Analytical applications of nano-baskets of calix[4]pyrroles

Calix[4]pyrrole is one such class which holds a great promise in the fields of sensors and their unique behavior as sensors owes to its structural flexibility. Anion binding ability of calix[4]pyrrole has been modified in a variety of ways. Introduction of electron releasing and electron withdrawing groups at the meso position or at β-pyrrolic positions leads to calix[4]pyrrole with deep cavities and fixed walls which shows increased selectivity and modified binding effects. Strapping of calix[4]pyrrole is another way to modify its structural behavior which is responsible for its binding behavior. Choice of strap could play a profound role not only in increasing the intrinsic anion binding affinity of calix[4] pyrrole, but also in modulating the receptor anion stoichiometry, thereby modifying potentially the inherent anion binding selectivity. Calix[n]pyrroles with extended cavities have also been synthesized. Such as calix[3]bipyrrole binds bromide substantially with high affinity than calix[4]pyrrole. Calix[4]pyrrole has also been used to produce anion sensors that can report the presence of anion by means of a color change. The medium effect on the complexation of calix[4]pyrrole and anion has been investigated in various solvents. Calix[4]pyrrole has also been used to increase the ionic conductivity of solid polymer electrolyte by anion complexation of the metal salt. Calix[4]pyrrole has been used to obtain optical sensors using surface plasmon resonance technique. Composite films of cellulose acetate containing calix[4]pyrrole has also been reported which has potential usage in packaging, storage and preservation. In nut shell, calix[4]pyrrole can be modified in a variety of ways to form versatile sensors which can be used in variety of ways in various areas.     

Separation of Phenolic Compounds and Organic Anions Using ODS Column Coated with Meso-octamethylcalix[4]pyrrole and Water as Eluent

Octadecylsilanized silica (ODS) was coated with meso-octarnethylcalix[4]pyrrole to obtain a novel calix[4]pyrrole containing stationary phase for HPLC. Compared to ODS, the new stationary phase showed a relatively large retention and an improved separation for phenolic compounds and organic anions, using pure water as mobile phase. The results can be ascribed to the interaction between analytes and calix[4]pyrrole.     

Synthesis and Properties of Functionalized Schiff Bases of 5α,10α-Di（4-hydroxylphenyl）calix[4]pyrrole and 5α,15β-Di（4-hydroxylphenyl）calix｜4] pyrrole

A series of calix[4]pyrrole meso-substituted Schiff bases was synthesized with 5α,10α-di（4- hydroxylphenyl）calix[4]pyrrole or 5α,15β-di（4-hydroxylphenyl）calix[4]pyrrole as starting materials. The synthetic routes included alkylation with methyl a-chlroroaceate, ammonolysis with alkylene diamine, and condensation with salieylladehyde or 2-hydroxynaphthaldehyde. The crystal structures of the new calix[4]pyrroles and their Schiff bases were determined by X-ray diffraction. The coordination properties of the representative ealix[4]pyrrole Sehiff bases to transition metal ions were also investigated by UV-Vis spectra.     

Strain-Induced Ring Expansion Reactions of Calix[3]pyrrole-Related Macrocycles

The recent discovery of calix[3]pyrrole, a porphyrinogen-like tripyrrolic macrocycle, has provided an unprecedented strain-induced ring expansion reaction into calix[6]pyrrole. Here, we synthesized calix[n]furan[3-n]pyrrole (n=1∼3) macrocycles to investigate the reaction scope and mechanism of the ring expansion. Single crystal X-ray analysis and theoretical calculations revealed that macrocyclic ring strain increases as the number of inner NH sites increases. While calix[1]furan[2]pyrrole exhibited almost quantitative conversion into calix[2]furan[4]pyrrole within 5 minutes, less-strained calix[2]furan[1]pyrrole and calix[3]furan were inert. However, N-methylation of calix[2]furan[1]pyrrole induced a ring-expansion reaction that enabled the isolation of a linear reaction intermediate. The mechanism analysis revealed that the ring expansion consists of regioselective ring cleavage and subsequent cyclodimerization. This reaction was further utilized for synthesis of calix[6]-type macrocycles.     

