Crown Ether Moiety Research Articles

Synthesis and Applications of Crown Ether-Linked Metal–Organic Framework Composite-Based Sensors for Stripping Voltammetric Determination of Lead

The present work demonstrates the construction and electrochemical characterization of novel disposable screen-printed carbon sensors integrated with functionalized MIL-53-NH2 metal–organic framework cross-linked with crown ethers and calixarene macrocyclic compounds for sensitive differential pulse voltammetric determination of lead ions in tap, surface water and biological fluid samples. The electroanalytical parameters were optimized regarding the nature of the electrode modifier, supporting electrolyte, the working pH value, scan rate, deposition potential, deposition time, reproducibility of measurement and the operational lifetime. Working electrodes dropcasted with MIL-53-NH2-dibenzo-24-crown-8-ether nanocomposite (MOFCE24) showed the proper performance within the lead concentration ranging from 13.75 to 217.83 ppb with LOD and LOQ values of 3.18 and 9.62 ppb, respectively. The synergistic effect of the metal–organic frameworks as transducer and dibenzo 24-crown-8-ether as sensing elements accelerates the electron transfer process at the electrode surface and improves the sensor selectivity through complexation of the lead ions with the crown ether moiety. The fabricated sensors showed high measuring reproducibility with long operational life time (60 days), which can be attributed to the formation of cross-linked ionophore/metal–organic framework with limited leaching of the sensing element in the measuring solution. The introduced sensors were utilized for the onsite voltammetric determination of lead in environmental and biological samples with acceptable average recoveries comparable with the graphite furnace atomic absorption spectrometric method.

A Doubly Dissipative System Driven by Chemical and Radiative Stimuli.

The operation of a dissipative network composed of two or three different crown-ether receptors and an alkali metal cation can be temporally driven by the use (combined or not) of two orthogonal stimuli of a different nature. More specifically, irradiation with light at a proper wavelength and/or addition of an activated carboxylic acid, are used to modulate the binding capability of the above crown-ethers towards the metal ion, allowing to control over time the occupancy of the metal cation in the crown-ether moiety of a given ligand. Thus, application of either or both of the stimuli to an initially equilibrated system, where the metal cation is distributed among the crown-ether receptors depending on the different affinities, causes a programmable change in the receptor occupancies. Consequently, the system is induced to evolve to one or more out-of-equilibrium states with different distributions of the metal cation among the different receptors. When the fuel is exhausted or/and the irradiation interrupted, the system reversibly and autonomously goes back to the initial equilibrium state. Such results may contribute to the achievement of new dissipative systems that, taking advantage of multiple and orthogonal stimuli, are featured with more sophisticated operating mechanisms and time programmability.

Novel Crown Ether Amino Acids as Fluorescent Reporters for Metal Ions.

Unnatural amino acids with enhanced properties, such as increased complexing ability and luminescence, are considered to be highly attractive building blocks for bioinspired frameworks, such as probes for biomolecule dynamics, sensitive fluorescent chemosensors, and peptides for molecular imaging, among others. Therefore, a novel series of highly emissive heterocyclic alanines bearing a benzo[d]oxazolyl unit functionalized with different heterocyclic π-spacers and (aza)crown ether moieties was synthesized. The new compounds were completely characterized using the usual spectroscopic techniques and evaluated as fluorimetric chemosensors in acetonitrile and aqueous mixtures in the presence of various alkaline, alkaline-earth, and transition metal ions. The different crown ether binding moieties as well as the electronic nature of the π-bridge allowed for fine tuning of the sensory properties of these unnatural amino acids towards Pd2+ and Fe3+, as seen by spectrofluorimetric titrations.

