Synthesis,p-Nitrophenolate Complexation and Competitive Anion Signaling of Novel Calixpyrrole Dimer

A novel dimeric calixpyrrole has been synthesized. The dimer forms stable complexes with p-nitrophenolate ion with a concomitant reduction in extinction coefficient. The chromogenic anion is displaced by the addition of various anions like fluoride and acetate. Effective optical sensing of these anions is accomplished using the calixpyrrole dimer.     

A new calix[4]pyrrole derivative and its anion (fluoride)/cation (mercury and silver) recognition

A new calix[4]pyrrole-based macrocycle, meso-tetramethyl-tetrakis{4-[2-(ethylthio)ethoxy]phenyl}calix[4]pyrrole, 7, has been synthesized and fully characterized. Unlike other calixpyrrole derivatives that show selective interaction with anions, calixpyrrole 7 described in the present work forms stable complexes with both metal cations and anions. The thermodynamics of complexation of this ditopic calixpyrrole derivative with metal cations (Hg2+ and Ag+) and the fluoride anion in nonaqueous solutions have been determined by titration calorimetry, and the host-guest composition has been investigated by using conductance measurements at 298.15 K. 1H NMR studies provide clear evidence about the sites of complexation of 7 with the ionic species, which show that the NH groups are taking part in the complexation of this ligand with the fluoride anion while the sulfur donor atoms are responsible for the interaction with metal cations. Using the present data on 7 and structurally related analogues (1-6), the complexation behavior is discussed comparatively from the thermodynamic point of view. Possessing four sulfur-containing pendent arms, 7 displays an enhanced hosting ability for Hg2+ in acetonitrile. As compared with 1, the calixpyrrole derivative, 7, shows a unique interaction with fluoride among the anions investigated in acetonitrile and dimethyl sulfoxide. As far as the fluoride complex is concerned, the medium effect is assessed in terms of the thermodynamics of the transfer of reactants and product from acetonitrile (reference solvent) to dimethyl sulfoxide.     

Nanosecond pulse radiolysis studies in benzene-methanol mixtures using a rigid 7-amino 4-trifluoro methyl coumarin as a probe

Pulse radiolysis of benzene-methanol mixtures leads to the formation of triplets and dimer anions of the solute C153. The spectra of the dimer anions and triplet in these solvent mixtures are reported. The relative G-values of these transients are determined. Dimer anion yield was found to vary exponentially with the reciprocal of the dielectric constant of the mixture.     

Sulfur-containing hetero-calix[4]pyrroles as mercury(II) cation-selective receptors: thermodynamic aspects

Two sulfur-containing hybrid calix[4]pyrrole derivatives (III and IV) have been synthesized and fully characterized. Several analytical techniques (1H NMR, conductance measurements, UV-vis spectrophotometry, titration potentiometry, and titration calorimetry) have been used to assess the interaction between these hybrid calixpyrrole receptors and metal cations in acetonitrile and dimethylsulfoxide. The partition constants of calix[4]pyrrole, I, II, and IV in the acetonitrile-hexane solvent system and the solubilities of the ligands in various solvents at 298.15 K were determined. 1H NMR measurements reveal the sites of interaction of calixpyrrole ligands with metal cations in CD3CN. Conductance and UV-vis spectrophotometric measurements were performed to establish the composition of mercury(II) calixpyrrole complexes in acetonitrile at 298.15 K. Titration calorimetry was used to quantitatively assess Hg(II)-calixpyrrole interactions. Thus the thermodynamics of complexation of calixpyrrole ligands with the mercury(II) cation in acetonitrile at 298.15 K are reported. Potentiometric titrations were also used to establish the stepwise stability constants for the complexation of calix[3]thieno[1]pyrrole with the Hg(II) cation in acetonitrile at 298.15 K. The results show that replacement of one or more pyrrole units by thiophene rings in calix[4]pyrrole has tuned significantly the discrimination ability of these ligands between anions and enables the produced hybrid calixpyrroles to bind selectively with Hg(II) in acetonitrile. No interaction was observed between these ligands and other metal cations in acetonitrile.     

