Solid-State Gas Adsorption Studies with Discrete Palladium(II) [Pd2 (L)4 ]4+ Cages

A Reduced-Symmetry Pd2L4 Cage from a Heterotopic Dipyridyl Ligand

Two dipyridyl ligands, L3,3 and L3,4, have been used in combination with palladium(II) in the construction of metallosupramolecular species that show anion-dependent behavior in solution. A rare example of a low-symmetry (C2h) lantern-type cage is formed in one instance, [Pd2(L3,3)4]4+, while the isomeric ligand yields a larger double-walled square complex, [Pd4(L3,4)8]8+. [Pd2(L3,3)4](NO3)4 was isolated in crystalline form revealing two anions within the interior of the C2h-symmetry cage. The cage itself is held together by hydrogen bonding between "head-to-tail" pairs of ligands that reinforces the symmetry generated by the ditopic ligands. In solution, the cage with NO3- has sharp 1H nuclear magnetic resonance (NMR) signals at room temperature, while the BF4- analogue has broad signals that sharpen at higher temperatures or upon addition of (Bu4N)(NO3), highlighting the importance of the anion in templating or otherwise influencing self-assembly in solution. Altering the substitution position of one of the pyridyl rings yields a more "open" complex, with [Pd4(L3,4)8](NO3)8 being isolated as a crystalline solid. The double-walled square complex has a greater Pd···Pd separation due to the increased angle that the pyridyl groups subtend at the core of the ligand. NMR spectroscopy and mass spectrometry studies suggest a single species in the presence of nitrate but multiple species with tetrafluoroborate.

Metal-organic cages for gas adsorption and separation

The unique high surface area and tunable cavity size endow metal-organic cages (MOCs) with superior performance and broad application in gas adsorption and separation. Over the past three decades, for instance, numerous MOCs have been widely explored in adsorbing diverse types of gas including energy gases, greenhouse gases, toxic gases, noble gases, etc. To gain a better understanding of the structure-performance relationships, great endeavors have been devoted to ligand design, metal node regulation, active metal site construction, cavity size adjustment, and function-oriented ligand modification, thus opening up routes toward rationally designed MOCs with enhanced capabilities. Focusing on the unveiled structure-performance relationships of MOCs towards target gas molecules, this review consists of two parts, gas adsorption and gas separation, which are discussed separately. Each part discusses the cage assembly process, gas adsorption strategies, host-guest chemistry, and adsorption properties. Finally, we briefly overviewed the challenges and future directions in the rational development of MOC-based sorbents for application in challenging gas adsorption and separation, including the development of high adsorption capacity MOCs oriented by adsorbability and the development of highly selective adsorption MOCs oriented by separation performance.

Porous Metal-Organic Cages Based on Rigid Bicyclo[2.2.2]oct-7-ene Type Ligands: Synthesis, Structure, and Gas Uptake Properties

Three new ligands containing a bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxydiimide unit have been used to assemble lantern-type metal-organic cages with the general formula [Cu4 L4 ]. Functionalisation of the backbone of the ligands leads to distinct crystal packing motifs between the three cages, as observed with single-crystal X-ray diffraction. The three cages vary in their gas sorption behaviour, and the capacity of the materials for CO2 is found to depend on the activation conditions: softer activation conditions lead to superior uptake, and one of the cages displays the highest BET surface area found for lantern-type cages so far.

Ferrocene-Derived Palladium(II)-Based Metallosupramolecular Structures: Synthesis, Guest Interaction, and Stimulus-Responsiveness Studies

Using ferrocene-based ligand systems, a series of heterobimetallic architectures of the general formula [Pd_mL_n]^x+ were designed with the aim of installing an opening and closing mechanism that would allow the release and binding of guest molecules. Palladium complex formation was achieved through coordination to pyridyl groups, and using 2-, 3-, and 4-pyridyl derivatives provided access to defined PdL, PdL₂, and Pd₂L₄ structures, respectively. The supramolecular complexes were characterized using nuclear magnetic resonance (NMR) and infrared spectroscopy, mass spectrometry, and elemental analysis, and for some examples density functional theory calculations and single-crystal X-ray diffraction analysis. ¹H NMR spectroscopy was used to investigate disassembly and reassembly of the metallosupramolecular structures. The former was induced by cleavage of the relatively labile Pd-N_pyridyl bonds with the introduction of the competing ligands N,N'-dimethylaminopyridine (DMAP) and Cl^- (using tetrabutylammonium chloride) to yield [Pd(DMAP)₄]²⁺ and [PdCl₄]^2-, respectively. The process was found to be reversible for several of the heterodimetallic compounds, with the addition of H⁺ or Ag⁺ triggering complex reassembly. Guest binding studies with several architectures revealed interactions with the anionic guests p-toluenesulfonate and octyl sulfate, but not with neutral molecules. Furthermore, the release of guests was reversibly induced with Cl^- ions as a stimulus.

