Four‐State Molecular Shuttling of [2]Rotaxanes in Response to Acid/Base and Alkali‐Metal Cation Stimuli

Abstract

In this study, we synthesized [2]rotaxanes possessing three recognition sites-a dialkylammonium, an alkylarylamine, and a tetra(ethylene glycol) stations-in their dumbbell-like axle component and dibenzo[24]crown-8 (DB24C8) as their macrocyclic component. These [2]rotaxanes behaved as four-state molecular shuttles: i) under acidic conditions, the DB24C8 unit encircled both the dialkylammonium and alkylarylammonium stations; ii) under neutral conditions, the dialkylammonium unit was the predominant station for the DB24C8 component; iii) under basic conditions, when both ammonium centers were deprotonated, the alkylarylamine unit became a suitable station for the DB24C8 component; and iv) under basic conditions in the presence of an alkali-metal cation, the tetra(ethylene glycol) unit recognized the DB24C8 component through cooperative binding of the alkali-metal ion. In addition, we observed that the [2]rotaxanes exhibited selective recognition for metal cations. These shuttling motions of the macrocyclic component proceeded reversibly.

Ring-Over-Ring Deslipping From Imine-Bridged Heterorotaxanes

“…The rotaxane moiety with [24]C8 was evidenced by the appearance of benzylanilinium CH 2 and + NH 2 moieties at large downfield shifted positions of 5.0 and 9.0 ppm, which were similar to the values observed for other rotaxanes with [24]C8 ( Kimura et al, 2017 ) ( Figure 3 ). In addition, the threaded [24]C8 part appeared as a large singlet at 3.3 ppm, which overlapped with the signal of the central imine-bridged station.…”

Utilization of a Crown Ether/Amine‐Type Rotaxane as a Probe for the Versatile Detection of Anions and Acids by Thin‐Layer Chromatography

“…As similar deprotonated rotaxane bearing dialkylamine and aniline moieties in its axle component exists as a mixture of translational isomers in solution, VT NMR experiments (CD 2 Cl 2 ) were performed to reveal the translational isomerism of 1/DB24C8 (Figure S3). Although significant signals based on the isomers were not observed above −90 °C, small signals (ca 3%), which can be assigned to the benzyl protons in translational isomer were detected at 4.16, 4.35, and 4.50 ppm at −50 °C.…”

Distinguishing Two Ammonium and Triazolium Sites of Interaction in a Three‐Station [2]Rotaxane Molecular Shuttle

“…R f = 0.65 (CH 2 Cl 2 /CH 3 OH, 9:1); 1 HNMR (600 MHz, CD 3 2.11 (m, 2H,H 3' ), 1.90 (m, 2H,H 13 ), 1.59-1.08 (m, 35 H, H 4' ,H 5' ,H 6' ,H 7' , H 2 ,H 4 ,H 5 ,H 6 ,H 7 ,H 8 ,H 9 ,H 10 ,H 11 ,H 12 ), 1.27 ppm (s, 18 H, H 15' ); 13 CNMR (151 MHz,CD 3 CN,298 K): d = 152.4 &1 52.2 &1 49.7 & 148.7 &1 48.6 &1 33.9 (C 2' C 10' C 13' C A C C C F C M C R C IV,arom,DB24C8 C IV,BOC ), 129.7 (C 1' ), 122.9 &1 22.4 &1 21.8 &1 15.8 &1 15.3 &1 12.9 (C A' C B' C B C D C E C N C O C P C Q ), 112.0 (C 12' ), 108.1 (C 11' ), 79.9 (C IV,BOC ), 71.9 &7 1.3 &7 0.8 &7 0.7 &7 0.4 &6 9.9 &6 9.7 &6 9.4 &6 9.1 (C C' C D' C E' C G C H C I C J C K C L C S C T C U C V C W C X ), 54.4 (C 14 ), 50.2 (C 1 ), 47.4 (C 3 ), 44.4 (C 8' ), 37.0 (C 16' ), 35.6 (C 14' ), 31.8 (C 15' ), 30.2 &3 0.0 &2 9.4 &2 8.7 &2 7.5 & 27.4 &2 7.1 ppm (C 3' C 4' C 5' C 6' C 7' C 4 C 5 C 6 C 7 C 8 C 9 C 10 C 11 C 12 Preparation of the fully protonated three-station [2]rotaxane molecular shuttle 7: From rotaxane 5:C ompound 5 (1 equiv, 40.9 mg, 22.1 mmol) was added to a2 5mLf lask and solubilized in dry CH 2 Cl 2 (0.5 mL) at 0 8C. An excess of hydrogen chloride (2 m) in diethylether (6 mL) was added to the solution, and the reaction mixture was stirred at 0 8Cf or 2h.T he solution was concentrated before being co-evaporated ( 3) with CH 2 Cl 2 .T he residue was diluted in CH 2 Cl 2 (3 mL), and as olution of ammonium hexafluorophosphate (3.9 equiv,1 4mg, 85.9 mmol) in Milli-Q water (3 mL) was added.…”

“…Very recently,t he same authorr eported av ery appealing metallo-and pH-responsive three-station rotaxane shuttle, which demonstrated the selectiver ecognition of ac rown ether for the aniline, the dialkylammonium, and atetraethyleneglycol spacer,the latter throught he help of asodium cation. [12] Herein, the targeted three-station molecular shuttle can switch betweent hree main co-conformational states through independentc hemical reactions owing to the presence of three molecular stations (an alkylbenzylammonium, an arylammonium,and an N-methyltriazolium) of different binding affinities for the encircling DB24C8 macrocycle (Figure 1b). In the fully deprotonated state, the DB24C8 macrocyclei sl ocated aroundt he triazolium moiety,b ecause it is am uch betterm olecular station than neutral aniline and alkylbenzylamine.…”

“…Strongly acidic conditions yielded mixtures of [2]rotaxane translational isomers, in which the crown ether interacts with both protonated aryl and dialkylammonium sites with a population distribution that was dependant on their structures. Very recently, the same author reported a very appealing metallo‐ and pH‐responsive three‐station rotaxane shuttle, which demonstrated the selective recognition of a crown ether for the aniline, the dialkylammonium, and a tetraethyleneglycol spacer, the latter through the help of a sodium cation …”