Calix[4]arenes bearing a tropylium substituent as hosts for organic cations

Four novel calix[4]arenes bearing one cycloheptatrienyl substituent and two calix[4]arenes with one tropylium substituent at the wider rim were synthesized. Cycloheptatrienyl- and tropylium-calixarenes represent two states of a potentially photoswitchable calixarene host. The complexation of selected hosts with organic cations such as quinolinium, ammonium and tropylium ions was studied. It was found that the complexation of organic cations by the tropylium-substituted host was much stronger in CDCl₃ solution than by the related cycloheptatrienyl-host. Aryltropylium ions are bound by tropylium hosts. Accordingly, dimers of the host itself are formed both in CDCl₃ solution and in the gas phase. Because of the intramolecular charge transfer tropyliumcalix[4]arenes are qualified as chromogenic hosts. These undergo two acid-base equilibriums depending on the concentration and the solvent. Electronic Supplementary Material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Synthesis and Complexation Studies of a Tetra(N,N-dimethyl) Aminoethylamide p-tert-butyl Calix[4]arene and Its Tetramethylammonium Derivative

We describe the synthesis of two functionalised p-tert-butyl calix[4]arenes: tetra(N,N-dimethyl) aminoethylamide derivative 1 and related tetramethylammonium 2. Their complexation properties towards alkali and zinc metal cations are reported along with complexation of perchlorate anion by 2.     

Potassium selective calix[4]semitubes

A new class of ionophore consisting of two calix[4]arene units linked through the lower rim by two ethylene chains, in combination with propyl ether and phenolic functional groups, has been developed. These calix[4]semitube molecules exhibit remarkable selectivity and fast complexation kinetics for potassium over all Group 1 metal cations. Molecular modelling studies, using structural models derived from crystallographic data, suggest the potassium cation is complexed by a horizontal, side-on route and not through the calix[4]arene annulus. The length of the bridging alkylene chain between the respective calix[4]arenes of the semitube structure dictates the strength and selectivity of alkali metal cation binding.     

Synthesis of Ionizable Calix[4]arenes for Chelation of Selected Divalent Cations

Two sets of functionalised calix[4]arenes, either with a 1,3-crown ether bridge or with an open-chain oligo ether moiety in 1,3-position were prepared and further equipped with additional deprotonisable sulfonamide groups to establish chelating systems for selected cations Sr²⁺, Ba²⁺, and Pb²⁺ ions. To improve the complexation behaviour towards these cations, calix[4]arenes with oligo ether groups and modified crowns of different sizes were synthesized. Association constants were determined by UV/Vis titration in acetonitrile using the respective perchlorate salts and logK values between 3.2 and 8.0 were obtained. These findings were supported by the calculation of the binding energies exemplarily for selected complexes with Ba²⁺.     

A paradigm shift in the construction of heterobimetallic complexes: synthesis of group 2 & 4 metal-calix[6]arene complexes

Deprotonation of calix[6]arenes with barium in methanol followed by the addition of [Ti(OPr(i))(4)] or [Zr(OBu(n))(4)] is effective in the formation of novel dimeric 2:1 barium-titanium(IV)/zirconium(IV) calix[6]arene complexes. In these complexes a central Ti(IV)/Zr(IV) coordinated in the exo-position connects the two calix[6]arenes in the 1,3-alternate conformation, each with an endo-barium sharing common phenolate groups with the titanium/zirconium centre and participating in cation-pi interactions. A homometallic barium calix[6]arene dimer was also prepared wherein the calix[6]arenes are in the 1,3-alternate conformation with each coordinating one endo- and one exo-barium centre. The exo-barium cations connect the two calix[6]arenes through bridging methanol ligands. In this and the heterometallic complexes, cation-pi complexation of the Ba(2+) ion within the 1,3 alternate conformation of calix[6]arene facilitates the formation of the dimeric complexes in methanol. In contrast, the smaller Sr(2+) ion did not form similar complexes in methanol, and the formation of an analogous 2:1 strontium-titanium calixarene complex required the use of the more sterically demanding donor alcohol, isopropanol, the resulting complex being devoid of cation-pi interaction. The results show (i) that a subtle interplay of solvation strength, coordination array type and cavity/cation size influences the accessibility of heterobimetallic complexes based on calix[6]arenes, and (ii) a synergistic endo-exo binding behaviour.     