Exploiting Supramolecular Interactions to Control Isomer Distributions in Reduced-Symmetry [Pd2L4]4+ Cages

High-symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis, and drug delivery. Recently, there have been increasing efforts to enhance those applications by generating reduced-symmetry MSAs. Here we report our attempts to use supramolecular (dispersion and hydrogen-bonding) forces and solvophobic effects to generate isomerically pure [Pd₂(L)₄]⁴⁺ cage architectures from a family of new reduced-symmetry ditopic tripyridyl ligands. The reduced-symmetry tripyridyl ligands featured either solvophilic polyether chains, solvophobic alkyl chains, or amino substituents. We show using NMR spectroscopy, high-performance liquid chromatography, X-ray diffraction data, and density functional theory calculations that the combination of dispersion forces and solvophobic effects does not provide any control of the [Pd₂(L)₄]⁴⁺ isomer distribution with mixtures of all four cage isomers (HHHH, HHHT, cis-HHTT, or trans-HTHT, where H = head and T = tail) obtained in each case. More control was obtained by exploiting hydrogen-bonding interactions between amino units. While the cage assembly with a 3-amino-substituted tripyridyl ligand leads to a mixture of all four possible isomers, the related 2-amino-substituted tripyridyl ligand generated a cis-HHTT cage architecture. Formation of the cis-HHTT [Pd₂(L)₄]⁴⁺ cage was confirmed using NMR studies and X-ray crystallography.

Conjoined and non-conjoined coordination cages with palladium(II) vertices: structural diversity, solution dynamics, and intermolecular interactions

Self-assembled coordination complexes prepared from a combination of Pd(II) components with one or more types of high-symmetry or low-symmetry bis/tris/tetrakis-monodentate ligands are considered in this review. The structures of these complexes are viewed in terms of the presence of a metallo-macromonocycle or conjoined metallo-macromonocycles/metallocages in the frameworks. Analysis of the typical molecular structures revealed an open truth that one or more units of metallo-macromonocycles can be conjoined to afford planar or non-planar systems. In the same line, the enveloping surface of a 3D cage can be considered as a multiple number of conjoined metallomacrocycles that embrace a 3D space from all directions. However, two or more units of cages are conjoined in a multi-3D-cavity cage system and such a system is considered as a conjoined cage. Construction of such conjoined cages having a finite but multiple number of 3D-cavities unified in a single molecular architecture is a challenging task when compared to that of single-3D-cavity based compounds. Conjoining of as many as four units of 3D cages is known so far. Single- as well as multi-cavity cages of lower symmetry have become a very recent trend in this regard where low-symmetry ligands or mixed ligand ensembles are crafted in the framework of the cages. Other structural diversities like helicity in cages, and supramolecular isomerism are also included in this assorted literature work. Although isomerism in classical coordination complexes is well known, it is very less studied in self-assembled coordination complexes. Ligand isomerism is one such feature that is reviewed here. The dynamic behavior of the cages results in interesting reactivity aspects. A large variety of dynamic processes are collected under an umbrella, i.e., "ligand exchange reactions" and described with examples. Intermolecular interaction among the already self-assembled molecules is possible in solution, solid, and gel-phases as discussed in the last part of this review. The understanding of intermolecular interaction is likely to influence different areas of research including crystal engineering, and materials chemistry.

