Hydrogen Bonding and Solvent Effects on Complexation of Alkali Metal Cations by Lower Rim Calix[4]arene Tetra(O-[N -acetyl-D -phenylglycine methyl ester]) Derivative

Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-D-α-phenylglycylcarbonylmethoxy)calix[4]arene (L) was studied by means of spectrophotometric, conductometric and potentiometric titrations at 25 °C. The solvent effect on the binding ability of L was examined by using two solvents with different affinities for hydrogen bonding, viz. methanol and acetonitrile. Despite the presence of intramolecular NH···O=C hydrogen bonds in L, which need to be disrupted to allow metal ion binding, this calix[4]arene amino acid derivative was shown to be an efficient binder for smaller Li⁺ and Na⁺ cations in acetonitrile (lg K_LiL > 5, lg K_NaL = 7.66), moderately efficient for K⁺ (lg K_KL = 4.62), whereas larger Rb⁺ and Cs⁺ did not fit in its hydrophilic cavity. The complex stabilities in methanol were significantly lower (lg K_NaL = 4.45, lg K_KL = 2.48). That could be explained by different solvation of the cations and by competition between the cations and methanol molecules (via hydrogen bonds) for amide carbonyl oxygens. The influence of cation solvation on complex stability was most pronounced in the case of Li⁺ for which, contrary to the quite stable LiL⁺ complex in acetonitrile, no complexation was observed in methanol under the conditions used.     

Ditopic Receptors based on Lower‐ and Upper‐Rim Substituted Hexahomotrioxacalix[3]arenes: Cation‐Controlled Hydrogen Bonding of Anion

Heteroditopic hexahomotrioxacalix[3]arene receptors that are capable of binding an anion and a cation simultaneously in a cooperative fashion were synthesized. The structure of one of the triamide derivatives was confirmed by single‐crystal X‐ray diffraction. The binding of alkali metals at the lower rim, and the binding of anions (chloride, bromide) at the upper rim, has been investigated by using ¹H NMR titration experiments. Alkali metal binding at the lower rim controls the calix cavity. Li⁺‐ion binding to the lower rim can improve the binding ability of anions at the upper rim amide moiety by a factor of 15, thus suggesting a strong positive allosteric effect for anion recognition. However, when a Na⁺ cation is bound to the ionophoric site on the lower rim, the calix cavity is changed from a “flattened cone” to a more‐upright form, which is favored for intramolecular hydrogen bonding between the neighboring NH and C?O groups; this change can block the inclusion of anions onto the amide moiety at the upper rim, which strongly suggests a negative allosteric effect of Na⁺‐ion binding, which controls the cooperative recognition system.     

Thermodynamics of Complexation of Alkali Metal Cations by a Lower-Rim Calix[4]arene Amino Acid Derivative

Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-d-α-phenylglycylcarbonylmethoxy)calix[4]arene (L) in methanol and acetonitrile was studied by means of direct and competitive microcalorimetric titrations at 25 °C. The thermodynamic parameters of complexation reactions showed that all the reactions investigated were enthalpically controlled. In both solvents the reaction enthalpy was most favorable for Na⁺ binding with L leading to the highest affinity of the examined calix[4]arene derivative towards this cation. The solubilities (and consequently the solution Gibbs energies) of the ligand were determined, as were the corresponding solution enthalpies and entropies. No significant difference was observed between the solution thermodynamic quantities of L in the two solvents, whereas the transfer of complex species from methanol to acetonitrile was found to be quite favorable. The interactions of solvent molecules with the free and the complexed ligand were investigated by ¹H NMR spectroscopy. It was concluded that in both cases inclusion of an acetonitrile molecule into the hydrophobic cavity of L occurred, which significantly affected the cation complexation in this solvent. The thermodynamic data were discussed regarding the structural properties of the ligand, the free and the complexed cations as well as the solvation abilities of the solvents examined. In this respect, the specific solvent-solute interactions and the intramolecular NH⋅⋅⋅O=C hydrogen bonds at the lower rim of L were particularly addressed.     

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.     

