Crown Ether Host Research Articles

One pot synthesis of crystalline covalent organic polymers with tunable pores for the removal of gold and toxic organic pollutants

The construction of crystalline covalent organic framework from flexible building blocks, especially macrocyclic hosts, rare in literature. Further, tuning the number of pores and their sizes in the final covalent organic frameworks (COFs)/covalent organic polymers (COPs) is challenging and exciting. Recently, attempts have been made to synthesise the crystalline covalent organic framework starting from a macrocyclic crown ether host. However, the synthetic strategies involve multi-step organic reactions. Herein, we report the construction of a crystalline covalent organic framework/polymers from a single-step reaction. Further, this approach will allow us to pre-determine the pore sizes and their properties to attain the focused covalent organic frameworks (COFs)/covalent organic polymers (COPs) materials of specific surfaces and functions. Based on this approach, we have constructed two triphenylene-based crown ether COPs, COP-TPC6 and COP-TPC8, from dibenzo-18-crown-6 and dibenzo-24-crown-8, respectively. The obtained COPs displayed remarkable removal capacity toward gold (COP-TPC6 = 1157 mg/g and COP-TPC8 = 943 mg/g) with high selectivity over a wide pH range. Furthermore, the obtained COP-TPCs displayed good removal capacity towards a wide range of organic dyes like thiazine dye, triarylmethane dye and azo dye via supramolecular non-covalent interaction. COP-TPC6 displayed good removal capacity toward Neutral red (NR) (Qmax = 500 mg/g), Malachite green (MG) (Qmax = 364 mg/g) and Mordant orange (MoO) (Qmax = 193.8 mg/g). Similarly, COP-TPC8 displayed a removal capacity of Qmax of 307 mg/g for NR, 544 mg/g for MoO and 144.77 mg/g for MG. This work will help the scientific communities to fabricate adsorbents with combined physical and chemical features of triphenylene and crown ethers units.

Crown Ether Modulation Enables over 23% Efficient Formamidinium-Based Perovskite Solar Cells.

The use of molecular modulators to reduce the defect density at the surface and grain boundaries of perovskite materials has been demonstrated to be an effective approach to enhance the photovoltaic performance and device stability of perovskite solar cells. Herein, we employ crown ethers to modulate perovskite films, affording passivation of undercoordinated surface defects. This interaction has been elucidated by solid-state nuclear magnetic resonance and density functional theory calculations. The crown ether hosts induce the formation of host-guest complexes on the surface of the perovskite films, which reduces the concentration of surface electronic defects and suppresses nonradiative recombination by 40%, while minimizing moisture permeation. As a result, we achieved substantially improved photovoltaic performance with power conversion efficiencies exceeding 23%, accompanied by enhanced stability under ambient and operational conditions. This work opens a new avenue to improve the performance and stability of perovskite-based optoelectronic devices through supramolecular chemistry.

Cover Picture: Construction of Crown Ether‐Stoppering [3]Rotaxanes Based on N‐Hetero Crown Ether Host (Chin. J. Chem. 7/2017)

The cover picture shows shows the construction of crown etherstoppering [3]rotaxanes based on N‐hetero crown ether host. Usually, crown ethers play the role of host macrocycles to combine with the guest molecules in the construction of rotaxanes. Based on the fact that crown ethers have large dimension, two [3]rotaxanes containing four crown ether units were designed and synthesized, of which, two N‐hetero crown ether components were employed as the macrocyclic hosts to assemble the mechanically interlocked framework by using a template‐directed clipping reaction while bis (metaphenylene‐26‐crown‐8) located on two sides of template diammonium acting as the stoppering groups of [3]rotaxanes. More details are discussed in the article by Yin et al. on page 1050–1056. image

Rotaxane‐type hyperbranched polymers from a crown ether host and paraquat guests containing blocking groups

