ARTICLES PUBLISHED ONLINE: 4 MAY 2015 | DOI: 10.1038/NCHEM.2258
A molecular shuttle that operates inside a metal–organic framework Kelong Zhu, Christopher A. O’Keefe, V. Nicholas Vukotic, Robert W. Schurko* and Stephen J. Loeb* A ‘molecular shuttle’ is an interlocked molecular assembly in which a macrocyclic ring is able to move back and forth between two recognition sites. This large-amplitude translational motion was first characterized in solution in 1991. Since that report, many mechanically interlocked molecules (MIMs) have been designed, synthesized and shown to mimic the complex functions of macroscopic switches and machines. Here, we show that this fundamental concept—the translational motion of a molecular shuttle—can be organized, initiated and made to operate inside a crystalline, solid-state material. A metal–organic framework (MOF) designated UWDM-4 was prepared that contains a rigid linker that is a molecular shuttle. It was demonstrated by variable-temperature 1H-13C cross-polarization/magic-angle spinning (CP/MAS) and 13 C 2D exchange correlation spectroscopy (EXSY) solid-state NMR at 21.1 T on a 13C-enriched sample that the macrocyclic ring undergoes rapid shuttling along the rigid axle built between struts of the framework.
T
he first example of a ‘molecular shuttle’—an interlocked molecular assembly in which a macrocyclic ring was able to move back and forth between two recognition sites on a linear track terminated by bulky stoppers—was reported in 1991 (Fig. 1)1. This pioneering work demonstrated that mechanically interlocked molecules (MIMs) could be designed to mimic the motion of macroscopic switches and machines, and can be considered the prototype for the construction of a variety of more sophisticated molecular assemblies2–8 such as non-degenerate molecular shuttles9–13 and other molecular switches14–19, including recent examples that feature molecular machines designed to operate away from equilibrium and accomplish ‘work’ at the molecular level17–19. Ultimately, it is envisioned that these types of molecular systems will lead to the transmission of information in a highly controlled manner at the molecular level20,21, making Feynman’s dream of a ‘bottom-up approach’ to nanotechnology a real-world concept22. One of the major challenges in the future development of this chemistry is that of organization—how to arrange ‘smart’ molecules in patterns or in condensed phases—as almost all examples to date have been characterized in solution where the molecular motion is random and incoherent23. One type of solid-state material that has received an enormous amount of attention in recent years comprises the metal–organic frameworks (MOFs)24,25, because the versatile node and linker design of the crystalline lattice is compatible with the incorporation of a myriad of functional groups26,27. To address the issue of organizing MIMs in the solid state, a number of MOFs have been studied that include interlocked components as linkers28–35. In particular, we recently reported the successful ordering of a rudimentary [2]rotaxane—a MIM with one ring and a single recognition site—in the solid state by utilizing the MIM as a linker in a MOF material36,37. It was demonstrated, using variable-temperature (VT) 2H solid-state NMR (SSNMR), that rotational motion of the soft and flexible macrocyclic component could be driven by a thermal input without any perturbation of the rigid MOF skeleton or degradation of the crystalline material36,37. In this Article, the same design approach36 is applied to a much more sophisticated problem—making the concept of a molecular
shuttle and its large-amplitude translational motion function inside the crystalline lattice of a MOF material. To this end, we report herein the preparation of a MOF designated UWDM-4 (University of Windsor Dynamic Material) containing a [2]rotaxane molecular shuttle as a linker, and demonstrate that the interlocked macrocyclic wheel can undergo repetitive, translational motion along a rigid track built between two struts of the MOF framework. The structural design components used to create such a material are illustrated in Fig. 2. The key design element is a crossbar, which provides clear separation of the carboxylate struts that make up the skeletal framework of the MOF from the axle and wheel of the [2]rotaxane molecular shuttle, which is the source of the translational dynamics.
Results and discussion A molecular shuttle linker, 5, was designed that incorporates two benzimidazole recognition sites and a single [24]crown-8 ether (24C8) macrocycle as a crossbar unit between two triphenyldicarboxylic acid struts33,38,39. The major steps in the synthesis of 5 are outlined in Fig. 3a. In a one-pot reaction, the preformed [2]pseudorotaxane [2-H⊂24C8]+ containing a terminal aldehyde functional group was condensed with 1 equiv. of diamine 1 and then oxidized in the presence of a catalytic amount of ZrCl4 to produce, after treatment with Et3N, the neutral tetraethyl ester [2]rotaxane, 340,41. This was followed by hydrolysis of 3 to give the MIM linker 5 in excellent yield (see Supplementary Section I for details). The carbon atoms at the 2-positions of the benzimidazole rings of 5 (shown with asterisks in Fig. 3a) were enriched to 50% 13C by using 13C-labelled 1,4-benzenedialdehdye prepared as outlined in Fig. 3b. This isotopic enrichment was used to aid in identifying the shuttling motion in the MOF by 13 C SSNMR (vide infra). A representation of the single-crystal X-ray structure of the neutral tetramethyl ester MIM 4 is shown in Fig. 3c. The axle and wheel components are clearly interpenetrated and the 24C8 macrocycle interacts with only one of the benzimidazole groups by way of a single hydrogen bond between a benzimidazole N–H group and an oxygen atom of the crown ether (d(N-H···O) = 3.09 Å, ∠(NHO) = 167°). MIM 4 is an excellent model for the structure of
Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4, Canada. * e-mail: [email protected]; [email protected] NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry
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Figure 1 | Stoddart’s original molecular shuttle operating in acetone solution. The electron-poor cyclophane macrocycle (blue) moves to and fro (shuttles) between the two electron-rich hydroquinol rings (red), but is prevented from sliding off the polyether chain because of the large size of the triisopropylsilyl groups (green) that act as stoppers. The energy barrier to this translational motion was determined by VT 1H NMR spectroscopy in acetone-d6 solution.
