Cone Conformation In Solution Research Articles

Anion Binding by Fluorescent Ureido-Hexahomotrioxacalix[3]arene Receptors: An NMR, Absorption and Emission Spectroscopic Study.

Fluorescent receptors (4a–4c) based on (thio)ureido-functionalized hexahomotrioxacalix[3]arenes were synthesised and obtained in the partial cone conformation in solution. Naphthyl or pyrenyl fluorogenic units were introduced at the lower rim of the calixarene skeleton via a butyl spacer. The binding of biologically and environmentally relevant anions was studied with NMR, UV–vis absorption, and fluorescence titrations. Fluorescence of the pyrenyl receptor 4c displays both monomer and excimer fluorescence. The thermodynamics of complexation was determined in acetonitrile and was entropy-driven. Computational studies were also performed to bring further insight into the binding process. The data showed that association constants increase with the anion basicity, and AcO−, BzO− and F− were the best bound anions for all receptors. Pyrenylurea 4c is a slightly better receptor than naphthylurea 4a, and both are more efficient than naphthyl thiourea 4b. In addition, ureas 4a and 4c were also tested as ditopic receptors in the recognition of alkylammonium salts.

Dihomooxacalix[4]arene-Based Fluorescent Receptors for Anion and Organic Ion Pair Recognition.

Fluorescent dihomooxacalix[4]arene-based receptors 5a–5c, bearing two naphthyl(thio)ureido groups at the lower rim via a butyl spacer, were synthesised and obtained in the cone conformation in solution. The X-ray crystal structures of 1,3- (5a) and 3,4-dinaphthylurea (5b) derivatives are reported. Their binding properties towards several anions of different geometries were assessed by 1H-NMR, UV-Vis absorption and fluorescence titrations. Structural and energetic insights of the naphthylurea 5a and 5b complexes were also obtained using quantum mechanical calculations. The data showed that all receptors follow the same trend, the association constants increase with the anion basicity, and the strongest complexes were obtained with F−, followed by the oxoanions AcO− and BzO−. Proximal urea 5b is a better anion receptor compared to distal urea 5a, and both are more efficient than thiourea 5c. Compounds 5a and 5b were also investigated as heteroditopic receptors for biologically relevant alkylammonium salts, such as the neurotransmitter γ-aminobutyric acid (GABA·HCl) and the betaine deoxycarnitine·HCl. Chiral recognition towards the guest sec-butylamine·HCl was also tested, and a 5:2 selectivity for (R)-sec-BuNH3+·Cl− towards (P) or (M) enantiomers of the inherently chiral receptor 5a was shown. Based on DFT calculations, the complex [(S)-sec-BuNH3+·Cl−/(M)-5a] was indicated as the more stable.

Recognition of Anions, Monoamine Neurotransmitter and Trace Amine Hydrochlorides by Ureido‐Hexahomotrioxacalix[3]arene Ditopic Receptors

Hexahomotrioxacalix[3]arene‐based receptors (phenylurea 4a, phenylthiourea 4b and tert‐butylurea 4c) were synthesised and obtained in the partial cone conformation in solution. Phenylurea 4a was also obtained in the cone conformation and its X‐ray crystal structure is reported. Their binding properties towards several anions of different geometries were studied by 1H NMR titrations. The data showed that phenylurea cone 4a is the best anion receptor, displaying a very high affinity for the carboxylates AcO– and BzO– anions (log Kass = 4.12 and 4.00, respectively). Phenylurea partial cone 4a and phenylthiourea 4b, although weaker, are still reasonably good receptors, showing the same trend as cone 4a. tert‐Butylurea 4c exhibited very low affinity for all the anions, confirming that the association constants strongly depend on the nature of the substituent (aryl/alkyl) at the urea moiety. In addition, cone and partial cone 4a derivatives were also tested in the recognition of n‐alkylammonium salts and cone 4a also as a ditopic receptor for biogenic amine hydrochlorides by 1H NMR titrations. Cone 4a showed an outstanding affinity for the guest alkylammonium and amines, even at 298 K and in a more competitive solvent such as CDCl3/[D6]DMSO mixture. DFT calculations corroborated the NMR observations.

Ditopic Receptors Based on Dihomooxacalix[4]arenes Bearing Phenylurea Moieties With Electron-Withdrawing Groups for Anions and Organic Ion Pairs.

