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.

Nanocapsules of unprecedented internal volume seamed by calcium ions.

The inception of an unprecedented class of voluminous Platonic solids displaying hierarchical geometry based on pyrogallol[4]arene moieties seamed by divalent calcium ion is described. Single-crystal X-ray structural determination has established the highly conserved geometry of two original Ca2+-seamed nanocapsules to be essentially cubic in shape with C-ethylpyrogallol[4]arene units located along the twelve edges of the cube which are then bridged by metallic polyatomic cations ([Ca4Cl]7+ or [Ca(HCO2)Na4]5+) at the six cube faces. The accessible volume of the nanocapsules is ca. 3500 Å3 and 2500 Å3 and is completely isolated from the exterior of the capsules. These remarkable nanocapsule discoveries cast a spotlight on a marginalized area of synthetic materials chemistry and encourage future exploration of diversiform supramolecular assemblies, networks, and capsules built on calcium, with clear benefits deriving from the intrinsic biocompatibility of calcium. Finally, a proof-of-concept is demonstrated for fluorescent reporter encapsulation and sustained release from the calcium-seamed nanocapsules, suggesting their potential as delivery vehicles for drugs, nutrients, preservatives, or antioxidants.

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.

A copper-seamed coordination nanocapsule as a semiconductor photocatalyst for molecular oxygen activation.

Here we report that a Cu2+-seamed coordination nanocapsule can serve as an efficient semiconductor photocatalyst for molecular oxygen activation. This capsule was constructed through a redox reaction facilitated self-assembly of cuprous bromide and C-pentyl-pyrogallol[4]arene. Photophysical and electrochemical studies revealed its strong visible-light absorption and photocurrent polarity switching effect. This novel molecular solid material is capable of activating molecular oxygen into reactive oxygen species under simulated sunlight irradiation. The oxygen activation process has been exploited for catalyzing aerobic oxidation reactions. The present work provides new insights into designing nonporous discrete metal-organic supramolecular assemblies for solar-driven molecular oxygen activation.

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.

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.

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.

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.

Solution structure of zinc-seamed C-alkylpyrogallol[4]arene dimeric nanocapsules.

The structural stability and solution geometry of zinc-seamed-C-propylpyrogallol[4]arene dimers has been studied in solution using in situ neutron scattering and 2D-DOSY NMR methods. In comparison with the structures of the analogous copper-/nickel-seamed dimeric entities, the spherical geometry of the PgC3Zn species (R = 9.4 Å; diffusion coefficient = 1.05 × 10-10 m2 s-1) is larger due to the presence of ligands at the periphery in solution. This enhanced radius in solution due to ligation is also consistent with the findings of model molecular dynamics simulations of the zinc-seamed dimers.

Novel keto-enol tautomerism in 1,3,5-trihydroxybenzene systems.

We report a new synthesis of the water-soluble compound 1,3,5-trihydroxy-2,4,6-trimethylsulfonic acid (1), which exists in two tautomeric forms (60 : 40::enol%:keto%) and can be used as a proton conductor. Quantum chemical calculations show the importance of intramolecular hydrogen bonding and the presence of implicit MeOH solvent on the relative stabilities of the tautomers. 1 complexes with lanthanides through its sulfonato groups and forms a layered cage-like structure with one intramolecular and two intermolecular hydrogen bonds.

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.

Controlled hierarchical self-assembly of networked coordination nanocapsules via the use of molecular chaperones.

Supramolecular chaperones play an important role in directing the assembly of multiple protein subunits and redox-active metal ions into precise, complex and functional quaternary structures. Here we report that hydroxyl tailed C-alkylpyrogallol[4]arene ligands and redox-active MnII ions, with the assistance of proline chaperone molecules, can assemble into two-dimensional (2D) and/or three-dimensional (3D) networked nanocapsules. Dimensionality is controlled by coordination between the exterior of nanocapsule subunits, and endohedral functionalization within the 2D system is achieved via chaperone guest encapsulation. The tailoring of surface properties of nanocapsules via coordination chemistry is also shown as an effective method for the fine-tuning magnetic properties, and electrochemical and spectroscopic studies support that the nanocapsule is an effective homogeneous water-oxidation electrocatalyst, operating at pH 6.07 with an exceptionally low overpotential of 368 mV.

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.

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.

Facile, one-pot, in aqua synthesis of catalytically competent gold nanoparticles using pyrogallol[4]arene as the sole reagent.

We report a simple, single-pot, room temperature synthetic route to prepare colloidally-stable pyrogallol[4]arene-capped gold nanoparticles. The number of carbon atoms in the pyrogallol[4]arene pendent alkyl chain and the addition of base (e.g., NaOH) are both found to significantly impact the resulting colloid size, stability, and catalytic activity for nitroarene reduction.

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.

Borohydride stabilized gold-silver bimetallic nanocatalysts for highly efficient 4-nitrophenol reduction.

Bimetallic Au x Ag1-x nanoparticles, prepared using sodium borohydride as the sole reducing and capping agent for various NaBH4 : metal molar ratios, were investigated as catalysts for 4-nitrophenol reduction. This approach yielded the highest catalytic activities observed for this model reaction to date.

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.

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.

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).