Synthesis, characterization and electrochemical properties of two new calix[4]arene derivatives bearing two ferrocene imine or ferrocene amine units at the upper rim

A novel calix[4]arene derivative with two ferrocenyl Schiff-base groups at the upper rim 3 has been synthesized from 5,17-diformyl-25,27-dipropoxy-26,28-dihydroxy calix[4]arene and 4-ferrocenylaniline via condensation reaction. Reduction of 3 with sodium borohydride led to calix[4]arene derivative 4 with two amino ferrocenyl groups at the upper rim. The ferrocenyl Schiff-base calix[4]arene and its corresponding reduced amine have been purified and characterized by elemental analysis,¹H NMR, FTIR, Mass and UV-vis spectral data. Electrochemical properties of compounds 3 and 4 have been investigated. Cyclic voltammograms of 3 and 4 show reversible redox couples of ferrocene/ferrocinium at E_1/2=0.401 V and 0.346 V, respectively. Electrochemical studies show these redox active compounds electrochemically recognize trivalent lanthanides La³⁺ and Ce³⁺ and divalent Pb²⁺ and Cu²⁺cations. With ferrocenyl Schiff-base calix[4]arene 3 an anodic shift as large as 130 mV is observed on addition of one equivalent of Ce³⁺ ion. Also extraction properties of compound 4 towards some metal cations have been described. It has been observed that compound 4 has a good selectivity for metal cations Fe³⁺, Cu²⁺, Pb²⁺ and Cd²⁺ against Ni²⁺ and Co²⁺.     

Selective metal detection in an unsymmetrical 1,3-alternate calix[4]biscrown chemosensor

Calix[4]crown-based chemosensors 1, 2, and reference 3 have been synthesized. The weak fluorescence intensity of 1,5-naphthalene of 1 suggests that the benzene rings of the calix[4]arene as well as the oxygen atoms of the crown-5 ring take part in PET. The complexation of two K⁺ ions by both crown-5 and 1,5-naphthalene-crown-6 loops of 1 caused fluorescence enhancement of the naphthalene unit by CHEF. Pb²⁺ acted as a quenching metal ion.     

Synthesis and evaluation of a novel pyrenyl-appended triazole-based thiacalix[4]arene as a fluorescent sensor for Ag ion

New fluorescent chemosensors 1,3-alternate-1 and 2 with pyrenyl-appended triazole-based on thiacalix[4]arene were synthesized. The fluorescence spectra changes suggested that chemosensors 1 and 2 are highly selective for Ag⁺ over other metal ions by enhancing the monomer emission of pyrene in neutral solution. However, other heavy metal ions, such as Cu²⁺, and Hg²⁺ quench both the monomer and excimer emission of pyrene acutely. The ¹H NMR results indicated that Ag⁺ can be selectively recognized by the triazole moieties on the receptors 1 and 2 together with the ionophoricity cavity formed by the two inverted benzene rings and sulfur atoms of the thiacalix[4]arene.     

A specific and ratiometric chemosensor for Hg based on triazole coupled ortho-methoxyphenylazocalix[4]arene

Calix[4]arene 3, which contains two distal triazole groups on the lower rim and two distal o-methoxyphenylazo groups on the upper rim, was synthesized and found to be a specific and ratiometric sensor for Hg²⁺ in a polar protic solvent. A series of o-methoxyphenylazo derivatives (3, 4, 5, 7, and 9) were synthesized, which proved that the lower-rim triazoles and the hydroxyl azophenol(s) were the major ligands for metal ion binding. Though analogues 4 and 10 showed some sensitivity for Hg²⁺, compound 3 was the only ratiometric chemosensor for Hg²⁺ among the series of azocalix[4]arenes synthesized in this work. The formation of 3·Hg²⁺ complex was supported by UV/vis and NMR titration studies and Mass spectrometry. Based on the symmetrical features of NMR spectra of 3·Hg²⁺, the complex is believed to be symmetrical with respect to the calix[4]arene cavity. Furthermore, the complex was determined to be 1:1 binding stoichiometry by Job’s plot, and the association constant was determined to be 4.02×10³ M⁻¹ using Benesi-Hildebrand plot.     

