The complexation of ferrocene derivatives by a water-soluble calix[6]arene

The complexation of several ferrocene derivatives by the water-soluble hostp-sulfonato-calix[6]arene was investigated using electrochemical and¹H-NMR spectroscopic techniques. The electrochemical results indicate that both oxidation states of the guests are bound to the calixarene host, although the oxidized (ferrocenium) forms are complexed more strongly than the reduced (ferrocene) species.¹H-NMR spectroscopic data indicate that the complexation phenomena involves the inclusion of the guest's ferrocene moiety into the flexible calixarene cavity.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.     

NMR studies of inclusion complexation of the pyrrolizidine alkaloid retronecine and p-sulfonic acid calix[6]arene

Accidental or intentional drug toxicity in humans and animals is a major concern and the search for detoxificant agents is a challenge. Pyrrolizidine-producing forages are a threat not only to livestock, but also to humans as a consequence of food contamination. Supramolecular systems are promising as detoxificant agents by decreasing the bioavailability of toxic compounds in biological environment. Cyclodextrin and calix[n]arenes are well known hosts for a variety of molecules and/or ions. Surprisingly, only few studies describe the potential of calix[n]arenes as host for toxic molecules. This study focused on the use of NMR techniques as tools for the investigation of the interactions between p-sulfonic acid calix[6]arene and retronecine, a toxic pyrrolizidine alkaloid.     

Computational study of calix[4]arene derivatives and complexation with Zn

High level DFT was applied to study structure and conformational equilibrium of amino and mercaptocalix[4]arenes possessing methylene groups or sulfur atoms at their four junctions. All the calculations point to cone and 1,3-alternate as the most stable conformers for sulfur and methylene bridged compounds, respectively. The presence of four sulfur atoms in place of the methylene bridges can lead to novel features in the complexation with transition metals. The host–guest 1:1 complexes between Zn²⁺ and each conformer of tetraaminothiacalix[4]arene were also investigated at the same DFT level. The four nitrogens of amino groups play a dominant role in the interaction with Zn²⁺ via distorted tetrahedral coordination for 1,3-alternate, or distorted square planar for partial cone and 1,2-alternate. On the other hand, the 2:1 complexes with cone show that the sulfur bridges can also bind Zn²⁺ and contribute to form two sets of five-membered chelated rings with two N donor atoms.     

Association and orientation of C70 on complexation with calix[5]arene

Calix[5]arene and C70 in p-xylene form a ball-and-socket supramolecular complex with the C5 axis of the fullerene tilted 40 degrees relative to the symmetry axis of the calixarene, the extended structure is comprised of well-separated zigzag sheets of C70 molecules.     

Synthesis and structure of a novel molybdenum imido calix[6]arene

Treatment of p-tert-butylcalix[6]areneH₆ (H₆L) with [Mo(OBu^t)₂{[2,2′-(N)-C₆H₄]₂(CH₂CH₂)}] in refluxing toluene affords, after work-up, the complex [Mo(2-NC₆H₄CH₂CH₂C₆H₄NHC(Me)NH-2^/)LH₂]·4MeCN (1), which contains an 11-membered metallocyclic ring as characterised by Synchrotron X-radiation.     

A new fluorescent double-cavity calix[4]arene: synthesis and complexation studies toward nitroanilines

A novel bicyclic calix[4]arene (4) molecular receptor has been synthesized, and its sensory abilities toward noxious aromatic nitroamines evaluated in apolar media. Molecular host–guest interactions examined through fluorescence and ¹H NMR spectroscopies showed the formation of 1:1 type endo-inclusion complexes with all the nitroaniline guests. The potential of 4 as a chemical sensor is established.     

Effect of varying bases on preferential crystallization of C-methylresorcin[4]arene and calix[6]arene

Preferential crystallization from a mixture of C-methylresorcin[4]arene (RsC1) and calix[6]arene (Calix6) in the presence of different bases has been investigated. In the presence of pyridine, a boat conformer of RsC1 crystallizes, whereas in the presence of triethylamine, Calix6 crystallizes in a symmetrically distorted conformation. The packing arrangements of the macrocycles show discrete solvent pockets for calixarenes and channels for resorcinarenes.     