molecular shuttle 5 inside a MOF, as it contains all the essential core components: two benzimidazole groups separated by a phenyl spacer (N···N = 7.1 Å, distance between centroids of the benzimidazole phenyl rings = 12.1 Å) and a single 24C8 wheel that can occupy either of the two benzimidazole recognition sites. As a first step towards understanding the dynamics of this MIM design, a 13C enriched sample of the tetraethyl ester MIM, 3*, was subjected to a VT 13C NMR study in a toluene-d8 solution; the 13 C labelled site (asterisk in Fig. 3a) was monitored as a function of temperature (Supplementary Fig. 12). Only a single resonance was observed for both the ‘occupied’ and ‘empty’ sites at room temperature, indicating that the 24C8 macrocycle undergoes rapid molecular shuttling between the two benzimidazole recognition sites at a rate that is fast on the NMR timescale. At lower temperature, as the rate of the translational motion slows, two separate resonances for the occupied and empty recognition sites become discernible. These data allow for an estimation of the rate and energy barrier to molecular shuttling for 3*: ΔG ‡ = 7.7 kcal mol–1 and shuttling rate = 1.4 × 107 s−1 at 298 K in toluene-d8. Free volume
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Figure 2 | MOF materials are commonly constructed from a combination of rigid linking struts (green) and metal nodes (brown). In this design, the components of a MIM—the axle (blue) and wheel (red) of a molecular shuttle—are inserted as a crossbar between the struts. This provides the single wheel component of the MIM with the free volume necessary to undergo unencumbered translational motion between two recognition sites while inside the pores of the MOF. 2
To address whether or not the large-amplitude translational motion of a macrocyclic ring in a molecular shuttle could be transferred into a solid-state material, we prepared a Zn(II) based MOF utilizing tetracarboxylic acid MIM 5 as the linker. Zinc(II) tetrafluoroborate hydrate was combined with 5 in a 1:1:2 mixture of N,N-dimethylformamide/dimethylsulfoxide/ethanol with a drop of HBF4 and heated in a temperature-controlled oven at 85 °C for 48 h followed by slow cooling to room temperature at 0.1 °C min−1. Crops of pale yellow crystalline material designated UWDM-4·HBF4 could be repeatedly produced in this manner with isolated yields of >70%. Single-crystal X-ray analysis determined the formula of the as-synthesized material to be [(Zn4O)2(5-4H)3(HBF4)3]·16EtOH. Figure 4a presents a ball-and-stick representation of the repeating unit in which each of the four carboxylate groups is coordinated to a Zn4O cluster25. The triphenyl struts form the sides of a cube, and the benzimidazole containing crossbars connect the cubes at the benzimidazole phenyl rings as illustrated in Fig. 4b. This pseudo-interpenetrated cubic arrangement is reminiscent of IRMOF-1538, which utilizes simpler triphenyl-dicarboxylate linkers to bridge Zn4O clusters to produce a two-fold interpenetrated, cubic lattice in which the two frameworks are not connected covalently. Figure 4c,d shows how these repeating units assemble, and illustrates the large channels along the c-axis of the crystal, which contain molecules of solvent (not shown). Calculations using PLATON42 indicate the material contains 47% void space for inclusion of solvent. Although linker 5 was used as a neutral species in the MOF synthesis, we suspected that the as-synthesized MOF material contained the linker in the mono-protonated state—hence the designation of this material as UWDM-4·HBF4. The overall charge on the linker is an extremely important property, as the interaction of the macrocycle with the charged (+1) benzimidazolium group is orders of magnitude stronger than with the neutral, benzimidazole unit40,43. The practical consequence of having one neutral and one charged recognition site per axle is that the macrocycle would exclusively favour occupying the charged site—that is, there could be no molecular shuttling as envisaged. We therefore developed a nondestructive method to render the as-synthesized material neutral, thus allowing operation of molecular shuttling inside the lattice. Accordingly, as-synthesized UWDM-4·HBF4 was treated with the strong base N,N,N′,N′-tetramethylnaphthalene-1,8-diamine (proton-sponge) in ethanol. The degree of neutralization (loss of HBF4) was determined by monitoring the amount of BF4−
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Figure 3 | Synthesis and X-ray structural characterization of a molecular shuttle linker. a, Major steps in the synthesis of a molecular shuttle MOF linker comprising a rigid H-shaped axle with two benzimidazole recognition sites, four carboxylic acid groups for coordination to metal ions and a [24]crown-8 ether wheel (Supplementary Fig. 1; see Supplementary pages 4–13 for full synthetic details and pages 25–38 for spectral details). b, The synthetic route used to label the MOF linker with 13C enrichment to aid in characterization of the shuttling motion by 1H-13C CP/MAS SSNMR. c, A ball-and-stick representation of the single-crystal X-ray of 4, the tetra-methylester version of the tetra-carboxylic acid MIM linker 5. THF, tetrahydrofuran; TFA, trifluoroacetic acid.
anion present in the sample, utilizing both solution and solid-state 19 F NMR spectroscopy (Fig. 5a,b). As-synthesized samples tested positive for the presence of BF4−, but after treatment with base no evidence for BF4− was found. Deprotonation was also indicated by the lack of a B–F stretch in the infrared spectrum of UWDM-4 (Supplementary Fig. 4), and supported by inductively coupled plasma optical emission spectroscopy (ICP-OES) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/ EDX) measurements (Supplementary Figs 5 and 6). Gratifyingly, optical inspection (Fig. 5c,d) and powder X-ray diffraction (PXRD) experiments (Fig. 5e) showed no significant changes to the crystalline material, inferring that the basic internal structure a
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of UWDM-4 could be described by the X-ray structure of UWDM-4·HBF4 (shown in Fig. 4). The PXRD pattern of UWDM-4 (solvent exchanged to m-xylene) showed no changes from 298 to 413 K, verifying the stability of the neutral material over this temperature range (Supplementary Fig. 9). To determine if linker 5 could function as a molecular shuttle inside the solid-state lattice of the MOF, a 13C-enriched sample of UWDM-4 was subjected to VT 13C SSNMR44,45 experiments in a 21.1 T field (see Supplementary Section VII for details). Figure 6a shows that the solid-state NMR spectra are fundamentally the same as those observed in solution for the MIM precursor 3*. That is, at high temperatures, only a single peak (154.0 ppm) is
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Figure 4 | Structure of UWDM-4·HBF4 determined by single-crystal X-ray diffraction. a, Ball-and-stick representation of a single unit of the MIM linker 5 coordinated to four Zn4O clusters (only the NH hydrogen atoms are shown for clarity). b, Two cubes of the lattice formed by the triphenyl struts (green and yellow) with connecting crossbar MIMs (axle in blue and wheel in red); only crossbars actually connecting these two cubes are shown. c, View of the open channels in the lattice down the c axis with macrocycles. d, Same view as in c, with macrocycles omitted. NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry
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Fig. 6c. Accordingly, it was determined that the rate of shuttling inside the MOF lattice is 283 s−1 at 298 K with an activation barrier (ΔG ‡) to this translational motion of 14.1 kcal mol–1. The results of 2D SSNMR EXSY46,47 experiments (Fig. 6b) at different mixing times provides further evidence that the dynamic process of molecular shuttling is occurring inside the MOF. It is interesting to compare the energy barrier and rate of shuttling in the solid state (14.1 kcal mol–1 and 283 s−1) with that in solution (7.7 kcal mol–1 and 1.4 × 107 s−1). The slower rate of shuttling in the solid state means that the ΔG ‡ associated with this translational motion must be higher; this is presumably the result of both enthalpic (ΔH ‡) and entropic (ΔS ‡) contributions (Supplementary Table 3). The higher ΔH ‡ associated with the shuttling motion in the solid state is consistent with the presence of a surrounding framework that would potentially induce steric and/or electrostatic hindrances to the motion. The lower ΔS ‡ in the solid state could arise from several factors that restrict the number of accessible configurations or states. For example, in solution, if the axle/wheel
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Figure 5 | UWDM-4·HBF4 can be efficiently converted to UWDM-4 without loss of crystallinity. a, 19F solution NMR spectra of UWDM-4 (top) and UWDM-4·HBF4 (bottom) after digestion in DMSO-d6/0.5%TFA. b, 19F SSNMR spectra of solid samples of UWDM-4 (top) and UWDM-4·HBF4 (bottom) (shaded bar, BF4− signal; filled diamonds, spinning sidebands; open squares, Teflon peak from spacers and cap). c, Optical micrograph of as-synthesized crystals of UWDM-4·HBF4. d, Optical micrograph of crystals of UWDM-4 prepared by treating UWDM-4·HBF4 with base in ethanol. e, PXRD (bottom to top) simulated from the singlecrystal X-ray structure of UWDM-4·HBF4 , as-synthesized UWDM-4·HBF4 , UWDM-4 prepared by treating UWDM-4·HBF4 with base in ethanol and UWDM-4 after soaking in m-xylenes.
