Modes of binding interaction between viologen guests and the cucurbit[7]uril host

[structure: see text] Host-guest interactions between cucurbit[7]uril (CB7) and a series of dialkyl-4,4'-bipyridinium (viologen) dicationic guests were investigated by NMR spectroscopy. CB7 includes the aromatic nucleus of short chain viologens, but the mode of interaction is different with longer chain viologens due to the favorable hydrophobic interactions between the terminal alkyl substituents and the inner cavity of the host. A new pseudorotaxane was designed and synthesized on the basis of viologen-CB7 binding interactions.     

Cucurbit[7]uril: a very effective host for viologens and their cation radicals

[reaction: see text] The host cucurbit[7]uril (CB7) forms very stable inclusion complexes with simple 4,4'-bipyridinium (viologen) dication guests in aqueous solution. The binding constants were measured by electronic absorption spectroscopy and found to be as high as 1 x 10(5) L/mol. One-electron reduction of the viologen guest results in a modest 2-fold decrease of the binding constant. The rate of the heterogeneous electron-transfer reaction between the complexed viologen dication and cation radical remained fast in the voltammetric time scale.     

Inclusion of Carboxyl Function Inside of Cucurbiturils and its Use in Molecular Switches

We have prepared organic guest molecules in which two pyridinium rings are connected through an aromatic/aliphatic bridge bearing a carboxyl group. The supramolecular interactions between these guests and macrocyclic hosts cucurbit[7]uril ( CB7 ) and cucurbit[8]uril ( CB8 ) has been studied. We have demonstrated that the binding modes of the complexes depend on the type of central bridge present in the guest molecules and the size of the macrocycle. We have also showed that the binding mode between cucurbiturils and guests with aromatic bridges is pH independent. On the other hand, a guest containing an aliphatic bridge and CB7 formed a pseudorotaxane, which behaved as a pH‐driven molecular switch.     

Complexation of 1,4-bis(pyridinium)butanes by negatively charged carboxylatopillar[5]arene

The binding behavior of substituted 1,4-bis(pyridinium)butane derivatives (X-Py(CH(2))(4)Py-X, X = H, 2-methyl, 3-methyl, 4-methyl, 2,6-dimethyl, 4-pyridyl, and 4-COOEthyl) 1(2+)-7(2+), with negatively charged carboxylatopillar[5]arene (CP5A) has been comprehensively investigated by (1)H NMR and 2D ROESY and UV absorption and fluorescence spectroscopy in aqueous phosphate buffer solution (pH 7.2). The results indicated that the position of the substituents attached on pyridinium ring dramatically affects the association constants and binding modes. 3- and 4-Substituted guests (1(2+), 3(2+), 4(2+), 6(2+), 7(2+)) form [2]pseudorotaxane geometries with CP5A host, giving very large association constants (>10(5) M(-1)), while 2,6-dimethyl-substituted 5(2+) forms external complex with relatively small K(a) values [(2.4 ± 0.3) × 10(3) M(-1)] because the 2,6-dimethylpyridinium unit is too bulky to thread the host cavity. Both of the binding geometries mentioned above are observed for 2(2+), having one methyl group in the 2-position of pyridinium. Typically, the association constant of [2]pseudorotaxane 1(2+)?CP5A exceeds 10(6) M(-1) in water, which is significantly higher than those of previously reported analogues in organic solvents. The remarkably improved complexation of bis(pyridinium) guests by the anionic host was due to electrostatic attraction forces and hydrophobic interactions.     

Cyclodextrin-driven movement of cucurbit[7]uril

The movement of cucurbit[7]uril (CB[7]) driven by alpha-cyclodextrin (alpha-CD) is investigated by various experimental techniques including NMR, ESI-MS, UV-vis, and ITC. CB[7] can form stable pseudorotaxanes with N-methyl-N'-octyl-4,4'-bipyridinium (MVO2+) and N,N'-dioctyl-4,4'-bipyridinium (OV2+) dication in aqueous solution. CB[7] shuttles between the octyl and bipyridinium moieties in MVO2+, but docks at one of the octyl moieties in OV2+. The addition of alpha-CD pushes CB[7] from the octyl moiety of MVO2+ or OV2+ to the bipyridinium moiety. Thermodynamically, the movement of CB[7] is mainly driven by exothermic enthalpy changes coming from the complexation of the octyl moiety of MVO2+ or OV2+ with alpha-CD.     