Complexation of phenols by calix[4]arene Diethers in a low-permittivity solvent. Self-switched complexation by 25,27-Dibenzyloxycalix[4]arene

To improve the selectivity of sensing, the thermodynamics of the complex formation of some calix[4]arene hosts with neutral phenol guests was studied in carbon tetrachloride as nonpolar solvent. The molecular shape of calixarenes was varied by the selective functionalization with tBu and O-CH2-Ph (O-benzyl) or OPr groups at the upper and lower rim, respectively. To vary the electron density on the guest's aromatic rings, the parent phenol was functionalized in the para position with electron-withdrawing Cl, as well as H, and electron-releasing CH3 and tBu groups. To study the interaction between calixarene and the guests, PL and quantum-chemical methods were applied. The results revealed an overall 1:1 complex stoichiometry except for the parent dibenzyloxycalix[4]arene, where 1:2 host-guest stoichiometries were observed irrespective of the quality of phenol. In the latter case, the complex formation shows a self-switched character: the first phenol molecule is included in the calixarene cavity, and only afterward, a second guest molecule is bound by the two benzyloxy aromatics. Although the enthalpy change predicts strong interaction between the host and the guest, the Gibbs free energy change of the complex formation is small, resulting in a relatively low complex stability. The solvent-relaxation measurements support that the unexpected entropy change could be the consequence of the reorientation of solvent molecules around the calixarene building block. The reorientation is assisted by dispersive forces between solute and solvent molecules. IR and RAMAN analysis of the complexes exclude a considerable participation of the phenolic OH group in the stabilization of the complex. This result is in agreement with earlier findings where deterministic role of pi-pi interaction in the complex stability was assumed.     

Self-assembly of a double calix[6]arene pseudorotaxane in oriented channels

A synthetic study to disclose the more appropriate manner by which two calix[6]arene units could be connected for the construction of an extended tubular structure was undertaken. As a result, a head-to-tail double calix[6]arene having the structure of an oriented nanotube that is about 2.6 nm long and 1.6 nm wide was prepared and characterized. This molecule is able to act as a wheel-type host and forms a supramolecular complex with an axle-type molecule, derived from 4,4'-bipyridinium (viologen), through very efficient self-assembly in solution. The properties of such a pseudorotaxane-type complex, which is stabilized by a combination of noncovalent interactions, were investigated in solution by UV/Vis absorption spectroscopy and voltammetric methods. These observations provide a clue about the location of the bipyridinium unit along the nanotube. In the solid state, the complex undergoes a further stage of self-assembly, thereby initiating extended oriented tubular structures. Crystallographic studies revealed that the positioning of the viologen dication in this asymmetric wheel is addressed by a complicated pattern of cooperative noncovalent intermolecular interactions that involve only one half of the host, whereas the remaining (more polar) half of the host is exploited to create long-range structural order that leads to a "secondary" structure of extended supramolecular channels that, in turn, self-assemble in the lattice, thus giving rise to a "tertiary" structure of parallel sandwiches of nanotubes.     

Highly selective fluorescent sensing of Pb by a new calix[4]arene derivative

A new chemosensor was synthesized by the alkylation of p-tert-butyl-calix[4]arene with three N,N-diethylacetamide groups and one methyl 3-ethoxy-naphthalene-2-carboxylate, which exhibited highly selective fluorescent response to Pb²⁺ over alkali, alkali earth metal ions and some transition metal ions in CH₃CN-H₂O.     

一种新型杯[4]芳烃的合成

本文通过对特丁基杯[4]芳烃的酚羟基烷基化和苯环特丁基位上的IPSO-硝化制备了一系列对硝基杯[4]芳烃烷基醚,烷基分别是n-C~4H~9(3a)、n-C~8H~1~7(3b)、n-C~1~2H~2~5(3c)和n-C~1~6H~3~3(3d)。^1HNMR表明所有新的杯[4]芳烃都具有锥形(cone)构象。     

Crystal and molecular structure of two calix[6]arenes:p-Isopropylcalix[6]arene andp-tert-butylcalix[6]arene—benzene(1∶3) complex