Competitive anion binding aptitude of benzimidazole and amide functionality of a non-symmetrical receptor in solid state and solution phase

A novel non-symmetric tripodal receptor (L) with benzimidazole and amide –NH functionalities has been synthesised and a comparative study for anion binding aptitude of those functionalities has been performed in solid state and solution phase. The protonated receptors in solid state tend to form a pseudo-cavity around the octahedral hexafluorosilicate anion whilst in second complex, binding of one sulphate and two bisulphates in unison by two protonated units arises. The benzimidazole –NHs are the strongest hydrogen bond donors in both the crystals controlling dominance over amide –NHs. The neutral receptor being selective towards fluoride anion induces a naked eye visible colorimetric change. The ¹H NMR studies with the neutral and protonated form of the receptor suggest the strong involvement of amide –NH in anion binding in solution phase.     

The Synthesis of Novel Macrocycles,Part V. The Coumarin Crown Ethers and Cation Binding with Fluorescence Spectra

Abstract

The 4-H, 4-methyl and 4-phenyl derivatives of benzo-α-pyrone of 12-crown-4 and 15-crown-5 were synthesised starting from 4-substituted-6,7-dihydroxy- and 7,8-dihydroxybenzo-α-pyrones which reacted with dichloropolyethylene glycols in DMF/water/alkali carbonate. The coumarin-macrocycles were identified by elemental analysis, IR, EI-GC-MS as well as ¹H, ¹³C NMR spectroscopy. The full experimental and spectral data is reported along with ion binding data studied in acetonitrile using fluorescence spectroscopy. The binding of the fluorogenic coumarin-crowns with Li⁺, Na⁺ and K⁺ were recognized as specific alterations on their fluorescence spectra that strongly originated from the structures. The observed CEQFS depending on the bound cation radii and macrocycle size evidenced the rules of cationic recognition of macrocycles. Some 15-crown-5 derivatives exhibited interesting Li⁺ and Na⁺ binding selectivities.     

Hydrogen Bonding and Solvent Effects on Complexation of Alkali Metal Cations by Lower Rim Calix[4]arene Tetra(O-[N-acetyl-D-phenylglycine methyl ester]) Derivative

Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-D-α-phenylglycylcarbonylmethoxy)calix[4]arene (L) was studied by means of spectrophotometric, conductometric and potentiometric titrations at 25 °C. The solvent effect on the binding ability of L was examined by using two solvents with different affinities for hydrogen bonding, viz. methanol and acetonitrile. Despite the presence of intramolecular NH···O=C hydrogen bonds in L, which need to be disrupted to allow metal ion binding, this calix[4]arene amino acid derivative was shown to be an efficient binder for smaller Li⁺ and Na⁺ cations in acetonitrile (lg K _LiL  > 5, lg K _NaL  = 7.66), moderately efficient for K⁺ (lg K _KL  = 4.62), whereas larger Rb⁺ and Cs⁺ did not fit in its hydrophilic cavity. The complex stabilities in methanol were significantly lower (lg K _NaL  =  4.45, lg K _KL  = 2.48). That could be explained by different solvation of the cations and by competition between the cations and methanol molecules (via hydrogen bonds) for amide carbonyl oxygens. The influence of cation solvation on complex stability was most pronounced in the case of Li⁺ for which, contrary to the quite stable LiL ⁺ complex in acetonitrile, no complexation was observed in methanol under the conditions used.     

Experimental and computational studies of the binding properties of lower rim tetra- and di-substituted calix[4]arene amide derivatives with metal ions

Abstract

Experimental and theoretical binding studies of representative alkali, alkaline earth, transition, heavy metal and lanthanide cations by tetra- and di-substituted calix[4]arene amide derivatives (diethyl amide 1a–c and morpholide amide 2a–c) in the cone conformation were carried out. Binding was assessed by extraction experiments of the metal picrates from water to dichloromethane and proton NMR titrations. Density functional theory calculations were also performed to determine the binding energy of the complexes formed and to analyse the host–guest interaction modes. In the cases of ligands 1b and 2c with Na⁺ and Ag⁺ picrates, the extraction energy was also determined using the polarisable continuum model. The results are discussed in terms of the nature of the amide residue and the substitution pattern (1,3 vs. 1,2). Both tetra-amide derivatives are good extractants, showing preference for Na⁺, Ca²⁺, Ag⁺ and Pb²⁺ cations, mainly di-ethylamide 1a. Concerning di-amide derivatives, the relative position of the substituents seems to be more important than the nature of the amide group in the extraction process. Proton NMR studies indicate the formation of 1:1 complexes between the amides and the cations studied, and DFT data show that all ligands form the most stable complexes with La³⁺.     

