Isolation by crystallization of translational isomers of a bistable donor-acceptor [2]catenane

Abstract

The template-directed synthesis of a bistable donor-acceptor [2]catenane wherein both translational isomers—one in which a tetrathiafulvalene unit in a mechanically interlocked crown ether occupies the cavity of a cyclobis(paraquat-p-phenylene) ring and the other in which a 1,5-dioxynaphthalene unit in the crown ether resides inside the cavity of the tetracationic cyclophane—exist in equilibrium in solution, has led to the isolation and separation by hand picking of single crystals colored red and green, respectively. These two crystalline co-conformations have been characterized separately at both the molecular and supramolecular levels, and also by dynamic NMR spectroscopy in solution where there is compelling evidence that the mechanically interlocked molecules are present as a complex mixture of translational, configurational, and conformational isomers wherein the isomerization is best described as being a highly dynamic and adaptable phenomenon.

The dynamics and stereochemical behavior of mechanically interlocked molecules (1–3) (MIMs) in solution require a fundamental understanding of the tenets of stereochemistry that reach beyond (4, 5) the commoner garden stereogenic centers that are normally how stereochemistry expresses itself—usually in a kinetically stable fashion—in organic compounds. In the 1980s Schill et al. (6) drew attention to the existence of translational isomerism (7–14) in catenanes and rotaxanes (15, 16). Subsequently, we proposed (17, 18) the use of the term co-conformation to describe different relative spatial arrangements of the components in MIMs—such as bistable [2]catenanes and [2]rotaxanes—in the context of molecular electronic devices (MEDs) (19). The reason we coined this variant of the term conformation is that the translational isomers of catenanes and rotaxanes are frequently (4, 5) separated by free energies of activation in the range of 8–24 kcal mol^-1—that is, the magnitudes of free energy barriers we associate with NMR observable conformational changes and isomerism over the experimentally accessible range of temperatures on commercially available spectrometers.

Whereas translational isomerism has some of the structural hallmarks (e.g., observable changes in shape and obvious alterations in structure) of conformational isomerism, and bears some superficial resemblance to it, it is not strictly correct—in the light of generally accepted definitions (20)—to refer to the relative movements between mechanically interlocked components in MIMs as a change in conformation. The reason is that the widely recognized definition (21) of the term conformation is as follows: The conformations of a molecule of defined configuration are the various arrangements of its atoms in space that differ only as after rotation about single bonds. This definition is now commonly extended to include rotation about π-bonds or bonds of partial order between one and two. Translational isomerism does not fall under this umbrella: hence, our introduction (17, 18) of the term co-conformation to describe a particular kind of translational isomerism that is observable using dynamic NMR spectroscopy (22). Just as dynamic NMR spectroscopy was employed (23, 24) in the 1960s to probe the equilibration of conformational diastereoisomers; e.g., the axial and equatorial isomers of chlorocyclohexane (25) and to study the conformational behavior of medium-sized ring compounds (26) in the 1980s, it has become possible, during the past decade, to investigate co-conformational equilibria in bistable donor-acceptor [2]catenanes (27) and [2]rotaxanes (28–30). The first really robust and efficient switchable [2]catenane (27) incorporating a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) one was employed subsequently to good effect in molecular switch tunnel junctions (MSTJs) in two-dimensional molecular electronic devices (2D MEDs) with crossbar architectures (31, 32). On account of the much stronger binding (33) of TTF to cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) compared with DNP, the bistable [2]catenane (Fig. 1) exists preferentially (> 9∶1) as the translational isomer in which the TTF unit is included inside the CBPQT⁴⁺ ring in the so-called (34) ground-state co-conformation (GSCC) rather than in the metastable state co-conformation (MSCC) where the DNP unit occupies the cavity inside the CBPQT⁴⁺ ring. Oxidation of the TTF unit, however, alters the GSCC: MSCC exclusively in favor of the DNP unit residing inside the cavity of the CBPQT⁴⁺ ring. To date, however, we have not been able to isolate and characterize the MSCC. The same scenario holds true for many (28–30) but not all (35, 36) the bistable [2]rotaxanes that have been designed and synthesized for incorporation (37, 38) into MSTJs in 2D MEDs, and more recently at metal (39, 40) nanoparticle and silica (41, 42) nanoparticle-solvent interfaces.

