Synthesis, NMR, X-ray structural analyses and complexation studies of new Ag selective calix[4]arene based dipodal hosts—a co-complexation of neutral and charged species

New podands based on the p-tert-butylcalix[4]arene unit with substitution at the lower rim incorporating imine units, have been synthesized in high yield by simple condensation method. These podands have been shown to extract and transport Ag⁺ selectively over alkali, alkaline earth metal cations, Zn²⁺, Pb²⁺ and Hg²⁺ ions, from neutral aqueous phase to organic phase. In all the ligands the calix unit has been found to be present in a cone conformation except for the one having pyridine as end group, at the ortho position. It has been isolated in two conformations; cone and 1,2-alternate. To the best of our knowledge, this may be the first 1,3-lower rim substituted calix[4]arene to exist in a 1,2-alternate conformation and is among a few known compounds with this conformation in the general class of calix[4]arenes. A complex of this ligand, which happens to be the highest extractant of Ag⁺ has been isolated and characterized using mass, ¹H and ¹³C NMR spectroscopy's and elemental analysis. The spectroscopic evidence and molecular modelling studies performed on the complex suggest a participation of the imine and pyridine nitrogens and two of the ether oxygens in coordination to the metal ion. The X-ray crystal structures of three of the ligands establish the formation of inclusion complexes with polar acetonitrile solvent molecules. The ¹H and ¹³C NMR spectra of all the compounds, taken in CDCl₃, show the presence of acetonitrile molecules in the cavity of the calix[4]arene, indicating inclusion of the neutral guest molecules in the solution phase as well. For one of the podands X-ray crystal structure has shown a formation of clatharate complex of chloroform with the ligand which has rarely been found in the case of calix[4]arenes.     

Synthesis of （p—Substituted phenyl）azo Calix[4] arenes

A novel method for the preparation of chromogenic calixarenes with azo groups was reported.p-Substituted(-NO2,-CH3,-Cl)amilines were diazotized with isoamyl nitrite in EtONa/EtOH under refluxing condition.Fifteen mono-,bis-,tris-and tetrakis(p-substituted phenyl)azo calix[4]arenes (including proximal and distal isomers) were obtainged respectively by diazo-coupling in different molar ratio to calix[4]arenes(1) under pH=7.5-9.0 in non-aqueous solution at 0-5℃.^1H NMR and ^13C NMR spectra of (p-substtituted phenyl)azo calix[4]-arenes indicated that they existed in cone conformation in solution.     

Azocalixarenes. 6: Synthesis, complexation, extraction and thermal behaviour of four new azocalix[4]arenes

Four new azocalix[4]arenes {5,11,17,23-tetrakis[(2-hydroxy-5-tert-butylphenylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (1), 5,11,17,23-tetrakis[(2-hydroxy-5-nitro phenylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (2), 5,11,17,23-tetrakis[(2-amino-5-carboxylphenylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (3) and 5,11,17,23-tetrakis[(1-amino-2-hydroxy-4-sulfonicacidnapthylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (4)} have been synthesized from p-tert-butylphenol, p-nitrophenol, p-aminobenzoic acid and 1-amino-2-hydroxy-4-sulphonic acid by diazo coupling reaction with p-aminocalix[4]arene. The resulting ligands (1–4) were treated with three transition metal salts (e.g., CuCl₂·2H₂O, NiCl₂·6H₂O or CoCl₂·6H₂O). Cu(II), Ni(II) and Co(II) complexes of the azocalix[4]arene derivatives were obtained and characterized by UV-vis, IR, ¹H-NMR spectroscopic techniques and elemental analysis. All the complexes have a metal:ligand ratio of 2:1. The Cu(II) and Ni(II) complexes of azocalix[4]arenes are square-planar, while the Co(II) complexes of azocalix[4]arenes are octahedral with water molecules as axial ligands. The solvent extraction of various transition metal cations from the aqueous phase to the organic phase was carried out by using azocalix[4]arenes (1–4). It was found that, azocalix[4]arenes 1, 2 and 3 examined selectivity for transition metal cations such as Ag⁺, Hg⁺ and Hg²⁺. In addition, the thermal stability of metal:azocalix[4]arene complexes were also reported. Dedicated to Prof. Dr. Mustafa Yılmaz on the occasion of his 50th birthday     

