Upper rim tetrathiafulvalene-bridged calix[4]arenes

The synthesis of novel upper rim calix[4]arene-tetrathiafulvalene conjugates 1a-d has been performed by bridging the tetrachloromethylated calix[4]arene derivative 4 with the corresponding tetrathiafulvalene-dithiolates. The cyclic voltammetry of 1a-d shows a two-step oxidation behavior, whereas NMR binding titrations showed their binding affinity to pyridinium salts. X-ray structure of 4 features calixarene fixed in the pinched cone conformation; its crystal packing is defined by the network of C-H···Cl weak hydrogen bonds.     

Synthesis and structure of lower rim C-linked tetra-N-tosyl peptidocalix[4]arenes

Chiral p-tert-butylcalix[4]arenes perfunctionalised at the lower rim with amino acid residues have been prepared. The ¹H and ¹³C NMR spectra indicate that the macrocycles adopt a cone conformation. Calix[4]arenes bearing amino acid moieties 5a shows strong complexation towards Cl⁻, Br⁻, HSO₄⁻, H₂PO₄⁻ and N-tosyl-(l)-alaninate.     

Adamantylcalixarenes with CMPO groups at the wide rim: synthesis and extraction of lanthanides and actinides

Starting from p-adamantylcalix[4]- and [6]arenes functionalized with carboxylic acid or ester groups at the adamantane nuclei, carbamoylmethylphosphine oxide (CMPO)-containing ligands of a novel type were synthesized. They were studied as extractants for a series of f-block elements including radioactive ¹⁵²Eu(III), ²⁴¹Am(III), ²³³U(VI), and ²³⁹Pu(IV). Tetrameric ligand 4b in which CMPO residues are connected to adamantane nuclei through methylene groups gave the best extraction results for lanthanides and actinides. For all the ligands the extraction efficiency does not decrease at higher nitric acid concentration. Although the discrimination between trivalent actinides and lanthanides is not good, all ligands are highly selective for thorium(IV) with the best separation factor achieved in the case of hexameric ligand 5 (D_Th/D_Ln>24).     

Investigations into cluster formation with alkyl-tethered bis-calix[4]arenes

Abstract

Calix[4]arenes are versatile ligands capable of forming a wide range of cluster motifs when reacted with 3d, 4f or 3d/4f metal ions. Synthetic modification at the calix[4]arene methylene bridge offers a unique opportunity to explore cluster formation with these alternative building blocks. Here, we present the synthesis of a range of bis-calix[4]arenes that are tethered by alkyl chains, as well as exploratory structural studies into cluster-forming reactions. Single crystals were formed in four cases, and from the X-ray structures elucidated it is possible to conclude that sufficiently long alkyl tethers allow for the formation of established cluster topologies without disruption to the core coordination chemistry.     

Metal-free synthesis of azacalix[4]arenes

The facile preparation of N(H)-bridged azacalix[4]arenes has been achieved by stepwise nucleophilic aromatic substitutions assisted by hydrogen bonding interactions. The synthesis is uncatalyzed and affords previously unknown tetranitroazacalix[4]arenes.     

Lanthanide complexation with CMPO and CMPO-calix[4]arenes in solution: spectrophotometric and electrospray mass spectrometric approaches

The binding of lanthanide(III) cations with organophosphorous ligands like CMPO and related calix[4]arene-based derivatives have been investigated using two experimental methods. The stability constants of the lanthanum, europium and ytterbium complexes were first determined in methanol by UV absorption spectrophotometry in the presence of nitrate or chloride anions. The results showed that the stoichiometry and the stability of the complexes formed depend on the position of the CMPO moieties either on the wide or the narrow rim of the calixarene scaffold, the nature of the medium, the conformational mobility of the ligands. Complexation of lanthanum was also followed by ESI-mass spectrometry in the same solvent. This method confirmed the stoichiometry of the complexes, giving also structural information, like coordination of anion or solvent molecules to the complexes, and allowed the calculation of distribution curves in good agreement with those derived from the spectrophotometric results. This is an important result showing that ESI-MS can be used to provide with quantitative information when absorption spectrophotometry is not applicable, i.e. for systems where complexation leads to weak spectral changes.     

Modification of calix[4]arenes with CMPO-functions at the wide rim. Synthesis, solution behavior, and separation of actinides from lanthanides

Two calix[4]arene tetraethers (Y = C5H11, C14H29) bearing on their wide rim four -N(Me)-CO-CH2-P(O)Ph2 residues were synthesized for the first time. Their ability to extract lanthanides and actinides from an acidic aqueous phase to organic phases (CH2Cl2, NPHE) was studied. In comparison to the corresponding -NH-analogs, they are less efficient extractants, the selectivity for the light over the heavy lanthanides is less pronounced, while there is still an interesting selectivity of Am3+ over Eu3+. Stability constants for selected lanthanide salts were determined also in homogenous phase (methanol, acetonitrile) but do not account for the different extraction results. The complexation of Gd3+ was also followed by relaxivity (NM RD) measurements, which suggest an even stronger aggregation for the N-methyl compound while the 1:1 complex is reached for a smaller ratio [L]/[Gd3+] compared to the NH analog. The formation of aggregates is also supported by dynamic light scattering measurements. A single crystal X-ray structure of the pentyl ether reveals a C2-symmetrical pinched cone conformation for the free ligand.     