Fig. 1.

(A) Structural formulas of the two translational isomers with different co-conformations—the ground state co-conformation (GSCC) and the metastable state co-conformation (MSCC)—of the first bistable [2]catenane to be obtained by template-directed synthesis. (B) The graphical representations of the GSCC and MSCC, illustrating the fact that the former is preferred over the latter in solution by greater than a 9∶1 ratio. This predominance of one translational isomer over the other is in sharp contrast to 1⁴⁺ where 1R⁴⁺ and 1G⁴⁺ are present in approximately equimolar amounts in solution.

Here we report on (i) the design of a TTF/DNP/CBPQT⁴⁺-[2]catenane that adopts both co-conformations in almost equal proportions in solution, (ii) the template-directed synthesis of this bistable donor-acceptor [2]catenane, (iii) its characterization by UV/Vis and ¹H NMR spectroscopies, as well as by electrochemical methods, (iv) crystallization of the [2]catenane in MeCN-i Pr ₂O and the manual separation of two crystalline forms—both rhomboidal in shape, with one set of single crystals colored red and the other green—which were examined separately by X-ray crystallography, (v) the outcome of computational studies performed on the two translational isomers identified in the solid state, and (vi) the measurement of the relaxation of one of the translational isomers to an equilibrium mixture of both. It transpires that the mechanically interlocked molecules of the bistable [2]catenane populate the solution as a complex mixture of six enantiomeric pairs of translational, configurational, and conformational isomers from which two of the enantiomeric forms crystallize as one translational isomer where the TTF unit is located outside the CBPQT⁴⁺ ring and adopts the cis configuration whereas the other translational isomer, where the TTF unit is located inside the CBPQT⁴⁺ ring, adopts the trans configuration.

Results and Discussion

The bistable [2]catenane 1·4PF₆, whose template-directed synthesis is summarized in Fig. 2, and outlined in Materials and Methods is composed of a π-electron-deficient CBPQT⁴⁺ ring and a crown ether containing two different π-electron-rich recognition sites. Investigation of the difference (34) in the free energies of binding (ΔΔG°) between the two recognition sites present in 1·4PF₆—namely, 4,4′,(5′)-bis[2-(2-(2-(2-hydroxyethoxy)-ethoxy)ethoxy)ethylthio]tetrathiafulvalene (43, 44) (STTFS-TEG) and 1,5-bis[2-(2-(2-(2-hydroxyethoxy)ethoxy)ethoxy)ethoxy]-naphthalene (45) (DNP-TEG)—with the tetracationic receptor CBPQT⁴⁺ by isothermal titration microcalorimetry indicates that the STTFS unit binds (SI Appendix) slightly more weakly to CBPQT⁴⁺ than does the tetracationic cyclophane to the DNP unit. Because the ΔΔG° value comparing these two binding events is in the range of less than 1.0 kcal mol^-1 (SI Appendix), the two co-conformations of 1·4PF₆ are expected to be present in roughly equal concentrations at room temperature: In 1R·4PF₆ the CBPQT⁴⁺ ring encircles the STTFS unit and in 1G·4PF₆ the CBPQT⁴⁺ ring encircles DNP unit. These two translational isomers are both stabilized by the π-electron donor–acceptor interactions between the CBPQT⁴⁺ rings and the STTFS/DNP units, as well as by [C - H⋯O] interactions (7) between the oxygen atoms in the polyether loops of the crown ether and the hydrogen atoms on carbons α to nitrogen on the CBPQT⁴⁺ ring.

Fig. 2.