Hydrazone- and imine-containing [PdPtL4]4+ cages: a comparative study of the stability and host-guest chemistry

A new [PdPtL₄]⁴⁺ heterobimetallic cage containing hydrazone linkages has been synthesised using the sub-component self-assembly approach. ¹H and DOSY nuclear magnetic resonance (NMR) spectroscopy and electrospray ionisation mass spectrometry (ESIMS) data were consistent with the formation of the [PdPtL₄]⁴⁺ architecture. The cage was stimulus-responsive and could be partially disassembled and reassembled by the addition of dimethylaminopyridine (DMAP) and p-tolenesulfonic acid (TsOH), respectively. Additionally, the stability of the hydrazone cage against hydrolysis in the presence of water and nucleophilic decomposition in the presence of guest molecules was compared to a previously synthesised imine-containing [PdPtL₄]⁴⁺ cage. It was established that the hydrazone linkage was more resistant to hydrolysis. Furthermore, the host-guest (HG) chemistry with a series of drug and drug-like molecules was examined. The hydrazone cage was shown to interact with cisplatin while the smaller imine cage was shown to interact with 5-fluorouracil and oxaliplatin in CD₃CN. No HG interactions were observed in the more polar d₆-DMSO. In vitro antiproliferative activity studies demonstrated both cages were active against the cancer cell lines tested and displayed half-maximal inhibitory (IC₅₀) values in the range of 25-35 μM. Most [PdPtL₄]⁴⁺-drug mixtures tested had higher IC₅₀ values than the hosts. However, the [PdPtL₄]⁴⁺ cages, and [PdPtL₄]⁴⁺:drug mixtures were less cytotoxic than the well established anticancer drugs cisplatin, oxaliplatin and 5-fluorouracil.

Theoretical investigation of C1-C4 hydrocarbons adsorption and separation in a porous metallocavitand

The purification of light hydrocarbons is one of the most important chemical processes globally which consumes substantial energy. Porous materials are likely to improve the efficiency of the separation process by acting as regenerable solid adsorbents. To investigate such translational systems, the underlying mechanism of adsorption in the porous materials must be taken into account. Herein we report the adsorption and selective separation of C1-C4 hydrocarbons in the coinage metal-based macrocyclic metallocavitand Pillarplex, which exhibits excellent performance in the adsorption of CH4 at the ambient conditions with a binding energy of -17.9 kcal mol-1. In addition, the endohedral adsorption of C2-C4 hydrocarbon is impressive. The CH4, C2H4, C3H4, and 1,3-butadiene have potential uptake of 2.57, 4.26, 3.60, and 2.95 mmol g-1, respectively at ambient conditions are highest from their respective isomers. Selective separation of C1-C4 hydrocarbons is studied using ideal adsorption solution theory demonstrating its potential for one-step purification of C1-C3 hydrocarbons.

Pseudo-heterolepticity in Low-Symmetry Metal-Organic Cages

Heteroleptic metal-organic cages, formed through integrative self-assembly of ligand mixtures, are highly attractive as reduced symmetry supramolecular hosts. Ensuring high-fidelity, non-statistical self-assembly, however, presents a significant challenge in molecular engineering due to the inherent difficulty in predicting thermodynamic energy landscapes. In this work, two conceptual strategies are described that circumvent this issue, using ligand design strategies to access structurally sophisticated metal-organic hosts. Using these approaches, it was possible to realise cavity environments described by two inequivalent, unsymmetrical ligand frameworks, representing a significant step forward in the construction of highly anisotropic confined spaces.

Metal-organic cages against toxic chemicals and pollutants

The continuous release of toxic chemicals and pollutants into the atmosphere and natural waters threatens, directly and indirectly, human health, the sustainability of the planet, and the future of society. Materials capable of capturing or chemically inactivating hazardous substances, which are harmful to humans and the environment, are critical in the modern age. Metal-organic cages (MOCs) show great promise as materials against harmful agents both in solution and in solid state. This Highlight features examples of MOCs that selectively encapsulate, adsorb, or remove from a medium noxious gases, toxic organophosphorus compounds, water pollutant oxoanions, and some emerging organic contaminants. Remarkably, the toxicity of interacting contaminants may be lowered by MOCs as well. Specific cases pertaining to the use of these cages for the chemical degradation of some harmful substances are presented. This Highlight thus aims to provide an overview of the possibilities of MOCs in this area and new methodological insights into their operation for enhancing their activity and the engineering of further remediation applications.

