Capsule-Capsule Conversion by Guest Encapsulation

Dynamic Metallosupramolecular Cages Containing 12 Adaptable Pockets for High-Order Guest Binding Beyond Biomimicry

Molecular recognition lies at the heart of biological functions, which inspires lasting research in artificial host syntheses to mimic biomolecules that can recognize, process, and transport molecules with the highest level of complexity; nonetheless, the design principle and quantifying methodology of artificial hosts for multiple guests (≥4) remain a formidable task. Herein, we report two rhombic dodecahedral cages [(Zn/Fe)8Pd6-MOC-16], which embrace 12 adaptive pockets for multiguest binding with distinct conformational dynamics inherent in metal-center lability and are able to capture 4-24 guests to manifest a surprising complexity of binding scenarios. The exceptional high-order and hierarchical encapsulation phenomena suggest a wide host-guest dynamic-fit, enabling conformational adjustment and adaptation beyond the duality of induced-fit and conformational selection in protein interactions. A critical inspection of the host-guest binding events in solution has been performed by NMR and ESI-MS spectra, highlighting the importance of acquiring a reliable binding repertoire from different techniques and the uncertainty of quantifying the binding affinities of multiplying guests by an oversimplified method.

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.

Transformation networks of metal-organic cages controlled by chemical stimuli

The flexibility of biomolecules enables them to adapt and transform as a result of signals received from the external environment, expressing different functions in different contexts. In similar fashion, coordination cages can undergo stimuli-triggered transformations owing to the dynamic nature of the metal-ligand bonds that hold them together. Different types of stimuli can trigger dynamic reconfiguration of these metal-organic assemblies, to switch on or off desired functionalities. Such adaptable systems are of interest for applications in switchable catalysis, selective molecular recognition or as transformable materials. This review highlights recent advances in the transformation of cages using chemical stimuli, providing a catalogue of reported strategies to transform cages and thus allow the creation of new architectures. Firstly we focus on strategies for transformation through the introduction of new cage components, which trigger reconstitution of the initial set of components. Secondly we summarize conversions triggered by external stimuli such as guests, concentration, solvent or pH, highlighting the adaptation processes that coordination cages can undergo. Finally, systems capable of responding to multiple stimuli are described. Such systems constitute composite chemical networks with the potential for more complex behaviour. We aim to offer new perspectives on how to design transformation networks, in order to shed light on signal-driven transformation processes that lead to the preparation of new functional metal-organic architectures.

LiBF4 -Induced Rearrangement and Desymmetrization of a Palladium-Ligand Assembly

Thirteen palladium-ligand assemblies with different structures and topologies were investigated for the ability to bind lithium ions. In one case, the addition of LiBF4 resulted in a profound structural rearrangement, converting a dincluclear [Pd2 L4 ]4+ complex into a low-symmetry [Pd4 L8 ]8+ assembly with two binding pockets for solvated LiBF4 ion pairs. The rearrangement could only be induced by Li+ , indicating highly specific host-guest interactions. A structural analysis of the [Pd4 L8 ]8+ receptor revealed a compact structure with multiple intramolecular interactions, reminiscent of what is seen for natural and synthetic foldamers.

Ion Mobility Mass Spectrometry Uncovers Guest-Induced Distortions in a Supramolecular Organometallic Metallosquare

The encapsulation of the tetracationic palladium metallosquare with four pyrene-bis-imidazolylidene ligands [1]4+ with a series of organic molecules was studied by Electrospray ionization Travelling Wave Ion-Mobility Mass Spectrometry (ESI TWIM-MS). The method allowed to determine the Collision Cross Sections (CCSs), which were used to assess the size changes experienced by the host upon encapsulation of the guest molecules. When fullerenes were used as guests, the host is expanded ΔCCS 13 Å2 and 23 Å2 , for C60 or C70 , respectively. The metallorectangle [1]4+ was also used for the encapsulation of a series of polycyclic aromatic hydrocarbons (PAHs) and naphthalenetetracarboxylic diimide (NTCDI), to form complexes of formula [(NTCDI)2 (PAH)@1]4+ . For these host:guest adducts, the ESI IM-MS studies revealed that [1]4+ is expanded by 47-49 Å2 .. The energy-minimized structures of [1]4+ , [C60 @1]4+ , [C70 @1]4+ , [(NTCDI)2 (corannulene)@1]4+ in the gas phase were obtained by DFT calculations.Introduction.

