Self-assembled Molecular Capsule Research Articles

Harnessing a Pd4 Water-Soluble Molecular Capsule as a Size-Selective Catalyst for Targeted Oxidation of Alkyl Aromatics.

Molecular hosts with functional cavities can emulate enzymatic behavior through selective encapsulation of substrates, resulting in high chemo-, regio-, and stereoselective product formation. It is still challenging to synthesize enzyme-mimicking hosts that exhibit a narrow substrate scope that relies upon the recognition of substrates based on the molecular size. Herein, we introduce a Pd4 self-assembled water-soluble molecular capsule [M 4 L 2] (MC) that was formed through the self-assembly of a ligand L (4',4‴'-(1,4-phenylene)bis(1',4'-dihydro-[4,2':6',4″-terpyridine]-3',5'-dicarbonitrile)) with the acceptor cis-[(en)Pd(NO3)2] [en = ethane-1,2-diamine] (M). The molecular capsule MC showed size-selective recognition towards xylene isomers. The redox property of MC was explored for efficient and selective oxidation of one of the alkyl groups of m-xylene and p-xylene to their corresponding toluic acids using molecular O2 as an oxidant upon photoirradiation. Employing host-guest chemistry, we demonstrate the homogeneous catalysis of alkyl aromatics to the corresponding monocarboxylic acids in water under mild conditions. Despite homogeneous catalysis, the products were separated from the reaction mixtures by simple filtration/extraction, and the catalyst was reused. The larger analogues of the alkyl aromatics failed to bind within the MC's hydrophobic cavity, resulting in a lower/negligible reaction outcome. The present study represents a facile approach for selective photo-oxidation of xylene isomers to their corresponding toluic acids in an aqueous medium under mild conditions.

Molecular Capsule Catalysis: Ready to Address Current Challenges in Synthetic Organic Chemistry?

Self-assembled molecular capsules, host structures that form spontaneously when their building blocks are mixed, have been known since the 1990s. They share some basic similarities with enzyme pockets, as they feature defined hydrophobic binding pockets that are able to bind molecules of appropriate size and shape. The potential to utilize such host structures for catalysis has been explored since their discovery; however, applications that solve current challenges in synthetic organic chemistry have remained limited. In this short article, we discuss the challenges associated with the use of molecular capsules as catalysts, and highlight some recent applications of supramolecular capsules to overcome challenges in synthetic organic chemistry.

Specific Encapsulation of Acetylcholine Chloride by a Self-Assembled Molecular Capsule with Sulfonamido Moiety.

New molecular capsule 12, which encapsulates only acetylcholine chloride in the ion-pair form, has been developed. Cavitand 1 with four sulfonamido moieties on the upper rim of tetraimino-cavitand self-assembled to form a stable molecular capsule in the presence of an acetylcholine chloride guest through eight intermolecular -NH···O═S hydrogen bonds, two from each of the four paired sulfonamide units.

ChemInform Abstract: Advantages of Catalysis in Self‐Assembled Molecular Capsules

AbstractReview: [29 refs.

Advantages of Catalysis in Self-Assembled Molecular Capsules.

Control over the local chemical environment of a molecule can be achieved by encapsulation in supramolecular host systems. In supramolecular catalysis, this control is used to gain advantages over classical homogeneous catalysis in bulk solution. Two of the main advantages concern influencing reactions in terms of substrate and product selectivity. Due to size and/or shape recognition, substrate selective conversion can be realized. Additionally, noncovalent interactions with the host environment facilitate alternative reaction pathways and can yield unusual products. This Concept article discusses and highlights literature examples utilizing self-assembled molecular capsules to achieve catalytic transformations displaying a high degree of substrate and/or product selectivity. Furthermore, the advantage of supramolecular hosts in multicatalyst tandem reactions is covered.

A chromogenic molecular capsule attributable to dipolar amide resonance structure.

A new chromogenic, self-assembled molecular capsule G@22 is developed by introducing four (N,N-dimethyl-4-aminophenyl) azobenzyl moieties on the upper rim of a resorcin[4]arene-based amidoimino-cavitand. The tuning of conjugation between amido and (N,N-dimethyl-4-aminophenyl)azobenzyl groups by acid-base titration allows naked-eye detection of molecular capsule formation.