Separation of lanthanides and actinides using magnetic silica particles bearing covalently attached tetra-CMPO-calix[4]arenes

Calix[4]arene tetraethers in the cone conformation bearing four -NH-CO-CH2-P(O)Ph2 (= CMPO) residues on their wide rim and one, two or four omega-amino alkyl residues of various lengths at the narrow rim were synthesized. Reaction with dichlorotriazinyl (DCT) functionalized magnetic particles led to complete coverage of the available surface by covalently linked CMPO-calix[4]arenes in all cases. Magnetically assisted removal of Eu(iii) and Am(iii) from acidic solutions was distinctly more efficient with these particles in comparison to analogous particles bearing the same amount of analogous single-chain CMPO-functions. The best result, an increase of the extraction efficiency by a factor of 140-160, was obtained for attachment via two propyl spacers. The selectivity Am/Eu was in the range of 1.9-2.8. No decrease of the extraction ability was observed, when the particles were repeatedly used, after simple back extraction with water.     

Regioselective deuteration of 25,27-dialkoxycalix[4]arenes

A direct deuteration of the upper rim of calix[4]arene has been carried out for the first time. 25,27-Dialkoxy derivatives of calix[4]arene (R = Me, Et, n-Pr, n-Bu) were regioselectively deuterated at the para positions of unsubstituted phenolic rings using DCl/D₂O in tetrachloroethane. Interestingly, identical reaction conditions do not lead to deuteration of mono- or tri-substituted derivatives where only simple cleavage of alkyl substituents was observed.     

Di-ionizable p-tert-butylcalix[4]arene-1,3-crown-4 ligands: synthesis and alkaline earth metal cation extraction

A series of di-ionizable calix[4]arene-1,3-crown-4 compounds has been synthesized. The ionizable groups are oxyacetic acid and N-(X)sulfonyl oxyacetamide groups with X=methyl, phenyl, 4-nitrophenyl and trifluoromethyl, which ‘tunes’ the acidity of the latter. The efficiency and selectivity of these novel ligands are assessed with competitive solvent extractions of alkaline earth metal cations from aqueous solutions into chloroform. The results are compared with those reported previously for calix[4]arene-crown analogues with crown-6 and crown-5 rings.     

Di-ionizable p-tert-butylcalix[4]arene-1,2-monothiacrown-3 ligands in the cone conformation: synthesis and metal ion extraction

A series of di-ionizable calix[4]arene-1,2-crown-3 compounds with a sulfur-containing unit bridging the proximate phenolic oxygens have been synthesized. The ionizable groups are oxyacetic acid and N-(X)sulfonyl oxyacetamide groups with X=methyl, phenyl, 4-nitrophenyl, and trifluoromethyl, which ‘tunes’ the acidity of the latter. The efficiency and selectivity of these novel ligands are assessed for competitive solvent extractions of alkali metal cations and of alkaline earth metal cations from aqueous solutions into chloroform. Also the efficiencies for single species extractions of Pb²⁺ and of Hg²⁺ are determined. The results are compared with those reported previously for related ligands in which only oxygen heteroatoms are present in the crown ether unit.     

Regioselective upper rim substitution of calix[4]arenes

The electrophilic substitution of 25,27-dipropoxy-26,28-dinosyloxycalix[4]arene leads exclusively to the para-substitution of the alkylated phenol rings, while in the next step, the protecting nosyl group can be easily removed using a basic hydrolysis. The overall process yields dialkoxycalix[4]arenes with the substitution on the alkylated rings—the substitution pattern, which is complementary to the common dialkoxycalix[4]arenes with substituted nonalkylated phenolic units. The usefulness of this protection/deprotection procedure was documented by the synthesis of novel type of calixarene dipropoxy derivatives, and by the preparation of a novel anion receptor based on this substitution pattern.     

Polymer-supported calix[4]arenes for sensing and conversion of NO2/N2O4

The use of simple calix[4]arenes 1a,b for NO₂/N₂O₄ sensing and conversion is demonstrated, both in solution and in the solid state. Upon reacting with these gases, compounds 1a,b encapsulate reactive NO⁺ cations within their cavities with the formation of deeply colored (λ_max∼570 nm) charge-transfer complexes 2a,b. Further functionalization of the calix[4]arene platform is reported for attachment to solid supports. Polymer-supported calixarene material 3 was prepared, which reversibly traps NO₂/N₂O₄ with the formation of nitrosonium storing polymer 4. Material 4 was effectively used for nitrosation of amides.     