Construction and Hierarchical Self-Assembly of Multifunctional Coordination Cages with Triangular Metal–Metal-Bonded Units

Herein, a series of face-capped (Tr2M3)4L4 (Tr = cycloheptatrienyl cationic ring; M = metal; L = organosulfur ligand) tetrahedral cages 1-3 functionalized with 12 appended crown ether moieties were designed and synthesized. The reversible binding of ammonium cations with peripheral crown ether moieties to adjust internal guest-binding was realized. Combination of a bisammonium linker and cage 3 led to the formation of a supramolecular gel SPN1 via host-guest interactions between the crown ether moieties and ammonium salts. The obtained supramolecular gel exhibited multiple-stimuli responsiveness, injectability, and excellent self-healing properties and could be further developed to a SPN1-based drug delivery system. In addition, the storage modulus of SPN1 was 20 times higher than that of the model gel without Pd-Pd bonded blocks, and SPN1 had better self-healing properties compared with the latter, demonstrating the importance of such cages in improving mechanical strength without losing the dynamic properties of the material. The cytotoxicity in vitro of the drug-loaded (doxorubicin or methotrexate) SPN1 was significantly improved compared to that of free drugs.

Novel class of crown ether functionalized ionic liquids with multiple binding sites for efficient separation of lithium isotopes

Lithium isotopes (6Li, 7Li) separation is an industrially important but challenging process due to their very similar chemical and physical properties. Crown ethers (CEs) have great potential in the isotopes separation for their typical cavity structures. However, the low distribution of Li+ has restricted their industrialuse. In this work, a novel class of functionalized ionic liquids with grafted crown ether-moieties (CE-FILs) for efficient separation of 6Li and 7Li are first reported. The enhanced distribution of Li+ could be achieved by the synergistic effect of ion–dipole interaction, hydrogen bonding and coordination interaction. As a result, an exceptionally high distribution coefficient for Li+ (DLi, up to 136) is obtained using 0.3 M [C6(B15C5)mim][NTf2] with a satisfied separation factor of 1.034 (at 273 K) for 6Li/7Li selectivity from an extremely hydrophilic chloride solutions, which strikes an excellent balance between Li distribution and 6Li/7Li separation. Theoretical calculations and spectroscopic characterization confirm that [C6(B15C5)mim][NTf2] supplies as an ion pair receptor to bind Li+, forming a 1:1 ion-pair complex [C6(B15C5)mim][NTf2]…Li+FeCl4- with double cations and anions. The separation performance of 6Li/7Li mainly depends on the weak interactions associated with Li+. The CE-FILs pave a new way for Li isotopes separation.

(1,6)Pyrenophanes containing crown ether moieties as fluorescence sensors for metal and ammonium ions. Formation of sandwich, dumbbell, and pseudorotaxane complexes

(1,6)Pyrenophanes 1 and 2 in which 1,6-positions of pyrene are bridged with respective tri- and tetra-ethylene glycol chains were synthesized.

Cyclometallated Platinum(II) Complexes with Small Crown Ether Rings: Appropriate Choice of the Bridging Diphosphane to Coordinate Potassium Cations.

This account reports the synthesis and structural characterization of the first cyclometallated platinum(II) complex that coordinates a potassium cation in a sandwich arrangement via two 15-crown-5 ether rings within the same molecule. The cooperation of the two small crown ether moieties allows the entrapment of the non-ideal potassium ion. The reaction of the parent thiosemicarbazone ligand 3,4-(C8H16O5)C6H3C(Me)=NN–(H)C(=S)NHMe, 1, containing the crown ether ring, with K2[PtCl4], or alternatively with PtCl2(DMSO)2, and subsequent treatment with the diphosphanes Ph2PCH2PPh2 (dppm) and Ph2PC(=CH2)PPh2 (vdpp) produced the double nuclear platinacycles 3a, 3b, and 4, probably via formation of the 2a and 2b intermediates. Complex 3a with the K+ cation in a sandwich coordination was slightly mixed with 3b lacking any K+. Alternatively, reaction of 1 with K2[PtCl4] or with PtCl2(DMSO)2 followed by the diphosphane Ph2PC(=CH2)PPh2 (vdpp) only gave the dinuclear phosphane-bridged compound 4; this highlights the importance of choosing the right diphosphane ligand. Density functional theory calculations (B3LYP-D3/LANL2DZ-ECP-6.311++G**) revealed similar affinities for both dppm and vdpp derivatives to coordinate potassium cations. Crystal structure analysis was performed for compounds 3a and 4.

Nanostructuring with Surfactants: The Self-Assembly of a New Poly(thiophene-phenylene) Conjugated Polymer Bearing Azacrown Ether Pendant Groups.

