Abstract
Rotaxanes are typically prepared using covalent bonds to trap a wheel component onto an axle molecule, and rotaxane-type wheel–axle assembly using only noncovalent interactions has been far less explored. Here we show that a dinickel(II) metallomacrocycle forms two different types of wheel–axle assemblies with a dibenzylammonium axle molecule based only on noncovalent interactions. The non-threaded assembly was obtained by introduction of Ni2+ into the macrocycle before the complexation with the axle molecule (metal-first method). The non-threaded assembly was in rapid equilibrium with each of the components in solution. The threaded assembly was obtained by introduction of Ni2+ after the formation of a pseudorotaxane from the non-metalated wheel and the axle molecule (axle-first method). The threaded assembly was not in equilibrium with the dissociated species even though it was maintained only by noncovalent interactions. Thus, formation of one of the non-threaded and threaded wheel–axle assemblies over the other is governed by the assembly pathway.
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Introduction
Rotaxanes are mechanically interlocked molecules in which a molecular wheel is trapped on a dumbbell-shaped axle1,2,3,4. To date, various kinds of rotaxanes have been synthesized and used as components of molecular machines5,6,7,8,9,10,11,12, catalysts13,14,15,16,17,18,19,20, entangled polymer materials21,22,23,24,25,26,27,28,29,30,31, etc. In principle, the synthesis of rotaxanes requires rational strategies to suitably assemble the wheel and axle components before interlocking; the typical and well-established methods include capping and clipping.
In order to make a rotaxane structure consisting of a wheel and an axle component, two different types of interactions/bonds are necessary; one is to preassemble the two components and the other is to lock the preassembled structure. In general, noncovalent interactions are used for the preassembly, while covalent bond formation is used in the second step for complete interlocking, e.g., introduction of bulky stoppers in the capping method and ring closure in the clipping method. However, noncovalent interactions have also been used for interlocking when these have a less reversible feature. In fact, there have been various rotaxane structures containing metal atoms in the wheel and/or axle components32,33,34,35,36,37,38. In some of these structures, the metal constituents are essential, because the interlocked rotaxane structures cannot be maintained without the metal atoms. For example, the metal atoms are introduced at both ends of the axle as a part of the stopper groups39,40,41,42, at the center of the axle as the interacting motif with the wheel43,44,45,46,47,48, or in the wheel as the constituent metal to complete the metallomacrocycle49,50,51,52,53. These metal-containing rotaxanes are obtained by coordination-based capping and clipping, which keeps the wheel and axle interlocked39,40,41,42,49,50,51,52,53. Shrinking is also a unique method to obtain an interlocked rotaxane structure from the corresponding preassembled components or to control the shuttling motions54,55,56,57. For example, a pseudorotaxane is converted into a genuine rotaxane by incorporating metal ions into the coordination sites of the wheel component while keeping the axle threaded54. In general, intermolecular association based on noncovalent interactions produces assembled products under thermodynamic control; the thermodynamically most favored product is usually obtained due to the dynamic nature of the noncovalent interactions. In this context, it should be challenging to selectively obtain two types of assemblies based only on noncovalent intermolecular interactions. It is essential to use noncovalent interactions that would suppress the interconversion between the two types of assemblies. We expected that relatively inert coordination bonds can be used in addition to the dynamic hydrogen bonds and electrostatic interactions.
In this study, we successfully obtained two types of wheel–axle assemblies, i.e., the non-threaded and threaded assemblies, based only on the noncovalent interactions (Fig. 1). We focused on a 32-membered macrocycle H4L1[,58 or H4L2 as the wheel unit, which shrinks to afford a 22-membered metallomacrocycle upon metalation of the two N,N′-disalicylidene-o-phenylenediamine (H2saloph) moieties with Ni2+, and a dibenzylammonium derivative as the axle unit, which strongly interacts with large-sized crown ethers. The non-threaded wheel–axle assembly, in which the wheel and axle components are arranged in a face-to-face fashion, was obtained by the metal-first method (Fig. 1a, (i)), i.e., by introduction of Ni2+ into the H4L2 macrocycle before the complexation with G1•PF6. On the other hand, the rotaxane-type threaded assembly was obtained by the axle-first method (Fig. 1a, (ii)), i.e., by introduction of Ni2+ after the formation of a pseudorotaxane from the H4L2 wheel and the G1•PF6 axle. Thus, both the non-threaded and threaded wheel–axle assemblies were formed depending on the order of the threading and shrinking steps. (Fig. 1a). We also investigated the different dissociation behaviors of these two types of wheel–axle assemblies upon the addition of Cs+; only the rotaxane-type threaded assembly remained intact even in the presence of the guest Cs+.
a (i) Metal-first method for non-threaded assembly and (ii) axle-first method for rotaxane-type threaded assembly. b Molecular components used in this study, H4L1/H4L2, L1Ni2/L2Ni2, DB24C8, and axles (G1•PF6, G2•PF6, G3•PF6, and G4•PF6).