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observed, but as the temperature is lowered this peak broadens, coalesces and then splits into two distinct resonances (152.7 and 155.2 ppm) at the lowest temperature. Although the peaks are much broader in the SSNMR spectra due to the nature of the sample phase (crystalline solid versus solution) the interpretation of the data is the same. The observance of a single peak is due to averaging of the signals from the two recognition sites: one that is open and another that is occupied by a 24C8 macrocycle. These data unequivocally demonstrate that rapid molecular shuttling of the macrocyclic ring between the two recognition sites is occurring inside the MOF. When the sample is cooled, the rate of this translational motion slows and both occupied and unoccupied sites are observed. Fitting of the experimental spectra with simulations allowed determination of the rates of shuttling at various temperatures (Fig. 6a). An Eyring plot of ln(k/T ) versus 1/T is shown in
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Figure 6 | 13C SSNMR studies verify that the designed molecular shuttle can undergo translational motion (molecular shuttling) inside the lattice of the MOF. a, 1H-13C CP/MAS NMR spectra (left, experimental; right, simulation) of a 13C-enriched sample of UWDM-4 (the uncertainties for the simulated rates are estimated to be 10%). b, 13C 2D EXSY SSNMR spectrum of UWDM-4. c, Eyring plot created from the VT NMR data in a (see Supplementary Section VII for complete analysis).
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DOI: 10.1038/NCHEM.2258
configuration is thought of as a small molecule, it undergoes a large number of collisions with non-hydrogen-bonding solvent molecules (for example, toluene), on the order of 1011 to 1012 times per second at room temperature. By contrast, solvent molecules in the MOF are not moving as isotropically or rapidly. The effect of the faster collisions in solution is to induce a wider range of ring conformations and increased relative shuttling motions of the rings and axles than would be experienced in the solid state. Another intriguing possibility for the lower ΔS ‡ in the solid state is that cooperativity is required for the motion of multiple rings to occur; that is, one ring cannot move without its motion requiring the next ring to move.
Conclusions We have prepared UWDM-4, a MOF material with a molecular shuttle as part of its internal structure. VT 13C SSNMR experiments demonstrate that the macrocyclic ring can undergo large-amplitude translational motion along the rigid skeleton of the MOF to which it is interlocked. This establishes, for the first time, that the dynamics of a molecular shuttle—the to and fro motion that is fundamental to many MIM switches and molecular machines—can be successfully made to function in a highly organized and dense material with ∼1021 units per cm3. The ability to arrange mobile and switchable MIM components in a highly dense and predictable array is a crucial step towards the generation of solid-state nanoscale devices based on mechanically interlocked molecules.
Methods Preparation of tetra-carboxylic acid linker 5. In an initial step (not shown in Fig. 3a) 4,7-bis( p-ethoxycarbonylphenyl)-2,1,3-benzothiodiazole was prepared via a Suzuki coupling reaction between 4,7-dibromo-2,1,3-benzothiodiazole and 4-ethoxycarbonylphenylboronic acid, using Cs2CO3 as the base and [Pd(PPh3)4] as the catalyst in a 1:1 DMF/toluene solution (yield 91%). The thiodiazole was then converted to diamine 1 using NaBH4 and CoCl2·6H2O in 1:3 THF/ethanol (yield 85%) and the diamine reacted with excess terephthalaldehyde in the presence of ZrCl4 in CH3CN to produce aldehyde 2 (yield 62%). The T-shaped benzimidazole 2 was subsequently protonated with HBF4 (yield 92%) and mixed with 2 equiv. of 24C8 in chloroform to form the [2]pseudorotaxane [2-H⊂24C8]+ (Ka = 1.9 × 104 M−1; 1H NMR, CDCl3 , 2.0 × 10−3 M). Condensation with a further equivalent of diamine 1 and then oxidization in the presence of a catalytic amount of ZrCl4 produced, after treatment with Et3N, the neutral tetraethyl-ester [2]rotaxane 3 (yields for two steps, 88% and 91%)40,41. Finally, the four ester groups were hydrolysed to carboxylic acids with 1 M NaOH in a 2:1 mixture of ethanol/THF to give the MIM linker 5 (yield 95%). Mp > 250 °C (dec). 1H NMR (500 MHz, DMSO-d6): δ = 13.00 (br s, 4H, COO-H), 12.46 (s, 2H, N-H), 8.63 (s, 4H, Ar-H), 8.40 (d, 4H, Ar-H, 3J = 8.0 Hz), 8.15 (d, 4H, Ar-H, 3J = 8.0 Hz), 8.11 (d, 4H, Ar-H, 3 J = 8.0 Hz), 7.86 (d, 4H, Ar-H, 3J = 8.0 Hz), 7.64 (d, 2H, Ar-H, 3J = 8.0 Hz), 7.40 (d, 2H, Ar-H, 3J = 8.0 Hz), 3.13 (s, 32H, O-CH2CH2-O). 