A facial microwave-assisted synthesis, spectroscopic characterization and preliminary complexation studies of calix[4]pyrroles containing the hydroxamic-acid moiety

The synthesis, spectroscopic characterization and preliminary complexation properties of functionalized calix[4]pyrroles are described. To date, two generalized preparative approaches have been pursued (i) modifying the basic pyrrole-plus-ketone synthesis of calix[4]pyrrole by using microwave irradiation protocol, (ii) the basic meso-tetra(methyl) meso-tetra(p-nitrophenyl) calix[4]pyrrole skeleton was functionalized to give hydroxamic acids, especially in the meso-position of the macrocycles. The structures of novel calix[4]pyrrole hydroxamic acid derivatives were confirmed on the basis of various physico-chemical techniques such as elemental analysis, FT-IR, ¹H NMR and FAB-Mass. The results of preliminary studies on the extraction of vanadium (V) with the host calix[4]pyrrole hydroxamic acids were elucidated by significant examination of UV–Vis spectroscopy and ICP-AES. Single crystal structure of basic meso-tetra(methyl) meso-tetra (p-nitro phenyl) calix[4]pyrrole moiety has also been reported.     

Tetra‐proline‐modified calix[4]arene‐bonded silica stationary phase for simultaneous reversed‐phase/hydrophilic interaction mixed‐mode chromatography

A new water‐soluble tetra‐proline‐modified calix[4]arene‐bonded silica stationary phase was prepared straightforwardly by an indirect method and characterized by elemental analysis, energy dispersive Spectrometry, solid‐state ¹³C NMR spectroscopy, Fourier‐transform infrared spectroscopy, and thermogravimetric analysis. Due to the simultaneous introduction of polar tetra‐proline and nonpolar calix[4]arene, the developed column possessing a double retention mode of reverse‐phase liquid chromatography and hydrophilic interaction liquid chromatography. A series of hydrophobic and hydrophilic test samples, including nucleosides and nucleotides, amines, monosubstituted benzenes, chiral compounds, and phenols, were used to evaluate the developed stationary phase. A rapid separation capability, high separation efficiency, and selectivity were achieved based on the multiple interactions between solutes and tetra‐proline‐modified calix[4]arene‐bonded silica stationary phase. Moreover, the developed stationary phase was further used to detect and separate hexamethylenetetramine in rice flour. All the results indicated the potential merits of the developed stationary phase for simultaneous separation of complex hydrophobic and hydrophilic samples with high selectivity.     

Transport of solutes through calix[4]pyrrole-containing cellulose acetate films

Films of cellulose acetate containing different concentrations of meso-octamethyl-porphyrinogen (calix[4]pyrrole) were prepared and characterized using UV-vis and FTIR spectroscopy, DSC and SEM. Incorporation of calix[4]pyrrole into cellulose acetate leads to a decrease in the degree of hydrophilicity of the polymeric matrix. However, a slight increase in the percentage of water uptake of the polymer is found with an increase of the initial amount of calix[4]pyrrole in the composite composition. This effect can be related to the plasticizing effect that the calix[4]pyrrole provokes in the cellulose acetate. A comparative study of transport parameters of oxygen and some non-associated electrolytes (sodium, copper(II) and nickel chlorides) was carried out. The diffusion coefficients of molecular oxygen through cellulose acetate films decrease with an increase of the concentration of calix[4]pyrrole in the composite films. The transport (diffusion and permeation) of the above mentioned electrolytes through a set of composite films shows a decrease of permeability and diffusion coefficients with an increase of calix[4]pyrrole concentration. Such behaviour is typical of systems where interactions between the polymer and diffusing species occur. However, from the analysis of the distribution coefficient, it was found that those interactions are only dependent on the calix[4]pyrrole content for 2:1 electrolytes.     

Cation- and anion-assisted binding of triethylenediamine by zinc bisporphyrinates

The complexation of zinc calix[4]arene or calix[4]pyrrole bisporphyrinates with alkali metal cations, halide anions, and triethylenediamine was studied by ¹H NMR spectroscopy. It was established that the binding of molecules and/or charged particles by various fragments of calix[4]arene and calix[4]pyrrole porphyrins are interrelated processes. This makes it possible to use one process (for example, complexation of the calix[4]arene fragment of the macrocycle with alkali metal cations or complexation of the calix[4]pyrrole fragment with halide ions) as a tool for controlling another process (complexation of the porphyrin fragments of the macrocycle with neutral molecules).     