We report the synthesis of a new conjugated polymer bearing crown ether moieties, poly[(N(1-aza-[18]crown-6)carbamido)thiophene-2,5-diyl-alt-1,4-phenylene] (BG2). In water, BG2 forms a dispersion with a slightly cloudy appearance. We have studied the effect of adding surfactants, with different polar head groups, on these polymer-polymer aggregates. Special attention is given to the system with the anionic surfactant, sodium dodecyl sulfate (SDS). The combination of photophysical techniques with electrical conductivity, NMR (1H, 13C, and 27Na), DFT calculations, molecular dynamics simulations, and small-angle neutron scattering (SANS) provides a detailed picture on the behavior of the SDS/BG2 system in aqueous solution and in thin films. NMR, electric conductivity, and DFT results suggest that hydrophilic interactions occur between the polar headgroup of the surfactant (OSO3- Na+) and the aza-[18]-crown-6 moiety. DFT calculations confirmed the capability of BG2 to form stable complexes with the Na+ cations, where the cation can be either inside the azacrown cavity or sandwiched between the cavity and the polymer chain, which seem to determine the position of the surfactant hydrocarbon chain and, therefore, be responsible for the disruption of the BG2 aggregates and subsequent increase in the photoluminescence quantum yields. SANS measurements, made with hydrogenated and deuterated SDS in D2O, clearly show how micron-sized aggregates of BG2 are broken down by SDS and then how BG2 becomes preferentially incorporated within joint colloidal particles of BG2 and SDS with increasing [SDS]/[BG2] molar ratio.

Molecular Tweezer Based on Perylene and Crown Ether for Selective Recognition of Fullerenes.

A molecular tweezer trans-di(perylene-3-ylmethanaminobenzo)-18-crown-6 (DP-18C6) incorporating two perylene subunits in a single crown ether core was designed and synthesized as a host for fullerenes. Through the cooperative effect of the perylene subunits and the crown ether moiety, DP-18C6 can efficiently recognize fullerenes including C60, C70, and C76. 1H NMR titration and fluorescence titration experiments demonstrated that DP-18C6 can effectively grasp the fullerene molecule to form a 1:1 host–guest complex. Density functional theory calculations revealed the presence of intermolecular π–π interactions between the perylene subunits of DP-18C6 and the fullerene molecule. More importantly, DP-18C6 exhibited remarkably high binding selectivity for higher fullerenes over C60, revealing potential application for the separation of fullerenes by means of host–guest interactions.

Non-Globular Organic Ionic Plastic Crystal Containing a Crown-Ether Moiety - Tuning Its Behaviour Using Sodium Salts.

Organic ionic plastic crystals (OIPCs) are a class of soft materials showing positional order while still allowing orientational freedom. Due to their motional freedom in the solid state, they possess plasticity, non‐flammability and high ionic conductivity. OIPC behavior is typically exhibited by ‘simple’ globular molecules allowing molecular rotation, whereas the interactions that govern the formation of OIPC phases in complex non‐globular molecules are less understood. To better understand these interactions, a new family of non‐globular OIPCs containing a 15‐crown‐5 ether moiety was synthetized and characterized. The 15C5BA molecule prepared does not exhibit the sought‐after behavior because of its non‐globular nature and strong intermolecular H‐bonds that restrict orientational motion. However, the OIPC behavior was successfully obtained through complexation of the crown‐ether moiety with sodium salts containing chaotropic anions. Those anions weaken the interactions between the molecules, allowing rotational freedom and tuning of the thermal and morphological properties of the OIPC.

Synthesis of Pyridine-Containing Crowned Fused Expanded Porphyrins.

