Partial Cone Conformation Research Articles

Calix[4]arene bisphosphite ligands bearing two distal 2,2′-biphenyldioxy or 2,2′-binaphthyldioxy moieties: conformational flexibility and allyl–palladium complexes

Achiral and chiral calix[4]arene bisphosphite ligands ( 2 and 3) bearing two distal 2,2′-biphenyldioxyphosphinoxy and 2,2′-binaphthyldioxyphosphinoxy moieties, respectively, have been synthesized. Each of these ligands exists in two pairs of interconverting conformations in solution. The partial cone conformer (A) of the (bis)biphenyldioxyphosphinoxy ligand 2 has been separated by fractional crystallization and its structure established by X-ray crystallography. The mechanism of interconversion of the pairs of conformers (A/B and C/D) has been probed by two-dimensional NMR spectroscopy. The 1H and 31P NMR evidence strongly supports a similar kind of exchange mechanism for ligand 3. Freezing of the cone conformer from the interconverting C/D pair of conformers of ligand 2 has been achieved by complexation with (allyl)palladium moieties. The methyl–allyl complex ( 2d) is moderately effective for catalytic regioselective allylic alkylation of crotyl acetate.

Ratiometric Sensing of Hg2+ Based on the Calix[4]arene of Partial Cone Conformation Possessing a Dansyl Moiety

A new fluorescent chemosensor based on the calix[4]arene of partial cone conformation possessing a dansyl moiety has been synthesized. The chemosensor demonstrates selective optical recognition of Hg(2+) and Cu(2+) in two contrasting modes. The receptor exhibited ratiometric sensing of Hg(2+) and "ON-OFF" type of fluorescence behavior in the presence of Cu(2+). The compound behaves as a fluorescent molecular switch upon chemical inputs of Hg(2+) and Cu(2+) ions.

Synthesis and Self-Assembly of Thio Derivatives of Calix[4]arene on Noble Metal Surfaces

Self-assembled monolayers (SAMs) provide a simple route to functionalize electrode surfaces with organic molecules. Herein we use cavity-containing derivatives of calix[4]arenes in SAMs. Bound to noble metal surface, the assembled molecules are candidates to serve as molecular sieves for H 2 molecules and H (+) ions, which could have relevance for fuel cell applications. Tetra- O-alkylated calix[4]arenes with thiolacetate and thiolamide wide-rim anchoring groups in cone and partial-cone conformations were designed, synthesized and self-assembled onto Au, Pt, and Pd surfaces. The resulting SAMs were systematically examined. Single crystal X-ray diffraction of 5,11,17,23-tetrakis(thioacetyl)-25,26,27,28-tetra- i-propoxycalix[4]arene confirmed the cone conformation and revealed the cavity dimensions of the SAMs that were formed by immersing noble metal substrates (Au, Pt and Pd deposited on Si-wafers) in solutions of calix[4]arenes. Surface characterization techniques including ellipsometry, cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) were used, indicating that the metal surface is terminated with a monomolecular layer. Experimental thicknesses obtained from the ellipsometry are consistent with the calculated values. CV results showed 50 to 80% physical passivation against the Fe(CN) 6 (3-/4-) couple, implying an overall relatively low concentration of defects and pinholes in the films. The binding energies of the S2p core level in the XPS were consistent with the literature values and revealed that up to 3.2 out of four anchoring groups were bonded to the noble metal surface.