Triazole- and azo-coupled calix[4]arene as a highly sensitive chromogenic sensor for Ca and Pb ions

A novel chromogenic calix[4]arene 3, which has within a molecule both the triazoles and the hydroxyl azophenols as the metal-binding sites and the azophenol moiety as a coloration sites was designed and synthesized. Calix[4]arene 3 is highly sensitive to Ca²⁺ and Pb²⁺ ions, which can be detected by the naked eye. Furthermore, the association constants for the 1:1 complexes of 3·Ca²⁺ and 3·Pb²⁺ were determined to be 7.06 × 10⁴ M⁻¹ and 8.57 × 10³ M⁻¹, respectively.     

Regulation of metal ion recognition by allosteric effects in thiacalix[4]crown based receptors

Three new ditopic receptors 3a-c based on thiacalix[4]arene of 1,3 alternate conformation possessing two different complexation sites have been designed and synthesized for both soft and hard metal ions. The imino nitrogens bind soft metal ion (Ag⁺/Pb²⁺/Cu²⁺) and the crown moiety binds K⁺ ion. The preliminary investigations show that 3a-c behave as ditopic receptors for Ag⁺/K⁺, Pb²⁺/K⁺, and Cu²⁺/K⁺ ions, respectively. In all the three receptors it was observed that the formation of 3a·Ag⁺/3b·Pb²⁺/3c·Cu²⁺ complex triggers the decomplexation of K⁺ ion from crown moiety and acts as a gateway, which regulates the binding of alkali metal to crown moiety. Thus, allosteric binding between metal ions ‘switch off’ the recognition ability of crown ether ring.     

Synthesis of novel p-tert-butyl-calix[4]arene derivatives and their cation binding ability: chromogenic effect upon side arms binding

A series of novel double-armed calix[4]arene derivatives, i.e. 5,11,17,23-tetra-tert-butyl -25,27-bis[2-[(2-hydroxy-5-(4-nitroazo)benzylidene)amino]ethoxy]-26,28-dihydroxy-calix[4]-arene (4), 5,11,17,23-tetra-tert-butyl-25,27-bis[2-[(2-hydroxy-5-(2-nitroazo)benzylidene) amino]ethoxy]-26,28-dihydroxycalix[4]arene (5), 5,11,17,23-tetra-tert-butyl-25,27-bis[2-[(2-hydroxy-5-(4-chloroazo)benzylidene)amino]ethoxy]-26,28-dihydroxycalix[4]arene (6), have been synthesized as an selective chromoionophore for Na⁺. The complexation behavior of ligands 4-6 with alkali metal ions Na⁺, K⁺, Rb⁺and Cs⁺ has been evaluated by using UV-Vis spectrometry in CH₃CN-H₂O (99:1/V:V) solution at 25°C. The UV-Vis spectra show that the complexation of 4-6 with Na⁺exhibits obvious bathochromic shifts (λ_max 379→480 nm) and there is a unique color change in the solution from yellow to red upon complexation. The binding constants for Na⁺ are higher than that of other alkali metal ions, giving the highest cation selectivity up to 7 for Na⁺/K⁺. The binding ability and photophysical behavior of alkali cations by calix[4]arene derivatives 4-6 are discussed from the point of view of substituted effects at the lower rim of parent calix[4]arene and size-fit concept between host calix[4]arenes and guest cations.     

New receptors for anions in water: Synthesis, characterization, X-ray structures of new derivatives of 5,11,17,23-tetraamino-25,26,27,28-tetrapropyloxycalix[4]arene

Syntheses and characterization of ten new compounds from the calixarene family, cone - 5,11,17,23- tetrakis(2-pyridylmethylamino)-25,26,27,28-tetrapropyloxycalix[4]arene 4a; cone - 5,11,17,23-tetrakis(3-pyridylmethylamino)-25,26,27,28-tetrapropyloxycalix[4]arene 4b; cone - 5,11,17,23-tetrakis(4-pyridylmethylamino)-25,26,27,28-tetrapropyloxycalix[4]arene 4c; cone - 5,11,17,23-tetrakis(ferrocenylmethylamino)-25,26,27,28-tetrapropyloxycalix[4]arene 4d; cone - 5,11,17,23-tetrakis(2-pyridylmethimino)-25,26,27,28-tetrapropyloxycalix[4]arene 3a; cone - 5,11,17,23-tetrakis(3-pyridylmethimino)-25,26,27,28-tetrapropyloxycalix[4]arene 3b; cone - 5,11,17,23-tetrakis(4-pyridylmethimino)-25,26,27,28-tetrapropyloxycalix[4]arene 3c; cone - 5,11,17,23-tetrakis(ferrocenylmethimino)-25,26,27,28-tetrapropyloxycalix[4]arene 3d; cone - 5,11,17,23-tetrakis(2-thienylmethimino)-25,26,27,28-tetrapropyloxycalix[4]arene 3e and cone - 5,11,17,23-tetrakis(2-pyrrolylmethimino)-25,26,27,28-tetrapropyloxycalix[4]arene 3f are reported. The target compounds 4a-4d were designed to form complexes with anions based on hydrogen bonds and electrostatic interactions in acidic aqueous solutions and the interaction constant 1770 mol⁻¹ dm³ of a 1:1 complex was obtained for the interaction of 4c with sulfate anion in 5 × 10⁻³ M aqueous HCl. The solid state structures of the compounds 3b, 3e and 3f were determined, their stereochemistry and the stereochemistry of the calix[4]arene frame is generally discussed. Raman, infrared and UV-vis spectra of the target compounds and some intermediates are reported, too.     