13C NMR (125 MHz, DMSO-d6): δ = 167.9, 167.8, 153.9, 143.0, 142.7, 142.6, 133.8, 131.4, 130.5, 130.2, 129.9, 129.8, 129.6, 129.4, 129.1, 125.5, 123.9, 122.0, 69.4. HR-MS (ESI-TOF): calculated for [M + H]+ [C64H63N4O16]+ m/z = 1,143.4239; found m/z = 1,143.4252. Preparation of UWDM-4·HBF4. Neutral MIM linker 5 and Zn(BF4)2.6H2O were dissolved in a solution of DMF/DMSO (1:1 vol/vol), to which one drop of HBF4 was added. The solution was then injected through a syringe filter into a 20 ml borosilicate scintillation vial and ethanol was added to give a solution of DMF/DMSO/EtOH (1:1:2 vol/vol/vol). The vial was then heated at a constant rate of 1 °C min−1 to 85 °C, kept at that temperature for 48 h and cooled to room temperature at a constant heating rate of 0.1 °C min−1. A pale yellow crystalline product was collected and washed with ethanol several times to yield pure UWDM-4·HBF4 (yield 74% based on 5). Single-crystal X-ray diffraction. Reflection data were collected for UWDM-4·HBF4 on a Bruker APEX diffractometer using MoKα radiation with an APEX chargecoupled device detector and for 4 on a Bruker D8 Venture diffractometer using CuKα radiation with a Photon-100 CMOS detector. Attempts to grow crystals of either the neutral tetra-carboxylic acid 5 or the neutral tetraethyl-ester precursor 3 were not successful. Tetramethyl-ester 4 was prepared specifically to obtain X-ray-quality crystals and those grown from dimethoxyethane (DME) were of good quality. The unit cell contained two molecules of the neutral [2]rotaxane (C68H70N4O16) and two molecules of DME. The structure was solved in the triclinic space group P–1 with Z = 2. All the non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were placed in idealized positions and refined using a riding model. As-synthesized crystals of UWDM-4·HBF4 were of
good quality and size. The crystals were soaked in fresh DMF and then soaked in ethanol before undertaking the single-crystal X-ray diffraction experiment to remove unwanted side products of the solvothermal synthesis from the pores and ensure the homogeneity of the solvent in the pores. Similar to other porous MOFs, the large amounts of highly disordered solvent (ethanol) in the crystals of UWDM-4·HBF4 resulted in poor diffraction, but in addition there are large disordered 24C8 macrocycles that also contribute to the poor diffraction. The structure was solved in the hexagonal space group R32 with Z = 3 for a formula of [(Zn4O)2(5-4H)3(HBF4)3]·16EtOH. The non-hydrogen atoms of the framework were easily located and well behaved, while the non-hydrogen atoms of the macrocycle were much more difficult to locate. As there are two benzimidazole groups but only one 24C8 macrocycle, the crown ether was disordered over the two sites and input at 50% occupancy. Assignment of the O atoms in the ring was made by comparing displacement parameters and hydrogen-bonding distances to the framework. The molecules of ethanol were treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON42. Detailed descriptions of the synthesis and characterization of all compounds as well as all SSNMR and X-ray diffraction data and modelling can be found in the Supplementary Information. Crystal structure data for tetramethyl-ester 4 and UWDM-4·HBF4 can be accessed at the Cambridge Crystallographic Data Centre (CCDC; www.ccdc.cam.ac.uk) and have been allocated accession nos. CCDC 1035586 and 1035587.
Received 4 December 2014; accepted 31 March 2015; published online 4 May 2015
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24. Zhou, H-C., Long, J. R. & Yaghi, O. M. Introduction to metal–organic frameworks. Chem. Rev. 112, 673–674 (2012). 25. Li, H., Eddaoudi, M., O’Keeffe, M. & Yaghi, O. M. Design and synthesis of an exceptionally stable and highly porous metal–organic framework. Nature 402, 276–279 (1999). 26. Furukawa, H., Cordova, K. E., O’Keeffe, M. & Yaghi, O. M. The chemistry and applications of metal–organic frameworks. Science 341, 1230444 (2013). 27. Lu, W. et al. Tuning the structure and function of metal–organic frameworks via linker design. Chem. Soc. Rev. 43, 5561–5593 (2014). 28. Li, Q. et al. Docking in metal–organic frameworks. Science 325, 855–859 (2009). 29. Li, Q. et al. A catenated strut in a catenated metal–organic framework. Angew. Chem. Int. Ed. 49, 6751–6755 (2010). 30. Li, Q. et al. A metal–organic framework replete with ordered donor–acceptor catenanes. Chem. Commun. 380–382 (2010). 31. Coskun, A. et al. Metal–organic frameworks incorporating copper–complexed rotaxanes. Angew. Chem. Int. Ed. 51, 2160–2163 (2012). 32. Cao, D. et al. Three-dimensional architectures incorporating stereoregular donor–acceptor stacks. Chem. Eur. J. 19, 8457–8465 (2013). 33. Strutt, N. L. et al. Incorporation of an A1/A2-difunctionalized pillar[5]arene into a metal–organic framework. J. Am. Chem. Soc. 134, 17436–17439 (2012). 34. Vukotic, V. N. & Loeb, S. J. Coordination polymers containing rotaxanes as linkers. Chem. Soc. Rev. 41, 5896–5906 (2012). 35. Loeb, S. J. Rotaxanes as ligands: from molecules to materials. Chem. Soc. Rev. 36, 226–235 (2007). 36. Vukotic, V. N., Harris, K. J., Zhu, K., Schurko, R. W. & Loeb, S. J. Metal–organic frameworks with dynamic interlocked components. Nature Chem. 4, 456–460 (2012). 37. Zhu, K., Vukotic, V. N., O’Keefe, C. A., Schurko, R. W. & Loeb, S. J. Metal–organic frameworks with mechanically interlocked pillars: controlling ring dynamics in the solid-state via a reversible phase change. J. Am. Chem. Soc. 136, 7403–7409 (2014). 38. Eddaoudi, M. et al. Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage. Science 295, 469–472 (2002). 39. Oisaki, K., Li, Q., Furukawa, H., Czaja, A. U. & Yaghi, O. M. A metal–organic framework with covalently bound organometallic complexes. J. Am. Chem. Soc. 132, 9262–9264 (2010). 40. Zhu, K., Vukotic, V. N. & Loeb, S. J. Molecular shuttling of a compact and rigid, H-shaped [2]rotaxane. Angew. Chem Int. Ed. 51, 2168–2172 (2012). 41. Zhu, K., Vukotic, V. N., Noujeim, N. & Loeb, S. J. Bis(benzimidazolium) axles and crown ether wheels: a versatile templating pair for the formation of [2]rotaxane molecular shuttles. Chem. Sci. 3, 3265–3271 (2012). 42. Vandersluis, P. & Spek, A. L. Bypass—an effective method for the refinement of crystal-structures containing disordered solvent regions. Acta Crystallogr. A46, 194–201 (1990).
