Selective Co-Encapsulation Inside an M6 L4 Cage

Guest Segregation in Heteromeric Multicage Systems

Dynamically interconvertible metallo-supramolecular multicomponent assemblies, coexisting orthogonally in solution, serve as simplified mimics for complex networks found in biological systems. Building on recent advances in controlling the nonstatistical self-assembly of heteroleptic coordination cages and heteromeric completive self-sorting, i.e., the coformation of multiple low-symmetry assemblies, we herein describe the first instance of emerging functionality from a heteromeric multicage system. In particular, one heteroleptic cage coexists in equilibrium with one or two different homoleptic cages, each capable of preferentially encapsulating one out of three guests within their cavities. Guest segregation occurs under thermodynamic control and can be reached following several distinct paths. Each pathway involves different sequential transformations, eventually leading to the same outcome. We found that the most complex mixture of components (three ligands, three guests, plus palladium cations) always yields an ordered, pathway-independent final state of three coexisting host-guest species, thus representing a case of simplexity in self-sorted systems. This study forms a basis for expanding the scope of dynamic transformation processes implemented in multicage host-guest systems. Together with the incorporation of stimuli-responsive elements and other functionality, complex cage populations with bioinspired applications in controllable compound separation schemes, compartmentalized cascade reactions, and multistep (even bifurcated) molecular information processing will become accessible.

Chemically Reversible Translational Moiré Superlattice Formations in the Two-Dimensional Films of the Zinc Phthalate Complex

The exciting electronic properties of two-dimensional layered materials introduced by twist angle and lattice mismatch between two consecutive layers are well-recognized. The major challenge lies in the control over the moiré periodicity. Herein, we report a chemically directed way to achieve the desired precision over moiré periodicity of the assembly of a complex of zinc ions and o-phthalic acid utilizing soft bonds. The reaction of o-phthalic acid with zinc ions in the aqueous medium at 80 °C for 1 h followed by preserving the reaction mixture at ambient temperature (35 °C) resulted in translational moiré superlattices with precise periods. Those lattices are further stacked angularly to give rotational super-moiré lattices. Further, the role of the π-zinc ion interaction in the origin of moiré fringes was examined with the reaction with 8-hydroxyquinoline-5-sulfonic acid (HQS) that also established the reversible nature of the superlattices. The present observations also suggest the formation of moiré superlattice through ion insertion in a chemical lattice.

Modulation of the isomerization of iminothioindoxyl switches by supramolecular confinement

Here we present the formation of an iminothioindoxyl (ITI)⊂Cage complex that retains the photochemical properties of the photoswitch within a confined environment in water. At the same time, besides ultrafast switching inside the cage, the ITI photoswitch displays an intriguing bifurcation of the excited state isomerization pathway when encapsulated.

Multipocket Cage Enables the Binding of High-Order Bulky and Drug Guests Uncovered by MS Methodology

Achieving high guest loading and multiguest-binding capacity holds crucial significance for advancement in separation, catalysis, and drug delivery with synthetic receptors; however, it remains a challenging bottleneck in characterization of high-stoichiometry guest-binding events. Herein, we describe a large-sized coordination cage (MOC-70-Zn8Pd6) possessing 12 peripheral pockets capable of accommodating multiple guests and a high-resolution electrospray ionization mass spectrometry (HR-ESI-MS)-based method to understand the solution host-guest chemistry. A diverse range of bulky guests, varying from drug molecules to rigid fullerenes as well as flexible host molecules of crown ethers and calixarenes, could be loaded into open pockets with high capacities. Notably, these hollow cage pockets provide multisites to capture different guests, showing heteroguest coloading behavior to capture binary, ternary, or even quaternary guests. Moreover, a pair of commercially applied drugs for the combination therapy of chronic lymphocytic leukemia (CLL) has been tested, highlighting its potential in multidrug delivery for combined treatment.

Design and Synthesis of a Novel Binuclear Palladium Complex as a Turn on Fluorescent Receptor for Neurotransmitter Dopamine

A novel binuclear palladium complex has been synthesized by the reaction of tetramethylethylenediamine palladium nitrate and sodium salt of pyromellitic acid. UV vis, NMR as well as fluorescent titration techniques show that this binuclear palladium complex interacts with neurotransmitter dopamine. The fluorescence of the palladium complex in aqueous solution gets enhanced with the gradual addition of the neurotransmitter dopamine which makes this complex to act as a turn on fluorescent sensor for neurotransmitter dopamine.

