Arene Capsules Research Articles

Hexameric Capsules Studied by Magic Angle Spinning Solid‐State NMR Spectroscopy: Identifying Solvent Molecules in Pyrogallol[4]arene Capsules

AbstractPowders of pyrogallol[4]arene hexamers were produced by evaporation from organic solvents and were studied, for the first time, by magic angle spinning solid‐state NMR (MAS ssNMR). Evaporation selectively removed non‐encapsulated solvent molecules leaving stable hexameric capsules encapsulating solvent molecules. After exposure of the powder to solvent vapors, 1H/13C heteronuclear correlation MAS ssNMR experiments were used to assign the signals of the external and encapsulated solvent molecules. The formed capsules were stable for months and the process of solvent encapsulation was reversible. According to the ssNMR experiments, the encapsulated solvent molecules occupy different sites and those sites differ in their mobility. The presented approach paves the way for studying guest exchange, guest affinity, and gas storage in hexamers of this type in the solid state.

Hexameric Capsules Studied by Magic Angle Spinning Solid-State NMR Spectroscopy: Identifying Solvent Molecules in Pyrogallol[4]arene Capsules.

Powders of pyrogallol[4]arene hexamers were produced by evaporation from organic solvents and were studied, for the first time, by magic angle spinning solid-state NMR (MAS ssNMR). Evaporation selectively removed non-encapsulated solvent molecules leaving stable hexameric capsules encapsulating solvent molecules. After exposure of the powder to solvent vapors, (1)H/(13)C heteronuclear correlation MAS ssNMR experiments were used to assign the signals of the external and encapsulated solvent molecules. The formed capsules were stable for months and the process of solvent encapsulation was reversible. According to the ssNMR experiments, the encapsulated solvent molecules occupy different sites and those sites differ in their mobility. The presented approach paves the way for studying guest exchange, guest affinity, and gas storage in hexamers of this type in the solid state.

α,ω-Alkanediyldiammonium dications sealed within calix[5]arene capsules with a hydrophobic bayonet-mount fastening

A combined XRD and 1H NMR study revealed that two molecules of tetrakis-(tert-butyloxycarbonylmethoxy)-calix[5]arene carboxylic acid are able to efficiently encapsulate α,ω-diaminoalkanes of suitable length as a result of proton-transfer-mediated recognition. In the case of 1,10-diaminodecane and 1,11-diaminoundecane, rim-to-rim attractive van der Waals interactions drive the formation of capsular complexes with a packing coefficient close to 70%. The two calixarene bowls in these capsules are seen in an eclipsed conformation. On the other hand, when the longer 1,12-diaminododecane guest is used, a quasi-capsular complex is formed in which the capsule seal is lost and the two calixarenes are arranged in a staggered conformation as a result of a reciprocal rotation of the two facing macrocyclic cavities. A comparison with similar capsular complexes confirms that a hydrophobic ‘bayonet-mount’ fastening is mandatory for the encapsulation of suitably sized guests.

Guest loading, chromatographic purification and controlled release from kinetically trapped, hydrogen-bonded pyrogallol[4]arene capsules

Pyrogallol[4]arene and hydrocarbon guest molecules form kinetically trapped encapsulation complexes when assembled in the absence of solvent. While solvent encapsulation complexes are more stable, these pre-loaded capsules can be purified by gel permeation chromatography and guest release can be induced later, at a chosen time, by heating. This result is the first example of chromatographic purification and subsequent guest release from kinetically trapped, artificial, hydrogen-bonded molecular capsules.

Encapsulation Studies of Cationic Gold Complexes within a Self‐Assembled Hexameric Resorcin[4]arene Capsule

AbstractAs the field of supramolecular catalysis develops, a sound fundamental understanding of the catalytic assemblies used will be essential. There is a lack of knowledge regarding systems based on transition‐metal catalysts as guests of supramolecular capsules. We investigate the inclusion behavior of gold complexes based on well‐known cationic Au(I) catalysts within a self‐assembled hexameric resorcin[4]arene capsule. The results shed light on the nature of the encapsulated species and the way in which those species are encapsulated. We introduce an encapsulation methodology that functions through the transient introduction of an excess of water. Through small modifications in the encapsulation methodology, we show that the inclusion process can be controlled to give different encapsulated species stemming from the same gold(I) precursor.

Efficient isonitrile hydration through encapsulation within a hexameric self-assembled capsule and selective inhibition by a photo-controllable competitive guest

Catalytic hydration of neutral isonitriles to yield the corresponding N-formylamides was achieved by reversible encapsulation in a self-assembled hexameric resorcin[4]arene capsule. Encapsulation of a photochromic dithienylethene bis-cation provides different levels of competitive inhibition depending on the geometry assumed by the cationic inhibitor.

Switching the activity of a photoredox catalyst through reversible encapsulation and release

Reversible encapsulation of [Ru(bpy)(3)](2+) within a self-assembled hexameric resorcin[4]arene capsule turns off the photocatalytic aerobic oxidation of an aliphatic sulfide. Upon addition of a competitive cationic guest, the Ru(II) catalyst is released into solution where its catalytic activity is restored.

Coordination Polymer Chains of Dimeric Pyrogallol[4]arene Capsules

Copper seamed dimeric metal-organic pyrogallol[4]arene capsules have been synthesized and engineered into coordination polymer chains, either by direct coordination or with additional bridging ligands. The formation of this copper capsule also demonstrates selective formation of either hexameric or dimeric metal-organic capsules.