Synthesis,characterization and metal ion complexation and extraction capabilities of calix[4]arene Schiff base compounds

The syntheses, characterization and metal ion complexation and extraction capabilities of six new calix[4]arene Schiff base compounds, 5–10, are reported. The preparation of the compounds was achieved by the condensation of 5,17-diamino-11,23-di-tert-butyl-25,27-di-n-butoxy-26,28-dihydroxycalix[4]arene with the appropriate aldehyde (5-bromosalicylaldehyde for 5, 4-anisaldehyde for 6, 4-(dimethylamino)benzaldehyde for 7, 9-anthracenecarboxaldehyde for 8, 1-pyrenecarboxaldehyde for 9, and 9-fluorenecarboxaldehyde for 10) in refluxing ethanol. The compounds were characterized by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, high-resolution mass spectrometry and elemental analysis. The X-ray crystal structures of 7, 8 and 9 (as dichloromethane solvates) revealed that the calixarene molecules adopt H-bond stabilized, distorted-cone conformations and form centrosymmetric dimers in the solid state. Compounds 5–10 did not form host–guest complexes with NEt₄[(bdt)MoO₂(OSiPh₃)] (bdt^2–=benzene-1,2-dithiolate), a potential precursor for biologically relevant oxosulfido-Mo(VI/V) enzyme models; such host–guest complexes have the potential to stabilize these sought-after but highly reactive model compounds. In addition, the capabilities of 5–10 to extract selected metal ions (Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺, Ag⁺, Pb²⁺, Cd²⁺ and Hg²⁺) from an aqueous into an organic phase have been assessed by picrate extraction experiments. Compound 5 displayed exceptional selectivity towards Ni²⁺, compound 7 exhibited enhanced extraction towards all of the metal ions tested and compounds 6, 9 and 10 showed very high selectivity towards Hg²⁺. On the other hand, compound 8 exhibited negligible capacity to extract any of the metal ions tested.     

Intramolecular hydrogen bonding effect on metal ion complexation of homooxacalix[4]arene bearing tetraamides

N,N-Dipentylamido homooxacalix[4]arene (3) in the C-1,2-alternate conformation provided Pb²⁺ ion selectivity over other metal cations. N-Monopentylamido homooxacalix[4]arene in C-1,2-alternate conformation has an intramolecular hydrogen bonding, causing decrease of the metal ion complex ability.     

Metal ion induced FRET On-Off in naphthyl-pyrenyl pendent tetrahomodioxacalix[4]arene

A novel tetrahomodioxacalix[4]arene (7) bearing both naphthyl- and pyrenyl-amide pendants was synthesized as FRET-based fluorometric sensor for Cu²⁺ ion. Intramolecular FRET from the naphthalene emission to the pyrene absorption affords Cu²⁺ ion selectivity over other metal ions. Upon addition of Cu²⁺ ion, the complex solution of 7 gave a significantly decreased pyrene acceptor emission along with an enhancement of naphthalene donor emission via FRET On-Off event.     

Water-soluble pentasulfonatocalix[5]arene: selective recognition of ditopic trimethylammonium cations by a triple non-covalent interaction

The inclusion of tetramethylammonium and ditopic trimethylammonium cations by the water-soluble pentasulfonatocalix[5]arene 1 has been studied at neutral pH by ¹H NMR and compared with the homologous tetrasulfonatocalix[4]arene 2. Unlike host 2, host 1 selectively binds the ditopic trimethylammonium ions by three different non-covalent interactions. Remarkably the flexible host 1 exhibits both more efficiency and selectivity in the complexation of ditopic methylammonium ions with respect to similar more preorganised calix[4]arene receptors.     

