Coordination Cages Research Articles

Engineering Permanent Porosity into Liquids.

The possibility of engineering well-defined pores into liquid materials is fascinating from both a conceptual and an applications point of view. Although the concept of porous liquids was proposed in 2007, these materials had remained hypothetical due to the technical challenges associated with their synthesis. Over the past five years, however, reports of the successful construction of porous liquids based on existing porous scaffolds, such as coordination cages, organic cages, metal-organic frameworks, porous carbons, zeolites, and porous polymers, have started to emerge. Here, the focus is on these early reports of porous liquids as prototypes in the field, classified according to the previously defined types of porous liquids. Particular attention will be paid to design strategies and structure-property relationships. Porous liquids have already exhibited promising applications in gas storage, transportation, and chemical separations. Thus, they show great potential for use in the chemical industry. The challenges of preparation, scale-up, volatility, thermal and chemical stability, and competition with porous solids will also be discussed.

A Novel Fishbone-Like Lead(II) Supramolecular Polymer: Synthesis, Characterization, and Application for Producing Nano Metal Oxide

The nanorods of [Pb(L)Br2]n (1) (L = 1,2-bis (pyridin-3-ylmethylene)hydrazine) underwent ultrasound irradiation and were synthesized as a novel three-dimensional fishbone-like Pb(II)–organic coordination supramolecular compound. The morphology and nanostructure of the synthesized compound were determined through SEM, FTIR, elemental analyses, and XRD. Compound 1 was structurally characterized by single-crystal X-ray diffraction and revealed six-coordinated Pb (II) ions bonded to two N atoms from two L ligands and four bromine anions, forming a one-dimensional fishbone-like coordination polymer, which extended into a 3D supramolecular structure through weak intermolecular interactions. The bulk thermal stability of compound 1 was examined using thermogravimetric analysis (TGA). Moreover, PbO nanoparticles with sizes of 40–80 nm were obtained through the thermolysis of 1 at 180 °C using oleic acid as a surfactant.

Self-assembly of novel copper(II) macrocyclic and luminescent europium(III) triple-stranded supramolecules with new anthracene-based bis-β-diketonate ligands

Two novel bis-(β-diketonate) ligands, 1,1′-(anthracene-1,8-diyl)bis(butane-1,3-dione) (H2L1), 1,1′-(anthracene-1,5-diyl)bis(4,4,4-trifluorobutane-1,3-dione) (H2L2) have been designed and synthesized for self-assembly of the luminescent supramolecular coordination compounds with transition and lanthanide metals. Two neutral dinuclear supramolecular assemblies Cu(II)-bis-β-diketonate macrocyclic complex [Cu2(L1)2](CH3OH)2 (1·2CH3OH) and Eu(III) bis-β-diketonate triple-stranded complex [Eu2(L2)3](CH3OH)4 (2·4CH3OH) have been characterized by various spectroscopic techniques. Their photophysical properties were further investigated. X-ray single crystal structural analysis revealed that complex 1·2CH3OH adopts a 1D chain arrangement which is constructed with a pitch of 1.951 Å through intermolecular hydrogen bonds along the b axis. The slightly distorted parallelogram of 1 is a neutral [M2L2]-type macrocycle with a size of 5.40 Å × 8.40 Å. The triple-stranded helicate CPK (Corey-Pauling-Koltun) diagram was calculated using Scigress 2.6 program.

Metal‐Organic Cages with Missing Linker Defects

AbstractWe present here the controlled synthesis of defective coordination cages by employing steric hindrance of organic linkers to manipulate coordination modes of the assembled metal ions. Three chiral 1,1′‐bi‐2‐naphthol (BINOL) derived bis‐tridentate ligands L1–L3 with pyridine‐2,6‐dicarboxamides (pcam) chelating moieties are therefore designed and synthesized, among which L3 has a smaller steric hindrance on the coordinating sites relative to the other two linkers. Complexes of L1 and L2 with lanthanides afford the irregular Ln8(L1)10 hexahedra with two missing edges and Ln4(L2)5 tetrahedra with one missing edge, respectively, both of which contain a 1:1 mixture of Ln(pcam)2 and Ln(pcam)3. In contrast, complex of L3 produces the regular twisted Ln6(L3)9 trigonal prisms without missing edges that contain only Ln(pcam)3 vertices. The defective cage has more freedom to adjust its structural conformation, affording adaptable cavity to accommodate a range of guest molecules with sizes comparable or much larger than the cavity portals.

Metal-Organic Cages with Missing Linker Defects.