ABSTRACTRotaxane‐type hyperbranched polymers are synthesized for the first time from A2B type semi‐rotaxane monomers formed in situ via complexation of bis(m‐phenylene)‐32‐crown‐10 dimethanol (1) and two paraquat ω‐n‐alkylenecarboxylic acid derivatives with tris(p‐t‐butylphenyl)methylphenylalkylene stoppers (8 and 9). Rotaxane and taco complexes exist in solutions of the hyperbranched polyesters in CD3CN/CDCl3 as confirmed by NMR spectroscopy, but the taco complexes, which derive from non‐rotaxanated paraquat units, disappear in DMSO‐d6. NMR spectroscopy indicates the portion of rotaxanes strongly interlocked by the environment (inner rotaxanes) is larger in HP1•9, which has longer alkylene spacers, perhaps indicating a higher degree of polymerization. The molecular size increases upon formation of the hyperbranched polymers are confirmed by dynamic light scattering and by viscometry. As with covalent hyperbranched polymers a number of potential applications exist; the unique mechanically linked character and the presence of uncomplexed host and guest moieties foreshadow the use of such systems for their responses to external stimuli with the added benefit of providing molecular recognition sites useful as delivery vehicles. Use of other host‐guest motifs to form the semirotaxane A2B monomers is possible and complementary systems with higher binding constants will enable efficient syntheses of high molecular weight, mechanically linked hyperbranched polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1647–1658

A molecular pulley based on a triply interlocked [2]rotaxane

A novel triply interlocked [2]rotaxane based on a triptycene-derived tris(crown ether) host and a guest embedded with three dibenzylammonium and three N-methyltriazolium recognition sites was designed and synthesized, which showed pulley-like shuttling motion controlled by acid and base.

Iptycene-derived crown ether hosts for molecular recognition and self-assembly.

CONSPECTUS: Synthetic macrocyclic hosts have played key roles in the development of host-guest chemistry. Crown ethers are a class of macrocyclic molecules with unique flexible structures. They have served as the first generation of synthetic hosts, and researchers have extensively studied them in molecular recognition. However, the flexible structures of simple crown ethers and their relatively limited modes of complexation with guests have limited the further applications of these molecules. In recent years, researchers have moved toward fabricating interlocking molecules, supramolecular polymers, and other assemblies with specific structures and properties. Therefore, researchers have developed more complex crown ether-based macrocyclic hosts with multicavity structures and multicomplexation modes that provide more diverse and sophisticated host-guest systems. In this Account, we summarize our research on the synthesis and characterization of iptycene-derived crown ether hosts, their use as host molecules, and their applications in self-assembled complexes. Iptycenes including triptycenes and pentiptycenes are a class of aromatic compounds with unique rigid three-dimensional structures. As a result, they are promising building blocks for the synthesis of novel macrocyclic hosts and the construction of novel self-assembled complexes with specific structures and properties. During the last several years, we have designed and synthesized a new class of iptycene-derived crown ether hosts including macrotricyclic polyethers, molecular tweezer-like hosts, and tritopic tris(crown ether) hosts, which are all composed of rigid iptycene building blocks linked by flexible crown ether chains. We have examined the complexation behavior of these hosts with different types of organic guest molecules. Unlike with conventional crown ethers, the combination of iptycene moieties and crown ether chains provides the iptycene-derived crown ether hosts with complexation properties that differ based on the structure of the guests. The rigid iptycene moieties within these synthetic host molecules both maintain their inherent three-dimensional cavities and generate multicavity structures. The flexible crown ether chains allow the iptycene-derived hosts to adjust their conformations as they encapsulate guest molecules. Moreover, the expanded complexation properties also allow the host-guest systems based on the iptycene-derived crown ethers to respond to multiple external stimuli, resulting in a variety of supramolecular assemblies. Finally, we also describe the construction of mechanically interlocked self-assemblies, molecular switches/molecular machines, and supramolecular polymers using these new host molecules. We expect that the unique structural features and diverse complexation properties of these iptycene-derived crown ether hosts will lead to increasing interest in this field and in supramolecular chemistry overall.