Binding modes of cucurbit[6]uril and cucurbit[7]uril with a tetracationic bis(viologen) guest

Binding behaviors of two cucurbit[n]urils (CB[n]) hosts with the [CH3bpy(CH2)6bpyCH3]4+ (bpy = 4,4'-bipyridinium) guest were investigated by 1H NMR and MALDI-TOF-MS experiments. While the CB[6] and CB[7] form [2]pseudorotaxanes with the host located over the hexamethylene chain of the guest, only the CB[7] forms a [3]pseudorotaxane with both host molecules residing over the bipyridinium groups. The initial CB[7] host vacates the inclusion of the hexamethylene chain as a result of the electrostatic and steric repulsions that would arise in simultaneous binding of adjacent aliphatic and aromatic portions of the guest.     

Recognition properties of cucurbit[7]uril self-assembled monolayers studied with force spectroscopy

Specific (host-guest) and unspecific (substrate-guest) interactions between self-assembled monolayers of cucurbit[7]uril (CB[7]) on gold (Au) substrates and neutral adamantyl (Ad) guests were resolved by studying these interactions at the single molecule level using dynamic force spectroscopy. The dissociation rate constants of the Ad-Au and the Ad-CB[7] interactions were 0.3 s(-1) and 0.03 s(-1), respectively, indicating that the specific binding is more stable. The probability of observing a specific interaction (40 ± 9%) is similar to the reported surface coverages of CB[7] monolayers on Au substrates. The higher strength and stability of the Ad-CB[7] interactions explains why, although presenting an imperfect coverage, CB[n] monolayers can be used successfully as a platform for surface immobilization.     

Complexation of ferrocene derivatives by the cucurbit[7]uril host: a comparative study of the cucurbituril and cyclodextrin host families

The formation of inclusion complexes between cucurbit[7]uril (CB[7]) and ferrocene and its derivatives has been investigated. The X-ray crystal structure of the 1:1 inclusion complex between ferrocene and CB[7] revealed that the guest molecule resides in the host cavity with two different orientations. Inclusion of a set of five water-soluble ferrocene derivatives in CB[7] was investigated by 1H NMR spectroscopy and calorimetric and voltammetric techniques. Our data indicate that all neutral and cationic guests form highly stable inclusion complexes with CB[7], with binding constants in the 10(9)-10(10) M(-)(1) and 10(12)-10(13) M(-1) ranges, respectively. However, the anionic ferrocenecarboxylate, the only negatively charged guest among those surveyed, was not bound by CB[7] at all. These results are in sharp contrast to the known binding behavior of the same guests to beta-cyclodextrin (beta-CD), since all the guests form stable inclusion complexes with beta-CD, with binding constants in the range 10(3)-10(4) M(-1). The electrostatic surface potentials of CB[6], CB[7], and CB[8] and their size-equivalent CDs were calculated and compared. The CD portals and cavities exhibit low surface potential values, whereas the regions around the carbonyl oxygens in CBs are significantly negative, which explains the strong affinity of CBs for positively charged guests and also provides a rationalization for the rejection of anionic guests. Taken together, our data suggest that cucurbiturils may form very stable complexes. However, the host-guest interactions are very sensitive to some structural features, such as a negatively charged carboxylate group attached to the ferrocene residue, which may completely disrupt the stability of the complexes.     

Binding modes of cucurbit[6]uril and cucurbit[7]uril with a series of bis-pyridinium compounds

Binding behaviors of cucurbit[6]uril (CB[6]) and cucurbit[7]uril (CB[7]) with a series of bis-pyridinium compounds N, N’-hexamethylenebis(1-alkyl-4-carbamoyl pyridinium bromide) (HBPB-n) (alkyl chain length, n = 6, 8 and 10) guests were investigated using ¹H-NMR, ESI–MS and single crystal X-ray diffraction methods. The results show that CB[6] and CB[7] can form [2]pseudorotaxanes with HBPB-n easily. When increasing the length of tail alkyl chain, the binding site of CB[6] at guest molecules changed from the tail to the middle part, while CB[7] remained located over the tail chain. As CB[6] and CB[7] were added in HBPB-8 aqueous solution, a [3]pseudorotaxane was formed by the inclusion of the internal middle site in CB[6] and the tail chain in CB[7].     

Cationic gemini guests promote the efficient self-assembly of capsular entities in water at neutral pH

Abstract

The binding features of a sulfonato-calix[4]arene with pyridinium-based gemini guests have been investigated in water at physiological condition by NMR spectroscopy. Results provided converging evidence showing that guests having different size, shape and flexibility formed water-soluble homodimeric supramolecular capsules. The remarkably large encapsulation efficiency of the anionic calixarene host towards these bis(pyridinium) gemini guests results from the favourable combination of electrostatic and hydrophobic interactions. The introduction of short and rigid spacers between the two positively charged heads of the guest improves the efficiency of the capsule formation. The insertion of a substituent into the pyridinium ring of the guest dramatically affects both the association constants and the binding modes.     