Synthesis of terpyridine-substituted calix[n]arenes

Calix[n]arenes (n = 4,5) comprising 4-(2,2′:6′,2″-terpyridyl)-phenyl substituents at the upper rim were synthesized for the first time, employing Suzuki-type coupling reactions. All calix[n]arene derivatives were prepared as cone conformers. The single crystal X-ray structure of cone-5,11,17,23-tetra{4-(2,2′:6′,2″-terpyridyl)-phenyl}-25,26,27,28-tetrabutoxycalix[4]arene 4 is analyzed in terms of structural rigidity and potential use of these ligands as novel synthons of cage-type metallosupramolecular assemblies.     

Synthesis of amine, halide, and pyridinium terminated 2-alkyl-p-tert-butylcalix[4]arenes

The synthesis of calix[4]arenes with functional groups tethered to a single methylene bridge has been explored. Mono-lithiated calix[4]arenes react with 1,ω-bromochloroalkanes to give 2-(ω-chloroalkyl)-calix[4]arenes, which function as key intermediates in the synthesis of a variety of tether-functionalized calix[4]arenes. Subsequent reactivity of these chloroalkyl species has allowed for successful synthesis of 2-(ω-iodoalkyl)-calix[4]arenes, 2-(ω-pyridiniumalkyl)-calix[4]arenes, and 2-(ω-aminoalkyl)-calix[4]arenes. This latter group of amine-terminated calix[4]arenes are especially significant as they may serve as useful entries to a wide range of subsequent chemistry, including metal coordination or attachment to a solid support. Both alkyl and aryl amines have been incorporated at the end of the tether, and a 2,6-diisopropylaniline derivative has been analyzed by X-ray diffraction.     

Synthesis of calix[4]arene library substituted with peptides at the upper rim

A fluorescence-labeled calix[4]arene library substituted with peptides at the upper rim was synthesized. Screening of the library for binding a dye-labeled oligopeptide indicated that some peptidocalix[4]arenes selectively bind the oligopeptide. The chemosensitivity of the library members for a target peptide was also investigated.     

Tetrazolecalix[4]arenes as new ligands for palladium(II)

The synthesis of new calix[4]arenes bearing two or four tetrazole liganting groups at the upper rim is decribed. The structures of tetrakis-tetrazolecalix[4]arene and its palladium dichloride (2:2) complex are examined by X-ray crystallography.     

Pyrrolamidocalix[4]arenes: new receptors for anion recognition

Pyrrolamidocalix[4]arenes 1-4, members of a new class of anion receptors bearing pyrrolic units at the upper rim of calix[4]arene macrocycle, have been readily synthesized in good yields. Derivatives 1 and 3, with unsubstituted pyrrole units, show a good selectivity for over F⁻ and AcO⁻, while the presence of electron-withdrawing NO₂ substituents in 2 and 4 inverts the selectivity favoring more basic AcO⁻ and F⁻. In addition, it is demonstrated that the flexibility of calix[4]arene skeleton, present in 1 but absent in 3, is very important in the fitting process that leads the amidopyrrole moieties to wrap the tetrahedral guest.     

Delayed lanthanide luminescent Tb(III) complexes formed from lower rim amide functionalised calix[4]arenes

The synthesis and the photophysical studies of a new generation of time resolved luminescent systems based on calix[4]arenes alkylated at the lower rim, capable of hosting lanthanide (III) ions such as terbium and sensitising its emission, are described. Two series of ligands were designed to provide an ideal cavity to host terbium (Tb(III)) and were synthesised in high yields following two novel approaches. The tetra-alkylation, which was achieved in one step using with piperidino- and morpholino-acetamide pendant arms, provides eight donor atoms forming a binding ‘pocket’ at an ideal distance from the metal core to favour the sensitisation via the antenna effect. Of the two ligand series developed, compounds 3 and 4 possess a short spacer between the calix and the amide receptor site. The second series of ligands 6–7, designed with longer pendant amide arms, was synthesised from 2 in two steps through the ester analogue 5. The crystal structure of 3 (and 6 as shown in Supporting Information, available online) is presented. The synthesis and the photophysical studies of the four resulting complexes 3.Tb, 4.Tb, 6.Tb and 7.Tb are described in detail and in each case, successful sensitisation of the terbium emission occurred upon excitation of the phenolic scaffold of the calixarene.     

Synthesis,Conformational Analysis and Extraction Studies of p-isopropylcalix[n]arene Derivatives (n = 4, 6, 8). A New Family of Molecular Receptors

Abstract

Ethers, esters and ketones, derivatives of p-isopropylcalix[n]arenes (n = 4,6,8), have been prepared; 33 new molecular receptors have been isolated and fully characterized. ¹H NMR and ¹³C NMR measurements reveal that tetraester and tetraketone derivatives are in cone conformation at room temperature when hexa and octa derivatives look like flexible flattened cones. Extraction studies with metal picrates from aqueous solution into dichloromethane were used to assess the ionophoric activity of the p- isopropylcalix[n]arene derivatives. The better E% are obtained with tetrameric derivatives in cone conformation, fully functionalized on the phenolic oxygens by ester or ketonic groups. The tetraamide derivative is the most efficient binder; the E% values are around 100 for Li, Na, K, Rb, Cs, Ca and Ba.