Solvent and Steric Influences on Rotational Dynamics in Porphyrinic

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Solvent and Steric Influences on Rotational Dynamics in Porphyrinic Metal−Organic Frameworks with Mechanically Interlocked Pillars Pablo Martinez-Bulit, Christopher A. O’Keefe, Kelong Zhu,† Robert W. Schurko,* and Stephen J. Loeb* Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4 Canada

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S Supporting Information *

ABSTRACT: Zinc(II) metal−organic framework (MOF) materials were prepared using a porphyrin-based tetracarboxylate linker and pyridine-terminated [2]rotaxane linkers as pillars. Two pillaring rotaxanes (R = H or CH3 at pyridine 2-position) were used to create two MOFs α-UWDM-6 and α-UWDM-7, and their structures were determined by single-crystal X-ray diffraction. Both MOFs undergo reversible phase transitions upon solvent removal to generate γ-UWDM-6 and β-UWDM-7. The dynamics of the 24-membered macrocycle ([24]crown-6) inside each MOF phase was studied using variable-temperature 2H solid-state NMR spectroscopy. Interestingly, only solvated α-UWDM-7 demonstrates unrestricted rotation of the macrocycle inside the MOF pores; studies on α-UWDM-7 with various solvents showed that the dynamics of the wheel is essentially independent of the solvent.

1. INTRODUCTION Organizing mechanically interlocked molecules (MIMs) into crystalline arrays allows for the investigation of motional modes that are not accessible when studying these molecules in solution. Eliminating the random tumbling of the molecules in the liquid phase and minimizing the thermal vibrations of the static components by placing them in a rigid framework also allow for the study of the effects that the framework structure, nature of the guest molecules, and external stimuli have on the dynamics of these materials. This information can then be used in the bottom-up design of molecular systems capable of ordered, switchable motion, and their eventual incorporation into solid-state molecular machinery. Following guidelines proposed by Garcia-Garibay1 and Loeb,2 our objective is to integrate rotaxanes into crystalline materials with rigid frameworks that minimize the motion of all the components except that of the interlocked macrocycles and with sufficient void space around these mobile components to allow for their unhindered motion or amphidynamic3 character. Metal−organic frameworks (MOFs) are an ideal class of materials for this purpose4 and have been actively used with this goal in mind.2,5−7 The emblematic MOF UWDM-12 (University of Windsor Dynamic Material), containing a rotaxane with a 24-crown-6 ether (24C6) macrocycle and having a fog topology, was the first example of rotational dynamics of a rotaxane wheel inside a MOF. Three other MOFs in this series, UWDM-2,6 UWDM-3,6 and UWDM-5,5 also have macrocyclic rings that undergo pirouetting motions. UWDM-2 and UWDM-3 form interpenetrated pcu lattices, which decrease their available free volumes and severely restrict the dynamics of the rings. A phase change that occurs for UWDM-3 results in full rotation of the ring, but © XXXX American Chemical Society

unfortunately, the structure of the new phase is unknown, limiting our understanding of the correlation between structure and dynamics. Here, we attempt to address some of the problems encountered when studying these pillared pcu MOFs, while advancing the understanding of the influences of structure and guest molecules on rotational dynamics in these systems. We chose to investigate porphyrin linkers to determine if they would produce a stable, non-interpenetrated structure. These molecules have been used as robust building blocks for the synthesis of MOFs for more than two decades.8−10 The rigidity, and the thermal and chemical stability of porphyrins, are ideal characteristics for MOF linkers, and they can be incorporated both as structural units and as active linkers (e.g., for catalysis11). One of the most prevalent porphyrins in MOF synthesis, meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP), produces a wide variety of topologies and networks, both as a single linker12−20 or in the construction of pillared threedimensional (3D) structures.21−26 The predictability of the MOF structure when combining this porphyrin, a linear bipyridine strut, and metal paddlewheel as secondary building unit (SBU), has been used in studies regarding the formation of particular stacking arrangements,27−29 making light-harvesting materials,25,26 and in photochromic MOFs.21 Herein, we report the synthesis of two new MOFs (UWDM6 and UWDM-7) fabricated from a porphyrin tetracarboxylate, and ZnII ions with linear pillaring rotaxane linkers. The dynamics of the rings and the influence of different solvent Received: May 22, 2019 Revised: August 8, 2019

