Study on thermal decomposition of calix[6]arene and calix[8]arene

Thermal decomposition kinetics of calix[6]arene (C6) and calix[8]arene (C8) were studied by Thermogravimetry analysis (TG) and Differential thermal analysis (DTA). TG was done under static air atmosphere with dynamic heating rates of 1.0, 2.5, 5.0, and 10.0 K min⁻¹. Model-free methods such as Friedman and Ozawa–Flynn–Wall were used to evaluate the kinetic parameters such as activation energy (E _a) and pre-exponential factors (ln A). Model-fitting method such as linear regression was used for the evaluation of optimum kinetic triplets. The kinetic parameters obtained are comparable with both the model-free and model-fitting methods. Within the tested models, the thermal decomposition of C6 and C8 are best described by a three dimensional Jander’s type diffusion. The antioxidant efficiency of C6 and C8 was tested for the decomposition of polypropylene (PP).     

Spectroscopic studies of water-soluble sulfonated calix[6]arene

Fluorescence and absorption studies of water-soluble sulfonated calix[6]arene (SCX6) are reported. Water-soluble calixarenes are potentially useful as host molecules for luminophores, and studies of their spectroscopic characteristics are therefore crucial. The absorption and fluorescence spectra of these molecules in aqueous solution were collected, analyzed, and compared with 4-hydroxybenzene sulfonate at different pHs. A red shift in the absorption spectrum and a change in the fluorescence spectrum of the calixarenes are observed upon an increase in pH from 2.0 to 13.0. Some of these spectroscopic changes are attributed to intramolecular hydrogen bonding between adjacent hydroxyl groups of SCX6 after proton disassociation. The formation of excimers between phenolic groups in the calixarene molecule is proposed. In addition, inner-filter effects of SCX6 are discussed. These inner-filter effects prove to be a disadvantage for the use of SCX6 as a host molecule for complexation studies by use of fluorescence probes whose absorption spectra overlap with those of calixarenes.     

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.     

Allosterically driven self-assemblies of interlocked calix[6]arene receptors

The construction of self-assembled receptors based on flexible concave subunits is a challenging task and constitutes an interesting approach to mimic binding processes occurring in biological systems. The receptors studied herein are based on flexible calix[6]arene skeletons bearing three (or more) acid-base functionalities at their narrow rim. When complementary, they self-assemble in a tail-to-tail manner to give a diabolo-like complex, provided that each calixarene subunit hosts a guest. The allosterically-driven multi-recognition pattern is highly selective and leads to stable quaternary adducts. In order to evaluate the scope of this system, various polyamino and polyacidic calix[6]arenes have been studied. It is shown that modifications of the nature of the wide rim substituents do not alter the efficiency of the quaternary self-assembling process, even with the more flexible macrocycles that lack tBu substituents. On the contrary, the replacement of the latter by smaller groups led to receptors with broader scope, as larger guests such as tryptamine and dopamine derivatives were stabilized in the cavities. Implementation of extra-functionalities at the narrow rim were revealed also to be of high interest. Indeed, it is shown that secondary interactions take place between the two calix-subunits when they present additional and complementary functions such as carboxylate and ureido moieties. The ureido arms are also capable of binding the counter anion Cl(-) of the ammonium guest, thus leading to a quinternary neutral complex. Such remarkable behavior is due to the versatility of the calix[6]arene platform, which allows the implementation of a high number of functions, leading to multiple non-covalent attractive interactions, whereas the macrocycle remains flexible, thus allowing induced-fit processes to occur.     

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.     

A divalent germanium complex of calix[5]arene

The reaction of the germylene Ge[N(SiMe(3))(2)](2) with calix[5]arene yields the first example of a group 14 calix[5]arene complex. The crystal structure of this material has been obtained and contains two calix[5]arene macrocycles held together by a Ge(2)O(2) rhombus.     

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.     

Unsymmetrical calix[4]arene bisporphyrin Pacman

[structure: see text] The synthesis of a Pacman heterobisporphyrin associating an octaethyl porphyrinatozinc (ZnOEP) energy donor and a triaryl porphyrinatozinc (ZntPP) energy acceptor around a calix[4]arene spacer is described. Contrary to previous symmetrical architectures, correlations between the chromophores in the unsymmetrical calixarene bisporphyrin Pacman scaffold can now be observed in ROESY experiments. Independent of the excitation wavelength, only the luminescence of the ZntPP moiety was observed corresponding to quenching of the OEP moiety's emission.     