Chiral Recognition of Amino Acid Enantiomers by a Crown Ether: Chiroptical IR-VCD Response and Computational Study

We report on a combined experimental and computational study of the chiral recognition of the amino acid serine in protonated form (L/D-SerH(+)), by the crown ether (all-S)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (S-18c6H4). Infrared and vibrational circular dichroism spectroscopies (IR-VCD) are employed to characterize the chiroptical response of the complexes formed by S-18c6H4 with the L-SerH(+) and D-SerH(+) enantiomers in dried thin films obtained from aqueous solutions. The study focuses on vibrational modes directly related to the intermolecular hydrogen bonds between the crown ether derivative and serine, responsible for crown-serine binding, namely, the C═O and C-O stretching modes, and on the C-O-H bending mode, which yield intense IR and VCD signals in the range of wavenumbers 900-2000 cm(-1). The experimental spectra are analyzed in combination with a computational structural survey and optimization at different levels of density functional theory. The conformational landscape of the complexes is found to be primarily governed by a bowl-like structure of the crown ether host and a tripodal coordination of the protonated R-NH3(+) group of serine with the oxygen atoms of the central ether ring. Additionally, one or two of the carboxylic side groups of the crown ether interact with the -COH and -COOH groups of serine. Chiral selectivity is probed by recording the IR and VCD spectra of dried thin films obtained from aqueous solutions with equimolar concentrations of the two serine enantiomers and the macrocycle. The results demonstrate a marked chiral recognition of L-SerH(+) relative to D-SerH(+) by the S-18c6H4 substrate, which arises from the favorable host-guest coordination through H-bonds at optimum distances and collinear orientations, also involving a limited distortion of the crown ether backbone.

Ammonium–crown ether based host–guest systems: N–H⋯O hydrogen bond directed guest inclusion featuring N–H donor functionalities in angular geometry

This article demonstrates crown ether based host–guest adduct formation in a series of crystalline solids 1–7, in which ammonium-type cationic guests have been integrated with various crown ethers. Addition of more than two equivalents of perchloric acid to an acetonitrile solution of 3-aminopyridine (3AmPy), 4,4′-diaminodiphenylmethane (DADPM) or 4,4′-diaminodiphenylether (DADPE) generates the doubly protonated anilinium (or ammonium) cationic species (written as 3AmPyH2, DADPMH2 and DADPEH2 respectively) which act as guests to the various crown ether hosts present in solution. The relevant compounds crystallize as their perchlorate salts and they have been structurally characterized through X-ray diffraction. The major driving force towards host–guest complexation in the compounds 1–7 is found to be the N–H⋯O hydrogen-bond interactions between the aforementioned guests and the crown ether hosts.

Chiral Resolution of Ionic Compounds by Thin Layer Extraction

The increasing demand for enantiomer intermediates has attracted in recent years attention to liquid–liquid extraction as a scalable chiral resolution method. Thin layer extraction, a simple and effective form of liquid–liquid extraction that uses little extractant, promises economic utilization of the most selective chiral hosts. The role played by various parameters when applying thin layer extraction to the resolution of an ionic racemic mixture into its enantiomer components is analyzed. This analysis provides a critical insight that facilitates the design of a successful separation scheme. It transpires that, given a suitable host, certain conditions concerning the host concentration and the pH and rate of the feed and strip solutions must be balanced to obtain optimum enantioseparation or even to obtain any separation at all. A calculated example that is based on experimental data from the published literature indicates that a simple thin layer extractor, using a crown ether host, is capable of separating an ionic racemic mixture into substantially pure enantiomer products at a high yield. It appears that thin layer extraction has some promising features to become an interesting alternative for other extraction-based techniques.