Selective encapsulation of carboxylic acid dimers within a size-regulable resorcinarene-based hemicarcerand

A cavity within a resorcinarene-based hemicarcerand was contracted and expanded through conformational changes induced by the complexation and decomplexation, allowing self-sorting of homo- and heterodimeric carboxylic acid pairs.

Cage Match: Comparing the Anion Binding Ability of Isostructural Versus Isofunctional Pairs of Metal-Organic Nanocages

Affinities of six anions (mesylate, acetate, trifluoroacetate, p-toluenecarboxylate, p-toluenesulfonate, and perfluorooctanoate) for three related Pt2+ -linked porphyrin nanocages were measured to probe the influence of different noncovalent recognition motifs (e. g., hydrogen bonding, electrostatics, π bonding) on anion binding. Two new hosts of M6 L3 12+ (1b) and M4 L2 8+ (2) composition (M=(en)Pt2+ , L=(3-py)4 porphyrin) were prepared in a one-pot synthesis and allowed comparison of hosts that differ in structure while maintaining similar N-H hydrogen-bond donor ability. Comparisons of isostructural hosts that differ in hydrogen-bonding ability were made between 1b and a related M6 L3 12+ nanoprism (1a, M=(tmeda)Pt2+ ) that lacks N-H groups. Considerable variation in association constants (K1 =1.6×103  M-1 to 1.3×108  M-1 ) and binding mode (exo vs. endo) were found for different host-guest combinations. Strongest binding was seen between p-toluenecarboxylate and 1b, but surprisingly, association of this guest with 1a was only slightly weaker despite the absence of NH⋅⋅⋅O interactions. The high affinity between p-toluenecarboxylate and 1a could be turned off by protonation, and this behavior was used to toggle between the binding of this guest and the environmental pollutant perfluorooctanoate, which otherwise has a lower affinity for the host.

Metal-Organic Framework-Derived Atomically Dispersed Co-N-C Electrocatalyst for Efficient Oxygen Reduction Reaction

In this work, an atomically dispersed cobalt-nitrogen-carbon (Co-N-C) catalyst is prepared for the oxygen reduction reaction (ORR) by using a metal-organic framework (MOF) as a self-sacrifice template under high-temperature pyrolysis. Spherical aberration-corrected electron microscopy is employed to confirm the atomic dispersion of high-density Co atoms on the nitrogen-doped carbon scaffold. The X-ray photoelectron spectroscopy results verify the existence of Co-N-C active sites and their content changes with the Co content. The electrochemical results show that the electrocatalytic activity shows a volcano-shaped relationship, which increases with the Co content from 0 to 0.99 wt.% and then decreases when the presence of Co nanoparticles at 1.61 wt.%. The atomically dispersed Co-N-C catalyst with Co content of 0.99 wt.% shows an onset potential of 0.96 V vs. reversible hydrogen electrode (RHE) and a half-wave potential of 0.89 V vs. RHE toward ORR. The excellent ORR activity is attributed to the high density of the Co-N-C sites with high intrinsic activity and high specific surface area to expose more active sites.

Encapsulation within a coordination cage modulates the reactivity of redox-active dyes

Confining molecules within well-defined nanosized spaces can profoundly alter their physicochemical characteristics. For example, the controlled aggregation of chromophores into discrete oligomers has been shown to tune their optical properties whereas encapsulation of reactive species within molecular hosts can increase their stability. The resazurin/resorufin pair has been widely used for detecting redox processes in biological settings; yet, how tight confinement affects the properties of these two dyes remains to be explored. Here, we show that a flexible Pd^II₆L₄ coordination cage can efficiently encapsulate both resorufin and resazurin in the form of dimers, dramatically modulating their optical properties. Furthermore, binding within the cage significantly decreases the reduction rate of resazurin to resorufin, and the rate of the subsequent reduction of resorufin to dihydroresorufin. During our studies, we also found that upon dilution, the Pd^II₆L₄ cage disassembles to afford Pd^II₂L₂ species, which lacks the ability to form inclusion complexes - a process that can be reversed upon the addition of the strongly binding resorufin/resazurin guests. We expect that the herein disclosed ability of a water-soluble cage to reversibly modulate the optical and chemical properties of a molecular redox probe will expand the versatility of synthetic fluorescent probes in biologically relevant environments.