Metal-organic rotaxane frameworks; MORFs

Linear exodentate pyridinium ligands such as 1,2-bis(4,4'-bipyridinium)ethane or its bis N-oxide derivative can be used as axles for the formation of [2]pseudorotaxanes utilising 24-membered crown ethers such as dibenzo-24-crown-8 ether (DB24C8) as the wheel. These [2]pseudorotaxanes can be used to construct coordination networks using transition or lanthanide metal ions as the connecting nodes. 1-, 2- and 3D metal-organic rotaxane frameworks (MORFs) are possible. The resulting materials contain mechanically interlocked units and may be the forerunners of unique solids which contain machine-like components in an ordered array.     

Synthesis and Characterization of Two [2]Catenanes Based on Phenylene-diacetylene Crown Ethers

Two charged donor-acceptor [2]catenanes were synthesized by the cyclization of the π-acceptor 1,1 ＇-[1, 4-phenylenebis-（methylene）]bis（4,4＇-bipyridinium）bis（hexafluorophosphate）（2） and bis（bromomethyl）benzene with the templates of the π-donor phenylene-diacetylene crown ethers la and lb in the yields of 31%, 35%, respectively. The mass spectra, 1H and 13C NMR spectra confirm that the phenylene-diacetylene crown ethers, with a nonaromatic ~r-system, successfully template the cyclization of CBPQT4＋. The X-ray crystallography analysis shows that the CBPQT4＋ ring encircles around the phenylene motif of the crown ethers, leaving the 1,3-butadiyne fragment out of the cavity of CBPQT4＋ ring. The mechanically interlocked structure of [2]catenanes was stabilized by the cooperative effects of π-stacking and hydrogen bonding interactions.     

Cucurbit[7]uril host-guest complexes with small polar organic guests in aqueous solution

The host-guest stability constants for the inclusion of a series of small neutral polar organic guests in cucurbit[7]uril (CB[7]) have been determined in aqueous solution by (1)H NMR titrations. The dependence of the stability constant on the nature of the guests indicates that hydrophobic and dipole-quadrupole interactions are responsible for the binding. The complexation-induced chemical shift changes in the guest proton resonances, coupled with energy-minimization calculations, suggest that the guests are located such that their dipole moment is aligned perpendicular with the quadrupole moment of the CB[7] host. The stability constants for acetone and acetophenone decrease in the presence of Na(+) or K(+) cations as a result of cation capping of the CB[7] portals.     

Cucurbituril-modulated supramolecular assemblies: from cyclic oligomers to linear polymers

Employing bis(p-sulfonatocalix[4]arenes) (bisSC4A) and N',N'hexamethylenebis(1-methyl-4,4'-bipyridinium) (HBV(4+)) as monomer building blocks, the assembly morphologies can be modulated by cucurbit[n]uril (CB[n]) (n = 7, 8), achieving the interesting topological conversion from cyclic oligomers to linear polymers. The binary supramolecular assembly fabricated by HBV(4+) and bisSC4A units, forms an oligomeric structure, which was characterized by NMR spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and gel permeation chromatography (GPC) experiments. The ternary supramolecular polymer participated by CB[8] is constructed on the basis of host-guest interactions by bisSC4A and the [2]pseudorotaxane HBV(4+)@CB[8], which is characterized by means of AFM, DLS, NMR spectroscopy, thermogravimetric analysis (TGA), UV/Vis spectroscopy, and elemental analysis. CB[n] plays vital roles in rigidifying the conformation of HBV(4+), and reinforcing the host-guest inclusion of bisSC4A with HBV(4+), which prompts the formation of a linear polymer. Moreover, the CB[8]-participated ternary assembly could disassemble into the molecular loop HBV(2+)@CB[8] and free bisSC4A after reduction of HBV(4+) to HBV(2+), whereas the CB[7]-based assembly remained unchanged after the reduction. CB[8] not only controlled the topological conversion of the supramolecular assemblies, but also improved the redox-responsive assembly/disassembly property practically.     

3]Pseudorotaxane-like complexes formed between bipyridinium dications and bis-p-xylyl[26]crown-6

[structure: see text]. The crown ether BPX26C6 forms a [3]pseudorotaxane-like complex with the N,N'-dimethyl-4,4'-bipyridinium dication both in solution and in the solid state. The facile one-pot synthesis of a [2]rotaxane from neutral precursors-BPX26C6, 4,4'-dipyridyl, and 3,5-di-tert-butylbenzyl bromide-suggests that BPX26C6 may bind to (mono)pyridinium cations in a [2]pseudorotaxane-like manner.     