Supramolecular Pseudorotaxane Graft Copolymer from a Crown Ether Polyester and a Complementary Paraquat-Terminated Polystyrene Guest

The self-assembly of a polyester containing crown ether host repeat units and a paraquat-terminated polystyrene guest provides a supramolecular pseudorotaxane graft copolymer based on the bis(m-phenylene)-32-crown-10/paraquat recognition motif. NMR chemical shift changes in solution gave evidence of the complexation of the crown ether and paraquat moieties. Solutions of the two polymers at 1:1 host:guest stoichiometry (crown ether:paraquat), but only 3 mass % crown polymer, possessed an intrinsic viscosity 1.9 times that of the paraquat-terminated polystyrene and 2.7 times that of the polyester. Differential scanning calorimetry revealed only one glass transition for a solid from evaporation of a solution at 3:1 host:guest (crown ether:paraquat) stoichiometry; this indicated complete phase mixing, as confirmed by optical microscopy and small-angle laser light scattering.

Efficient Syntheses and Complexation Studies of Diacetylene‐Containing Macrocyclic Polyethers

AbstractA series of novel macrocyclic polyethers, phenylene–diacetylene‐containing macrocycles was synthesized in high yields by employing copper(II)‐mediated Eglinton coupling as the key macrocyclization (80–92 %). As an analogue of the bisphenylene crown ether hosts, the binding behavior of these new macrocycles was investigated by 1H NMR spectroscopy, mass spectrometry (ESI), and X‐ray crystallography. The results obtained indicate that macrocycles with tetraethylene glycol chains (3c–e) bind a paraquat guest to form [2]pseudorotaxane‐like complexes in solution and in the solid state, whereas macrocycles with triethylene glycol chains (i.e., 3f–h) bind a paraquat guest to form [3]pseudorotaxane‐like complexes, which implies that macrocycles 3f–h may be able to complex to (mono)pyridinium cations in a [2]pseudorotaxane‐like fashion in solution.

Negatively charged crown ethers for binding paraquat in water

A water soluble negatively charged fluorescent 1,4-benzo-1,5-naphtho-36-crown-10-based host has been devised and synthesized. As shown by proton NMR, ESI mass spectrometry and UV-vis spectroscopy, it binds paraquat with a 1:1 stoichiometry and an association constant of 4.50 (± 0.02) × 103 M−1 in water. Its complexation with paraquat in water was further investigated by fluorescence emission spectroscopy. The results revealed that when paraquat was added to the water solution of the host, the fluorescence emission of the host was quenched by the charge transfer between the crown ether host and paraquat guest. A similar bis-p-phenylene-34-crown-10-based host was synthesized as a contrasting host. Lacking the naphthalene unit, the second host binds paraquat with a 1:1 stoichiometry and a lower association constant, 1.04 (± 0.04) ×103 M−1, in water. This demonstrated that the naphthalene unit could not only act as a strong fluorescence group but also improve the π-π stacking interactions between the host and guest.

Characterization of second coordination shell ionophore–siderophore host–guest assemblies and binding selectivities in binary and complex mixtures by electrospray ionization mass spectrometry

An ESI-MS method for the determination of host–guest association constants and binding selectivities for systems of binary and complex mixtures, respectively, is presented in an effort to quantitatively study the strength of host–guest interactions, while accounting for the different ESI-MS spray efficiencies of each component. Association constants were obtained for host–guest complexes formed between a series of crown ether and linear ionophore hosts and various guests, including protonated amines, and an apo- and ferri-siderophore, ferrioxamine B.

Alternative Donor−Acceptor Stacks from Crown Ethers and Naphthalene Diimide Derivatives: Rapid, Selective Formation from Solution and Solid State Grinding

The atypical 1:2 complexation between an electron-rich crown ether host and electron-deficient naphthalene diimide-based guests led to the formation of alternative donor-acceptor (ADA) stacks. The ADA stacks can be expediently obtained in high yield as polycrystalline aggregates from solution. More remarkably, the high degree of organization has also been realized in a simple solid-to-solid mechanical grinding process. The solid-state structures have been verified by solid-state NMR spectroscopy, single crystal, and powder X-ray diffraction analysis. The current findings not only provide convenient ways of obtaining novel donor-acceptor stacks involving a macrocyclic host but also represent an important step in transferring electroactive host-guest systems from solution to the solid state.

