Solvent and Steric Influences on Rotational Dynamics in Porphyrinic Metal-Organic Frameworks with Mechanically Interlocked Pillars

Article abstract of DOI:10.1021/acs.cgd.9b00669

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 cr

Full text of DOI:10.1021/acs.cgd.9b00669

Article
pubs.acs.org/crystal
Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
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
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.
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 three-
dimensional (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-harvest-
ing materials,25,26 and in photochromic MOFs.21
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
Herein, we report the synthesis of two new MOFs (UWDM-
6 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
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.9b00669
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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
guest molecules inside the frameworks were studied using
variable-temperature (VT) H solid-state NMR spectroscopy
2
(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.
1
isotopomers. Yield: 820 mg, 82%. MP: 114−117 °C. H 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
2
(s, 3H), 2.03 (s, 6H), 1.96 (s, 6H,), 1.51−1.06 (m, 10H). H NMR
2. EXPERIMENTAL SECTION
(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.
2.1. Precursors. 4-Bromo-3,5-dimethylaniline,30 4-bromo-3,5-
dimethylbenzaldehyde,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
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).
3. RESULTS AND DISCUSSION
3.1. [2]Rotaxane Synthesis. Scheme 1 outlines the
synthesis of the pillaring linkers L1 and L2. The dibromo-
substituted [2]rotaxane synthon was synthesized via ring-
closing 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, bipyridine-
like 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 chromatog-
raphy (SiO2, ethyl acetate/CH2Cl2/CH3OH 1:1:0.1 v/v) Yield: 170
1
mg, 90%. (E/Z mixture). MP: 99−103 °C. H 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
DOI: 10.1021/acs.cgd.9b00669
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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 α-
UWDM-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.
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].
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
C
DOI: 10.1021/acs.cgd.9b00669
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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).
small changes might have on the rotational dynamics inside the
MOF, vide infra.
2
3.5. Ring Dynamics Studied by H SSNMR Spectros-
copy. 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 2H
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
3.4. MOF Phases and Stability. The stabilities of both as-
synthesized 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 β-
UWDM-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 re-
exposure to DMF. On the other hand, activation of α-UWDM-
6 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
2
SML are too slow to produce changes in the H powder
2
patterns, whereas motions with rates in the FML produce H
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 solid-
state: (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
2
the data collection and simulation of the H powder patterns
can be found in the Supporting Information).
Although both UWDM-6 and UWDM-7 materials have a 2-
fold 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
Table 1. Unit Cell Parameters of MOFs Measured Using
SC-XRD Data on Single Crystals
2
MOFs using H SSNMR. The spectra for α-UWDM-7 and β-
UWDM-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.
α-UWDM-6 γ-UWDM-6
α-UWDM-7
β-UWDM-7
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
17.051(4)
16.254(4)
24.725(6)
90
16.901(3)
16.390(3)
24.577(4)
90
16.8477(4)
16.4056(4)
24.7321(7)
90
94.169(2)
90
16.8219(6)
16.4164(6)
49.137(2)
90
91.786(3)
90
90.867(7)
90
92.421(7)
90
2
For α-UWDM-6 (as-synthesized material) the H SSNMR
6852(3)
6808(2)
6817.6(3)
12962(9)
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.
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
2H 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).
D
DOI: 10.1021/acs.cgd.9b00669
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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
2
temperature does not result in any further changes in the H
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-UWDM-
7.
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.
2
Figure 3. (a) Experimental VT H 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
2
