Recognition of guests bearing donor and acceptor hydrogen bonding groups by heteroditopic calix[4]arene receptors

The synthesis of new hosts specifically designed for the recognition of neutral guests bearing donor-acceptor hydrogen bonding groups is described. These hosts are characterized by the presence of two distinct binding region in close proximity: the rigid π-donor cavity and the H-bond donor N-methylene-N′-phenylureido group inserted onto the upper rim of the calix[4]arene skeleton. The binding abilities of these receptors were investigated toward a series of neutral ditopic organic molecules in CDCl₃ solution by ¹H NMR spectroscopy. The results obtained show that rigidity of the calix[4]arene apolar cavity is the control element in determining efficiency. In fact, compared with the more rigid 2, host 10, where the rigidity of the cone structure is maintained by hydrogen bonding of the OH of the lower rim, a decrease of efficiency of almost one order of magnitude was observed. The cooperative effect of the two binding region of host 2 was verified with different classes of ditopic guests. Good efficiency in the recognition of urea derivatives and dimethylsulfoxide was achieved.     

Anion effects on the recognition of ion pairs by calix[4]arene-based heteroditopic receptors

A novel heteroditopic receptor (5) based on a rigid calix[4]arene cavity bearing at the upper rim four arylsulfonamido binding sites has been synthesized. The binding abilities of this new host have been investigated in apolar solvents toward a series of tetramethylammonium salts (tosylate, chloride, acetate, trifluoroacetate, and picrate) and compared with those of monotopic and heteroditopic calix[4]arene-bis(crown-3)-based receptors 1 and 2 in order to evaluate the role of the anion on ion-pair recognition. While monotopic host 1 shows an efficiency order toward the different salts that increases when the anion is less interactive (Hofmeister trend), an opposite role of the counteranion on the recognition process was observed with host 5 (anti-Hofmeister trend). A more complex behavior is experienced by host 2, which shows a high and leveled efficiency for all the anions tested. The results obtained were explained on the basis of the different types of ion pairs present in the recognition process. Further information on the role of the anion were obtained by the "dual host" strategy utilizing the tri-n-butylthioureido derivative of tren 7, which forms a stable complex with chloride anion. The very high efficiency shown by these heteroditopic hosts opens new routes in supramolecular projects and is a very interesting tool in the molecular recognition of ion-pairs and its applications.     

Cooperative AND ion-pair recognition by heteroditopic calix[4]diquinone receptors

A new family of heteroditopic calix[4]diquinone receptors capable of the cooperative recognition of ion-pair species through a contact binding mechanism has been developed. The receptors bind contact ion pairs cooperatively, with an unprecedented AND recognition phenomenon being observed to operate in certain cases, in which receptors display no affinity for either of the individual "free" cation or anion, but bind the cation and anion ion-pair strongly. X-ray crystallographic, solution-state, and computational methods rationalize the observed recognition behavior of the receptors. It is shown that the contact ion-pair interaction occurs through a pi-stacking-mediated folding of the receptors such that the anion and cation binding sites are arranged in close proximity, while in the solid state an unusual ion-mediated receptor dimerization is observed. Molecular dynamics simulations are further used to explain the observed trends in the association constants of different ion-pair species and the mechanism of interaction.     

Stilbene-bridged 1,3-alternate calix[4]arene crown ether for selective alkali ion extraction

A series of stilbene-bridged calix[4]arenes was synthesized through an intramolecular reductive McMurry coupling of bisbenzaldehyde calix[4]arene in high yields. Tetra- and pentaethylene glycol chains were tethered to the phenolic groups of calix[4]arene to form stilbene-bridged calix[4]arene crown-5 and crown-6, respectively. The presence of stilbene bridge over the calix[4]arene rim effectively prevented the connection of the polyether chains in the cone conformation resulting in the exclusive formation of 1,3-alternate stilbene-bridged calix[4]arene crown product. Compared to the cone analogues, the 1,3-alternate calix[4]arene crown ethers showed a greater extraction ability and selectivity toward Cs⁺.     