We present here the controlled synthesis of defective coordination cages by employing steric hindrance of organic linkers to manipulate coordination modes of the assembled metal ions. Three chiral 1,1'-bi-2-naphthol (BINOL) derived bis-tridentate ligands L1 -L3 with pyridine-2,6-dicarboxamides (pcam) chelating moieties are therefore designed and synthesized, among which L3 has a smaller steric hindrance on the coordinating sites relative to the other two linkers. Complexes of L1 and L2 with lanthanides afford the irregular Ln8 (L1 )10 hexahedra with two missing edges and Ln4 (L2 )5 tetrahedra with one missing edge, respectively, both of which contain a 1:1 mixture of Ln(pcam)2 and Ln(pcam)3 . In contrast, complex of L3 produces the regular twisted Ln6 (L3 )9 trigonal prisms without missing edges that contain only Ln(pcam)3 vertices. The defective cage has more freedom to adjust its structural conformation, affording adaptable cavity to accommodate a range of guest molecules with sizes comparable or much larger than the cavity portals.

Development and implementation of a project for an open-air cage complex for keeping wild animals in captivity

The article discusses the construction of enclosure complexes for keeping wild animals in captivity. The absence of a clear definition and regulation of the term “open-air cage keeping” is noted, which complicates the organization of the open-air cage keeping of animals. It is indicated that for the development and implementation of an open-air cage complex, one should be guided by the building codes adopted in Russia, as well as the norms for organizing zoo nurseries, and the norms for keeping animals. The step-by-step rules for choosing a place, construction of cages, corrals, enclosures and other premises for keeping animals, features of electrical wiring, water supply, sewerage, heating, ventilation, and features of office premises are analyzed in detail. The selection and training of personnel for enclosure complexes should be based on the number and types of animals kept in the enclosure and working conditions (degree of mechanization, electrification). It is concluded that modern enclosures are a complex of structures for keeping animals, as well as their maintenance (administrative, economic, research and production, veterinary zone).

Taming the Topology of Calix[4]arene-Based 2D-Covalent Organic Frameworks: Interpenetrated vs Noninterpenetrated Frameworks and Their Selective Removal of Cationic Dyes

A bowl-shaped calix[4]arene with its exciting host-guest chemistry is a versatile supramolecular building block for the synthesis of distinct coordination cages or metal-organic frameworks. However, its utility in the synthesis of crystalline covalent organic frameworks (COFs) remains challenging, presumably due to its conformational flexibility. Here, we report the synthesis of a periodic 2D extended organic network of calix[4]arenes joined by a linear benzidine linker via dynamic imine bonds. By tuning the interaction among neighboring calixarene units through varying the concentration in the reaction mixture, we show the selective formation of interpenetrated (CX4-BD-1) and non-interpenetrated (CX4-BD-2) frameworks. The cone-shaped calixarene moiety in the structural backbone allows for the interweaving of two neighboring layers in CX4-BD-1, making it a unique example of interpenetrated 2D layers. Due to the high negative surface charge from calixarene units, both COFs have shown high performance in charge-selective dye removal and an exceptional selectivity for cationic dyes irrespective of their molecular size. The charge distribution of the COFs and the resulting selectivity for the cationic dyes were further investigated using computational methods.

Application of Crystalline Matrices for the Structural Determination of Organic Molecules.

While single-crystal X-ray diffraction (SC-XRD) is one of the most powerful structural determination techniques for organic molecules, the requirement of obtaining a suitable crystal for analysis limits its applicability, particularly for liquids and amorphous solids. The emergent use of preformed porous crystalline matrices that can absorb organic compounds and stabilize them via host–guest interactions for observation via SC-XRD offers a way to overcome this hindrance. A topical and current discussion of SC-XRD in organic chemistry and the use of preformed matrices for the in crystallo analysis of organic compounds, with a particular focus on the absolute structure determination of chiral molecules, is presented. Preformed crystalline matrices that are covered include metal–organic frameworks (MOFs) as used in the crystalline sponge method, metal–organic polyhedra (MOPs, coordination cages), porous organic materials (POMs)/porous organic molecular crystals (POMCs), and biological scaffolds. An outlook and perspective on the current technology and on its future directions is provided.

Supermolecule Cucurbituril Subnanoporous Carbon Supercapacitor (SCSCS).