Ion Interactions with a New Ditopic Naphthalimide‐Based Receptor: A Photophysical,NMR and Theoretical (DFT) Study

A new multi‐component chemosensor system comprising a naphthalimide moiety as fluorophore is designed and developed to investigate receptor–analyte binding interactions in the presence of metal and non‐metal ions. A dimethylamino moiety is utilized as receptor for metal ions and a thiourea receptor, having acidic protons, for binding anions. The system is characterized by conventional analytical methods. The absorption and fluorescence spectra of the system consist of a broad band typical for an intramolecular charge transfer (ICT). The effects of various metal‐ion additives on the spectral behavior of the present sensor system are examined in acetonitrile. It is found that among the metal ions studied, alkali/alkaline earth‐metal ions and transition‐metal ions modulate the absorption and fluorescence spectra of the system. As an additional feature, the anion signaling behavior of the system in acetonitrile is studied. A decrease in fluorescence efficiency of the system is observed upon addition of fluoride and acetate anions. Fluorescence quenching is most effective in the case of fluoride ions. This is attributed to the enhancement of the photoinduced electron transfer from the anion receptor to the fluorophore moiety. Hydrogen‐bond interactions between the acidic NH protons of the thiourea moiety and the F^? anions are primarily attributed to the fluoride‐selective signaling behavior. Interestingly, a negative cooperativity for the binding event is observed when the interactions of the system are studied in the presence of both Zn²⁺ and F^? ions. NMR spectroscopy and theoretical calculations are also carried out to better understand the receptor–analyte binding.     

A Novel Colorimetric Reagent for Potassium Based on Crown Ether Complex Formation

Abstract

The extraction study of alkali metal ions was made with a new type of crown ether, 4′-picrylaminobenzo-15-crown-5 (HL). Upon dissociation in alkaline medium orange-colored HL gives blood-red anion, L^?, and extracts selectively K⁺ (and to a lesser extent Rb⁺) ion into chloroform as a colored complex of composition ML·HL. A colorimetric determination of 10 - 400 ppm K⁺ in the presence of < 2000 ppm Na⁺ was possible using this new crown ether reagent.     

The cation binding of benzo crown ethers in acetonitrile using fluorescence spectroscopy

The effects of Na⁺, K⁺ and Li⁺ cations on the fluorescence spectra of benzo[15]crown-5, benzo[18]crown-6 and dibenzo[18]crown-6 were investigated in acetonitrile. The alkali cation role observed was usually the complexation-enhanced quenching fluorescence effect (CEQF) in acetonitrile due to the increased fluorescence quenching rate of the complexed fluoroionophore. The association constants for 1 :1 stoichiometry InK _a have been obtained using the relationship 1/K _a[L ₀] = (1 –P)²/P. It was shown that the preferential interaction rule of compatibility of cationic radii and macrocyclic ring size is in excellent agreement with the association constants obtained by fluorescence spectroscopy. The order of InK _a found for benzo[15]crown-5 complexation was Li⁺ > Na⁺ > K⁺ and K⁺ > Na⁺ > Li⁺ for benzo[18]crown-6 in acetonitrile.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August, 1995.     

Simple indole-based colorimetric sensors with electron-withdrawing chromophores: Tuning selectivity in anion sensing

Three simple colorimetric anion sensors (1, 2, and 3) containing anthrone, 1,3-indanedione, and malononitrile as signaling chromophores and an indole binding site have been designed and synthesized. The introduction of electron-withdrawing groups can not only provide chromogenic signal output, but also tune the sensitivity and selectivity of indole-based anion sensors by electron push–pull features. Their anion binding and sensing properties were investigated in detail by dramatic color changes, UV–vis absorption, and ¹H NMR. As results revealed, sensor 1 showed high selectivity for F⁻ over AcO⁻ and H₂PO₄⁻ with a distinct change in color due to the deprotonation of indole NH group. The excellent selectivity of 1 for F⁻ can be attributed to the fitness in the acidity of its NH-group, which is tuned to be able to distinguish the subtle difference in the affinity of F⁻, AcO⁻, and H₂PO₄⁻ to NH proton.     

