Revisiting the Formation and Tunable Dissociation of a [2]Pseudorotaxane Formed by Slippage Approach

A new [2]pseudorotaxane DB24C8⊃1-H·PF6 with dibenzo[24]crown-8 (DB24C8) crown ether-dibenzylammonium (1-H·PF6) binding which was formed by slippage approach at different solvents and temperature, had been isolated and characterized by NMR spectroscopy and mass spectrometry. The [2]pseudorotaxane DB24C8⊃1-H·PF6 was stable at room temperature. The dissociation rate of [2]pseudorotaxane DB24C8⊃1-H·PF6 could be tuned by using different stimuli such as triethylamine (TEA)/diisopropylethylamine (DIPEA) and dimethyl sulfoxide (DMSO). In particular, the dissociation of [2]pseudorotaxane DB24C8⊃1-H·PF6 by an excess of TEA/DIPEA base mixture possessed a long and sustained, complete dissociation over 60 days. Other stimuli by DMSO possessed a relatively fast dissociation over 24 h.


Introduction
Rotaxanes contain a linear dumbbell-shaped component bearing bulky end-groups or stoppers around which one or more macrocycles are trapped. On the other hand, pseudorotaxanes are OPEN ACCESS temporally encircled around an unstoppered thread through noncovalent interactions from which they are readily susceptible to dissociation without breaking a covalent bond [1][2][3][4]. Rotaxanes and pseudorotaxanes have been studied extensively for the ability of the interlocked ring to be switched on demand by external stimuli such as pH [5,6], electrochemical reagents [7], heat [8,9], moisture [10,11], salt [12], light [13], etc. Coupled with their ability to be customized and optimized for nanoscale functions, these interlocked molecules are excellent candidates as movable elements in the construction of nanovalves [14] based on a porous, solid-phase support, for controlled substrate release. Many examples have been demonstrated about their relatively fast substrate releases within 24 h by tuning the pH values in the solution [12]. Currently, no pseudorotaxane or rotaxane building blocks are available and suitable for the construction of vehicles for sustained substrate release over a week.
The construction of rotaxane-like assemblies has recently relied upon thermodynamic, templated reactions with enhanced efficiencies [15][16][17][18][19][20][21][22][23][24]. The term "slippage" has been coined [25][26][27][28] for pseudorotaxane synthesis that employs thermodynamic threading of macrocycle to a "dumbbell". This strategy utilizes (1) the size complementarily between the macrocycle and the "dumbbell's" stoppers; and (2) the stabilizing noncovalent bonding interactions between the macrocycle and the "dumbbell's" rod. In this strategy, the macrocycle and the dumbbell have been separately synthesized, prior to heating them together in solution so that the free energy of activation for the thermodynamic threading (slippage) of macrocycle to dumbbell can be overcome. The presence of a template on the dumbbell's rod renders the pseudorotaxane structure more stable so that the free energy of activation for its dissociation becomes insurmountable when the solution has been cooled to ambient temperature. There are several disadvantages of the slippage approach in terms of reaction time and stability. Generally, it requires a long reaction time with a large slippage rate (kon) for pseudorotaxane formation from usually a few days up to 90 days [27]. However, the dissociation of pseudorotaxane will sometimes have an uncontrollable, small slippage rate (koff). Therefore, there is a need to study a better reaction condition for pseudorotaxane formation by slippage and to tune the pseudorotaxane dissociation rate (koff << kon) from days to hours.

