Ion-pair Receptor Research Articles

Bambus[6]uril as a novel example of a ditopic ion-pair molecular receptor for sodium iodide

By using quantum mechanical DFT calculations, the most probable structure of the bambus[6]uril-Na+I− electroneutral complex species was derived. In this complex with a plane of symmetry, the considered anion I−, included in the macrocyclic cavity, is bound by 12 weak hydrogen bonds between methine hydrogen atoms on the convex face of glycoluril units and the I− anion, while the sodium cation Na+ interacts only with two carbonyl oxygen atoms of the parent macrocyclic receptor. The interaction energy of the resulting complex, involving the Boys-Bernardi counterpoise corrections of the basis set superposition error, was found to be −593.8 kJ mol−1.

Exploring the mechanism of ion-pair recognition by new calix[4]pyrrole bis-phosphonate receptors: insights from quantum mechanics study

Three diastereomeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole can act as ion-pair receptors for alkylammonium/phosphonium chloride salts. In this contribution, the ion-pair binding mode and binding affinity were investigated using density functional theory (DFT) calculations and new nonconvalent weak interaction analysis method. One of the three receptors was chosen as the host and two guests were chosen to model quaternary phosphonium chloride salts and primary ammonium chloride salts respectively, and two types of arrangements – separated and contact – were taken into account for each guest. The binding energy suggests that contact arrangement is the favorable binding mode for this receptor and it prefers to bind primary ammonium chloride salts rather than quaternary phosphonium chloride salts both in the gas-phase and in solution, in agreement with the experiment. Moreover, geometry analysis and charge transfer based on natural bond orbital (NBO) analysis suggest that the binding modes and binding affinities of the receptor towards different ion-pairs are determined by the amount and strength of hydrogen bonds. Furthermore, nonconvalent weak interactions between the host and the guests have been explored to unravel the driving forces responsible for the ion-pair recognition. There are hydrogen bonding, van der Waals, cation–π, ion-induced dipole and charge–charge interactions in ion-pair recognition, and hydrogen bonding interactions play the dominant role in this process. This work unveils the mechanism of ion-pair recognition by new calix[4]pyrrole bis-phosphonate receptors, while opening exciting perspectives for the design of novel calix[4]pyrrole-based ion-pair receptors.

Theoretical study on the ion–pair recognition of Na+/X− (X = F− , Cl− , Br− ) by urea calix[4]bis crown-3 derivative

The structure of tetraurea calix[4]bis crown-3 derivative (Ht) and its complexes with ion–pair Na+/X− (X = F− , Cl− , Br− ) were studied using quantum mechanical (QM) methods (at the B3LYP/6-31G(d,p) level) and molecular dynamics (MD) simulations under various conditions. Two types of complexes (host-separated type (S) and contact type (C)) were found based on QM calculations in the gas phase. The ion-binding sites in Ht, the interaction forces between Na+/X− and Ht and the recognition selectivity for ion pairs were discussed in detail. In the case of host-separated type (S), the MD simulation results showed that the halogen anion gradually broke away from the host molecule in water. However, the anion got into the calix[4]arene subunit and interacted with sodium cation in chloroform solvent. For the contact type, anion and cation were always embraced by the host molecule during a 1-ns simulation both in water and chloroform. These results indicated that Ht could be a potential selective ion–pair receptor, and the QM+MD strategy could provide reasonable descriptions for ion–pair recognitions.

Selective Ammonium Nitrate Recognition by a Heteroditopic Macrotricyclic Ion-Pair Receptor

The heteroditopic macrotricyclic molecular receptor 1, which bears a tripodal anion binding domain and 4,10,16-triaza-18-crown-6 cation recognition domain, proves to be an effective ion-pair receptor. In the absence of the cobound cation (TBA(+) salts) receptor 1 preferably binds nitrate and nitrite over other anions, including basic anions such as acetate or dihydrogenphosphate. Ammonium cation binding by the 4,10,16-triaza-18-crown-6 subunit significantly enhances the strength of the nitrate and nitrite complexation at the triamide recognition site of the receptor. In the presence of ammonium cations, the association constants of nitrate binding reach an impressive value of 1050 M(-1) in highly polar DMSO-d6. Interestingly, the binding of other anions such as chloride and bromide is not enhanced in the presence of a cobound NH4(+) cation. The increased affinity of [1·NH4(+)]PF6(-) for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the receptor 1 cavity, thus allowing for the formation of the ion pair. Under liquid/liquid conditions, receptor 1 is able to extract NH4NO3 from an aqueous to an organic phase, as inferred from (1)H NMR spectroscopic and nitrite/nitrate colorimetric analyses.