The template-directed synthesis of the [2]catenane 1·4PF₆ in which the crown ether 4—obtained from 2 and 3 under high dilution reaction conditions in the presence of CsOH—acts as the template for the formation of the catenated CBPQT⁴⁺ ring. Spectroscopic evidence indicates that the [2]catenane exists in solution as an approximately equimolar mixture of the two translational isomers, 1R·4PF₆ and 1G·4PF₆.

To understand rather better the implications of replacing the TTF unit by a STTFS unit in bistable donor-acceptor [2]catenanes containing the CBPQT⁴⁺ ring, we turned to density functional theory. Previously, we had shown (46) that the M06-suite of density functionals, especially the M06-L and the M06-HF variants, are a good choice for investigating (47) MIMs. The M06-L functional is adequate for geometry optimization in view of its reasonably good accuracy and relatively low cost—no Hartree–Fock exchange—whereas the M06-HF functional is a hybrid meta-GGA (generalized gradient approximation) with full Hartree–Fock exchange, especially accurate for long-range charge transfer (CT) and noncovalent bonding interactions. We used the M06-L functional in the gas phase to minimize both translational isomers 1R·4PF₆ and 1G·4PF₆ of 1·4PF₆ and the GSCC and MSCC of the bistable [2]catenane shown in Fig. 1A. We then calculated natural orbital populations (48) from the M06-HF wave functions. Natural populations show a +0.1 e charge difference for TTF when it is encircled by a CBPQT⁴⁺ ring than when DNP is encircled by the tetracationic cyclophane. By contrast, in the case of 1·4PF₆, the difference in natural populations is 10 times smaller (+0.01 e). This difference reveals that the tetrathiafulvalene moiety in the STTFS unit has a virtually identical electronic population when it is encircled by a CBPQT⁴⁺ ring than when it is not. However, when encircled by a CBPQT⁴⁺ ring, the DNP unit has a very similar electronic population in both the bistable [2]catenanes—the one containing TTF (Fig. 1A) and 1·4PF₆. Additionally, our calculations reveal higher positive charges on the distal S atoms in STTFS, which in turn, reduces the ability to induce attractive [π⋯π] stacking inside the tetracationic cyclophane in comparison with TTF. This observation could be interpreted as a “charge leak” that reduces the strength of the interaction.

The UV/Vis spectrum of 1·4PF₆, which was recorded in MeCN at 298 K (SI Appendix), reveals a CT absorption band centered on 800 nm, characteristic of co-conformations with the STTFS unit residing inside the CBPQT⁴⁺ ring. In addition, however, a CT band centered on 540 nm, which arises when a DNP unit is located inside the CBPQT⁴⁺ ring, is also observed for the other co-conformation. ¹H-¹H Gradient-selected Double-Quantum-Filtered Correlation Spectroscopy (¹H-¹H-g-DQF-COSY) was performed at 233 K on 1·4PF₆ in CD₃CN. The main proton resonances were assigned to these two sets of co-conformations (translational isomers) based on this COSY spectrum (SI Appendix). Both translational isomers (1R·4PF₆ and 1G·4PF₆) of 1·4PF₆ were identified in CD₃CN at 233 K in a conventional ¹H NMR spectrum. Using specific DNP protons as probes, the ratio of 1R·4PF₆ to 1G·4PF₆ was ∼1∶1. The presence of approximately equimolar amounts of these two translational isomers in solution was also confirmed by cyclic voltammetry (CV) (SI Appendix). For 1·4PF₆, two oxidation potentials were recorded at +415 and +646 mV. The first peak at +415 mV arises from the first oxidation process (STTFS → STTFS^+•) of the alongside STTFS unit in 1G·4PF₆ and the peak at +646 mV is a combination of the first oxidation process for the inside STTFS and the second oxidation process (STTFS^+• → STTFS²⁺) in both co-conformations. An attempt to follow the relaxation kinetics from 1G⁴⁺ back to the equilibrium mixture of 1G⁴⁺ and 1R⁴⁺ by varying the CV scan rate revealed that the interconversion was too fast to be monitored at room temperature. By comparison with a previous investigation (34) on the bistable [2]catenane illustrated in Fig. 1A, the relaxation process must be associated with a free energy of activation (ΔG^‡ value) of less than 16 kcal mol^-1.

