Complex Formation between Cucurbit[n]urils and Alkali, Alkaline Earth and Ammonium Ions in Aqueous Solution

The complex formation between cucurbit[5]uril, decamethylcucurbit[5]uril and cucurbit[6]uril and alkali, alkaline earth and ammonium cations is examined. The solubility of these ligands is rather small in aqueous solution. In the presence of salts the solubility normally increases due to the formation of complexes. The total concentration of the ligands can be easily measured from the total organic carbon content of the salt solutions saturated with the ligand. From these results it is possible to calculate the stability constants of the complexes formed even without the knowledge of the exact solubility of the ligand.     

Complexation behavior of cucurbit[6]uril with short polypeptides

The binding properties of cucurbit[6]uril towards various peptides have been investigated in acidic aqueous solution. Stability constants and thermodynamic values of the complex formation between following peptides: glycyl-l-alanine, l-leucyl-l-valine, glycyl-l-asparagine, l-leucyl-l-phenylalanine, l-leucyl-l-tryptophan, glycyl-l-histidine, l-glutathione reduced (γ-l-glutamyl-l-cysteinyl-glycine, GSH), and dl-leucyl-glycyl-dl-phenylalanine) with cucurbit[6]uril in aqueous formic acid (50%, v/v) have been calculated from calorimetric titrations. From these results it can be seen that the peptides form exclusion complexes with cucurbit[6]uril. Due to the polar peptide bond they are not included within the hydrophobic cavity of cucurbit[6]uril. The complex formation is favoured by entropic contributions. The release of water molecules from the polar amino groups of the peptides and the carbonyl groups of cucurbituril are responsible.     

Complex Formation of Crown Ethers with the Cucurbit[6]uril–Spermidine and Cucurbit[6]uril–Spermine Complex in Aqueous Solution

Alkyl amines are able to form complexes with either crown ethers or cyclodextrins or cucurbit[6]uril. The same is known for polyamines such as spermidine and spermine. However, the simultaneous formation of such polyamines with crown ethers and cucurbit[6]uril has not been studied. The ability of polyamines such as spermidine and spermine to form mixed complexes with different ligands, e.g. crown ethers and cucurbit[6]uril has been studied in aqueous solution using pH-metric and calorimetric titrations. The thermodynamic data of reaction between crown ethers with spermidine, spermine and their cucurbit[6]uril complexes have been determined. The presence of cucurbit[6]uril on the polyamines has no important influence upon the reaction of these amines with crown ethers. The reactions between polyamines, cucurbit[6]uril and crown ethers are simple examples for the self organization of molecules due to specific interactions. Received in final form: 26 January 2005     

Cucurbit[5]uril, Decamethylcucurbit[5]uril and Cucurbit[6]uril. Synthesis, Solubility and Amine Complex Formation

A simple way to prepare cucurbit[5]uril is described. The macrocycles of the cucurbituril type are nearly insoluble in water. The solubilities of cucurbit[5]uril, decamethylcucurbit[5]uril and cucurbit[6]uril in hydrochloric acid, formic acid and acetic acid of different concentrations have been investigated. Due to the formation of complexes between cucurbit[n]urils and protons the solubility increases in aqueous acids. The macrocyclic ligands are able to form complexes with several organic compounds. Thus, the complex formation of the cucurbituril macrocycles with different amines has beenstudied by means of calorimetric titrations. The reaction enthalpy gives noevidence of the formation of inclusion or exclusion complexes. ¹H-NMR measurements show that in the case of cucurbit[5]uril and cucurbit[6]uril the organic guest compound is included within the hydrophobic cavity. Decamethylcucurbit[5]uril forms only exclusion complexes with organicamines. This was confirmed by the crystal structure of the decamethylcucurbit[5]uril-1,6-diaminohexane complex.     

11-Aminoundecanoic acid, Cyclodextrins (α, β) and Cucurbit[n]urils (n = 6, 7) as Building Blocks for Supramolecular Assemblies: A Thermodynamic Study

The formation of 1:1 complexes of α-, β-cyclodextrin, cucurbit[6]uril, and cucurbit[7]uril with 11-aminoundecanoic acid have been studied using calorimetric titrations. The influence of solvent composition (aqueous formic acid) upon the complex stability and the values of the reaction enthalpies and entropies has been studied in the case of α-cyclodextrin (α-CD). With increasing concentration of formic acid the values of the reaction enthalpy decrease and of the reaction entropy increase. All ligands examined form 1:1 complexes with 11-aminoundecanoic acid under the experimental conditions. However, it is also possible to study the formation of 2:1 complexes (ligand:aminocarboxylic acid ratio). Even the formation of mixed 1:1:1 complexes with two different ligands (ligand(1):ligand(2):aminocarboxylic acid ratio) can be measured.     