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DOI: 10.1021/acs.cgd.9b00669 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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flask charged with 10% Pd/C (200 mg) under N2 atmosphere. The flask was degassed and flushed with D2 gas introduced via a balloon. The mixture was stirred for 3 h under ambient conditions. The reaction mixture was filtered through a Celite pad, and the solvents removed under reduced pressure. Recrystallization from hexanes gave the pure product as a white solid consisting of a mixture of isotopomers. Yield: 820 mg, 82%. MP: 114−117 °C. 1H NMR (500 MHz, CDCl3, 298 K) δ (ppm) = 8.55 (d, 1H, J = 5.0 Hz), 8.48 (d, 1H, J = 5.0 Hz), 7.49 (s, 2H), 7.00 (s, 1H), 6.95 (s, 1H), 6.94 (d, 1H, J = 5 Hz), 6.89 (d, 1H, J = 5 Hz), 6.69 (s, 2H), 5.19 (t, 1H, J = 4.5 Hz), 4.48 (d, 2H, J = 4.5 Hz), 3.60−3.25 (m, 24H), 2.57 (s, 3H) 2.54 (s, 3H), 2.03 (s, 6H), 1.96 (s, 6H,), 1.51−1.06 (m, 10H). 2H NMR (77 MHz, CDCl3, 298 K) δ (ppm) = 1.13. 13C NMR (125 MHz, CDCl3, 298 K) δ (ppm) = 158.6, 157.9, 151.5, 150.5, 149.3, 149.2, 148.8, 140.2, 137.4, 134.6, 134.2, 129.2, 126.6, 125.7, 124.3, 123.4, 122.1, 112.9, 77.4, 75.1, 71.8, 71.1, 70.7, 70.7, 70.6, 70.4, 47.3, 29.8, 28.9, 28.5, 25.7, 25.0, 24.7, 24.6, 21.1, 20.7. HR-MS (ESI+): Calculated for [M − H]+ [C47H66N3O6]+ m/z = 772.5228; found m/z = 772.5234. 2.4. Synthesis of UWDM-6. Ten 20 mL scintillation vials were each loaded with Zn(NO3)2·6H2O (6 mg, 0.02 mmol), L1 (30 mg, 0.04 mmol), and [Pd(TCPP)] (9 mg, 0.01 mmol). DMF (2 mL) and HBF4·Et2O (1 drop) were added to each vial, and the mixture was sonicated for 1 min. The mixture was heated in a programmable oven at 85 °C for 48 h then cooled to room temperature over 6 h. Red, plate-like crystals were collected by filtration and stored in fresh DMF. Yield based on porphyrin: 231 mg, 90%. To confirm the composition of the MOF, a small amount of the sample was digested in DMSO-d6 using CF3SO3D. The ratio of porphyrin linker to rotaxane linker matched the expected value of 1:1 (see Supporting Information for details). 2.5. Synthesis of UWDM-7. Ten 20 mL scintillation vials were each loaded with Zn(NO3)2·6H2O (6 mg, 0.02 mmol), L2 (30 mg, 0.04 mmol), and [Pd(TCPP)] (9 mg, 0.01 mmol). DMF (2 mL) and HBF4·Et2O (1 drop) were added to each vial, and the mixture was sonicated for 1 min. The mixture was heated in a programmable oven at 80 °C for 48 h and cooled to room temperature over 6 h. Red, plate-like crystals were collected by filtration and stored in fresh DMF. Yield based on porphyrin: 210 mg, 80%. The same digestion procedure used for UWDM-6 confirmed the composition of the material (see Supporting Information for details).

guest molecules inside the frameworks were studied using variable-temperature (VT) 2H solid-state NMR spectroscopy (SSNMR). UWDM-6 and UWDM-7 are robust enough to undergo activation and solvent exchange without collapsing the frameworks. By increasing the available data on rotational dynamics inside MOFs and exploring the role that secondary linkers, guest molecules, and structural variation have on such dynamics, we hope to make advances in the realization of molecular machines that function in the solid state.