Both the degenerate (49, 50) and nondegenerate (27) [2]catenanes (Fig. 3A), which have been studied in the dim and distant past crystallized (Fig. 3B) as one-dimensional polar stacks (Fig. 3 C and D). The bistable [2]catenane (27) has only been isolated in crystalline form wherein the molecules that make up the polar stacks have their TTF units encircled by the CBPQT⁴⁺ ring; i.e., it would appear to have undergone a crystallization-induced second-order transformation (51) insofar as the MSCC has not yet been obtained crystalline. We assumed that, during the crystallization of the GSCC, the small proportion of the MSCC at equilibrium presumably funnels through to the GSCC (Fig. 1A) during the kinetically controlled crystallization process. We hypothesized that, because the bistable [2]catenane 1·4PF₆ exists in solution as an approximately 1∶1 mixture of the translational isomers (SI Appendix), it might undergo a first-order transformation (51); i.e., a solid mixture of crystals of the two translational isomers might be obtained reflecting the proportions present at equilibrium in solution. This situation is one we have not observed previously.

Fig. 3.

(A) Structural formulas of the degenerate donor-acceptor [2]catenane containing two DNP units in its crown ether component, as well as of the bistable donor-acceptor [2]catenane mentioned in Fig. 1. It contains both a DNP and a TTF unit in its crown ether component. The TTF unit prefers to occupy the central cavity of CBPQT⁴⁺ ring much (> 9∶1) more so than does the DNP unit. (B) Stick diagrams of the solid-state structures of the degenerate and bistable [2]catenane obtained by X-ray crystallography. Note that only one—namely, that where the TTF unit is inside the cavity of the CBPQT⁴⁺ ring and the DNP unit is outside—of the two translational isomers of the bistable [2]catenane has been characterized by X-ray crystallography in the solid state. In addition, note that the TTF unit inside the CBPQT⁴⁺ cavity adopts the trans configuration. (C) and (D) Space-filling representations of the solid-state superstructures of the degenerate and bistable [2]catenanes. In both cases, the [2]catenane molecules form donor-acceptor stacks involving [π⋯π] interactions between the outside π-electron-deficient bipyridium units and the alongside π-electron-rich units in the adjacent molecules.

Slow vapor diffusion of i Pr ₂O into a MeCN solution of 1·4PF₆ at 298 K afforded a mixture (Fig. 4) of green and red crystals, both suitable for X-ray crystallography. The co-conformational assignments to the two isomers of 1·4PF₆ were made (Fig. 5A) following the isolation of a single red (1R·4PF₆) and a single green (1G·4PF₆) crystal from the mixture. It is worth noting here that the CT band for the TTF⊂CBPQT⁴⁺ complex recorded in MeCN at 298 K is situated (27, 52) around 800–900 nm, an absorption that leads to a deep emerald colored solution. This emerald green color is also witnessed in the solid state for crystal (super) structures containing TTF units encircled by CBPQT⁴⁺ rings; i.e., pseudorotaxanes, catenanes, and rotaxanes. The CT band for the DNP-TEG⊂CBPQT⁴⁺ complex is situated (50, 53) around 500–560 nm, resulting in a deep reddish-purple colored solution. Once again, this color is also expressed in the solid state where crystals containing DNP units encircled by CBPQT⁴⁺ rings are also a deep reddish-purple color. In the light of these observations, it was surprising to us that the red crystal turned out to correspond to the co-conformation in which the STTFS unit is encircled by the CBPQT⁴⁺ ring, whereas the green crystals correspond to the co-conformation in which the DNP unit is encircled by the tetracationic cyclophane. Remarkably, in the case of 1·4PF₆, the color of the crystal is derived principally from within the polar donor-acceptor stacks, rather than by the intramolecular CT interactions from within the [2]catenane itself; i.e., the solid-state (super)structure for 1R·4PF₆—the red crystals—corresponds to the co-conformation in which the STTFS units are encircled and bound by the CBPQT⁴⁺ ring. When dissolved in CD₃COCD₃, the red crystal produces a characteristic emerald green colored solution at 190 K. Likewise, the solid-state (super) structure for 1G·4PF₆—the green crystals—corresponds to the co-conformation in which the DNP unit is encircled by the CBPQT⁴⁺ ring.