A densely decorated disubstituted ferrocene as an ion-pair recognition receptor

A functionalized 1,1'-disubstituted ferrocene acts as an electro-optical ion-pair receptor for Mg(2+), Ca(2+), Cd(2+), and Zn(2+) cations in the presence of a co-bound H2PO4(-) anion, through perturbation of the redox potential of the ferrocene unit, enhancement of the fluorescence and a noticeable colour change.

Boolean operations mediated by an ion-pair receptor of a multi-readout molecular logic gate

A heteroditopic BODIPY dye that performs all basic Boolean operations with a cation (K(+)) and an anion (F(-)) as inputs and absorption, transmission and fluorescence as outputs is described. The molecular logic gate can also act as a digital comparator between the inputs.

Capture and metathesis-based release of potassium salts by a multitopic ion receptor

The multitopic ion-pair receptor 2 is able to recognize and extract various cesium and potassium salts via three different ion recognition modes. Furthermore, it is capable of extracting and then releasing KNO(3)via ion-pair metathesis with CsClO(4), allowing KNO(3) recovery.

KF and CsF Recognition and Extraction by a Calix[4]crown-5 Strapped Calix[4]pyrrole Multitopic Receptor

On the basis of (1)H NMR spectroscopic analyses and single crystal X-ray crystal structural data, the ion-pair receptor 1, bearing a calix[4]pyrrole for anion binding and calix[4]arene crown-5 for cation recognition, was found to act as a receptor for both CsF and KF ion-pairs. Both substrates are bound strongly but via different binding modes and with different complexation dynamics. Specifically, exposure to KF in 10% CD(3)OD in CDCl(3) leads first to complexation of the K(+) cation by the calix[4]arene crown-5 moiety. As the relative concentration of KF increases, then the calix[4]pyrrole subunit binds the F(-) anion. Once bound, the K(+) cation and the F(-) anion give rise to a stable 1:1 ion-pair complex that generally precipitates from solution. In contrast to what is seen with KF, the CsF ion-pair interacts with receptor 1 in two different modes in 10% CD(3)OD in CDCl(3). In the first of these, the Cs(+) cation interacts with the calix[4]arene crown-5 ring weakly. In the second interaction mode, which is thermodynamically more stable, the Cs(+) cation and the counteranion, F(-), are simultaneously bound to the receptor framework. Further proof that system 1 acts as a viable ion-pair receptor came from the finding that receptor 1 could extract KF from an aqueous phase into nitrobenzene, overcoming the high hydration energies of the K(+) and F(-) ions. It was more effective in this regard than a 1:1 mixture of the constituent cation and anion receptors (4 and 5).

An NBD-armed thiacalix[4]arene-derived colorimetric and fluorometric chemosensor for Ag+: a metal–ligand receptor of anions

A new type of thiacalix[4]arene-based chemosensor 2a and 2b containing amino groups as a binding site and 7-nitrobenzo-2-oxa-1,3-diazole (NBD) as a chromophore have been synthesized and characterized by single crystal X-ray diffraction and by NMR spectroscopic methods. Each receptor acted as a strong colorimetric and fluorometric sensor for silver ions and exhibited significant stepwise deprotonation of NH as evidenced by a bathochromic shift in both the UV-vis absorption and fluorescence emission spectra after addition of an Ag(+) cation. Furthermore, the metal-ligand complex 2a·Ag(+) can be exploited as an ion-pair receptor for the recognition of F(-), AcO(-) or I(-) anions through switching the detection signal output (can been distinguished by 'naked eye'). However, receptor 2b·Ag(+) displays a selective response only to iodide ion. These results could be attributed to the slightly different structural conformation between receptors 2a and 2b, which are derived from a 1,3-alternate-thiacalix[4]arene scaffold, suggesting they play a key role in the selective and binding ability of the host chemosensors.

Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated, Cation‐Binding Behavior

A new ditopic ion-pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl(3). Treatment of the halide-ion complexes of 1 with Group I and II metal ions (Li(+), Na(+), K(+), Cs(+), Mg(2+), and Ca(2+); studied as their perchlorate salts in CD(3)CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [1⋅F](-) (preformed as the tetrabutylammonium (TBA(+)) complex), little evidence of interaction with the K(+) ion was seen. In contrast, when this same complex (i.e., [1⋅F](-) as the TBA(+) salt) was treated with the Li(+) or Na(+) ions, complete decomplexation of the receptor-bound fluoride ion was observed. In sharp contrast to what was seen with Li(+), Na(+), and K(+), treating complex [1⋅F](-) with the Cs(+) ion gave rise to a stable, receptor-bound ion-pair complex [Cs⋅1⋅F] that contains the Cs(+) ion complexed within the cup-like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [1⋅Cl](-). In this case, no appreciable interaction was observed with Na(+) or K(+). In addition, treating [1⋅Cl](-) with Li(+) produces a tightly hydrated dimeric ion-pair complex [1⋅LiCl(H(2)O)](2) in which two Li(+) ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [1⋅F](-), exposure of [1⋅Cl](-) to the Cs(+) ion gives rise to an ion-pair complex [Cs⋅1⋅Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.