It is quite challenging to prepare subnanometer porous materials from traditional porous precursors, and use of supramolecules as carbon sources was seldom reported due to the complex preparation and purification processes. We explore a facile one-pot method to fabricate supramolecular coordination compounds as carbon sources. The resultant CB[6]-derived carbons (CBC) have a high N content of 7.0-22.0%, surface area of 552-861 m2 g-1, and subnano/mesopores. The CBC electrodes have a narrow size distribution at 5.9 Å, and the supercapacitor exhibits an energy density of 117.1 Wh kg-1 and a potential window of over 3.8 V in a two-electrode system in the ionic liquid (MMIMBF4) electrolyte with appropriate cationic (5.8 Å) and anionic (2.3 Å) diameter. This work presents the facile fabrication of novel supermolecule cucurbituril subnanoporous carbon materials and the smart design of "pores and balls" for high-performance energy storage systems.

Metal-organic cages for molecular separations.

Separation technology is central to industries as diverse as petroleum, pharmaceuticals, mining and life sciences. Metal-organic cages, a class of molecular containers formed via coordination-driven self-assembly, show great promise as separation agents. Precise control ofthe shape, size and functionalization of cage cavities enables them to selectively bind and distinguish a wide scope of physicochemically similar substances in solution. Extensive research has, thus, been performed involving separations of high-value targets using coordination cages, ranging from gases and liquids to compounds dissolved in solution. Enantiopure capsules also show great potential for the separation of chiral molecules. The use of crystalline cages as absorbents, or the incorporation of cages into polymer membranes, could increase the selectivity and efficiency of separation processes. This Review covers recent progress in using metal-organic cages to achieve separations, with discussion of the many methods of using them in this context. Challenges and potential future developments are also discussed.

Identification of a Heteroleptic Pd6L6L'6 Coordination Cage by Screening of a Virtual Combinatorial Library.

The design of structurally defined heteroleptic coordination cages is a challenging task, and only few examples are known to date. Here we describe a selection approach that allowed the identification of a novel hexanuclear Pd cage containing two types of dipyridyl ligands. A virtual combinatorial library of [PdnL2n](BF4)2n complexes was prepared by mixing six different dipyridyl ligands with substoichiometric amounts of [Pd(CH3CN)4](BF4)2. Analysis of the equilibrated reaction mixture revealed the preferential formation of a heteroleptic [Pd6L6L'6](BF4)12 assembly. The complex was prepared on a preparative scale by a targeted synthesis, and its structure was elucidated by single-crystal X-ray diffraction. It features an unprecedented trigonal-antiprismatic cage structure with two triangular Pd3L3 macrocycles bridged by six L' ligands. A related but significantly larger [Pd6L6L'6](BF4)12 cage was obtained by using metalloligands instead of organic dipyridyl ligands.

Coal-Tar Dye-based Coordination Cages and Helicates.

A strategy to implement four members of the classic coal‐tar dye family, Michler's ketone, methylene blue, rhodamine B, and crystal violet, into [Pd2L4] self‐assemblies is introduced. Chromophores were incorporated into bis‐monodentate ligands using piperazine linkers that allow to retain the auxochromic dialkyl amine functionalities required for intense colors deep in the visible spectrum. Upon palladium coordination, ligands with pyridine donors form lantern‐shaped dinuclear cages while quinoline donors lead to strongly twisted [Pd2L4] helicates in solution. In one case, single crystal X‐ray diffraction revealed rearrangement to a [Pd3L6] ring structure in the solid state. For nine examined derivatives, showing colors from yellow to deep violet, CD spectroscopy discloses different degrees of chiral induction by an enantiomerically pure guest. Ion mobility mass spectrometry allows to distinguish two binding modes. Self‐assemblies based on this new ligand class promise application in chiroptical recognition, photo‐redox catalysis and optical materials.

Phenol-triggered supramolecular transformation of titanium–oxo cluster based coordination capsules

Supramolecular transformations of coordination cage or capsule have received much attention recently, which help elucidate this natural self-assembly behavior in biological systems. The current study describes the first supramolecular transformation of titanium–organic coordination capsule triggered by phenol (and H3PO3). The structural alterations are accompanied by the reconstruction of 5-coordinated Ti centers to 6-coordinated ones. Meanwhile, different amounts of encapsulated phenol guest molecules can be identified, dependent on the sizes of the obtained cavities. In addition, they display much better visible light absorption and air stability than the isopropanol stabilized ones.

Allosteric Guest Binding in Chiral Zirconium(IV) Double Decker Porphyrin Cages.

