Dustin E. Gross

Crown-6-Calix(4)arene Capped Calix(4)pyrrole: An Ion Pair Receptor for Solvent Separated CsF Ions

An ion-pair receptor, 1, containing both cation- and anion-recognizing sites, has been synthesized and characterized. Single-crystal X-ray diffraction structural studies and (1)H NMR spectroscopic analyses confirmed that 1 forms stable 1:1 complexes with CsF in solution and in the solid state in spite of the large separation enforced between the receptor-bound anion and cation. In 9:1 CDCl3/CD3OD, binding of fluoride anion within the calix[4]pyrrole core of 1 was not observed in the absence of a cobound cesium cation; however, it was seen in this solvent mixture under conditions where a Cs(+) cation was bound to the crown ether-strapped calix[4]arene subunit.

Benzene-, pyrrole-, and furan-containing diametrically strapped calix[4]pyrroles - An experimental and theoretical study of hydrogen-bonding effects in chloride anion recognition

Weak hydrogen bonds recently have arisen as a topic of interest in supramolecular chemistry.[1] Among the various interactions being studied, the C-H···anion hydrogen bonds have drawn considerable attention. Positively-charged groups, such as imidazolium cations, provide strong C-H donors and have been used extensively in the design of numerous anionophore architectures.[2] There is increasing evidence, however, that even charge-neutral C-H donors may be strong enough to be exploited effectively in anion recognition chemistry. Such interactions, which involve both aliphatic and aryl C-H groups, have been inferred from gas phase studies,[3] deduced from NMR spectroscopic studies via, e.g., chemical shifts changes,[4-7] and observed in solid state structures.[4,5,8] A recent review of anion-arene adducts notes that C-H···anion hydrogen bonding, rather than interaction with the π-system, is by far the most prevalent bonding motif observed in the solid state.[9] These experimental observations are supported by theoretical analyses.[10,11] For instance, Hay and co-workers have calculated that benzene C-H···anion hydrogen bonds are significant,[11a] being roughly half the strength of typical neutral N-H···anion hydrogen bonds. In a subsequent theoretical report, it was noted that the aryl C-H···Cl binding energies in the gas phase can be tuned from –8 to –16 kcal/mol by altering the para substitution from NH2 to NO2.[11b] Although there are a couple examples where complementary aryl C-H anion interactions have been deliberately incorporated into the design of anion receptors,[7,12] to the best of our knowledge, no efforts have been made to date to test theoretical predictions via the synthesis and experimental study of a matched series of anion receptors. We now report efforts along these lines.

Octafluorocalix[4]pyrrole: a chloride/bicarbonate antiport agent

meso-Octamethyloctafluorocalixpyrrole, a simple tetrapyrrolic macrocycle, has been shown to function as both a chloride/nitrate and a chloride/bicarbonate antiport agent for lipid bilayer transmembrane anion transport. This is the first example of a synthetic macrocyclic pyrrole-based receptor capable of transmembrane bicarbonate transport.

Cooperative Binding of Calix[4]pyrrole–Anion Complexes and Alkylammonium Cations in Halogenated Solvents

Calix[4]pyrrole-chloride interactions are affected not only by the choice of countercation in halogenated solvents, but show a specific dependence on the way in which these cations are bound within the electron rich, bowl-like calix[4]pyrrole cavity formed upon chloride anion complexation. In dichloromethane, the affinities of simple meso-octamethylcalix[4]pyrrole (1) for methyl-, ethyl-, and n-butylammonium chlorides are on the order of 10(5), 10(4), and 10(2) M(-1), respectively, as determined from isothermal titration calorimetry (ITC) analyses. These cation-dependent anion affinity effects, while clearly evident, are less pronounced in other halogenated solvents, such as 1,2-dichloroethane. Support for the proposed cation complexation selectivity is provided by solid state X-ray crystallographic analyses.

Real-time determination of chloride anion concentration in aqueous-DMSO using a pyrrole-strapped calixpyrrole anion receptor

The use of a pyrrole-strapped calix[4]pyrrole () permits the determination of chloride anion concentrations in mixed aqueous DMSO-d(6)-H(2)O environments via proton NMR spectroscopy.

N-Tosylpyrrolidine calix[4]pyrrole: synthesis and ion binding studies.