Results and Discussion
Herein, we employ a slippage approach in one pot (Figure 1) to yield a thermodynamically stable [2]pseudorotaxane [29,30] by mixing an ammonium thread (dumbbell) 1-H·PF6 with dibenzo [24]crown-8 (DB24C8). Benzo-crown ether DB24C8 is capable to recognize with secondary ammonium ions by virtue of [N + -H···O] and [N + -C-H···O] hydrogen bonds, electrostatic interactions, and augmented with some aromatic π-π interactions [31][32][33][34][35][36]. The 3,5-dimethoxyaryl moiety at one end of the dumbbell 1-H·PF6, is a relatively bulky stopper that can effectively block the passageway of DB24C8 at high reaction temperature. On the other hand, the cyclohexyl ring of the dumbbell-like thread, in contrast, allow the crown ether to thread through to the dumbbell's rod at elevated reaction temperature with the molecular flipping of between their chair-boat-chair forms [31][32][33][34][35][36][37]. The reaction time of the pseudorotaxane synthesis was fixed in 4 days with reasonable yields (~50%). The percentage yields (Table 1) of individual [2]pseudorotaxane after a reaction time of 4 days, have been determined by 1 H NMR spectroscopy according to their "bound" and "free" signal intensities [36][37][38]. From the results, the reaction yields and rates in synthesizing the [2]pseudorotaxanes are sensitive to different solvents and temperatures. In particular, the percentage yield of the [2]pseudorotaxane DB24C8⊃1-H·PF6 was found to be the highest (46%) after a reaction temperature at 70 °C with MeCN compared to using PhMe (70 °C) and CH2Cl2 (40 °C). This is partially because, in polar solvents, the [2]pseudorotaxane might undergo intra and/or intermolecular aggregation of the hydrophobic alkyl chains and this will overcome the extrusion effect. The [2]pseudorotaxane DB24C8⊃1-H·PF6 could be isolated by flash chromatography on silica gel (CH2Cl2/THF = 7/1) and characterized by NMR spectroscopy and mass spectrometry.  High-resolution electrospray ionization mass spectrometry (ESI-MS) has been employed ( Figure 3) to further characterize the [2]pseudorotaxane. The molecular ion peak at m/z 740 which is the most abundant peak in the spectrum, is corresponded to the [M-PF6] + ion of the [2]pseudorotaxane. Furthermore, the dissociation and stabilities of [2]pseudorotaxane towards organic amine bases and a hydrogen bond disrupting solvent were evaluated. In particular, pure [2]pseudorotaxane DB24C8⊃1-H·PF6 was dissolved in CD3CN and was treated with an excess of triethylamine (TEA)/diisopropylethylamine (DIPEA) mixture [5,6,12]. The ammonium ion of the thread 1-H·PF6 could be successfully deprotonated by the bases wherein the DB24C8 loss its binding affinity towards the deprotonated, amine thread 1. Since the template effect is lost, the [2]pseudorotaxane is no longer stable whereas extrusion of macrocycle occurs with the molecular flipping of the cyclohexyl ring. This extrusion behavior was monitored (Figure 4) by observing a significant decrease of characteristic signal at δ = 6.16 ppm (bound ArH of 1-H·PF6) as well as an increase of signal at δ = 6.38 ppm (free ArH of 1) from their 1 H NMR spectra over time. Interestingly, the [2]pseudorotaxane requires almost 60 days to dissociate completely into two separate components at ambient temperature. The half-life of dissociation was determined to be approximately 2 days (Figure 6).  On the other hand, by dissolving the pure [2]pseudorotaxane DB24C8⊃1-H·PF6 in a hydrogen bond disfavored solvent-dimethyl sulfoxide (DMSO) [1,27], extrusion of DB24C8 from the thread 1-H·PF6 occurred. Since the template effect is lost, the [2]pseudorotaxane is no longer stable whereas extrusion of macrocycle occurs with the molecular flipping of the cyclohexyl ring. This extrusion behavior was monitored ( Figure 5) by observing the decreases of characteristic signals at δ = 3.81, and 4.14 ppm (bound -OCH2CH2O-of DB24C8) as well as the increases of signals at δ = 3.74, and 4.07 ppm (free -OCH2CH2O-of DB24C8) from their 1 H NMR spectra (in DMSO-d6) over time. The [2]pseudorotaxane required only 24 h for a complete dissociation. The half-life of dissociation was determined to be approximately 4 h ( Figure 6).

Experimental Section
General Information. 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were recorded at room temperature in CDCl3 unless otherwise stated. Each solvent residual signal was used as the internal standard. Chemical shifts were reported as parts per million (ppm) in δ scale and coupling constants (J) were reported in hertz. Mass spectra were obtained on a double focusing sector mass spectrometer with electrospray ionization (ESI) technique. The reported molecular mass (m/z) values, unless otherwise specified, were mono-isotopic mass. All reactions were carried out under N2. All reactions were monitored by thin layer chromatography (TLC) performed on pre-coated silica gel 60 F254 plates, and compounds were visualized with a spray of 5% (w/v) dodecamolybdophosphoric acid in ethanol and subsequent heating. Flash chromatography was carried out on columns of silica gel (230-400 mesh). Tetrahydrofuran (THF) was freshly distilled prior to use from sodium/benzophenone ketyl under N2. CH2Cl2 was freshly distilled from CaH2. Cyclohexanemethylamine (3) and dibenzo [24]crown-8 (DB24C8) were commercially available from Sigma-Aldrich (St. Louis, MO, USA) while the starting compound (2) was synthesized according to the literature procedures [39]. The synthetic scheme of new compounds is shown in Scheme 1.

Conclusions
In summary, a [2]pseudorotaxane DB24C8⊃1-H·PF6 which was synthesized and isolated from a slippage approach, was found to be stable at room temperature. The [2]pseudorotaxane DB24C8⊃1-H·PF6 was characterized by NMR spectroscopy and mass spectrometry. The dissociation rate of [2]pseudorotaxane DB24C8⊃1-H·PF6 could be tuned by amine bases and DMSO solvent at room temperature from which the supramolecular interactions between the crown ether DB24C8 and the ammonium can be tuned. When the crown ether DB24C8 was lost its noncovalent interactions with the ammonium/amine, it could be detached from the dumbbell 1-H·PF6 via the molecular flipping of between the chair-boat-chair forms of the cyclohexyl ring. In particular, the dissociation of [2]pseudorotaxane DB24C8⊃1-H·PF6 with amine bases possessed a long and sustained, complete dissociation over 60 days. The use of DMSO possessed relatively fast and complete dissociation (24 h). The selectively tunable dissociation rates by potentially varying the ratios of stimuli for macrocycle extrusion from the novel [2]pseudorotaxane provide avenues for constructing novel functional nanovalves that sustained release substrates for a much longer period of time.