Anion recognition and cation-induced molecular motion in a heteroditopic [2]rotaxane

A heteroditopic [2]rotaxane consisting of a calix[4]diquinone–isophthalamide macrocycle and 3,5‐bis‐amide pyridinium axle components with the capability of switching between two positional isomers in response to barium cation recognition is synthesised. The anion binding properties of the rotaxane’s interlocked cavity together with Na⁺, K⁺, NH₄⁺ and Ba²⁺ cation recognition capabilities are elucidated by ¹H NMR and UV‐visible spectroscopic titration experiments. Upon binding of Ba²⁺, molecular displacement of the axle’s positively charged pyridinium group from the rotaxane’s macrocyclic cavity occurs, whereas the monovalent cations Na⁺, K⁺ and NH₄⁺ are bound without causing significant co‐conformational change. The barium cation induced shuttling motion can be reversed on addition of tetrabutylammonium sulfate.     

Polarography of Alkali Metal ion Complexes of Macrotetrolides: Complex ion Size and Stability

Abstract

The stability constants of several alkali metal ion complexes with the macrotetrolides were determined polarographically in acetonitrile. For any single alkali metal ion the stability constant increased in the order nonactin < monactin < dinactin < trinactin. Their values are larger than those found in methanol. The Stoke's radii estimated from the limiting diffusion currents of the complexes increase with increasing crystallographic radius of the cation. The increasing strain on the ligand causes a decrease of stability constant for Rb⁺ and Cs⁺.     

Synthesis of a new colorimetric receptor based on calix[4]pyrrole binding oxime for selective recognition of fluoride ions

A colorimetric anion sensor α-meso-heptaethylcalix[4]pyrrole-meso-3-aminophenyl-p-nitrobenzaldoxime was synthesised and characterised by various spectroscopic techniques. Anion-binding studies were carried out using UV–vis, and ¹H NMR titrations, revealing that the receptor exhibits selective recognition towards F^?  over other anions. The selectivity for F^?  among the halides is attributed mainly to the hydrogen bond interaction of the receptor with F^? . Receptor showed colour change from colourless to yellow in the presence of tetrabutylammonium fluoride with 1:2 stoichiometry. Cyclic voltammetry studies, carried out in CH₃CN, provided evidence of an anion-dependent electrochemical response with F^?  ion. This response was particularly dramatic in the case of receptor after the addition of ～1 equiv. of F^?  ion.     

A coumarin-based fluorescent and colorimetric chemosensor for rapid detection of fluoride ion

A novel coumarin-based compound 1 featuring thiosemicarbazone as binding unit, was reported as a colorimetric and fluorescent probe for the detection of fluoride anion. The addition of F^? to a solution of probe 1 in tetrahydrofuran resulted in evident naked-eye color change from green-yellow to orange-red under daylight and obvious fluorescence quenching within 3 s. And the detection limit toward F^? was calculated to be as low as 2.16 × 10^?7 mol/L. ¹H NMR titrations proved that the interaction between 1 and fluoride ion: hydrogen bond at low fluoride ion concentration, deprotonation at high fluoride ion concentration. Besides, it exhibited highly sensitivity and selectivity for F^? over other examined ions (Cl^?, Br^?, I^?, AcO^?, NO₃^?, HSO₄^?, H₂PO₄^?) in tetrahydrofuran solution.     

An Interplay of Cooperativity between Cation⋅⋅⋅π, Anion⋅⋅⋅π and C?H⋅⋅⋅Anion Interactions

Mixed cation (Li⁺, Na⁺ and K⁺) and anion (F^?, Cl^?, Br^?) complexes of the aromatic π‐surfaces (top and bottom) are studied by using dispersion‐corrected density functional theory. The selectivity of the aromatic surface to interact with a cation or an anion can be tuned and even reversed by the electron‐donating/electron‐accepting nature of the side groups. The presence of a methyl group in the ? OCH₃, ? SCH₃, ? OC₂H₅ in the side groups of the aromatic ring leads to further cooperative stabilization of the otherwise unstable/weakly stable anion???π complexes by bending of the side groups towards the anion to facilitate C? H???anion interactions. The cooperativity among the interactions is found to be as large as 100 kcal mol^?1 quantified by dissection of the three individual forces from the total interaction energy. The crystal structures of the fluoride binding tripodal and hexapodal ligands provide experimental evidence for such cooperative interactions.     