2. EXPERIMENTAL SECTION 2.1. Precursors. 4-Bromo-3,5-dimethylaniline,30 4-bromo-3,5dimethylbenzaldehyde,31 pentaethyleneglycoldipent-4-enyl ether (2),32 [1-H][BF4],6 (3), and [Pd(TCPP)]25 were synthesized following modified literature methods. See Scheme 1 and Supporting Information for general experimental details.

Scheme 1. Synthesis of [2]Rotaxane Linkers L1 and L2

3. RESULTS AND DISCUSSION 3.1. [2]Rotaxane Synthesis. Scheme 1 outlines the synthesis of the pillaring linkers L1 and L2. The dibromosubstituted [2]rotaxane synthon was synthesized via ringclosing metathesis32 of a preorganized polyethylene glycol precursor around an axle incorporating an anilinium recognition site. The methyl groups on the 3,5-positions of the aromatic core are bulky enough to prevent the slippage of the macrocyclic ring under the solvothermal conditions required for MOF synthesis.6 Suzuki coupling between the dibromo synthon and 4-pyridinylboronic acid pinacolate (L1) or 2-methyl-4-pyridinylboronic acid (L2) was used to incorporate the pyridine groups. The double bond on the macrocyclic ring (E/Z isomers) was reduced using D2 gas and Pd/C, to yield the linkers L1 or L2 as a mixture of isotopomers. 3.2. MOF Synthesis. The combination of a planar, square tetratopic linker, a metal paddlewheel, and a linear, bipyridinelike pillaring linker is known to produce an fsc topology.33 Varying the nature of the pillaring linker gives rise to isoreticular networks of varying sizes.28 Conversely, different tetratopic linkers can be used in the construction of the 2D grid, resulting in interpenetrated and non-interpenetrated structures.29,34 Two new MOFs, labeled UWDM-6 and UWDM-7, were synthesized under solvothermal conditions,

2.2. Synthesis of 4b. Compound 3 (200 mg, 0.27 mmol, E/Z mixture) was dissolved in DMF/toluene (1:1) (20 mL) under N2 atmosphere. 2-Methylpyridine-4-boronic acid (177 mg, 0.81 mmol), Cs2CO3 (263 mg, 0.81 mmol), and Pd(PPh3)4 (32 mg, 0.03 mmol) were added, and the reaction mixture was heated at 110 °C for 12 h. The reaction mixture was cooled and filtered, and the solid residue was rinsed with CHCl3. The solvents were removed under reduced pressure and the crude product was purified by column chromatography (SiO2, ethyl acetate/CH2Cl2/CH3OH 1:1:0.1 v/v) Yield: 170 mg, 90%. (E/Z mixture). MP: 99−103 °C. 1H NMR (500 MHz, CDCl3, 298 K) δ (ppm) = 8.55−8.54 (m, 1H), 8.49−8.48 (m, 1H), 7.48−7.47 (m, 2H), 7.01−7.00 (m, 1H), 6.96−6.95 (m, 2H), 6.90− 6.89 (m, 1H), 6.71−6.66 (m, 2H), 5.32−5.22 (m, 3H), 4.52−4.42 (m, 2H), 3.60−3.30 (m, 24H), 2.61 (s, 3H), 2.58 (s, 3H), 2.28−1.90 (m, 16H), 1.71−1.38 (m, 4H). 13C NMR (125 MHz, CDCl3, 298 K) δ (ppm) = 158.6, 158.0, 151.5, 151.4, 150.4, 149.4, 149.3, 148.8, 140.4, 140.0, 137.5, 137.4, 134.8, 134.6, 134.2, 130.3, 129.8, 129.4, 129.1, 126.7, 125.6, 124.3, 123.4, 122.0, 113.0, 112.7, 72.2, 71.1, 71.0, 70.7, 70.6, 70.5, 70.4, 70.3, 47.5, 47.3, 30.6, 29.3, 28.8, 25.3, 24.7, 24.6, 21.2, 20.7. HR-MS (ESI + ): Calculated for [M − H] + [C47H66N3O6]+ m/z = 768.4946; found m/z = 768.4929. 2.3. Synthesis of L2. Compound 4b (1.00 g, 1.3 mmol, E/Z mixture) was dissolved in CH3OH (50 mL) and added to a Schlenk B

