Interpenetrated Cage Structures

Host-Guest Chemistry of Truncated Tetrahedral Imine Cages with Ammonium Ions

Three shape-persistent [4+4] imine cages with truncated tetrahedral geometry with different window sizes were studied as hosts for the encapsulation of tetra-n-alkylammonium salts of various bulkiness. In various solvents the cages behave differently. For instance, in dichloromethane the cage with smallest window size takes up NEt4+ but not NMe4+, which is in contrast to the two cages with larger windows hosting both ions. To find out the reason for this, kinetic experiments were carried out to determine the velocity of uptake but also to deduce the activation barriers for these processes. To support the experimental results, calculations for the guest uptakes have been performed by molecular mechanics' simulations. Finally, the complexation of pharmaceutical interested compounds, such as acetylcholine, muscarine or denatonium have been determined by NMR experiments.

Singlet oxygen stimulus for switchable functional organic cages

Molecular cages 1a and 2a incorporating a 9,10-diphenylanthracene (DPA) chromophore were synthesized through a templated ring-closure metathesis approach that allows variation in cavity size through the introduction of up to three different pillars. Reversible Diels–Alder reaction between the DPA moiety and photogenerated singlet oxygen smoothly converted 1a and 2a to the corresponding endoperoxide cages 1b and 2b, which are converted back to 1a and 2a upon heating. Endoperoxide formation constitutes a reversible covalent signal that combines structural changes in the interior of the cage with introduction of two additional coordination sites. This results in a large modulation of the binding ability of the receptors attributed to a change in the location of the preferred binding site owing to the added coordination by the endoperoxide oxygen lone pairs. Cages 1a and 2a form complexes with sodium and cesium whose association constants are modified by 4–20 fold for Na⁺ and 200–450 fold for Cs⁺ upon conversion to 1b and 2b. DFT calculations show that in the anthracene form, cages 1a and 2a can bind 2 metal cations in their periphery so that each cation is coordinated by 4 oxygens and one amine nitrogen, whereas the endoperoxide cages 1b and 2b bind cations centrally in a geometry that favors coordination to the endoperoxide oxygens.

Allosteric switchable organic cages allow variability in cation recognition.

Tuning the structure and the properties of dithiafulvene metalla-assembled tweezers

An electroactive M₂L₂ metalla-macrocycle constructed through coordination driven self-assembly dimerizes upon oxidation and binds an electro-deficient substrate with a high association constant.

Structural elucidation of microcrystalline MOFs from powder X-ray diffraction

Ab initio powder XRD structure solution and MOFs.

Chiral Self-Sorting in Truxene-Based Metallacages

Two chiral face-rotating metalla-assembled polyhedra were constructed upon self-assembling achiral components, i.e., a tritopic ligand based on a truxene core (10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene) and two different hydroxyquinonato–bridged diruthenium complexes. Both polyhedra were characterized in solution as well as in the solid state by X-ray crystallography. In both cases, the self-sorting process leading to only two homo-chiral enantiomers was governed by non-covalent interactions between both truxene units that faced each other.

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.

A Zn(II) Metallocycle as Platform to Assemble a 1D + 1D → 1D Polyrotaxane via π···π Stacking of an Ancillary Ligand

A new [Zn2L2] metallocycle bearing two metal centers that can coordinate ancillary ligands and a pocket suitable to host guest molecules is reported. These two features are exploited by reacting the metallocycle with a pyridine ligand to self-assemble in the solid state an extended intertwined system with the rare 1D + 1D → 1D topology. This interpenetrated architecture is supported by π···π stacking between two pyridine units of two different metallocycles in the pocket of a third metallocycle.

One-Pot Self-Assembly of Stellated Metallosupramolecules from Multivalent and Complementary Terpyridine-Based Ligands

A series of stellated metallosupramolecular architectures have been assembled through three-component integrative self-sorting. Building on the complementary ligand pairing, the initial attempts to synthesize the hexagram complex from a combination of X-shaped tetrakis- and V-shaped bis-terpyridine ligands, and Cd^II ions, resulted in an unprecedented mixture of stellated octanuclear and dodecanuclear metallocages, which were further isolated by column chromatography. To overcome the unexpected obstacle, the multivalent ligand design along with spontaneous heteroleptic complexation was applied to realization of the one-pot synthesis of the intricate topology. A centrally situated triangle served as a prop for quantitative formation of the six-pointed stellated complex. Notably, in the absence of the triangular prop, a four-pointed star was produced.