Two bidentate dihomooxacalix[4]arene receptors bearing phenylurea moieties substituted with electron-withdrawing groups at the lower rim via a butyl spacer (CF3-Phurea 5b and NO2 Phurea 5c) were obtained in the cone conformation in solution, as shown by NMR. The X-ray crystal structure of 5b is reported. The binding affinity of these receptors toward several relevant anions was investigated by 1H NMR, UV-Vis absorption in different solvents, and fluorescence titrations. Compounds 5b and 5c were also tested as ditopic receptors for organic ion pairs, namely monoamine neurotransmitters and trace amine hydrochlorides by 1H NMR studies. The data showed that both receptors follow the same trend and, in comparison with the unsubstituted phenylurea 5a, they exhibit a significant enhancement on their host-guest properties, owing to the increased acidity of their urea NH protons. NO2-Phurea 5c is the best anion receptor, displaying the strongest complexation for F−, closely followed by the oxoanions BzO−, AcO−, and HS. Concerning ion pair recognition, both ditopic receptors presented an outstanding efficiency for the amine hydrochlorides, mainly 5c, with association constants higher than 109 M−2 in the case of phenylethylamine and tyramine.

Anion Recognition by Partial Cone Dihomooxacalix[4]arene‐Based Receptors Bearing Urea Groups: Remarkable Affinity for Benzoate Ion

Tetraureido‐dihomooxacalix[4]arenes (tert‐butyl 4a and phenyl 4b) were synthesised and obtained in a partial cone conformation in solution and in the solid state, as confirmed by NMR and X‐ray diffraction studies. Their complexation ability towards halides, pseudo‐halides and oxoanions was assessed by 1H NMR and UV/Vis titrations. Structural and energetic insights of phenylurea 4b complexes were also obtained using molecular dynamics (MD) simulations. The binding data showed that the association constants are strongly dependent on the nature of the substituent (alkyl/aryl) at the urea unit. tert‐Butyl urea 4a is a much weaker receptor than phenylurea 4b, and showed association constants that decrease with decreasing of anion basicity. Phenylurea 4b is a highly efficient anion receptor, exhibiting a remarkable binding ability towards BzO– ion (log Kass = 4.81). In comparison to the phenylurea analogue bearing a butyl spacer and a cone conformation, receptor 4b containing a shorter spacer (three carbon atoms) and a partial cone conformation is more pre‐organized, displaying a strong enhancement of its binding efficiency. MD simulations have shown that the anions are preferentially bound to the urea moieties of the macrocycle lower rim, in agreement with the ROESY spectrum carried out with phenylurea 4b and BzO– anion.

Selective Binding of Spherical and Linear Anions by Tetraphenyl(thio)urea-Based Dihomooxacalix[4]arene Receptors.

Three novel tetra(thio)ureido dihomooxacalix[4]arene anion receptors (phenylurea 4a, phenylthiourea 4b, and tert-butylurea 4c) were synthesized and obtained in the cone conformation in solution, as shown by NMR studies. The X-ray crystal structure of 4c is reported. The host-guest properties of these receptors toward several anions were investigated by 1H NMR titrations. Phenylurea 4a displayed a very efficient binding toward the spherical F- and Cl- anions, and the linear CN- (log Kass = 3.46, 3.50, and 4.02, respectively). In comparison to related bidentate phenylurea dihomooxacalix[4]arenes, tetraphenylurea 4a is more preorganized and the higher number of hydrogen bond donor sites provides a remarkable enhancement of its binding efficiency.

Synthesis, binding properties and theoretical studies of p- tert-butylhexahomotrioxacalix[3]arene tri(adamantyl)ketone with alkali, alkaline earth, transition, heavy metal and lanthanide cations

p- tert-Butylhexahomotrioxacalix[3]arene tri(adamantyl)ketone ( 1b) was synthesized for the first time. Compound 1b was obtained in a cone conformation in solution at room temperature, as established by NMR spectroscopy ( 1H and 13C). The binding properties of ligand 1b for alkali, alkaline earth, transition, heavy metal and lanthanide cations have been assessed by phase transfer and proton NMR titration experiments. Molecular mechanics and ab initio techniques were also employed to complement the NMR data. The results are compared to those obtained with other closely related homooxacalixarene derivatives. Although triketone 1b is a weak extractant, it shows a strong peak selectivity for Na + and also some preference for Ag +. Proton NMR titrations indicate the formation of 1:1 complexes between 1b and the cations studied, and also that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. Although the molecular mechanics results show little correlation with the NMR data, a good agreement was obtained with the ab initio models.

