Electrostatic Repulsion between Cucurbit[7]urils Can Be Overcome in [3]Pseudorotaxane without Adding Salts

Abstract

The host-guest chemistry between cucurbit[7]uril (CB7) and a series of bolaform (Bn) surfactants with different chain lengths, n = 12-22, was the target of our study. [3]Pseudorotaxanes are formed when the alkyl chain of the bolaform has more than 14 carbon atoms. In these cases, two CB7 molecules can be accommodated between the two head groups of the bolaform without addition of electrolytes to the medium. In the case of a bolaform with 12 carbon atoms, the electrostatic repulsion between the carbonyl groups of the CB7 molecules avoids the threading of a second CB7 molecule yielding a mixed structure formed by a [2]pseudorotaxane and an external host-guest complex. The assembly behavior was investigated using NMR spectroscopy, isothermal titration calorimetry (ITC), and kinetic measurements.

Host–Guest Complexes of Flavylium Cations and Cucurbit[7]uril: The Influence of Flavylium Substituents on the Structure and Stability of the Complex

“…Information on the location of guests inside cucurbituril cavities can be obtained from the complexation‐induced chemical shifts (Δδ=δ complex −δ free ) of 1 H NMR signals upon complexation . The guest's protons located inside the hydrophobic cavity usually exhibit upfield shifts (Δδ<0) while protons located outside of the cavity near the carbonyl portals display downfield shifts (Δδ>0).…”

Excited-State Proton Transfer in Confined Medium. 4-Methyl-7-hydroxyflavylium and β-Naphthol Incorporated in Cucurbit[7]uril

“…It is accepted that the signals of the guest protons located near the carbonyl portals of the host are displaced downfield while the signals corresponding to the protons included inside the cavity are displaced upfield. 39 As can be observed in Figure 8, the 1 H NMR signals corresponding to the phenyl group of 4-methyl-7-hydroxyflavylium are displaced upfield (Δδ(4′) = −0.593 ppm, Δδ(3′,5′) = −0.926 ppm, and Δδ(2′,6′) = −0.882 ppm), suggesting that this group is included inside the cavity of CB7 while the signals corresponding to the protons of the benzopyrylium group are affected to a lesser extent. Protons 5 and 6 are pratically unaffected, while proton 3 is slightly displaced upfield (−0.09 ppm) and proton 8 displaced downfield (0.965 ppm).…”

Radical-radical rotaxanes with tetracationic cyclophane ring, and quaternary azaaromatic rotaxanes with cage macrocycles

“…Pseudo [4]rotaxanes containing three rings, two or three different, have been synthesized and characterized. In the experiments the thread 41 incorporating a central azobenzene unit and two terminal viologen units was used, and as rings served α-cyclodextrin (α-CD), p-sulfonatocalixarene (cal) and cucurbit [7]uril (CB7). The syntheses have been performed in aqueous solution.…”

“…The second part deals with rotaxanes consisting of quaternary azaaromatic thread and cage macrocycles serving as rings; for cage macrocycles cyclodextrins, 4 calixarenes, 5 pillararenes 6 and cucurbiturils 7 have been chosen. As examples, the rotaxanes containing one or more rings of the same kind, as well as rotaxanes containing different rings are presented, pointing out their application possibilities on various fields.…”