Chiral Self-Discrimination and Guest Recognition in Helicene-Based Coordination Cages

Chiral nanosized confinements play a major role for enantioselective recognition and reaction control in biological systems. Supramolecular self-assembly gives access to artificial mimics with tunable sizes and properties. Herein, a new family of [Pd2 L4 ] coordination cages based on a chiral [6]helicene backbone is introduced. A racemic mixture of the bis-monodentate pyridyl ligand L1 selectively assembles with PdII cations under chiral self-discrimination to an achiral meso cage, cis-[Pd2 L1P2 L1M2 ]. Enantiopure L1 forms homochiral cages [Pd2 L1P/M4 ]. A longer derivative L2 forms chiral cages [Pd2 L2P/M4 ] with larger cavities, which bind optical isomers of chiral guests with different affinities. Owing to its distinct chiroptical properties, this cage can distinguish non-chiral guests of different lengths, as they were found to squeeze or elongate the cavity under modulation of the helical pitch of the helicenes. The CD spectroscopic results were supported by ion mobility mass spectrometry.

Unraveling Confined Dynamics of Guests Trapped in Self-Assembled Pd6L4 Nanocages by Ultrafast Mid-IR Polarization-Dependent Spectroscopy

Self-assembled coordination cages form host-guest complexes through weak noncovalent interactions. Knowledge of how these weak interactions affect the structure, reactivity, and dynamics of guest molecules is important to further the design principles of current systems and optimize their specific functions. We apply ultrafast mid-IR polarization-dependent pump-probe spectroscopy to probe the effects of two Pd₆L₄ self-assembled nanocages on the properties and dynamics of fluxional group-VIII metal carbonyl guest molecules. We find that the interactions between the Pd₆L₄ nanocages and guest molecules act to alter the ultrafast dynamics of the guests, restricting rotational diffusional motion and decreasing the vibrational lifetime.

Palladium-Based Metal-Ligand Assemblies: The Contrasting Behavior upon Addition of Pyridine or Acid

The stability of five different [Pd _n(N-donor) _m]^{2 n+} assemblies was examined by performing disassembly experiments with pyridine and with trifluoroacetic acid. Pyridine-induced disassembly was found to be most pronounced for Pd complexes containing N-donor ligands of low basicity. At the same time, these assemblies displayed high acid resistance. The contrasting stability in the presence of acid or pyridine can be used for the pH-controlled switching between different metallosupramolecular structures.

A pore-expanded supramolecular organic framework and its enrichment of photosensitizers and catalysts for visible-light-induced hydrogen production

A 3.6 nm-pore SOF is constructed, which adsorbs both photosensitizers and polyoxometallates for visible light-induced proton reduction to produce H₂.

A luminescent edge-interlocked prismatic heteroleptic metallocage assembled through a ligand replacement reaction

A luminescent edge-interlocked heteroleptic metallocages based on Cu₃(pyrazolate)₃ was prepared through a ligand replacement reaction from a homoleptic metallocage and a new ligand.

A Cu(ii) metallocycle for the reversible self-assembly of coordination-driven polyrotaxane-like architectures

We report the design and synthesis of a Cu(ii) metallocycle (1) and use the possibility to expand the Cu(ii) coordination sphere to self-assemble mechanically interlocked species via interpenetration. Metallocycle 1 can be used as a platform to reversibly assemble a 1D + 1D → 1D coordination-driven polyrotaxane (3), where 1 acts as the hosting ring as well as the stopper, and 4,4'-bipyridine is the guest-axle. A coordinating solvent can substitute the 4,4'-bipyridine axle to disassemble the polyrotaxane (3 → 2) that is easily restored by further adding 4,4'-bipyridine (2 → 3). Other polyrotaxanes can be isolated by reacting 1 with pyridine (4) and phenylpyridine (5). Interconversion among the presented species is demonstrated and ensured by the open position of each copper center in platform 1.