Chloride anion-induced dimer capsule based on a polyfluorinated macrocycle meta-WreathArene

[1_n]metacyclophanes are a class of important building blocks for supramolecular assembly of artificial capsules. Herein we present the preparation and properties of a novel polyfluorinated macrocycle meta-WreathArene, a C₂-symmetrical [1₄]metacyclophane. Adopting a cone conformation in acetone solution, the macrocycle can form dimer capsules through hydrogen bonds induced by chloride anions. Each dimer capsule consists of two meta-WreathArene and two chloride anions, and has been unambiguously characterized both in solution and in solid state.     

Electron transfer collisions between sulfur dioxide clusters and laser-excited Rydberg atoms

Electron attachment to SO₂ clusters is studied in a pulsed crossed beam apparatus, using laser-excited nf Rydberg atoms as a low energy electron source. The results are interpreted as an attachment to a dimer subcluster followed by a rapid impulsive dissociation of the nascent dimer anion. The remaining cluster anions possess a large amount of internal energy. At low principal quantum numbersn, the influence of the Rydberg ionic core leads to an important evaporation process interpreted with simple model calculations.     

杯吡咯和杂杯吡咯的结构、合成与应用

杯吡咯以其特殊的结构和优异性能, 在主客体化学尤其是分子识别等方面的应用前景已备受关注, 合成新型功能化杯吡咯及其衍生物已成为超分子化学领域的研究热点之一. 近年来通过引入新型芳烃基体合成了结构和性能多样的杂杯吡咯. 主要介绍了功能化的杯吡咯和杂杯吡咯的结构、合成及其在分子识别等领域的应用.     

Strapped and other topographically nonplanar calixpyrrole analogues. Improved anion receptors

Calixpyrroles and related macrocycles are non-aromatic synthetic anion receptors that have attracted considerable attention in recent years. The unfunctionalized, parent calix[4]pyrrole system, also known as octamethylporphyrinogen, may be prepared in one step and in high yield from pyrrole and acetone, and is an effective anion receptor, showing a preference for fluoride, phosphate, carboxylate and chloride anions in organic media. Efforts to improve the anion binding affinity of calix[4]pyrrole and to modify its inherent selectivity have led to the synthesis of a variety of new, modified calixpyrroles. Among the most effective of these are derivatives that contain bridging "straps". In this Feature Article, the preparation and properties of these and other topographically nonplanar calixpyrrole analogues are reviewed from the perspective of the anion recognition chemist.     

Calix[4]pyrrole[2]carbazole: a new kind of expanded calixpyrrole

The synthesis and anion binding properties of a new class of calixpyrrole analogue, containing two carbazole subunits in lieu of two of the four acetone bridging elements normally found in calix[4]pyrrole, is described. The compound exists in a winglike structure in the solid state, as judged from single-crystal X-ray diffraction analyses of both the free system and the corresponding benzoate anion complex. Evidence for anion binding in dichloromethane solution was obtained from static fluorescent quenching experiments; these latter revealed a slight preference for acetate relative to other carboxylate anions (e.g., benzoate, oxalate, succinate), as well as various other anionic substrates (i.e., chloride and dihydrogen phosphate). No evidence of binding was observed in the case of bromide, nitrate, and hydrogen sulfate.     

The fluoride dilemma

The implications of environmental contamination by fluoride on human health call upon the need for the development of monitoring systems for the ‘in situ’ detection of fluoride in contaminated sources and new technologies approaches for their removal. This paper reports recent work on the design of calixpyrrole receptors selective for the fluoride anion. The various steps undertaken for the thermodynamic characterization of these receptors and their anionic complexes are discussed. Thus based on thermodynamic data, the medium and ligand effects on selectivity are quantitatively assessed using representative calixpyrrole derivatives.     

Crystal structures and magnetic properties of (m- or p-MPYNN)2 NiII(tdas)2

Novel Ni^II(tdas)₂ complexes, (m- or p-MPYNN)₂Ni(tdas)₂ (MPYNN=N-methylpyridinium α-nitronyl nitroxide and tdas=1,2,5-thiadiazole-3,4-dithiolate), were prepared. The Ni(tdas)₂ anions are located on the lattice points and at the lattice center in the crystal of (m-MPYNN)₂Ni(tdas)₂. The m-MPYNN cations form a centrosymmetric dimer, and the anion is sandwiched by a pair of the dimers, forming alternate stacks along the a axis. In the crystal of the (p-MPYNN)₂Ni(tdas)₂ complex, the Ni(tdas)₂ anions are located on the lattice points and at the center of the bc plane. The anion is surrounded by four p-MPYNN cations. The p-MPYNN cations also form a centrosymmetric dimer. Each cation has a short contact with a different Ni(tdas)₂ anion. The magnetic susceptibilities of both complexes decrease with decreasing temperature, indicating antiferromagnetic interaction within the radicals. The behavior of (m- and p-MPYNN)₂Ni(tdas)₂ was understood with the singlet–triplet model and the Curie–Weiss law.     

PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING II: REACTIONS INVOLVING PYRIMIDINE RADICAL ANION INTERMEDIATES

A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor Nα-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1′-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product.     

Do Photolyases Need To Provide Considerable Activation Energy for the Splitting of Cyclobutane Pyrimidine Dimer Radical Anions?

cis-syn Cyclobutane pyrimidine dimers, major UV-induced DNA lesions, are efficiently repaired by DNA photolyases. The key step of the repair reaction is a light-driven electron transfer from the FADH(-) cofactor to the dimer; the resulting radical anion splits spontaneously. Whether the splitting reaction requires considerable activation energy is still under dispute. Recent reports show that the splitting reaction of a dimer radical anion has a significant activation barrier (0.45 eV), and so photolyases have to provide considerable energy. However, these results contradict observations that cis-syn dimer radical anions split into monomers at -196 degrees C, and that the full process of DNA photoreactivation was fast (1.5-2 ns). To investigate the activation energies of dimer radical anions, three model compounds 1-3 were prepared. These include a covalently linked cyclobutane thymine dimer and a tryptophan residue (1) or a flavin unit (3), and the covalently linked uracil dimer and tryptophan (2). Their properties of photosensitised splitting of the dimer units by tryptophan or flavin unit were investigated over a large temperature range, -196 to 70 degrees C. The activation energies were obtained from the temperature dependency of splitting reactions for 1 and 2, 1.9 kJ mol(-1) and 0.9 kJ mol(-1) for the thymine and uracil dimer radical anions, respectively. These values are much lower than that obtained for E. coli photolyase (0.45 eV), and are surmountable at -196 degrees C. The activation energies provide support for previous observations that repair efficiencies for uracil dimers are higher than thymine dimers, both in enzymatic and model systems. The mechanisms of highly efficient enzymatic DNA repair are discussed.     

Anion-binding behavior of hybrid calixpyrroles

Hybrid calixpyrrole systems are calixpyrrole-like macrocycles that are based on more than one type of small molecule building block. Structurally, these "mixed-breed" macrocycles differ from calixpyrroles in that some pyrrolic units in the latter are replaced by other hetereocyclic units such as furan, thiophene, bipyrrole, and bithiophene. Although several such systems have been reported in recent years, only a few have been studied as possible anion receptors. In this paper, the results of detailed anion binding studies involving several prototypic systems are reported. Taken in concert, these results highlight the fact that some hybrid systems, including compounds 2-5, display anion affinities that are considerably weaker than those of the parent system 1. On the other hand, they also show that compounds 6-8 are good receptors for "Y-shaped" anions, such as carboxylates, and that they bind these species with high affinity. These findings are strongly supported by solid-state structural studies, which reveal an interesting "cross binding mode" for the binding of carboxylate anions by the bis-thiophene, bis-pyrrole system 7.     

Anion-induced switching of the helical orientation of a chiral indolocarbazole dimer

An indolocarbazole dimer, containing chiral urea appendages, that adopts a helically folded conformation by intramolecular hydrogen bonds as proven by ¹H NMR and circular dichroism (CD) spectroscopy has been prepared. Owing to the preferential formation of one helical conformer, strong CD signals appear in relatively non-polar solvents such as chloroform (CHCl₃) and dichloromethane (CH₂Cl₂) but the signal is negligible in dimethyl sulfoxide (DMSO). In addition, the optical rotation of the dimer is highly sensitive to the polarity of solvents. For example, the magnitude of the specific rotation ([α]_D) is ? 934° in CH₂Cl₂ and ? 657° in CHCl₃ but it is only ? 75° in DMSO. These observations suggest that the dimer folds to a helical structure by intramolecular hydrogen bonds in relatively non-polar solvents but exists in an unfolded extended conformation in polar solvents such as DMSO. The dimer strongly binds anions such as chloride, acetate and sulfate by multiple hydrogen bonds. In addition, anion binding leads to considerable CD spectral changes with the different pattern and degree of Cotton effects depending on the kind of anions. The dimer may be therefore utilised for the construction of an anion-responsive chiroptical sensor or switch.     