Molecular Assemblies Offering Hydrogen-Bonding Cavities: Influence of Macrocyclic Cavity and Hydrogen Bonding on Dye Adsorption

This work presents a set of Hg macrocycles of amide-phosphine-based ligands offering H-bonding cavities of different dimensions. Such macrocycles are shown to selectively adsorb anionic dyes followed by neutral dyes as well as Prontosil, a biologically relevant antibiotic, within their cavities with the aid of H-bonding-assisted encapsulation. Kinetic experiments supported by spectroscopic and docking studies illustrate the importance of the cavity structure as well as H-bonds for the selective adsorption of dyes.

Conformational Control of a Metallo-Supramolecular Cage via the Dissymmetrical Modulation of Ligands

In nature as well as life systems, the presence of asymmetrical and dissymmetrical structures with specific functions is extremely common. However, the construction of metallo-supramolecular assemblies based on dissymmetrical ligands still remains a considerable challenge for avoiding the generation of unexpected isomers with similar thermodynamic stabilities, especially for three-dimensional supramolecular structures. In this study, a strategy for the conformational control of metallo-supramolecular cages via the enhancement of ligand dissymmetry was proposed. Four dissymmetrical ditopic ligands were designed and synthesized. By increasing the dissymmetry of length or angle, conformations of assemblies were precisely controlled to form discrete cis-Pd_n L_2n molecular cages.

A [Pd2L4]4+ cage complex for n-octyl-β-d-glycoside recognition

The cage complex [Pd294]4+ (3') binds n-octyl glycosides in DCM/DMSO (9 : 1) solution with Ka ≈ 51 M-1 for n-Oct-β-d-Glc and Ka ≈ 29 M-1 for n-Oct-β-d-Gal.

A Synthetic Galectin Mimic

Galectins are a galactoside specific subclass of carbohydrate binding proteins (lectins) involved in various cellular activities, certain cancers, infections, inflammations, and many other biological processes. The molecular basis for the selectivity of galectins is well-documented and revolves around appropriate interaction complementarity: an aromatic residue for C-H⋅⋅⋅π interactions and polar residues for (charge assisted) hydrogen bonds with the axial hydroxyl group of a galactoside. However, no synthetic mimics are currently available. We now report on the design and synthesis of the first galectin mimic (6), and show that it has a higher than 65-fold preference for n-octyl-β-galactoside (8) over n-octyl-β-glucoside (7) in CD2 Cl2 containing 5 % [D6 ]DMSO (with Ka ≥4500 M-1 for 6:8). Molecular modeling informed by nOe studies reveal a high degree of interaction complementarity between 6 and galactoside 8, which is very similar to the interaction complementarity found in natural galectins.

Cyclic monoterpenes trapped in a polyaromatic capsule: unusual selectivity, isomerization, and volatility suppression

Cyclic monoterpenes (CMTs) are intractable natural products with high volatility and strong odors so that there has been no molecular receptor capable of selectively and tightly trapping CMTs in both solution and the solid state. We herein report that a polyaromatic capsule acts as a functional nanoflask for CMTs with the following five features: (i) the capsule can selectively bind menthone from mixtures with other saturated CMTs in water. In contrast, (ii) treatment of the capsule with mixtures of menthone and π-conjugated CMTs gives rise to ternary host–guest complexes with high pair-selectivity. Notably, (iii) the encapsulated menthone displays unusual isomerization from a typical chair conformer to otherwise unstable conformers upon heating. (iv) The selective binding of volatilized CMTs is demonstrated by the capsule even in the solid state at atmospheric pressure. Furthermore, (v) the volatilities of CMTs are significantly suppressed at elevated temperatures by the capsule upon encapsulation in solution as well as in the solid state.

A polyaromatic capsule demonstrated its unique host functions toward cyclic monoterpenes, i.e., selective binding in water, pair-selective encapsulation, unusual isomerization, selective binding in the solid state, and remarkable volatility suppression.

Functional Material Systems Based on Soft Cages

Discrete molecular soft cages integrate multiple functionalities in one molecule. They express their functions from the confined space in their cavity, functional groups in the cavity interior wall and exterior wall, and the chelating nodes in many chelating cages. Such functional integrity render cage molecules special applications in material engineering. Increasing applications of cage molecules in material design have been reported in recent years. Compared with other cavity-rich molecular structures such as metal-organic framework (MOF) or covalent organic frameworks (COF), discrete soft cages present the unique advantage of material design flexibility, that they can easily composite with nanoparticles or polymers and exist in materials of various forms. We document the development of cage-based materials in recent years and expect to further inspire materials engineering to integrate contribution from the functionality specificity of cage molecules and ultimately promote the development of functional materials and thus human life qualities.