Switching from Separated to Contact Ion-Pair Binding Modes with Diastereomeric Calix[4]pyrrole Bis-phosphonate Receptors

We describe the design, synthesis and conformational assignment of three diasteromeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole tetrol scaffold. The diastereoisomers differ in the relative spatial orientation of the P═O groups installed at their upper rims. We demonstrate that these compounds act as heteroditopic receptors for ion pairs forming ion-paired 1:1 complexes with alkylammonium (quaternary and primary) chloride salts in dichloromethane (DCM) solution and in the solid-state. (1)H NMR titrations indicate that the complexes are highly stable thermodynamically and kinetically. In the case of tetraalkyl-phosphonium/ammonium chloride guests, the host featuring the two P═O groups directed outwardly with respect to the aromatic cavity, 4oo, produces the most thermodynamically stable complexes. Conversely, for the primary alkyl ammonium chloride, the most effective receptor is the diastereoisomer 4ii with the two P═O groups converging on top of the aromatic cavity. In the nonpolar DCM solvent, the size of the quaternary cation has a strong impact in the thermodynamic stability of the complexes and their binding geometry. We use 2D-ROESY experiments to map out the binding geometries of the 1:1 complexes formed in solution. The 1:1 complexes of the 4oo host with the chloride salts have a separated arrangement of the bound ion-pair. In contrast, those of the 4ii host display a close-contact arrangement. We also investigate the same complexation processes in acetonitrile (ACN) solution. Both the salt and the initially formed anionic complex are fully dissociated in this more polar solvent. The receptors show an analogous trend in their binding affinities for quaternary phosphonium/ammonium chloride salts to the one seen in DCM solution. However, in ACN solution, the magnitudes of the binding affinities are reduced significantly and the size of the cation does not play a role. In addition, the inversion in the trend of relative binding affinities of the complexes, which was revealed in DCM solution, is eradicated in ACN when changing the cation substitution from quaternary to primary.

ChemInform Abstract: Heteroditopic Receptors for Ion‐Pair Recognition.

AbstractReview: 46 refs.

A Phosphorus/Aluminum‐Based Frustrated Lewis Pair as an Ion Pair Receptor: Alkali Metal Hydride Adducts and Phase‐Transfer Catalysis

A Phosphorus/Aluminum‐Based Frustrated Lewis Pair as an Ion Pair Receptor: Alkali Metal Hydride Adducts and Phase‐Transfer Catalysis

A Phosphorus/Aluminum‐Based Frustrated Lewis Pair as an Ion Pair Receptor: Alkali Metal Hydride Adducts and Phase‐Transfer Catalysis

Hilfreiche Frustration: Das frustrierte geminale Phosphor-Aluminium-Lewis-Paar (Mes2P)(tBu2Al)CC(H)Ph (Mes=2,4,6-Me3C6H2) bildet stabile Addukte mit Alkalimetallhydriden (LiH, NaH, KH). Die erhöhte Reaktivität dieser molekularen Hydridkomplexe zeigt sich bei der katalytischen Umwandlung von Chlortriphenylsilan in das entsprechende Hydrid, die über einen SN2-f@Si-Vorderseitenangriff verläuft.

A new ion pair receptor fulfilling a dual function as a chromogenic molecular switch for F− and ratiometric selective recognition of HSO4−

A new ditopic ion pair receptor 1 containing two biindole moieties and a bis-benzocrown ether unit shows a remarkable color switching (ON- and -OFF) function induced by anion (F−) and cation (K+) recognition. The ditopic receptor 1 binds in a cooperative fashion to HSO4− in the presence of 1·K+ and acts as a selective ditopic receptor to recognize ion pairs with a wavelength-ratiometric manner.