Bio-inspired self-assembled molecular capsules

Bowl shaped molecules are useful for making molecular capsules with suitable non-covalent bonds. We appended cyclotriguaiacylene with biologically important adenine and thymine to make capsule in solution by hydrogen bonding.

Diffusion NMR of molecular cages and capsules.

In the last decade diffusion NMR and diffusion ordered spectroscopy (DOSY) have become important analytical tools for the characterization of supramolecular systems in solution. Diffusion NMR can be used to glean information on the (effective) size and shape of molecular species, as well as to probe inter-molecular interactions and can be used to estimate the association constant (Ka) of a complex. In addition, the diffusion coefficient, as obtained from diffusion NMR, is a much more intuitive parameter than the chemical shift for probing self-association, aggregation and inter-molecular interactions. The diffusion coefficient may be an even more important analytical parameter in systems in which the formed supramolecular entity has the same symmetry as its building units, when there is a large change in the molecular weight, where many molecular species are involved in the formation of the supramolecular systems, and when proton transfer may occur which, in turn, may affect the chemical shift. Some of the self-assembled molecular capsules and cages prepared in the last decade represent such supramolecular systems and in the present review, following a short introduction on diffusion NMR, we survey the contribution of diffusion NMR and DOSY in the field of molecular containers and capsules. We will first focus on the role played by diffusion NMR in the field of hydrogen bond driven self-assembled capsules. We then survey the contributions of diffusion NMR and DOSY to the study and characterization of metal-ligand cages and capsules. Finally, we describe a few recent applications of diffusion NMR in the field of hydrophobic, electrostatic and covalent containers.

Propeller-Shaped Self-Assembled Molecular Capsules: Synthesis and Guest Entrapment

Molecular containers or supramolecular capsules are a subject of great interest because of their potential utilizations in various fields of chemistry, medicine, and nanoscience. They have been utilized as nanovessels for reactions and targeted drug delivery in recent years. Cyclotricatechylene (CTC) is a bowl-shaped molecule in its crown structure. It has six phenolic groups as potential hydrogen bond donors. In this work, we utilize this hydrogen bonding capability of CTC to generate supramolecular complexes with hydrogen bond acceptors such as 4,4′-bipyridine, pyrazine, 2,2′-bipyridine, and phenanthroline. These supramolecular entities were studied in the solid state by X-ray crystallography and scanning electron microscopy. They are not soluble in low polar solvents. Therefore, the hydrogen bonding behavior in solution could not be studied by NMR. Except in the case of pyrazine, capsular supramolecular assemblies were formed in all the cases. One of these capsules was studied for guest entrapment. Pla...

Mechanistic Insight on the Diels–Alder Reaction Catalyzed by a Self-Assembled Molecular Capsule

We combined Monte Carlo simulations and density functional theory calculations to study the mechanism of the Diels-Alder reaction of p-quinone and cyclohexadiene catalyzed by a self-assembled molecular capsule. Our calculations show that the encapsulation of the reactants into the cage is driven by hydrogen-bonding interactions and π-π stacking interactions between two reactants and the capsule. The encapsulated Diels-Alder reaction at different locations inside the capsule may have quite different reactivity due to different guest-host interactions. A comparison of the free energy profiles of the Diels-Alder reaction in the capsule and in the bulk solution reveals that the Diels-Alder reaction in the capsule is accelerated because the host-guest interaction leads to a relatively smaller barrier for the cycloaddition step.

Selective Stabilization of Self-Assembled Hydrogen-Bonded Molecular Capsules Through π–π Interactions

Subtle noncovalent forces such as π-π interactions play an import role in the folding of biological macromolecules such as DNA and proteins. We describe here a system where such interactions on the outside of a molecular capsule trigger a selective change of its structure as a self-assembled receptor.

25 Years Full of Chemical Discovery

25 Years Full of Chemical Discovery

Self‐Assembled Molecular Reaction Vessels Reloaded

Enzyme-like catalysis of Nazarov cyclizations has been achieved by encapsulation of pentadienols into a metallosupramolecular capsule. Constrictive binding of the substrate and functional-group activation give rise to a tremendous rate acceleration of up to a factor of 2.1×106, thus, pushing the efficiency of self-assembled molecular capsules as catalysts to a new level.