The formation of homogeneous and heterogeneous 2:1 complexes between dialkyl- and diarylammonium ions and α-cyclodextrin and cucurbit[6]uril in aqueous formic acid

Dialkyl- and diarylammonium ions are able to form complexes with α-cyclodextrin and cucurbit[6]uril. These amines are able to complex two guest molecules simultaneously resulting in the formation of homogeneous or heterogeneous 1:2 (ratio of dialkylammonium to ligand) complexes. The stability constants and reaction enthalpies for the formation of 1:1 complexes have been measured using potentiometric and calorimetric titrations. Differences between the values obtained by these methods can be attributed to solvent composition. Only for the 1:2 complex formation with cucurbit[6]uril, the ligands influenced each other. The polar carbonyl groups at each portal of the cucurbit[6]urils interacted simultaneously with the protonated amino group resulting in an electrostatic repulsion between both molecules. No further interactions between two complexed molecules of α-cyclodextrin or cucurbit[6]uril and α-cyclodextrin were observed. The absence of polar groups in the case of α-cyclodextrin led to unaffected formation of homogeneous and even heterogeneous 1:2 complexes.     

Cucurbit[6]uril as Ligand for the Complexation of Diamines, Diazacrown Ethers and Cryptands in Aqueous Formic Acid

The complex formation between cucurbit[6]uril and different diamines, diazacrown ethers, and cryptands has been studied in aqueous formic acid solution. The complex stabilities and the thermodynamic values for the complex formation of diamines are reduced if any further donor atom (e.g., sulfur, oxygen, or nitrogen) is present in the molecules. The inclusion of this polar group inside the cavity of cucurbit[6]uril has a negative effect upon the complex formation. Diazacrown ethers and cryptands do not form inclusive complexes. One nitrogen donor atom interacts with the carbonyl groups at one of the portals.     

Cucurbit[6]uril as Ligand for the Complexation of Diamines, Diazacrown Ethers and Cryptands in Aqueous Formic Acid

The complex formation between cucurbit[6]uril and different diamines, diazacrown ethers, and cryptands has been studied in aqueous formic acid solution. The complex stabilities and the thermodynamic values for the complex formation of diamines are reduced if any further donor atom (e.g., sulfur, oxygen, or nitrogen) is present in the molecules. The inclusion of this polar group inside the cavity of cucurbit[6]uril has a negative effect upon the complex formation. Diazacrown ethers and cryptands do not form inclusive complexes. One nitrogen donor atom interacts with the carbonyl groups at one of the portals.     

Host–guest interactions of thiabendazole with normal and modified cucurbituril: 1H NMR,phase solubility and antifungal activity studies

Guest–host inclusion complexes between thiabendazole (TBZ) and cucurbit[7]uril (Q[7]), symmetrical tetra-methylcucurbit[6]uril (TMeQ[6]) and meta-hexamethyl-substituted cucurbit[6]uril (HMeQ[6]) in aqueous solution were investigated by ¹H NMR spectroscopy and phase solubility studies. The antifungal activities of the inclusion complexes were also determined. Analysis of the ¹H NMR spectra revealed that the host Q[7] selectively binds the benzimidazole ring moiety of the guest molecule and that the thiazole ring is encapsulated into the cavities of TMeQ[6] and HMeQ[6]. Phase solubility diagrams were analysed using rigorous procedures to obtain estimates of the complex formation constants for Q[n]-TBZ complexation. The phase solubility studies showed that TBZ solubility increased as a function of Q[7], TMeQ[6] and HMeQ[6] concentrations. We found that complexation of TBZ with Q[n] increased the inhibitory effect of TBZ on the growth of Fusarium graminearum. Our results thus demonstrate that complexation of TBZ with Q[n] could be used to improve the solubility and antifungal activity of TBZ.     