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by combining 1 equiv of [Pd(TCPP)], 2 equiv of Zn(NO3)2· 6H2O, and 4 equiv of the respective rotaxane linker, L1 or L2, in DMF. [Pd(TCPP)] was employed rather than a complex with a more common metal ion such as ZnII or NiII in order to eliminate axial coordination to the metal center, thus simplifying layering. The MOF materials UWDM-6 and UWDM-7 were isolated in good yields, > 80% based on the amount of porphyrin used, and their structures were determined by single-crystal X-ray diffraction (SC-XRD) to have formulas [Zn2(Pd(TCPP))(L1)](DMF)14 and [Zn2(Pd(TCPP))(L2)](DMF)10, respectively. 3.3. MOF Structures. UWDM-6 and UWDM-7 both adopt a 2-fold interpenetrated fsc net, where the porphyrins form a two-dimensional grid with the zinc paddlewheels distributed at ∼17 Å intervals, which then is pillared by the [2]rotaxane linkers, producing an interlayer distance of ∼25 Å (Figure 1). Although it has been shown that when using long

Figure 1. Ball-and-stick representation of the SC-XRD structure of αUWDM-7 showing how the [2]rotaxane pillars the 2D layers formed by the tetracarboxylate porphyrin and Zn(II) paddlewheel nodes only a single lattice is shown. Color code: porphyrin linker = green, rotaxane axle pillar = blue, 24C6 macrocyclic ring = red, Zn = dark blue, Pd = green.

pillaring linkers, the structures tend to be interpenetrated, it was thought that the combination of these two particular linkers might prevent interpenetration due to the steric bulk of the interlocked 24C6 rings. Structural analysis shows that even if the macrocyclic ring does not fit through the openings in the 2D grid (an ∼8 Å wide opening vs an ∼11 Å wide macrocycle), the narrower section of the axle can reside in this opening. This allows a second framework to grow within the original lattice, resulting in a 2-fold interpenetrated structure. In this structure, the rotaxane linkers are spaced by ∼8 Å along the crystallographic axis a and ∼14 Å apart along the b axis. As a consequence, the crown ether rings are also spaced at ∼8 Å intervals and sit ∼4 Å from the closest adjacent axle fragment. Because of interpenetration, the macrocyclic rings reside between two planes defined by the porphyrin linkers of the two independent frameworks (Figure 2a). Even with this structural

Figure 2. Ball-and-stick representations of the SC-XRD structure of as-synthesized α-UWDM-7 showing the 2-fold interpenetration of the lattices: (a) view down the c-axis, (b) view down the b-axis. (c) view down the a-axis showing the positioning of the voids in the lattice. Color code: independent lattices blue and green and associated macrocyclic rings red and orange respectively; yellow spheres representing voids drawn with diameters of 8.0 and 12.0 Å.

arrangement, there is no obvious steric hindrance that would prevent rotation of the interlocked rings (Figure 2b). Analysis of these structures using PLATON35 showed solvent accessible spaces totalling 49% and 47% for UWDM-6 and UWDM-7, respectively. For both, there are two voids, centered at [0.5, 0.85, 0] and [0.5, 0.85, 0.5]. C