Donor-Site-Directed Rational Assembly of Heteroleptic cis-[Pd2 L2 L'2 ] Coordination Cages from Picolyl Ligands

A donor-site engineering approach facilitates the formation of heteroleptic [Pd2 L2 L'2 ]4+ cage structures through a favored cis-'in2 /out2 ' spatial configuration of the methyl groups of 5- and 3-substituted bis-monodentate picolyl ligands with flat acridone and bent phenothiazine backbones. The heteroleptic cages were confirmed by ESI-MS and 2D NMR experiments as well as DFT calculations, which pointed toward a cis-configuration being energetically favored. This was further supported by the synthesis and X-ray structure of a previously unreported cis-[Pd(2-picoline)4 ]2+ complex. The formation of homoleptic structures, however, was met with considerable steric hindrance at the PdII centers, as observed by the formation of [Pd2 L3 (solvent)2 ]4+ and [Pd2 L2 (solvent)4 ]4+ species when only one type of acridone-based ligand was offered. In contrast, bent phenothiazine ligands with outside-pointing methyl groups showed the ability to form interpenetrated double-cages, as revealed by X-ray crystallography. The general route presented herein enables the assembly of uniform cis-[Pd2 L2 L'2 ]4+ coordination cages, thus furthering the possibility to increase structural and functional complexity in supramolecular systems.

Inflating face-capped Pd6L8 coordination cages

Tritopic metalloligands were used to form two Pd6L8-type coordination cages. With molecular weights of more than 15 kDa and PdPd distances of up to 4.2 nm, these complexes are among the largest palladium cages described to date.

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.

Hierarchical Assembly of an Interlocked M8 L16 Container

The self-assembly of eight PdII cations and sixteen phenanthrene-derived bridging ligands with 60° bite angles yielded a novel M8 L16 metallosupramolecular architecture composed of two interlocked D4h -symmetric barrel-shaped containers. Mass spectrometry, NMR spectroscopy, and X-ray analysis revealed this self-assembled structure to be a very large "Hopf link" catenane featuring channel-like cavities, which are occupied by NO3- anions. The importance of the anions as catenation templates became imminent when we observed the nitrate-triggered structural rearrangement of a mixture of M3 L6 and M4 L8 assemblies formed in the presence of BF4- anions into the same interlocked molecule. Furthermore, the densely packed structure of the M8 L16 catenane was exploited in the preparation of a hexyloxy-functionalized analogue, which further self-assembled into vesicle-like aggregates in a reversible manner.

Flexibility of components alters the self-assembly pathway of Pd2L4 coordination cages

The self-assembly process of a Pd₂L₄ cage consisting of flexible ditopic ligands and Pd(ii) ions was revealed by QASAP (quantitative analysis of self-assembly process), which enables one to obtain information about the intermediates transiently produced during the self-assembly as the average composition of all the intermediates. It was found that the dominant pathway to the cage is the formation of a submicrometre-sized sheet structure, which was characterized by dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM), followed by the addition of free ditopic ligands to the Pd(ii) centres of the sheet structure to trigger the cage formation. This assembly process is completely different from that of a Pd₂L₄ cage composed of rigid ditopic ligands, indicating that the flexibility of the components strongly affects the self-assembly process.

Site-selective anion recognition of an interlocked dimer

Interlocked dimer 2, which is composed of two physically interlocked monomers 1, has three cavities (cavity A × 2 and cavity B × 1) and can encapsulate three anions, such as NO₃^- and BF₄^-, one anion per cavity. There are six possible encapsulation patterns, A-F; two (A and F) contain only one kind of anion and the others (B-E) contain both NO₃^- and BF₄^- at the same time with different ratios and with different positions. Anion competition experiments showed that in addition to F, which encapsulates three NO₃^- ions, C, in which NO₃^- and BF₄^- ions are captured in cavities A and cavity B, respectively, was selectively formed. Detailed investigations have revealed that B-E were formed by dimerization, but three of the four were subjected to anion exchange and converged into C or F. This selective formation can be explained by the fact that NO₃^- is a better anion template than BF₄^-, as well as the molecular structure of the interlocked dimer; cavities A are surrounded by four bridging ligands and can be accessed by free anions, whereas no space available for anion exchange is present around cavity B because this cavity is surrounded by eight bridging ligands. Therefore, the BF₄^- ions in cavities A are expelled by free NO₃^-, but the BF₄^- ion in cavity B is not, resulting in the selection of C and F. We have found that the volume of the cavity influenced anion recognition. New interlocked dimer 3, which has smaller cavities than those of 2, captured three NO₃^- ions to form F, whereas only a small amount of an interlocked dimer that contains both NO₃^- and BF₄^- was formed.