葫芦[8]脲的环套环分子组装

以4,4'-联吡啶鎓、 2,6-萘二酚和2,7-萘二酚为基本结构单元, 设计合成了2种带有分子内给受体相互作用的大环化合物, 并采用紫外光谱和核磁共振等手段研究了其与葫芦[8]脲的键合行为. 研究结果表明, 在水溶液中大环的2,6-萘二酚和2,7-萘二酚与4,4'-联吡啶鎓之间存在分子内的电荷转移相互作用, 而葫芦[8]脲可以键合这2种大环化合物, 导致电荷转移吸收峰增强并红移, 表明葫芦[8]脲能增强这种分子内的电荷转移相互作用, 形成稳定的环套环分子组装体. 这种结构是基于葫芦[8]脲的新型拓扑结构.     

Regioselective photodimerization of cinnamic acids in water: Templation with cucurbiturils

Cinnamic acids upon irradiation in solution undergo geometric isomerization while dimerizing to different dimers in the crystalline state. Controlling the nature of the dimer formed upon irradiation remains a challenging task. We have aligned a variety of cinnamic acid molecules in a head-head fashion employing cucurbit[8]uril, a weakly water soluble host as a template. The water solubility of cucurbit[8]uril is enhanced by inclusion of water soluble cinnamic acids and positions the olefins in an arrangement that favors the formation of syn head-head cyclobutanes in near quantitative yields. This methodology works in both solid state as well as in aqueous solution. Irradiation of cinnamic acid complexes with gamma-cyclodextrin has been carried out as a comparison. We find that while cucurbit[8]uril functions well both in solid state and aqueous solution, cyclodextrin works best as solid complexes only. Consistent with the postulated requirement of large cavities for templating olefins to dimerization, irradiation of complexes of cinnamic acid with cucurbit[7]uril resulted in only the corresponding cis isomers.     

Mononuclear and Dinuclear Cobalt Complexes and its Dioxygen Adduct with a New 24-membered Hexaaza Macrocyclic Ligand

Abstract

Dinucleating 24-membered hexaazadiphenol macrocyclic ligand 3,6,9,17,20,23-hexaaza-29,30-dihydroxy-13,27-dimethyl-tricyclo[23,3,1,1^11,15] triaconta-1(29), 11,13,15(30),25,27-hexaene (L or BDBPH), is prepared by the NaBH₄ reduction of the Schiff base obtained from [2+2] template condensation of 2,6-diformyl-p-cresol with diethylenetriamine. The ligand maintains dinuclear integrity for cobalt (II) while facilitating the formation of bridging phenolate dicobalt cores. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear cobalt (II) complexes form from p[H] 2 through 11 in aqueous solution. The protonation constants of this ligand and stability constants of the 1:1, 1:2 ligand: cobalt(II) complexes were determined in KCl supporting electrolyte (μ = 0.100 M) at 25°C. The mechanism for the formation of dinuclear dioxygen cobalt(II) complexes is also described. The stability constants of mononuclear and dinuclear cobalt complexes were determined under nitrogen. Preliminary results show that the dinuclear dioxygen cobalt (II) complexes do not catalyze hydroxylation of adamantane in the presence of H₂S as two-electron reductant.     

Primary photoprocesses of 3,3’-diethylthiacarbocyanine in the presence of cucurbit[7]uril: Laser flash photolysis and quantum chemical calculations

The effect of cucurbit[7]uril on the phototransformation of 3,3′-diethylthiacarbocyanine iodide dye in aqueous solution has been investigated by nanosecond laser flash photolysis. The presence of cucurbit[7]uril results in the formation of its complex with the dye molecule producing a dimer. The dimer formation is evident from the ground and triplet-triplet absorption spectra. The dimers in the triplet state are capable of electron transfer. The structure of the complexes is suggested on the basis of quantum chemical calculations.     

New bis-cresol-bridged bis(1,4,7-triazacyclononane) ligand as receptor for metal cations and phosphate anions

The synthesis and characterization of a new bis([9]aneN3) ligand (H2L) containing two [9]aneN3 macrocyclic moieties separated by a 2,2'-methylene-bis-cresol (cresol = 4-methyl-phenol) unit is reported. A potentiometric and (1)H NMR study in aqueous solution reveals that H2L is in a zwitterionic form, and protonation of the cresolate oxygens occurs only with the formation of the highly charged (H5L)(3+) and (H6L)(4+) species at acidic pH values. The coordination properties of H2L toward Cu(II), Zn(II), Cd(II), and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand gives both mono- and binuclear complexes in aqueous solution. At acidic pH values the ligand forms stable binuclear [M2H2L](4+) complexes, where each metal is coordinated by two amine groups of [9]aneN3 and the deprotonated oxygen of the adjacent cresol unit; the remaining amine group is protonated. Deprotonation of the [M2H2L](4+) species at alkaline pH values affords [M2L](2+) complexes, where all amine groups of the [9]aneN3 moieties are involved in metal coordination. Binding of mono-, di- and triphosphate, and adenosine triphosphate (ATP) was studied by means of potentiometric, (1)H and (31)P NMR measurements and by molecular dynamics simulations. The receptor forms stable 1:1 adducts with di-, triphosphate, and ATP, while the interaction with monophosphate is too low to be detected. In the complexes both the [9]aneN3 moieties act cooperatively in the substrate binding process. The stability of the adducts increases in the order diphosphate < triphosphate < ATP. This trend is explained in terms of increasing number of charge-charge interactions between the phosphate chains and the protonated [9]aneN3 subunits and, in the case of ATP, of stacking interactions between the adenine and cresol units.     

