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.     

Calix[n]bispyrrolylbenzenes: synthesis, characterization, and preliminary anion binding studies

A series of novel calixpyrrole-like macrocycles, calix[n]bis(pyrrol-2-yl)benzene (calix[n]BPBs, n=2-4) 9 a-11 a, have been synthesized by means of the TFA-catalyzed condensation reaction of bis(pyrrol-2-yl)benzene 8 a with acetone. Calix[2]BPB 9 a represents an expanded version of calix[4]pyrrole in which two of the four meso bridges are replaced by benzene rings. By contrast, systems 10 a and 11 a, which bear great considerable to calixbipyrroles 2 and 3, represent higher homologues of the basic calix[n]BPB motif. Solution-phase anion binding studies, carried out by means of (1)H NMR spectroscopic titrations in [D2]dichloromethane and isothermal titration calorimetry (ITC) in 1,2-dichloroethane, reveal that 9 a binds typical small anions with substantially higher affinities than 1, even though the same number of hydrogen bonding donor groups are found in both compounds. The basic building block for 9 a, benzene dipyrrole 8 a, also displays a higher affinity for anions than the building block for 1, dimethyldipyrromethane 16. Structural studies, carried out by single-crystal X-ray diffraction analyses, are consistent with the solution-phase results and reveal that 9 a is able to stabilize complexes with chloride and nitrate in the solid state. Structures of the PF6- and NO3- complexes of 10 a were also solved as were those of the acetone adduct of 9 a and the ethyl acetate adduct of 11 a.     

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.     

Conformational features and anion-binding properties of calix[4]pyrrole: a theoretical study

The conformational preference of calix[4]pyrrole and its fluoride and chloride anion-binding properties have been investigated by density functional theory calculations. Geometries were optimized by the BLYP/3-21G and BLYP/6-31G methods, and energies were evaluated with the BLYP/6-31+G method. To model the effect of medium, the SCIPCM solvent model was also employed. Four typical conformations of the parent substituent-free calix[4]pyrrole were studied. Both in the gas phase and in CH(2)Cl(2) solution, the stability sequence is predicted to be 1,3-alternate > partial cone > 1,2-alternate > cone. The cone conformation is predicted to be about 16.0 and 11.4 kcal/mol less stable in the gas phase and CH(2)Cl(2) solution, respectively. This is mainly due to electrostatic repulsions arising from the all-syn pyrrole/pyrrole/pyrrole/pyrrole arrangement present in this conformer. The existence of possible 1:1 and 1:2 anion-binding modes were explored in the case of fluoride anion, and the factors favoring the 1:1 binding mode are discussed. The calculated binding energy for fluoride anion is about 15 kcal/mol larger than that for chloride anion. The calculated binding energy for chloride anion agrees with the experimental value very well. The presence of meso-alkyl substituents destabilizes the cone conformer with respect to the 1,3-alternate conformer and, therefore, reduces the anion-binding affinity by 3-4 kcal/mol. The strength of N-H- - -anion hydrogen bonds in the various structures subject to study were estimated on the basis of the calculated anion-binding energies and the predicted structural deformation energies of substituent-free calix[4]pyrrole.     

Activation of C--H... Halogen (Cl, Br, and I) hydrogen bonds at the organic/inorganic interface in fluorinated tetrathiafulvalenes salts

The electrocrystallization of fluorinated bis(2,2'-difluoropropylenedithio)tetrathiafulvalene (1) in the presence of linear (ICl2-, IBr2-, I2Br-) or cluster ([Mo6Cl14]2-) anions affords 1:1 and 2:1 cation radical salts such as [1][ICl2] and [1]2[Mo6Cl14].(CH3CN)2. In both salts, the 1*+ radical ion adopts a boat conformation and envelops the anion through C-H...Hal(anion) (Hal(anion) = Cl, Br, I) hydrogen bonds. This demonstrates the activating role of the neighboring electron-withdrawing CF2 moieties in the stabilization of bi- or trimolecular neutral entities. With smaller linear anions, fluorine segregation controls the solid-state associations of the bimolecular [1]*+[X] entities, and gives rise to layered materials with a limited overlap interaction between the open-shell organic cations and magnetic spin chain behavior. With the larger [Mo6Cl14]2 ions, a strong overlap interaction between radical cations gives rise to diamagnetic [1]2(2+) dimers, which alternate with the cluster anions to form hybrid organic/inorganic ...[1]2(2+)[Mo6Cl14]2... chains. This behavior is also observed in [2]2(2+)[Mo6Cl14]2-.(CH2Cl2)2, in which compound 2 is the unsymmetrically substituted (ethylenedithio)(2,2'-difluoropropylenedithio)tetrathiafulvalene. On the other hand, the unsymmetrically substituted 2,2'-difluoropropylenedithiotetrathiafulvalene (3) affords a mixed-valence 4:1 salt with [Mo6Cl14]2, which is formulated as [3]4[Mo6Cl14].(CH3CN)2. This semiconducting salt is characterized by the coexistence of both the fluorine/fluorine segregation (with solvent inclusion) and the organic/inorganic segregation (with delocalized overlap interactions). Both Csp2-H...Cl and Csp3-H...Cl hydrogen bonds facilitate the stabilization of the organic/inorganic interface and the presence of conducting organic slabs.     