Calix[4]Arenes in the partial cone conformation as ionophores in silver ion-selective electrodes

Two calix[4]arene derivatives, in the partial cone conformation, with sulfur-containing functionalities, were tested as neutral carrier ionophores in potentiometric silver-selective electrodes of conventional membrane and membrane-coated glassy carbon electrode types. Comparison with a calix[4]arene in the cone conformation was made. The membranes were prepared using either 2-nitrophenyl octyl ether or bis(ethylhexyl)sebacate as plasticizers and potassium tetrakis(p-chlorophenyl)borate as the lipophilic salt in a poly(vinyl chloride) matrix. Both calix[4]arenes yielded electrodes of good sensitivity (approx. 47 mV dec⁻¹) in the range 10⁻⁴–10⁻¹ M and excellent selectivity [log K_Ag,_MH+ < −1.5] of transition, alkali and heavy metal cations, including sodium, mercury(II) and lead(II) cations. Temperature effects and reproducibility of response were determined and the interfering effects of mercury(II) and lead (II) ions on the membranes were noted. The partial cone conformation allows improved selectivity over certain cations relative to calix[4]arenes in the cone conformation.     

A Quantum Chemical Study on Recognition of Nitro-phenols by Calix[n]arenes

A PM3 calculation in vacuum of the inclusion complexation ofo-, m-, p-nitro-phenol with calix[n]arenes is performedsuccessfully. The pathways for inclusion process are describedand the most probable structures of the 1:1 complex are soughtthrough a potential energy scan. The energy differences betweenthe inclusion complexes and the hosts, by calculation, show thatthe most stable complexation is calix[4]-p-nitro phenol andcalix[6]-m-nitro phenol.     

富勒烯的杯芳烃包合物的研究现状与展望

介绍了[60/70]富勒烯与杯[8],[6],[5]芳烃、高氧杂杯[3]芳烃及双杯芳烃形成包合物的研究进展和应用前景。     

Transition metal-directed self-assembly of calix[4]arene based dithiocarbamate ligands

The transition metal-directed self-assembly of dithiocarbamate ligand functionalized upper and lower rim calix[4]arenes affords novel dimeric bimetallic bis(calix[4]arene) species as determined by a combination of analytical methods including X-ray crystallography. An exception is a zinc(II) dithiocarbamate upper rim calix[4]arene assembly which is monomeric in nature. Electrochemical investigations reveal the bimetallic copper(II) bis(calix[4]arene) systems can electrochemically sense dihydrogen phosphate and carboxylate anions via significant cathodic perturbations of the respective copper(II)/(III) dithiocarbamate oxidation wave.     

Complexation of fullerenes by calix[n]arenes: molecular dynamic simulations and benesi-hildebrandequilibrium constants

MD simulations using a combination of AMI and AMBER are performed to elucidate the origin of complexation between fullerenes and calix[n]arenes (n = 4,6,8). Only calix[6]arenes and calix[8]arenes are suitable for complex formation. By introducing benzyl substitutents on the upper rim of the calixarenes the interaction energy between host and guest is even improved. Analysis of the data also reveals a strong stabilization by t-butyl or benzyl substitutents beside π-π interactions between the arene units of the calixarene and fullerenes. The calculated data perfectly correlate with the experimentally observed equilibrium constants.     

New artificial receptors from selectively functionalized calix[4]arenes

Abstract

The development of new synthetic methods for the monoalkylation of calix[4]arenes at the lower rim allows the synthesis of a new class of trihydroxamate siderophores. Three chelating hydroxamic acid units are introduced through a sequence of reactions which blocks the macrocycle in the cone conformation. The new ligands obtained form neutral 1:1 complexes (FeL) with iron (III), which are stable in EtOH/H₂O 9:1 at pH 2–7. Calix[4]arene bis-crown ethers are prepared by exploiting the selective 1,2-(proximal) functionalization of calix[4]arenes at the lower rim. These ligands are, however, less effective in complexing alkali metal cations compared with the 1,3-calix[4]arene crown-ethers which, in their partial cone structure, offer a better shielding for the complexed cations. Rigid upper rim-bridged calix[4]arenes potentially useful for the inclusion of neutral molecules are prepared by exploiting the selective 1,3-diformylation of calix[4]arene at the upper rim. Finally a new chloromethylation method for calix[4]arenes blocked in the cone conformation is described together with the synthesis of new cavitands.     