2]Catenane assembly from calix[4]arene crown ethers [In Process Citation

A variety of novel calix[4]arene-incorporating crown ethers with or without intramolecular hydrogen bonding have been prepared by two efficient methods and utilized as donor rings to assemble calix[4]arene [2]catenanes based on pi-stacking interaction between hydroquinone and bipyridinium units. Treatment of calix[4]arene crown ethers 4, 10a, or 10b, whose cone conformation was fixed by intramolecular hydrogen bonding within the calix[4]arene moiety, with dicationic salt 15 x 2PF6 and dibromide 16 afforded the corresponding [2]catenanes 17a x 4PF6, 17b x 4PF6, and 17c x 4PF6 in 20%, 53%, and 55% yields, respectively, whereas from the reactions of 15 x 2PF6 and dibromide 16 in the presence of conformationally flexible 11 or 12 with a cone conformation kept by two propyl groups, [2]catenanes 18 x 4PF6 and 19 x 4PF6 were obtained in 12% and 6% yields. [2]Catenanes 21a x 4Cl, 21b x 4Cl, and 21c x 4Cl, incorporating calix[4]arene in both the donor and acceptor rings, were also successfully assembled from 10a or 10b, 16, and dicationic salts 20a x 2PF6 or 20b x 2PF6. The dynamic 1H NMR and absorption spectra of the [2]catenanes have been investigated, which revealed a strongest donor-acceptor interaction in 17a x 4PF6 and that the cone [2]catenanes 17a-c x 4PF6 can isomerize to the partial cone isomer at high temperature. The difference of the dynamic properties of these catenanes was discussed. The results demonstrate that catenation is one new general method to change the conformational distributions of calix[4]arenes.     

Topological and metal ion effects on the anion binding abilities of new heteroditopic receptors derived from p-tert-butylcalix[4]arene

Two derivatives of p-tert-butylcalix[4]arenes 1 and 2 containing diethyl ester and amidoferrocene units as cation and anion binding sites, respectively, have been synthesized. Both compounds were isomeric with different topology for accommodating ions. ¹H NMR spectroscopy, cyclic voltammetry, and square-wave voltammetry were used to study the binding abilities of receptors 1 and 2 toward anions. Both receptors were found to bind Br⁻ and I⁻ selectively in the presence of Na⁺. The electrochemically oxidized ferrocenium form of para-isomer 2 in the free form was found to sense AcO⁻ selectively, but demonstrated a negative sensing in the presence of Na⁺. In contrast, the electrochemically oxidized ferrocenium form of meta-isomer 1 was found to enhance sensing of AcO⁻ and Cl⁻ in the presence of Na⁺.     

A di-aza-benzo crown ether derived fromp-tert-butyl calix[4]arene. Synthesis and complexation of zinc cation

Di-aza-benzo crown etherp-tert-butyl calix[4]arene (1) has been prepared by hydrogenating the already known Schiff-base precursor (2). The metal ion complexing ability of (1) toward zinc cation is presented. The formation of a 2:1 (Zn: (1) complex and the location of zinc cations were deduced from¹H-NMR investigations.     

Networked calix[4]arene polymers with unusual mechanical properties

Polymeric networks built from calix[4]arenes that form a three dimensional folded structure have been predicted to exhibit negative Poisson's ratios (auxetic), an unusual property which makes them superior to conventional materials in many practical applications.     

Two triethylammonium ion complexes of monoanionic calix[4]arene

Two complexes between mono‐deprotonated calix­[4]­arene and Et₃HN⁺ are reported. The first, triethylammonium 26,27,28‐tri­hydroxy­penta­cyclo­[19.3.1.1^3,7.1^9,13.1^15,19]­octacosa‐1(25),3,5,7(28),9,11,13(27),15,17,19(26),21,23‐dodecaen‐25‐olate, C₆H₁₆N⁺·C₂₈H₂₃O₄^?, comprises only the cationic and anionic species, whereas the second, tris­(triethyl­ammonium) tris[26,27,28‐tri­hydroxy­penta­cyclo­[19.3.1.1^3,7.1^9,13.1^15,19]­octacosa‐1(25),3,5,7(28),9,11,13(27),15,17,19(26),21,23‐dodecaen‐25‐olate] aceto­nitrile solvate, 3C₆H₁₆N⁺·3C₂₈H₂₃O₄^?·C₂H₃N, comprises one aceto­nitrile solvent mol­ecule for three such units. In both cases, the units are stacked in columns so that the ammonium cation is hydrogen bonded to a phenolic or phenolate O atom of one mol­ecule and included in the hydro­phobic cavity of another neighbouring calixarene. The short contacts present indicate that cation?π and C—H?π interactions are likely to be involved in the inclusion phenomena.     