Dimeric Resorcin[4]arene Capsules in the Solid State

AbstractSupramolecular chemistry research is focused on the study of weak non‐covalent intermolecular — that is, supramolecular — interactions as the driving force in self‐assembly and molecular recognition. Dimeric resorcin[4]arenes capsules have been a focus of our research for the last 15 years. This review describes the solid state complexation studies of unsubstituted phenolic resorcin[4]arenes and pyrogall[4]arenes towards the formation of dimeric capsules and assemblies using ionic and neutral species as guest molecules and templates. The multitude of different crystal structures obtained during these studies demonstrates the versatile nature of resorcin[4]arenes and pyrogall[4]arenes (2‐hydroxy‐resorcin[4]arenes) as supramolecular hosts in crystal engineering.

Recent advances in hydrogen-bonded hexameric encapsulation complexes

Basic research in the chemistry of hexameric resorcin[4]arenes and pyrogallol[4]arenes during the last decade is reviewed. Applications of NMR methods to determine solution structures, host guest properties and exchange dynamics are discussed. The scientific issue is the behavior of molecules in small spaces; the challenge is to translate this information to practical applications in, say, catalysis or transport.

Electrochemical response of ferrocenium filled calix[4]arene capsules in solution and immobilized on gold

Hydrogen-bonded dimers of tetra-urea calix[4]arenes are able to include ferrocenium ions as a guest. Heterodimeric capsules are exclusively formed in 1:1 mixtures of tetra-arylurea calix[4]arene 1 with tetra-tosylurea calix[4]arene 2. Sulfide groups attached via spacers on the urea rim of 1 allow their binding to metal surfaces rendering simultaneously their dissociation more difficult. Thus, the electrochemical response of encapsulated ferrocenium could be studied in a self-assembled monolayer (SAM) in addition to the solution electrochemistry with cyclic voltammetry. The stability of the electrochemically active monolayers was tested after a heat treatment (225 °C for 30 min in N 2 atmosphere) and multiple cycling (up to 1200 cycles), which could be required for a possible application as molecular based information storage.

The Underestimated Role of Counter Ions in Electrostatic Self‐Assembly:[1+1] Cavitand‐Calix[4]arene Capsules Based on Azinium–Sulfonate Interactions

AbstractThe apparent Ka values of the formation of [1+1] cavitand‐calix[4]arene capsules, based on azinium–sulfonate electrostatic interactions, obtained by direct titration experiments are concentration dependent due to the influence of the interaction of the charged capsule components with their initial counterions. 1H NMR and UV dilution experiments, in which this ion‐pairing is excluded, gave Ka‐values larger than 2·106 M–1 in methanol and methanol/water (1:1) for the capsule formation. The capsule encapsulates small guests like methanol, ethanol, etc., as proven by ESI‐MS. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

An achiral form of the hexameric resorcin[4]arene capsule sustained by hydrogen bonding with alcohols

The well-known hexameric capsules sustained by self-assembly of resorcin[4]arenes 1 with water molecules (1(6).(H2O)8) are shown to assemble similarly with (+/-)-2-ethylhexanol (2EH) as an achiral 1(6).(2EH)6.(H2O)2 species which further encapsulates 2EH.

Guest-Induced Molecular Capsule Assembly of p-Sulfonatothiacalix[4]arene

Two new compact molecular capsules of p-sulfonatothiacalix[4]arene with sodium ions have been synthesized by inducing suitable guest molecules, 1,2-bis(imidazol-1‘-yl)ethane or 2,2‘-bipyridine. Single-crystal X-ray diffraction analyses reveal that these supramolecular compounds possess open or close capsule conformations in a zigzag-like chain or a straight-line chain array. 1H NMR spectroscopic studies confirm that the above stable molecular capsules are present in solution. Their guest molecules in both compounds are well accommodated by p-sulfonatothiacalix[4]arene capsules via a variety of supramolecular interactions.

Fluorescent Guest Molecules Report Ordered Inner Phase of Host Capsules in Solution

Despite recent strides in the synthesis of elaborate nanometer-scale molecular hosts, the internal structure of these self-assembled cages remains ill characterized. We used fluorescent probe molecules, pyrene butyric acid (PBA), as guests in C-hexylpyrogallol[4]arene capsules to relay information about the chemical environment on the interior of the assemblies. Spectroscopic and single-crystal x-ray diffraction studies show that, in both solution and the solid state, the host can encapsulate two PBA guests and keep them well separated through specific interactions with the capsule walls.

Self-recognition, structure, stability, and guest affinity of pyrogallol[4]arene and resorcin[4]arene capsules in solution.

In the present study, we used diffusion NMR to probe the structures and characteristics of the products obtained from the self-assembly of resorcin[4]arenes 1a and 1b and pyrogallol[4]arenes 2a and 2b in CDCl(3) solutions. It was found that all four molecules self-assemble into hexameric capsules. The hexameric capsules of pyrogallol[4]arenes 2a and 2b were found to be more stable than the capsules of resorcin[4]arenes 1a and 1b in polar media. We also studied the role of water molecules in the self-assembly of the different capsules and found that water molecules are part of the hexameric capsules of resorcin[4]arenes 1a and 1b but not in the capsules of pyrogallol[4]arenes 2a and 2b. It was found that the self-assembly process between the resorcin[4]arenes and pyrogallol[4]arenes proceeds with self-recognition. When mixing two macrocycles of different types in a chloroform solution, no heterohexamers are formed, only the capsule constructed from the same macrocycle is detected. However, when two resorcin[4]arenes (i.e., 1a and 1b) or two pyrogallol[4]arenes (i.e., 2a and 2b) are mixed, heterohexamers are formed over time. In addition, we found that resorcin[4]arenes and pyrogallol[4]arenes differ significantly in their guest affinity. The capsules of 1a and 1b can accommodate both the tertiary alkylamines and their respective ammonium salts, while the capsules of 2a and 2b encapsulate only the neutral tertiary alkylamines.