Permittivity-Dependent Entropy Driven Complexation Ability of Cone and Paco Tetranitro-calix[4]arene toward para-Substituted Phenols

Considering the importance of the polarizability of the rings of calixarenes in the entropy-driven interaction processes, we examined the effect of entropy compensation on the complex formation of cone and partial cone (paco) conformers of tetranitro-calix [4]arene, possessing O-ethyl substituents at the lower rim. Both calixarene conformers were fully characterized including X-ray crystallography. Various para-substituted phenols were used as guest molecules. Photoluminescence (PL) measurements and quantum-chemical (QC) investigations were used. A permittivity dependence of the molecular interactions was obtained in different alcohols as solvents. It was found that the cone conformer of the title calixarene derivative forms stable complexes with all phenols of the p-substituted series. The free enthalpy changes show very high complex stability of cone calixarene with p-nitro and p-chloro-substituted phenols. In the cases of parent phenol, p-cresol and p- tBu-phenol, the stability is significantly lower; however, it slightly increases with the increasing electron density on the aromatic ring of guest molecules. Similarly, the entropy changes are significantly different for these two separated groups: the entropy changes obtained in the former cases are nearly the same, while large differences in the formation entropy were obtained in the latter cases. Both the experimental and theoretical investigations revealed that no considerable interaction exists between phenols and the paco conformer of the title calixarene. It is probably due to the locking of the calixarene cavity by the bent O-ethyl chain.

DFT Study for Cage-annulated p-tert-Butylcalix[4]crown-ether Complexed with Potassium Ion

Using DFT B3LYP/6-31+G(d,p)//B3LYP/6-31G(d,p) calculation method, stable molecular structures were optimized for the p-tert-butylcalix[4]arene functionalized at lower rim by cage-annulated crown ether (1) in two different conformers and their potassium-ion complexes. Cone conformer of free host 1 was slightly more stable than partial-cone conformer. For two different kinds of complexation mode, the potassium ion in benzene-rings (bz) pocket showed comparable complexation efficiency with the cation in cage-annulated crown-ether (cr) for the cone and partial-cone conformers of 1. The complex (1<TEX>${\bullet}K^+$</TEX>) in the cr-binding mode for the partial-cone conformer was more stable than the cone conformer for B3LYP/6-31G(d,p) geometry optimization. However, <TEX>$1_{(cone)}{\bullet}K^+$</TEX>(cr) showed lower single-point energy than the <TEX>$1_{(pc)}{\bullet}K^+$</TEX>(cr) for B3LYP/6- 31+G(d,p) calculation method.

Conformation-Selective Synthesis and Binding Properties of N-Benzylhexahomotriaza-p-chlorocalix[3]trinaphthylamide

N-benzylhexahomotriaza-p-chlorocalix[3]trinaphthylamide in partial cone conformation was selectively synthesized by appropriately controlling the steric effect during the amidation reactions of N-benzylhexahomotriaza-p-chlorocalix[3]tri(ethyl acetate) in cone or partial cone conformations with use of 1-aminomethylnaphthalene. The conformation was confirmed by (1)H, (13)C, and 2D NMR and X-ray single-crystal analysis. Analyses of the complexes revealed that recognition is strongly affected by Cd(2+), Pb(2+), and F(-) ions.

Synthesis, 1H-NMR conformational analysis and complexation studies of two di-imidazolyl acetamido p-tert-butylcalix[4]arenes

The synthesis of a new di-imidazolyl-di-methoxy acetamido p-tert-butylcalix[4]arene 4 is reported. 4 has been prepared by reacting the corresponding di-methyl ester di-methoxy derivative with histamine in 1:1 mixture of methanol: toluene. The binding properties of 4 towards alkali, alkaline earth, transition (Zn2+, Co2+) and heavy (Pb2+, Cd2+) metals have been investigated along with the complexes stoichiometries. The 1H-NMR spectra of complexes show the location of cations in receptor 4. Partial cone conformation is observed only with strontium and calcium whereas the cone conformation is detected with most of the cations. Comparison of the complexation results with those obtained for di-imidazolyl acetamido p-tert-butylcalix[4]arene 3 missing the methyl groups is also reported.