Complex formation of Ni(II), Cu(II), Pd(II), and Co(III) with 1,2,3,4-tetraaminobutane

Complex formation of the two tetraamine ligands (2S,3S)-1,2,3,4-tetraaminobutane (threo-tetraaminobutane, ttab) and (2R,3S)-1,2,3,4-tetraaminobutane (erythro-tetraaminobutane, etab) with Co(III), Ni(II), Cu(II), and Pd(II) was investigated in aqueous solution and in the solid state. For Ni(II) and Cu(II), the pH-dependent formation of a variety of species [Mn(II)xLyHz](2x+z)+ was established by potentiometric titrations and UV/Vis spectroscopy. In sufficiently acidic solutions the divalent cations formed a mononuclear complex with the doubly protonated ligand of composition [M(H2L)]4+. An example of such a complex was characterized in the crystal structure of [Pd(H2ttab)Cl2]Cl2.H2O. If the metal cation was present in excess, increase of pH resulted in the formation of dinuclear complexes [M2L]4+. Such a species was found in the crystal structure of [Cu2(ttab)Br4].H2O. Excess ligand, on the other hand, lead to the formation of a series of mononuclear bis-complexes [Mq(HxL)(HyL)](q+x+y)+. The crystal structure of [Co(Hetab)2][ZnCl4]2Cl. H2O with the inert, trivalent Co(III) center served as a model to illustrate the structural features of this class of complexes. By using an approximately equimolar ratio of the ligand and the metal cation, a variety of polymeric aggregates both in dilute aqueous solution and in the solid state were observed. The crystal structure of Cu2(ttab)3Br4, which exhibits a two-dimensional, infinite network, and that of [Ni8(ttab)12]Br16.17.5H2O, which contains discrete chiral [Ni8(ttab)12]16+ cubes with approximate T symmetry, are representative examples of such polymers. The energy of different diastereomeric forms of such complexes with the two tetraamine ligands were analyzed by means of molecular mechanics calculations, and the implications of these calculations for the different structures are discussed.     

Encapsulation of metal cations and anions within the cavity of bis(1,4,7-triazacyclononane) receptors

The synthesis and characterisation of a new bis([9]aneN3) ligand (L4) containing two [9]aneN3 macrocyclic moieties separated by a 2,6-dimethylenepyridine unit is reported. A potentiometric and 1H NMR study in aqueous solution reveals that ligand protonation occurs on the secondary amine groups and does not involve the pyridine nitrogen. The coordination properties toward Cu(II), Zn(II), Cd(II) and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand can form mono- and binuclear complexes in aqueous solution. In the 1 : 1 complexes, the metal is sandwiched between the two [9]aneN3 moieties and the pyridine N-donor is coordinated to the metal, as actually shown by the crystal structure of the compound [ZnL4](NO3)2.CH3NO2. L4 shows a higher binding ability for Cd(II) with respect to Zn(II), probably due to a better fitting of Cd(II) ion inside the cavity generated by the two facing [9]aneN3 units. The formation of binuclear complexes is accompanied by the assembly of OH-bridged M2(OH)x (x = 1-3) clusters inside the cavity defined by the two facing [9]aneN3 units, and pyridine is not involved in metal coordination. A potentiometric and (1)H NMR study on the coordination of halogenide anions by L4 and its structural analogous L3 in which the two [9]aneN3 units are separated by a shorter quinoxaline linkage, shows that bromide is selectively recognised by L4, while chloride is selectively bound by L3. Such a behaviour is discussed in terms of dimensional matching between the spherical anions and the cavities generated by the two [9]aneN3 units of the receptors.     