Synthesis and binding properties of calix[4]arene telomers

Two calix[4]arene derivatives (5 and 8) and their telomers are synthesized to estimate selective extraction of alkali and transition metal cations from the aqueous to the organic phase (chloroform). Compound 5 shows selectivity toward Hg²⁺. Compound 8 and telomers 6 and 9 are not selective but are good extractants for all used transition metal ions. The observations suggest that ethylene glycol bridges are efficient for carrying transition metal cations in a two-phase solvent system. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4351–4355, 1999     

Acid-base modulation of a versatile heteroditopic calix[6]arene based receptor

A new calix[6]crypturea (3) has been efficiently synthesized through a domino Staudinger/aza-Wittig reaction followed by a [1 + 1] macrocyclization step. In comparison to the previously reported tren-based calix[6]crypturea, this heteroditopic receptor 3 displays a more flexible and larger tris-ureido cap. Due to this structural alteration, 3 exhibits unique host-guest properties: (i) the protonation of its basic cap leads to a rigidification of the whole structure and, thus, allosteric control of the binding properties and selective guest switching processes are possible, (ii) its versatility is unprecedented in the literature since it can bind either neutral molecules, anions, primary/secondary ammonium ions, quaternary ammonium ions or contact ion pairs according to different modes of recognition and with a remarkable selectivity within each family of guest, (iii) cascade complexes even stable in a protic environment can be obtained. These remarkable features are nicely illustrated by the fact that, according to the nature of its counterion, an ammonium ion R(1)R(2)NH(2)(+) can be accommodated into the cavity either as an independent guest, as a contact ion-pair or as a cascade complex. All these results are reminiscent of biological receptors and validate the strategy that consists of designing receptors presenting a high flexibility that can be controlled by an external stimulus.     

New water-soluble host molecules derived from calix[6]arene

4-Sulfonate derivatives of calix[6]arene and O-n-hexylcalix[6]arene (1a and 1b, respectively) were synthesized. Measurements using fluorescence and absorption spectral probes indicated that the cavity of calix-[6]arene can accept a naphthalene molecule but not a pyrene molecule and that (1b) has a hydrophobic cavity constructed from the aggregate of the hexyl groups while (1a) provides a reaction field apparently more polar than water. This is the first example for water-soluble host molecules derived from calix[6]arene.     

New preorganized calix[4]arenes. Part II. Double Schiff-based calix[4]arenes derived from 4,6,10,12,16,18,22,24,25,26,27,28-dodecamethyl-5,11,17,23-tetrahydroxycalix[4]arene

We describe the condensation of an appropriate tetraaldehyde derivative of mesitol calix[4]arene with 1,n-diaminoalkanes to afford double Schiff-base calix[4]arene receptors in the 1,3-alternate conformation.     

Second generation of calix[6]aza-Cryptands: synthesis of heteroditopic receptors for organic ion pairs

The efficient syntheses of calix[6]azacryptands decorated with anion-binding groups on the narrow rim have been achieved from an 1,3,5-tris-protected calix[6]hexa-amine. These heteroditopic receptors can bind ammonium ions or organic ion pair salts with a positive cooperativity. In regard to their functionalization at the 1,3,5-phenolic positions, these compounds constitute the first examples of a second generation of C3 v symmetrical calix[6]azacryptands.     