Chiral zirconium(IV) double cage sandwich complex Zr(1)2 has been synthesized in one step from porphyrin cage H21. Zr(1)2 was obtained as a racemate, which was resolved by HPLC and the enantiomers were isolated in >99.5 % ee. Their absolute configurations were assigned on the basis of X‐ray crystallography and circular dichroism spectroscopy. Vibrational circular dichroism (VCD) experiments on the enantiomers of Zr(1)2 revealed that the chirality around the zirconium center is propagated throughout the whole cage structure. The axial conformational chirality of the double cage complex displayed a VCD fingerprint similar to the one observed previously for a related chiral cage compound with planar and point chirality. Zr(1)2 shows fluorescence, which is quenched when viologen guests bind in its cavities. The binding of viologen and dihydroxybenzene derivatives in the two cavities of Zr(1)2 occurs with negative allostery, the cooperativity factors α (=4 K2/K1) being as low as 0.0076 for the binding of N,N’‐dimethylviologen. These allosteric effects are attributed to a pinching of the second cavity as a result of guest binding in the first cavity.

Self-assembly of a water-soluble endohedrally functionalized coordination cage including polar guests.

Coordination cages containing endohedrally functionalized aromatic cavities are scarce in the literature. Herein, we report the self-assembly of a tetra-cationic super aryl-extended calix[4]pyrrole tetra-pyridyl ligand into a water-soluble Pd(ii)-cage featuring two endohedral polar binding sites. They are defined by the four pyrrole NHs of the calix[4]pyrrole unit and the four inwardly directed α-protons of the coordinated pyridyl groups. The efficient assembly of the Pd(ii)-cage requires the inclusion of mono- and ditopic pyridyl N-oxide and aliphatic formamide guests. The monotopic guests only partially fill the cage's cavity and require the co-inclusion of a water molecule that is likely hydrogen-bonded to the endohedral α-pyridyl protons. The ditopic guests are able to completely fill the cage's cavity and complement both binding sites. We observed high conformational selectivity in the inclusion of the isomers of α,ω-bis-formamides. We briefly investigate the uptake and release mechanism/kinetics of selected polar guests by the Pd(ii)-cage using pair-wise competition experiments.

Outside the box: quantifying interactions of anions with the exterior surface of a cationic coordination cage.

We describe a study of the binding of anions to the surface of an octanuclear coordination cage HW, which carries a 16+ charge, in aqueous solution. Anionic aromatic fluorophores such as fluorescein (and derivatives) and hydroxypyrene tris-sulfonate (HPTS) bind strongly to an extent depending on their charge and hydrophobicity. Job plots indicated binding of up to six such fluorescent anions to HW, implying that one anion can bind to each face of the cubic cage, as previously demonstrated crystallographically with small anions such as halides. The quenching of these fluorophores on association with the cage provides the basis of a fluorescence displacement assay to investigate binding of other anions: addition of analyte (organic or inorganic) anions in titration experiments to an HW/fluorescein combination results in displacement and restoration of the fluorescence from the bound fluorescein, allowing calculation of 1 : 1 binding constants for the HW/anion combinations. Relative binding affinities of simple anions for the cage surface can be approximately rationalised on the basis of ease of desolvation (e.g. F- < Cl- < Br-), electrostatic factors given the 16+ charge on the cage (monoanions < dianions), and extent of hydrophobic surface. The interaction of a di-anionic pH indicator (bromocresol purple) with HW results in a pKa shift, with the surface-bound di-anionic form stabilised by approximately 1 pKa unit compared to the non-bound neutral form due to the charge on the cage.

Orthogonal binding and displacement of different guest types using a coordination cage host with cavity-based and surface-based binding sites.

The octanuclear Co(ii) cubic coordination cage system H (or HW if it bears external water-solubilising substituents) has two types of binding site for guests. These are (i) the partially-enclosed central cavity where neutral hydrophobic organic species can bind, and (ii) the six 'portals' in the centres of each of the faces of the cubic cage where anions bind via formation of a network of CH⋯X hydrogen bonds between the anion and CH units on the positively-charged cage surface, as demonstrated by a set of crystal structures. The near-orthogonality of these guest binding modes provides the basis for an unusual dual-probe fluorescence displacement assay in which either a cavity-bound fluorophore (4-methyl-7-amino-coumarin, MAC; λem = 440 nm), or a surface-bound anionic fluorophore (fluorescein, FLU; λem = 515 nm), is displaced and has its emission ‘switched on’ according to whether the analyte under investigation is cavity-binding, surface binding, or a combination of both. A completely orthogonal system is demonstrated based using a Hw/MAC/FLU combination: addition of the anionic analyte ascorbate displaced solely FLU from the cage surface, increasing the 515 nm (green) emission component, whereas addition of a neutral hydrophobic guest such as cyclooctanone displaced solely MAC from the cage central cavity, increasing the 440 nm (blue) emission component. Addition of chloride results in some release of both components, and an intermediate colour change, as chloride is a rare example of a guest that shows both surface-binding and cavity-binding behaviour. Thus we have a colourimetric response based on differing contributions from blue and green emission components in which the specific colour change signals the binding mode of the analyte. Addition of a fixed red emission component from the complex [Ru(bipy)3]2+ (Ru) provides a baseline colour shift of the overall colour of the luminescence closer to neutral, meaning that different types of guest binding result in different colour changes which are easily distinguishable by eye.

