Arene Structure Research Articles

A study on the thermal behaviours of parent calix[4]arenes and some azocalix[4]arene derivatives

Thermal behaviours of two parents calix[4]arene ( 1– 2) and three azocalix[4]arene derivatives ( 3– 5) containing upper rim functionalized groups such as n-butyl, phenylazo and heterocyclic thiazol were investigated by means of thermogravimetry (TG), differential thermal analysis (DTA) and differential thermogravimetry (DTG). The thermal decomposition points and amounts of volatile pyrolysis products were determined in nitrogen and air atmosphere using TG, DTA and DTG curves. In the present study, the thermal analysis of azocalix[4]arenes demonstrated that the stability of the calix[4]arene depends on substituted groups and their position in the calix[4]arene structure.

Charge‐Transfer Effects on Arene Structure and Reactivity

AbstractFor Abstract see ChemInform Abstract in Full Text.

Bio-inspired Calix[6]Arene–Zinc Funnel Complexes

A bio-inspired supramolecular system is presented. A calix[6]arene possessing three imidazolyl arms on alternate phenolic positions binds a zinc ion. The resulting complex contains a hydrophobic pocket, which has a flattened conic shape. The system behaves as a selective molecular funnel for neutral guests that bind the metal centre. The exceptional stability of these tetrahedral dicationic complexes is exemplified by the acetaldehyde ternary adduct that was analysed by X-ray crystallography. The ligand is deeply buried in the heart of the calixarene cavity, pointing its methyl group selectively towards the centre of one of the aromatic walls, thereby establishing a stabilizing CH/π interaction. Protic guests undergo hydrogen bonding with the phenolic oxygens of the calixarene structure. The selectivity of the binding in the cavity is based on both the affinity of the donor atom of the guest ligand for the zinc ion and the relative host–guest geometries. The helical shape of the tris-imidazolyl groups binding the metal centre is the base of the chirality of the system. The twisted calix[6]arene structure of the zinc funnel complexes is shown to provide a new example of a cavity suitable for host–guest chiral induction.

ESI-FTICR investigation of triethylammonium ion-driven resorcin[4]arene dimer formation and structure.

In the course of mass spectrometric measurements a self-assembled hydrogen bonded resorcinarene dimer was observed, the formation of which was driven by the binding of triethylammonium ion as a guest and as an ionic label.

Calix[4]tubes: a new class of potassium-selective ionophore.

A new class of cryptand-like ionophore based on a bis calix[4]arene structure has been developed. These molecules proved highly selective for complexation of potassium over all group I metal cations and barium. A range of symmetric and asymmetric calix[4]tubes featuring either alkyl or phenyl substituents at the upper rim have been synthesized in exceptional yield. Alteration of the calix[4]arene upper rim environment enables fine-tuning of the rate of potassium uptake, which can be evaluated through the conformational change on binding by NMR studies. Selectivity of potassium complexation has been demonstrated using NMR and electrospray mass spectral techniques. Molecular modeling studies, derived from crystallographic data, confirm that the potassium metal cation is complexed via the axial route, passing through the calix[4]arene annulus, and provide evidence for complexation rate and selectivity.

Biomimetic copper(I)--CO complexes: a structural and dynamic study of a cali.

Four novel calix[6]arene-based cuprous complexes are described. They present a biomimetic tris(imidazole) coordination core associated with a hydrophobic cavity that wraps the apical binding site. Each differs from the other by the methyl or ethyl substituents present on the phenoxyl groups (OR1) and on the imidazole arms (NR2) of the calix[6]arene structure. In solution, stable CO complexes were obtained. We have investigated their geometrical and dynamic properties with respect to the steric demand. IR and NMR studies revealed that, in solution, these complexes adopted two distinct conformations. The preferred conformation was dictated only by the size of the OR1 group. When R1 was an ethyl group, the complex preferentially adopted a flattened C3-symmetrical structure. The corresponding helical enantiomers were in conformational equilibrium, which, however, was slow on the 1H NMR time scale at -80 degrees C. When R1 was a methyl group, the low-temperature NMR spectra revealed the partial inclusion of one tBu group. The complex wobbled between three dissymmetric but equivalent conformations. Hence, small differences in the steric demand of the calixarene's skeleton changed the geometry and dynamics of the system. Indeed, this supramolecular control was promoted by the strong conformational coupling between the metal center and the host structure. Interestingly, this was not only the result of a covalent preorganization, but also stemmed from weak interactions within the hydrophobic pocket. The vibrational spectra of the bound CO were revealed to be a sensitive gauge of this supramolecular behavior, similar to copper proteins in which allosteric effects are common.