Tri- and pentacalix[4]pyrroles: synthesis, characterization and their use in the extraction of halide salts

Calixpyrrole-based oligomeric compounds were synthesized by "click chemistry" from the corresponding alkyne- and azide-functionalized calix[4]pyrroles. Calix[4]pyrrole 3, possessing an alkyne functional group, was prepared through a mixed condensation of pyrrole with acetone and but-3-ynyl 4-oxopentanoate. Another alkyne-group-containing calix[4]pyrrole 5 was obtained by treatment of 4'-hydroxyphenyl-functionalized calixpyrrole 4 with propargyl bromide. Tetrakis(azidopentyl)-functionalized calix[4]pyrrole 7 was synthesized by reacting NaN(3) with tetrabromopentyltetraethylcalix[4]pyrrole 6, which was prepared through a condensation reaction of pyrrole and 7-bromohept-2-one. Oligomeric calixpyrrole compounds were found to be capable of extracting tetrabutylammonium chloride and fluoride salts from aqueous media. Extraction abilities of the oligomeric compounds were monitored by NMR and UV/Vis spectroscopy and thermogravimetric analysis.     

Meso-diacylated calix[4]pyrrole: structural diversities and enhanced binding towards dihydrogenphosphate ion

Meso-diacylated calix[4]pyrrole was obtained via acid catalysed condensation of meso-acylated dipyrromethane with acetone. Selective presence of flexible substituents at the calix[4]pyrrole periphery led to interesting structural motifs in the solid state along with enhanced binding towards anions, especially dihydrogenphosphate ion via anchoring.     

杯[4]吡咯-卤素、铵离子复合物的理论研究

采用密度泛函理论的M06-2X/6-31G(d, p)方法对杯[4]吡咯(CP)与卤素离子(X^-=F^-, Cl^-, Br^-)及卤素-铵根离子对的各种可能组装体系进行了系统研究. 详细讨论了各体系的结构、结合能、自然键轨道分析(NBO)和Multiwfn波函数分析的情况. 结果显示杯[4]吡咯与卤素阴离子的相互作用主要是氢键, 波函数分析显示在CPCl-和CP-Br-复合物中长程范德华力和空间位阻作用也明显存在. 杯[4]吡咯能与卤素-铵根离子形成稳定的复合物, 主要通过氢键作用、阴-阳离子的静电作用以及阳离子-π相互作用.从理论上探讨了杯[4]吡咯与离子或离子对的2:1组装体系,但相对于1:1组装体系来讲, 2:1体系并不占优势.本文结果进一步表明, 杯[4]吡咯不仅是一种阴离子受体,而且也是一种良好的离子对受体,尤其是对涉及氟离子的客体,更是如此.     

Anion Complexation by Meso-Octamethylcalix[4]pyrrole in Aqueous Solution

The ligand calix[4]pyrrole is quite insoluble in aqueous solution. In the presence of NaCl, KCl, CsCl, NH₄Cl, CaCl₂, and CdCl₂, the solubility of calix[4]pyrrole linearly increases with the salt concentration. The ligand calix[4]pyrrole forms stable complexes with the chloride anion. All other halogen anions do not form complexes. The measurement of the amount of dissolved ligand allows the determination of the complex stabilities in aqueous solution. This experimental method is discussed in detail due to its general suitability to study the complexation reactions with nearly insoluble ligands.     

Anion binding studies of fluorinated expanded calixpyrroles

The anion binding properties of fluorinated calix[n]pyrroles (n = 4-6) in aprotic solvents (acetonitrile and DMSO) and modified reaction conditions allowing for the synthesis and isolation of the hitherto missing dodecafluorocalix[6]pyrrole from the condensation of 3,4-difluoro-1H-pyrrole and acetone are described. In acetonitrile solution containing 2% water, the association constants for the 1:1 binding interaction between octafluorocalix[4]pyrrole and chloride anion obtained with isothermal titration calorimetry (ITC) and (1)H NMR titration methods were found to match reasonably well. As compared to its nonfluorinated congener, octafluorocalix[4]pyrrole was found to display enhanced binding affinities for several representative anions in pure acetonitrile as judged from ITC analyses. Similar analyses of the fluorinated calix[n]pyrroles revealed an increase in the relative affinity for bromide over chloride with increasing macrocycle size, as manifest in a decrease in the binding ratio K(a(Cl))/K(a(Br)). Anion binding studies in the solid state, involving single-crystal X-ray diffraction analyses of the chloride and acetate anion complexes of octafluorocalix[4]pyrrole and decafluorocalix[5]pyrrole, respectively, confirmed the expected hydrogen bond interactions between the pyrrolic NH protons and the bound anions.     