Fullerene encapsulation with calix[5]arenes

This paper presents the synthesis of the fullerene hosts based on the calix[5]arenes and their binding properties. Calix[5]arenes 1a, 2, 3a bind C₆₀ or C₇₀ in organic solvents. The solvent effect of the fullerene complexation was clearly observed; the association constant decreases in a solvent with high solubility for C₆₀. Covalently linked double-calix[5]arenes 4-6 were also investigated on their binding properties for fullerenes in organic solvents. Their binding abilities for both C₆₀ and C₇₀ are extremely high in toluene solution. Higher binding selectivity toward C₇₀ is observed by all the double-calix[5]arenes. The selectivity of 5a toward C₇₀/C₆₀ is highest in toluene with a value of 10. The structures of the supramolecular complexes of the calix[5]arene hosts and C₆₀ or C₇₀ were investigated by using ¹H and ¹³C NMR studies. The molecular mechanics calculation and X-ray structure reveal that the interior of the calix[5]arene is complementary to the exterior of C₆₀ molecule. In contrast, the host-guest complexes of C₇₀ with the simple calix[5]arenes take many conformational options due to its less symmetric shape. The molecular mechanics calculation and our chemical shift simulation nicely worked to estimate the reliable structures; the calix[5]arene cavity takes up C₇₀ molecule, and the C₇₀ molecule tilts significantly from the C5 axis of the calix[5]arene. In the case of the host-guest complex of C₇₀ with the double-calix[5]arene, the molecular dynamics simulation of the host-guest complex represented the realistic movement of the bound C₇₀ inside the cavity. The combination of the molecular dynamics simulation and the chemical shift simulation of the host-guest complex suggested that the C₇₀ molecule rapidly moves inside the cavity.     

Sorbents based on calix[4]arenes for extraction of technetium(vii) from acidic and alkaline media

Sorption of Tc^VII from solutions of various compositions with new sorbents prepared by the noncovalent immobilization of (thia)calix[4]arenes on the Amberlite XAD-7™ support was studied. The sorbents studied efficiently extract technetium(vii) from both acidic and alkaline media. The sorption capacity of the sorbent with thiacalix[4]arene groups is superior to that of the sorbents with calix[4]arene groups and several times higher than that of the sorbents previously proposed for the sorption of Tc^VII. Technetium(vii) is sorbed by this sorbent as 1: 1 and 1: 2 thiacalix[4]arene—NH₄TcO₄ and 1: 1 and 1: 2 thiacalix[4]arene—NaTcO₄ complexes.     

A study of hydrogen bonds in <Emphasis Type="Italic">p</Emphasis>-substituted calix[4]-and calix[6]arenes by <Emphasis Type="Italic">ab initio</Emphasis> and electron density functional methods

RHF/3-21G and B3LYP/3-21G methods are used to calculate the hydrogen bond energies in calix[4]-, calix[6]-, p-fluorocalix[4]-, p-fluorocalix[6]-, p-chlorocalix[4]-, p-chlorocalix[6]-, p-bromocalix[4]-, pbromocalix[6]-, p-iodocalix[4]-, p-iodocalix[6]-arenes and a number of other p-substituted calix[4]- and calix[6]arenes (R = Me, OMe, NO₂, Ac, NH₂, CN, N₂⁺). The calculations along with the structural data give evidence of the cooperative effect of hydrogen bonding. Multiple correlation (p ≥ 0.9) between the pairs of Hammett substituent constants and the calculated values of hydrogen bond energies in the corresponding p-substituted calixarenes is found. It is predicted that in the presence of weak bases and in aprotic solvents as well as in the gas phase, the nucleophilic substitution reaction involving p-halogen calix[6]arenes should proceed through diastereomeric transition states.     

