Ditopic Covalent Cage for Ion-Pair Binding: Influence of Anion Complexation on the Cation Exchange Rate

A new hemicryptophane host with a ditopic molecular cavity combining a cyclotriveratrylene (CTV) unit with a tris-urea moiety was synthesized. The complexation of halides, tetramethylammonium (TMA+) cation, and ion pairs was investigated. A positive cooperativity was observed, since halides display a higher binding constant when a TMA+ cation is already present inside the cage. When TMA+ was complexed alone, a decrease of temperature from 298 K to 230 K was required to switch from a fast to a slow exchange regime on the NMR time scale. Nevertheless, the prior complexation of a halide guest in the lower part of the host resulted in significant decrease of the exchange rate of the subsequent complexation of the TMA+ cation. Under these conditions, the 1H NMR signals characteristic of a slow exchange regime were observed at 298 K. Addition of an excess of salts, increases the ionic strength of the solution, restoring the fast exchange dynamics. This result provides insight on how the exchange rate of a cation guest can be modulated by the complexation of a co-guest anion.

Molecular Recognition Process in Resorcinarene-based Coordination Capsules

Cu and Ag capsules can take up various organic molecules. Their molecular recognition possibly involves partial dissociation and slippage. We investigated molecular recognition processes in the Cu and Ag capsules by CD and 1 H NMR spectroscopy and employed 4,4'-diacetoxy biphenyl carrying two benzothiadiazole groups as a probe. CD and 1 H NMR measurements reveal that the host-guest complexation proceeds under second-order reactions and that these capsules undergo the partial dissociation to take up the probe in [D1 ]chloroform and [D8 ]THF. The slippage also contributes to host-guest complexation for a Cu capsule that carries p-methoxyphenyl groups on the 2,2'-bipyridiyl arms. DFT calculations suggest that π/π stacking interactions between the electron-rich p-methoxyphenyl group and the electron-poor 2,2'-bipyridyl arm elongate the capsule, allowing the guest to access the cavity without the partial dissociation.

Megalo-Cavitands: Synthesis of Acridane[4]arenes and Formation of Large, Deep Cavitands for Selective C70 Uptake

Deep cavitands, concave molecular containers, represent an important supramolecular host class that has been explored for a variety of applications ranging from sensing, switching, purification and adsorption to catalysis. A major limitation in the field has been the cavitand volume that is restricted by the size of the structural platform utilized (diameter approx. 7 Å). We here report the synthesis of a novel, unprecedentedly large structural platform, named acridane[4]arene (diameter approx. 14 Å), suitable for the construction of cavitands with volumes of up to 814 ų . These megalo-cavitands serve as size-selective hosts for fullerenes with mM to sub-μM binding affinity for C₆₀ and C₇₀ . Furthermore, the selective binding of fullerene C₇₀ in the presence of C₆₀ was demonstrated.

Dissipative Formation of Covalent Basket Cages

Living systems use chemical fuels to transiently assemble functional structures. As a step toward constructing abiotic mimics of such structures, we herein describe dissipative formation of covalent basket cage CBC 5 by reversible imine condensation of cup‐shaped aldehyde 2 (i.e., basket) with trivalent aromatic amine 4. This nanosized [4+4] cage (V=5 nm³, M_w=6150 Da) has shape of a truncated tetrahedron with four baskets at its vertices and four aromatic amines forming the faces. Importantly, tris‐aldehyde basket 2 and aliphatic tris‐amine 7 undergo condensation to give small [1+1] cage 6. The imine metathesis of 6 and aromatic tris‐amine 4 into CBC 5 was optimized to bias the equilibrium favouring 6. Addition of tribromoacetic acid (TBA) as a chemical fuel perturbs this equilibrium to result in the transient formation of CBC 5, with subsequent consumption of TBA via decarboxylation driving the system back to the starting state.

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.

Picking on Carbonate: Kinetic Selectivity in the Encapsulation of Anions

Supramolecular hosts bind to inorganic anions at a fast rate and select them in proportion with thermodynamic stability of the corresponding [anion⊂host] complexes, forming in a reversible manner. In this study, we describe the action of hexapodal capsule 1 and its remarkable ability to select anions based on a large span of rates by which they enter this host. The thermodynamic affinity of 1 toward eighteen anions extends over eight orders of magnitude (0<K_a <10⁸  M^-1 ; ¹ H NMR spectroscopy). The capsule would retain CO₃ ^2- (K_a =10⁷  M^-1 ) for hours in the presence of eleven competing anions, including stronger binding SO₄ ^2- , HAsO₄ ^2- and HPO₄ ^2- (K_a =10⁷ -10⁸  M^-1 ). The observed selection resulted from 1 possessing narrow apertures (ca. 3×6 Å) comparable in size to anions (d=3.5-7.1 Å) slowing down the encapsulation to last from seconds to days. The unorthodox mode of action of 1 sets the stage for creating hosts that pick anions by their ability to access the host.

