An Imine-Based Molecular Cage with Distinct Binding Sites for Small and Large Alkali Metal Cations

Porous Organic Cages

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

Recent advances in heteroleptic multiple-stranded metallosupramolecules

Well-ordered combination of defined coordination spheres and multiple types of ligands (heteroleptic) in a given structure can expand the structural complexity and functional diversity of the resulting metallosupramolecules. Such heteroleptic metallosupramolecular architectures are expected to afford advanced utility in a variety of applications. In this concise review article, recent advances in the development of multi-nuclear-cluster-based heteroleptic multiple-stranded (HLMS) metallosupramolecules are summarized and demonstrated. To construct HLMS metallosupramolecules, one type of multitopic ligands can be employed for building up multiple strands, while another type of ligands can be utilized to construct multi-nuclear clusters. Most HLMS metallosupramolecules adopt helical geometries and have high molecular symmetry, which can be key factors for the structural completion. HLMS metallosupramolecules can be used as basic building blocks for the fabrication of higher-order polymeric or discrete assembly architectures with well-defined geometries.

Structural and computational investigation of an imine-based propeller-shaped macrocyclic cage

In this study, we present the synthesis, spectroscopic and structural characterization of self-assembling gem-dimethyl imine based molecular cage (IMC). Self-assembling macrocycles and cages have well-defined cavities and have extensive functionalities ranging from energy storage, liquid crystals, and catalysts to water splitting photo absorber. IMC has large voids i.e., 25% of the total crystal volume thus could accommodate wide substrates. The synthesized imine-based molecular cages are stabilized by coaxial π bonded networks and long-range periodic van der Waal and non-bonded contacts as observed from the crystal structure. IMC also has typical properties of soft condensed matter materials, hence theoretical prediction of stress and strain tensor along with thermophysical properties were computed on crystal system and were found to be stable. Molecular dynamics revealed IMC is stabilized by, strong interactions between the interstitial phenyl rings. Density functional theory (DFT) based physicochemical properties were evaluated and has band gap of around 2.38ev (520 nm) similar to various photocatalytic band gap materials.

An N-linked disalicylaldehyde together with its caesium ion and dichloromethane sensing performances: ‘dual key & lock’ LMCT-enhanced fluorescence strategy

The disalicylaldehyde-Cs⁺sensing system, a novel approach for quick and reusable detection of Cs⁺together with convenient CH₂Cl₂monitoring.

Coordination-driven self-assembly vs dynamic covalent chemistry: versatile methods for the synthesis of molecular metallarectangles

Supramolecular coordination assemblies have a range of potential applications in chemical and biological sciences. Herein, simple modular methods for the synthesis of metallarectangles are described. The desired tetranuclear metallarectangles were synthesized by using coordination-driven self-assembly of half-sandwich rhodium-based organometallic clip units and organic ligands. The reaction of such an organometallic clip with 4-formylpyridine provided a dinuclear molecular tweezer with pendant aldehyde groups, and subsequent [4 + 4] condensation reactions with diamines provides another route to the target metallarectangles in good yields. The same assemblies can also be easily isolated in one-pot procedures by mixing the organometallic clip, diamines and 4-formylpyridine.

Porous Zr6L3 Metallocage with Synergetic Binding Centers for CO2

Coordination-driven assembly has been widely successful in the synthesis of metallocages and used for many applications, such as catalysis. However, studies on CO₂ adsorption with metallocages have been rarely conducted, compared to other well-known cage-type materials, such as porous organic cages. In this study, a rational choice of ligand and metal led to the synthesis of a Zr₆L₃-type metallocage, exhibiting exceptional CO₂ adsorption properties. CO₂ adsorption experiments revealed that the metallocage shows highly selective adsorption of CO₂ over N₂ with high CO₂ binding energy. Density functional theory calculations uncovered the origin of this exceptional affinity for CO₂ over N₂.

Host–guest capability of a three-dimensional heterometallic macrocycle

Three-dimensional heterometallic macrocycles with half-sandwich Rh corners were studied for their ability to trap planar and non-planar guests. Furthermore, these heterometallic macrocycles can be destroyed in the presence of a soft base to form hexanuclear triangular prism complexes.

Syntheses, Structures, and Solution Studies of Multicomponent Macrocycles and Cages Based on Versatile Ligands

Different types of multinuclear half-sandwich rhodium macrocycles and cages were designed and synthesized by using two similar multifunctional hydroxamate ligands (pyrazine-2-hydroxamic acid (NaHL¹ ) and 4,4'-bipyridine-2-hydroxamic acid (KHL² )) featuring one monodentate site and two pairs of chelating sites. The Rh^III -Pd^II heterometallic macrocycles were constructed by using the semi-open palladium(II) source [Pd(en)Cl₂ ] with two free acceptor sites. However, only one kind of macrocycle was found when the shorter ligand L¹ was used, while in for the larger ligand, various spectroscopic techniques demonstrated the coexistence of hexanuclear and octanuclear macrocycles in solution and the proportions of both components depended on concentration and temperature. The palladium salt Pd(NO₃ )₂ , as a source of "naked" Pd²⁺ , was introduced to assemble the cuboid-shaped cage composed of two types of metal ions and three types of organic ligands. In addition, two silver(I)-containing mixed-metal complexes bridged by pyrazine were obtained, in which two forms of decanuclear complex with C_2v and C_2h point symmetry cocrystallized-one is a polymeric structure and the other is a discrete cage. However, the third form, with D₂ point symmetry, was found in the larger cage.