The isopropyl derivative crystallizes from a mixture of carbon disulfide and benzene in the orthorhombic system: Space groupP2₁ nb, a=17.420(3),b=17.708(3),c=18.972(3) Å,V=5852(3) Å,Z=4. Thet-butyl derivative crystallizes from benzene, but the crystal is a complex (13), space groupP,a=15,065(5),b=19.103(3),c=13.878(3) Å, =106.95(2), =102.72(2), =80.61(2),V=3703(2) ų,Z=2. Refinement led toR=0.185 for 1512 reflections for the isopropyl derivative, a sufficiently high number to establish the conformation of the molecule; for thet-butyl complexR=0.12 for 7340 reflections. Intramolecular hydrogen bonds are given as well as comparison of the conformation of both compounds. Thet-butyl groups and the benzene molecules are disordered but the isopropyl groups are not. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82071 (57 pages).     

per-Hydroxylated pillar[6]arene: synthesis, X-ray crystal structure, and host-guest complexation

A per-hydroxylated pillar[6]arene was prepared. Single-crystal X-ray analysis demonstrated that its molecules are arranged in an up-to-down manner to form infinite channels in the solid state. Its host-guest complexation with a series of bispyridinium salts in solution was further investigated. It was found that the per-hydroxylated pillar[6]arene could form a 1:1 complex with paraquat in acetone with an association constant of 2.2 × 10(2) M(-1). This complex is a [2]pseudorotaxane as shown by its crystal structure, which is the first pillar[6]arene-based host-guest complex crystal structure.     

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.     

Shape recognition of alkylammonium ions by 1,3-bridged calix[5]arene crown-6 ethers: endo- vs exo-cavity complexation

A series of tri-O-substituted 1,3-bridged calix[5]arene crown-6 ethers bearing alkyl, arylalkyl, alkoxyalkyl, and alkoxycarbonylmethyl residues at the lower rim and either (t)()Bu or H substituents at the upper rim have been synthesized. (1)H NMR studies have shown that p-tert-butylcalix[5]crowns, irrespective of the size and nature of their lower rim pendant groups, adopt preorganized conelike conformations, whereas p-H-calix[5]crowns with bulky substituents preferentially exist in solution as partial cone conformers (C(1) symmetry). Calix[5]crown derivatives behave as mono- or ditopic receptors for isomeric butylammonium ions, forming endo-cavity (inside the calixarene cup) and/or exo-cavity (at the crown ether moiety) 1:1 complexes according to the shape of the guest. These two binding modes can be clearly distinguished and monitored by (1)H NMR titration experiments.     

Synergistic effect of ligating and ionic functions, prearranged on a calix[4]arene

The covalent attachment of two CMPO-functions and two anionic Cosan groups to the narrow rim of tert-butylcalix[4]arene leads to a dramatic increase of the extraction efficiency for the cone isomer; Am(3+) is removed from 5 x 10(-8) M solution to more than 99% by a single extraction step with a 3 x 10(-6) M solution of the calixarene.     

Dominant factors affecting extraction behavior of amino compounds by a calix[6]arene carboxylic acid derivative

A calix[6]arene carboxylic acid derivative was utilized as an extractant for various amino compounds including amino acids. The host compound exhibited a high extractability compared to that of commercially available extractants such as di(2-ethylhexyl)phosphoric acid (D2EHPA). The important factors for amino acid extraction were found to be its cyclic structure and its cavity size, the functional carboxylic groups, and the length of the alkyl chains. The hydrophobicity and the ionic property of guest molecules also affect the extraction efficiency. Calix[6]arene forms a stable complex with a guest molecule by entrapping it into the cavity. The molecular geometries of calixarene before and after extraction were also discussed using computational calculations with the PM3 type of semi-empirical method.     

Solvent-free multicomponent synthesis of 2-arylpyridines using p-sulfonic acid calix[6]arene as a reusable catalyst

An efficient methodology for obtaining 2-arylpyrimidines based on the use of p-sulfonic acid calix[6]arene as an organocatalyst is proposed. The methodology involves the formation of 1,2-dihydropyridine intermediates using a variety of aromatic aldehydes with methyl or ethyl acetoacetate and ammonium acetate, which are the same starting materials as in the Hantzsch reaction, under solvent-free reaction conditions, at 25 °C, followed by air oxidation for 12 h. The catalyst efficiency is not compromised after its successive use in reactions. Eleven examples were obtained with very good to excellent yields of 2-arylpyridines (92–62%). This is the first report about the use of calixarenes as catalysts in the multicomponent synthesis of 2-arylpyridines (molecules with potential biological activity).