A computational study of competing conformational selection and induced fit in an abiotic system

Host-guest complexations can be described by two competing mechanisms, conformational selection (CS) and induced fit (IF). In this work, we used a combination of nudged elastic band (NEB), adaptive steered molecular dynamics (ASMD), and density functional theory (DFT, with a correction for dispersion) to study the dynamics of the pathways (IF/CS) by which two conformers of basket B(+) and B(-) interconvert and trap CX₄ guests (X = Cl and Br). While the results from NEB/DFT studies disclosed host-guest noncovalent contacts reducing the basket's conformational dynamics, ASMD methodology suggested an associative mechanism for the guest complexation. With theory in excellent agreement with experiments, NEB and ASMD emerge as the methods of choice for studying dynamics of supramolecular systems.

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.

Self-assembly of a water-soluble endohedrally functionalized coordination cage including polar guests

Coordination cages containing endohedrally functionalized aromatic cavities are scarce in the literature. Herein, we report the self-assembly of a tetra-cationic super aryl-extended calix[4]pyrrole tetra-pyridyl ligand into a water-soluble Pd(ii)-cage featuring two endohedral polar binding sites. They are defined by the four pyrrole NHs of the calix[4]pyrrole unit and the four inwardly directed α-protons of the coordinated pyridyl groups. The efficient assembly of the Pd(ii)-cage requires the inclusion of mono- and ditopic pyridyl N-oxide and aliphatic formamide guests. The monotopic guests only partially fill the cage's cavity and require the co-inclusion of a water molecule that is likely hydrogen-bonded to the endohedral α-pyridyl protons. The ditopic guests are able to completely fill the cage's cavity and complement both binding sites. We observed high conformational selectivity in the inclusion of the isomers of α,ω-bis-formamides. We briefly investigate the uptake and release mechanism/kinetics of selected polar guests by the Pd(ii)-cage using pair-wise competition experiments.

A tetra-cationic calix[4]pyrrole tetra-pyridyl ligand self-assembles into a water-soluble Pd(ii)-cage featuring two endohedral polar binding sites. The Pd(ii)-cage encapsulates pyridyl N-oxide and aliphatic formamide guests in water.

Just Add Water: Modulating the Structure-Derived Acidity of Catalytic Hexameric Resorcinarene Capsules

The hexameric undecyl-resorcin[4]arene capsule (C11R6) features eight discrete structural water molecules located at the vertices of its cubic suprastructure. Combining NMR spectroscopy with classical molecular dynamics (MD), we identified and characterized two distinct species of this capsule, C11R6-A and C11R6-B, respectively featuring 8 and 15 water molecules incorporated into their respective hydrogen-bonded networks. Furthermore, we found that the ratio of the C11R6-A and C11R6-B found in solution can be modulated by controlling the water content of the sample. The importance of this supramolecular modulation in C11R6 capsules is highlighted by its ability to perform acid-catalyzed transformations, which is an emergent property arising from the hydrogen bonding within the suprastructure. We show that the conversion of C11R6-A to C11R6-B enhances the catalytic rate of a model Diels-Alder cyclization by 10-fold, demonstrating the cofactor-derived control of a supramolecular catalytic process that emulates natural enzymatic systems.

From Selection to Instruction and Back: Competing Conformational Selection and Induced Fit Pathways in Abiotic Hosts

Two limiting cases of molecular recognition, induced fit (IF) and conformational selection (CS), play a central role in allosteric regulation of natural systems. The IF paradigm states that a substrate "instructs" the host to change its shape after complexation, while CS asserts that a guest "selects" the optimal fit from an ensemble of preexisting host conformations. With no studies that quantitatively address the interplay of two limiting pathways in abiotic systems, we herein and for the first time describe the way by which twisted capsule M-1, encompassing two conformers M-1(+) and M-1(-), trap CX₄ (X=Cl, Br) to give CX₄ ⊂M-1(+) and CX₄ ⊂M-1(-), with all four states being in thermal equilibrium. With the assistance of 2D EXSY, we found that CBr₄ would, at its lower concentrations, bind M-1 via a M-1(+)→M-1(-)→CBr₄ ⊂M-1(-) pathway corresponding to conformational selection. For M-1 complexing CCl₄ though, data from 2D EXSY measurements and 1D NMR line-shape analysis suggested that lower CCl₄ concentrations would favor CS while the IF pathway prevailed at higher proportions of the guest. Since CS and IF are not mutually exclusive, we reason that our work sets the stage for characterizing the dynamics of a wide range of already existing hosts to broaden our fundamental understanding of their action. The objective is to master the way in which encapsulation takes place for designing novel and allosteric sequestering agents, catalysts and chemosensors akin to those found in nature.