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43. Noujeim, N., Zhu, K., Vukotic, V. N. & Loeb, S. J. [2]Pseudorotaxanes from T-shaped benzimidazolium axles and [24]crown-8 wheels. Org. Lett. 14, 2484–2487 (2012). 44. Bain, A. D. Chemical exchange in NMR. Prog. Nucl. Magn. Reson. Spectrosc. 43, 63–103 (2003). 45. Karlen, S. D. et al. Symmetry and dynamics of molecular rotors in amphidynamic molecular crystals. Proc. Natl Acad. Sci. USA 107, 14973–14977 (2010). 46. Jeener, J., Meier, B. H., Bachmann, P. & Ernst, R. R. Investigation of exchange processes by two-dimensional NMR spectroscopy. J. Chem. Phys. 71, 4546–4553 (1979). 47. Perrin, C. L. & Dwyer, T. J. Application of two-dimensional NMR to kinetics of chemical exchange. Chem. Rev. 90, 935–967 (1990).
Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through Discovery Grants and Accelerator Supplements to S.J.L. and R.W.S. and a Canada Research Chair award to S.J.L. R.W.S. and S.J.L. acknowledge support from NSERC, the Canadian Foundation for Innovation, the Ontario Innovation Trust and the University of Windsor, for the development and maintenance of the SSNMR and X-ray diffraction centres. V.N.V. acknowledges financial support provided by NSERC through an Alexander Graham Bell Canada Graduate Doctoral Scholarship and by the International Center for Diffraction Data for a Ludo Frevel Crystallography Scholarship. The authors thank M. Revington for technical assistance with solution NMR spectroscopy experiments, V. Terskikh for collecting the 21.1 T SSNMR data and J. Auld for recording electrospray mass spectrometry data. Access to the 900 MHz NMR spectrometer was provided by the National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada; http://www.uwindsor.ca).
Author contributions S.J.L. supervised the project. K.Z. designed the experiments with help from V.N.V. and C.O. K.Z. performed all the synthetic experiments. K.Z. and V.N.V. collected and analysed the PXRD, TGA and SCXRD data with assistance from S.J.L. C.O. collected and analysed the SSNMR data. R.W.S supervised all SSNMR data collection, analysis and interpretation. S.J.L. and K.Z. wrote the manuscript with input from V.N.V., C.O. and R.W.S.
Additional information Supplementary information and chemical compound information are available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to R.W.S. and S.J.L.
Competing financial interests
The authors declare no competing financial interests.
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A molecular shuttle that operates inside a metal–organic framework Kelong Zhu, Christopher A. O’Keefe, V. Nicholas Vukotic, Robert W. Schurko* and Stephen J. Loeb* A ‘molecular shuttle’ is an interlocked molecular assembly in which a macrocyclic ring is able to move back and forth between two recognition sites. This large-amplitude translational motion was first characterized in solution in 1991. Since that report, many mechanically interlocked molecules (MIMs) have been designed, synthesized and shown to mimic the complex functions of macroscopic switches and machines. Here, we show that this fundamental concept—the translational motion of a molecular shuttle—can be organized, initiated and made to operate inside a crystalline, solid-state material. A metal–organic framework (MOF) designated UWDM-4 was prepared that contains a rigid linker that is a molecular shuttle. It was demonstrated by variable-temperature 1H-13C cross-polarization/magic-angle spinning (CP/MAS) and 13 C 2D exchange correlation spectroscopy (EXSY) solid-state NMR at 21.1 T on a 13C-enriched sample that the macrocyclic ring undergoes rapid shuttling along the rigid axle built between struts of the framework.
T
he first example of a ‘molecular shuttle’—an interlocked molecular assembly in which a macrocyclic ring was able to move back and forth between two recognition sites on a linear track terminated by bulky stoppers—was reported in 1991 (Fig. 1)1. This pioneering work demonstrated that mechanically interlocked molecules (MIMs) could be designed to mimic the motion of macroscopic switches and machines, and can be considered the prototype for the construction of a variety of more sophisticated molecular assemblies2–8 such as non-degenerate molecular shuttles9–13 and other molecular switches14–19, including recent examples that feature molecular machines designed to operate away from equilibrium and accomplish ‘work’ at the molecular level17–19. Ultimately, it is envisioned that these types of molecular systems will lead to the transmission of information in a highly controlled manner at the molecular level20,21, making Feynman’s dream of a ‘bottom-up approach’ to nanotechnology a real-world concept22. One of the major challenges in the future development of this chemistry is that of organization—how to arrange ‘smart’ molecules in patterns or in condensed phases—as almost all examples to date have been characterized in solution where the molecular motion is random and incoherent23. One type of solid-state material that has received an enormous amount of attention in recent years comprises the metal–organic frameworks (MOFs)24,25, because the versatile node and linker design of the crystalline lattice is compatible with the incorporation of a myriad of functional groups26,27. To address the issue of organizing MIMs in the solid state, a number of MOFs have been studied that include interlocked components as linkers28–35. In particular, we recently reported the successful ordering of a rudimentary [2]rotaxane—a MIM with one ring and a single recognition site—in the solid state by utilizing the MIM as a linker in a MOF material36,37. It was demonstrated, using variable-temperature (VT) 2H solid-state NMR (SSNMR), that rotational motion of the soft and flexible macrocyclic component could be driven by a thermal input without any perturbation of the rigid MOF skeleton or degradation of the crystalline material36,37. In this Article, the same design approach36 is applied to a much more sophisticated problem—making the concept of a molecular
shuttle and its large-amplitude translational motion function inside the crystalline lattice of a MOF material. To this end, we report herein the preparation of a MOF designated UWDM-4 (University of Windsor Dynamic Material) containing a [2]rotaxane molecular shuttle as a linker, and demonstrate that the interlocked macrocyclic wheel can undergo repetitive, translational motion along a rigid track built between two struts of the MOF framework. The structural design components used to create such a material are illustrated in Fig. 2. The key design element is a crossbar, which provides clear separation of the carboxylate struts that make up the skeletal framework of the MOF from the axle and wheel of the [2]rotaxane molecular shuttle, which is the source of the translational dynamics.