1,3-Alternate and partial cone conformers of tetramethyl (5,11,17,23-tetra-tert-butyl-2,8,14,20-tetrathiacalix[4]arene-25,26,27,28-tetrayltetraoxy)tetraacetate and tetramethyl (5,11,17,23-tetrabromo-2,8,14,20-tetrathiacalix[4]arene-25,26,27,28-tetrayltetraoxy)tetraacetate

The crystal structures of the two title thiacalix[4]arene derivatives, C52H64O12S4, (I), and C36H28Br4O12S4, (II), are reported. Compound (I) has crystallographic C2 symmetry and adopts a 1,3-alternate conformation where the four -OCH2CO2Me groups are located alternately above and below the virtual plane (R) defined by the four bridging S atoms. The dihedral angles between the plane (R) and the aromatic rings are 87.17 (7) and 87.60 (8) degrees . Compound (II) has a partial cone conformation in which the pendant -OCH2CO2Me group of the rotated aryl ring is oriented away from the thiacalixarene cavity formed by the other three aryl rings. The dihedral angles between the plane (R) and the aryl rings range from 17.47 (10) to 85.98 (6) degrees . In the supramolecular structure of (II), the molecular components are linked into a two-dimensional framework by a combination of C-H...O hydrogen bonds and C-Br...C interactions. This study demonstrates the usefulness of these motif-generating interactions and thiacalix[4]arene derivatives in crystal engineering.

New Fluorogenic Dansyl-Containing Calix[4]arene in the Partial Cone Conformation for Highly Sensitive and Selective Recognition of Lead(II)

A new efficient and highly selective fluorescent chemosensor for determination of Pb (2+) has been obtained by covalent attachment of two pendent proton-ionizable dansylcarboxamide groups to the calix[4]arene preorganized in the partial cone conformation. This geometry of the calixarene moiety was chosen on the basis of the prior (1)H NMR study of conformations adopted by the flexible dansyl-containing prototype upon complexation with lead ion. In acidic MeCN-H2O (1:1 v/v) solutions, the partial cone fluoroionophore allowed for detection of Pb (2+) at the levels as low as 2.5 ppb, which is totally compatible with the regulations of the U.S. Environmental Protection Agency and the World Health Organization on the limiting content of this hazardous pollutant in drinking water.

Partial cone conformer of 25,27-bis[(methoxycarbonyl)methoxy]-26,28-dipropoxycalix[4]arene

Mol­ecules of the title compound, C40H44O8, adopt a partial cone conformation. The dihedral angles between the planes of the aromatic rings and the mean plane through the methyl­ene C atoms bridging the aromatic rings are 35.74 (7), 85.86 (5), 87.77 (4) and 89.95 (5)°. Two opposite aryl rings are approximately parallel to each other; the others are at an inter­planar angle of 52.41 (6)°. Intra- and inter­molecular C—H⋯O hydrogen bonds stabilize the mol­ecular conformation and the crystal packing. Two C atoms of one propoxy chain are disordered over two positions; the site occupancy factors are ca 0.66 and 0.34.

5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene dichloromethane hemisolvate

In the title compound, C48H64O4·0.5CH2Cl2, both crystallographically independent calixarene mol­ecules display a partial cone conformation. Their crystal packing is stabilized by C—H⋯π contacts involving the meth­oxy groups. The solvent mol­ecule is located inter­stitially between two calixarene units with C—H⋯Cl contacts to meth­oxy and tert-butyl groups. One tert-butyl residue of each calixarene mol­ecule is disordered over two positions (occupancies 0.60/0.40 and 0.63/0.37), resulting in bond distances that deviate from ideal values. The tetra­mer calixarene mol­ecules present models with approximate non-crystallographic Cs symmetry.

Supramolecular chain-like aggregates and polymeric sandwich complexes constructed from p-sulfonatocalix[4,6]arenes with (8-hydroxy)quinoline guests

Three crystalline complexes were prepared by the inclusion complexation of p-sulfonatocalix[4]arene with quinoline (1), p-sulfonatocalix[6]arene with quinoline (3) and 8-hydroxyquinoline (4), respectively. These crystals were compared to the reported p-sulfonatocalix[4]arene complex with 8-hydroxyquinoline (2). The results obtained show that p-sulfonatocalix[4]arene forms 1 : 1 included complexes for (8-hydroxy)quinoline guests with the pinched-cone conformation, whereas p-sulfonatocalix[6]arene forms 1 : 2 complexes with the centrosymmetric ‘up–down’ double partial cone conformation. Furthermore, the aggregation structures of complexes 1–4 are diverse from the common bilayer array for 2 to the chain-like aggregation for 1 and to the polymeric sandwich complexes for 3 and 4. These observations are carefully discussed from the viewpoints of host conformation, host–guest binding structure and stoichiometry, and the effect of counterion.

