Polycatenanes Formed of Self-Assembled Metal-Organic Cages

Poly-[n]-catenanes (PCs) self-assembled of three-dimensional (3D) metal organic cages (MOCs) (hereafter referred to as PCs-MOCs) are a relatively new class of mechanically interlocked molecules (MIMs) that combine the properties of MOCs and polymers. The synthesis of PCs-MOCs is challenging because of the difficulties associated with interlocking MOCs, the occurrence of multiple weak supramolecular electrostatic interactions between cages, and the importance of solvent templating effects. The high density of mechanical bonds interlocking the MOCs endows the MOCs with mechanical and physical properties such as enhanced stability, responsive dynamic behavior and low solubility, which can unlock new functional properties. In this Minireview, we highlight the benefit of interlocking MOCs in the formation of PCs-MOCs structures as well as the synthetic approaches exploited in their preparation, from thermodynamic to kinetic methods, both in the solution and solid-states. Examples of PCs-MOCs self-assembled from various types of nanosized cages (i.e., tetrahedral, trigonal prismatic, octahedral and icosahedral) are described in this article, providing an overview of the research carried out in this area. The focus is on the structure-property relationship with examples of functional applications such as electron conductivity, X-ray attenuation, gas adsorption and molecular sensing. We believe that the structural and functional aspects of the reviewed PCs-MOCs will attract chemists in this research field with great potential as new functional materials in nanotechnological disciplines such as gas adsorption, sensing and photophysical properties such as X-ray attenuation or electron conductivity.

Dynamic Interconversion and Induced-Fit Guest Binding with Two Macrocycle-Based Coordination Cages

We report the syntheses and host-guest chemistry of two interconvertible coordination cages, Pd₂L₂ and Pd₁L₁, from a dynamic macrocycle ligand (L) and a cis-blocking (tmen)Pd(NO₃)₂ (tmen = tetramethylethylenediamine) unit (Pd). The water-soluble macrocyclic L, which can bind various polycyclic aromatic hydrocarbon (PAH) guests in its cis-conformation, was constructed via four pyridinium bonds between two 2,4,6-tri(4-pyridyl)-1,3,5-triazine [TPT] panels and two p-xylene bridges. We selectively formed each cage either by changing the reaction concentration/solvent/temperature or through induced-fit guest encapsulation, while direct assembly of L and Pd resulted in a mixture of Pd₂L₂ and Pd₁L₁ in equilibrium. X-ray structures of the free ligand and the host-guest complexes confirmed the induce-fit adaptive changes in the ligand's conformation and the cage's cavity. This work demonstrates a useful strategy for designing multistimuli-responsive supramolecular hosts by coordination self-assembly with macrocyclic ligands featuring rich conformational freedom.

Highly Stable Europium(III) Tetrahedral (Eu4L4)(phen)4 Cage: Structure, Luminescence Properties, and Cellular Imaging

Luminescent lanthanide cages have many potential applications in guest recognition, sensing, magnetic resonance imaging (MRI), and bioimaging. However, these polynuclear lanthanide assemblies' poor stability, dispersity, and luminescence properties have significantly constrained their practical applications. Furthermore, it is still a huge challenge to simultaneously synthesize and design lanthanide organic polyhedra with high stability and quantum yield. Herein, we demonstrate a simple and robust strategy to improve the rigidity, chemical stability, and luminescence of an Eu(III) tetrahedral cage by introducing the conjugated planar auxiliary phen ligand. The self-assembled tetrahedral cage, (Eu₄L₄)(phen)₄ [L = (4,4',4″-tris(4,4,4-trifluoro-1,3-dioxobutyl)-triphenylamine), phen = 1,10-phenanthroline], exhibited characteristic luminescence of Eu³⁺ ions with high quantum yield (41%) and long lifetime (131 μs) in toluene (1.0 × 10^-6 M). Moreover, the Eu(III) cage was stable in water and even in an aqueous solution with a pH range of 1-14. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and cellular imaging revealed that the Pluronic F127-coated hybrid material, (Eu₄L₄)(phen)₄@F127, exhibited low cytotoxicity, good biocompatibility, and cellular imaging ability, which may inspire more insights into the development of lanthanide organic polyhedra (LOPs) for potential biomedical applications.