Calix[6]arenes and Zinc: Biomimetic Receptors for Neutral Molecules

A novel supramolecular system that acts as a biomimetic receptor for neutral guests is presented. The functionalization of t-butyl-calix[6]arene (X6H6) at the lower rim in alternate position by methyl (Me) and 2-methylene-1-methyl-1H-imidazole (Imme) yielded the new ligand X6Me3Imme3. It provided a neutral tris(imidazole) coordination site associated with a conic cavity, thereby mimicking the active center of zinc proteins. Upon reaction with Zn(ClO4)2(H2O)6, an air-stable dicationic zinc−aqua complex [Zn(X6Me3Imme3)(H2O)](ClO4)2 was obtained. The highly acidic Zn2+ center was constrained in a tetrahedral environment with a labile site oriented toward the inside of the calix[6]arene structure. The hydrophobic pocket acted as a selective molecular funnel for neutral molecules. 1H NMR spectroscopy studies showed the easy exchange of the aqua ligand for amines, alcohols, amides, or nitriles. The selectivity of the exchange process is based on both the guest's binding ability to the metal center and its shape...

Thermodynamics of C−H Activation in Multiple Oxidation States: Comparison of Benzylic C−H Acidities and C−H Bond Dissociation Energies in the Isostructural 16−20-Electron Complexes [Fex(η5-C5R5)(η6-arene)]n,x= 0−IV, R = H or Me,n= −1 to +3

The pKa of the 18-electron complexes [Fe(η5-C5R5)(η6-C6Me6)][PF6] {1a[PF6] (R = H) and 1b[PF6] (R = Me)} and [Fe(η5-C5R5)(η6-C6H5CHPh2)][PF6] {1c[PF6]} have been determined by the direct method in DMSO using bases with known pKa values and found to be 12−14 pKa units lower than the pKa of the free arenes, illustrating the electron-withdrawing character of the CpFe+ and Cp*Fe+ groups. Access to the pKa values for 16-, 17-, 19-, and 20-electron iron-sandwich complexes of this type with various arene structures was available by means of thermodynamic diagrams using the standard redox potentials of the oxidation and reduction of the 18-electron cations 1+ and the deprotonated complexes 2. For instance, the pKa of the 19-electron iron complex 1a (43.5) is about the same as that of free C6Me6 (43−44) whereas that of 1b is even slightly higher (46.4). The pKas of the anionic 20-electron complexes 1a- and 1b- are 7 and 12 units, respectively, higher than that of C6Me6. The pKas of 12+ are around −10, whereas thos...

Open dimeric and capped polymeric container molecules: calixarene and resorcinarene complexes of AgI co-ordinated by participation of the upper-rim carbon atoms

The reactions of C-methylcalix[4]resorcinarene (L1), calix[6]arene (L2) and calix[4]arene (L3) with silver(I) salts have resulted in the isolation and structural characterization of three dimeric and one polymeric silver derivatives of calixarene based on cation π interactions. The dinuclear cation in [Ag2(L1)(C6H6)2][ClO4]2·5C4H8O 1 contains two symmetry-related metal centers, each involving a rather distorted tetrahedral co-ordination with, in addition to one benzene molecule, both the aryl OH groups and the aromatic rings of resorcinarene. The two dimeric calix[6]arene structures, {[Ag(L2)X]·2C6H6}2, where X = ClO4– 2 or CF3SO3– 3, display similar characteristics with two ligand groups inversely linked by the co-ordination of the silver ion to the phenyl carbons. The solvent molecules or the counter anions are not included in the cavity generated by the dimeric unit, but intercalated into calixarene or resorcinarene bilayers. In the calix[4]arene complex [Ag2(L3)(ClO4)2] 4 each of the four phenyl rings is bound to one metal atom which in turn bridges between the two calixarenes leading to an infinite two-dimensional arrangement. The polyoxo anion bridging between silver ions partially caps the container cavity of the calixarene. The factors controlling the selectivity of complexation and inclusion properties of calixarenes are discussed.

Tethered Bis-Amidinates as Supporting Ligands: A Concerted Elimination/σ-π Rearrangement Reaction Forming an Unusual Titanium Arene Complex.

A 1,4-cyclohexadiene dianion resonance structure makes a significant contribution to the structure of the (arene)Ti complex [LTi(η6 -PhCH3 )] (see picture on the right). Labeling experiments have demonstrated that the structure forms through an unusual elimination/σ-π rearrangement reaction in which [LTi(CH2 Ph)2 ] reacts readily with hydrogen; the use of the cyclohexane-linked bis-amidinate ligand L with constrained geometry makes the transformation possible.