Uptake, Trapping, and Release of Organometallic Cations by Redox-Active Cationic Hosts

The host-guest chemistry of metal-organic nanocages is typically driven by thermodynamically favorable interactions with their guests such that uptake and release of guests can be controlled by switching this affinity on or off. Herein, we achieve this effect by reducing porphyrin-walled cationic nanoprisms 1a¹²⁺ and 1b¹²⁺ to zwitterionic states that rapidly uptake organometallic cations Cp*₂Co⁺ and Cp₂Co⁺, respectively. Cp*₂Co⁺ binds strongly (K_a = 1.3 × 10³ M^-1) in the neutral state 1a⁰ of host 1a¹²⁺, which has its three porphyrin walls doubly reduced and its six (bipy)Pt²⁺ linkers singly reduced (bipy = 2,2'-bipyridine). The less-reduced states of the host 1a³⁺ and 1a⁹⁺ also bind Cp*₂Co⁺, though with lower affinities. The smaller Cp₂Co⁺ cation binds strongly (K_a = 1.7 × 10³ M^-1) in the 3e^- reduced state 1b⁹⁺ of the (tmeda)Pt²⁺-linked host 1b¹²⁺ (tmeda = N,N,N',N'-tetramethylethylenediamine). Upon reoxidation of the hosts with Ag⁺, the guests become trapped to provide unprecedented metastable cation-in-cation complexes Cp*₂Co⁺@1a¹²⁺ and Cp₂Co⁺@1b¹²⁺ that persist for >1 month. Thus, dramatic kinetic effects reveal a way to confine the guests in thermodynamically unfavorable environments. Experimental and DFT studies indicate that PF₆^- anions kinetically stabilize Cp*₂Co⁺@1a¹²⁺ through electrostatic interactions and by influencing conformational changes of the host that open and close its apertures. However, when Cp*₂Co⁺@1a¹²⁺ was prepared using ferrocenium (Fc⁺) instead of Ag⁺ to reoxidize the host, dissociation was accelerated >200× even though neither Fc⁺ nor Fc have any observable affinity for 1a¹²⁺. This finding shows that metastable host-guest complexes can respond to subtler stimuli than those required to induce guest release from thermodynamically favorable complexes.

A Molecular Capsule with Revolving Doors Partitioning Its Inner Space

Covalent capsule 1 was designed to include two molecular baskets linked with three mobile pyridines tucked into its inner space. On the basis of both theory (DFT) and experiments (NMR and X-ray crystallography), we found that the pyridine "doors" split the chamber (380 ų ) of 1 so that two equally sizeable compartments (190 ų ) became joined through a conformationally flexible aromatic barrier. The compartments of such unique host could be populated with CCl₄ (88 ų ; PC=46 %), CBr₄ (106 ų ; 56 %) or their combination CCl₄ /CBr₄ (PC=51 %), with thermodynamic stabilities ΔG° tracking the values of packing coefficients (PC). Halogen (C-X⋅⋅⋅π) and hydrogen bonding (C-H⋅⋅⋅X) contacts held the haloalkane guests in the cavities of 1. The consecutive complexations were found to occur in a negative allosteric manner, which we propose to result from the induced-fit mode of complexation. Newly designed 1 opens a way for probing the effects of inner conformational dynamics on noncovalent interactions, reactivity and intramolecular translation in confined spaces of hollow molecules.

Interactions of Small-Molecule Guests with Interior and Exterior Surfaces of a Coordination Cage Host

Coordination cages are well-known to act as molecular containers that can bind small-molecule guests in their cavity. Such cavity binding is associated with interactions of the guests with the surrounding set of surfaces that define the cavity; a guest that is a good fit for the cavity will have many favourable interactions with the interior surfaces of the host. As cages have exterior as well as interior surfaces, possibilities also exist for ‘guests’ that are not well-bound in the cavity to interact with the exterior surface of the cage where spatial constraints are fewer. In this paper, we report a combined solid-state and solution study using an octanuclear cubic M8L12 coordination cage which illustrates the occurrence of both types of interaction. Firstly, crystallographic studies show that a range of guests bind inside the cavity (either singly or in stacked pairs) and/or interact with the cage exterior surface, depending on their size. Secondly, fluorescence titrations in aqueous solution show how some flexible aromatic disulfides show two separate types of interaction with the cage, having different spectroscopic consequences; we ascribe this to separate interactions with the exterior surface and the interior surface of the host cage with the former having a higher binding constant. Overall, it is clear that the idea of host/guest interactions in molecular containers needs to take more account of external surface interactions as well as the obvious cavity-based binding.