Beyond Platonic: How to Build Metal-Organic Polyhedra Capable of Binding Low-Symmetry, Information-Rich Molecular Cargoes

The field of metallosupramolecular chemistry has advanced rapidly in recent years. Much work in this area has focused on the formation of hollow self-assembled metal-organic architectures and exploration of the applications of their confined nanospaces. These discrete, soluble structures incorporate metal ions as ‘glue’ to link organic ligands together into polyhedra.Most of the architectures employed thus far have been highly symmetrical, as these have been the easiest to prepare. Such high-symmetry structures contain pseudospherical cavities, and so typically bind roughly spherical guests. Biomolecules and high-value synthetic compounds are rarely isotropic, highly-symmetrical species. To bind, sense, separate, and transform such substrates, new, lower-symmetry, metal-organic cages are needed. Herein we summarize recent approaches, which taken together form the first draft of a handbook for the design of higher-complexity, lower-symmetry, self-assembled metal-organic architectures.

Post-synthetic modifications of metal–organic cages

Metal-organic cages (MOCs) are discrete, supramolecular entities that consist of metal nodes and organic linkers, which can offer solution processability and high porosity. Thereby, their predesigned structures can undergo post-synthetic modifications (PSMs) to introduce new functional groups and properties by modifying the linker, metal node, pore or surface environment. This Review explores current PSM strategies used for MOCs, including covalent, coordination and noncovalent methods. The effects of newly introduced functional groups or generated complexes upon the PSMs of MOCs are also detailed, such as improving structural stability or endowing desired functionalities. The development of the aforementioned design principles has enabled systematic approaches for the development and characterization of families of MOCs and, thereby, provides insight into structure-function relationships that will guide future developments.

Metal-organic tube or layered assembly: reversible sheet-to-tube transformation and adaptive recognition

Rational preparation of an adaptive cavity-like enzyme is a great challenge for chemists. Herein, a new self-assembly strategy for the rational preparation of metal-organic tubes with nano-channels has been developed; both 1D metal-organic tube and corresponding 2D layered assemblies can be selectively synthesized driven by different templates; reversible sheet-to-tube transformation can be realized and the key intermediate has been identified. Furthermore, the newly formed nano-channel displays excellent polarity-selectivity for encapsulation of guest molecules, and can be further expanded or contracted through guest-driven adaptive deformation; even induced by very similar guest molecules, the adaptive deformations can also be obviously distinguished. Finally, the key chemicals benzene/hexane with a similar size can also be effectively separated by such nano-channels in the liquid phase. Our work not only provides a new synthetic strategy for the rational synthesis of metal-organic tubes with a reversible sheet-to-tube transformation character, but also gives a potential method for the construction of adaptive host-like enzymes and an in-depth understanding of the nature of adaptive host and guest molecules.

Conformation-Driven Self-Assembly: From a 1D Metal-Organic Polymer to an Infinite Double Nanotube

A novel conformation-driven self-assembly system from a one-dimensional polymer to a metal-organic double nanotube has been developed. DFT calculations indicate that the double nanotube with a syn-conformation of ligands is a thermodynamically favored stable product than a one-dimensional polymer. To the best of our knowledge, this is the first example of the infinite metal-organic double nanotube. In addition, a discrete M₆L₈-type cage has also been in situ self-assembled from rationally selected building blocks.

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.

Ring-to-Cage Structural Conversion via Template Effect in a Gold(I) Metallosupramolecular System