Guest Recognition Control Accompanied by Stepwise Gate Closing and Opening of a Macrocyclic Metallohost

Host-guest binding behavior of macrocyclic hosts is significantly influenced by the shapes and sizes of the hosts. In particular, closing/opening the apertures of the hosts controls the guest uptake/release. A post-metalation modification method was used to achieve the open/close conversions. The starting open complex, [LCo₂ (pip)₄ ](OTf)₂ , was efficiently converted to the closed complex, [LCo₂ (hda)₂ ](OTf)₂ , which has a doubly bridged structure. The conversion of this closed complex to the open complex [LCo₂ (hda)₂ (OAc)]⁺ was too slow to be completed, but this gate-opening was dramatically accelerated by the addition of Na⁺ . The Na⁺ binding was also significantly enhanced by the gate-opening, that is, conversion of [LCo₂ (hda)₂ ]²⁺ to [LCo₂ (hda)₂ (OAc)]⁺ .

Sierpiński Pyramids by Molecular Entanglement

Planar, terpyridine-based metal complexes with the Sierpiński triangular motif and alkylated corners undergo a second self-assembly event to give megastructural Sierpiński pyramids; assembly is driven by the facile lipophilic-lipophilic association of the alkyl moieties and complementary perfect fit of the triangular building blocks. Confirmation of the 3D, pyramidal structures was verified and supported by a combination of TEM, AFM, and multiscale simulation techniques.

Strapped calix[4]pyrroles: from syntheses to applications

Supramolecular chemistry is a central topic in modern chemistry. It touches on many traditional disciplines, such as organic chemistry, inorganic chemistry, physical chemistry, materials chemistry, environmental chemistry, and biological chemistry. Supramolecular hosts, inter alia macrocyclic hosts, play critical roles in supramolecular chemistry. Calix[4]pyrroles, non-aromatic tetrapyrrolic macrocycles defined by sp³ hybridized meso bridges, have proved to be versatile receptors for neutral species, anions, and cations, as well as ion pairs. Compared to the parent system, octamethylcalix[4]pyrrole and its derivatives bearing simple appended functionalities, strapped calix[4]pyrroles typically display enhanced binding affinities and selectivities. In this review, we summarize advances in the design and synthesis of strapped calix[4]pyrroles, as well as their broad utility in molecular recognition, supramolecular extraction, separation technology, ion transport, and as agents capable of inhibiting cancer cell proliferation. Future challenges within this sub-field are also discussed.

Controlled binding of organic guests by stimuli-responsive macrocycles

Synthetic supramolecular chemistry pursues not only the construction of new matter, but also control over its inherently dynamic behaviour.

A NIR-triggered gatekeeper of supramolecular conjugated unimicelles with two-photon absorption for controlled drug release

Near-infrared-sensitive supramolecular hyperbranched conjugated unimicelles were constructed for controlled drug release via two-photon excited fluorescence resonance energy transfer.

A mono-metallic Pd(ii)-cage featuring two different polar binding sites

A tetra-α super aryl-extended tetra-pyridyl calix[4]pyrrole self-assembles into a mono-Pd(ii) cage featuring two different and converging polar binding sites.

Anion-capped metallohost allows extremely slow guest uptake and on-demand acceleration of guest exchange

The switching of molecular recognition selectivity is important for tuning molecular functions based on host–guest binding. While the switching processes in artificial functional molecules are usually driven by changes of the thermodynamic stabilities, non-equilibrium phenomena also play an important role in biological systems. Thus, here we designed a host–guest system utilizing a non-equilibrium kinetically trapped state for on-demand and time-programmable control of molecular functions. We synthesized a bis(saloph) macrocyclic cobalt(III) metallohost 1(OTf)₂, which has anion caps at both sides of the cation-binding site. The anion caps effectively retard the guest uptake/release so that we can easily make a non-equilibrium kinetically trapped state. Indeed, we can obtain a long-lived kinetically trapped state {[1·K]³⁺+La³⁺} prior to the formation of the thermodynamically more stable state {[1·La]⁵⁺+K⁺}. The guest exchange to the more stable state from this kinetically trapped state is significantly accelerated by exchange of TfO⁻ anion caps by AcO⁻ in an on-demand manner.

Host—guest assemblies can exploit stimuli-responsive guest binding and release for molecular recognition, but are typically governed by thermodynamics alone. Here, the authors design macrocycles with removable and exchangeable anion caps, allowing for the kinetic trapping and on-demand exchange of guest ions.

Light-responsive molecular containers

This review highlights relevant studies of light-controlled molecular containers able to catch and release small molecules.

Moving systems of polar dimeric capsules out of thermal equilibrium by light irradiation

Heterodimeric capsules self-assembled from tetraurea calix[4]pyrrole and tetraurea calix[4]arene provide unique molecular containers for the organised inclusion of small polar molecules. By inserting stimuli-responsive groups (azobenzene) in the heterocapsule structure, we are able to modify the equilibrium state of the system or the exchange between different host-guest assemblies in a reversible manner.