Novel Two- and Three-Dimensional Organometallic-Organic Hybrid Materials Based on Polyphosphorus Complexes

The reaction of the silver salt Ag[Al{OC(CF3)3}4] (1) with the P2 ligand complex [Cp2Mo2(CO)4(η(2)-P2)] (2) and the organic ditopic linker trans-1,2-di(pyridine-4-yl)ethene (dpe) results in the formation of four novel organometallic-organic hybrid compounds. Depending on the reaction conditions, the two-dimensional networks [{Cp2Mo2(CO)4(μ4,η(1:1:2:2)-P2)}(μ,η(1:1)-C12H10N2)Ag]n[Al{OC(CF3)3}4]n·0.075nCH2Cl2·1.425nC6H6 (3) and [{Cp2Mo2(CO)4(μ3,η(2:2:2)-P2)}2(μ,η(1:1)-C12H10N2)3Ag2]n[Al{OC(CF3)3}4]2n·2nC7H8 (4) are accessible. The latter shows a two-dimensional (2D) → 2D interpenetration structure. Furthermore, the formation of a unique three-dimensional polymer [{Cp2Mo2(CO)4(μ4,η(1:1:2:2)-P2)}(μ,η(1:1)-C12H10N2)Ag]n[Al{OC(CF3)3}4]n·0.3nCH2Cl2 (5b) together with another 2D polymer [{Cp2Mo2(CO)4(μ4,η(1:1:2:2)-P2)}(μ,η(1:1)-C12H10N2)3Ag2]n[Al{OC(CF3)3}4]2n·0.75CH2Cl2·0.5C7H8 (5a) was observed. In three of these polymers, unprecedented organometallic nodes were realized including one, two, or even four silver cations. All products were characterized by X-ray structural analysis and classified by the structural characteristics in three different network topologies.

A heterometallic macrocycle as a redox-controlled molecular hinge

The ability to modify the structure of nanoscopic assemblies in a controlled fashion is an important prerequisite for the creation of functional supramolecular systems. Here, we describe a heterometallic Pt2Cu2-macrocycle which behaves as a molecular hinge. A square-planar Pt(ii) complex with pendent 2-formylpyridine groups was synthesized and structurally characterized. Condensation of the complex with benzylamine followed by reaction with Cu(MeCN)4BF4 resulted in the formation of a rectangular Pt2Cu2-macrocycle. Upon chemical oxidation of the Cu centers, the macrocycle folds up to adopt a butterfly-like geometry in which the Pt centers approach each other. This process can be reversed by chemical reduction.

Bulky metallocavitands with a chiral cavity constructed by aluminum and magnesium atrane-likes: enantioselective recognition and separation of racemic alcohols

Novel tetranuclear metallocavitands with a chiral cavity were synthesized via self-assembly of aluminum/magnesium atrane-likes and were successfully applied to the chiral resolution of 2-butanol.

Dynamic combinatorial/covalent chemistry: a tool to read, generate and modulate the bioactivity of compounds and compound mixtures

Reversible covalent bond formation under thermodynamic control adds reactivity to self-assembled supramolecular systems, and is therefore an ideal tool to assess complexity of chemical and biological systems. Dynamic combinatorial/covalent chemistry (DCC) has been used to read structural information by selectively assembling receptors with the optimum molecular fit around a given template from a mixture of reversibly reacting building blocks. This technique allows access to efficient sensing devices and the generation of new biomolecules, such as small molecule receptor binders for drug discovery, but also larger biomimetic polymers and macromolecules with particular three-dimensional structural architectures. Adding a kinetic factor to a thermodynamically controlled equilibrium results in dynamic resolution and in self-sorting and self-replicating systems, all of which are of major importance in biological systems. Furthermore, the temporary modification of bioactive compounds by reversible combinatorial/covalent derivatisation allows control of their release and facilitates their transport across amphiphilic self-assembled systems such as artificial membranes or cell walls. The goal of this review is to give a conceptual overview of how the impact of DCC on supramolecular assemblies at different levels can allow us to understand, predict and modulate the complexity of biological systems.

A tetrameric cage with D2h symmetry through alkyne metathesis

Shape-persistent covalent organic polyhedrons (COPs) with ethynylene linkers are usually prepared through kinetically controlled cross-coupling reactions. The high-yielding synthesis of ethynylene-linked rigid tetrameric cages via one-step alkyne metathesis from readily accessible triyne precursors is presented. The tetrameric cage contains two macrocyclic panels and exhibits D2h symmetry. The assembly of such a COP is a thermodynamically controlled process, which involves the initial formation of macrocycles as key intermediates followed by the connection of two macrocycles with ethynylene linkages. With a large internal cavity, the cage exhibits a high binding selectivity toward C70 (K = 3.9×10(3) L mol(-1)) over C60 (no noticeable binding).

Fluorophore incorporation allows nanomolar guest sensing and white-light emission in M4L6 cage complexes

M₄L₆ cages built from BODIPY- and pyrene-containing subcomponents perform multiple functions: sensing anions and amino acids, and forming a white-light emitting complex with perylene.