Complexation of sym-bis(benzimidazole)-2,2'-ethylene salts with cucurbit[6]uril derivatives:A potential axle molecule for pseudorotaxanes

Sym-bis（benzimidazole）-2,2＇-ethylene cations act as a new axle template for threading cucurbit[6]uril derivatives on,forming[2]pseudorotaxane and[3]pseudorotaxane.These new complexes have been studied using 1 H NMR,UV-vis absorption spectroscopy and X-ray analysis.Changes in the 1 H NMR spectra indicate that the two types of pseudorotaxane can be formed by varying the host concentration. UV-vis absorption titration experiments at different pH values demonstrate that interesting pK_a shifts of the bis-benzimidazole derivatives can be induced by the host-guest complexation.The associated constants were calculated to be 2.81×10^4 L/mol and 9.06×10^6 L/mol for the[2]pseudorotaxanes and ｜3]pseudorotaxanes,respectively.Furthermore,X-ray diffraction studies of the solid state structures provide unequivocal proof of the host concentration dependent pseudorotaxane,which is strongly in line with the evidences in solution.

A twin-axial[5]pseudorotaxane based on cucurbit[8]uril and a-cyclodextrin

A twin-axial hetero[5]pseudorotaxane was constructed based on 1-hexyl-4,40-bipyridinium guest 1 and cucurbit[8]uril（CB[8]）and a-cyclodextrin（a-CD）.In its structure,CB[8]included two bipyridinium units to realize the twin-axial mode,and the hexyl chain was threaded into the cavity of a-CD.The[5]pseudorotaxane contains two types of macrocyclic hosts while the single axial and twin axial modes co-exist in its structure.The transformation of[5]pseudorotaxane could be realized by the addition of acid and 2,6-dihydroxynaphthalene（HN）.

A new cryptand/paraquat [2]pseudorotaxane

A new cryptand/paraquat [2]pseudorotaxane and its crown ether analog were synthesized and characterized by proton NMR, mass spectrometry, and single crystal X-ray analysis. The Job plots based on proton NMR data demonstrated that both the complex between cryptand 1 and paraquat derivative 2a and the complex between bis(p-phenylene)-34-crown-10 (3) and paraquat derivative 2b have 1:1 stoichiometry in solution. The apparent association constants (Ka,exp) of 1·2a and 3·2b calculated for 1:1 complexes were 1.4 (±0.2) × 104 and 4.7 (± 0.5) × 102 M-1, respectively. X-ray analysis showed that 1·2a is stabilized by ten hydrogen bonds and face-to-face π-stacking interactions in the solid state, while 3·2b is stabilized by four hydrogen bonds and face-to-face π-stacking interactions. As well as in the X-ray structure of 1·2c, a water molecule acts as a hydrogen bridge between the β-pyridinium protons of 2a and the ether oxygen atoms of 1 in 1·2a. The successful preparation of 1·2a demonstrated that the tert-butyl group is not big enough to be a stopper for rotaxanes based on 1.

A Pillar[6]arene-[2]pseudorotaxane Based pH-Sensitive Molecular Switch

A novel pH-responsive[2]pseudorotaxane based on ethylated pillar[6]arene and bis(1,2,3-triazolium)butane cation was constructed.The dethreading and rethreading could be reversibly controlled by adding base and acid,i.e.,the deprotonation and reprotonation of the guest.In contrast,such responsible switching behavior does not occur for the complexation between the guest and ethylated pillar[5]arene,since both the cationic axle and the deprotonated one can be strongly bound by ethylated pillar[5]arene.Furthermore,the bis(1,2,3-triazolium)butane⊂pillar[6]-arene-[2]pseudorotaxane can also disassemble upon the addition of competitive ethylated pillar[5]arene.