A unique example of a ring-to-cage structural conversion in a multinuclear gold(I) coordination system with d-penicillamine (d-H2 pen) is reported. The reaction of [Au2 Cl2 (dppe)] (dppe=1,2-bis(diphenylphosphino)ethane) with d-H2 pen in a 1:1 ratio gave [Au4 (dppe)2 (d-pen)2 ] ([1]), in which two [Au2 (dppe)]2+ units are linked by two d-pen S atoms in a cyclic form so as to have two bidentate-N,O coordination arms. The subsequent reaction of [1] with Cu(OTf)2 afforded [Au4 Cu(dppe)2 (d-pen)2 ]2+ ([2]2+ ), in which a CuII ion is chelated by the two coordination arms in [1] to form an AuI4 CuII bicyclic metallocage. A similar reaction using Cu(NO3 )2 was accompanied by the ring expansion of [1] to [Au8 (dppe)4 (d-pen)4 ], leading to the production of [Au8 Cu2 (dppe)4 (d-pen)4 ]4+ ([3]4+ ). In [3]4+ , two CuII ions are each chelated by the two coordination arms to form an AuI8 CuII2 tricyclic metallocage, accommodating a nitrate ion. The use of Ni(NO3 )2 or Ni(OAc)2 instead of Cu(NO3 )2 commonly gave a tricyclic metallocage of [Au8 Ni2 (dppe)4 (d-pen)4 ]4+ ([4]4+ ), but a water molecule was accommodated inside the AuI8 NiII2 metallocage.

Vapour-driven crystal-to-crystal transformation showing an interlocking switch of the coordination polymer chains between 1D and 3D

A rare crystal-to-crystal transformation occurs between 1D coordination polymer chains and 3D mechanically interlocked structures via reversible opening and closing of the homogeneous chains without any change in the coordination mode and composition.

Synthesis of stable polymetalated aromatic complexes through metal-macrocycle capsule-triggered cyclization

Polymetalated aromatic compounds are of great interest because of their intermediate roles in many organic transformations. However, they are elusive and synthetically challenging. In this study, a dynamic coordination capsule constructed by a flexible macrocycle and silver(i) ions is applied to trigger one-step or cascade cyclization reactions for various alkyne substrates, finally leading to five unprecedented polysilver heteroaromatic intermediates (including indole, quinoline, benzocarbazole and 2,2'-biindole). The acquired heteroaromatic species is doubly charged, particularly at vicinal positions, and each is surrounded by a tetrasilver aggregate. The metal-macrocycle capsule holds a great potential of flexibly adjusting its conformation to adapt different polysilver heteroaromatic species. DFT calculations further reveal that metal-perturbed aromaticity and multi-centered bonding both contribute to stabilization of the polysilver heteroaromatic complexes.

PdII2L4-type coordination cages up to three nanometers in size

The utilization of large ligands in coordination-based self-assembly represents an attractive strategy for the construction of supramolecular assemblies more than two nanometers in size. However, the implementation of this strategy is hampered by the fact that the preparation of such ligands often requires substantial synthetic effort. Herein, we describe a simple one-step protocol, which allows large bipyridyl ligands with a bent shape to be synthesized from easily accessible and/or commercially available starting materials. The ligands were used to construct PdII2L4-type coordination cages of unprecedented size. Furthermore, we provide evidence that these cages may be stabilized by close intramolecular packing of lipophilic ligand side chains. Packing effects of this kind are frequently encountered in protein assemblies, but they are seldom used as a design element in metallasupramolecular chemistry.

Design of a flexible organometallic tecton: host–guest chemistry with picric acid and self-assembly of platinum macrocycles

A new “flexible” and ditopic Pt(ii) organometallic compound is a tecton for the self-assembly of neutral metallacycles. It also exhibits significant binding affinity for picric acid.

The fabrication of a supra-amphiphile for dissipative self-assembly

Dissipative self-assembly is a challenging but attractive field of supramolecular science, because it generally concerns complex systems but is more close to the self-assembly of living bodies. In this article, we realized dissipative self-assembly by coupling a supra-amphiphile with a chemical oscillator. The supra-amphiphile was fabricated with iodine and a double hydrophilic block copolymer containing PEG segments, as the non-covalent interaction between PEG and iodine could turn PEG hydrophobic, leading to the formation of the supra-amphiphile. The self-assembly and disassembly of the supra-amphiphile could be controlled by varying the concentration of iodine. Therefore, the dissipative self-assembly of the supra-amphiphile was realized when it was coupled with the IO3--NH3OH+-OH- chemical oscillator, which was able to produce iodine periodically. Meanwhile, the kinetic data of the self-assembly and disassembly of the supra-amphiphile could be estimated by the theoretical simulation of the chemical oscillations. This line of research promotes the self-assembly of supra-amphiphiles one step forward from thermodynamic statics to a dissipative system, and also suggests a new strategy to investigate the kinetics of stimuli-responsive molecular self-assembly.