New examples of nonaromatic fused expanded porphyrins containing both pyridine and crown ether moiety as a part of macrocyclic framework were synthesized by condensing pyridine based pentapyrrane with polyether-based diol in CH2 Cl2 in the presence of one equivalent of BF3 ⋅ OEt2 under inert conditions followed by oxidation with DDQ in open air. The condensation was expected to form pyridine-containing crowned expanded porphyrins but resulted in the formation of fused crowned expanded porphyrins due to intramolecular fusion of two pyrrole nitrogens with two adjacent inverted thiophene carbons as revealed by X-ray crystallography obtained for one of the macrocycle. HRMS and NMR studies supported the formation of fused crowned pyridine containing expanded porphyrins, and the macrocycles showed simple, well-resolved NMR spectra where all resonances were identified easily by 2D NMR spectroscopy. The macrocycles exhibited typical nonaromatic absorption features and showed one broad band with peak maxima at 535 nm and one or two shoulder bands in the higher energy region. The protonation studies resulted in clear colour change from purple to blue and absorption bands experienced bathochromic shifts with a broad band at 662 nm which was extended up to 800 nm. The electrochemical studies revealed that the macrocycles were easier to oxidize but difficult to reduce. DFT studies indicated that the macrocycle attains a very puckered and distorted 'U' shaped structure owing to the flexibility of the crown ether chain and TD-DFT studies corroborated experimental results. The preliminary studies indicated that the macrocycles could be used as colorimetric optical sensor for the detection of Cu2+ ion.

Counterintuitive Electrostatics upon Metal Ion Coordination to a Receptor with Two Homotopic Binding Sites.

The consecutive binding of two potassium ions to a bis(18-crown-6) analogue of Tröger’s base (BCETB) in water was studied by isothermal titration calorimetry using four different salts, KCl, KI, KSCN, and K2SO4. A counterintuitive result was observed: the enthalpy change associated with the binding of the second ion is more negative than that of the first (ΔHbind,2° < ΔHbind,1°). This remarkable finding is supported by continuum electrostatic theory as well as by atomic scale replica exchange molecular dynamics simulations, where the latter robustly reproduces experimental trends for all simulated salts, KCl, KI, and KSCN, using multiple force fields. While an enthalpic K+–K+attraction in water poses a small, but fundamentally important, contribution to the overall interaction, the probability of the collapsed conformation (COL) of BCETB, where both crown ether moieties (CEs) of BCETB are bent in toward the cavity, was found to increase successively upon binding of the first and second potassium ions. The promotion of the COL conformation reveals favorable intrinsic interactions between the potassium coordinated CEs, which further contribute to the observation that ΔHbind,2° < ΔHbind,1°. While the observed trend is independent of the counterion, the origin of the significantly larger magnitude of the difference ΔHbind,2° – ΔHbind,1° observed experimentally for KSCN was studied in light of the weaker hydration of the thiocyanate anion, resulting in an enrichment of thiocyanate ions close to BCETB compared to the other studied counterions.

Octopus-Type Crown-Bisphthalocyaninate Anchor for Bottom-Up Assembly of Supramolecular Bilayers with Expanded Redox-Switching Capability.

Achievement of information storage at molecular level remains a pressing task in miniaturization of computing technology. One of the promising approaches for its practical realization is development of nanoscale molecular switching materials including redox-active systems. The present work demonstrates a concept of expansion of a number of available redox-states of self-assembled monolayers through supramolecular approach. For this, the authors synthesized an octopus-like heteroleptic terbium(III) bisphthalocyaninate bearing one ligand with eight thioacetate-terminated "tentacles" (octopus-Pc) and a ligand with four crown-ether moieties (H2 [(15C5)4 Pc]). It is shown that octopus-Pc forms stable monolayers on gold, where its face-on orientation allows for subsequent binding of crown-phthalocyanine molecules via potassium ion bridges. This chemistry is utilized to form a heterogeneous bilayer, in which a single molecule thick adlayer brings an additional redox-state to the system, thus expanding the multistability of the system as a whole. All four redox states available to this system exhibit characteristic absorbance in visible range, allowing for the switching to be easily read out using optical density measurements. The proposed approach can be used in wide range of switchable materials-single-molecule magnets, conductive, and optical devices, etc.

Endohedral functionalization of chiral metal-organic cages for encapsulating achiral dyes to induce circularly polarized luminescence

<h2>Summary</h2> Circularly polarized luminescence (CPL) is useful for 3D optical imaging and optical storage, but it remains a big challenge to prepare chiral compounds with both strong emission and high-dissymmetry factor (<i>g</i>_lum). Here, we demonstrate that encapsulation of achiral dyes into hollow cages can inhibit the aggregation-caused quenching (ACQ) behavior and promote CPL induction and amplification. This leads to the synthesis of two chiral hexahedral Pd₆<b>L</b>₁₂ cages with crown ether or methoxymethyl moieties. Complexation of K⁺ with crown ethers allows inclusion of anionic achiral boron-dipyrromethene (BODIPY) dyes into the cage cavities to form stable host-guest complexes by cation-anion and host-guest interactions. Although both the cage and dyes exhibit no CPL, their host-guest complexes display strong CPL emissions with the <i>g</i>_lum factors in the order of 10⁻³ in solution. Upon crystallization, one order of magnitude amplification of the glum values in the cage-dye adducts (up to 2.4 × 10⁻²) is obtained.