Synthesis and Structure of the Potassium Salt with Monodeprotonated 1,2,3-Tri(ethoxycarbonylpropyl)-p-t-butylcalix[6]arene

Alkylation of p-t-butylcalix[6]arene with ethyl 4-bromobutanoate in the presence of K2CO3 results in a potassium salt of the monodeprotonated 1,2,3-tri(ethoxycarbonylpropyl)-p-t-butylcalix[6]arene in high yield. X-Ray crystallographic analysis shows that this compound exists as a double partial cone conformation. The potassium ion is positioned near the centre of the cavity of the macrocycle and binds to four phenolic oxygens. Close contacts between the potassium ion and phenyl rings observed in this compound are indicative of the presence of the cation–π interaction. The molecules are connected via intermolecular K+···O interactions into infinite chains.

The X-Ray Crystal Structures of p-Sulfonatothiacalix[6]arene Inclusion Complexes with Lanthanide Metal Ions (M=Gd,Ho and Er) and Water Molecules

Three crystal structures of p-sulfonatothiacalix [6] arene (1) inclusion complexes with octa-aqua lanthanide metal ions (M=Gd3+, Ho3+, and Er3+) have been determined by single crystal X-ray diffraction studies. Three structures show a same conformation as “up-down” double partial cone conformation. The aquated lanthanide metal ions were captured in the “up-down” cavities of thiacalixarene. In the overall their crystal structures, supramolecular assemblies are formed by hydrogen bonds and π-π staking interactions.

Synthesis and conformational behaviour of lower-rim tetraacetylated thiacalix[4]arenes

The acylation of thiacalix[4]arenes with AcCl or Ac 2O gave the corresponding lower-rim tetraacetoxy derivatives. In contrast to classical calix[4]arenes, tetraacetylated thiacalix[4]arenes are conformationally mobile in solution and represent a thermodynamic equilibrium of three different conformers at room temperature. As proven by a dynamic 1H NMR study, conformational preferences of acetylated thiacalix[4]arenes considerably depend on the upper-rim substitution. Hence, t-Bu thiacalixarene prefers 1, 3- alternate and 1, 2- alternate conformations (43% and 38%, respectively), while the upper-rim unsubstituted compound adopts preferably the partial cone conformation (70%).

Conformational Preferences of Bispicolyl-p-tert-butylcalix[4]arene Anions: Synthesis of Cone and Partial-cone Conformers of Bispicolyl-bis-(tert-butoxycarbonyl)methoxy-p-tert-butylcalix[4]arene

The disubstituted calix[4]arene derivative 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-26,28-bis(2-pyridylmethoxy)calix[4]arene 2, which crystallises in the monoclinic space group P2 1 /n, was functionalised in the phenolic O–H groups with (tert-butoxycarbonyl)methoxy groups. The yield of the isolated cone and partial-cone conformers of tetrasubstituted 5,11,17,23-tetra-tert-butyl-25,27-bis[(tert-butoxycarbonyl)methoxy]-26,28-bis(2-pyridylmethoxy)calix[4]arene 3a and 3b, depends on the identity of the alkali metal hydride employed to deprotonate 2. Partial-cone conformer 3b crystallises in the monoclinic space group P2 1 /n, with an inverted (tert-butoxycarbonyl)methoxy-containing phenol moiety. The product distribution of 3a and 3b was interpreted based on the relative stabilities of the alkali metal complexes with the doubly deprotonated derivative of 2, and the monoanionic derivative [(tert-butoxycarbonyl)methoxy-2]− , by molecular mechanics analysis.