Partially O-Alkylated Thiacalix[4]arenes: Synthesis, Molecular and Crystal Structures, Conformational Behavior

NMR spectroscopy, X-ray diffraction analysis, and quantum chemical calculations were used for conformational behavior study of partially alkylated thiacalix[4]arenes bearing methyl (1), ethyl (2), or propyl (3) groups at the lower rim. The conformational properties are governed by two basic effects: (i) stabilization by intramolecular hydrogen bonds, and (ii) sterical requirements of the alkoxy groups at the lower rim. While the monosubstituted derivatives 1a and 3a adopt the cone conformation in solution, distally disubstituted compounds 1b, 1'b, 2b, 2'b, 3b, and 3'b exhibit several interesting conformational features. They prefer pinched cone conformation in solution, and, except for 3'b, they form also 1,2-alternate conformation, which is flexible and undergoes rather fast transition between two identical structures. The crystal structures of the compounds 1b, 2b, 2'b, and 3b revealed yet quite rare 1,2-alternate conformation forming molecular channels held together by pi-pi interactions. Different channels-with hexagonal symmetry, 0.26 nm wide-are formed in the crystal structure of the pinched cone conformation of 3b. An uncommon hydrogen bonding pattern was found in dimethoxy and dipropoxy derivatives 1'b and 3'b that adopt distorted cone conformations in crystal. Trialkoxy-substituted compounds 1c and 3c adopt the partial cone conformation in solution. A higher mobility of methyl derivative 1c enables also existence of the cone conformer.

Exchange Coupling Mediated Through-Bonds and Through-Space in Conformationally Constrained Polyradical Scaffolds: Calix[4]arene Nitroxide Tetraradicals and Diradical

Calix[4]arenes constrained to the 1,3-alternate conformation and functionalized at the upper rim with four and two tert-butylnitroxides have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and (1)H NMR) spectroscopy, and magnetic studies. The 1,3-alternate nitroxide tetraradical and diradical provide unique polyradical scaffolds for dissection of the through-bond and through-space intramolecular exchange couplings. In addition, detailed magnetic studies of the previously reported calix[4]arene nitroxide tetraradical, which possesses cone conformation in solution, reveal conformational dependence of exchange coupling. Through-bond coupling between the adjacent nitroxide radicals is mediated by the nitroxide-m-phenylene-CH(2)-m-phenylene-nitroxide coupling pathway, and through-space coupling is found between the diagonal nitroxide radicals at the conformationally constrained N...N distance of 5-6 A. Magnetic studies of the calix[4]arene polyradical scaffolds in frozen solutions show that the through-bond exchange coupling in the 1,3-alternate calix[4]arene tetraradical is antiferromagnetic, while that in cone calix[4]arene tetraradical is ferromagnetic. The through-space exchange couplings are antiferromagnetic in both cone and 1,3-alternate calix[4]arene tetraradical, as well as in the 1,3-alternate calix[4]arene diradical. The exchange coupling constants (|J/k|) are of the order of 1 K.

Synthesis, NMR characterization and ion binding properties of 1,3-bridged p- tert-butyldihomooxacalix[4]crown-6 bearing pyridyl pendant groups

1,3-Di(2-pyridylmethoxy)- p- tert-butyldihomooxacalix[4]arene-crown-6 ( 2 ) was synthesized for the first time. 2 was isolated in a cone conformation in solution at room temperature, as established by NMR spectroscopy ( 1H, 13C and NOESY). Complete assignment of both proton and carbon NMR spectra was achieved by a combination of COSY, HSQC and HMBC experiments. The binding properties of ligand 2 towards alkali, alkaline earth, transition and heavy metal cations have been assessed by phase transfer and proton NMR titration experiments. The results are compared to those obtained with other dihomooxacalix[4]arene-crowns-6 and closely-related calix[4]arene-crown derivatives. 2 shows a preference for the soft heavy metal cations (except for Cd 2+), with a very strong affinity for Ag +. Some transition metal cations are also well extracted. 2 forms 1:1 complexes with K +, Ca 2+ and Ag +, and 1H NMR titrations indicate that they should be encapsulated into the cavity defined by the crown ether unit and by the two pyridyl pendant arms. A 1:2 (ML 2) complex is formed with Zn 2+ and two species, probably 1:1 and 1:2 complexes, are obtained with Pb 2+.