Cavity-based applications of metallo-supramolecular coordination cages (MSCCs)

This review describes cavity-based applications of cage-like SCCs such as molecular recognition and separation, stabilization of reactive species by encapsulation, as drug delivery systems and as molecular flasks.

Supramolecular Host-Selective Activation of Iodoarenes by Encapsulated Organometallics

Supramolecular hosts offer defined microenvironments that facilitate selective host-guest interactions, enabling reactivity that would otherwise be challenging in bulk solution. While impressive rate enhancements and selectivities have been reported, similar reactivity can often be accessed through modifications of reaction conditions even in the absence of the host. We report here an oxidative addition of aryl halides across the metal centers in Cu(I) and Pd(II) organometallics that is assisted by the presence of a supramolecular host, realized via electrostatic stabilization and increased local substrate concentrations. When reaction conditions were screened to assess background reactivity, alternative reactivity (typically decomposition) resulted, indicating that encapsulation led to host-selective reaction trajectories.

Coordination-Driven Self-Assembled Metallacycles Incorporating Pyrene: Fluorescence Mutability, Tunability, and Aromatic Amine Sensing

Constructing polycyclic aromatics-based, highly emissive fluorophores with good solubility and tunable aggregated structures and properties is of great importance for film fabrication, solution processing, and relevant functionality studies. Herein, we describe a general strategy to endow conventional organic fluorophores with enhanced solubility and modulated fluorescent properties via their incorporation into coordination-driven self-assembled metallacycles. A widely used fluorophore, pyrene, was decorated with two pyridyl groups to yield functionalized pyrene 4. Mixing 4 with three aromatic dicarboxylates with different lengths and a 90° Pt(II) metal acceptor in a 2:2:4 stoichiometric ratio resulted in the formation of three metallacycles, 1, 2, and 3. The metallacycles display good solubility in polar organic solvents, highly aggregation-dependent fluorescence, and size-dependent emissions at higher concentrations. Moreover, metallacycle 2-based, silica-gel-supported film as fabricated not only is more emissive than the ligand 4-based one but also displays much improved sensing properties for amines in the vapor state, as demonstrated by significantly increased response speed and decreased recovery time. The enhanced solubility, unique fluorescence behavior, and multi-factor modulation character show that coordination-driven self-assembly can be utilized for the development of new fluorophores through simple modification of conventional fluorophores. The fluorophores synthesized this way possess not only complex topological structures but also good modularity and tunability in fluorescence behavior, which are important for grafting multi-stage energy-transfer systems necessary for the development of high-performance sensing materials.

Reversible multi-electron storage in dual-site redox-active supramolecular cages

Redox-active M₆L₄ cages display multiple reversible redox-events, enabling switching from overall +12 to −4 charged species with reversible storage of 16 electrons.

Coordination-driven self-assembly of chiral palladium(ii)-based supramolecular triangle structures

Chiral structures of palladium(ii)-based triangular supramolecular complexes, with achiral corners and edges, have been characterized in solution.

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.

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.

Selective Co-Encapsulation Inside an M6 L4 Cage

There is broad interest in molecular encapsulation as such systems can be utilized to stabilize guests, facilitate reactions inside a cavity, or give rise to energy-transfer processes in a confined space. Detailed understanding of encapsulation events is required to facilitate functional molecular encapsulation. In this contribution, it is demonstrated that Ir and Rh-Cp-type metal complexes can be encapsulated inside a self-assembled M6 L4 metallocage only in the presence of an aromatic compound as a second guest. The individual guests are not encapsulated, suggesting that only the pair of guests can fill the void of the cage. Hence, selective co-encapsulation is observed. This principle is demonstrated by co-encapsulation of a variety of combinations of metal complexes and aromatic guests, leading to several ternary complexes. These experiments demonstrate that the efficiency of formation of the ternary complexes depends on the individual components. Moreover, selective exchange of the components is possible, leading to formation of the most favorable complex. Besides the obvious size effect, a charge-transfer interaction may also contribute to this effect. Charge-transfer bands are clearly observed by UV/Vis spectrophotometry. A change in the oxidation potential of the encapsulated electron donor also leads to a shift in the charge-transfer energy bands. As expected, metal complexes with a higher oxidation potential give rise to a higher charge-transfer energy and a larger hypsochromic shift in the UV/Vis spectrum. These subtle energy differences may potentially be used to control the binding and reactivity of the complexes bound in a confined space.