Electrochemical and Guest Binding Properties of Fréchet- and Newkome-Type Dendrimers with a Single Viologen Unit Located at Their Apical Positions

Several new series of dendrimers containing a single redox-active 4,4'-bipyridinium (viologen) residue were synthesized and characterized. In these dendrimers, the viologen group is covalently attached to the apical position of a Newkome- or Frechet-type dendron, ranging in size from first to third generation of growth. The half-wave potentials corresponding to the two consecutive one-electron reductions of the viologen residue are affected by the size of the dendritic component. The size effects are more pronounced in the Newkome-type dendrimers and seem to result from the polarity contrast between the microenvironments provided by the solution and the internal phase of the dendrimer. Unlike in many other dendrimers having a redox-active core, the voltammetric behavior remains fast (reversible) even in third generation dendrimers. Pulse gradient stimulated echo NMR diffusion coefficient measurements on the Newkome-type dendrimers reveal that their hydrodynamic radii are relatively invariant in solvents of widely different polarities (dichloromethane to dimethyl sulfoxide). The host-guest binding interactions between the viologen residue in these dendrimers and the crown ether host bis-p-phenylene-34-crown-8 were also investigated. While in Newkome-type dendrimers the growth of the dendron caused a substantial attenuation of the binding constant values, this size effect was not observed in the Frechet-type dendrimers. These electrochemical and binding measurements underscore some of the structural differences between these two common types of dendritic architectures.

Positive ion pair cooperativity exhibited for the binding of phosphate under physiological conditions

The synthesis of a heteroditopic receptor which exhibits positive cooperativity for the binding of phosphate ion pairs under physiological conditions. Optimised complementarity between crown ether host and metal guest leads to increased binding affinity, Ka.

The influence of constitutional isomerism and change on molecular recognition processes.

Three constitutionally isomeric bis(naphthylmethyl)ammonium ions, in which the two naphthyl groups are substituted 1) both at their 1-positions, 2) one at its 1-position and the other at its 2-position, and 3) both at their 2-positions, have been investigated separately in solution for their propensities to undergo spontaneous self-assembly with three different [24]crown-8 derivatives, namely, pyrido[24]crown-8 (P24C8), dipyrido[24]crown-8 (DP24C8) and dibenzo[24]crown-8 (DB24C8), in turn to form [2]pseudorotaxanes. The strengths of the 1:1 complexes depend on the composition of the secondary dialkylammonium ions and on the nature of the crown ether hosts; generally, as far as the guest cation is concerned, the 1/1- and 2/2-isomers form stronger complexes, as indicated by stability constant measurements, than the 1/2-isomer and, as far as the crown ethers are concerned, the more flexible P24C8 is a much more efficient host than either DP24C8 or DB24C8. The rates of formation of the [2]pseudorotaxanes are fast (i.e., taking no more than a few minutes) in solution with the exception of one case, that is, in which the crown ether host is DB24C8 and the guest cation is the 1/1-isomer, when it can take upwards of one month for the complexation-decomplexation equilibrium to be established at room temperature. In all cases, the equilibrium between complexed and uncomplexed species is slow on the NMR timescale, allowing the determination of stability constants to be made readily using the single-point method. X-ray crystallography and molecular modeling have been used to gain insight into ground and transition state interactions, respectively, in some of the [2]pseudorotaxanes. The relative stabilities of the three [2]pseudorotaxanes formed by each guest cation in the presence of the three crown ether hosts were also evaluated in solution by competition experiments that were monitored by (1)H NMR spectroscopy. By and large the results of the competition experiments could be predicted on the basis of the derived stability constants for the individual [2]pseudorotaxanes.

Depression of the apparent chiral recognition ability obtained in the host-guest complexation systems by electrospray and nano-electrospray ionization mass spectrometry.