Synthesis of chiral calix[4]arenes bearing tartaric ester moieties

The synthesis of chiral calix[4]arenes with tartaric acid ester moieties has been achieved by the reactions of tartaric ester chloroacetates with calix[4]arenes in moderate yields. All the chiral calix[4]arene derivatives are in a cone conformation according to the ¹H NMR doublet–doublet pattern of the protons of the methylene groups between the phenol rings. The results of NMR and specific rotations indicate that the molecules have C₂ symmetry with asymmetric features.     

Synthesis and anion recognition of neutral receptors based on multiamide calix[4]arene

Two multiamide calix[4]arenes (5, 6) were synthesized and characterized by NMR, MS and elemental analysis. The binding properties of receptors with some anions (π-O₂NPhOPO₃²⁻, π-O₂NPhO⁻, H₂PO₄⁻, Ac⁻, Cl⁻, Br⁻ and I⁻) were studied by UV-Vis spectra. The results indicate that the tetraamide calix[4]arenes (5, 6) have a good selectivity to the anions containing aromatic ring (π-O₂NPhOPO₃ ²⁻, π-O₂NPhO⁻). The 1 : 1 complexes between host and guest were formed through multiple hydrogen bonding and π-π interactions. The hosts 5 and 6 also show a definite binding ability for the anions (H₂PO₄⁻, Ac⁻, Cl⁻) that have no ultraviolet absorption, which provides a simple method of spectrum detection for these anions.     

Electrochemical properties of outer-sphere associates of bipyridyl and sepulchrate metal complexes with (thia)calix[4]arenes

The electrochemical one-electron reduction (oxidation) of bipyridyl metal complexes ([Co(bipy)₃]³⁺, [Cr(bipy)₃]³⁺, [Fe(bipy)₃]²⁺, [Ru(bipy)₃]²⁺ (as well as Co(III) sepulcrate)) with water-soluble (thia)calix[4]arenes has been studied by means of cyclic voltammetry. It has been shown that [M(bipy)₃]^3+/2+ bind to (thia)calix[4]arenes via sulfonate groups of the upper rim. Oxidized forms bind stronger than reduced ones leading to reduction (oxidation) of half-wave cathodic shift. The effect of predominant stabilization of oxidized forms of metal complexes for carboxylated calix[4]arene is stronger than for thiacalix[4]arene (ΔΔG₀?=???7.8?÷???12.5 and ??3.7 kJ/mol, respectively). The redox-switchable outer-sphere binding of Co(III) sepulchrate via lower rim of carboxylated calix[4]arene has been revealed using cyclic voltammetry. The binding constants of outer-sphere associates based on calix[4]arenes and unstable metal complexes ([Co(sep)]²⁺, [Ru(bipy)₃]³⁺, [Co(bipy)₃]²⁺) have been calculated for the first time using ¹H NMR titration and cyclic voltammetry data.

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Stoichiometric complexes of calix[4]arenes with solvent molecules

Some calix[4]arenes were found to form complexes with solvent molecules which retained their stoichiometric composition when sublimed. Saturated vapor pressures of calix[4]arenes and their complexes with solvents were for the first time determined by the Knudsen effusion method over a wide temperature range. Changes in the standard thermodynamic parameters of complex formation in the transfer from the condensed to gas phase were calculated. The enthalpy contribution to the Gibbs energy grew as conformational ligand mobility increased, and the entropy contribution increased as flexibility decreased and rigidity increased.     

Synthesis of New Calix[4]arenes Functionalizated by Acetylhydrazide Groups

A series of new calix[4]phenols, calix[4]resorcinols and calix[4]pyrogallols with acetylhydrazide substitutes has been␣synthesized with high yields by hydrazinolysis of ester group containing calix[4]arenes. The synthesized calix[4]phenols adopt the cone conformation while the calix[4]resorcinol and calix[4]pyrogallol derivatives prefer the boat conformation. The amide fragment of the hydrazide groups predominantly exists in the trans-conformation. The binding ability of synthesized calix[4]arenes toward transition and alkali metals by solvent extraction has been investigated.     