Synthesis and structure of a chelating arene–ruthenium complex [RuCl 2(PPh 2(CH 2) 3- η6-C 6H 5)

The synthesis of an arene–phosphine ligand, Ph 2P(CH 2) 3Ph, has provided a route into a new chelating arene–phosphine–ruthenium complex, [RuCl 2(PPh 2(CH 2) 3- η 6-C 6H 5)] which has been structurally characterized. This complex can be made by thermolysis of [( η 6-MeC 6H 4CHMe 2)RuCl 2(PPh 2(CH 2) 3Ph)] or electrochemically utilizing the formation of the labile seventeen-electron ruthenium(III) species [( η 6-MeC 6H 4CHMe 2)RuCl 2(PPh 2(CH 2) 3Ph)] + which can be converted in good yield into [RuCl 2(PPh 2(CH 2) 3- η 6-C 6H 5)].

A doubly annelated triple calix[4]arene in the cone/1,2-alternate/cone conformation †

The doubly annelated triple calix[4]arene 2a, substituted by four nitro groups was synthesised by fragment condensation of the known exo-calix[4]arene 3 and the bisbromomethylated dimer of p-nitrophenol 4. Single crystals of its calcium salt [({[Ca·(DMSO)4·(H2O)2]2+(4DMSO)}(2a – 2H)2–) 2DMSO] were obtained from DMSO–water containing Ca(HCO3)2. Their X-ray analysis, the first crystallographic proof of the constitution of an annelated calixarene, shows both monoprotonated endo-calix[4]arene structures in the expected cone conformation, while the exo-calix[4]arene in the middle of the molecule is fixed in the 1,2-alternate conformation. The Ca2+ ion is completely solvated by four DMSO and two water molecules in a slightly distorted octahedral fashion. Each water binds two further DMSO molecules, one of which is included in the cavity of the endo-calix[4]arene parts, leading in this way to infinite chains of anions and cations. No direct contact between the Ca2+-ion and the phenolate oxygens or the aromatic π-electron systems of 2a is present. The pKa-values for the first deprotonation step of each endo-calix[4]arene part of 2a are similar to an analogous calix[4]arene 5 with two adjacent p-nitrophenol units, while in contrast to 5 no further deprotonation can be observed.

Optimization of phase-transfer catalysts designed from calix[4]arene

To optimize the calix[4]arene structure for a phase-transfer catalyst, we modified the upper rim with lipophilic tert-octyl groups and the lower rim with O(CH 2CH 2O) mMe (m=1, 2, and 3). Through two-phase solvent extraction, 1H NMR studies on the metal-binding site, and phase-transfer catalysis we have reached conclusions that (i) lipophilic groups introduced into the upper rim effectively enhance the catalytic activity whereas (ii) to compose an effective ionophoric cavity on the lower rim, OCH 2CH 2OMe ( i.e., m=1) suffices and m=2 and m=3 rather decrease the catalytic activity and cause emulsification. The optimized calix[4]arene-based phase-transfer catalyst showed the catalytic activity comparable with dicyclohexyl-18-crown-6.

Double partial cone conformation for Na8{calix[6]arene sulfonate}.cntdot.20.5H2O and its parent acid

The chemistry of calixarenes has recently become a very active area of endeavor. Of the numerous stimulating findings to appear thus far in the literature, one of the most intriguing concerns the discovery of certain uranophiles by Shinkai. It was reported that calix[5]arene sulfonate, calix[6]arene sulfonate, and the two corresponding derivatives substituted at the base by carboxymethoxy groups, 1 with R{prime} = CH{sub 2}COOH, exhibited stability constants for the uranyl ion of K = 10{sup 18.4-19.2}. Indeed, in competition experiments these calixarenes showed selectivity factors of 10{sup 12-17} for uranyl over the Ni{sup 2+}, Zn{sup 2+}, and Cu{sup 2+} ions. This selectivity was attributed to a moderately rigid calix[6]arene structure which was preorganized to match the rather unusual pseudoplanar hexacoordination needs of the UO{sub 2}{sup 2+} ion. However, on the basis of this study this premise appears untenable. Considering the importance of the above mentioned findings, the structural chemistry of derivatives of calix[6]arene has been slow to develop. In the available selection it is difficult to find general conformational features, apart from the elliptical cone conformation exhibited by the parent p-tert-butycalix[6]arene. Since our group has found a rich and often surprising inclusion chemistry for calix[4]arene sulfonates, we decided to investigatemore » the structure of calix[6]arene sulfonate and its alkali metal salts. 28 refs., 1 fig.« less

Effects of drying methods on the low temperature reactivity of Victorian brown coal to oxygen