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small changes might have on the rotational dynamics inside the MOF, vide infra. 3.5. Ring Dynamics Studied by 2H SSNMR Spectroscopy. 2H SSNMR spectroscopy is an excellent tool to monitor rotational dynamics in mechanically interlocked molecules, as the powder patterns are highly sensitive to molecular motion and their corresponding rates. Three motional regimes are defined based on the relative magnitudes of the 2 H quadrupolar frequency (νQ) and the exchange rate (νex): the slow motion limit (SML), where νex ≪ νQ (i.e., νex < 103 Hz); the intermediate motion regime (IMR), where νex ≈ νQ (i.e., 103 Hz ≤ νex ≤ 107 Hz); and the fast motion limit (FML), where νex ≫ νQ (i.e., νex > 107 Hz). Motions with rates in the SML are too slow to produce changes in the 2H powder patterns, whereas motions with rates in the FML produce 2H powder patterns that are easily described by consideration of motional types and averaged effective 2H electric field gradient (EFG) tensors. However, motions with rates in the IMR result in powder patterns that change drastically with temperature and can be diagnostic in the determination of motional models. As previously demonstrated,2,5,6 the 24C6 macrocyclic rings can undergo three distinct types of motion in the solidstate: (1) a two-site jump of the deuterons about an axis that is collinear with the C−C bonds, (2) partial rotation of the ring in which the hydrogen-bond donor on the axle interacts with each of the oxygen atoms on the ring, and (3) full (but nondirectional) rotation of the ring about the axle (details of the data collection and simulation of the 2H powder patterns can be found in the Supporting Information). Although both UWDM-6 and UWDM-7 materials have a 2fold interpenetrated network, analysis of their structures shows the macrocycles are located in relatively unhindered positions. Following PXRD analysis, it was decided to investigate the dynamics of both as-synthesized and activated samples of these MOFs using 2H SSNMR. The spectra for α-UWDM-7 and βUWDM-7 are shown in Figure 3those for α-UWDM-6 and γ-UWDM-6 can be found in the Supporting Informationand Table 2 summarizes the onset temperatures and modes of motion for these and other MOFs in this series that contain imbedded 24C6 macrocycles. In all cases, the spectra acquired at 185 K were simulated with the absence of motions, or equivalently, motions occurring at rates within the SML. For α-UWDM-6 (as-synthesized material) the 2H SSNMR spectrum acquired at 208 K was simulated as a two-site jump motion with an angle of β = 75°. The onset of partial rotational motion is observed in the spectrum acquired at 253 K. The spectrum collected at 285 K was simulated as the FML of the combined two-site jump and partial rotational motions. Increasing the temperature to 411 K (i.e., highest temperature the sample could be heated) did not produce any significant changes to the patterns, and therefore, no additional modes of motion occur in α-UWDM-6. It is important to remember that α-UWDM-6 is converted to β-UWDM-6 upon heating, so the SSNMR does not represent the motions of a phase-pure material. 2 H SSNMR spectra for γ-UWDM-6 (desolvated/activated material) acquired above 185 K were simulated as a two-site jump motion with rates in the IMR. The FML for the two-site jump motion occurs at 321 K. Increasing the temperature further results in the onset of the partial rotational motion and the spectrum acquired at 366 K was simulated as the FML of the combined two-site jump and partial rotational motions (see Supporting Information).

The smaller pore is located between two porphyrin planes separated by ∼8 Å, while the larger pore is located in the vicinity of the crown ethers (Figure 2c). Thermogravimetric analysis is consistent with these spaces being occupied by DMF molecules, with an initial weight loss of 10% above 110 °C (not observed for the activated samples), followed by the decomposition of the crown ether rings around 200 °C, and finally, the decomposition of the framework above 350 °C (see Supporting Information). 3.4. MOF Phases and Stability. The stabilities of both assynthesized and activated (desolvated) samples of UWDM-6 and UWDM-7 were studied by variable-temperature powder X-ray diffraction (VT-PXRD) (see Supporting Information). It was found that UWDM-6 has three different phases in the temperature range studied. The first, α-UWDM-6, corresponds to the as-synthesized material with DMF inside the cavities. Upon heating, a gradual phase change to β-UWDM-6 is observed and attributed to a deformation or tilting of the pillars inside the framework. Although the exact structure of βUWDM-6 is unknown, this hypothesis is consistent with observations made for similar pillared systems,36 and this new phase reverts to the original α-UWDM-6 form upon reexposure to DMF. On the other hand, activation of α-UWDM6 by exchanging DMF for CH2Cl2 and air-drying produces a desolvated material that is not β-UWDM-6. This third phase, γ-UWDM-6, displays a powder pattern at room temperature that does not match those for α-UWDM-6 or β-UWDM-6 and remains unchanged in the temperature range of 25−200 °C. The activation process does not visibly reduce the crystallinity of the sample, but it does affect the quality of the single crystals (i.e., cracking), making unambiguous determination of the structure of γ-UWDM-6 difficult (Table 1). Despite the Table 1. Unit Cell Parameters of MOFs Measured Using SC-XRD Data on Single Crystals a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3)