Binding of anions in triply interlocked coordination catenanes and dynamic allostery for dehalogenation reactions

By synergistic combination of multicomponent self-assembly and template-directed approaches, triply interlocked metal organic catenanes that consist of two isolated chirally identical tetrahedrons were constructed and stabilized as thermodynamic minima. In the presence of suitable template anions, the structural conversion from the isolated tetrahedral conformers into locked catenanes occurred via the cleavage of an intrinsically reversible coordination bond in each of the tetrahedrons, followed by the reengineering and interlocking of two fragments with the regeneration of the broken coordination bonds. The presence of several kinds of individual pocket that were attributed to the triply interlocked patterns enabled the possibility of encapsulating different anions, allowing the dynamic allostery between the unlocked/locked conformers to promote the dehalogenation reaction of 3-bromo-cyclohexene efficiently, as with the use of dehalogenase enzymes. The interlocked structures could be unlocked into two individual tetrahedrons through removal of the well-matched anion templates. The stability and reversibility of the locked/unlocked structures were further confirmed by the catching/releasing process that accompanied emission switching, providing opportunities for the system to be a dynamic molecular logic system.

Concentration dependent supramolecular interconversions of triptycene-based cubic, prismatic, and tetrahedral structures

New insight into the molecular fission–fusion process is obtained with the characterization of a stable intermediate prismatic cage.

Terpyridine-based metallosupramolecular constructs: tailored monomers to precise 2D-motifs and 3D-metallocages

Comprehensive summary of the recent developments in the growing field of terpyridine-based, discrete metallosupramolecular architectures.

The Promise of Self-Assembled 3D Supramolecular Coordination Complexes for Biomedical Applications

In the supramolecular chemistry field, coordination-driven self-assembly has provided the basis for tremendous growth across many subdisciplines, spanning from fundamental investigations regarding the design and synthesis of new architectures to defining different practical applications. Within this framework, supramolecular coordination complexes (SCCs), defined as large chemical entities formed from smaller precursor building blocks of ionic metal nodes and organic multidentate ligands, resulting in intricate and well-defined supramolecular structures, hold great promise. Notably, interest in the construction of discrete 3D molecular architectures, such as those offered by SCCs, has experienced extraordinary progress because of their potential application as sensors, catalysts, probes, and containers and in basic host-guest chemistry. Despite numerous synthetic efforts and a number of inherent favorable properties, the field of 3D SCCs for biomedical applications is still in its infancy. This Viewpoint focuses on 3D SCCs, specifically metallacages and helicates, first briefly presenting the fundamentals in terms of the synthesis and characterization of their host-guest properties, followed by an overview of the possible biological applications with representative examples. Thus, emphasis will be given in particular to metallacages as drug delivery systems and to chiral helicates as DNA recognition domains. Overall, we will provide an update on the state-of-the-art literature and will define the challenges in this fascinating research area at the interface of different disciplines.

Anion-Directed Metallocages: A Study on the Tendency of Anion Templation

Self-assembly of Cu(NO₃ )₂ ⋅3 H₂ O and di(3-pyridylmethyl)amine (dpma) with addition of different acids (HNO₃ , HOAc, HCl, HClO₄ , HOTf, HPF₆ , HBF₄ , and H₂ SO₄ ) afforded a family of anion-templated tetragonal metallocages with a cationic prismatic structure of [(G^n- )⊂{Cu₂ (Hdpma)₄ }]^(8-n)+ (G^n- =NO₃^- , PF₆^- , SiF₆^2- ) with different ligating anions/solvents (NO₃^- , Cl^- , ClO₄^- , OTf^- , H₂ O) outside the cage. Systematic competitive experiments have rationalized the tendency of anion templation towards the formation of metallocages [(G^n- )⊂{Cu₂ (Hdpma)₄ }]^(8-n)+ as occurring in the order SiF₆^2- ≈PF₆^- >NO₃^- >SO₄^2- ≈ClO₄^- ≈BF₄^- . This sequence is mostly elucidated by shape control over size selectivity and electrostatic attraction between the cationic {Cu₂ (Hdpma)₄ }⁸⁺ host and the anionic guests. In addition, these results have also roughly ranked the anion coordination ability in the order Cl^- , ClO₄^- , OTf^- >NO₃^- >BF₄^- , CH₃ SO₄^- . Magnetic studies of metallocages 1 t and 2-4 suggest that the fitted magnetic interaction, being weakly magnetically coupled overall, is interpreted as a result of the combination of intracage ferromagnetic coupling integrals and intercage antiferromagnetic exchange; both contributions are very weak and comparable in strength.