The synthesis and preliminary solution phase ion binding properties of the N-tosylpyrrolidine calix[4]pyrrole 2 are reported. This β-octaalkyl-substituted calix[4]pyrrole, the first to be prepared via a direct condensation reaction, was obtained by reacting the 3,4-alkyl-functionalized pyrrole 8 with acetone in the presence of an acid catalyst. On the basis of (1)H NMR spectroscopic analyses and isothermal titration calorimetry, it was concluded that, compared with the parent, β-unsubstituted calix[4]pyrrole (1), compound 2 possesses significantly enhanced binding ability for halide anions in chloroform. Furthermore, 2 proved capable of solubilizing in chloroform solution the otherwise insoluble salts, CsF and CsCl. These effects are ascribed to the interactions between the four tosyl groups present in 2 and the counter cations of the halide anion salts.

Enhanced liquid–liquid anion exchange using macrocyclic anion receptors: effect of receptor structure on sulphate–nitrate exchange selectivity

When certain macrocyclic anion receptors are added to a chloroform solution of the nitrate form of a lipophilic quaternary ammonium salt (methyltri-C8,10-ammonium nitrate, Aliquat 336N), the extraction of sulphate from an aqueous sodium nitrate solution via exchange with the organic-phase nitrate is significantly enhanced. Eight macrocycles were surveyed, including two derivatives of a tetraamide macrocycle, five derivatives of calix[4]pyrrole and β-decafluorocalix[5]pyrrole. Under the hypothesis that the enhancement originates from sulphate binding by the anion receptors in the chloroform phase, it was possible to obtain reasonable fits to the sulphate distribution survey data based on the formation of 1:1 and 2:1 receptor:sulphate complexes in the chloroform phase. Apparent 1:1 sulphate-binding constants obtained from the model in this system fell in the range . Comparison of the results for the various anion receptors included in this study reveals that sulphate binding is sensitive to the nature of the substituents on the parent macrocycle scaffolds in a way that does not follow straightforwardly from simple chemical expectations, such as electron-withdrawing effects on hydrogen-bond donor strength.

Anion Transfer at a Micro-Water/1,2-Dichloroethane Interface Facilitated by β-Octafluoro-meso-octamethylcalix[4]pyrrole

The facilitated transfer of four hydrophilic anions, i.e., Cl-, Br-, NO2-, and CH3CO2-, at the micro-water/1,2-dichloroethane interface supported at the tip of a micropipet has been observed successfully using beta-octafluoro-meso-octamethylcalix[4]pyrrole 2 as the receptor. We have also shown for the first time that the dynamics of this process can be studied by micropipet voltammetry. The standard kinetic rate constants (kdegrees) for facilitated anion transfer at such an interface were determined to be (2.11 +/- 0.90) x 10(-2) and (0.75 +/- 0.50) x 10(-2) cm/s in the case of Cl- and CH3CO2-, respectively. These values are much smaller than those associated with the facilitated transfer of analogous alkali metal ions. This difference is thought to reflect a number of underlying factors, including the higher hydration of anions as compared to similar sized cations. Studies such as these are expected to be useful in understanding the mechanism of anion transport at soft interfaces and for the design of yet-improved anion receptors and carriers.

Bis-amidopyrrolyl Receptors Based on Anthracene and Carbazole

A new set of diamide receptors containing anthracene and carbazole bridging subunits and either pyrrole or phenyl substituents were synthesised. The four systems produced in this way were shown to bind representative anions in DMSO-d 6 solution and in the solid state. A higher relative affinity for two test oxoanions, namely dihydrogen phosphate and benzoate, over chloride anion was seen in solution, with the anions in question being studied in the form of their respective tetrabutylammonium salts. However, the specifics of the anion recognition process were seen to depend on structure, with the pyrrole-containing systems displaying higher relative affinities than their corresponding phenyl-containing congeners, and the carbazole receptors proving more effective than the anthracene analogues. Such observations provide support for the notion that both the carbazole NH and the pyrrolic NH protons play an important role in stabilising the receptor-bound anions in solution. Structural analyses of several anion complexes of the diamidopyrrole carbazole receptor reveal that this is not necessarily the case in the solid state; specifically, the pyrrole NH protons are seen to interact with the amide oxygen of another molecule. The net result is an extended one-dimensional coordination polymer.

Bis-cation salt complexation by meso-octamethylcalix[4]pyrrole: linking complexes in solution and in the solid state

Pyridinium and imidazolium bis-cations are shown to link calix[4]pyrrole anion complexes both in solution and in the solid state. This is accomplished by binding of the bis-cations to the electron-rich bowl shaped cavities formed by two separate calixpyrrole-anion complexes. These resulting sandwich-type structures provide a new way of organising calix[4]pyrrole anion complexes in space.