Results and discussion
Design and synthesis of wheel and axle components
As the wheel unit for the rotaxane structures with dibenzylammonium derivatives, the dibenzo-24-crown-8 (DB24C8) derivatives are known to be the most suitable molecule59,60,61,62,63. We expected that the bis(saloph) macrocycle, H4L1, in which two H2saloph units are connected with two O–CH2CH2–O linkers, would be a suitable candidate for the wheel component in this study, because its metalated form, L1Ni2, has a binding site similar to DB24C8 (Fig. 1b) but shows a significantly higher binding affinity toward cationic species due to the negatively polarized phenoxo groups of the O–Ni–O moieties64,65,66,67,68. We have already demonstrated that the H4L1 macrocycle shrinks upon metalation due to the formation of two O–Ni–O linkages, and that the resultant DB24C8-like O8 cavity exhibited an excellent binding affinity to Cs+,58. As the axle component, we expected that the parent dibenzylammonium (G3•PF6) could be used. The unsubstituted benzyl group would act as a sufficiently bulky stopper to prevent the axle from passing through the relatively smaller oval cavity of L1Ni2, whereas the flexible DB24C8 is known to require a bulkier stopper such as the 3,5-disubstituted benzyl groups69,70,71,72,73. However, these wheel (L1Ni2) and axle (G3•PF6) components showed a low solubility in nonpolar solvents, which are essential for strengthening the wheel–axle interaction and increasing the efficiency of the pseudorotaxane formation59,60,61. Therefore, we used their lipophilic analogues, L2Ni2 and G1•PF6, which have four peripheral octyloxy groups and two butyl groups, respectively.
The H4L2 macrocycle was obtained by the reaction of bis(salicylaldehyde) 158 with an equimolar amount of 4,5-dioctyloxy-1,2-phenylenediamine (2)74 in chloroform/dimethyl sulfoxide (DMSO) at room temperature in 61% yield (Fig. 2a (i)), which basically follows the synthetic procedure of the unsubstituted analogue H4L1[,58. The formation of the [2 + 2] macrocycle, H4L2, was confirmed by the electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) (m/z = 1261.75 for [H4L2 + H]+). The axle G1•PF6 was synthesized via a well-established procedure (Fig. 2b) by the reduction of the imine 375 with LiAlH4 followed by protonation with hydrogen chloride and the subsequent counter anion exchange with ammonium hexafluorophosphate. The axle G2•PF6 containing bulkier stoppers was synthesized according to the literature76.
a Wheel components (H4L2 and L2Ni2) and b axle component (G1•PF6).
As already mentioned, the H4L2 macrocycle would shrink to afford the metallomacrocycle, L2Ni2, upon the metalation of the H2saloph coordination sites. Thus, we investigated the shrinking of the macrocycle in the absence of the axle, which corresponds to the first step of the metal-first method (Fig. 1a (i)). Indeed, the reaction of the H4L2 macrocycle with nickel(II) acetate gave the L2Ni2 metallomacrocycle in 71% yield (Fig. 2a (ii)), which was characterized by spectroscopic techniques.
The X-ray crystallographic analysis revealed the structure of L2Ni2 (Fig. 3), which has a 22-membered macrocyclic structure. The nickel(II) ions have a square planar geometry as observed for the unsubstituted analogue L1Ni258. The two [Ni(saloph)] moieties in the L2Ni2 macrocycle are situated in an almost coplanar fashion. This relatively planar structure was in sharp contrast to the V-shaped conformation observed for the unsubstituted analogue L1Ni2 in the crystalline state58. Since introduction of the peripheral alkyl/alkoxy groups does not usually have a significant influence on the most stable conformation in solution, this structural diversity implies the sufficient flexibility of the L2Ni2 wheel, which allows adoption of various conformations from the planar to the V-shaped structures in solution.
a top view and b side view.
Formation of non-threaded wheel–axle assembly
We then investigated the formation of the wheel–axle assembly according to the metal-first method (Fig. 1a (i)) using the pre-formed L2Ni2 as the synthetic intermediate. The interaction of the wheel L2Ni2 with the axle G1•PF6 was clearly demonstrated by the 1H nuclear magnetic resonance (NMR) titration experiments in CDCl3/CD3CN (4:1). The addition of G1•PF6 to L2Ni2 caused significant chemical shift changes indicative of the strong wheel–axle interaction (Fig. 4a (i)–(iv)). The binding constant for the 1:1 complexation was determined to be logKa = 4.0 ± 0.2 by a nonlinear least-squares regression (Supplementary Fig. S1).
a 1H NMR spectra of non-threaded and threaded wheel–axle assemblies of L2Ni2 and G1•PF6 (400 MHz, CDCl3/CD3CN (4:1)). (i–iv) Spectral changes of L2Ni2 upon the addition of G1•PF6 (i, 0 equiv, ii, 0.5 equiv, iii, 1.0 equiv, iv, 2.0 equiv) indicative of the formation of the non-threaded assembly, nthr-L2Ni2•G1•PF6; (v) isolated rotaxane-type threaded assembly, rot-L2Ni2•G1•PF6; (vi) nthr-L2Ni2•G1•PF6 in the presence of CsOTf (2 equiv); (vii) rot-L2Ni2•G1•PF6 in the presence of CsOTf (2 equiv); (viii) uncomplexed G1•PF6. b Formation of nthr-L2Ni2•G1•PF6 from L2Ni2 and G1•PF6 (metal-first method). The 1H NMR signal assignments of nthr- and rot-L2Ni2•G1•PF6 are also shown.