Synthesis of novel pillar-shaped cavitands “Pillar[5]arenes” and their application for supramolecular materials

In 2008, we reported a new class of macrocyclic hosts and named “Pillar[5]arenes”. They combine the advantages and aspects of traditional hosts and have a composition similar to those of typical calix[n]arenes. Pillar[5]arenes have repeating units connected by methylene bridges at the para-position, and thus they have a unique symmetrical pillar architecture differing from the basket-shaped structure of meta-bridged calix[n]arenes. Pillar[5]arenes show high functionality similar to cyclodextrins, and can capture electron accepting guest molecules within their cavity similarly to cucurbit[n]urils. In this review, the synthesis, structure, rotation, host–guest properties, planar chirality and functionality of pillar[5]arenes are discussed, along with pillar[5]arene-based supramolecular architectures and the challenges in synthesizing pillar[6]arenes.     

Sulfur-containing hetero-calix[4]pyrroles as mercury(II) cation-selective receptors: thermodynamic aspects

Two sulfur-containing hybrid calix[4]pyrrole derivatives (III and IV) have been synthesized and fully characterized. Several analytical techniques (1H NMR, conductance measurements, UV-vis spectrophotometry, titration potentiometry, and titration calorimetry) have been used to assess the interaction between these hybrid calixpyrrole receptors and metal cations in acetonitrile and dimethylsulfoxide. The partition constants of calix[4]pyrrole, I, II, and IV in the acetonitrile-hexane solvent system and the solubilities of the ligands in various solvents at 298.15 K were determined. 1H NMR measurements reveal the sites of interaction of calixpyrrole ligands with metal cations in CD3CN. Conductance and UV-vis spectrophotometric measurements were performed to establish the composition of mercury(II) calixpyrrole complexes in acetonitrile at 298.15 K. Titration calorimetry was used to quantitatively assess Hg(II)-calixpyrrole interactions. Thus the thermodynamics of complexation of calixpyrrole ligands with the mercury(II) cation in acetonitrile at 298.15 K are reported. Potentiometric titrations were also used to establish the stepwise stability constants for the complexation of calix[3]thieno[1]pyrrole with the Hg(II) cation in acetonitrile at 298.15 K. The results show that replacement of one or more pyrrole units by thiophene rings in calix[4]pyrrole has tuned significantly the discrimination ability of these ligands between anions and enables the produced hybrid calixpyrroles to bind selectively with Hg(II) in acetonitrile. No interaction was observed between these ligands and other metal cations in acetonitrile.     

Evaluation and solubility improvement of Carvedilol: PSC[n]arene inclusion complexes with acute oral toxicity studies

The aqua phobic molecules that are practically insoluble in aqueous media demonstrate a staggeringly slow intrinsic dissolution rate. In this work, we exemplify the utility of calixarenes as a tool to form inclusion complexes with Carvedilol (CDL). It is poorly water soluble drug. CDL is a Biopharmaceutical Classification System (BCS) Class II drug and it is a nonselective β-adrenegenic blocking agent with α1-blocking activity. It is mainly used in the management of hypertension. The maximum complexation of the drug was accomplished after 48?h of stirring with para sulphonato calix[4]arene (PSC[4]arene) and para sulphonato calix[6]arene (PSC[6]arene) in water and evaporation of water to acquire solid complexes. The study includes characterisation of both the complexes—physical mixtures of drug and PSC[4]arene and PSC[6]arenes by different methods like Fourier-transform infra red spectroscopy, differential scanning calorimetry and powder X-ray diffraction, proton nuclear magnetic resonance. This studies shows that there is electrostatic interaction between drug and PSC[n]arenes. The complexation was determined by phase solubility study. The prepared complexes exhibited improved in vitro dissolution profile and decreased in vivo acute oral toxicity compared to the pure drug.     

Mass spectroscopic investigation of bis-1,3-urea calix[4]arenes and their ability to complex N-protected α-amino acids

We report the ability of urea’s appended onto the upper-rim of conformationally locked ‘cone’ calix[4]arenes to show a preference for binding specific N-protected α-amino acids. Superior complexation (as judged by mass spectroscopy) between N-protected α-amino results and bis-1,3-N-benzylureas calix[4]arenes was observed when methylene bridges were present between the calix[4]arene ‘host’ and the urea motif. Interestingly we also demonstrate that subjecting mixtures of structurally diverse N-Fmoc-α-amino acids to a single bis-1,3-N-benzylurea derived calix[4]arene allows, in some cases, the calix[4]arene ‘host’ to selectively ‘pick out’ and complex a specific N-Fmoc amino acid from the mixture.     