Results and discussion A molecular shuttle linker, 5, was designed that incorporates two benzimidazole recognition sites and a single [24]crown-8 ether (24C8) macrocycle as a crossbar unit between two triphenyldicarboxylic acid struts33,38,39. The major steps in the synthesis of 5 are outlined in Fig. 3a. In a one-pot reaction, the preformed [2]pseudorotaxane [2-H⊂24C8]+ containing a terminal aldehyde functional group was condensed with 1 equiv. of diamine 1 and then oxidized in the presence of a catalytic amount of ZrCl4 to produce, after treatment with Et3N, the neutral tetraethyl ester [2]rotaxane, 340,41. This was followed by hydrolysis of 3 to give the MIM linker 5 in excellent yield (see Supplementary Section I for details). The carbon atoms at the 2-positions of the benzimidazole rings of 5 (shown with asterisks in Fig. 3a) were enriched to 50% 13C by using 13C-labelled 1,4-benzenedialdehdye prepared as outlined in Fig. 3b. This isotopic enrichment was used to aid in identifying the shuttling motion in the MOF by 13 C SSNMR (vide infra). A representation of the single-crystal X-ray structure of the neutral tetramethyl ester MIM 4 is shown in Fig. 3c. The axle and wheel components are clearly interpenetrated and the 24C8 macrocycle interacts with only one of the benzimidazole groups by way of a single hydrogen bond between a benzimidazole N–H group and an oxygen atom of the crown ether (d(N-H···O) = 3.09 Å, ∠(NHO) = 167°). MIM 4 is an excellent model for the structure of
Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4, Canada. * e-mail: [email protected]; [email protected] NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry
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Figure 1 | Stoddart’s original molecular shuttle operating in acetone solution. The electron-poor cyclophane macrocycle (blue) moves to and fro (shuttles) between the two electron-rich hydroquinol rings (red), but is prevented from sliding off the polyether chain because of the large size of the triisopropylsilyl groups (green) that act as stoppers. The energy barrier to this translational motion was determined by VT 1H NMR spectroscopy in acetone-d6 solution.
molecular shuttle 5 inside a MOF, as it contains all the essential core components: two benzimidazole groups separated by a phenyl spacer (N···N = 7.1 Å, distance between centroids of the benzimidazole phenyl rings = 12.1 Å) and a single 24C8 wheel that can occupy either of the two benzimidazole recognition sites. As a first step towards understanding the dynamics of this MIM design, a 13C enriched sample of the tetraethyl ester MIM, 3*, was subjected to a VT 13C NMR study in a toluene-d8 solution; the 13 C labelled site (asterisk in Fig. 3a) was monitored as a function of temperature (Supplementary Fig. 12). Only a single resonance was observed for both the ‘occupied’ and ‘empty’ sites at room temperature, indicating that the 24C8 macrocycle undergoes rapid molecular shuttling between the two benzimidazole recognition sites at a rate that is fast on the NMR timescale. At lower temperature, as the rate of the translational motion slows, two separate resonances for the occupied and empty recognition sites become discernible. These data allow for an estimation of the rate and energy barrier to molecular shuttling for 3*: ΔG ‡ = 7.7 kcal mol–1 and shuttling rate = 1.4 × 107 s−1 at 298 K in toluene-d8. Free volume
Metal node
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Figure 2 | MOF materials are commonly constructed from a combination of rigid linking struts (green) and metal nodes (brown). In this design, the components of a MIM—the axle (blue) and wheel (red) of a molecular shuttle—are inserted as a crossbar between the struts. This provides the single wheel component of the MIM with the free volume necessary to undergo unencumbered translational motion between two recognition sites while inside the pores of the MOF. 2
To address whether or not the large-amplitude translational motion of a macrocyclic ring in a molecular shuttle could be transferred into a solid-state material, we prepared a Zn(II) based MOF utilizing tetracarboxylic acid MIM 5 as the linker. Zinc(II) tetrafluoroborate hydrate was combined with 5 in a 1:1:2 mixture of N,N-dimethylformamide/dimethylsulfoxide/ethanol with a drop of HBF4 and heated in a temperature-controlled oven at 85 °C for 48 h followed by slow cooling to room temperature at 0.1 °C min−1. Crops of pale yellow crystalline material designated UWDM-4·HBF4 could be repeatedly produced in this manner with isolated yields of >70%. Single-crystal X-ray analysis determined the formula of the as-synthesized material to be [(Zn4O)2(5-4H)3(HBF4)3]·16EtOH. Figure 4a presents a ball-and-stick representation of the repeating unit in which each of the four carboxylate groups is coordinated to a Zn4O cluster25. The triphenyl struts form the sides of a cube, and the benzimidazole containing crossbars connect the cubes at the benzimidazole phenyl rings as illustrated in Fig. 4b. This pseudo-interpenetrated cubic arrangement is reminiscent of IRMOF-1538, which utilizes simpler triphenyl-dicarboxylate linkers to bridge Zn4O clusters to produce a two-fold interpenetrated, cubic lattice in which the two frameworks are not connected covalently. Figure 4c,d shows how these repeating units assemble, and illustrates the large channels along the c-axis of the crystal, which contain molecules of solvent (not shown). Calculations using PLATON42 indicate the material contains 47% void space for inclusion of solvent. Although linker 5 was used as a neutral species in the MOF synthesis, we suspected that the as-synthesized MOF material contained the linker in the mono-protonated state—hence the designation of this material as UWDM-4·HBF4. The overall charge on the linker is an extremely important property, as the interaction of the macrocycle with the charged (+1) benzimidazolium group is orders of magnitude stronger than with the neutral, benzimidazole unit40,43. The practical consequence of having one neutral and one charged recognition site per axle is that the macrocycle would exclusively favour occupying the charged site—that is, there could be no molecular shuttling as envisaged. We therefore developed a nondestructive method to render the as-synthesized material neutral, thus allowing operation of molecular shuttling inside the lattice. Accordingly, as-synthesized UWDM-4·HBF4 was treated with the strong base N,N,N′,N′-tetramethylnaphthalene-1,8-diamine (proton-sponge) in ethanol. The degree of neutralization (loss of HBF4) was determined by monitoring the amount of BF4−
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Figure 3 | Synthesis and X-ray structural characterization of a molecular shuttle linker. a, Major steps in the synthesis of a molecular shuttle MOF linker comprising a rigid H-shaped axle with two benzimidazole recognition sites, four carboxylic acid groups for coordination to metal ions and a [24]crown-8 ether wheel (Supplementary Fig. 1; see Supplementary pages 4–13 for full synthetic details and pages 25–38 for spectral details). b, The synthetic route used to label the MOF linker with 13C enrichment to aid in characterization of the shuttling motion by 1H-13C CP/MAS SSNMR. c, A ball-and-stick representation of the single-crystal X-ray of 4, the tetra-methylester version of the tetra-carboxylic acid MIM linker 5. THF, tetrahydrofuran; TFA, trifluoroacetic acid.