α-UWDM-6

γ-UWDM-6

α-UWDM-7

β-UWDM-7

17.051(4) 16.254(4) 24.725(6) 90 90.867(7) 90 6852(3)

16.901(3) 16.390(3) 24.577(4) 90 92.421(7) 90 6808(2)

16.8477(4) 16.4056(4) 24.7321(7) 90 94.169(2) 90 6817.6(3)

16.8219(6) 16.4164(6) 49.137(2) 90 91.786(3) 90 12962(9)

structural changes that must be occurring in these materials, none of the phases of UWDM-6 show irreversible collapse of the framework or total loss of crystallinity, both of which are fairly common for pillared structures,36 and would greatly hinder the dynamics of the interlocked macrocycles. UWDM-7 is somewhat more robust. The as-synthesized sample α-UWDM-7 does not undergo any structural changes upon heating, and the PXRD pattern only exhibits broadening at high temperatures. After activation, the powder pattern exhibits changes in the intensity of some peaks; however, crystallinity is retained, and it was possible to determine a unit cell for β-UWDM-7 from SC-XRD data (Table 1). The change in unit cell parameters implies that the differences in the PXRD patterns correspond to a distortion of the framework that causes a change in the space group due to loss of symmetry. These transformations are helpful in understanding the effect of guest removal for this type of interpenetrated porous material, as well as the effect that such D

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site jump motion with a jump angle of 70° and rates in the FML (Figure 3b). Increasing the temperature to 298 K results in an increase in the jump angle (β = 75°) and the onset of partial rotation. The FML of the combined two-site jump and partial rotational motions occurs at 343 K and increasing the temperature does not result in any further changes in the 2H powder patterns (Figure 3b), indicating that full rotational motion does not occur. It is clear that the small structural changes induced by removal of the solvent have pronounced effects on the motion. For α-UWDM-6, the onset temperature of the two-site jump motion is 208 K and the FML is reached by 253 K. Conversely, for γ-UWDM-6, the FML of the two-site jump occurs at 321 K, a difference of almost 75 K. An analogous observation is made for the partial rotational motion with FML temperatures of 285 and 366 K for α-UWDM-6 and γ-UWDM-6, respectively. The effect is less pronounced for UWDM-7, where the FML for the two-site jump and partial rotation occurs at similar temperatures. However, increasing the temperature further results in full rotation of the macrocyclic rings in α-UWDM-7 but no additional motion for β-UWDM7. 3.6. Influence of Solvent on Dynamics in Pores. Activation results in a decrease in the rates of motion of the rings in both UWDM-6 and UWDM-7, but it is unclear whether this is driven by structural changes or is a consequence of the different chemical environments of the rings (i.e., interactions with solvent molecules). To address this, multiple solvent-exchanges were conducted on α-UWDM-7, where solvents were chosen to reflect a variety of polarities and viscosities.37 The samples were submerged in the new solvent, and the solvent was replaced every 2−3 h, for at least 24 h. Samples were then filtered and air-dried briefly before obtaining SSNMR data. VT 2H SSNMR spectra for five different solvents are shown in Figure 4. With the exception of some of the lower temperature spectra with poor signal-to-noise, it appears that the motion of the rings in α-UWDM-7 is the same, and full rotation occurs at high temperature regardless of the solvent within the pores.

Figure 3. (a) Experimental VT 2H SSNMR spectra for α-UWDM-7 and corresponding simulations showing the (i) SML, (ii) FML of the two-site jump (β = 78°) and onset of partial rotation, (iii) FML of the two-site jump and partial rotation, and (iv) FML of the two-site jump and full rotation. (b) Experimental VT 2H SSNMR spectra for βUWDM-7 and corresponding simulations showing the (i) SML, (ii) FML for the two-site jump (β = 70°), (iii) FML of the two-site jump (β = 75°) and onset of partial rotation, and (iv) FML of the two-site jump and partial rotation.