In the 1H NMR titration experiment, the wheel–axle assembly was observed at the chemical shifts that are averaged with the wheel or axle component; for example, only one set of signals for the L2Ni2 macrocycle was observed when 0.5 equiv of G1•PF6 was present. This suggests the fast exchange kinetics for the formation/dissociation equilibrium of this wheel–axle assembly on the 1H NMR time scale. This observation is in contrast to the behavior observed for the interaction of the fully organic DB24C8 wheel with the same axle G1•PF6, which showed signals for the pseudorotaxane DB24C8•G1•PF6 separate from each of the axle and wheel components (Supplementary Fig. S2). This independently observed set of signals including the downfield shifted axle benzyl proton (4.56 ppm) is an indication of the rotaxane-type threaded structure59,60,61,62,63. The spectral changes for the L2Ni2–G1•PF6 complexation without emergence of the separate set of signals are rather similar to those observed for the DB24C8–G2•PF6 titration experiments showing only slight changes in the chemical shifts (Supplementary Fig. S3); in this case, the 3,5-dimethylbenzyl groups in the axle G2•PF6 are too large to be threaded/dethreaded into the DB24C8 macrocycle70,71,72,73.
Based on these experimental results, it is reasonable to conclude that the L2Ni2 wheel and the G1•PF6 axle form a loosely-bound non-threaded wheel–axle assembly77,78,79,80,81 (Fig. 1a (i)), i.e., nthr-L2Ni2•G1•PF6 (Fig. 4b (i)), in which the association/dissociation processes are fast on the 1H NMR time scale, rather than the rotaxane-type threaded assembly, rot-L2Ni2•G1•PF6 (Fig. 4b (ii)). The loose character of the non-threaded wheel–axle assembly, nthr-L2Ni2•G1•PF6, was consistent with the mass spectrometric observation, which mainly showed the dissociated species, whereas the corresponding rotaxane-type threaded assembly only exhibited the bound species under the same conditions (see below). Nevertheless, the wheel–axle interaction for the nthr-L2Ni2•G1•PF6 assembly (logKa = 4.0) was approximately two orders of magnitude stronger than that for the DB24C8•G2•PF6 (logKa = 2.37 Supplementary Fig. S3), which makes the non-threaded assembly nthr-L2Ni2•G1•PF6 more clearly observable than the crown ether analogues showing very small chemical shift changes. The formation efficiency was presumably enhanced by the negatively polarized phenoxo groups of the [Ni(saloph)] moieties in L2Ni2.
Formation of rotaxane-type threaded wheel-axle assembly
As already described, the wheel–axle assembly obtained by mixing the L2Ni2 and G1•PF6 does not have a rotaxane-type threaded structure but a non-threaded structure (Fig. 4b). This means that either the p-substituted benzyl group in the axle G1•PF6 is sufficiently large with respect to the small oval cavity of the L2Ni2 macrocycle to prevent the rotaxane formation or this is just because the non-threaded assembly is more thermodynamically stable than the rotaxane-type threaded assembly. Thus, we attempted the synthesis of the rotaxane according to the axle-first method (Fig. 1a (ii)), i.e., the metalation-driven shrinking strategy54 starting from the non-metalated H4L2 macrocycle and G1•PF6 axle.
In fact, in order to obtain the targeted rotaxane according to the axle-first method, the pseudorotaxane intermediate between H4L2 and G1•PF6 should be efficiently formed before the metalation-driven shrinking. As expected, the pseudorotaxane was almost quantitatively formed simply by mixing the two components (Fig. 5a (i)). The 1H NMR titration experiment exhibited signals of the pseudorotaxane separately from those of its constituents, H4L2 and G1•PF6 (Supplementary Fig. S4), which is indicative of the rotaxane-type threaded structure of H4L2•G1•PF6 with a slow formation/dissociation equilibrium. The axle benzyl proton Ha was significantly downfield shifted (from 4.13 ppm to 4.68 ppm) upon complexation, which could be attributed to the strong C–H‧‧‧O interactions with the oxygen atoms in the H4L2 macrocycle, and the wheel–axle ROE correlations agreed with the rotaxane-type threaded structure (Supplementary Fig. S5). The tightly-bound structure was evidenced by the mass spectrometric measurement (m/z = 1572.00 for [H4L2 + G1]+; Supplementary Fig. S6). It is noteworthy that 1 equiv of G1•PF6 was sufficient for the complete conversion of H4L2 to this pseudorotaxane, indicating the very strong wheel–axle binding. Therefore, this pseudorotaxane, H4L2•G1•PF6, would be a suitable precursor for the rotaxane-type threaded wheel–axle assembly by mechanically interlocking based on the shrinking strategy (axle-first method; Fig. 1a (ii)).
a Formation of rot-L2Ni2•G1•PF6 by metalation of the pseudorotaxane H4L2•G1•PF6 (axle-first method). b ESI-TOF mass spectra of (i) nthr-L2Ni2•G1•PF6, (ii) rot-L2Ni2•G1•PF6, (iii) nthr-L2Ni2•G1•PF6 + CsOTf (2 equiv), and (iv) rot-L2Ni2•G1•PF6 + CsOTf (2 equiv). c–e X-ray crystal structure of rot-L2Ni2•G1•PF6 (only one of the crystallographically independent molecules is shown). c Thermal ellipsoid plot (15% probability level), d space-filling model, and e capped stick model with peripheral groups omitted to show N–H‧‧‧O and C–H‧‧‧O interactions.