Structurally Strained Cyclic Xanthene Dimers: A Model for the Rigid Crown Ether Moiety in the Reduced Graphene Oxide Framework

Abstract Highly strained cyclic xanthene dimers, which serve as models for reduced graphene oxide (RGO), were synthesized by Ni(0) mediated homo-coupling reactions. The dimers are obtained as highly stable colorless solids. X-ray crystallographic analysis of a tetra-tert-butyl derivative demonstrates that it possesses a highly strained structure with an extremely short non-bonded O⋯O distance (2.5662(44) Å). The results of variable temperature NMR experiments show that no signal broadening occurs even at 398 K, indicating that conformational flipping requires considerably high energy.

NMR thermosensing properties on binuclear triple-decker complexes of terbium(III) and dysprosium(III) with 15-crown-5-phthalocyanine

In this paper, we have firstly demonstrated the thermosensing properties of binuclear triple-decker terbium(III) and dysprosium(III) complexes with 15-crown-5-phthalocyanine Tb2[(15C5)4Pc]3 (I) and Dy2[(15C5)4Pc]3 (II) in the physiological temperature range (303–323 K). Measuring the LIS values for I and II in CDCl3 has revealed high temperature sensitivity for the signals of the aromatic protons of the inner deck of the complexes: ∂δ/∂T = 1.1 and 0.55 ppm/K for I and II, respectively. Terbium-containing complex I has the highest sensitivity of the LIS among the published complexes of 4f and d elements with polydentate ligands in organic media. A significant impact of the Curie-spin contribution to the paramagnetic spin–spin relaxation rate enhancement has been found, which has a pronounced temperature dependence.The results obtained for kinetically and thermodynamically stable Ln2[(15C5)4Pc]3 complexes allow one to consider them pronounced NMR thermosensors in nonpolar solutions. The possibility of the solubilization of complex I in aqueous media due to the presence of crown ether moieties has been also demonstrated.

Synthesis of crown ether appended 25-Oxasmaragdyrins and their BF2-Complexes

Three examples of crown ether appended 25-oxasmaragdyrins were synthesized in 15–20% yields by [3 + 2] oxidative condensation of 16-oxatripyrrane and crown ether appended meso-aryl dipyrromethane in CH2Cl2 in the presence of catalytic amount TFA under inert atmosphere followed by oxidation with DDQ in open air. The crown ether appended 25-oxasmaragdyrins were treated with BF3.OEt2 in CH2Cl2/TEA on hot water bath with 50 °C for 30 min to afford crown ether appended BF2-oxasmaragdyrins in 60–70% yields. The identities of crown ether appended 25-oxasmaragdyrins and their BF2 complexes were confirmed by molecular ion peak in mass spectra and detailed 1D & 2D NMR, absorption, fluorescence and DFT/TD-DFT techniques were used to study the macrocycles. DFT studies revealed that the crown ether appended 25-oxasmaragdyrin and its BF2 complex were almost planar with the meso-phenyl ring containing the crown ether moiety deviated by an angle of 54.86◦ with respect to mean plane defined by three meso carbon atoms. The absorption studies indicated that crown ether appended smaragdyrins and their BF2 complexes showed one or two strong Soret bands in the region of 440–480 nm and four weak Q-bands in 500–750 nm region. The crown ether appended 25-oxasmaragdyrins and their BF2 complexes were weakly fluorescent.