Missing-link macrocycles: hybrid heterocalixarene analogues formed from several different building blocks

The synthesis and structural characterization of hybrid heterocalix[4]arene analogues containing pyrrole, benzene, methoxy-substituted benzene, and pyridine subunits is described. Macrocycles 1 and 2, examples of calix[2]benzene[2]pyrrole and calix[1]benzene[3]pyrrole systems, respectively, are synthesized by the condensation of pyrrole and an appropriate phenylbis(carbinol). Macrocycles 3 and 7, examples of calix[2]benzene[1]pyridine[1]pyrrole and calix[1]pyridine[3]pyrrole, respectively, are synthesized by the use of a carbene-based pyrrole-to-pyridine ring-expansion procedure. Single-crystal X-ray analysis reveals that compounds 1a, 1b, and 2b adopt 1,3-alternate conformations in the solid state, whereas compounds 3 and 7 display structures that are best described as "flattened partial cones" in terms of their conformation. (Series a refers to pure benzene-derived systems, whereas series b indicates macrocycles containing 5-methoxyphenyl subunits). In the solid state, the methoxy-functionalized macrocycles 1b and 2b, and the chloropyridine-containing macrocycle 7 exist as dimers. In the case of 1a and 7, these compounds interact with neutral solvent in the solid state. The conformations of the macrocycles in solution were explored by temperature-dependent proton NMR and NOESY spectral analysis. At 188 K, macrocycles 1a and 2a adopt flattened 1,3-alternate conformations, whereas macrocycles 3 and 7 exist in the form of flattened partial-cone conformations. Standard proton NMR titration analyses were carried out in the case of macrocycles 1a and 2a, and reveal that at least the second of these systems is capable of binding fluoride and chloride anions in CD(2)Cl(2) solution at room temperature (K(a)=571 and 17M(-1) in the case of 2a and F(-) and Cl(-), respectively).     

杯[4]吡咯在不同溶液中的分子动力学模拟研究

运用分子动力学(MD)模拟方法对杯[4]吡咯与不同溶剂之间的相互作用能、杯[4]吡咯在不同溶液中的构象变化以及杯[4]吡咯与溶剂分子之间的氢键相互作用进行了计算研究.模拟发现,杯[4]吡咯与不同溶剂间的相互作用能受溶剂分子偶极矩和杯[4]吡咯-溶剂分子间氢键相互作用影响.杯[4]吡咯在不同溶液中的构象发生翻转的主导因素是杯[4]吡咯与溶剂分子间形成氢键相互作用,溶剂分子的偶极矩不是杯[4]吡咯发生构象转化的主要因素.     

Theoretical study on nonlinear optical properties of the Li+[calix[4]pyrrole]Li−dimer,trimer and its polymer with diffuse excess electrons

The static (hyper)polarizabilities of the dimer and trimer with diffuse excess electrons, [Li⁺[calix[4]pyrrole]Li^?]_n, are firstly investigated by the DFT(B3LYP) method in detail. For the dimer and trimer, a Li atom inside each calix[4]pyrrole unit is ionized to form a diffuse excess electron. The results show that the dimer and trimer containing two and three excess electrons, respectively, have very large first hyperpolarizablities as 2.3 × 10⁴ and 4.0 × 10⁴ au, which are 30 and 40 times larger than that of the corresponding [calix[4]pyrrole]_n (n = 2, 3) without Li atom. Also, β values of dimer and trimer are twice and four times as large as that of monomer containing one excess electron. Obviously, not only excess electron but also the number of excess electron plays an important role in increasing the first hyperpolarizability. Moreover, the (hyper)polarizabilities of the [Li⁺[calix[4]pyrrole]Li^?]_n polymer are investigated at ab initio level by using the elongation finite‐field (elongation FF) method. All the oligomers of the [Li⁺[calix[4]pyrrole]Li^?]_n with many excess electrons exhibit very large first hyperpolarizability and large second hyperpolarizability. The present investigation shows that by introducing several and more excess electrons into the nonlinear optical (NLO) materials will be an important strategy for improving their NLO properties, which will be helpful for design of NLO materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Synthesis and characterisation of a calix[4]pyrrole functional polystyrene via ‘click chemistry’ and its use in the extraction of halide anion salts