Anion binding behavior of heterocycle-strapped calix[4]pyrroles

A comparative study of the halide and benzoate anion binding properties of a series of phenyl, pyrrole, and furan-strapped calix[4]pyrroles has been carried out. These receptors, which have previously been shown to bind the chloride anion (Yoon et al., Angew. Chem., Int. Ed. 47(27):5038–5042, 2008), were found to bind bromide and benzoate anion (studied as the corresponding tetrabutylammonium salts) with near equal affinity in acetonitrile, albeit less well than chloride, as determined from ITC measurements or NMR spectroscopic titrations. This stands in marked contrast to the parent octamethylcalix[4]pyrrole, where the carboxylate anion affinities are substantially higher than those for bromide anion under identical conditions. This finding is rationalized in terms of tighter binding cavity present in the strapped systems. For all three anions for which quantitative data could be obtained (i.e., Cl^?, Br^?, PhCO₂ ^?), the pyrrole-strapped system displayed the highest affinity, although the relative enhancement was found to depend on the anion in question. In the specific case of fluoride anion binding to the pyrrole-strapped receptor, two modes of interaction are inferred, with the first consisting of binding to the calix[4]pyrrole via NH-anion hydrogen bonds, followed by a process that involves deprotonation of the strapped pyrrolic NH proton. A single crystal X-ray diffraction analysis provides support for the first of these modes and further reveals the presence of a methanol molecule bound to the fluoride anion.     

Influence of the Attachment of a Gold(I) Phosphine Moiety at the Upper Rim of a Calix[4]pyrrole on the Binding of Tetraalkylammonium Chloride Salts

We report the synthesis of an unprecedented mono-gold(I) phosphine complex based on a “two-wall” aryl-ethynyl extended calix[4]pyrrole. We describe and compare the binding properties of the parent 10α,20α-bis-aryl-ethynyl calix[4]pyrrole ligand and the prepared organometallic compound as receptors for tetraalkylammonium chloride salts in dichloromethane and acetone. We describe the results of ¹H NMR, UV–Vis titrations and isothermal titration calorimetry (ITC) experiments in dichloromethane and acetone, aiming to thermodynamically characterize the formed complexes. The obtained results indicate a noticeable decrease in the binding affinity of the chloride for the mono-gold(I) receptor 1 compared to the parent ligand 2 . The increase in the negative value of the electrostatic surface potential at the center of the aromatic ring of the gold(I) meso-aryl-ethynyl substituent serves to explain the observed results and the presence in solution of the chloride complex of 1 as a mixture of two conformers.     

Selective anion binding by a cofacial binuclear zinc complex of a Schiff-base pyrrole macrocycle

The synthesis of the new cofacial binuclear zinc complex [Zn(2)(L)] of a Schiff-base pyrrole macrocycle is reported. It was discovered that the binuclear microenvironment between the two metals of [Zn(2)(L)] is suited for the encapsulation of anions, leading to the formation of [K(THF)(6)][Zn(2)(μ-Cl)(L)]·2THF and [Bu(n)(4)N][Zn(2)(μ-OH)(L)] which were characterized by X-ray crystallography. Unusually obtuse Zn-X-Zn angles (X = Cl: 150.54(9)° and OH: 157.4(3)°) illustrate the weak character of these interactions and the importance of the cleft preorganization to stabilize the host. In the absence of added anion, aggregation of [Zn(2)(L)] was inferred and investigated by successive dilutions and by the addition of coordinating solvents to [Zn(2)(L)] solutions using NMR spectroscopy as well as isothermal microcalorimetry (ITC). On anion addition, evidence for deaggregation of [Zn(2)(L)], combined with the formation of the 1:1 host-guest complex, was observed by NMR spectroscopy and ITC titrations. Furthermore, [Zn(2)(L)] binds to Cl(-) selectively in THF as deduced from the ITC analyses, while other halides induce only deaggregation. These conclusions were reinforced by density functional theory (DFT) calculations, which indicated that the binding energies of OH(-) and Cl(-) were significantly greater than for the other halides.     

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.     