We then investigated the introduction of Ni2+ into the coordination sites of the H4L2 macrocycle while keeping the threaded structure of the pseudorotaxane, H4L2•G1•PF6. As expected, this conversion quite efficiently proceeded to give the targeted rotaxane, rot-L2Ni2•G1•PF6, in 75% isolated yield simply by the reaction with nickel(II) acetate (Fig. 5a (ii)). The 1H NMR spectrum of the product no longer showed the phenolic OH signal, confirming the metalation of the H2saloph coordination sites. The axle benzyl proton Ha was observed in the downfield shifted region (5.25 ppm), which is consistent with the threaded rotaxane structure (Fig. 4a (v)). The wheel–axle ROE correlations also support the threaded structure (Supplementary Fig. S7).
It should be noted that the 1H NMR spectrum of this rot-L2Ni2•G1•PF6 was completely different from that of the non-threaded wheel–axle assembly, nthr-L2Ni2•G1•PF6, which was formed simply by mixing the L2Ni2 wheel and the G1•PF6 axle (Fig. 4a (iv, v)). The ESI-TOF mass spectrum was also supportive of the firmly-interlocked structure, showing the [L2Ni2 + G1]+ assembly as an intense peak at m/z = 1684.87 without showing the dissociated species (Fig. 5b (ii)). This is in contrast to the mass spectrum of the non-threaded assembly, nthr-L2Ni2•G1•PF6, which predominantly showed the dissociated species at m/z = 1397.57 [L2Ni2 + Na]+ (Fig. 5b (i)). These mass spectrometric investigations are in good agreement with the rotaxane-type threaded structure of rot-L2Ni2•G1•PF6, which is not in equilibrium with the non-threaded assembly, nthr-L2Ni2•G1•PF6.
The rotaxane-type threaded structure was unambiguously confirmed by X-ray crystallography (Fig. 5c). The crystal contains two crystallographically independent assemblies of rot-L2Ni2•G1•PF6 with a similar conformation, in which the G1+ axle is threaded into the L2Ni2 wheel (Supplementary Fig. S8). The space-filling representation of the molecular structure clearly showed the bulkiness of the p-butylbenzyl stopper groups with respect to the L2Ni2 cavity (Fig. 5d), enabling the isolation of the interlocked rotaxane species. The two interlocked entities, L2Ni2 and G1+, are noncovalently bound through the N–H‧‧‧O, C–H‧‧‧O, C–H‧‧‧π, and π-stacking interactions (Fig. 5e). Unlike the relatively planar conformation of the uncomplexed L2Ni2 wheel in the crystalline state (Fig. 3), the L2Ni2 wheel in the rotaxane structure adopted a V-shaped bent conformation with the dihedral angle of 84.1° or 106.1° between the two [Ni(saloph)] units.
As already described, the non-threaded wheel–axle assembly was formed by the metal-first method (Fig. 1a (i)), i.e., simply by mixing the pre-metalated L2Ni2 macrocycle with the G1•PF6 axle in solution, while the rotaxane-type threaded assembly was formed by the axle-first method (Fig. 1a (ii)), i.e., by the metal-induced shrinking of the pre-formed H4L2•G1•PF6 pseudorotaxane. Thus, two kinds of wheel–axle assemblies, non-threaded and threaded, were selectively formed by changing the order of addition of the metal and axle, even though both the metal and axle are bound via noncovalent interactions.
We found that the phenyl groups in the axle G1•PF6 were essential to obtain the rotaxane-type threaded assembly based on the following experiments using the less bulky axle, G4•PF6, without a phenyl group. When G4•PF6 was mixed with the L2Ni2 macrocycle according to the metal-first method, chemical shift changes were observed in the 1H NMR spectra, suggesting an interaction between the two species with a fast association/dissociation equilibrium (Supplementary Fig. S9). The signal of the NH2–CH2 group did not show the downfield shift indicative of the threaded structure. This suggested that the non-threaded assembly, nthr-L2Ni2•G4•PF6, is dominating in the mixture. The axle-first method was also employed as an alternative method, but only the unreacted L2Ni2 macrocycle was recovered without producing the rotaxane. These results suggested that the threaded assembly, rot-L2Ni2•G4•PF6, readily undergoes dethreading, because it is less stable than the non-threaded assembly, nthr-L2Ni2•G4•PF6, and it has no phenyl groups as the stopper. Therefore, the phenyl groups of the axle G1•PF6 are essential for the formation of the rotaxane-type threaded assembly.
Computational studies of the wheel–axle assemblies
The structures and stabilities of the non-threaded and threaded wheel–axle assemblies were investigated by density functional theory (DFT) calculations for the unsubstituted analogue L1Ni2•G3+ for simplicity (Fig. 6; Supplementary Figs. S10 and S11). For the two types of assemblies, several different conformations were optimized in order to find the global-minimum structures, and the interaction energies were evaluated by the supermolecule method taking into account the counterpoise basis set superposition error (BSSE) correction.
a nthr-a-L1Ni2•G3+, b rot-a-L1Ni2•G3+, and c rot-b-L1Ni2•G3+ (DFT, wB97XD, 6-31 G(d,p)).