Structure, Stability, Electronic Properties and NMR-Shielding of the Cucurbit[6]uril–Spermine-Complex

Geometries, stabilities, electronic properties and NMR-shielding of cucurbit[6]uril–spermine host-ligand complexes are investigated with DFT calculations and compared to experimental results. Cucurbit[6]uril and spermine can form complexes with two different minimum energy geometries and corresponding characteristic differences in NMR shielding. The energetically preferred complex geometry has a perfect inversion symmetry and its proton NMR shielding agrees very well with experimental results. The cucurbit[6]uril host molecule shows a distinct geometrical flexibility in ligand binding which allows an induced fit of the spermine ligand. The energetic barrier for the rotation of spermine in the favourable complex is approximated to be in the order of a few kilocalories per mole.     

Encapsulation of 2,2′-(decane-1,10-diyl)-diisoquinoline into cucurbit[6]uril and α,α′,δ,δ′-tetramethyl-cucurbit[6]uril: formation of pseudorotaxanes and polypseudorotaxanes†

Host–guest complexation of cucurbit[6]uril and α,α′,δ,δ′-tetramethyl-cucurbit[6]uril with 2,2′-(decane-1,10-diyl)-diisoquinolinium has been investigated by ¹H NMR, UV and fluorescence spectroscopy in aqueous solution and by X-ray crystallography in solid state. Experimental data suggest that in the aqueous solution, both host molecules form [2]pseudorotaxanes with the host located over the decyl chain of the guest. In the solid state, neighbouring [2]pseudorotaxanes are linked by π?π and C–H?π interactions, eventually generating polypseudorotaxanes.     

Host–guest interactions of 6-benzyladenine with normal and modified cucurbituril: 1H NMR,UV absorption spectroscopy and phase solubility methods

Guest–host inclusion complexes between 6-benzyladenine (6-BA), cucurbit[7]uril (Q[7]), symmetrical tetramethylcucurbit[6]uril (TMeQ[6]) and meta-hexamethyl-substituted cucurbit[6]uril (HMeQ[6]) in aqueous solution were investigated by ¹H NMR, UV absorption spectroscopy and phase solubility studies. The ¹H NMR spectra analysis revealed that the hosts selectively bound the phenyl moiety of the guests. Absorption spectroscopic analysis defined the stability of the host–guest inclusion complexes. A host:guest ratio of 1:1 was measured quantitatively as (5.63 ± 0.26) × 10⁴, (1.94 ± 0.17) × 10³ and (2.89 ± 0.23) × 10³ mol L^? 1 for the Q[7]-6-BA, TMeQ[6]-6-BA and HMeQ[6]-6-BA systems, respectively. Phase solubility diagrams were analysed through rigorous procedures to obtain estimates of the complex formation constants for Q[n]-6-BA complexation. The formation constants were (1.29 ± 0.24) × 10⁴ L mol^? 1 for Q[7]-6-BA, (3.20 ± 0.17) × 10³ L mol^? 1 for TMeQ[6]-6-BA and (3.52 ± 1.01) × 10³ L mol^? 1 for TMeQ[6]-6-BA. Furthermore, phase solubility studies showed that 6-BA solubility increased as a function of Q[7], TMeQ[6] and HMeQ[6] concentrations. The thermodynamic parameters of the complex formation were also determined. The formation of inclusion complexes between 6-BA and Q[7] was enthalpy controlled, suggesting that hydrophobic and van der Waals interactions were the main driving forces. Our results demonstrated that the complexation of 6-BA with Q[n] could be used to improve the solubility of 6-BA.     

IBX Oxidation of Benzenedimethanols in the Presence of Cucurbit[8]uril

The cucurbit[8]uril (Q[8]) mediated oxidation of benzenedimethanols with o‐iodoxybenzoic acid (IBX) in aqueous solution has been investigated, and the results reveal the supramolecular catalysis depends on the electronic and geometric structure of substrate. In the cases of o‐benzenedimethanol ( 1a ) and m‐benzenedimethanol ( 1b ), the IBX oxidation could be obviously enhanced by the addition of Q[8] at different extent. There is no observation of the catalytic activity of Q[8] when p‐benzenedimethanol ( 1c ) is subjected to the IBX oxidation. The addition amount of Q[8] is discussed herein, and the addition of more than 10% mol catalyst cannot improve the oxidation much more. The investigation of host‐guest interactions by isothermal titration calorimetry implies the supramolecular catalysis is related to the formation of complexes between benzenedimethanols and cucurbit[8]uril.