Cation-Anion Arrangement Patterns in Self-Assembled Pd2L4 and Pd4L8 Coordination Cages

Compounds featuring one-dimensional regular arrangements of stacked metal complexes and alternating [cation-anion]_∞ sequences have raised considerable interest owing to their peculiar electronic and optical properties as well as guest inclusion capabilities. While traditional ways to realize these structural motifs rely on crystalline compounds, exclusively existing in the solid state, recent progress in the area of metal-mediated supramolecular self-assembly allows for the rational synthesis of structurally well-defined short stretches of stacked metal complexes and cation-anion arrangements. Therefore, metal cations, counteranions, and suitably designed organic bridges are allowed to self-assemble in solution. While the bridges can be designed as cross-linkers to yield extended two- or three-dimensional networks such as layered materials, metal-organic frameworks (MOFs), or porous coordination polymers (PCPs), they can also be tailored to lead to discrete nanoscopic objects. Supramolecular helicates, grids, and knots belong to this class of compounds, and a particularly interesting subfamily are coordination cages and capsules, which possess nanosized cavities with the ability to encapsulate guest molecules. Here, we focus on coordination cages consisting of two or more square-planar Pd(II) or Pt(II) metal cations, bridged by banana-shaped bis-monodentate pyridyl ligands that encapsulate various guest molecules, usually anions, in their cavities. Monoanions as well as dianions with localized or delocalized charges can be bound with remarkable complementarity between cage and guest in terms of size and shape. We show how dimerization of the prototypical [Pd₂L₄] cages into their interpenetrated dimers [Pd₄L₈] leads to an increase in cavity number from one to three while the cavity volume decreases. Usually, all three pockets of these double cages are filled with monoanions such as BF₄^- or Cl^-, thus leading to well-defined linear [Pd-anion]₃Pd stacks, as observed by X-ray studies. The ligand-based mechanical coupling of the linearly aligned cavities leads to interesting effects concerning guest encapsulation cooperativity, such as allosteric binding and triggered sequential uptake. While most of the so far reported coordination cages consist of only a single type of ligand, recent advances in rational assembly strategies allow for high-yielding syntheses of structurally defined multicomponent architectures by integrative self-sorting mechanisms. One family of heteroleptic [Pd₂L₂L'₂] cages whose formation is based on shape-complementarity between two different ligands, L and L', is introduced. Furthermore, the implementation of ligand-based functions such as redox activity, photochromic behavior, specific binding sites, chirality, and catalytic activity allows us to study systems with properties far beyond basic structural features. We showcase selected examples of self-assembled cages whose guest uptake or even overall structural integrity is reversibly switched by light or small molecules with potential application in stimuli responsive materials (e.g., for sequestration of pollutants or stabilization of reactive compounds) up to functional nanosystems (e.g., diagnostic devices or supramolecular catalysts) and molecular machines.

Multicavity Metallosupramolecular Architectures

Discrete metallosupramolecular systems are often macrocyclic or cage-like architectures with an accessible internal cavity. Guest molecules can reside within these cavities and much of the interest in these systems is derived from these fascinating host-guest interactions. A range of potential applications stem from the ability of these metallosupramolecular architectures to encapsulate guests. These applications include catalysis or acting as molecular reaction flasks, the molecular scavenging of pollutants, storage of reactive species, and drug delivery. Multicavity metallosupramolecular architectures combine the ability of large hollow assemblies to bind multiple guests concurrently with the binding specificity associated with small cages. A variety of different approaches to generating separate compartments within a single metallosupramolecular assembly have emerged. These include interpenetrated cages, cages with polytopic ligands that have a long backbone, and molecules that have two or more clefts. This review examines these approaches, and highlights key contributions to the field.

Excitation Energy Delocalization and Transfer to Guests within MII4L6 Cage Frameworks

We have prepared a series of M^II₄L₆ tetrahedral cages containing one or the other of two distinct BODIPY moieties, as well as mixed cages that contain both BODIPY chromophores. The photophysical properties of these cages and their fullerene-encapsulated adducts were studied in depth. Upon cage formation, the charge-transfer character exhibited by the bis(aminophenyl)-BODIPY subcomponents disappeared. Strong excitonic interactions were instead observed between at least two BODIPY chromophores along the edges of the cages, arising from the electronic delocalization through the metal centers. This excited-state delocalization contrasts with previously reported cages. All cages exhibited the same progression from an initial bright singlet state (species I) to a delocalized dark state (species II), driven by interactions between the transition dipoles of the ligands, and subsequently into geometrically relaxed species III. In the case of cages loaded with C₆₀ or C₇₀ fullerenes, ultrafast host-to-guest electron transfer was observed to compete with the excitonic interactions, short-circuiting the I → II → III sequence.