Monophosphanylcalix[6]arene Ligands: Synthesis Characterization, Complexation, and Their Use in Catalysis

AbstractNovel phosphanylcalix[6]arenes having mono‐O‐diphenylphosphanylmethyl (3) and mono‐O‐(4‐diphenylphosphanylphenyl)methyl substituents (5) have been synthesized. The structures of these monophosphanylcalix[6]arenes were determined by NMR spectroscopy, mass spectrometry, and X‐ray crystal structure analysis. The X‐ray structure reveals that 3 adopts a flattened 1,2,3‐alternate conformation in the crystalline state, while the NMR spectra show that 3 and 5 have a cone conformation in solution. Structure optimization and energy calculations for 3 and 5 at the B3LYP/LANL2DZ‐CONFLEX5/MMFF94s level of theory show that the cone conformation is slightly more stable than the 1,2,3‐alternate conformation by 0.36 kcal mol–1 for 3 and 0.96 kcal mol–1 for 5. Complexation of 3 with [PtCl2(COD)] and [Rh(COD)2]BF4 gives cis‐coordinated [PtCl2(3)2] and [Rh(COD)(3)2]BF4, respectively. The X‐ray analysis of [PtCl2(3)2] shows that 3 adopts a cone conformation upon complexation. Combination of 3 and 5 with [Rh(COD)2]BF4 provides an active catalyst for the hydroformylation of a variety of terminal alkenes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis, structures, and conformational characteristics of calixarene monoanions and dianions.

The synthesis, complete characterization, and solid state structural and solution conformation determination of calix[n]arenes (n = 4, 6, 8) is reported. A complete series of X-ray structures of the alkali metal salts of calix[4]arene (HC4) illustrate the great influence of the alkali metal ion on the solid state structure of calixanions (e.g., the Li salt of monoanionic HC4 is a monomer; the Na salt of monoanionic HC4 forms a dimer; and the K, Rb, and Cs salts exist in polymeric forms). Solution NMR spectra of alkali metal salts of monoanionic calix[4]arenes indicate that they have the cone conformation in solution. Variable-temperature NMR spectra of salts HC4.M (M = Li, Na, K, Rb, Cs) show that they possess similar coalescence temperatures, all higher than that of HC4. Due to steric hindrance from tert-butyl groups in the para position of p-tert-butylcalix[4]arene (Bu(t)C4), the alkali metal salts of monoanionic Bu(t)C4 exist in monomeric or dimeric form in the solid state. Calix[6]arene (HC6) and p-tert-butylcalix[6]arene (Bu(t)C6) were treated with a 2:1 molar ratio of M(2)CO(3) (M = K, Rb, Cs) or a 1:1 molar ratio of MOC(CH(3))(3) (M = Li, Na) to give calix[6]arene monoanions, but calix[6]arenes react in a 1:1 molar ratio with M(2)CO(3) (M = K, Rb, Cs) to afford calix[6]arene dianions. Calix[8]arene (HC8) and p-tert-butylcalix[8]arene (Bu(t)()C8) have similar reactivity. The alkali metal salts of monoanionic calix[6]arenes are more conformationally flexible than the alkali metal salts of dianionic calix[6]arenes, which has been shown by their solution NMR spectra. X-ray crystal structures of HC6.Li and HC6.Cs indicate that the size of the alkali metal has some influence on the conformation of calixanions; for example, HC6.Li has a cone-like conformation, and HC6.Cs has a 1,2,3-alternate conformation. The calix[6]arene dianions show roughly the same structural architecture, and the salts tend to form polymeric chains. For most calixarene salts cation-pi arene interactions were observed.

Synthesis and Chiral Recognition of a New Type of Chiral Calix[4]‐arene Derivatives

AbstractTwo new chiral calix[4]arenes bearing chiral pendants, which were from by‐product of the antibiotic industry, were synthesized and characterized by 1H NMR, MS‐FAB and elemental analysis. Studies of 1H NMR of the two calix[4] arene derivatives indicate that they exist in cone conformation in solution. Results of chiral recognition of the two chiral ligands 2a and 2b towards the tartaric add derivative 3 show that ligand 2a exhibited good chiral recognition abilities compared to ligand 2b.

Synthesis of (p‐Substituted phenyl)azo Calix[4]arenes

AbstractA novel method for the preparation of chromogenic calixarenes with azo groups was reported. p‐Substituted (‐ NO2, ‐ CH3, ‐ Cl) anilines were diazotized with isoamyl nitrite in EtONa/EtOH under refluxing condition. Fifteen mono‐, bis‐, tris‐ and tetrakis(p‐substituted phenyl)azo calix[4]arenes (including proximal and distal isomers) were obtained respectively by diazo‐coupling in different molar ratio to calix[4]arenes (1) under pH = 7.5–9.0 in non‐aqueous solution at 0‐5 °C. 1H NMR anduC NMR spectra of (α‐substituted phenyl)azo calix[4]‐arenes Indicated that they existed in cone conformation in solution.