A Cavity-Tailored Metal-Organic Cage Entraps Gases Selectively in Solution and the Amorphous Solid State

Here we report the subcomponent self-assembly of a truxene-faced Zn4 L4 tetrahedron, which is capable of binding the smallest hydrocarbons in solution. By deliberately incorporating inward-facing ethyl groups on the truxene faces, the resulting partially-filled cage cavity was tailored to encapsulate methane, ethane, and ethene via van der Waals interactions at atmospheric pressure in acetonitrile, and also in the amorphous solid state. Interestingly, gas capture showed divergent selectivities in solution and the amorphous solid state. The selective binding may prove useful in designing new processes for the purification of methane and ethane as feedstocks for chemical synthesis.

SO2 Capture and Oxidation in a Metal-Organic Cage

The facile and green preparation of novel materials that capture sulfur dioxide (SO₂) with significant uptake at room temperature remains challenging, but it is crucial for public health and the environment. Herein, we explored for the first time the SO₂ adsorption within microporous metal-organic cages using the palladium(II)-based [Pd₆L₈](NO₃)₃₆ tetragonal prism 1, assembled in water under mild conditions. Notably and despite the low BET surface area of 1 (111 m² g^-1), sulfur dioxide was found to be irreversibly and strongly adsorbed within the activated cage at 298 K (up to 6.07 mmol g^-1). The measured values for the molar enthalpy of adsorption (ΔH_ads) coupled to the FTIR analyses imply a chemisorption process that involves the direct interaction of SO₂ with Pd(II) sites and the subsequent oxidation of this toxic chemical by the action of the nitrate anions in 1. To the best of our knowledge, this is the first reported metal-organic cage that proves useful for SO₂ adsorption. Metallosupramolecular adsorbents such as 1 could enable new detection applications and suggest that the integration of soft metal ions and self-assembly of molecular cages are a potential means for the easy tuning of SO₂ adsorption capabilities and behavior.

Self-Assembly of a Redox Active, Metallosupramolecular [Pd3 L6 ]6+ Complex Using a Rotationally Flexible Ferrocene Ligand

A new ferrocene-containing [Pd₃ (L_4EFc )₆ ]⁶⁺ (X^- )₆ (C ⋅ BF₄ and C ⋅ SbF₆ where X=BF₄ ^- or SbF₆ ^- ) self-assembled double-walled triangle has been synthesized from the known, rotationally flexible, 1,1'-bis(4-pyridylethynyl)ferrocene ligand (L_4EFc ), and characterized by ¹ H, ¹³ C and diffusion ordered (DOSY) NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), X-ray crystallography and cyclic voltammetry (CV). The molecular structures confirmed that double-walled triangle cage systems (C ⋅ BF₄ and C ⋅ SbF₆ ) were generated. C ⋅ BF₄ was shown to interact with the anionic guest, p-toluenesulfonate. CV experiments revealed that the triangles were redox active, however addition of the guest did not influence the redox potentials.

A New Mechanically-Interlocked [Pd2 L4 ] Cage Motif by Dimerization of two Peptide-based Lemniscates

Most metallo-supramolecular assemblies of low nuclearity adopt simple topologies, with bridging ligands spanning neighboring metal centers in a direct fashion. Here we contribute a new structural motif to the family of host compounds with low metal count (two) that consists of a pair of doubly-interlocked, Figure-eight-shaped subunits, also termed "lemniscates". Each metal is chelated by two chiral bidentate ligands, composed of a peptidic macrocycle that resembles a natural product with two pyridyl-terminated arms. DFT calculation results suggest that dimerization of the mononuclear halves is driven by a combination of 1) Coulomb interaction with a central anion, 2) π-stacking between intertwined ligand arms and 3) dispersive interactions between the structure's compact inner core bedded into an outer shell composed of the cavitand-type macrocycles. The resulting cage-like architecture was characterized by NMR, MS and X-ray structure analyses. This new mechanically bonded system highlights the scope of structural variety accessible in metal-mediated self-assemblies composed of only a few constituents.