anion present in the sample, utilizing both solution and solid-state 19 F NMR spectroscopy (Fig. 5a,b). As-synthesized samples tested positive for the presence of BF4−, but after treatment with base no evidence for BF4− was found. Deprotonation was also indicated by the lack of a B–F stretch in the infrared spectrum of UWDM-4 (Supplementary Fig. 4), and supported by inductively coupled plasma optical emission spectroscopy (ICP-OES) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/ EDX) measurements (Supplementary Figs 5 and 6). Gratifyingly, optical inspection (Fig. 5c,d) and powder X-ray diffraction (PXRD) experiments (Fig. 5e) showed no significant changes to the crystalline material, inferring that the basic internal structure a
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of UWDM-4 could be described by the X-ray structure of UWDM-4·HBF4 (shown in Fig. 4). The PXRD pattern of UWDM-4 (solvent exchanged to m-xylene) showed no changes from 298 to 413 K, verifying the stability of the neutral material over this temperature range (Supplementary Fig. 9). To determine if linker 5 could function as a molecular shuttle inside the solid-state lattice of the MOF, a 13C-enriched sample of UWDM-4 was subjected to VT 13C SSNMR44,45 experiments in a 21.1 T field (see Supplementary Section VII for details). Figure 6a shows that the solid-state NMR spectra are fundamentally the same as those observed in solution for the MIM precursor 3*. That is, at high temperatures, only a single peak (154.0 ppm) is
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Figure 4 | Structure of UWDM-4·HBF4 determined by single-crystal X-ray diffraction. a, Ball-and-stick representation of a single unit of the MIM linker 5 coordinated to four Zn4O clusters (only the NH hydrogen atoms are shown for clarity). b, Two cubes of the lattice formed by the triphenyl struts (green and yellow) with connecting crossbar MIMs (axle in blue and wheel in red); only crossbars actually connecting these two cubes are shown. c, View of the open channels in the lattice down the c axis with macrocycles. d, Same view as in c, with macrocycles omitted. NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry
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Fig. 6c. Accordingly, it was determined that the rate of shuttling inside the MOF lattice is 283 s−1 at 298 K with an activation barrier (ΔG ‡) to this translational motion of 14.1 kcal mol–1. The results of 2D SSNMR EXSY46,47 experiments (Fig. 6b) at different mixing times provides further evidence that the dynamic process of molecular shuttling is occurring inside the MOF. It is interesting to compare the energy barrier and rate of shuttling in the solid state (14.1 kcal mol–1 and 283 s−1) with that in solution (7.7 kcal mol–1 and 1.4 × 107 s−1). The slower rate of shuttling in the solid state means that the ΔG ‡ associated with this translational motion must be higher; this is presumably the result of both enthalpic (ΔH ‡) and entropic (ΔS ‡) contributions (Supplementary Table 3). The higher ΔH ‡ associated with the shuttling motion in the solid state is consistent with the presence of a surrounding framework that would potentially induce steric and/or electrostatic hindrances to the motion. The lower ΔS ‡ in the solid state could arise from several factors that restrict the number of accessible configurations or states. For example, in solution, if the axle/wheel
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Figure 5 | UWDM-4·HBF4 can be efficiently converted to UWDM-4 without loss of crystallinity. a, 19F solution NMR spectra of UWDM-4 (top) and UWDM-4·HBF4 (bottom) after digestion in DMSO-d6/0.5%TFA. b, 19F SSNMR spectra of solid samples of UWDM-4 (top) and UWDM-4·HBF4 (bottom) (shaded bar, BF4− signal; filled diamonds, spinning sidebands; open squares, Teflon peak from spacers and cap). c, Optical micrograph of as-synthesized crystals of UWDM-4·HBF4. d, Optical micrograph of crystals of UWDM-4 prepared by treating UWDM-4·HBF4 with base in ethanol. e, PXRD (bottom to top) simulated from the singlecrystal X-ray structure of UWDM-4·HBF4 , as-synthesized UWDM-4·HBF4 , UWDM-4 prepared by treating UWDM-4·HBF4 with base in ethanol and UWDM-4 after soaking in m-xylenes.
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152 ppm
3 2 ppm
observed, but as the temperature is lowered this peak broadens, coalesces and then splits into two distinct resonances (152.7 and 155.2 ppm) at the lowest temperature. Although the peaks are much broader in the SSNMR spectra due to the nature of the sample phase (crystalline solid versus solution) the interpretation of the data is the same. The observance of a single peak is due to averaging of the signals from the two recognition sites: one that is open and another that is occupied by a 24C8 macrocycle. These data unequivocally demonstrate that rapid molecular shuttling of the macrocyclic ring between the two recognition sites is occurring inside the MOF. When the sample is cooled, the rate of this translational motion slows and both occupied and unoccupied sites are observed. Fitting of the experimental spectra with simulations allowed determination of the rates of shuttling at various temperatures (Fig. 6a). An Eyring plot of ln(k/T ) versus 1/T is shown in
160
ln(k/T )
5
1 0 –1 0.0030
0.0032
0.0034
1/T (K–1)
Figure 6 | 13C SSNMR studies verify that the designed molecular shuttle can undergo translational motion (molecular shuttling) inside the lattice of the MOF. a, 1H-13C CP/MAS NMR spectra (left, experimental; right, simulation) of a 13C-enriched sample of UWDM-4 (the uncertainties for the simulated rates are estimated to be 10%). b, 13C 2D EXSY SSNMR spectrum of UWDM-4. c, Eyring plot created from the VT NMR data in a (see Supplementary Section VII for complete analysis).
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configuration is thought of as a small molecule, it undergoes a large number of collisions with non-hydrogen-bonding solvent molecules (for example, toluene), on the order of 1011 to 1012 times per second at room temperature. By contrast, solvent molecules in the MOF are not moving as isotropically or rapidly. The effect of the faster collisions in solution is to induce a wider range of ring conformations and increased relative shuttling motions of the rings and axles than would be experienced in the solid state. Another intriguing possibility for the lower ΔS ‡ in the solid state is that cooperativity is required for the motion of multiple rings to occur; that is, one ring cannot move without its motion requiring the next ring to move.
Conclusions We have prepared UWDM-4, a MOF material with a molecular shuttle as part of its internal structure. VT 13C SSNMR experiments demonstrate that the macrocyclic ring can undergo large-amplitude translational motion along the rigid skeleton of the MOF to which it is interlocked. This establishes, for the first time, that the dynamics of a molecular shuttle—the to and fro motion that is fundamental to many MIM switches and molecular machines—can be successfully made to function in a highly organized and dense material with ∼1021 units per cm3. The ability to arrange mobile and switchable MIM components in a highly dense and predictable array is a crucial step towards the generation of solid-state nanoscale devices based on mechanically interlocked molecules.
