Molecular Switch Based on Very Weak Association between BPX26C6 and Two Recognition Units

Controlling molecular shuttling in a rotaxane with weak ring recognition sites

We describe a rotaxane molecular shuttle encompassing triazolium and tertiary ammonium units as weak recognition sites for the ring. Such a design, which differs from that of typical controllable rotaxanes, allows the precise tuning of the ring distribution among the two sites - i.e., the coconformational equilibrium - by changing the solvent polarity or the nature of the counteranions. Shuttling of the ring between the two stations can also be toggled by acid-base stimuli. Such an approach is paradigmatic to obtain rotaxanes that can sense environmental changes and transduce them into a coconformational response and opens avenues for novel applications in sensing and stimuli-responsive materials.

Rotaxanes comprising cyclic phenylenedioxydiacetamides and secondary mono- and bis-dialkylammonium ions: effect of macrocyclic ring size on pseudorotaxane formation

[2]Rotaxanes, stabilized through multiple and cooperative hydrogen bonding system, were synthesized from dialkylammonium ions and macrocycle possessing two phenylenedioxydiacetamide units and appropriate spacers.

Using Alkali Metal Ions To Template the Synthesis of Interlocked Molecules

In 1987, Pedersen, Cram, and Lehn were awarded the Nobel Prize in Chemistry to honor their achievements in, among other things, the selective recognition of alkali metal ions by synthetic hosts. Almost three decades later, the 2016 Nobel Prize went to Stoddart, Sauvage, and Feringa for the development of artificial molecular machines, in which interlocked molecules play a significant role. Surprisingly, although many rotaxane- and catenane-based molecular machines have been constructed using various templating approaches, alkali metal ions, which are good templates for crown ether synthesis, have only rarely been applied as templates for the assembly of these interlocked molecules. This paucity of examples is probably due to the less well defined coordination numbers and geometries in the complexation of alkali metal ions to common oxygen-containing ligands, resulting in much weaker metal-ligand interactions and less predictable structures for their complexes compared with those formed between transition metal ions and common pyridine-containing ligands. Nevertheless, the ease of removing alkali metal ions from interlocked compounds and their much lower toxicity compared with that of transition metal ions are attractive features that have inspired their use as templates in the synthesis of interlocked molecules. About a decade ago, we began investigating the feasibility of using alkali metal ions to template the formation of catenanes and rotaxanes, with the hope of developing facile, broadly applicable, green, and efficient methods for their construction. We noticed that the interactions between oxygen-containing ligands and alkali metal ions can be strengthened by minimizing the effects of competing interactions from solvent molecules and counteranions. Thus, to increase the solubility of the metal ion salts in less polar solvents (e.g., CH₂Cl₂, CHCl₃) and minimize ion pairing, we chose tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), a weakly coordinating anion, as the counteranion for the alkali metal ions applied as templates. Our strategy has been based on the association of simple and general recognition units: (i) the orthogonal arrangement of two oligo(ethylene glycol) chains around an alkali metal ion and (ii) the encircling of a single urea/amide unit by an oligo(ethylene glycol)-containing macrocycle in the presence of a templating alkali metal ion. The former recognition system has allowed the facile construction of many interesting interlocked structures, including cyclic [2]catenane trimers and tetramers; the latter has provided several rotaxanes, including some incorporating monomers of practically important (macro)molecules (e.g., peptides, polymers) and some that behave as switches with unique functions (e.g., catalysis, gelation). The components in these recognition systems possess high flexibility in terms of their structures and the choice of suitable alkali metal ion templates. This Account tells the story of the concept behind this alkali metal ion-templating approach as well as its elaboration, scope, and recent advances. We hope to convince the reader that alkali metal ions are powerful templates for assembling interlocked structures and compounds and also to demonstrate the range of possibilities that they provide for future endeavors.

Size effects in the alkali metal ion-templated formation of oligo(ethylene glycol)-containing [2]catenanes

The most suitable alkali metal ions for templating the assembly of various homo- and hetero-[2]catenanes from the diamines containing central di-, tri-, and tetra(ethylene glycol) motifs, and isophthalaldehyde are investigated.

A redox-controllable molecular switch based on weak recognition of BPX26C6 at a diphenylurea station

The Na⁺ ion–assisted recognition of urea derivatives by BPX26C6 has allowed the construction of a redox-controllable [2]rotaxane-type molecular switch based on two originally very weakly interacting host/guest systems. Using NOBF₄ to oxidize the triarylamine terminus into a corresponding radical cation attracted the macrocyclic component toward its adjacent carbamate station; subsequent addition of Zn powder moved the macrocyclic component back to its urea station.

A ring-flippable sugar as a stimuli-responsive component of liposomes

For the development of a liposome that takes in and out a drug in response to stimuli, 2,4-diaminoxylose (Xyl), which allows stimuli-responsive conformational switches between (4)C1 and (1)C4, was incorporated into a lipid structure: Xyl derivatives with C8 and C16 methylene chains at the 1,3-positions (C8Xyl and C16Xyl) were synthesized. (1)H NMR spectroscopy indicates that the addition of Zn(2+) and then H(+) induces conformational switches from the chair ((4)C1) to the reverse chair ((1)C4) and (1)C4-to-(4)C1, respectively, at Xyl; this leads to transformation of the lipids between linear and bent structures. Osmotic pressure and electron microscopy studies demonstrate that C8Xyl in water forms spherical solid aggregates (C8Xyl-Zn), which are converted into liposomes (C8Xyl+Zn) upon the addition of Zn(2+), and C16Xyl forms liposomes regardless of the presence of Zn(2+). The aggregates of C8Xyl±Zn incorporated a fluorophore and only C8Xyl+Zn released the content upon the addition of HCl. This study shows that Xyl could be a stimuli-responsive component of a liposome.

A novel pentiptycene bis(crown ether)-based [2](2)rotaxane whose two DB24C8 rings act as flapping wings of a butterfly

A novel [2](2)rotaxane based on pentiptycene-derived bis(crown ether) can be efficiently synthesized via a "click chemistry" method and the subsequent N-methylation. Due to the different affinities of DB24C8 with the ammonium and triazolium stations, the wing-flapping movement of the DB24C8 "wings" in the [2](2)rotaxane can be easily achieved by acid/base stimulus.

Five additional macrocycles that allow Na+ ion-templated threading of guest units featuring a single urea or amide functionality

Five analogues of the macrocycle BPX26C6 are also capable of recognizing single urea and/or amide functionalities in the presence of templating Na⁺ ions.