Anion responsive TTF-appended calix[4]arenes. Synthesis and study of two different conformers

Two new cone- and 1,3-alternate-calix[4]arenes (cone-1 and 1,3-alt-1), bearing four modified TTF (tetrathiafulvalene) substituents on the upper rim, have been synthesized. The binding ability of these two sets of conformers for various anions, including F(-), Cl(-), Br(-), I(-), PF6(-), ClO4(-), HSO4(-), CH3COO(-), H2PO4(-), and HP2O7(3-), was tested in organic media by monitoring the changes in their UV/vis and (1)H NMR spectra as a function of added anion, as well as via cyclovoltammetry (CV) (all anions studied as their respective TBA salts). On the basis of the present findings, we propose that incorporation of four TTF units within an overall calix[4]arene-based recognition framework produces a preorganized receptor system that displays a modest preference for the pyrophosphate (HP2O7(3-)) anion.     

Syntheses, structures, and anion-binding properties of two novel calix[2]benzo[4]pyrroles

Calix[2]benzo[4]pyrrole m-6 and p-6, each containing two dipyrromethane moieties and two m-phenylene or p-phenylene units, respectively, were readily synthesised from pyrrole, 1,3- and 1,4-bis(1,1'-dimethylhydroxymethyl)benzene, (m-4 and p-4, respectively) and acetone. Macrocycles m-6 and p-6 were tested as receptors for a selection of anions, such as acetate, dihydrogenphosphate and fluoride. The X-ray structures of m-6 and p-6 and those of the complexes m-6F(-), m-6Cl(-) and m-6CH(3)COO(-) (with an nBu(4)N(+) counterion) were also determined.     

Theoretical study of anion binding to calix[4]pyrrole: the effects of solvent,fluorine substitution,cosolute, and water traces

The binding of different anions to calix[4]pyrrole has been studied by means of molecular dynamics coupled to thermodynamic integration calculations. The effect of different apolar solvents, octafluoro substitution, and the change in binding free energy derived from the presence of cosolute and water traces (the hydrated salt used to introduce the anion in the solution) were examined. Calculations allow us to rationalize the differential binding of ions to calix[4]pyrrole and octafluorocalix[4]pyrrole as well as to predict the behavior in new solvents for which experimental data are not available yet. It is found that both calix[4]pyrrole and octafluorocalix[4]pyrrole have a dramatic preference for F- in the gas phase and pure aprotic solvents, but the situation can change dramatically in protic solvents or in the presence of the hydrated cation which is used as cosolute of the anion. Overall, our results provide interesting clues for a better understanding of the process detected experimentally as "binding".     

Bis(tetrathiafulvalene)-calix[2]pyrrole[2]- thiophene and its complexation with TCNQ

The synthesis of a rare example of a calix[2]pyrrole[2]thiophene derivative incorporating two tetrathiafulvalene units is described along with its X-ray crystal structure. Complexation studies between the bis(tetrathiafulvalene)-calix[2]pyrrole[2]thiophene and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) were carried out in solution using absorption and ESR spectroscopies as well as in the solid state using X-ray crystallography and IR spectroscopy.     

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.     

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.     

Ion-controlled on-off switch of electron transfer from tetrathiafulvalene calix[4]pyrroles to Li+@C60

Binding of chloride anion to a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) donor results in ET to Li(+)@C(60) to produce the radical pair (TTF-C4P(?+)/Li(+)@C(60)(?-)), the structure of which was characterized by X-ray crystallographic analysis. The addition of tetraethylammonium cation, which binds more effectively than Li(+)@C(60)(?-) as a guest within the TTF-C4P cavity, leads to electron back-transfer, restoring the initial oxidation states of the donor and acceptor pair.     

Binding cooperativity of two different Lewis acid groups at the edge of ferrocene

The binding properties of heteronuclear bidentate Lewis acids, in which an organoboron and an organotin moiety are attached adjacent to each other at one of the Cp rings of ferrocene, have been studied. Treatment of [1,2-fc(SnMe2Cl)(BClMe)] (1-Cl) (fc = ferrocenediyl) with one equivalent of pyridine or 4-dimethylaminopyridine (DMAP) resulted in diastereoselective complexation of boron. Adducts 2 and 3 have been studied by multinuclear NMR, and the stereoselectivity of complexation was further confirmed by single crystal X-ray diffraction of 2. The importance of cooperative effects that involve an intramolecular B-ClSn interaction on the diastereoselectivity is evident from comparison with binding studies on the phenyl-substituted analogue [1,2-fc(SnMe2Cl)(BPhMe)] (1-Ph). Complexation of 1-Ph led to diastereomeric mixtures of adducts 4 and 5, respectively, which were identified by multinuclear NMR including NOESY experiments. The solid-state structure of one of the diastereomers of 5 was confirmed by X-ray crystallography. Facile isomerization was found in solution and the barrier of activation was determined by VT NMR studies (4: Delta(#)(298) = 54.9+/-0.4 kJ mol(-1); 5: Delta(#)(298) = 70.3+/-0.1 kJ mol(-1)). Competitive binding of pyridine to 1-Cl and [FcB(Cl)Me] (Fc = ferrocenyl) showed that cooperative effects between tin and boron lead to significant Lewis acidity enhancement. Binding of a second nucleophile in the presence of excess of base occurred also at boron. The novel zwitterionic complexes [1,2-fc(BMe(py)2)(SnMe2Cl2)] (6) and [1,2-fc(BMe(dmap)(2))(SnMe(2)Cl2)] (7) formed, which consist of boronium cation and stannate anion moieties. The structure of 7 in the solid-state was confirmed by X-ray crystallography. Multinuclear NMR data and competition experiments indicate weak binding of chloride to tin in 7 and partial dissociation in solution.     