Coordination chemistry of platinum(II) and rhodium(I) complexes containing chiral monophosphacrown ether ligands

Coordination properties of recently synthesized chiral phospha-modified crown ethers were studied in the presence of platinum(II) and rhodium(I) precursors. The prepared and in situ formed complexes were characterized by 1H and 31P NMR spectroscopies. Platinum(II)-complexes were dominantly formed as transPP arrangements, even in the case of diphosphines, possessing large bite angles, enabling to act as trans-chelating ligands. Isomers of trans-complexes were identified and explained by ligand back-bone interactions, supported by DFT calculations. Basically different complexes were formed in the presence of rhodium(I) precursors, since only one of the two phosphorous donor atoms was coordinated to the metal centre. The addition of Na[BPh4] was also examined showing dramatically different effects: sodium ion coordination to the crown ether moiety can be supposed in case of platinum complexes, while the use of rhodium(I) precursors brought about the formation of ionic complexes.

Dibenzo-21-crown-7-ether contained 6FDA-based polyimide membrane with improved gas selectivity

In the present work, a novel crown ether contained polyimide membrane was synthesized by 4, 4′- (hexafluoro - isopropylidene) diphthalic anhydride, 1, 4 - Bis (4 - aminophenoxy) benzene, and di(aminobenze)-21-Crown-7 ether. The physical property and chemical structure were studied by FTIR, 1HNMR, XRD, TGA, GPC, and tensile testing. Molecular dynamic simulations were applied to investigate the effects of crown ether content on free volume property of membranes. The permselectivity was studied employing He, H2, CO2, N2, and CH4 pure gas as feed. The effects of crown ether content, operation temperature, feed gas pressure, and aging effect on membranes were investigated. Results show that introducing crown ether into polymeric backbone can increase the chains packing density, which lead to reduce d-spacing as well as accessible fractional free volume. These permeabilities followed the order of He > H2 > CO2 > N2 > CH4, indicating a typical molecular sieving separation mechanism in membranes. The introduction of DB21C7 can improve the diffusion selectivity and solubility selectivity. For example, CO2/N2 and CO2/CH4 diffusion selectivity display impressing increase of 53% and 84% respectively by incorporation of 30 mol% DB21C7. As a consequence, the membrane with 30 mol% DB21C7 exhibit remarkable selectivity increase of about 173.5% (for He/CH4), 156.9% (for H2/CH4), 109.4% (for CO2/N2), 88.5% (for CO2/CH4) than non-crown ether contained polyimide membrane. Hence, future membrane material design for high purity gas requirement may be considered as a promising alternative membrane by introducing the crown ether moiety into the polymeric backbone.

Design and Synthesis of Supramolecular Phosphatases Formed from a Bis(Zn2+‐Cyclen) Complex, Barbital‐Crown‐K+ Conjugate and Cu2+ for the Catalytic Hydrolysis of Phosphate Monoester

AbstractThe development of artificial mimics of natural enzymes such as hydrolases and phosphatases is one of the great challenges in bioorganic and bioinorganic chemistry and related sciences. Supramolecular strategies are one of the useful methods to construct artificial catalysts as mimics of natural enzymes and to understand their reaction mechanisms. Herein, we report on the formation of amphiphilic supramolecular phosphatases by the 2 : 2 : 2 self‐assembly of a bis(Zn2+‐cyclen) complex (cyclen=1,4,7,10‐teraazacyclododecane) containing a 2,2′‐bipyridyl (bpy) linker and one long alkyl chain (Zn2L3), 5,5‐diethylbarbituric acid (Bar) derivative functionalized with 1‐aza‐18‐crown‐6 ether and Cu2+ in a two‐phase solvent system (CHCl3/H2O). We hypothesized that crown ether moiety of the Bar‐crown ether conjugate would form complexes with alkaline ions and other metal ions such as Li+, Na+, K+, Rb+, Mg2+ and La3+ in organic phase to mimic the Mg2+ found as the third metal ion in the active site of alkaline phosphatase (AP). The results indicate that the 2 : 2 : 2 : 4 complexes of Zn2L3, a Bar block equipped with the 18‐crown‐6 ether, Cu2+ and alkaline metal are constructed in a two‐phase solvent system. The resulting complexes have a higher hydrolysis activity for mono(4‐nitrophenyl)phosphate (MNP) in the presence of K+ than that in the presence of Li+, Na+, Rb+, Mg2+ and La3+ and a greater hydrolysis activity than our previous supermolecules having no crown ether part, suggesting that crown ether‐K+ complex located in close proximity to the Cu2(μ‐OH)2 core contributes to the acceleration of the MNP hydrolysis.