The most stable structure of the non-threaded assembly, nthr-a-L1Ni2•G3+, contains the G3+ axle with a folded conformation, which was located inside the cleft of the V-shaped L1Ni2 wheel (Fig. 6a). The four protons of the NH2–CH2 moiety in the G3+ axle effectively interacted with the L1Ni2 oxygen atoms through the N–H‧‧O and C–H‧‧‧O hydrogen bonds. The L1Ni2 macrocycle adopted a twisted V-shaped conformation, and six out of their eight oxygen atoms participated in the hydrogen bonds. In addition, one of the benzene rings of the G3+ axle was stacked on top of the square planar nickel(II) at a distance of 3.2–3.3 Å. These noncovalent interactions probably contributed to the strong association between the L1Ni2 and G3+ components even though they did not form a threaded structure.
For the rotaxane-type threaded assemblies, we obtained two different stable structures within 2 kJ mol–1, rot-a- and rot-b-L1Ni2•G3+, which have the G3+ axle in different orientations (Fig. 6b, c). The rot-a-L1Ni2•G3+ is the global-minimum structure, and the second stable rot-b-L1Ni2•G3+ has a conformation similar to the X-ray crystal structure of rot-L2Ni2•G1•PF6. Both structures have the L1Ni2 macrocycle with a V-shaped conformation, in which the four protons of the NH2–CH2 moiety were hydrogen bonded to the L1Ni2 oxygen atoms. Unlike the non-threaded structure, all the eight oxygen atoms in the L1Ni2 macrocycle participated in the hydrogen bonds to the G3+ axle with the H‧‧‧O distances in the range of 1.8–2.5 Å.
These threaded assemblies, rot-a- and rot-b-L1Ni2•G3+, are approximately 10 kJ mol–1 less stable than the non-threaded assembly, nthr-a-L1Ni2•G3+ (Table 1). The formation energies Eform for rot-a- and rot-b-L1Ni2•G3+ are also 10 kJ mol–1 smaller than that for nthr-a-L1Ni2•G3+, because these two types of assemblies consist of the same wheel and axle constituents. In contrast, the interaction energy Eint between the two constituents, which was estimated by the supermolecule method, showed a different trend. The wheel–axle interactions of nthr-a-L1Ni2•G3+ and rot-a-L1Ni2•G3+ are similar, but that of rot-b-L1Ni2•G3+ is significantly stronger by more than 20 kJ mol–1.
The stability difference between the non-threaded and threaded assemblies of L1Ni2•G3+ can be reasonably explained by the strain energies of each constituent, Edef(W) and Edef(A) (Table 1). In particular, the strain energies for the wheel component of rot-a- and rot-b-L1Ni2•G3+ are more than 80 kJ mol–1, which are 22–25 kJ mol–1 higher than that of nthr-a-L1Ni2•G3+. Thus, the intercomponent interaction between the L1Ni2 wheel and the G3+ axle can be maximized in the threaded structure, but this is energetically less favorable mainly because its elliptical cavity needs to deform to accommodate the G3+ axle, leading to the increased strain energies.
As suggested by the computational investigations, the experimentally obtained non-threaded assembly, nthr-L2Ni2•G1•PF6, should be energetically more favorable than the threaded assembly, rot-L2Ni2•G1•PF6. Before the metalation of the macrocycle H4L2, however, the threaded assembly, pseudorotaxane H4L2•G1•PF6, was spontaneously formed simply by mixing the H4L2 and G1•PF6 (Fig. 5a). Obviously, the introduction of nickel(II) ions destabilizes the threaded structure, resulting in the reversed relative stability of the non-threaded and threaded assemblies. At the same time, this metalation suppresses the interconversion between the non-threaded and threaded assemblies by making the macrocycle smaller so that it does not allow the phenyl groups to pass through. This is consistent with the observation that the rotaxane-type threaded assembly was kinetically trapped only when phenyl substituted G1•PF6 was used as the axle component while the dialkylammonium axle G4•PF6 did not give the rotaxane structure.
Dissociation of the non-threaded assembly by Cs+
We have previously reported that the unsubstituted L1Ni2 macrocycle showed a strong binding affinity to Cs+ even in a polar solvent that could diminish the electrostatic interactions (logKa = 4.94 in DMSO-d6)58. This interaction with Cs+ could be sufficiently strong to replace the G1+ axle to demonstrate the labile nature of the non-threaded nthr-L2Ni2•G1•PF6 assembly, but not to replace if the G1+ axle was mechanically interlocked in the rotaxane-type threaded assembly, rot-L2Ni2•G1•PF6.
When CsOTf was added to the non-threaded assembly, nthr-L2Ni2•G1•PF6, the 1H NMR spectra showed significant changes in the chemical shifts, which are indicative of the interaction with Cs+(Fig. 4a (vi)). The ESI-TOF mass spectrum of the mixture showed the peak at m/z = 1507.53 assignable to [L2Ni2 + Cs]+ without showing intense peaks for the wheel–axle assembly (Fig. 5b (iii)). Therefore, Cs+ almost completely replaced the G1+ axle of the non-threaded wheel–axle assembly, nthr-L2Ni2•G1•PF6 (Fig. 7 (i)). In contrast, the addition of CsOTf to the rotaxane-type threaded wheel–axle assembly, rot-L2Ni2•G1•PF6, caused no change in the 1H NMR and ESI-TOF mass spectra (Figs. 4a (vii) and 5b (iv)). These results indicated that the G1+ axle in the rotaxane was firmly interlocked with the L2Ni2 macrocycle even in the presence of the Cs+ that could more strongly interact with the L2Ni2 macrocycle (Fig. 7 (ii)). This confirms the kinetic stability of the rotaxane-type threaded assembly, rot-L2Ni2•G1•PF6, which is maintained only by coordination bonds used for the shrinking step in the axle-first method.
