Abstract
Two types of mono-ester-functionalized pillar[5]arenes, P1 and P2, bearing different side-chain groups, were synthesized. Their host–guest complexation and self-inclusion properties were studied by 1H NMR and 2D nuclear overhauser effect spectroscopy (NOESY) NMR measurements. The results showed that the substituents on their phenolic units have a great influence on the self-assembly of both pillar[5]arenes, although they both could form stable pseudo[1]rotaxanes at room temperature. When eight bulky 4-brombutyloxy groups were capped on the cavity, instead of methoxy groups, pseudo[1]rotaxane P1 became less stable and its locked ester group in the inner space of cavity was not as deep as P2, leading to distinctly different host–guest properties between P1 and P2 with 1,6-dibromohexane. Moreover, pillar[5]arene P1 displayed effective molecular recognition toward 1,6-dichlorohexane and 1,2-bromoethane among the guest dihalides. In addition, the self-complex models and stabilities between P1 and P2 were also studied by computational modeling and experimental calculations.
Highlights
Pseudo[n]rotaxanes (n ≥ 2), a lasso-type of complexes, are two molecules or multiple molecules mechanically interlocked
The proton signals of the ethyl acetate chain (Hj and Hk ) on pillar[5]arene showed a very substantial upfield shift (∆δHj = 1.26 ppm, ∆δHk = 1.98 ppm) compared to M, indicating that the ethyl acetate moiety was locked in the shielded space of cavity of the pillar[5]arene in CDCl3. This result was consistent with the 2D nuclear overhauser effect spectroscopy (NOESY) NMR spectroscopy of P1, as shown in Figure S15, based on the strong nuclear overhauser effect (NOE) correlation of the proton Hj with the protons Hg and Hd, respectively
When eight bulky 4-brombutyloxy groups were capped on the cavity, instead of methoxy groups, the locked ethyl acetate group of P1 was inhibited from threading into the deeper cavity, leading to a less-stable self-inclusion structure
Summary
Pseudo[n]rotaxanes (n ≥ 2), a lasso-type of complexes, are two molecules or multiple molecules mechanically interlocked. Pseudo[1]rotaxanes of pillar[n]arene-type have an important application in molecular switches and host–guest complexation Their mono-substituent groups play an essential role in the self-assembly of the formed pseudo[1]rotaxanes. In our previous studies [30,31,32,33,34], we found that pillar[5]arene showed good binding properties to linear guests such as α,ω-dihaloalkane Their complexation behavior with the guest (for instance, with 1,4-dibromobutane) was affected by the different substituents on the rings. We synthesized a pillar[5]arene P1 bearing an acetate chain and multi-bromoalkyl chain groups and investigated its self-inclusion behavior and host–guest properties with dihalides such as 1,6-dibromohexane (G1), 1,4-dibromobutane (G2), 1,2-bromoethane (G3), 1,6-dichlorohexane (G4), and 1,6-diiodohexane (G5) (see Scheme 1) by 1 H NMR and 2D nuclear overhauser effect spectroscopy (2D NOESY) NMR measurements.
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