Metal-Organic Frameworks and Metal-Organic Cages - A Perspective

The fields of metal-organic cages (MOCs) and metal-organic frameworks (MOFs) are both highly topical and continue to develop at a rapid pace. Despite clear synergies between the two fields, overlap is rarely observed. This article discusses the peculiarities and similarities of MOCs and MOFs in terms of synthetic strategies and approaches to system characterisation. The stability of both classes of material is compared, particularly in relation to their applications in guest storage and catalysis. Lastly, suggestions are made for opportunities for each field to learn and develop in partnership with the other.

Hysteresis in the gas sorption isotherms of metal–organic cages accompanied by subtle changes in molecular packing

Cooperative gas uptake in metal–organic cages is tuned using supramolecular chemistry.

Long-cavity [Pd2L4]4+ cages and designer 1,8-naphthalimide sulfonate guests: rich variation in affinity and differentiated binding stoichiometry

Long cavity dual domain [Pd₂L₄]⁴⁺ cages bind long, dual domain guests, with tunable binding affinities and stoichiometries.

Synthesis and characterization of low-nuclearity lantern-type porous coordination cages

This work presents the design, synthesis, and characterization of small lanterns with BET surface areas in excess of 200 m² g⁻¹. These cages represent the lower size limit for permanently microporous coordination cages.

Conformational control of Pd2L4 assemblies with unsymmetrical ligands

With increasing interest in the potential utility of metallo-supramolecular architectures for applications as diverse as catalysis and drug delivery, the ability to develop more complex assemblies is keenly sought after. Despite this, symmetrical ligands have been utilised almost exclusively to simplify the self-assembly process as without a significant driving foa mixture of isomeric products will be obtained. Although a small number of unsymmetrical ligands have been shown to serendipitously form well-defined metallo-supramolecular assemblies, a more systematic study could provide generally applicable information to assist in the design of lower symmetry architectures. Pd2L4 cages are a popular class of metallo-supramolecular assembly; research seeking to introduce added complexity into their structure to further their functionality has resulted in a handful of examples of heteroleptic structures, whilst the use of unsymmetrical ligands remains underexplored. Herein we show that it is possible to design unsymmetrical ligands in which either steric or geometric constraints, or both, can be incorporated into ligand frameworks to ensure exclusive formation of single isomers of three-dimensional Pd2L4 metallo-supramolecular assemblies with high fidelity. In this manner it is possible to access Pd2L4 cage architectures of reduced symmetry, a concept that could allow for the controlled spatial segregation of different functionalities within these systems. The introduction of steric directing groups was also seen to have a profound effect on the cage structures, suggesting that simple ligand modifications could be used to engineer structural properties.

Redox active [Pd2L4]4+ cages constructed from rotationally flexible 1,1'-disubstituted ferrocene ligands

Two new ferrocene-containing [Pd₂(L_Fc)₄]⁴⁺(X^-)₄ (where X^- = BF₄^- or SbF₆^-) self-assembled cages (C·BF₄ and C·SbF₆) were synthesised from the known, rotationally flexible, 1,1'-bis(3-pyridylethynyl)ferrocene ligand (L_Fc), and characterised by ¹H, ¹³C and diffusion ordered (DOSY) NMR and UV-visible absorption spectroscopies, high resolution electrospray ionisation mass spectrometry (HR-ESI-MS), elemental analysis, X-ray crystallography and cyclic voltammetry (CV). The molecular structures confirmed that cage-like systems (C·BF₄ and C·SbF₆) were generated. Similar to related [Pd₂L₄]⁴⁺(X^-)₄, C·SbF₆ was able to interact with a range of neutral and anionic guests, with p-toluenesulfonate showing the strongest association constant. Cyclic voltammetry studies revealed that the cage systems were redox active. However, the redox potential of the cage was unperturbed upon the addition of guests.