(i) The non-threaded assembly, nthr-L2Ni2•G1•PF6, readily dissociates into its components to give L2Ni2•Cs+; (ii) The rotaxane-type threaded assembly, rot-L2Ni2•G1•PF6, remains intact in the presence of Cs+.
Conclusions
In conclusion, we have selectively obtained two kinds of wheel–axle assemblies, the non-threaded assembly (nthr-L2Ni2•G1•PF6) and the rotaxane-type threaded assembly (rot-L2Ni2•G1•PF6), simply by changing the order of the wheel–axle complexation and the metal-induced shrinking of the H4L2 macrocycle. The complexation of the pre-formed L2Ni2 wheel with the G1•PF6 axle (metal-first method) resulted in the formation of a non-threaded assembly (nthr-L2Ni2•G1•PF6), which can easily dissociate to give L2Ni2•Cs+ by the addition of Cs+. The formation efficiency of the non-threaded assembly was presumably enhanced by the negatively polarized phenoxo groups of the [Ni(saloph)] moieties in L2Ni2. On the other hand, the rotaxane-type threaded assembly (rot-L2Ni2•G1•PF6) was efficiently obtained by the metal-induced shrinking of the pre-formed pseudorotaxane, H4L2•G1•PF6 (axle-first method). This threaded assembly did not dissociate into its constituents even in the presence of Cs+, confirming the firmly interlocked nature of the rotaxane-type threaded assembly, rot-L2Ni2•G1•PF6.
In general, coordination bonds are categorized as noncovalent interactions that provide reversibility in the supramolecular self-assembling processes82,83. In fact, several metal-containing rotaxanes whose interlocked structure is maintained only by coordination bonds32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53 showed a sufficient dynamic feature that affords the thermodynamically most stable structure due to the equilibration. In this study, thanks to this dynamic feature, we can easily change the order of the wheel–axle complexation and the metal-induced shrinking to obtain two different kinds of wheel–axle assemblies. Nevertheless, both of these isomeric assemblies can be obtained due to the relatively inert nature of the Ni–N and Ni–O coordination bonds used for the shrinking step. It is well known that the dynamic nature of coordination bonds can be easily tuned by changing the external and environmental factors, e.g., solvents, additives, and redox reactions, which could facilitate switching between the two types of wheel–axle assemblies and enabling/disabling the interconversions between them. Further studies of the stimuli-responsive structural conversions of dynamic metallorotaxanes are currently underway.
Methods
Materials and methods
The reagents and solvents were purchased from Fujifilm Wako Pure Chemical or TCI and used without further purification. The 1H NMR spectra were recorded on a JEOL JNM-ECS 400 (400 MHz), a Bruker Avance Neo 400 (400 MHz), or a Bruker Avance Neo 600 (600 MHz). The 13C NMR spectra were recorded on a JEOL JNM-ECS 400 (100 MHz) or a Bruker Avance Neo 600 (150 MHz). Chemical shifts were referenced with respect to tetramethylsilane (0 ppm for 1H and 13C) as an internal standard or the solvent residual peaks. The ESI-TOF mass spectra were recorded on a Bruker Daltonics micrOTOF II. The synthetic precursors, bis(salicylaldehyde) 158, 4,5-dioctyloxy-1,2-phenylenediamine (2)74, and ammonium salt G2•PF676 were prepared according to the literature. The 1H and 13C NMR spectra of new compounds were shown in Supplementary Information (Supplementary Figs. S12–S21).
Synthesis of 3
A solution containing 4-butylbenzylamine84 (483 mg, 2.96 mmol) and 4-butylbenzaldehyde (486 mg, 2.98 mmol) in dehydrated toluene (6 mL) was heated to reflux for 20 h. After cooling to room temperature, the mixture was concentrated to dryness to give the imine 375 (875 mg, 2.85 mmol, 96%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ 8.34 (s, 1H), 7.68 (d, J = 8.0 Hz, 2H), 7.22 (d, J = 8.0 Hz, 2H), 7.21 (d, J = 8.0 Hz, 2H), 7.14 (d, J = 8.0 Hz, 2H), 4.77 (s, 2H), 2.63 (t, J = 7.7 Hz, 2H), 2.57 (t, J = 7.7 Hz, 2H), 1.60 (quint, J = 7.7 Hz, 2H), 1.58 (quint, J = 7.6 Hz, 2H), 1.35 (sext, J = 7.4 Hz, 2H), 1.34 (sext, J = 7.4 Hz, 2H), 0.92 (t, J = 7.3 Hz, 3H), 0.91 (t, J = 7.3 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 161.76, 145.97, 141.54, 136.51, 133.75, 128.64, 128.48, 128.23, 127.87, 64.82, 35.61, 35.31, 33.70, 33.42, 22.34, 22.32, 13.95, 13.92 (Supplementary Data 1,2).