Binding properties of solvatochromic indicators [Cu(X)(acac)(tmen)] (X = PF6- and BF4-, acac- = Acetylacetonate, tmen = N,N,N',N'-tetramethylethylenediamine) in solution and the solid state

The solvatochromic indicator [Cu(acac)(tmen)(H 2O)].PF 6 ( 1.H 2O) has been synthesized and crystallographically characterized. 1.H 2O binds an H 2O molecule at the Cu(II) axial site, while the PF 6 (-) anion is coordination free. The binding properties of [Cu(PF 6)(acac)(tmen)] ( 1) and [Cu(BF 4)(acac)(tmen)] ( 2) have been investigated in solution and the solid state. The donor number of the PF 6 (-) anion (DN PF6) was determined from the UV-vis spectra of 1 in 1,2-dichloroethane. The value of DN PF6 of the PF 6 (-) anion is slightly larger than that of the tetraphenylborate anion (BPh 4 (-)), which is known as a noncoordinating anion. In the solid state, 1 and 2 reversibly bind and release H 2O molecules at the Cu(II) axial sites. The coordinated H 2O molecules in 2 are more easily removed than those in 1 because of the strong Lewis basicity of the BF 4 (-) anion compared to the PF 6 (-) ion. The lower melting point of 1 versus 2 is attributed to the loose binding of the PF 6 (-) anions to the Cu(II) centers, which induces the dynamic nature of the crystal.     

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.     

New series of asymmetrically substituted Bis(1,2-dithiolato)-nickel(III) complexes exhibiting near IR absorption and structural diversity

The synthesis, structural characterization, and properties of a new series of asymmetrically substituted bis(dithiolene) nickel(III) compounds [Bu4N][Ni(Phdt)2] (1) (Phdt = 2-Phenyl-1,2-dithiolate), [Bu4N][Ni(NO2Phdt)2] (2) (NO2Phdt = 2-( p-nitrophenyl)-1,2-dithiolate), [Bu4N][Ni(FPhdt)2] (3) (FPhdt = 2-( p-fluorophenyl)-1,2-dithiolate), [Bu4N][Ni(ClPhdt)2] (4) (ClPhdt = 2-( p-chlorophenyl)-1,2-dithiolate), and [Bu4N][Ni(BrPhdt)2] (5) (BrPhdt = 2-( p-bromophenyl)-1,2-dithiolate) have been described. All complexes 1- 5 exhibit absorptions in the near-infrared region; the shift of these absorption bands can be tuned by the choice of the substituents on the relevant dithiolene moieties. The substituents on the dithiolene moiety are also responsible for their structural diversities. The nature of the substituents on the dithiolene moiety play an important role in tuning the redox potentials along this series. The nitro derivative (compound 2) exhibits several redox couples in its cyclic voltammogram in contrast to the other compounds in this series. The synthesis and characterization of two asymmetrically halogen substituted tetrathiafulvalene (TTF) derivatives 4,4'-bis(4-chlorophenyl)-tetrathiafulvalene ClPhTTF (6) and 4,4'-bis(4-bromophenyl)-tetrathiafulvalene (BrPhTTF) (7) have been described. One of these compounds has been structurally characterized. Iodine treatment of the monoanionic Ni(III) compound [Bu4N][Ni(ClPhdt)2] (4) results in the formation of a neutral Ni(IV) complex [Ni(ClPhdt)2] (8). All monoanionic compounds 1- 5 are Ni(III) complexes, as evidenced by electron spin resonance spectroscopy. Interestingly, strong Cl...Cl interactions are observed in the solid state structures of the chlorinated compounds 6 and 8. Finally, the structural features of compound [Ni(ClPhdt)2] (8) and the TTF derivative ClPhTTF (6) are compared based on their enormous structural similarities, and the neutral compound [Ni(ClPhdt)2] (8) is classed as the "an inorganic counterpart of TTF".