Fig. 4.

Photographs of the single crystals grown by the vapor diffusion of i Pr ₂O into a solution of 1·4PF₆ in MeCN at 298 K. (A) Culture tube with two kinds of crystals growing on its surface. (B) Magnification of the previous photograph. (C)–(E) A selection of photographs taken at even higher magnification, showing the morphology of the two kinds of crystals, one red and the other green.

Fig. 5.

(A) Structural formulas of the translational isomers 1R⁴⁺ and 1G⁴⁺ of the bistable [2]catenane 1⁴⁺ containing DNP and STTFS units in the crown ether mechanically interlocked by the CBPQT⁴⁺ ring. In 1R⁴⁺, the STTFS unit is located inside the CBPQT⁴⁺ ring, whereas, in 1G⁴⁺, the encircled unit is a DNP one. (B) Stick diagrams of the solid-state structure of 1R⁴⁺ and 1G⁴⁺ determined by X-ray crystallography. Note that, although the inside STTFS unit in 1R⁴⁺ assumes a trans configuration, the alongside STTFS unit in 1G⁴⁺ adopts a cis configuration. (C) and (D) Space-filling representations of the solid-state superstructures of 1R⁴⁺ and 1G⁴⁺ highlighting their donor-acceptor stacks.

The solid-state structures of 1R⁴⁺ and 1G⁴⁺ are shown in Fig. 5B. Both translational isomers are stabilized by the same [π⋯π], [C - H⋯π] and [C - H⋯O] interactions that have been shown to be present in the past in their degenerate (49, 50) and nondegenerate (27) counterparts (Fig. 3B). For 1R·4PF₆, the interplanar separations between the inside STTFS unit and the outside and inside bipyridinium units are 3.33 and 3.40 Å, respectively, whereas the plane-to-plane distance between the outside DNP unit and the inside bipyridinium unit is 3.47 Å. For 1G·4PF₆, the interplanar separations between the inside DNP unit and the outside and inside bipyridinium units are 3.58 and 3.20 Å, respectively, whereas the plane-to-plane distance between the outside STTFS unit and the inside bipyridinium unit is 3.56 Å. The packing (Fig. 5 C and D) in the solid state of the 1R⁴⁺ and 1G⁴⁺ tetracations (mean interplanar separations of 3.38 and 3.47 Å, respectively) is comprised of intermolecular donor–acceptor [π⋯π] interactions between the outside π-electron-deficient bipyridinium units and the alongside π-electron-rich DNP (in 1R⁴⁺ polar stacks) and STTFS (in 1G⁴⁺ polar stacks) units of adjacent bistable [2]catenane molecules, resulting in conventional polar donor-acceptor stacks. It is intriguing that 1⁴⁺, which, in principle at least, is capable of forming three different types of polar donor–acceptor stacks—namely, (i) random stacks, (ii) alternating stacks, 1R·4PF₆⋯1G·4PF₆⋯1R·4PF₆, and (iii) homostacks, where 1R·4PF₆⋯1R·4PF₆⋯1R·4PF₆ stacks (Fig. 5C) constitute the red crystals, and where 1G·4PF₆⋯1G·4PF₆⋯1G·4PF₆ stacks (Fig. 5D) constitute the green crystals—opts to undergo a first-order transformation (51).