The effects of air drying and thermal dewatering on the low temperature oxygen reactivity of Victorian brown coal have been investigated in the temperature range 35–55 °C and at 100 kPa oxygen pressure using coal samples ground to < 100 mesh. An attempt has also been made to relate the low temperature oxygen reactivity of the coal to its free radical concentration as measured prior to oxidation. Two rate models, the Schmidt and Winmill models, have been adapted to include the initial free radical concentration of the coal samples as the drying method sensitivity parameter in lieu of the concentration of oxygen-reactive sites in the coal material. The experimental results show that air drying, which reduces the free radical concentration of the coal, causes a decline in its oxygen reactivity whereas thermal dewatering, which causes an increase in the free radical concentration of the coal, enhances its oxygen reactivity. Air drying does not affect the distribution of the consumed oxygen in the oxidation products. A difference is observed in the case of the thermally dewatered coal samples. The correlation of the two rate models adopted is considered equally satisfactory. However, only the values obtained for the two activation energies in the Winmill model reflect the changes caused by thermal dewatering in the oxidation pattern of the coal. The activation energy values obtained from the two models are within the range of those quoted in the literature for the abstraction of hydrogen from various arene structures by free radicals.

Molecular design of calixarene-based ion-selective electrodes

In order to obtain insights into relationships between the calix[4]arene structure and the ion selectivity in the electrode system, 20 ionophoric calix[4]arenes were synthesized and their ion selectivity (with Na+ as a standard) estimated. Among these ionophoric calix[4]arenes, 25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]-p-t-octylcalix[4]arene afforded the highest logKNA,Mpot value (−3.1) in the presence of 2-fluorophenyl-2′-nitrophenylether (10) as the best of 13 plasticizers. This is the first example in which the Na+/K+ selectivity exceeds a factor of 103 in the electrode system based on the neutral carrier. The high Na+ selectivity is attributed to modification of the upper rim which ostensibly has no relation with the component of the cavity. This paper demonstrates the potential relationships between the unique structure of the calix[4]arene-based ligands and selectivity performance for the design of ion-selective electrodes.

Reaction of simple arenes with FSO3H.cntdot.SbF5/SO2: one-pot synthesis of aromatic sulfoxides. Mechanistic aspects and synthetic utility

In a simple one-pot reaction, mono-, di-, tri-, and polyalkylbenzenes, isomeric alkylhalobenzenes, and fluoro-, (trifluoromethyl)-, and 1,3,5-trifluorobenzene were converted to their corresponding diaryl sulfoxides with FSO 3 H•SbF 5 (1:1) (magic acid)/SO 2 . Dependency of the yields on the acidity (H 0 ) and the arene structure was demonstrated. The reduction is superacid-catalyzed, and protonated sulfoxides are the key intermediates en route to sulfides. Protonation of several functionalized diaryl sulfoxides was also studied in magic acid/SO 2 under stable ion conditions

Structural consequences of electron-transfer reactions. 19. NMR evidence for a planar arene intermediate in the electron-transfer-induced .eta.6 to .eta.4 hapticity change of a rhodium arene complex

Abstract : Hydrogen 1 NMR spectra have been obtained for the Rhodium (II) complex (eta-C6Me6)RhCp*)+, Cp* = 5-C5Me5, at 200 MHz, and 470 MHZ. A single resonance was observed for te arene CH3 protons between 298 K and 183 K in Methylene Chloride. The temperature dependence of the resonance was consistent with a doublet state formulation of the Rh(II) complex. At 298 K, the CH3 chemical shifts were 56 ppm for the arene substituents and 123 ppm for the cyclopentadienyl substituents. The magnetic equivalence of the methyl arene resonances is consistent with a planar arene structure in the complex. It is shown that a structural formulation including a highly fluxional bent arene is unlikely. The results demonstrate eta 4-arene hapticity, the one-electron intermediate can retain the planar arene structure, even if this results in a 19 e- metal.

Ab initio study on the molecular structure of the naphthalene metabolite, trans-1,2-dihydroxy-1,2-dihydronaphthalene.

The molecular geometries of three conformations of trans-1,2-dihydroxy-1,2-dihydronaphthalene have been refined by an ab initio gradient procedure at the 4-21G level to determine the effect of dihydrodiol conformation on arene structure. The preferred conformation is an equatorial form similar to the most stable conformation of ethylene glycol. All the structures investigated have similar arene geometries. The effect of the various conformations on metabolism of dihydrodiols to dihydrodiol epoxides is considered.

Photochemical reactions of arenecarbonitriles with aliphatic amines. 1. Effect of arene structure on aminyl vs. .alpha.-aminoalkyl radical formation

Photochemical reactions of arenecarbonitriles with aliphatic amines. 1. Effect of arene structure on aminyl vs. .alpha.-aminoalkyl radical formation