Anion binding in covalent and self-assembled molecular capsules

This critical review describes selected examples extracted from the extensive literature generated during the past 42 years on the topic of anion binding in molecular capsules. The goal of including anions in molecular capsules emerges from the idea of incorporating the traits exhibited by biological receptors into synthetic ones. At the outset of this research area the capsules were unimolecular. The scaffold of the receptor was designed to covalently link a series of functional groups that could converge into a cavity and to avoid its collapse. The initial examples involved the encapsulation of one monoatomic spherical anion. With time, the cavity size of the receptor was increased and encapsulation of polyatomic anions and co-encapsulation became a reality. Synthetic economy fueled the use of aggregates of self-complementary molecules rather than one large molecule as capsules. The main purpose of this review is to give a general overview of the topic which might be of interest to supramolecular or non supramolecular chemists alike (149 references).

Removal of Perchlorate Anion from an Aqueous Solution by Encapsulation in an Anion-templated Self-assembled Molecular Capsule

Abstract A new M2L4-type molecular capsule was prepared and characterized. It is shown that the anion-templated self-assembled construction is useful for selective removal of perchlorate anion from aqueous solution.

テトラホウ酸キャビタンドとビス(カテコール)リンカーとの動的共有結合に基づく自己集合カプセル

テトラホウ酸キャビタンドとビス(カテコール)リンカーとの動的共有結合に基づく自己集合カプセル

Self-Assembled Molecular Capsules for C70 Separation

Self-Assembled Molecular Capsules for C₇₀ Separation

Charge transfer and encapsulation in a synthetic, self-assembled receptor

Tetrathiafulvalene derivatives show charge transfer and exceptional binding within a self-assembled molecular capsule.

Molecular Engineering. Part 13. Formation of Hemicarcerand Dimer by Metal Coordination

Container molecules such as carcerand, hemicarcerand, and self-assembled molecular capsule have been characterized as molecular scavengers, molecular storages, molecular reactors and controlled-releasing systems. Various heterobridged hemicarceplexes in which the fourth bridging unit differs from the other three bridging units were reported by Cram et al. and the fourth bridging unit has been used to adopt an additional binding site or to connect with another hemicarcerand to obtain covalently linked dimeric hemicarceplexes. The characteristics of container molecules can be accumulated when they are assembled to highly ordered supramolecular systems. Dimeric container system could duplex the functions of monomeric container molecule and a wellordered multiple container system would result in a new high density information storage system. Metal coordination has become an important synthetic strategy for the self-assembly of high-ordered and welldefined supramolecular architectures because it allows well defined geometry, coordination number, and a range of binding strengths. Recently the interesting guest’s size and shape selectivities of cyanohemicarcerand 1 was reported. 8 But the stability of Pd(II) or Pt(II)-coordinated dimeric assembly 1-ML2-1 was too weak to be observed by H NMR spectrometry. Here we report on the synthesis of hemicarcerand 4 which has a metal coordinating p-pyridylphenyl unit on a pillar and its formation of dimeric self-assemblies 5a and 5b by Pd(II) and Pt(II)-coordination, respectively.

Relative binding affinities of molecular capsules investigated by ESI-mass spectrometry.

ESI-MS(/MS) has been used as a method which allows the fast, unambiguous and sensitive simultaneous detection and relative stability approximation of supramolecular assemblies in mixtures. In spite of the obvious fundamental differences between solution and gas phase, ESI-MS in the case of self-assembled molecular capsules has been shown to produce very similar results to single binding experiments monitored by NMR titrations as well as conformational searches performed by Monte-Carlo simulations. MS/MS experiments reveal the same relative order of gas phase stabilities as previously found in solution. Moreover, proton transfer reactions which lead to new molecular capsules, are not detectable in the time-averaged NMR spectrum. However, the newly produced species are found in the complex mixtures by ESI-MS and can be conveniently characterized by subsequent MS/MS experiments: in a collision-induced dissociation the single half-spheres are easily discovered and structurally assigned. Thus, ESI-MS has worked as a valuable tool for the rapid screening of complex supramolecular mixtures and in combination with MS/MS experiments elucidated both the path of unexpected side reactions as well as the thermodynamic gas-phase stabilities of all components in the mixture.