When the red crystals of 1R·4PF₆ are dissolved in CD₃COCD₃ at 220 K, it is possible to follow the first-order decay by ¹H NMR spectroscopy (SI Appendix) of this translational isomer to an equilibrium mixture of the two isomers in which 1G·4PF₆ predominates. ¹H NMR spectra recorded at suitable time intervals during almost 17 h provided us with data, related to the changing proportions of 1R⁴⁺ and 1G⁴⁺ with time, from which a ΔG^‡ value of ca. 16 kcal mol^-1 for the isomerism from 1R⁴⁺ to 1G⁴⁺ could be obtained. The ΔG^‡ value is very much in the expected (34) range for circumrotation of a crown ether containing tetrathiafulvalene and 1,5-dioxynaphthalene units through the cavity of the tetracationic cyclophane.

Conclusion

In stereochemical terms, the equilibration between 1R⁴⁺ and 1G⁴⁺ is far from being a simple and straightforward process. The complexity is brought about by 1⁴⁺ having four sources of isomerism (Fig. 6), all of which are happening rapidly on the laboratory timescale and three of which are occurring at different rates on the ¹H NMR timescale. The cis-trans isomerism involving the STTFS units is a slow process on this timescale at room temperature. Whereas the circumrotation of the crown ether through the tetracationic cyclophane is sufficiently slow to give rise to two different sets of ¹H NMR signals at low temperature in solution, the other two processes, which involve the inversion of the planes of chirality (54) associated with the trans-STTFS and the DNP units, are occurring rapidly on the ¹H NMR timescale, even at 220 K.

Fig. 6.

(A) A graphical representation of the translational isomers 1R⁴⁺ and 1G⁴⁺ at equilibrium. (B) The interconversion between the cis and trans isomers of disubstituted TTF units. (C) The inversion of the plane of chirality between the (pR)- and (pS)- isomers for STTFS units. (D) The inversion of the plane of chirality between the (pR)- and (pS)-isomers for DNP units. In the case of both the STTFS and DNP units, the absolute chiralities have been defined as shown in (C) and (D).

The stereochemical situation as a whole is summarized in Fig. 7. There are a total of 12 isomers (six pairs of enantiomers) that are inverting (between enantiomeric forms) and interconverting (between isomers that are not enantiomers) rapidly on the laboratory timescale. We propose a hierarchy of isomerism that locates (i) the translational isomerism between 1R⁴⁺ and 1G⁴⁺ at the top of the tree, followed by (ii) the configurational isomerism between cis and trans STTFS units, and finally (iii) the conformational isomerism associated with the element of planar (p) chirality between (pR)- and (pS)-trans-STTFS units and likewise the conformational isomers associated with the element of planar (p) chirality between (pR)- and (pS)-DNP units. Thus, we classify the two isomers 1R⁴⁺ and 1G⁴⁺, which have crystallized out separately as, first and foremost, translational isomers (STTFSi/DNPo and STTFSo/DNPi), and then secondly, as configurational isomers (c and t) resulting from rotation about the C = C double bond in the middle of the STTFS unit, followed by the isomers, pR_STTFS and pS_STTFS, created as a result of the planar chirality associated with the STTFS unit and, finally, the isomers, pR_DNP and pS_DNP, created as a consequence of the planar chirality associated with the DNP unit. From this dynamic pool of 12 isomers, two pairs of enantiomers—namely ( and t-pR_STTFSi-pR_DNPo/t-pS_STTFSi-pS_DNPo)—crystallize out by a first-order transformation event.

Fig. 7.

A chart and supporting graphical display showing the dynamic interplay between translational, configurational and conformational isomerism in the bistable [2]catenane 1⁴⁺ molecule that experiences (i) cis-trans isomerism of its STTFS units, (ii) translational isomerism depending upon whether its STTFS or DNP unit is located inside the cavity of the CBPQT⁴⁺ ring, and (iii,iv) two conformational processes involving the inversion of the planes of chirality associated with both its STTFS and DNP units. The configurational isomerism is denoted by “c” for the cis-isomers and “t” for the trans-isomers. The subscripts “i” and “o” following the acronyms STTFS and DNP indicate whether these units are inside or outside the CBPQT⁴⁺ ring in the translational isomers. The dynamic planar chirality is designated, according to the Cahn–Ingold–Prelog rules, as (pR) or (pS). See Fig. 6. This dynamic stereochemistry yields 12 possible isomers in solution. The designations of their structures to stereochemical descriptors indicates that there are actually six pairs of enantiomers. The two pairs of enantiomers that have yielded single crystals simultaneously from crystal growing experiments, and for which the solid-state structures have been solved by X-ray crystallography, are surrounded by two black rectangles. Note that the equilibrium arrows do not necessarily represent a specific inversion or interconversion process. E identifies the six pairs of enantiomers.