Shape-Adaptability and Redox-Switching Properties of a Di-Gold Metallotweezer

The use of a carbazolyl-connected di-gold(I) metallotweezer for the encapsulation of several electron-poor organic substrates, and a planar Au(III) complex containing a CNC pincer ligand, is described. The binding affinity of the receptor depends on the electron-deficient character of the planar guest, with larger association constants found for the more electron-poor guests. The X-ray diffraction molecular structures of two host:guest adducts show that the host approaches its arms in order to facilitate the optimum interaction with the surface of the planar guests, in a clear example of an guest-induced fit conformational arrangement. The electrochemical studies of the encapsulation of N,N'-dimethyl-naphthalenetetracarboxy diimide (NTCDI) show that the redox active guest is released from the receptor upon one electron reduction, thus constituting an example of redox-switchable binding.

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.

Guest-Reaction Driven Cage to Conjoined Twin-Cage Mitosis-Like Host Transformation

We report here a guest-reaction-induced mitosis-like host transformation from a known Pd₄ L₂ cage 1 to a conjoined Pd₆ L₃ twin-cage 2 featuring two separate cavities. The encapsulation of 1-hydroxymethyl-2-naphthol (G1), a known ortho-quinone methide (o-QMs) precursor, within the hydrophobic cavity of cage 1 is found crucial to realize the cage to twin-cage conversion. Confined G1 molecules within the nanocavity undergo self-coupling dimerization reaction to form 2,2'-dihydroxy-1,1'-dinaphthylmethane (G2) which then triggers the cage to twin-cage mitosis. The same conversion also proceeds, in a much faster rate, via the direct templation of G2, confirming the induced-fit transformation mechanism. The structure of the (G2)₂ ⊂2 host-guest complex has been established by X-ray crystallographic study, where cis- to trans- conformational switch on one bridging ligand is revealed.

Dimensional Matching versus Induced-Fit Distortions: Binding Affinities of Planar and Curved Polyaromatic Hydrocarbons with a Tetragold Metallorectangle

A tetragold(I) rectangle-like metallocage containing two pyrene-bis-imidazolylidene ligands and two carbazolyl-bis-alkynyl linkers is used for the encapsulation of a series of polycyclic aromatic hydrocarbons (PAHs), including corannulene. The binding affinities obtained for the encapsulation of the planar PAHs guests in CD₂ Cl₂ are found to exponentially increase with the number of π-electrons of the guest (1.3 > logK >6.6). For the bowl-shaped molecule of corannulene, the association constant is much lower than the expected one according to its number of electrons. The molecular structure of the host-guest complex formed with corannulene shows that the molecule of the guest is compressed, while the host is expanded, thus showing an interesting case of artificial mutual induced-fit arrangement.

Selective Anion Extraction and Recovery Using a FeII4 L4 Cage

Selective anion extraction is useful for the recovery and purification of valuable chemicals, and in the removal of pollutants from the environment. Here we report that Fe^II₄L₄ cage 1 is able to extract an equimolar amount of ReO₄⁻, a high‐value anion and a nonradioactive surrogate of TcO₄⁻, from water into nitromethane. Importantly, the extraction was efficiently performed even in the presence of 10 other common anions in water, highlighting the high selectivity of 1 for ReO₄⁻. The extracted guest could be released into water as the cage disassembled in ethyl acetate, and then 1 could be recycled by switching the solvent to acetonitrile. The versatile solubility of the cage also enabled complete extraction of ReO₄⁻ (as the tetrabutylammonium salt) from an organic phase into water by using the sulfate salt of 1 as the extractant.

Dynamic Interconversion between Boroxine Cages Based on Pyridine Ligation

Dynamic interconversion between large covalent organic cages was achieved simply by heating or acid/base treatment. A mixture of the boroxine cages 12-mer and 15-mer was cleanly converted into a pyridine adduct of the 9-mer boroxine cage upon treatment with pyridine, and the geometry of N-coordinated boron atoms changed from trigonal to tetrahedral. The reverse reaction was achieved by heating or acid treatment. In this process, the larger boroxine cages 12-mer and 15-mer were found to be entropically favored owing to the release of free pyridine molecules from 9-mer⋅6 Py.

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.