Synthesis and spectral characterization of the first fluorescein-tagged iron(ii) clathrochelates, their supramolecular interactions with globular proteins, and cellular uptake.

A fluorescein-tagged iron(ii) cage complex was obtained in a moderate total yield using a two-step synthetic procedure starting from its propargylamine-containing clathrochelate precursor. An 11-fold decrease in fluorescence quantum yield is observed in passing from the given fluorescein-based dye to its clathrochelate derivative. An excitation energy transfer from the terminal fluorescent group of the macrobicyclic molecule to its quasiaromatic highly π-conjugated clathrochelate framework can explain this effect. The kinetics of the hydrolysis of the acetyl groups of acetylated fluorescein azide and its clathrochelate derivative in the presence of one equivalent of BSA evidenced no strong supramolecular host–guest interactions between BSA and the tested compounds. Study of a chemical stability of the deacetylated iron(ii) clathrochelate suggested the formation of a supramolecular 1 : 1 BSA–clathrochelate assembly. Moreover, an addition of BSA or HSA to its solution caused the appearance of strong clathrochelate-based ICD outputs. The fluorescence emission anisotropy studies also evidenced the supramolecular binding of the fluorescein-tagged iron(ii) clathrochelate to the BSA macromolecule, leading to a high increase in this type of anisotropy. Subcellular uptake of the fluorescein-tagged molecules was visualized using fluorescence microscopy and showed its distribution to be mainly in the cytosol without entering the nucleus or accumulating in any other organelle. An X-rayed crystal of the above propargylamide macrobicyclic precursor with a reactive terminal C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond contains the clathrochelate molecules of two types, A and B. The encapsulated iron(ii) ion in these molecules is situated in the center of its FeN6-coordination polyhedron, the geometry of which is intermediate between a trigonal prism (TP) and a trigonal antiprism (TAP). The Fe–N distances vary from 1.8754(6) to 1.9286(4) Å and the heights h of their distorted TP–TAP polyhedra are very similar (2.30 and 2.31 Å); their values of φ are equal to 25.3 and 26.6°. In this crystal, the molecules of types A and B participate in different types of hydrogen bonding, giving H-bonded clathrochelate tetramers through their carboxylic and amide groups, respectively; these tetramers are connected to H-bonded chains.

Soluble imidazolium-functionalized coordination cages for efficient homogeneous catalysis of CO2 cycloaddition reactions.

A new strategy to prepare soluble homogeneous catalysts is developed by introducing imidazolium into cationic calix[4]arene-based metal-organic cages (MOCs). The soluble MOCs show high activity and recyclability in the cycloaddition reaction of CO2 without the addition of any co-catalysts. This method provides new inspiration to design highly efficient catalysts by combining the advantages of homogeneous and heterogeneous catalysis.

Increasing structural and functional complexity in self-assembled coordination cages.

Progress in metallo-supramolecular chemistry creates potential to synthesize functional nano systems and intelligent materials of increasing complexity. In the past four decades, metal-mediated self-assembly has produced a wide range of structural motifs such as helicates, grids, links, knots, spheres and cages, with particularly the latter ones catching growing attention, owing to their nano-scale cavities. Assemblies serving as hosts allow application as selective receptors, confined reaction environments and more. Recently, the field has made big steps forward by implementing dedicated functionality, e.g. catalytic centres or photoswitches to allow stimuli control. Besides incorporation in homoleptic systems, composed of one type of ligand, desire arose to include more than one function within the same assembly. Inspiration comes from natural enzymes that congregate, for example, a substrate recognition site, an allosteric regulator element and a reaction centre. Combining several functionalities without creating statistical mixtures, however, requires a toolbox of sophisticated assembly strategies. This review showcases the implementation of function into self-assembled cages and devises strategies to selectively form heteroleptic structures. We discuss first examples resulting from a combination of both principles, namely multicomponent multifunctional host–guest complexes, and their potential in application in areas such as sensing, catalysis, and photo-redox systems.