Magnetism in metal-organic capsules

Nanotoroids or Coordination Networks: Molecular Constructs at the Mercy of Anions

Pyrogallol[4]arene (PgC) macrocycles, owing to their preorganized structures, are known to form dimeric (M8L2), hexameric (M24L6), and dodecameric (M24L12) nanocapsules, as well as coordination networks. However, the role of anions in the formation of varied geometries and stoichiometries of PgCs is understudied. In this study, we explore the assembly of pyrogallol[4]arene with strontium iodide, forming metal-organic nanotoroids (M10L8), and with strontium nitrate, which forms coordination networks. Preliminary fluorescence spectroscopy experiments with dyes such as coumarin 153 and Nile red confirm that these nanotoroidal channels can effectively host dye molecules, making them suitable for diverse applications, including drug delivery. Furthermore, the nanotoroids and surface defects of the crystals can serve as a template to grow gold nanosheets.

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.

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 Å cylinder 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.

Photo/Electrochromic Dual Responsive Behavior of a Cage-like Zr(IV)-Viologen Metal-Organic Polyhedron (MOP)

Stable stimulus-responsive materials are highly desirable due to their widespread potential applications and growing demand in recent decades. Despite the fact that viologen derivatives have long been known as excellent photochromic and electrochromic materials, the development of stable viologen-based multifunctional smart materials with short coloration times remains an exciting topic. To obtain photochromic and electrochromic dual responsive materials, embedding the viologen ligand into a robust metal oxide cluster to increase its stability and sensitivity is an effective strategy. Herein, a viologen-based metal-organic polyhedron (MOP) {[Zr₆L₃(μ₃-O)₂(μ₂-OH)₆Cp₆]·8Cl·CH₃OH·DMF} [Zr-MOP-1; H₂L·2Cl = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium dichloride, and Cp = η⁵-C₅H₅] was successfully prepared and characterized. It consists of trinuclear Zr-oxygen secondary building units and exhibits reversible photochromic and electrochromic dual responsive behaviors. As expected, the designed robust viologen-based nanocage with a V₂E₃ (V = vertex, and E = edge) topology can maintain its stability and rapid photo/electrochromic behaviors with an obvious reversible change in color from purple (brown) to green, mainly due to the enclosed cluster structure and the abundant free viologen radicals that originate from the effective Cl → N and O → N electron transfers. Spectroelectrochemistry and theoretical calculations of this Zr-MOP were also performed to verify the chromic mechanism.

Heterometal-Organic Cages as Photo-Fenton-like Catalysts

Metal-organic cages, a class of supramolecular containers constructed by the self-assembly of metal ions and organic ligands, show great promise as catalytic agents. In this work, we designed and synthesized a series of rhombic dodecahedral Ni-Cu heterometal imidazolate cages (Ni₈Cu₆L₂₄) that can act as highly active photo-Fenton-like catalysts. These cages possess a high ability to generate hydroxyl radicals (^•OH) under visible light in the presence of H₂O₂, which can rapidly degrade organic pollutants (e.g., rhodamine B, methylene blue, and methyl orange) into CO₂ and H₂O. Besides, they are robust catalysts, with high catalytic activity and reusability under conditions in high H₂O₂ concentration, providing potentially advanced materials for degrading persistent organic pollutants.

Exploiting host-guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages

Reaction of Ni(OTf)₂ with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [Ni^II₄L₆]⁸⁺. By selectively exchanging the bound triflate from [OTf⊂Ni^II₄L₆](OTf)₇ (1), we have been able to prepare a series of host–guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [M^IIX₄⊂Ni^II₄L₆](OTf)₆, where M^IIX₄²⁻ = MnCl₄²⁻ (2), CoCl₄²⁻ (5), CoBr₄²⁻ (6), NiCl₄²⁻ (7), and CuBr₄²⁻ (8) or [M^IIIX₄⊂Ni^II₄L₆](OTf)₇, where M^IIIX₄⁻ = FeCl₄⁻ (3) and FeBr₄⁻ (4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host–guest complexes 1–8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN₆} vertexes within a single Ni₄L₆ unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [Ni^II₄] complex, and between the host and the MX₄ⁿ⁻ guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX₄ⁿ⁻ guest molecules lead to stronger host–guest magnetic exchange interactions.

The tetrahedral [Ni^II₄L₆]⁸⁺ cage can reversibly bind paramagnetic MX₄^1/2− guests, inducing magnetic exchange interactions between host and guest.

Two Zr-based heterometal–organic frameworks for efficient CO2 reduction under visible light

Two novel zirconium-based heterometal–organic framework catalysts can effectively improve photocatalytic activities for CO2 photoreduction under visible light.

Self-assembly of zirconocene-based metal–organic capsules: the structure, luminescence sensing of Fe3+ and iodine capture

Three new nanosized Zr-based MCs were synthesized under solvothermal conditions with excellent selectivity for luminescence sensing of Fe³⁺ and high performance for I₂ sorption and release.