In the event, 1R⁴⁺ and 1G⁴⁺ are simultaneously translational, configurational, and conformational isomers in a situation where isomerism is a highly dynamic phenomenon. Put another way, this research has established the concept of dynamic isomerism in a bistable [2]catenane containing four sources of isomerism, which are equilibrating rapidly with one another in solution at room temperature. One can speculate that such a molecule should be highly adaptive to its surroundings, solution or otherwise, chiral or achiral. It is a molecule that “wears a coat of many colors,” depending on its environment in solution from which it can produce spontaneously coats of two different colors on selective crystallization under the appropriate conditions.

Materials and Methods

Synthesis and Characterization of the Bistable [2]Catenane 1·4PF₆.

A mixture of 4,4′-bis(bromomethyl)benzene (105 mg, 0.40 mmol), the macrocycle 4 (100 mg, 0.13 mmol), and 1,1′-[1,4-phenylenebis(methylene)]-bis(4,4′-bipyridinium) bis(hexafluoro phospate) (55) (285 mg, 0.40 mmol) was dissolved in N-N-dimethylformamide (10 mL). The reaction mixture was subjected to 15 kbar in an ultra high pressure reaction chamber at room temperature for 3 d, the resulting green solution was subjected to column chromatography [SiO₂∶Me₂CO/NH₄PF₆(100∶1,v/w)]. The green band was collected and most of the solvent was removed under reduced pressure, followed by the addition of H₂O (10 mL). The precipitate was collected by filtration and washed with H₂O (3 × 20 mL), to give the pure product 1·4PF₆ as a green solid (132 mg, 55%). ¹H NMR (CD₃CN, 600 MHz, 233 K, ppm): δ = 2.29 (d, J = 8.0 Hz, ∼1H), 2.76–2.82 (m, ∼2H), 2.91–3.00 (m, ∼2H), 3.40–4.30 (m, 28H), 5.54–5.94 (m, 10H), 6.17 (t, J = 7.9 Hz, ∼1H), 6.20–6.25 (m, 1H), 6.43 (d, J = 7.6 Hz, ∼0.95H), 6.54 (d, J = 7.7 Hz, ∼0.05H), 7.14–8.09 (m, 18H), 8.58–9.01 (m, 8H). HR MS (ESI): Calculated for C₆₈H₇₂F₂₄N₄O₈P₄S₆ m/z = 1699.2594 [M - PF₆]⁺, 777.1474 [M - 2PF₆]²⁺, found m/z = 1699.2559 [M - PF₆]⁺, 777.1475 [M - 2PF₆]²⁺.

Crystallization of the Bistable [2]Catenane 1·4PF₆.

The green solid was dissolved in MeCN in a small test tube and was subjected to slow (∼2 weeks) vapor diffusion with i Pr ₂O contained in a larger sealed vial. Under these crystallization conditions, batches of both red and green single crystals formed on the sides of the test tube. With the aid of a stereomicroscope, the crystals were detached with considerable care from the sides of the test tube using a dissecting needle. The freed crystals were transferred into a glass Petri dish (60 × 15 mm), which was covered to prevent solvent evaporation. Using tweezers and a stereomicroscope, the red and green crystals were separated mechanically and placed in separate vials. Signal crystals, suitable for X-ray analysis were investigated to determine (Fig. 5) their solid-state structures. The data were corrected for Lorentz and polarization effects.