A polystyrene with pendant calix[4]pyrroles was prepared via ‘click reaction’ strategy. First, a poly(styrene-co-chloromethylstyrene) with approximately 12% of chloro groups was prepared by conventional free radical polymerisation. The chloro groups were then converted to azido groups using NaN₃ in N,N-dimethylformamide. An alkyne-functionalised calix[4]pyrrole was then coupled to the azido-functionalised polystyrene by click chemistry with high efficiency. The resulting polystyrene with pendant calix[4]pyrroles was used to extract fluoride and chloride anions (as their tetrabutylammonium salts) from their aqueous solutions to organic media.     

Substituent Effects on the Structural Features and Nonlinear Optical Properties of the Organic Alkalide Li+(calix[4]pyrrole)Li−

The effects of substituents on the structure, character, and nonlinear optical (NLO) properties of the organic alkalide Li⁺(calix[4]pyrrole)Li^? were studied by density functional theory. Natural bond orbital analysis and vertical ionization energies reveal that electron‐donating substituents strengthen the alkalide character of Li⁺(calix[4]pyrrole)Li^? and that they are beneficial for a larger first hyperpolarizability (β₀) value. However, electron‐withdrawing substituents have the opposite effect. The dependence of the NLO properties on the number of substituents and their relative position was detected in multisubstituted Li⁺(calix[4]pyrrole)Li^? compounds. For both the amino‐ and methyl‐substituted derivatives, the polarizabilities and the first hyperpolarizabilities increase as more pyrrole β‐H atoms are substituted. Moreover, distribution of the substituents so that they are as far away from each other as possible resulted in an increase in the β₀ value. The new knowledge obtained in this study may provide an effective approach to enhance the NLO responses of alkalides by employing pyrrole derivatives as complexants.     

Tuning the anion binding properties of calixpyrroles by means of p-nitrophenyl substituents at their meso-positions

Extended cavity calix[4]pyrroles and a calix[6]pyrrole were synthesized by cyclization of 5-methyl-5-(4-nitrophenyl)dipyrromethane with acetone in the presence of acid. The solid-state structures of the novel macrocycles were determined by X-ray crystallography. The host-guest chemistry of these receptors towards halide ions was investigated in solution by ¹H NMR titration techniques and compared with those of the meso-octamethylcalix[4]pyrrole and meso-dodecamethylcalix[6]pyrrole. The binding of chloride anions was observed to occur with different affinities on the two faces of the novel calix[6]pyrrole derivative described here.     

Perfluoroalkylated Calix[4]pyrroles: Fluoride Ion Extraction from an Aqueous Medium

Octaalkenyl calix[4]pyrrole ((CH₂=CH(CH₂)₂)₈C4P) is highly useful for the postfunctionalization of different calix[4]pyrroles with desired functionalities. Functionalization with perfluoroalkyl chains [CF₃(CF₂)_n ; R_fn ] gave perfluoroalkyl calix[4]pyrroles (R_fn (CH₂)₄)₈C4P; n =6, 8), having >60 % fluorine content, which created a hydrophobic environment inside the calix[4]pyrrole cavity and recognized fluoride and chloride ions in solution as well as in the solid state. The fluoride ion is extracted efficiently from aqueous CsF and TBAF solutions by using (R_f6(CH₂)₄)₈C4P, as droplets. The fluorinated chain generated a hydrophobic environment which broke the hydration shell associated with the anion and separated out fluoride ions as droplets from aqueous medium. Furthermore, the fluoride ions competitively replaced chloride ions from the (R_f6(CH₂)₄)₈C4P cavity.