Complexation and extraction of non-ferrous metals by calix[n]arene phosphine oxides

The extraction of non-ferrous metal (M²⁺) nitrates by the calix[4,6]arenes (L), bearing four or six phosphine oxide donor groups at the upper or at the lower rim, was quantitatively described in the form of [M_n(NO₃)_2nL] (n = 1, 2) complexes. The extraction constants (Zn²⁺ > Cu²⁺ > Co²⁺ > Ni²⁺) for the both types of L coincide with Irving-Williams sequence. Calix[4]arenes, phosphorylated at the lower (narrow) rim, provide better stability of ML complexes because of the best spatial fitting of M²⁺ by the donor groups. For the upper (wide) rim phosphorylated calix[4]arenes M₂L and ML₂ complexes are more stable. Unusual zwitterionic [Co₂(NO₃)₄L] complex of the lower rim tetraphosphorylated calix[4]arene 1 was determined by X-ray structural analysis.     

Thermodynamics of sublimation of calix[4]arene complexes with solvent molecules

Some calix[4]arenes form complexes with solvent molecules, preserving the stoichiometric composition upon sublimation. The saturated vapor pressures of calix[4]arenes and their complexes with solvents were determined by the Knudsen effusion method in a wide temperature range. The changes in the standard thermodynamic functions of complexation in transfer from the condensed state to the gas phase were determined. It was found that the enthalpy term of the Gibbs energy of the complex formation increases with an increase in the conformational mobility of the ligand, and the entropy term increases as the ligand becomes less flexible and more rigid.     

Calix[4]arenes as molecules for second order nonlinear optics

Abstract

The synthesis of a novel class of molecules for second order nonlinear optics, i.e. calix[4]arenes with extended π-systems, is described. These compounds are obtained via Wittig-Horner reactions of the formylated calix[4]arenes 5 and 6 to give the stilbene derivatives 7–9, or by diazotization of calix[4]arene, 1, followed by alkylation to give the phenylazocalix[4]arenes 11 and 12. The molecular second order nonlinear optical properties (β_z) of these calix[4]arenes have been measured by electric field-induced second harmonic generation. The influence of different acceptors as well as the influence of the different conformations of the calix[4]arenes on β_z values were determined. Surprisingly, the wavelength of the charge-transfer band λ_max is lower when β_z increases upon increasing the number of acceptors.     

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.     

Fluorescence Spectral Studies on the Coordination of Calix[4]‐arenes Bearing Boronic Acid Moieties with Monosaccharides

Coordination of ten calix[4]arenes bearing boronic acid moieties with five monosaccharides was studied by fluorescence spectrometry. The stability constants (K₂) of the complexes and Gibbs free energy change ( ‐ ΔG⁰) of the coordination reactions were calculated according to the modified Hilderbrand‐Benesi equation. The results obtained indicated that the coordination ability of D‐( ‐ )‐fructose with calix[4] arenes bearing boronic acid moieties was stronger than that of the other monosaccharides. And these calix[4]arene derivatives might be used for identification of L‐( ‐ )‐sorbose.     

Extraction properties of 25,27-bis(carbonylmethoxy)calix[4]arenes towards Sr2+: competitive extraction and extraction in a synthetic groundwater

The Sr²⁺ extraction properties of some 25,27-bis(carbonylmethoxy)calix[4]arenes, in which ester (–CH₂CO₂Et), keto (–CH₂COCH₃) and acid functionalities (–CH₂CO₂H) have been attached to the lower rim, are reported. Strontium ion extraction experiments were performed in a chloroform/water system, and the extraction performance analyzed by radiotracing using the short-lived radio nuclide ⁸⁵Sr. Effects of pH value, ligand to metal ratio, temperature, extraction time and strontium species on the extraction behavior were examined. It was observed that the 25,27-bis(carboxymethoxy)calix[4]arene derivatives are potent extracting agents towards the strontium ion, showing remarkable extraction performance in competition with a series of organic and inorganic impurities and in a synthetic groundwater. The crystal structure of 25,27-bis(acetonyloxy)calix[4]arene is also reported.     