Synthesis of novel chromogenic bi- and tri-functionalized calix[4]arenes

Novel azo-calix[4]arenes have been prepared by the incorporation of both ester and amide groups. Six bis(phenylazo)- O-substituted calix[4]arene derivatives were isolated and fully characterized. NMR analyses show that these compounds are in cone conformation in solution at room temperature.

The synthesis of a new type of calix[4]arene with chiral pendant

Two new chiral calixarenes were synthesized by condensing 1,3-bis-chlorocarbomethyl-calix[4] arenes with (1S, 2S)-2-amino-1-(4-nitrophenyl)-1,3-propanediol, a by-product from antibiotic industry, and were characterized by1H-NMR, MS-FAB, and elemental analysis. The1H-NMR spectra show that those molecules assume a cone conformation in solution.

Lower rim tetra-substituted and upper rim ferrocene amide calix[4]arenes: synthesis, conformation and anion-binding properties

Calix[4]arenes containing ferrocene amide at the upper rim and methoxy or ethoxycarbonylmethoxy (ethyl ester) groups at the lower rim ( 5a , 5b and 5c ) have been synthesized. It was found that tetramethoxy 5a and dimethoxy diethyl ester 5c were conformationally labile and existed in both cone and partial cone conformation in solution. Anion-binding studies by 1H NMR titration in CDCl 3 showed that 5b bound Cl − selectively with high stability. Compound 5c formed complexes with Cl − and H 2PO 4 − where the former was more stable. The ratio of cone to partial cone conformation in compound 5a was found to decrease upon binding Cl − and H 2PO 4 − and in a polar solvent.

Synthesis, NMR conformational analysis, complexation and transport studies of an inherently chiral dihomooxacalix[4]arene triester

The synthesis of the inherently chiral triethyl ester monomethyl ether of p-tert-butyldihomooxacalix[4]arene (3) is reported. A distorted cone conformation in solution at room temperature has been established for triester 3 by NMR measurements (1H, 13C, NOE 1D and 2D). The extracting and complexing properties of 3 towards the entire alkali and alkaline earth cation series have been assessed by picrate extraction and stability constant determinations. Transport experiments with metal picrates through a CH2Cl2 membrane have also been performed. Comparison is made with tetraethyl ester of p-tert-butylcalix[4]arene (4) described in literature and also studied by us. 3 is a poor phase transfer agent and binder. In contrast, it is a good carrier, mainly for the alkali cations and displays a selective transport for Ba2+.

Studies on Properties of Tetra‐p‐nitro‐tetra‐O‐alkylcalix[4]arenes

AbstractIbis paper reports the properties of the novel tetra‐p‐nitro‐tetra‐O‐alkyl‐calix[4]arenes (alkyl= n‐C4H9, 1; n‐C8H17 2; n‐C12H25, 3; n‐C16H33, 4). X‐ray crystallographic analysis and 1H NMR revealed that they exist as pinched‐cone conformation in crystal or cone conformation in solution. EFISH experiments at 1064 nm in CHCl3, indicated that tetra‐p‐nitro‐tetra‐O‐butyl‐calix[4]arene (1) has higher hyperpolarizability β, values than the corresponding reference compound p‐nitro‐phenyl butyl ether, without red shift of the charge transfer band. Compounds 2, 3 and 4 with longer alkyl chains can form monolayer at the air/water.

Synthesis, characterization, and C—N bond cleavage of tungsten(VI)arylimido-calix[4]arene complexes

The tetradentate, partially flattened cone tungsten(VI) imido-calix[4]arene derivative [W(NC6H5)(t-BucalixH)]+Cl–, (4), is formed in good yield from reaction of p-tert-butyl-calix[4]arene (t-BucalixH4), and W(NC6H5)Cl4. A reaction mechanism involving a putative, bidentate calixarene intermediate W(NC6H5)(t-BucalixH2)Cl2 (3), which partitions by reversible cleavage of the imido group to give the known tetradentate calixarene complex W(t-Bucalix)Cl2 (2), and elimination of HCl to form the thermodynamic imido product 4 is proposed. A series of iso structural [W(NAr)(t-BucalixH)]+Cl– (Ar = C6H5 or o,m,p-C6H4CH3) imido derivatives (4–7), which adopt a partially flattened cone conformation in solution has been prepared by reaction of dichloride 2 with H2NAr (Ar = C6H5, o,m,p-C6H4CH3). Reaction of W(t-Bucalix)Cl2 with aliphatic amines affords a red product formulated as the nitrido complex, 9, which results from facile cleavage of the imido C—N bond.Key words: calix[4]arene complex, imido complex, nitrido complex, C—N bond cleavage, conformation, metallocalixarene.