Effects of solvation and core switching on the photoelectron angular distributions from (CO2)n(-) and (CO2)(n)(-).H2O

Photoelectron images are recorded in the photodetachment of two series of cluster anions, (CO(2))(n)(-), n=4-9 and (CO(2))(n)(-).H(2)O, n=2-7, with linearly polarized 400 nm light. The energetics of the observed photodetachment bands compare well with previous studies, showing evidence for switching between two anionic core structures: The CO(2)(-) monomer and covalent (CO(2))(2)(-) dimer anions. The systematic study of photoelectron angular distributions (PADs) sheds light on the electronic structure of the different core anions and indicates that solvation by several CO(2) molecules and/or one water molecule has only moderate effect on the excess-electron orbitals. The observed PAD character is reconciled with the symmetry properties of the parent molecular orbitals. The most intriguing result concerns the PADs showing remarkable similarities between the monomer and dimer anion cluster-core types. This observation is explained by treating the highest-occupied molecular orbital of the covalent dimer anion as a linear combination of two spatially separated monomeric orbitals.     

Primary processes in the photosensitized redox reaction of dimers of thiacarbocyanine dyes

The kinetics and the mechanism of the reaction of donor (ascorbic acid) oxidation by electron acceptors (methylviologen and p-nitroacetophenone) photosensitized by dimers of sulfoalkyl-9-ethylthiacarbocyanine dyes (Dye1, Dye2, and Dye3) were studied in aqueous solutions. Dimers of the dyes (dianions) are capable of transition to the triplet state that is mainly quenched by acceptors to form radical anions of dimers, which are unstable and dissociate within 10–12 μs into the monomer (anion) and its radical (the limiting reaction stage). The presence of a donor in the dye-acceptor mixture leads to one-electron reduction of the monomer radical to its anion followed by the dimerization reaction. The results of the analysis of the experimental data obtained by the laser photolysis technique are in good agreement with the calculated kinetic curves for the formation and the decay of the dimer radical anions.     

Efficient organocatalysis with a calix[4]pyrrole derivative

The 10α,20β-bis(4-nitrophenyl)-calix[4]pyrrole was found to act as an effective organocatalyst for the hetero Diels-Alder reaction of Danishefsky’s diene with aromatic aldehydes. This discovery is the first reported case of a calixpyrrole that exhibits organocatalytic activity.     

Synthesis, structure, and properties of benzoquinone dimer and trimers bearing t-Bu substituents

Highly soluble and stable quinone dimer and trimers were successfully yielded by introduction of t-Bu substituents. In X-ray structure analysis, the dimer quinone moiety was distorted into the boat shape, which was clarified by MO calculations. X-ray and UV/vis studies indicated that the covalently linked quinone moieties bear a large torsional angle. Nevertheless, the reduction potentials rose significantly with the order of monomer < dimer < trimer, indicating that the negative charge was efficiently delocalized within the radical anions.     

Design of a magnetic bistability molecular system constructed by H-bonding and pi...pi-stacking interactions

Crystal structures and magnetic properties were determined for two novel polymorphs of the complex [H2DABCO][Ni(mnt)2] [(H2DABCO)2+ = diprotonated 1,4-diazabicyclo[2.2.2]octane; mnt2- = maleonitriledithiolate]. For each polymorph, anions form a layered structure in which two kinds of dimers were observed. The adjacent anionic sheets are held together by cations via H-bonding interactions between protons of cations and CN groups of anions. Two polymorphs possess spin bistability; namely, upon cooling, a magnetic transition happens at around 120 K with about 1 K hysteresis on heating for the alpha phase and at 112 K with about 10 K hysteresis for the beta phase. Above the transition, the magnetic behaviors of two polymorphs can be approximately interpreted by a singlet-triplet model of an antiferromagnetically coupled S = 1/2 dimer, which is supported by the crystal structures and spin dimer analyses based on extended Hückel molecular orbital calculations.