Methods Preparation of tetra-carboxylic acid linker 5. In an initial step (not shown in Fig. 3a) 4,7-bis( p-ethoxycarbonylphenyl)-2,1,3-benzothiodiazole was prepared via a Suzuki coupling reaction between 4,7-dibromo-2,1,3-benzothiodiazole and 4-ethoxycarbonylphenylboronic acid, using Cs2CO3 as the base and [Pd(PPh3)4] as the catalyst in a 1:1 DMF/toluene solution (yield 91%). The thiodiazole was then converted to diamine 1 using NaBH4 and CoCl2·6H2O in 1:3 THF/ethanol (yield 85%) and the diamine reacted with excess terephthalaldehyde in the presence of ZrCl4 in CH3CN to produce aldehyde 2 (yield 62%). The T-shaped benzimidazole 2 was subsequently protonated with HBF4 (yield 92%) and mixed with 2 equiv. of 24C8 in chloroform to form the [2]pseudorotaxane [2-H⊂24C8]+ (Ka = 1.9 × 104 M−1; 1H NMR, CDCl3 , 2.0 × 10−3 M). Condensation with a further equivalent of diamine 1 and then oxidization in the presence of a catalytic amount of ZrCl4 produced, after treatment with Et3N, the neutral tetraethyl-ester [2]rotaxane 3 (yields for two steps, 88% and 91%)40,41. Finally, the four ester groups were hydrolysed to carboxylic acids with 1 M NaOH in a 2:1 mixture of ethanol/THF to give the MIM linker 5 (yield 95%). Mp > 250 °C (dec). 1H NMR (500 MHz, DMSO-d6): δ = 13.00 (br s, 4H, COO-H), 12.46 (s, 2H, N-H), 8.63 (s, 4H, Ar-H), 8.40 (d, 4H, Ar-H, 3J = 8.0 Hz), 8.15 (d, 4H, Ar-H, 3J = 8.0 Hz), 8.11 (d, 4H, Ar-H, 3 J = 8.0 Hz), 7.86 (d, 4H, Ar-H, 3J = 8.0 Hz), 7.64 (d, 2H, Ar-H, 3J = 8.0 Hz), 7.40 (d, 2H, Ar-H, 3J = 8.0 Hz), 3.13 (s, 32H, O-CH2CH2-O). 13C NMR (125 MHz, DMSO-d6): δ = 167.9, 167.8, 153.9, 143.0, 142.7, 142.6, 133.8, 131.4, 130.5, 130.2, 129.9, 129.8, 129.6, 129.4, 129.1, 125.5, 123.9, 122.0, 69.4. HR-MS (ESI-TOF): calculated for [M + H]+ [C64H63N4O16]+ m/z = 1,143.4239; found m/z = 1,143.4252. Preparation of UWDM-4·HBF4. Neutral MIM linker 5 and Zn(BF4)2.6H2O were dissolved in a solution of DMF/DMSO (1:1 vol/vol), to which one drop of HBF4 was added. The solution was then injected through a syringe filter into a 20 ml borosilicate scintillation vial and ethanol was added to give a solution of DMF/DMSO/EtOH (1:1:2 vol/vol/vol). The vial was then heated at a constant rate of 1 °C min−1 to 85 °C, kept at that temperature for 48 h and cooled to room temperature at a constant heating rate of 0.1 °C min−1. A pale yellow crystalline product was collected and washed with ethanol several times to yield pure UWDM-4·HBF4 (yield 74% based on 5). Single-crystal X-ray diffraction. Reflection data were collected for UWDM-4·HBF4 on a Bruker APEX diffractometer using MoKα radiation with an APEX chargecoupled device detector and for 4 on a Bruker D8 Venture diffractometer using CuKα radiation with a Photon-100 CMOS detector. Attempts to grow crystals of either the neutral tetra-carboxylic acid 5 or the neutral tetraethyl-ester precursor 3 were not successful. Tetramethyl-ester 4 was prepared specifically to obtain X-ray-quality crystals and those grown from dimethoxyethane (DME) were of good quality. The unit cell contained two molecules of the neutral [2]rotaxane (C68H70N4O16) and two molecules of DME. The structure was solved in the triclinic space group P–1 with Z = 2. All the non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were placed in idealized positions and refined using a riding model. As-synthesized crystals of UWDM-4·HBF4 were of
good quality and size. The crystals were soaked in fresh DMF and then soaked in ethanol before undertaking the single-crystal X-ray diffraction experiment to remove unwanted side products of the solvothermal synthesis from the pores and ensure the homogeneity of the solvent in the pores. Similar to other porous MOFs, the large amounts of highly disordered solvent (ethanol) in the crystals of UWDM-4·HBF4 resulted in poor diffraction, but in addition there are large disordered 24C8 macrocycles that also contribute to the poor diffraction. The structure was solved in the hexagonal space group R32 with Z = 3 for a formula of [(Zn4O)2(5-4H)3(HBF4)3]·16EtOH. The non-hydrogen atoms of the framework were easily located and well behaved, while the non-hydrogen atoms of the macrocycle were much more difficult to locate. As there are two benzimidazole groups but only one 24C8 macrocycle, the crown ether was disordered over the two sites and input at 50% occupancy. Assignment of the O atoms in the ring was made by comparing displacement parameters and hydrogen-bonding distances to the framework. The molecules of ethanol were treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON42. Detailed descriptions of the synthesis and characterization of all compounds as well as all SSNMR and X-ray diffraction data and modelling can be found in the Supplementary Information. Crystal structure data for tetramethyl-ester 4 and UWDM-4·HBF4 can be accessed at the Cambridge Crystallographic Data Centre (CCDC; www.ccdc.cam.ac.uk) and have been allocated accession nos. CCDC 1035586 and 1035587.
Received 4 December 2014; accepted 31 March 2015; published online 4 May 2015
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Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through Discovery Grants and Accelerator Supplements to S.J.L. and R.W.S. and a Canada Research Chair award to S.J.L. R.W.S. and S.J.L. acknowledge support from NSERC, the Canadian Foundation for Innovation, the Ontario Innovation Trust and the University of Windsor, for the development and maintenance of the SSNMR and X-ray diffraction centres. V.N.V. acknowledges financial support provided by NSERC through an Alexander Graham Bell Canada Graduate Doctoral Scholarship and by the International Center for Diffraction Data for a Ludo Frevel Crystallography Scholarship. The authors thank M. Revington for technical assistance with solution NMR spectroscopy experiments, V. Terskikh for collecting the 21.1 T SSNMR data and J. Auld for recording electrospray mass spectrometry data. Access to the 900 MHz NMR spectrometer was provided by the National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada; http://www.uwindsor.ca).
Author contributions S.J.L. supervised the project. K.Z. designed the experiments with help from V.N.V. and C.O. K.Z. performed all the synthetic experiments. K.Z. and V.N.V. collected and analysed the PXRD, TGA and SCXRD data with assistance from S.J.L. C.O. collected and analysed the SSNMR data. R.W.S supervised all SSNMR data collection, analysis and interpretation. S.J.L. and K.Z. wrote the manuscript with input from V.N.V., C.O. and R.W.S.
Additional information Supplementary information and chemical compound information are available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to R.W.S. and S.J.L.
Competing financial interests
The authors declare no competing financial interests.
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