[2]Pseudorotaxanes Based on Naphtho-21-crown-7 and Secondary Dialkylammonium Salts:Remarkably Improved Asso-ciation Constants Among Four Threaded Structures

This research article is focused on the recognition interaction of a new host naphtho-21-crown-7 and four sec-ondary dialkylammonium salts.In acetone,they can form 1∶1 host-guest complexes which belong to slow-exchange systems.We also found the differences of binding affinity and binding selectivity between the host and its complementary guest moieties,which could be ascribed to the aromaticπ-πstacking effect and the acidity increase of N-methylene and ammonium hydrogens due to the increasing electron withdrawing ability from butyl to methoxyphenyl to phenyl to p-cyanophenyl substituents in the recognition motif.

Negatively charged crown ethers for binding paraquat in water

A water soluble negatively charged fluorescent 1,4-benzo-1,5-naphtho-36-crown-10-based host has been devised and synthesized.As shown by proton NMR,ESI mass spectrometry and UV-vis spectroscopy,it binds paraquat with a 1:1 stoichiometry and an association constant of 4.50(±0.02) ×103 M-1 in water.Its complexation with paraquat in water was further investigated by fluorescence emission spectroscopy.The results revealed that when paraquat was added to the water solution of the host,the fluorescence emission of the host was quenched by the charge transfer between the crown ether host and paraquat guest.A similar bis-p-phenylene-34-crown-10-based host was synthesized as a contrasting host.Lacking the naphthalene unit,the second host binds paraquat with a 1:1 stoichiometry and a lower association constant,1.04(±0.04) ×103 M-1,in water.This demonstrated that the naphthalene unit could not only act as a strong fluorescence group but also improve the π-πstacking interactions between the host and guest.

Stable pillar[5]arene-based pseudo[1]rotaxanes formed in polar solution

Mono-alkyl-functionalized pillar[5]arenes PI, P2, and P3 were synthesized by click reaction, which exhibited different self-assembly behavior in polar solvent DMSO. Stable pseudo[ 1 ]rotaxane was formed by the self-complexation from P1 or P2, whereas, concentration-dependent pseudorotaxane structures were generated by P3 which bearing more flexible side chain. Interestingly, the obtained pseudo[1]rotaxanes exhibited a dynamic fast assembly process upon adding NaBF4, resulting in the formation of Na＋-induced pseudorotaxanes.

Synthesis of perylene-bridged bis(dibenzo-24-crown-8) and its assembly behavior with a fullerene-based secondary dibenzylammonium salt

This paper reports the synthesis of a perylene-bridged bis(crown ether) through the condensation reaction between primary amine-armed dibenzo-24-crown-8 and 3,4,9,10-perylenetetracarboxylic dianhydride.Pseudorotaxane with a perylene moiety at the wheel and a fullerene unit in the middle of the axle was prepared,which was evidenced by 1H NMR and fluorescence experiments.Subsequent investigations show that the formation and disassociation of the pseudorotaxane can be controlled by the alternating addition of KPF6 and 18-crown-6,following the change of the florescence intensity of perylene.

Supramolecular Self-Assembly of Cucurbit[6]uil and Ionic Liquid in Non-aqueous System

A novel[2]pseudorotaxane of cucurbit[6]uril(CB[6])and 1-butyl-3-methyl-imidazolium bromide([C4mim]Br)was synthesized by directly mixing the host and the guest molecules in non-aqueous system.Structural characteri-zations of the[2]pseudorotaxane were carried out by 1D,2D NMR and X-ray crystallography techniques both in solution and in crystal structure.The crystal structure demonstrated that CB[6]and[C4mim]Br formed a complex with the ratio 1∶1,in which one guest[C4mim]Br was included inside the CB[6],while two other[C4mim]Br molecules were free and surrounded the[2]pseudorotaxane as solvent molecules,which could stabilize the crystal structure through hydrogen bonds.Moreover,parallel solvent channels consisting by free[C4mim]Br molecules occupied the pores among the frame of the pseudorotaxanes and formed zigzag lines in the crystal structure.[C4mim]Br can serve as not only the guest reactant but also the solvent in the formation of[2]pseudorotaxane for-mation.