Syntheses of two diamine substituted 1,3-distal calix[4]arene-based magnetite nanoparticles for extraction of dichromate, arsenate and uranyl ions

The synthesis of two new calixarene derivatives 4 and 5, functionalized at the lower rim with 4-amino-1-benzylpiperidine to give diamide and diamine derivatives of p-tert-butylcalix[4]arene, is described. They were obtained by the reaction of both the diester derivative of p-tert-butylcalix[4]arene (2) and the dialkyl bromide derivative of p-tert-butylcalix[4]arene (3) with 4-amino-1-benzylpiperidine. The ¹H NMR spectra of calixarene derivatives show that 4 and 5 exist in the cone conformation. Moreover, these diamide and diamine derivatives of p-tert-butylcalix[4]arene (4 and 5) have been immobilized onto [3-(2,3-epoxypropoxy)-propyl]-trimethoxysilane-modified Fe₃O₄ magnetite nanoparticles to obtain calixarene-based magnetic nanoparticles M-DADBP-Calix (6) and M-DABP-Calix (7). The calix[4]arene immobilized materials were characterized by a combination of Fourier Transform Infrared Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and Thermogravimetric Analyses (TGA) and elemental analysis. Additionally, the studies regarding the removal of As(V)/Cr(VI) ions as well as U(VI) ion from aqueous solutions were also carried out by using these compounds in liquid-liquid/solid-liquid extraction experiments.     

Novel 2,2′-bipyridine-modified calix[4]arenes: ratiometric fluorescent chemosensors for Zn ion

Two calix[4]arene derivatives with modified bipyridine as binding sites have been designed and synthesized. Compounds 1 and 2 are the first 2,2′-bipyridine-modified calix[4]arene-based sensors that can detect Zn²⁺ selectively with respect to ratiometric fluorescent changes and red shift. A binuclear complex structure has been demonstrated in the binding modes of 1-Zn²⁺ and 2-Zn²⁺ complexes.     

Highly selective fluorescent sensing of Cu ion by an arylisoxazole modified calix[4]arene

A novel fluorescent chemosensor 1 with two anthraceneisoxazolymethyl groups at the lower rim of calix[4]arene has been synthesized, which revealed a dual emission (monomer and excimer) when excited at 375 nm. This chemosensor displayed a selective fluorescence quenching only with Cu²⁺ ion over all other metal ions examined. When Cu²⁺ ion was bound to 1, the fluorescence intensities of both monomer and excimer were quenched. Furthermore, the association constant for the 1:1 complex of 1·Cu²⁺ was determined to be (1.58 ± 0.03) × 10⁴ M⁻¹.     

Highly selective complexation of metal ions by the self-tuning tetraazacalixpyridine macrocycles

By means of UV-vis and ¹H NMR titrations and X-ray crystallography, complexation of tetramethylazacalix[4]pyridine L1 and tetramethylazacalix[2]arene[2]pyridine L2 with metal ions was studied. While no interaction was observed with alkali and alkaline earth metal ions, both ligands have been found to act as powerful and selective macrocyclic hosts to complex a number of transition and heavy metal ions. Due to the intrinsic nature of the bridging nitrogen atoms that can adopt different electronic configurations and form varied degrees of conjugations with their adjacent pyridine rings, tetramethylazacalix[4]pyridine L1 regulated its conformation and cavity structure to best fit the guest metal ion species, yielding a 1:1 square planar L1-Mⁿ⁺ complex with binding constants log K_1:1 ranging from 2.7(1) to 8.2(8).     

Synthesis of crownophanes possessing bipyridine moieties: bipyridinocrownophanes exhibiting perfect extractability toward Ag ion

Novel crownophanes with two bipyridine moieties (bipyridinocrownophanes 1a and 1b) were conveniently prepared by means of intramolecular [2+2] photocycloaddition of vinylbipyridine derivatives. In the liquid-liquid extraction of heavy metal cations, 1a and 1b exhibited perfect selectivity toward Ag⁺ with high efficiency. It was found that the ethereal oxygen atoms and the four nitrogen atoms in 1a and 1b acted as ligating sites, according to the high extractability and complexing stability constant for Ag⁺ compared to those of the corresponding pyridinocrownophanes 4a and 4b. ¹H NMR and ESI-MS analyses suggested that the crownophanes formed a 1:1 complexes with the Ag⁺ ion.     