Synthesis of 4
Under a nitrogen atmosphere, a solution of the imine 3 (1.52 g, 4.93 mmol) in THF (15 mL) was added dropwise to a suspension of lithium aluminum hydride (1.38 g, 36.4 mmol) in THF (20 mL) at 0 °C and the mixture was heated to reflux for 20 h. After cooling to room temperature, the reaction mixture was quenched with an aqueous sodium sulfate solution. The resultant solid was filtered off and the filtrate was extracted with diethyl ether. The organic layer was washed with water, then brine, dried over anhydrous magnesium sulfate, filtered, and concentrated to dryness to obtain 4 (870 mg, 2.81 mmol, 57%) as a colorless oil. 1H NMR (400 MHz, CDCl3): δ 7.23 (d, J = 8.0 Hz, 4H), 7.13 (d, J = 8.0 Hz, 4H), 3.77 (s, 4H), 2.59 (t, J = 7.7 Hz, 4H), 1.6 (br), 1.59 (quint, J = 7.6 Hz, 4H), 1.35 (sext, J = 7.4 Hz, 4H), 0.92 (t, J = 7.3 Hz, 6H); 13C NMR (100 MHz, CDCl3): δ 141.52, 137.46, 128.39, 128.04, 52.93, 35.31, 33.71, 22.37, 13.96 (Supplementary Data 3,4).
Synthesis of G1•PF6
Hydrogen chloride gas, which was generated by N2 bubbling into concentrated hydrochloric acid, was bubbled into a solution of the amine 4 (349 mg, 1.13 mmol) in diethyl ether (12 mL) for 5 min. The white precipitates of G1•Cl were collected by filtration. This G1•Cl was dissolved in methanol (8 mL), then an aqueous solution of ammonium hexafluorophosphate (9.13 g, 56.0 mmol in 35 mL) was added. The resultant white precipitates were collected by filtration, thoroughly washed with water, and dried in vacuo to obtain G1•PF6 (406 mg, 0.89 mmol, 79%) as colorless crystals. 1H NMR (400 MHz, CDCl3) δ 10.09 (br, 2H), 7.39 (d, J = 8.0 Hz, 4H), 7.16 (d, J = 8.0 Hz, 4H), 3.79 (s, 4H), 2.52 (t, J = 7.8 Hz, 4H), 1.50 (quint, J = 7.6 Hz, 4H), 1.30 (sext, J = 7.4 Hz, 4H), 0.88 (t, J = 7.3 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ 145.34, 129.70, 129.62, 126.11, 50.64, 35.32, 33.27, 22.32, 13.89; ESI-MS (m/z): [G1]+ calcd. for C22H32N, 310.2535; found, 310.2552 (Supplementary Data 5,6).
Synthesis of H4L2
Under a nitrogen atmosphere, a solution of 4,5-dioctyloxy-1,2-phenylenediamine (2) (21 mM in dehydrated chloroform, 10 mL, 0.21 mmol) was added to a solution of the bis(salicylaldehyde) 1 (60.0 mg, 0.198 mmol) in dry DMSO (10 mL). The reaction mixture was left to stand at room temperature for 4 d. The reaction mixture was then filtered and concentrated to a minimal volume. The residue was poured into water and extracted with chloroform (50 mL × 3). The combined organic extract was washed with water (50 mL × 2), dried over anhydrous sodium sulfate, filtered, and concentrated to dryness. The crude product was purified by reprecipitation from chloroform/hexane followed by centrifugation (three times) of the methanol suspension to yield H4L2 (76.6 mg, 60.7 μmol, 61%) as yellow crystals. 1H NMR (600 MHz, CDCl3): δ 13.42 (s, 4H), 8.60 (s, 4H), 7.20 (dd, J = 7.8, 1.1 Hz, 4H), 7.07 (dd, J = 7.8, 1.1 Hz, 4H), 6.83 (t, J = 7.8 Hz, 4H), 6.83 (s, 4H), 4.43 (s, 8H), 4.07 (t, J = 6.6 Hz, 8H), 1.86 (quint, J = 7.3 Hz, 8H), 1.51 (quint, J = 7.3 Hz, 8H), 1.40–1.25 (m, 32H), 0.89 (t, J = 6.9 Hz, 12H); 13C NMR (150 MHz, CDCl3): δ 162.08, 153.05, 149.04, 147.45, 135.29, 125.78, 121.91, 120.05, 118.57, 105.08, 70.02, 69.84, 31.84, 29.39, 29.29 (×2), 26.05, 22.68, 14.11; HRMS (ESI-TOF) (m/z): [H4L2 + H]+ calcd. for C76H100N4O12H, 1261.7415; found 1261.7481 (Supplementary Data 7,8).