STUDIES ON BIPYRIDINIUM SALTS AND THEIR POLYMERS Ⅰ. SYNTHESIS OF 4, 4''-(1, 4-PHENYLENE-)BIS-[N-ALKYL (ARYL)-2, 6-DIPHENYL-] PYRIDINIUM SALTS AND THEIR POLYMERS FROM CORRESPONDING BIPYRYLIUM SALT*

The reaction of pyrylium salt with primary amine has been utilized to synthesize 4,4'-bipyri-dinium salts and pyridinium polymers. Seven new 4,4'-(1,4-phenylene-) bis-[N-alkyl(aryl)-2,6-diphenyl-] pyridinium perchlorates 4 and five corresponding bipyridinium polymers 5 were preparedwith good yields from 4,4'-(1,4-phenylene-) bis-(2,6-diphenyl-) pyrylium perchlorate 3. Preliminarytests have been made for their redox behavior and catalytic activity.     

Kinetics of formation of the host-guest complex of a viologen with cucurbit[7]uril

Host-guest complexation between the dicationic viologen 1-tri(ethylene glycol)-1'-methyl-m-xylyl-4,4'-bipyridinium and cucurbit[7]uril (CB7) was studied at pH = 4.5 in water. The stability constants of the mono- and bis-CB7 adducts were determined at 25 °C by UV-vis spectroscopy. Stopped-flow kinetic experiments were performed to measure the formation and dissociation rate constants of the monoadduct: k(1) = (6.01 ± 0.03) × 10(6) M(-1)s(-1) and k(-1) = 52.7 ± 0.4 s(-1), respectively. Possible mechanisms of complexation are discussed in view of the kinetic results.     

The cucurbit[n]uril family: prime components for self-sorting systems

We determined the values of Ka for a wide range of host-guest complexes of cucurbit[n]uril (CB[n]), where n = 6-8, using 1H NMR competition experiments referenced to absolute binding constants measured by UV/vis titration. We find that the larger homologues--CB[7] and CB[8]--individually maintain the size, shape, and functional group selectivity that typifies the recognition behavior of CB[6]. The cavity of CB[7] is found to effectively host trimethylsilyl groups. Remarkably, the values of Ka for the interaction of CB[7] with adamantane derivatives 22-24 exceeds 10(12) M(-1)! The high levels of selectivity observed for each CB[n] individually is also observed for the CB[n] family collectively. That is, the selectivities of CB[6], CB[7], and CB[8] toward a common guest can be remarkably large. For example, guests 1, 3, and 11 prefer CB[8] relative to CB[7] by factors greater than 10(7), 10(6), and 3000, respectively. Conversely, guests 23 and 24 prefer CB[7] relative to CB[8] by factors greater than 5100 and 990, respectively. The high levels of selectivity observed individually and collectively for the CB[n] family renders them prime components for the preparation of functional biomimetic self-sorting systems.     

七元瓜环与N,N′-二烷基-1,3-(4,4′-二吡啶基)丙烷衍生物的相互作用

以1,3-(4,4′-二吡啶基)丙烷为母体,合成了N,N′-二乙基\,二丁基\,二己基以及二辛基1,3-(4,4′-二吡啶基)丙烷衍生物.利用 1H NMR技术和紫外吸收光谱法,考察了Q[7]与上述链状吡啶衍生物的相互作用.实验结果表明,Q[7]与客体PC0,PC2作用,瓜环包结客体的二吡啶基丙烷部分形成1∶1的包结配合物;对于取代烷基碳链数大于4的N,N′-二烷基-1,3-(4,4′-二吡啶基)丙烷衍生物,随着主体与客体摩尔比值的增加,体系中主-客体相互作用的主导模式是Q[7]逐渐包结了客体二吡啶基丙烷部分,进而形成Q[7]包结客体两端取代烷基,甚至形成一个客体分子上"挂满"3个主体瓜环的包结物.     

七元瓜环与N,N’-二烷基-1,3-(4,4’-二吡啶基)丙烷衍生物的相互作用

以1,3-(4,4’-二吡啶基)丙烷为母体, 合成了N,N’-二乙基、二丁基、二己基以及二辛基1,3-(4,4’-二吡啶基)丙烷衍生物. 利用1H NMR技术和紫外吸收光谱法, 考察了Q[7]与上述链状吡啶衍生物的相互作用. 实验结果表明, Q[7]与客体PC0, PC2作用, 瓜环包结客体的二吡啶基丙烷部分形成1∶1的包结配合物; 对于取代烷基碳链数大于4的N,N’-二烷基-1,3-(4,4’-二吡啶基)丙烷衍生物, 随着主体与客体摩尔比值的增加, 体系中主-客体相互作用的主导模式是Q[7]逐渐包结了客体二吡啶基丙烷部分, 进而形成Q[7]包结客体两端取代烷基, 甚至形成一个客体分子上“挂满”3个主体瓜环的包结物.