Asymmetric calix[4]-thiacalix[4]arene tubes: Synthesis and ionophore properties

Asymmetric p-tert-butylcalix[4]-p-R-thiacalix[4]arene tubes (R = tert-Bu, H, 1-adamantyl) were prepared for the first time by reaction of tosyloxyethoxy derivative of p-tert-butylcalix-[4]arene and the corresponding p-R-thiacalix[4]arenas in acetonitrile in the presence of K₂CO₃. Complex formation of compounds obtained with sodium, potassium, and rubidium iodides in CDCl₃-CD₃OD, 4:1, was studied by means of ¹H NMR. The ionophore properties of the molecule were governed by the character of substituents on the upper rim of the thiacalixarene fragment, and only the molecular tube containing a fragment of the p-(1-adamantyl)-thiacalix[4]arene quantitatively bound potassium ions (quickly) and rubidium ions (slowly).     

Spectrophotometric study of the supramolecular complexes of [60]- and [70]Fullerenes with biscalix[6]arene and crown[4]calix[6]arene

[60]- and [70]Fullerenes have been shown to form 1:1 supramolecular complexes with bis[2-(5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41-pentahydroxycalix[6]arenyl-oxy ethyl ether) (1) and 5,11,17,23,29,35-hexa-tert-butyl-37,38,40,41-tetra hydroxyl-39,42-(crown-4)calix[6]arene (2) in CHCl3 medium by electronic absorption spectroscopy. Formation constants (K) of the complexes of [60]- and [70]fullerenes with 1 and 2 have been determined at room temperature from which free energy of formation values of the complexes have been estimated. The very high formation constant value of [60]fullerene/1 complex (5900 dm3 mol-1) in indicative of formation of inclusion complex. Moreover, PM3 calculations reveal that intermolecular interaction between [60]fullerene and 1 proceeds through quite deep energy molecular orbital.     

Cesium ion selective electrode based on calix[4]crown ether-ester

The potentiometric response characteristics of cesium ion selective PVC membrane electrode employing calix[4]crown ether-ester as an ionophore were investigated. The electrode exhibit a good response for cesium ion over wide concentration range of 5.0x10(-6)-1.0x10(-1) M with a Nernstian slope of 59 mV per decade. The detection limit of electrode is 5.0x10(-6) M. The electrode was found to have selectivity for cesium ion over alkali, alkaline and transition metals. The response time of the electrode is less than 20 s and can be used for more than 4 months without observing any divergence in potentiometric response. The electrode response was stable over wide pH range.     

Supramolecular interaction between water-soluble calix[4]arene and ATP--the catalysis of calix[4]arene for hydrolysis of ATP

The catalysis effect of water-soluble calix[4]arene C[4] (calix[4]arene-5,11,17,23-tetrasulfonate) on hydrolysis of ATP in aqueous solution was studied by HPLC. Using laser photolysis and pulse radiolysis, the supramolecular interaction between water-soluble calix[4]arene and ATP was investigated.     

Aminolysis reaction of calix [ 4 ] arene esters and crystal structures and conformational behaviors of calix[4]arene amides

We first make use of aminolysis of calix[4]arene esters to synthesize calix[4]arene amides. When the two ethyl esters of the calix[4]arene esters are aminolysized, the 1, 3-amide derivative is formed selectively. The crystal structures of the calix-[4]arene with two butyl amide (3b) and four butyl amide moieties (4b) were determined. The intermolecular hydrogen bonds make 4b form two-dimensional net work insolid state. The 1H NMR spectra prove that 3b is of a pinched cone conformation, while 4b and tetraheptylamide-calix[4]arene (6b) take fast interconversion between two C2v isomers in solution and appear an apparent cone conformation at room temperature. As decreasing temperature, the interconversion rate decreases gradually and, finally, the interconversion process is frozen at Tc = -10℃, which makes both conformations of 4b and 6b the pinched cone structures. The hydrogen bond improves the interconversion barrier, and the large different values of the potential barrier between 6b and 4b (or 6b) may     

Fluoride sensing with a PCT-based calix[4]arene

Novel calix[4]arene-based anion sensor 1 with two coumarin units attached via amido functions acting also as binding sites is presented. Complexation of F⁻ by PCT-based 1 causes selectively red-shift in UV-vis absorption and in fluorescence emission due to H-bonding followed by deprotonation of NH-amide groups.