Gated molecular baskets

In this review, we describe the construction of gated molecular baskets, discuss their mechanism of action in regulating the exchange of guests and illustrate the potential of these concave hosts to act as catalysts for controlling chemical reactions. Importantly, a number of computational and experimental studies have suggested that gated baskets ought to unfold their gates at the rim for permitting the passage of guests to/from their inner space. These dynamic hosts are therefore offered as useful models for investigating the process of gating in artificial systems. Furthermore, gated baskets should permit examining the benefit of controlling the rate by which reactants access a gated catalyst for promoting chemical reactions occurring in its confined space.

Multiaddressable molecular rectangles with reversible host-guest interactions: modulation of pH-controlled guest release and capture

A series of multiaddressable platinum(II) molecular rectangles with different rigidities and cavity sizes has been synthesized by endcapping the U-shaped diplatinum(II) terpyridine moiety with various bis-alkynyl ligands. The studies of the host-guest association with various square planar platinum(II), palladium(II), and gold(III) complexes and the related low-dimensional gold(I) complexes, most of which are potential anticancer therapeutics, have been performed. Excellent guest confinement and selectivity of the rectangular architecture have been shown. Introduction of pH-responsive functionalities to the ligand backbone generates multifunctional molecular rectangles that exhibit reversible guest release and capture on the addition of acids and bases, indicating their potential in controlled therapeutics delivery on pH modulation. The reversible host-guest interactions are found to be strongly perturbed by metal-metal and π-π interactions and to a certain extent, electrostatic interactions, giving rise to various spectroscopic changes depending on the nature of the guest molecules. Their binding mode and thermodynamic parameters have been determined by 2D NMR and van't Hoff analysis and supported by computational study.

Gas-phase chemistry of molecular containers

The use of mass spectrometry for the investigation of supramolecular capsules and containers in solution and the gas-phase is reviewed.

pH-dependent binding of guests in the cavity of a polyhedral coordination cage: reversible uptake and release of drug molecules

A range of organic molecules with acidic or basic groups exhibit strong pH-dependent binding inside the cavity of a polyhedral coordination cage. Guest binding in aqueous solution is dominated by a hydrophobic contribution which is compensated by stronger solvation when the guests become cationic (by protonation) or anionic (by deprotonation). The Parkinson's drug 1-amino-adamantane ('amantadine') binds with an association constant of 104 M-1 in the neutral form (pH greater than 11), but the stability of the complex is reduced by three orders of magnitude when the guest is protonated at lower pH. Monitoring the uptake of the guests into the cage cavity was facilitated by the large upfield shift for the 1H NMR signals of bound guests due to the paramagnetism of the host. Although the association constants are generally lower, guests of biological significance such as aspirin and nicotine show similar behaviour, with a substantial difference between neutral (strongly binding) and charged (weakly binding) forms, irrespective of the sign of the charged species. pH-dependent binding was observed for a range of guests with different functional groups (primary and tertiary amines, pyridine, imidazole and carboxylic acids), so that the pH-swing can be tuned anywhere in the range of 3.5-11. The structure of the adamantane-1-carboxylic acid complex was determined by X-ray crystallography: the oxygen atoms of the guest form CH···O hydrogen bonds with one of two equivalent pockets on the internal surface of the host. Reversible uptake and release of guests as a function of pH offers interesting possibilities in any application where controlled release of a molecule following an external stimulus is required.

On the nature of the transition state characterizing gated molecular encapsulations

Gated molecular encapsulations, with baskets of type 1, are postulated to occur by the mechanism in which solvent molecule penetrates the inner space of 1, through one of its apertures, while the residing guest simultaneously departs the cavity. In the transition state of the exchange, three pyridine-based gates are proposed to assume an open position with both incoming solvent and departing guest molecules interacting with the concave surface of the host. The More O'Ferrall-Jencks diagram and linear free energy relationships (LFERs) suggest a more advanced departure of the guest when bigger solvents partake in the displacement.

Towards chemical communication between gated nanoparticles

The design of comparatively simple and modularly configurable artificial systems able to communicate through the exchange of chemical messengers is, to the best of our knowledge, an unexplored field. As a proof-of-concept, we present here a family of nanoparticles that have been designed to communicate with one another in a hierarchical manner. The concept involves the use of capped mesoporous silica supports in which the messenger delivered by a first type of gated nanoparticle is used to open a second type of nanoparticle, which delivers another messenger that opens a third group of gated nanoobjects. We believe that the conceptual idea that nanodevices can be designed to communicate with one another may result in novel applications and will boost further advances towards cooperative systems with complex behavior as a result of the communication between simple abiotic individual components.