Table 2. Summary of Onset Temperatures for the Three Distinct Motion Modes Observed in UWDM MOFs with an Aniline-Based Rotaxane Linker and 24C6 Macrocycle MOF

two-site jump (K)

partial rotation (K)

full rotation (K)

UWDM-1 UWDM-2 α-UWDM-3 β-UWDM-3 α-UWDM-6 γ-UWDM-6 α-UWDM-7 β-UWDM-7

251 255 247 234 208 273 223 248

324 255 255 255 253 330 253 298

373

4. CONCLUSIONS It was concluded that while desolvation produces only minor changes in structure, this is sufficient to cause significant changes in the motion of the rings. This is very likely due to tilting of the framework structure such that the rings of one [2]rotaxane linker come into close contact with the static portion of the framework. The addition of the methyl group to L2 increases the rigidity of the pillars and reduces the possibility for thermally induced structural transformations (i.e., framework tilting) that increase the steric interactions between the rings and the framework, thereby allowing for unhindered motions of the rings in α-UWDM-7. Moreover, the dynamic behavior of the interlocked 24C6 macrocycle in α-UWDM-7 can be described as being analogous to the macroscopic stick−slip phenomenon, in which a moving object momentarily drags the objects surrounding itin this case solvent moleculesfollowed by the relaxation of these objects to their original position.38 Previous studies have shown that small modifications to the organic linker in a MOF such as a change in substituents from H to Me or CH2OH can result in topological changes, formation of a different secondary building unit (SBU),39

339

323

For α-UWDM-7 (as-synthesized material), increasing the temperature above 185 K results in drastic changes in the 2H powder patterns (Figure 3a) indicating a rapid onset of the two-site jump motion. The spectrum acquired at 253 K was simulated with the FML of the two-site jump and the onset of the partial rotational motion, the rate of which increases with temperature. At 343 K, an axially symmetric pattern is obtained, indicating the FML of the full rotational motion. Increasing the temperature further (to 411 K) results in no further changes to the spectra. The spectrum acquired at 273 K for β-UWDM-7 (desolvated/activated material) was simulated with the twoE

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Figure 4. Experimental VT 2H SSNMR spectra of UWDM-7 with (a) no solvent (β-UWDM-7), (b) dimethylformamide (α-UWDM-7), (c) mesytilene, (d) dioxane, (e) n-butanol, or (f) ethylene glycol, within the pores of the framework.

Notes

varying the degrees of interpenetration, or modifying the layering of a structure.28 This work represents another example of how seemingly small alterations of organic linkers can play a key role in the final properties of materials and should be carefully considered in their design. Future studies will be focused on the variation of the metal center in the porphyrin in order to investigate its effects on the rotational dynamics of the MIM component. This can potentially create systems where the actuation of the rotational motion is controlled not only thermally but also by the introduction of additional ligands coordinated to the porphyrin metal center.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, primarily through Discovery grants to S.J.L. and R.W.S. and partially through a Canada Research Chair program award to S.J.L. R.W.S. is also grateful for 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 centre. The authors acknowledge M. Revington for technical assistance with solution NMR spectroscopy and J. Auld for technical assistance with highresolution mass spectrometry.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.9b00669. Full description of synthesis for linkers and MOFs; all solution and solid-state NMR data; details of PXRD and SC-XRD experiments; TGA (PDF) Accession Codes

CCDC 1904563−1904564 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*(S.J.L.) E-mail: [email protected]. *(R.W.S.) E-mail: [email protected]. ORCID

Robert W. Schurko: 0000-0002-5093-400X Stephen J. Loeb: 0000-0002-8454-6443 Present Address †

(K.Z.) School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, P. R. China. E-mail: zhukelong@mail. sysu.edu.cn. F

DOI: 10.1021/acs.cgd.9b00669 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.9b00669 Cryst. Growth Des. XXXX, XXX, XXX−XXX