Recent Developments of Supramolecular Metal-based Structures for Applications in Cancer Therapy and Imaging

The biomedical application of discrete supramolecular metal-based structures, including supramolecular coordination complexes (SCCs), is still an emergent field of study. However, pioneering studies over the last 10 years demonstrated the potential of these supramolecular compounds as novel anticancer drugs, endowed with different mechanisms of action compared to classical small-molecules, often related to their peculiar molecular recognition properties. In addition, the robustness and modular composition of supramolecular metal-based structures allows for an incorporation of different functionalities in the same system to enable imaging in cells via different modalities, but also active tumor targeting and stimuli-responsiveness. Although most of the studies reported so far exploit these systems for therapy, supramolecular metal-based structures may also constitute ideal scaffolds to develop multimodal theranostic agents. Of note, the host-guest chemistry of 3D self-assembled supramolecular structures - within the metallacages family - can also be exploited to design novel drug delivery systems for anticancer chemotherapeutics. In this review, we aim at summarizing the pivotal concepts in this fascinating research area, starting with the main design principles and illustrating representative examples while providing a critical discussion of the state-of-the-art. A section is also included on supramolecular organometallic complexes (SOCs) whereby the (organic) linker is forming the organometallic bond to the metal node, whose biological applications are still to be explored. Certainly, the myriad of possible supramolecular metal-based structures and their almost limitless modularity and tunability suggests that the biomedical applications of such complex chemical entities will continue along this already promising path.

Construction of Metallacage-Cored Supramolecular Gel by Hierarchical Self-Assembly of Metal Coordination and Pillar[5]arene-Based Host-Guest Recognition

In this work, a Pd₂ L₄ metallacage 2•([BF₄ ]^- )₄ with four pillar[5]arene units is first prepared and characterized by 1D multinuclear NMR (¹ H, ¹¹ B, and ¹⁹ F NMR), 2D ¹ H-¹ H correlation spectra, ¹ H-¹³ C heteronuclear single quantum coherence, and diffusion-ordered NMR spectroscopy, and electrospray ionization time-of-flight mass spectrometry. By the introduction of a ditopic guest molecule 3 into a chloroform solution of 2•([BF₄ ]^- )₄ , a supramolecular polymer network gel is successfully constructed based on the metal coordination interactions and host-guest recognition between the pillar[5]arene units of 2•([BF₄ ]^- )₄ and neutral ditopic guest molecule 3. The temperature and pH responsivenesses of the supramolecular gel are studied, which are further employed for the controlled release of different cargos. As a demonstration, emodin and methylene blue are trapped in the cavities of the metallacage and in the pores of the supramolecular gel, respectively. Methylene blue is first released along with the gel-sol transition while emodin is then released by the further addition of acid to destroy the metallacage. This study explores the use of metallacage-cored supramolecular network gels for sequential controlled release and contributes to the development of smart and adaptive materials.

Selective Anion Recognition by a Dynamic Quadruple Helicate

An M₂ L₄ quadruple helicate, formed by wrapping four molecules of 1,4-bis(3-pyridyloxy)benzene (L₁ ) about two palladium(II) centers, is shown to bind anions within its internal cavity. ¹ H NMR exchange experiments provide a quantitative measure of anion selectivity and reveal a preference for ClO₄ ^- over the other tetrahedral anions BF₄ ^- and ReO₄ ^- and the octahedral anion PF₆ ^- . X-ray crystal structures of [Pd₂ (L₁ )₄ ]⁴⁺ helicates containing ClO₄ , BF₄ ^- and I^- reveal that the cavity size can dynamically change in response to the size of the guest.

A Rotaxane-like Cage-in-Ring Structural Motif for a Metallosupramolecular Pd6 L12 Aggregate

A BODIPY-based bis(3-pyridyl) ligand undergoes self-assembly upon coordination to tetravalent palladium(II) cations to form a Pd₆ L₁₂ metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane-like cage-in-ring arrangement. In this assembly the ligand adopts two different conformations-a C-shaped one to form a Pd₂ L₄ cage which is located in the center of a Pd₄ L₈ ring consisting of ligands in a W-shaped conformation. This assembly is not mechanically interlocked in the sense of catenation but it is stabilized only by attractive π-stacking between the peripheral BODIPY chromophores and the ligands' skeleton as well as attractive van der Waals interactions between the long alkoxy chains. As a result, the co-arrangement of the two components leads to a very efficient space filling. The overall structure can be described as a rotaxane-like assembly with a metallosupramolecular cage forming the axle in a metallosupramolecular ring. This unique structural motif could be characterized via ESI mass spectrometry, NMR spectroscopy, and X-ray crystallography.

A supramolecular porous material comprising Fe(ii) mesocates

The dinuclear mesocate [Fe₂L₃](BF₄)₄ is a supramolecular building block for a microporous material possessing 1D channels that are permanently accessible to incoming guest molecules showing a high selectivity for CO₂ over N₂.