novel solution-phase geometries. Further, we have demonstrated that both guest encapsulation and structural rearrangements may be studied by solution-phase techniques such as small-angle neutron scattering (SANS) and diffusion NMR. Application of these techniques to pyrogallol[4]arene-based nanoassemblies has allowed (1) differentiation among spherical, ellipsoidal, toroidal, and tubular structures in solution, (2) determination of factors that control the preferred geometrical shape and size of the nanoassemblies, and (3) detection of small variations in metric dimensions distinguishing similarly and differently shaped nanoassemblies in a given solution. Indeed, we have shown that the solution-phase structure of such nanoassemblies is often quite different from what one would predict based on solid-state studies, a result in disagreement with the frequently made assumption that these assemblies have similar structures in the two phases. We instead have predicted solid-state architectures from solution-phase structures by combining the solution-phase analysis with solid-state magnetic and elemental analyses. Specifically, the iron-seamed C-methylpyrogallol[4]arene nanoassembly was found to be tubular in solution and predicted to be tubular in the solid state, but it was found to undergo a rearrangement from a tubular to spherical geometry in solution as a function of base concentration. The absence of metal within a tubular framework affects its stability in both solution and the solid state; however, this instability is not necessarily characteristic of hydrogen-bonded capsular entities. Even metal seaming of the capsules does not guarantee similar solid-state and solution-phase architectures. The rugby ball-shaped gallium-seamed C-butylpyrogallol[4]arene hexamer becomes toroidal on dissolution, as does the spherically shaped gallium/zinc-seamed C-butylpyrogallol[4]arene hexamer. However, the arenes are arranged differently in the two toroids, a variation that accounts for the differences in their sizes and guest encapsulation. Guest encapsulation of biotemplates, such as insulin, has demonstrated the feasibility of synthesizing nanocapsules with a volume three times that of a hexamer. The solution-phase studies have also demonstrated that the self-assembly of dimers versus hexamers can be controlled by the choice of metal, solvent, and temperature. Controlling the size of the host, nature of the metal, and identity of the guest will allow construction of targeted host-guest assemblies having potential uses as drug delivery agents, nanoscale reaction vessels, and radioimaging/radiotherapy agents. Overall, the present series of solid- and solution-phase studies has begun to pave the way toward a more complete understanding of the properties and behavior of complex supramolecular nanoassemblies.