Synthesis of new calix[4]arenes containing nucleoside bases

A family of novel calix[4]arene derivatives containing nucleoside bases were designed and synthesized. Coupling reaction between para mono- or bis-amino calix[4]arenes 5, 6 or 7 and thymin-1-ylacetic acid in the presence of DCC afforded mono- or bis-thymine-substituted calix[4]arenes 8, 9 or 10 in over 70% yield. Owing to the low solubility of adenine-N⁹-ylacetic acid in DMF and DMSO and the weak nucleophilicity of aminocalix[4]arene derivatives, alternatively, the substitution reaction of bromoacetylated aminocalix[4]arenes derivatives 11, 12, 13 with adenine in the presence of sodium hydride was carried out to synthesize mono- or bis-adenine-substituted calix[4]arenes. Two kinds of isomers 15 and 16 or 17 and 18 were obtained due to the non-regiospecific alkylation of adenine, and their structures have been confirmed by ¹³C NMR and ¹H NMR spectra.     

Selective detection of pyrophosphate by new tripodal amine calix[4]arene-based Cu(II) complexes using indicator displacement strategy

Mono- and dinuclear Cu(II) complexes of p-tert-butylcalix[4]arene (CuL1 and CuL2, respectively) were synthesized, and their anion recognition abilities were explored. Recognition is efficiently signaled through the displacement of pyrocatechol violet bound to the receptor. For CuL2, recognition selectivity is ascribed to the tuning of the distance between donor atoms of anion guests and their ability to encompass the Cu²⁺-Cu²⁺ distance within the cleft of CuL2. In addition, the preorganization of calix[4]arene in the cone conformation and steric hindrance of two bulky tripodal amine moieties are important factors in controlling the Cu²⁺-Cu²⁺ distance. These factors caused CuL2 to recognize pyrophosphate selectively with respect to other inorganic anions in 80/20 (v/v%) MeCN/H₂O solution buffered with 10 mM HEPES at pH 6.4.     

Phosphite ligands derived from distally and proximally substituted dipropyloxy calix[4]arenes and their palladium complexes: Solution dynamics, solid-state structures and catalysis

Two bisphosphite ligands, 25,27-bis-(2,2′-biphenyldioxyphosphinoxy)-26,28-dipropyloxy-p-tert-butyl calix[4]arene (3) and 25,26-bis-(2,2′-biphenyldioxyphosphinoxy)-27,28-dipropyloxy-p-tert-butyl calix[4]arene (4) and two monophosphite ligands, 25-hydroxy-27-(2,2′-biphenyldioxyphosphinoxy)-26,28-dipropyloxy-p-tert-butyl calix[4]arene (5) and 25-hydroxy-26-(2,2′-biphenyldioxyphosphinoxy)-27,28-dipropyloxy- p-tert-butyl calix[4]arene (6) have been synthesized. Treatment of (allyl) palladium precursors [(η³-1,3-R,R′-C₃H₄)Pd(Cl)]₂ with ligand 3 in the presence of NH₄PF₆ gives a series of cationic allyl palladium complexes (3a-3d). Neutral allyl complexes (3e-3g) are obtained by the treatment of the allyl palladium precursors with ligand 3 in the absence of NH₄PF₆. The cationic allyl complexes [(η³-C₃H₅)Pd(4)]PF₆ (4a) and [(η³-Ph₂C₃H₃)Pd(4)]PF₆ (4b) have been synthesized from the proximally (1,2-) substituted bisphosphite ligand 4. Treatment of ligand 4 with [Pd(COD)Cl₂] gives the palladium dichloride complex, [PdCl₂(4)] (4c). The solid-state structures of [{(η³-1-CH₃-C₃H₄)Pd(Cl)}₂(3)] (3f) and [PdCl₂(4)] (4c) have been determined by X-ray crystallography; the calixarene framework in 3f adopts the pinched cone conformation whereas in 4c, the conformation is in between that of cone and pinched cone. Solution dynamics of 3f has been studied in detail with the help of two-dimensional NMR spectroscopy.The solid-state structures of the monophosphite ligands 5 and 6 have also been determined; the calix[4]arene framework in both molecules adopts the cone conformation. Reaction of the monophosphite ligands (5, 6) with (allyl) palladium precursors, in the absence of NH₄PF₆, yield a series of neutral allyl palladium complexes (5a-5c; 6a-6d). Allyl palladium complexes of proximally substituted ligand 6 showed two diastereomers in solution owing to the inherently chiral calix[4]arene framework. Ligands 3, 6 and the allyl palladium complex 3f have been tested for catalytic activity in allylic alkylation reactions.