Chiral recognition in the host-guest complexation systems of chiral crown ether hosts and amino ester guests was thoroughly examined using the electrospray ionization (ESI) mass spectrometry/enantiomer labeled (EL)-guest method. In this method, the mass spectra of a mixture of three components in a solution, a chiral host (H), an equal amount of an (S)-enantiomer guest labeled with deuterium atoms (G(S-dn)(+)) and an unlabeled (R)-enantiomer guest (G(R)+), were measured and the relative peak intensity value [I(H + G(R))(+) / I(H + G(S-dn))(+) = IRIS] of the host-guest complex ions, observed with an excess guest concentration, was taken to provide the chiral recognition ability of the host. In our earlier report (1996), we demonstrated that the apparent chiral recognition abilities using a mass spectrometer with a homemade ESI interface were depressed by about one tenth compared with the corresponding abilities obtained by fast-atom bombardment (FAB) MS. In the present study, the enantioselective complexation behaviors of various combinations of chiral crown hosts with chiral guests were further investigated in detail mainly using a modern commercial ESI/ion trap (IT) mass spectrometer. Consequently, it was found that the apparent IRIS values from the ESI-MS/EL-guest method changed significantly, depending upon the instrument used, and in particular, upon the ESI interfaces. Moreover, under the specific measuring conditions in ESI-IT-MS, the degrees of depression of the apparent chiral recognition abilities are roughly grouped into three classes, depending upon the number (or probably the type) of the hydrophobic substituents of the hosts. Representing the degrees by the slopes when plotting the apparent IRIS values in ESI-MS versus those in FAB-MS, the slopes for the three classes are (1) 1.0, (2) 0.7 and (3) 0.3; the higher the hydrophobicity of the hosts (and then, the host-guest complex ions), the lower the slope (the apparent enantioselectivity). Strengthening the degree of depression may be caused by an increase in the local concentration of the host close to the surface of the droplets produced during the electrospary ionization process. The chiral recognition ability (K(R )/ K(S)) in an equilibrated solution agrees quite well with the IRIS value in FAB-MS rather than that in ESI-MS.

Host-Guest Complexes. Selective Reaction of Crown Ether Hosts with Triphenylmethane Derivatives

One of priority tasks of guest-host chemistry is search for selective interactions of neutral molecules, leading to formation of molecular complexes to model processes widely occurring in biological systems [1]. In [2], crown ethers and triphenylmethane derivatives are referred to as host molecules capable of binding smaller size host molecules [3 5] into complexes. The aim of the present work was synthesize crystalline molecular complexes of crown ethers I III with triphenylmethane derivatives and examine selectivity of the complex-formation reaction between these partners.

Chiral Recognizable Host-Guest Interactions Detected by Fast-Atom Bombardment Mass Spectrometry: Application to the Enantiomeric Excess Determination of Primary Amines

The enantiomeric excess ( ee) of organic amine compounds has been determined using fast-atom bombardment (FAB) mass spectrometry based on the chiral host–guest complexation systems. The method uses a 1:1 mixture of the given chiral crown ether hosts (HRRRR and HSSSS-dn), one of whose enantiomers is isotopically labeled, for the ee-determination of a given amine salt guest (G+). The peak intensity ratio { I[(HRRRR + G)+] / I[(HSSSS-dn + G)+] = IRIS} of the two diastereomeric host–guest complex ions clearly changes with the change in the ee of the guest. The intensity excess ( Ie) of the two complex ion peaks is newly defined as Ie = ( IRIS −1) / ( IRIS + 1). The two sets of mass spectrometrically obtained Ie values vs a set of ee values of phenylalanine methyl ester and leucine methyl ester hydrochlorides show excellent linear relationships through the zero point ( R2 = 0.9989 and R2 = 0.9970, respectively). This indicates the potential utility of the present ee-determination method to within ± 3% ee. Furthermore, based on the solution equilibrium distributions of the complex ions, the linearity is mathematically justified. Therefore, the ee-determination can be simplified as ee (%) = (| Ie| / | Ie(100)|) × 100: here, Ie(100) is the corresponding Ie value obtained using an enantiomerically pure (100% ee) guest as a reference. The present chiral dimethoxyphenyl crown ether host pairs employed are effective ( IRIS ≥ 1.5) for the ee-determination of most α-amino acid esters, but not effective ( IRIS ≈ 1.0) for most simple alkylamines.