A Cd(ii) based metal organic framework: a photosensitive current conductor

A novel cadmium(ii) based metal organic framework, [Cd(3-bpd)(SCN)2]n () where 3-bpd = 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene has been synthesized and characterized by elemental analysis, various spectroscopic techniques, TGA and single crystal X-ray diffraction analysis. X-ray analysis shows the formation of an undulated polymeric two-dimensional network parallel to the (0 -1 1) plane. Current conduction properties of have been explored in the dark and in the presence of light. The study shows that current conduction of the complex increases with the increase of the incident light intensity. On progression of intensity of glancing radiation the photosensitivity of has been increased. The time dependent light response on charge carrier conduction reveals that complex may uncover new ground to explore in optoelectronic switching applications.

Conformation driven in situ interlock: from discrete metallocycles to infinite polycatenanes

A novel conformation driven self-assembly system, where four metallocycles with different conformations have been in situ self-assembled, has been reported. Interestingly, only square metallocycles can further interlock into polycatenanes. However, rectangular and rhombus metallocycles fail to overcome such an entropically unfavourable process, which constitutes an obstacle to the formation of polycatenanes.

Controllable Coordination Self-Assembly Based on Flexible Tripodal Ligands: From Finite Metallocages to Infinite Polycatenanes Step by Step

This article describes the developments in coordination self-assembly based on flexible tripodal ligands with different metal species. Various finite metallocages such as M3 L2 , M6 L8 , M6 L4 , M4 L4 and different catenanes based on discrete metallocages constructed from flexible tripodal ligands with suitable metal species are presented here. Many M3 L2 metallocages based on ligands L(1) -L(12) and different two-coordinated metal species have been prepared, in which various Ag(I) salts and other metal species that have been protected by suitable groups, such as Zn(OAc)2 , ZnBr2 , and PdBr2 , have been used as effective acceptors. All of the M6 L8 -type metallocages are constructed from ligands L(2) or L(12) -L(20) and different four-coordinated metal species, such as various palladium(II) salts or NiCl2 , and have similar topological structures. Only a few discrete M6 L4 -type metallocages, based on ligands L(21) -L(24) , have been reported, using different strategies such as protecting groups and steric hindrance. All of the M4 L4 -type cages have similar topological structures and are constructed from ligands L(25) -L(29) with multiple donor sites. More intriguing interlocking ensembles constructed from discrete metallocages are also described here in detail, namely, three [2]catenanes based on ligands L(30) -L(32) and four polycatenanes based on ligands L(33) -L(34) .

Catenanes: fifty years of molecular links

Half a century after Schill and Lüttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.

An interconverting family of coordination cages and a meso-helicate; effects of temperature, concentration, and solvent on the product distribution of a self-assembly process

The self-assembly between a water-soluble bis-bidentate ligand L(18w) and Co(II) salts in water affords three high-spin Co(II) products: a dinuclear meso-helicate [Co2(L(18w))3]X4; a tetrahedral cage [Co4(L(18w))6]X8; and a dodecanuclear truncated-tetrahedral cage [Co12(L(18w))18]X24 (X = BF4 or ClO4). All three products were crystallized under different conditions and structurally characterized. In [Co2(L(18w))3]X4 all three bridging ligands span a pair of metal ions; in the two larger products, there is a metal ion at each vertex of the Co4 or Co12 polyhedral cage array with a bridging ligand spanning a pair of metal ions along every edge. All three structural types are known: what is unusual here is the presence of all three from the same reaction. The assemblies Co2, Co4, and Co12 are in slow equilibrium (hours/days) in aqueous solution, and this can be conveniently monitored by (1)H NMR spectroscopy because (i) the paramagnetism of Co(II) disperses the signals over a range of ca. 200 ppm and (ii) the different symmetries of the three species give characteristically different numbers of independent (1)H NMR signals, which makes identification easy. From temperature- and concentration-dependent (1)H NMR studies it is clear that increasing temperature and increasing dilution favors fragmentation to give a larger proportion of the smaller assemblies for entropic reasons. High concentrations and low temperature favor the larger assembly despite the unfavorable entropic and electrostatic factors associated with its formation. We suggest that this arises from the hydrophobic effect: reorganization of several smaller complexes into one larger one results in a smaller proportion of the hydrophobic ligand surface being exposed to water, with a larger proportion of the ligand surface protected in the interior of the assembly. In agreement with this, (1)H NMR spectra in a nonaqueous solvent (MeNO2) show formation of only [Co2(L(18w))3]X4 because the driving force for reorganization into larger assemblies is now absent. Thus, we can identify the contributions of temperature, concentration, and solvent on the result of the metal/ligand self-assembly process and have determined the speciation behavior of the Co2/Co4/Co12 system in aqueous solution.