The structure of a valinomycin–hexaaquamagnesium trifluoromethanesulfonate compound

Valinomycin is a naturally occurring cyclic dodecadepsipeptide with the formula cyclo‐[d ‐HiVA→l ‐Val →l ‐LA→l ‐Val]₃ (d ‐HiVA is d ‐α‐hydroxyisovaleic acid, Val is valine and LA is lactic acid), which binds a K⁺ ion with high selectively. In the past, several cation‐binding modes have been revealed by X‐ray crystallography. In the K⁺, Rb⁺ and Cs⁺ complexes, the ester O atoms coordinate the cation with a trigonal antiprismatic geometry, while the six amide groups form intramolecular hydrogen bonds and the network that is formed has a bracelet‐like conformation (Type 1 binding). Type 2 binding is seen with the Na⁺ cation, in which the valinomycin molecule retains the bracelet conformation but the cations are coordinated by only three ester carbonyl groups and are not centrally located. In addition, a picrate counter‐ion and a water molecule is found at the center of the valinomycin bracelet. Type 3 binding is observed with divalent Ba²⁺, in which two cations are incorporated, bridged by two anions, and coordinated by amide carbonyl groups, and there are no intramolecular amide hydrogen bonds. In this paper, we present a new Type 4 cation‐binding mode, observed in valinomycin hexaaquamagnesium bis(trifluoromethanesulfonate) trihydrate, C₅₄H₉₀N₆O₁₈·[Mg(H₂O)₆](CF₃SO₃)₂·3H₂O, in which the valinomycin molecule incorporates a whole hexaaquamagnesium ion, [Mg(H₂O)₆]²⁺, via hydrogen bonding between the amide carbonyl groups and the hydrate water H atoms. In this complex, valinomycin retains the threefold symmetry observed in Type 1 binding, but the amide hydrogen‐bond network is lost; the hexaaquamagnesium cation is hydrogen bonded by six amide carbonyl groups. ¹H NMR titration data is consistent with the 1:1 binding stoichiometry in acetonitrile solution. This new cation‐binding mode of binding a whole hexaaquamagnesium ion by a cyclic polypeptide is likely to have important implications for the study of metal binding with biological models under physiological conditions.     

Comparison of ionization behaviors of ring and linear carbohydrates in MALDI-TOFMS

Ionization efficiencies of cyclodextrins and their linear compounds in matrix-assisted laser desorption and ionisation (MALDI) analysis were compared, and differences in the ionization efficiencies of α- and β-cyclodextrins were also studied. The mass spectra showed a series of the [M+cation]⁺ ions but not the [M+H]⁺ ions. Alkali metal salts of Li⁺, Na⁺, K⁺, and Cs⁺ were used as the cationizing agents to enhance the ionization efficiency. Relative ion intensities of the ring compounds (α- and β-cyclodextrins) were much larger than those of the linear ones (maltohexaose and maltoheptaose), and the difference showed an increasing trend with the size of the alkali metal cation. β-Cyclodextrin had higher ionization efficiency than α-cyclodextrin and the difference increased by increasing the size of the alkali metal cation. It was also found that the ionization efficiency was affected by the counter anion of the salt. The higher ionization efficiencies of cyclodextrins were explained with the number of coordination sites and the binding energies.     

Anion,cation and ion-pair recognition by bis-urea based receptors containing a polyether bridge

Two bis-urea type receptors were synthesized containing a polyether bridge and two 4-nitrophenyl groups as chromogenic units, R1 and R2. Molecular recognition studies of receptors towards different tetraalkylammonium and alkali metal salts were carried out in DMSO by UV-Vis and ¹H-NMR spectroscopy. The receptors were found to have high affinity for diverse anions and ion-pairs, showing the highest affinities towards the tetramethylammonium and sodium salts. The cation binding ability of the receptors was evidenced by means ¹H NMR, mass spectrometric ESI⁺ studies and the crystal structures of some precursors. Additionally, the molecular modelling at the DFT level of the tetramethylammonium acetate complexes illustrates the potential ion-pair binding ability of the receptors: the anion is recognized through strong hydrogen bonds from the NH─ groups from the two urea sites, while the cation is bound by a combination of cation─π, C-H···O and C-H···π interactions.