Ion‐Pair Recognition by a Heteroditopic Triazole‐Containing Receptor

A new heteroditopic calix[4]diquinone triazole containing receptor capable of recognising both cations and anions through Lewis base and C-H hydrogen-bonding modes, respectively, of the triazole motif has been prepared. This ion-pair receptor cooperatively binds halide/monovalent-cation combinations in an aqueous mixture, with selectivity trends being established by (1)H NMR and UV/Vis spectroscopy. Cation binding by the calix[4]diquinone oxygen and triazole nitrogen donors enhances the strength of the halide complexation at the isophthalamide recognition site of the receptor. Conversely, anions bound in the receptor's isophthalamide cavity enhance cation recognition. (1)H NMR investigations in solution suggest that the receptor's triazole motifs are capable of coordinating simultaneously to both cation and anion guest species. Solid-state X-ray crystallographic structural analysis of a variety of receptor ion-pair adducts further demonstrates the dual cation-anion binding role of the triazole group.

Heteroditopic Receptors for Ion‐Pair Recognition

Ion-pair recognition is a new field of research emerging from cation and anion coordination chemistry. Specific types of heteroditopic receptor designs for ion pairs and the complexity of ion-pair binding are discussed to illustrate key concepts such as cooperativity. The importance of this area of research is reflected by the wide variety of potential applications of ion-pair receptors, including applications as membrane transport and salt solubilization agents and sensors.

Ion vs. ion pair receptor: NMR and DFT study of the interaction of Thallium and Cesium ions and ion pairs with meso-octamethylcalix[4]pyrrole

Binding interaction of Tl+, Cs+, Li+Br−, and Cs+Br− with meso-octamethylcalix[4]pyrrole (1) in nitrobenzene-d5 was studied using 1H, 13C, 7Li, and 133Cs NMR spectroscopy and DFT quantum-chemical calculations. Although originally declared as typical anion and ion pair receptor, 1 is shown to bind fairly strongly Tl+ cations (equilibrium constant K=1600±80mol−1L−1). The binding of Cs+ is much weaker (K=370±18mol−1L−1). Neither of these cations invert the conformation of 1. Li+Br− binds to 1 (K=1790±160mol−1L−1) forming at least four energetically close complexes without much change in its conformation. Cs+ Br− prepared in situ from Li+Br− and the cesium salt of dicarbollyl cobaltate in the presence of 1 binds strongly to 1 (K=103514±4000mol−1L−1, log K=5.015) in spite of the slight competition of Li+ ions. The cooperative binding of Cs+ and Br− ions is shown, in accord with a recent suggestion in literature, to invert the natural conformation of 1, the four NH groups binding to Br− by hydrogen bonds and the aromatic rings forming a cup binding the embedded Cs+ by its interaction with the π electron orbitals.

Oligoether‐Strapped Calix[4]pyrrole: An Ion‐Pair Receptor Displaying Cation‐Dependent Chloride Anion Transport

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1·F](-) and [1·Cl](-)) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li(+), Na(+), K(+), Cs(+), as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1·F](-), no appreciable interaction with the K(+) ion was seen. On the other hand, when this complex was treated with Li(+) or Na(+) ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li(+), Na(+), K(+), treating [1·F](-) with Cs(+) ions gave rise to a stable, host-separated ion-pair complex, [F·1·Cs], which contains the Cs(+) ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1·Cl](-). Here, no appreciable interaction was observed with Na(+) or K(+). In contrast, treating with Li(+) produces a tight ion-pair complex, [1·Li·Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1·F](-), treatment of [1·Cl](-) with Cs(+) ions gives rise to a host-separated ion-pair complex, [Cl·1·Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.

Controlling Cesium Cation Recognition via Cation Metathesis within an Ion Pair Receptor

Ion pair receptor 3 bearing an anion binding site and multiple cation binding sites has been synthesized and shown to function in a novel binding-release cycle that does not necessarily require displacement to effect release. The receptor forms stable complexes with the test cesium salts, CsCl and CsNO(3), in solution (10% methanol-d(4) in chloroform-d) as inferred from (1)H NMR spectroscopic analyses. The addition of KClO(4) to these cesium salt complexes leads to a novel type of cation metathesis in which the "exchanged" cations occupy different binding sites. Specifically, K(+) becomes bound at the expense of the Cs(+) cation initially present in the complex. Under liquid-liquid conditions, receptor 3 is able to extract CsNO(3) and CsCl from an aqueous D(2)O layer into nitrobenzene-d(5) as inferred from (1)H NMR spectroscopic analyses and radiotracer measurements. The Cs(+) cation of the CsNO(3) extracted into the nitrobenzene phase by receptor 3 may be released into the aqueous phase by contacting the loaded nitrobenzene phase with an aqueous KClO(4) solution. Additional exposure of the nitrobenzene layer to chloroform and water gives 3 in its uncomplexed, ion-free form. This allows receptor 3 to be recovered for subsequent use. Support for the underlying complexation chemistry came from single-crystal X-ray diffraction analyses and gas-phase energy-minimization studies.