5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene tetrahydrofuran solvate

The asymmetric unit of the title compound, C44H64O4·C4H8O, comprises two crystallographically independent calixarene mol­ecules, which display a partial cone conformation, and two tetrahydrofuran molecules. The crystal packing is stabilized by C—H⋯π contacts involving the meth­oxy groups, while the solvent mol­ecules are located in voids between the calixarene molecules. Two of the tert-butyl residues of each calixarene mol­ecule are disordered over two positions [0.74/0.26 (ring B), 0.71/0.29 (ring C), 0.77/0.23 (ring C′), 0.67/0.33 (ring D′)], resulting in bond distances that deviate from ideal values.

Aminolysis of p- tert-butyltetrathiacalix[4]arene tetraethylacetates in cone, partial cone and 1,3-alternate conformation: synthesis of amide based receptors for oxyanions

Cone, partial cone and 1,3-alternate conformers of tetrathiacalix[4]arene tetraethylacetate were synthesized and subjected to aminolysis with alkyl amines [CH 3(CH 2) n NH 2; n=2, 3, 5] to yield mono-, di-, tri- or tetrasubstituted p- tert-butyltetrathiacalix[4]arene amides which were characterized by detailed analysis of their NMR spectral and single crystal X-ray crystallography. It has been observed that while the 1,3-alternate and cone conformers of the tetrathiacalix[4]arene tetraethylacetate gave corresponding tetrathiacalix[4]arene tetraamides under different experimental conditions, the corresponding partial cone conformer undergoes a cascade of regioselective reactions with the same amines. Variable temperature 1H NMR experiments allowed the determination of relative stability of different conformers within the temperature range of 298–333 K. The synthesized derivatives were evaluated as molecular extractants for cations and anions and were determined to facilitate extraction of oxyanions (CrO 4 2− and Cr 2O 7 2−) from aqueous to the organic phase. The studies have a significance in the design of tetrathiacalix[ n]arene based molecular receptors for innovative applications.

Conformational Behaviors of Tetra-O-methylsulfinylcalix[4]arenesAn Approach to Control the Conformation of Thiacalix[4]arenes by Oxidizing Sulfur Bridges

The conformational behaviors of all four stereoisomers [5(rctt), 5(rcct), 5(rtct), and 5(rccc)] of tetra-O-methylsulfinylcalix[4]arene were studied by the 1H NMR spectroscopic method. Variable-temperature (VT) NMR experiments of 5(rctt), 5(rcct), and 5(rtct) revealed that each compound adopted the same conformation as that in the crystals at low temperatures and exhibited a self-exchange between the two equivalent species of this conformation at elevated temperatures. The values of the activation enthalpy DeltaH for the self-exchange were similar (approximately 70 kJ mol-1). Further, the activation entropy DeltaS++ was more important for 5(rtct) (-40 J mol-1 K-1) than for 5(rctt) (-5 J mol-1 K-1) and 5(rcct) (-7 J mol-1 K-1); consequently, the exchange rate of 5(rtct) was 150-180 times less than that of the other isomers at 273 K. On the other hand, 5(rccc) was in an equilibrium state between cone and partial-cone conformers at 253 K with the molar ratio being 85:15, which was in reasonable agreement with the relative stability between the two conformers calculated by the ab initio molecular orbital method.

Calix[4]arenes with Siloxanes Bridging Opposite Rings

Several C-silylated (allyldimethylsilyl) calixarenes (1, 14, and 18) were treated with commercial TBAF in THF, giving the novel calixarenes 2, 15, and 19, in which the opposite rings were bridged by a Si-O-Si siloxane group. Compound 19 and the derived phenol 20 (as well as the dibromocalixarene precursor 17) were mixtures of cone and partial cone conformations according to NMR, while 2 and 15 (as well as 14 and the cone components of 17, 19, and 20) were in flattened cone conformations.