Merohedral icosahedral M48 (M = CoII, NiII) cage clusters supported by thiacalix[4]arene

Cage clusters are a discrete chemically and topologically diverse family of molecule-based functional materials. Presented here are two isostructural M48 (M = CoII for LSHU01, NiII for LSHU02) cage clusters with a merohedral icosahedral cage structure featuring 12 M4-TC4A (H4TC4A, p-tert-butylthiacalix[4]arene) second building units as vertices and 18 asymmetric 5-(1H-tetrazol-1-yl)isophthalate ligands as faces. They are the highest-nuclearity cage compounds of CoII and NiII. The activated Co48 cage exhibited high selectivity in the sorption of C3H8 over CH4 under ambient conditions. Frequency response experiments indicated that the extrinsic voids and matrix interface of the activated crystalline samples are primarily responsible for the observed gas adsorption performance.

Interconvertible vanadium-seamed hexameric pyrogallol[4]arene nanocapsules

Research into stimuli-responsive controlled self-assembly and reversible transformation of molecular architectures has received much attention recently, because it is important to understand and reproduce this natural self-assembly behavior. Here, we report two coordination nanocapsules with variable cavities: a contracted octahedral V₂₄ capsule and an expanded ball-shaped V₂₄ capsule, both of which are constructed from the same number of subcomponents. The assemblies of these two V₂₄ capsules are solvent-controlled, and capable of reversible conversion between contracted and expanded forms via control of the geometries of the metal centers by association and dissociation with axial water molecules. Following such structural interconversions, the magnetic properties are significantly changed. This work not only provides a strategy for the design and preparation of coordination nanocapsules with adaptable cavities, but also a unique example with which to understand the transformation process and their structure-property relationships.

Adapting the cavity of a coordination capsule generally involves the addition or removal of subcomponents. Here, the authors report two vanadium-organic coordination nanocapsules with the same number of components but variable cavity sizes—an expanded ball and contracted octahedron—whose solvent-controlled interconversion is attributed to the versatile coordination geometry of the vanadium centers.

A monomeric bowl-like pyrogallol[4]arene Ti12 coordination complex

Herein, an unprecedented monomeric bowl-like coordination complex, Ti₁₂PgC₃ (PgC₃ = C-propylpyrogallol[4]arene), has been successfully synthesized. To the best of our knowledge, Ti₁₂PgC₃ not only presents the first pyrogallol[4]arene-based titanium coordination complex, but also the highest nuclearity titanium coordination complex in the metal-calixarene system. In addition, this titanium coordination complex can effectively degrade the methylene blue (MB) dye under sunlight.

A M18L6 metal–organic nanocapsule with open windows using mixed macrocycles

Introducing defects into metal–organic nanocapsules creates two open windows.

Novel magnesium-seamed organic nanocapsules with hierarchical structural complexity

Novel magnesium-seamed organic nanocapsules with hierarchical structural complexity are prepared via endohedral and exterior functionalization.

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.

Engineering supramolecular organic frameworks (SOFs) of C-alkylpyrogallol[4]arene with bipyridine-based spacers

Supramolecular organic frameworks (SOFs) based on pyrogallol[4]arene and 4,4′-bipyridine-type spacer molecules have been investigated.

Directed synthesis of {CuII2ZnII2} and {CuII8ZnII8} heterometallic complexes

The polydentate ligand Bis–tris propane is used to assemble two Cu/Zn heterometallic complexes: a tetramer which displays an unusually strong Cu(ii)⋯Cu(ii) exchange given the Cu⋯Cu separation or, in the presence of base, a hexadecanuclear complex with a remarkable double-concentric ring structure.

Establishing trends based on solvent system changes in cocrystals containing pyrogallol[4]arenes and fluorescent probes rhodamine B and pyronin Y

Eight cocrystal structures containing a pyrogallol[4]arene of varying aliphatic tail lengths and either fluorescent probes rhodamine B or pyronin Y in a range of solvent systems are examined. Trends based on probe, solvent, and aliphatic tail length are investigated.

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.

Zinc-seamed pyrogallol[4]arene dimers as structural components in a two-dimensional MOF

Computational methods were used as a proof of principle in the synthesis of a two-dimensional coordination polymer constructed from zinc-seamed pyrogallol[4]arene dimeric nanocapsules and 4,4′-bipyridine.

Aqueous solubilization of hydrophobic supramolecular metal–organic nanocapsules

Micelles of surfactant solubilized metal-seamed pyrogallol[4]arene based organic nanocapsules are synthesized and characterized using in situ neutron scattering and dynamic light scattering techniques, which show trends in sizes as a function of alkyl tails of pyrogallols and surfactants.

Exploring the magnetic behavior of nickel-coordinated pyrogallol[4]arene nanocapsules

The magnetic behavior of nickel-seamed C-propylpyrogallol[4]arene dimeric and hexameric nanocapsular assemblies has been investigated in the solid state using a SQUID magnetometer. These dimeric and hexameric capsular entities show magnetic differentiation both in terms of moment per nanocapsule and potential antiferromagnetic interactions within individual nanocapsules. The weak antiferromagnetic behavior observed at low temperatures indicates dipolar interactions between neighboring nickel atoms; however, this effect is higher in the hexameric nickel-seamed assembly. The differences in magnetic behavior of dimer versus hexamer can be attributed to different coordination environments and metal arrangements in the two nanocapsular assemblies.