Controllable coordination-driven self-assembly: from discrete metallocages to infinite cage-based frameworks

CONSPECTUS: Nanosized supramolecular metallocages have a unique self-assembly process that allows chemists to both understand and control it. In addition, well-defined cavities of such supramolecular aggregates have various attractive applications including storage, separation, catalysis, recognition, drug delivery, and many others. Coordination-driven self-assembly of nanosized supramolecular metallocages is a powerful methodology to construct supramolecular metallocages with considerable size and desirable shapes. In this Account, we summarize our recent research on controllable coordination-driven assembly of supramolecular metallocages and infinite cage-based frameworks. To this end, we have chosen flexible ligands that can adopt various conformations and metal ions with suitable coordination sites for the rational design and assembly of metal-organic supramolecular ensembles. This has resulted in various types of metallocages including M3L2, M6L8, M6L4, and M12L8 with different sizes and shapes. Because the kinds of metal geometries are limited, we have found that we can replace single metal ions with metal clusters to alternatively increase molecular diversity and complexity. There are two clear-cut merits of this strategy. First, metal clusters are much bigger than single metal ions, which helps in the construction and stabilization of large metallocages, especially nanosized cages. Second, metal clusters can generate diverse assembly modes that chemists could not synthesize with single metal ions. This allows us to obtain a series of unprecedented supramolecular metallocages. The large cavities and potential unsaturated coordination sites of these discrete supramolecular cages offer opportunities to construct infinite cage-based frameworks. This in turn can offer us a new avenue to understand self-assembly and realize certain various functionalities. We introduce two types of infinite cage-based frameworks here: cage-based coordination polymers and cage-based polycatenanes, which we can construct through coordination bonds and mechanical bonds, respectively. Through either directly linking the unsaturated coordination sites of metallocages or replacing the labile terminal ligands with bridging ligands, we can produce infinite cage-based frameworks based on coordination bonds. We introduce several interesting cage-based coordination polymers, including a single-crystal-to-single-crystal transformation from a M6L8 cage to an infinite cage-based chain. Compared with discrete metallocages, these kinds of materials can give us higher structural stability and complexity, favoring the applications of metallocages. In addition, we discuss how we can use mechanical bonds, such as interlocking and interpenetrating, to construct extended cage-based frameworks. So far, study in this field has focused on polycatenanes constructed from M6L4 and M12L8 cages, as well as a controllable and dynamic self-assembly based on M6L4 metallocages. We also discuss cage-based polycatenanes, which can give dynamic properties to discrete metallocages. We hope that our investigations will bring new insights to the world of the supramolecular metallocages by enlarging its breadth and encourage us to devote more effort to this blossoming field in the future.

One-pot synthesis of oxamidato-bridged hexarhenium trigonal prisms adorned with ester functionality

The self-assembly of hexarhenium trigonal prisms using fac-Re(CO)₃ cores (pink), bischelating oxamide ligands (green) and ester-containing tritopic linkers (blue) has been accomplished via an orthogonal bonding approach.

Metal–organic frameworks based on flexible ligands (FL-MOFs): structures and applications

This review presents the recent developments on FL-MOFs, including their structures and applications in gas adsorption, catalysis and proton conduction.