Synthesis of L2Ni2
A solution of nickel(II) acetate tetrahydrate (13.7 mM in methanol, 4.2 mL, 58 μmol) was added to a solution of H4L2 (36.1 mg, 28.6 μmol) in chloroform (9.8 mL). The reaction mixture was left to stand for 6 h at room temperature and concentrated to dryness. The residue was washed several times with methanol and the product was collected by centrifugation to yield L2Ni2 (27.9 mg, 20.3 μmol, 71%) as dark red crystals. 1H NMR (400 MHz, CDCl3): δ 7.97 (s, 4H), 7.06 (s, 4H), 7.01 (d, J = 7.7 Hz, 4H), 6.92 (d, J = 7.7 Hz, 4H), 6.52 (t, J = 7.7 Hz, 4H), 4.56 (s, 8H), 4.01 (t, J = 6.4 Hz, 8H), 1.83 (quint, J = 7.0 Hz, 8H), 1.48 (quint, J = 7.0 Hz, 8H), 1.39–1.27 (m, 32H), 0.89 (t, J = 6.5 Hz, 12H); HRMS (ESI-TOF) (m/z): [L2Ni2 + H]+ calcd. for C76H96N4O12Ni2H, 1375.5801; found, 1375.5879 (Supplementary Data 9).
Synthesis of rot-L2Ni2•G1•PF6
A solution of nickel(II) acetate tetrahydrate (27 mM in methanol, 0.75 mL, 20 μmol) was added to a solution containing H4L2 (12.6 mg, 10 μmol) and G1•PF6 (46.9 mg, 103 μmol) in chloroform (1.8 mL). The mixture was stirred at room temperature for 3 h and was concentrated dryness. The residue was suspended in methanol, collected on a filter, and thoroughly washed with methanol to obtain rot-L2Ni2•G1•PF6 (13.6 mg, 7.4 μmol, 75%) as dark red crystals. 1H NMR (600 MHz, CDCl3/CD3CN (4:1)): δ 8.65 (br, 2H), 8.11 (s, 4H), 7.43 (d, J = 8.1 Hz, 4H), 7.14 (s, 4H), 7.11 (dd, J = 7.9, 1.4 Hz, 4H), 6.96 (d, J = 8.1 Hz, 4H), 6.82 (dd, J = 7.9, 1.4 Hz, 4H), 6.64 (t, J = 7.9 Hz, 4H), 5.26–5.24 (m, 4H), 4.07 (t, J = 6.5 Hz, 8H), 3.91 (s, 8H), 2.33 (t, J = 7.8 Hz, 4H), 1.85 (quint, J = 7.0 Hz, 8H), 1.50 (quint, J = 7.5 Hz, 8H), 1.42–1.25 (m, 36H), 1.21 (sext, J = 7.4 Hz, 4H), 0.89 (t, J = 7.0 Hz, 12H), 0.83 (t, J = 7.3 Hz, 6H); ESI-MS (m/z): [L2Ni2•G1]+ calcd. for C98H128N5O12Ni2, 1684.8270; found, 1684.8407 (Supplementary Data 10).
X-ray crystallography
The intensity data were collected on a Bruker SMART APEX II or a Bruker Venture diffractometer with Cu Kα radiation (λ = 1.54178 Å). The diffraction data were corrected for the Lorentz and polarization factors, and for absorption using the multi-scan methods. The structure was solved by direct methods (SHELXT85) and refined by full-matrix least squares on F2 using SHELXL 201486. The non-hydrogen atoms were refined anisotropically and hydrogen atoms were idealized using the riding models. The crystallographic data are summarized in Supplementary Tables S1 and S2. CCDC-2335486 (L2Ni2•2CH3CN) (Supplementary Data 11) and −2335487 (rot-L2Ni2•G1•PF6•5MeCN•0.5H2O) (Supplementary Data 12) contain the supplementary crystallographic data for this paper.
Computational study
The DFT calculations were performed using the Gaussian09 packages87 with the wB97XD functional and the 6-31 G(d,p) basis sets for C, H, N, O, and Ni. The geometry optimizations of nthr- and rot-L1Ni2•G3+ starting from different conformations gave seven and five different structures, respectively, four of which are shown in Supplementary Figs. S10 and S11 (Supplementary Data 13–22). The relative stabilities are obtained after correction of the zero-point energies. The interaction energies were calculated by the supermolecule methods and the basis set superposition error (BSSE) was corrected using the counterpoise method. The energies for nthr- and rot-L1Ni2•G3+ are summarized in the Supplementary Table S3.
Data availability
All data analyzed and generated throughout this study are included in this article and its accompanying Supplementary Information. The X-ray crystallographic data for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC 2335486 and 2335487. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
The research was partly supported by the Japan Society of the Promotion Science (KAKENHI Grants JP21H05477 and JP23H04021 [Condensed Conjugation] as well as JP16H06510 [Coordination Asymmetry], JP18H03913, JP20K21206, and JP23H01972) and the World Premier International Research Initiative, Ministry of Education, Culture, Sports, Science and Technology, Japan. This paper is dedicated to professor Takayuki Kawashima on the occasion of his 77th birthday.
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S.A. conceived and designed the research. S.K., M.Y., K.D., M.K., and S.Y. carried out the experimental work, analysis, and interpretation of the results. Y.S. and S.A. carried out the crystallographic analysis. Y.S. and S.A. supervised the project and supported the analysis and interpretation of the results. All the authors discussed the results and edited the manuscript.
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Sakata, Y., Kobayashi, S., Yamamoto, M. et al. Non-threaded and rotaxane-type threaded wheel–axle assemblies consisting of dinickel(II) metallomacrocycle and dibenzylammonium axle. Commun Chem 7, 166 (2024). https://doi.org/10.1038/s42004-024-01246-8
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DOI: https://doi.org/10.1038/s42004-024-01246-8
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