Stereochemical control of calixarenes useful as rigid and conformationally diversiform platforms for molecular design

Stereochemical problems and related functions of calix[4]arenes, calix[6]arenes and their chiral derivatives have been reviewed. In p-tert-butylcalix[4]arene (1H₄) and its mono-, di-, tri-, and tetra-O-alkyl derivatives (1H₃R, 1H₂R₂,1HR₃, and 1R₄, respectively), 23 different homologues can exist (including 1H₄). We found that the OH group in the unmodified phenol unit is permeable through the calix[4]arene ring. Thus, several conformational isomers become equivalent after the ‘oxygen-through-the-annulus’ rotation of the OH group and the number of possible homologues is reduced to 13 (including 1H₃). We report in this paper the syntheses of all of these possible conformational isomers using a protection-deprotection method with a benzyl group and metal template effects. On the other hand, all possible chiral isomers that can be derived from calix[4]arene by modification of the OH groups have been systematically classified. Molecular asymmetry can be generated not only by different substituents but also by conformational isomerism. The numbers of chiral isomers are 17 for tetra-O-substituted calix[4]arenes, 9 for tri-O-substituted calix[4]arenes, 3 for di-O-substituted calix[4]arenes, and 0 for mono-O-substituted calix[4]arenes. Chiral calix[4]arenes can also be designed by the introduction of a substituent into the m-position of a phenol unit or by the use of a dissymmetric ‘stapling reaction’ in proximal phenol units. In p-tert-butylcalix[6]arene, the conformational behaviour is totally different from that in p-tert-butylcalix[4]arene. A large degree of conformational freedom remains in the framework, and both ‘oxygen-through-the-annulus rotation’ and ‘para-substituent-through-the-annulus rotation’ can take place. However, when metal cations are bound to calix[6]aryl esters, the conformation is changed to a cone type. Bridging and capping are powerful methods to immobilize the conformation of calix[6]arenes. In addition, definitive evidence for ring immobilization was obtained from the absence of racemization in the chiral calix[6]arene. A successful example of chiral recognition for α-amino acid derivatives was achieved by using chiral homooxacalix[3]arene which has ‘pseudo C₂ symmetry’. These examples indicate that calixarenes serve as rigid and conformationally diversiform platforms for the design of novel functional supramolecules.     

The Synthesis of Ester and Ketone Derivatives of Azocalix[4]arene Containing Chromogenic Groups

Azocalix[4]arenes were prepared by linking 4-ethylaniline, 4-n-butylaniline, 4-acetamide aniline and 2-aminothiazol to calix[4]arene through a diazo-coupling reaction. A new family of azocalix[4]arenes, L1–L12, have been prepared by the incorporation of acetyl, benzoyl, and methyl ketone units to azocalix[4]arene. Characterization of the synthesized compounds was carried using elemental analyses, UV–Vis, IR and ¹H NMR spectroscopic studies.     

Triptycene‐Derived Calix[6]arenes: Synthesis,Structures, and Their Complexation with Fullerenes C60 and C70

Two pairs of novel triptycene‐derived calix[6]arenes 4 a , b and 5 a , b have been efficiently synthesized through both one‐pot and two‐step fragment‐coupling strategies starting from 2,7‐bis(hydroxymethyl)‐1,8‐dimethoxytriptycene 1 . Subsequent demethylation of 4 a , b and 5 a , b with BBr₃ in dry dichloromethane gave the macrocyclic compounds 6 a , b and 7 a , b . Treatment of either 4 a or 6 a with AlCl₃ resulted in the same debutylated product 8 , while 9 was similarly obtained from either 5 a or 7 a . Structural studies revealed that all of the macrocycles have well‐defined structures with fixed conformations both in solution and in the solid state owing to the introduction of the triptycene moiety with a rigid three‐dimensional (3D) structure, making them very different from their classical calix[6]arene counterparts. As a consequence, it was found that all of these the triptycene‐derived calix[6]arenes could encapsulate small neutral molecules in their cavities in the solid state. Moreover, it was also found that the macrocycles 4 b and 5 b showed highly efficient complexation abilities toward fullerenes C₆₀ and C₇₀, forming 1:1 complexes with association constants ranging from (5.22±0.20)×10⁴ to (8.68±0.30)×10⁴ M ^?1.