Supramolecular assemblies with calix[6]arenes and copper ions: from dinuclear to trinuclear linear arrangements of hydroxo-Cu(II) complexes

Complexation of copper(II) by calix[6]arene-based ligands bearing either two or three N-benzylimidazole coordinating arms under basic conditions has been studied. Whereas the tris(imidazole) derivative stabilizes dicationic 5-coordinate aqua complexes in a mononuclear state with an intracavity bound guest, in the presence of hydroxide ions, the latter undergo dimerization. An X-ray structure revealed decoordination of one imidazole arm and formation of a bis(hydroxo) bridged Cu(II) core with a square-planar geometry for both metal centers sandwiched by two empty calixarene cavities. Upon methanolysis, the dinuclear complex underwent an unexpected rearrangement leading to the clean formation of a trinuclear complex. X-ray diffraction analyses of this novel species revealed a trinuclear core constructed around a central Cu(II) ion that is doubly bridged through either methoxide or hydroxide anions to two Cu(II) ions hold by two calixarene units. The same complex could be directly synthesized by reacting the ligand with copper(II) perchlorate in a 2:3 ratio in the presence of base. In solution, the tetrahydroxo Cu(3) complex was characterized by UV-vis and (1)H NMR spectroscopies and displayed an electron paramagnetic resonance (EPR) signal only below 100 K that accounts for a S = 1/2 fundamental state. Formation of the same di- and trinuclear species was observed with a calix[6]arene-based bis(imidazole) ligand, which demonstrates the generality of the reaction schemes. All these results emphasize the versatility of the calix[6]arene scaffold for the stabilization of metal complexes with various nuclearities.     

Synthesis of novel cone-configurated hexa-tert-butyl-trimethoxy-calix[6]arenes bearing tris(bipyridyl) pendants and their use in recognition and ionic speciation

A series of novel cone-configurated p-tert-butyl-trimethoxycalix[6]arenes bearing three 2,2′-bipyridyl units at their lower rim have been synthesized. The ¹H NMR and ¹³C NMR spectra of synthesized derivatives revealed that the ring inversion in calix[6]arene could be suppressed by the introduction of three 2,2′-bipyridyl moieties at the lower rim of calix[6]arene scaffold which fixes it into its cone configuration. The complexation ability of the synthesized receptors (5a–d) towards Fe(II) ion was investigated by UV-Visible titration to reveal that the synthesized receptors interact with Fe(II) in a 1:1 binding stoichiometry and respond to a specific oxidation state of the metal ion. The observations have significance for studies directed at the design of molecular receptors for ionic speciation through molecular recognition.     

杯芳烃衍生物液－液萃取和液膜传输银的对比研究及机理探讨

以4类含硫、硒、氮等杂原子基团二取代的杯[4]芳烃五衍生物（1－14）为中 性载体，在H2O-CHCl3-苦味酸体系萃取银和H2O-CHCl3-H2O液膜体系中传输银进行 了对比研究。萃取和传输结果具有一致性，除了苯并噻唑取代的杯[4]芳烃衍生物 （3－6）外，其它10个含硫、硒、氮的杯[4]芳烃衍生物1－14均对软重金属银和汞 有很高的选择性，而吡啶取代的杯[4]芳烃衍生物7－10对铅有一定的萃取选择性， 其中羟基硫醚取代的杯[4]芳烃衍生物11－14对银的萃取率和传输速率最大。并且 就杯[4]芳烃衍生物对银的传输机理进行了探讨，发现传输速率随源相中金属离子 浓度和有机相中载体浓度的增加而增大，因此推论这是由金属离子浓度梯度推动下 的传输。     

Calix[4]tubes: An Approach to Functionalization

Adamantylcalix[4]arenes decorated with ester groups and calix[4]arene tetratosylates were used to prepare a series of calix[4]tubes bearing 3-methoxycarbonyl- and 3-methoxycarbonylmethyl-1-adamantyl units (up to eight) in good yield. These compounds were subjected to further chemical transformations giving a wide set of novel ester-, acid-, hydroxy-, amine-, and urea-functionalized calix[4]tubes. Introduction of urea groups into the calixtube core led not only to anion-targeted receptors, but also provided heteroditopic behavior of the hosts, which enriches the well-established potassium-uptake ability of calix[4]tubes.     

A spirobiscalix[4]azacrown: synthesis and complexing properties

The synthesis and complexation properties of a novel type of calix[4]azacrown, in which two cone calix[4]arenes are linked via a spiro C-atom incorporated into polyaminoalkylene chains are presented.