The selective functionalisation and difunctionalisation of p-substituted calix[6]arene and calix[8]arenes using hydrophilic moieties

Methodologies to access water soluble large ringed calixarenes in good yield using efficient synthetic procedures have been investigated. Symmetrical partial functionalisations at the lower rim are described using activated [n]ethylene glycol chains and the addition behaviour contrasted with that of bromoalkanenitriles which proceeds with no observed regioselectivity. Full functionalisations of the calixarenes bearing hydrophilic groups are then investigated and a two-step procedure established which appears to be generally applicable for the addition of different [n]ethylene glycol chains. Furthermore, difunctionalisation under different reaction conditions are described. Throughout, strategies for the characterisation of these high mass compounds are outlined.     

Novel tungsten calix[8]arene complexes

p-tert-Butylcalix[8]areneH8 reacts with WCl6 forming a number of new metallocalix[8]arenes, one of which is reduced to give the first example of a metal-metal triple bond supported by a large calixarene ligand; the degree of metallation and conformations adopted by the newly generated metallocalix[8]arene systems are determined by single crystal X-ray diffraction.     

A selective fluorescent probe for La and Y based on calix[6]arene

A new fluorescent probe based on calix[6]arene functionalized with three naphthoic acid groups was synthesized and showed selective fluorescence enhancement in the presence of La³⁺ and Y³⁺. In addition, the fluorescence enhancement behaviors depended on the pH values of the solution.     

A fluorescent probe for fluoride ion based on 2-aminopyridyl-bridged calix[6]arene

An 2-aminopyridyl-bridged calix[6]arene on the upper rim fixed in cone conformation (3) was successfully synthesized and its highly selective recognition towards fluoride ion was proved by fluorescent and ¹H NMR titration experiments.     

Open-tubular capillary electrochromatography using capillary columns chemically bonded with the new host molecules calix[6]crown,calix[6]arene

Two host molecules, p-tert-butylcalix[6]-1,4-crown-4 and p-tert-butylcalix[6]arene were prepared and attached onto the inner surfaces of capillaries for open-tubular electrochromatography with the aid of gamma-glycidoxypropyl-trimethoxysilane (KH-560) as bridging agent. The successful bonding was confirmed by infrared (IR) results and greatly decreased electroosmotic flow (EOF). Parameters affecting separation, such as buffer pH and organic modifier were studied. The two novel stationary phases were evaluated by the separation of isomeric toluidines, a mixture of pyridine and isomeric picolines, and isomeric dihydroxybenzenes; comparisons between capillaries coated with the two stationary phases and bare capillary were investigated. The special selectivity of these two novel stationary phases showed a certain extent of supramolecular interactions between stationary phases and solutes.     

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).     

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.     

Synthesis and Structure of Hexa(Diethoxyphosphoryloxy)calix[6]arene

Abstract

Hexapodand 1, which contains six phosphoryl groups at the lower rim of the macrocycle, was synthesized by the reaction of calix[6]arene with diethylchlorophosphate and sodium hydride. The structure is confirmed by X-ray studies: C₆₆H₉₀O₂₄P₆, Pbca, a = 14.867 (4), b = 18.577 (4), c = 26.510(6), D_c = 1.265g/cm³, Z = 4, and R = 0.063 for the 4600 observed reflections. The molecule 1 exists in a centrosymmetrical flattened 1,2,3-alternate conformation, in which diametrically opposed benzene rings are parallel. Four phosphoryl groups are oriented away from the cycle, two other groups are self-included in the macrocycle cavity.     

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.     

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.     

Hydrogen bonding nature between calix[6]arene and piperidine/triethylamine

We investigated the binding nature of the 1,2,3-alternate calix[6]arene with one piperidine, two piperidines, and two triethyl amines with a special emphasis on the hydrogen bonding networks by density functional theory calculations. The 1,2,3-alternate calix[6]arene strongly binds with piperidines and triethylamines at two different binding sites, exo and endo sites. In the two binding sites, the hydrogen bonding nature shows a characteristic difference. In the exo site, there formed only one hydrogen bond, while in the endo site, two hydrogen bonds except for the triethylamine. The proton transfer within the hydrogen bonding and the hydrogen bonding types, normal hydrogen bonding (NHB), short strong hydrogen bond (SSHB), and low barrier hydrogen bonding (LBHB), will be discussed in detail.