and full rotation. (b) Experimental VT H SSNMR spectra for β-
UWDM-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.
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.
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)
4. CONCLUSIONS
UWDM-1
UWDM-2
251
255
247
234
208
273
223
248
324
255
255
255
253
330
253
298
373
339
323
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
α-UWDM-3
β-UWDM-3
α-UWDM-6
γ-UWDM-6
α-UWDM-7
β-UWDM-7
For α-UWDM-7 (as-synthesized material), increasing the
temperature above 185 K results in drastic changes in the H
2
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.
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
The spectrum acquired at 273 K for β-UWDM-7
(desolvated/activated material) was simulated with the two-
E
DOI: 10.1021/acs.cgd.9b00669
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
2
Figure 4. Experimental VT H 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.
The authors declare no competing financial interest.
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 high-
resolution mass spectrometry.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.9b00669.
REFERENCES
■
Full description of synthesis for linkers and MOFs; all
solution and solid-state NMR data; details of PXRD and
SC-XRD experiments; TGA (PDF)
(1) Khuong, T. A. V.; Nunez, J. E.; Godinez, C. E.; Garcia-Garibay,
M. A. Crystalline Molecular Machines: A Quest toward Solid-State
Dynamics and Function. Acc. Chem. Res. 2006, 39, 413−422.
(2) Vukotic, V. N.; Harris, K. J.; Zhu, K.; Schurko, R. W.; Loeb, S. J.
Metal-Organic Frameworks with Dynamic Interlocked Components.
Nat. Chem. 2012, 4, 456−460.
(3) Karlen, S. D.; Garcia-Garibay, M. A. Amphidynamic Crystals:
Structural Blueprints for Molecular Machines. Top. Curr. Chem. 2005,
262, 179−227.
(4) Deng, H.; Olson, M. A.; Stoddart, J. F.; Yaghi, O. M. Robust
Dynamics. Nat. Chem. 2010, 2, 439−443.
Accession Codes
CCDC 1904563−1904564 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(5) Farahani, N.; Zhu, K.; O’Keefe, C. A.; Schurko, R. W.; Loeb, S. J.
Thermally Driven Dynamics of a Rotaxane Wheel about an
Imidazolium Axle inside a Metal−Organic Framework. ChemPlu-
sChem 2016, 81, 836−841.
AUTHOR INFORMATION
Corresponding Authors
*(S.J.L.) E-mail: loeb@uwindsor.ca.
*(R.W.S.) E-mail: rschurko@uwindsor.ca.
■
(6) 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. 2014, 136, 7403−7409.
ORCID
́
(7) Coskun, A.; Hmadeh, M.; Barin, G.; Gandara, F.; Li, Q.; Choi,
Robert W. Schurko: 0000-0002-5093-400X
Stephen J. Loeb: 0000-0002-8454-6443
E.; Strutt, N. L.; Cordes, D. B.; Slawin, A. M. Z.; Stoddart, J. F.; et al.
Metal-Organic Frameworks Incorporating Copper-Complexed Rotax-
anes. Angew. Chem., Int. Ed. 2012, 51, 2160−2163.
(8) Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R.
Assembly of Porphyrin Building Blocks into Network Structures with
Large Channels. Nature 1994, 369, 727−729.
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
(9) Huh, S.; Kim, S. J.; Kim, Y. Porphyrinic Metal-Organic
Frameworks from Custom-Designed Porphyrins. CrystEngComm
2016, 18, 345−368.
(MOFs): Efficient Strut-to-Strut Energy Transfer in Bodipy and
Porphyrin-Based MOFs. J. Am. Chem. Soc. 2011, 133, 15858−15861.
(27) Choi, E. Y.; Barron, P. M.; Novotny, R. W.; Son, H. T.; Hu, C.;
Choe, W. Pillared Porphyrin Homologous Series: Intergrowth in
Metal-Organic Frameworks. Inorg. Chem. 2009, 48, 426−428.
(28) Chung, H.; Barron, P. M.; Novotny, R. W.; Son, H. T.; Hu, C.;
Choe, W. Structural Variation in Porphyrin Pillared Homologous
Series: Influence of Distinct Coordination Centers for Pillars on
Framework Topology. Cryst. Growth Des. 2009, 9, 3327−3332.
(29) Barron, P. M.; Wray, C. A.; Hu, C.; Guo, Z.; Choe, W. A
Bioinspired Synthetic Approach for Building Metal-Organic Frame-
works with Accessible Metal Centers. Inorg. Chem. 2010, 49, 10217−
10219.
(30) Zysman-Colman, E.; Arias, K.; Siegel, J. S. Synthesis of
Arylbromides from Arenes and N -Bromosuccinimide (NBS) in
Acetonitrile  A Convenient Method for Aromatic Bromination.
Can. J. Chem. 2009, 87, 440−447.
(31) Kang, B.; Kurutz, J. W.; Youm, K. T.; Totten, R. K.; Hupp, J.
T.; Nguyen, S. T. Catalytically Active Supramolecular Porphyrin
Boxes: Acceleration of the Methanolysis of Phosphate Triesters via a
Combination of Increased Local Nucleophilicity and Reactant
Encapsulation. Chem. Sci. 2012, 3, 1938−1944.
(32) Kilbinger, A. F. M.; Cantrill, S. J.; Waltman, A. W.; Day, M. W.;
Grubbs, R. H. Magic Ring Rotaxanes by Olefin Metathesis. Angew.
Chem., Int. Ed. 2003, 42, 3281−3285.
(33) Bi, M.; Li, G.; Hua, J.; Liu, Y.; Liu, X.; Hu, Y.; Shi, Z.; Feng, S.
Two Isomers with FSC Topology Constructed from Cu6I6(DABCO)4
and Cu8I8(DABCO)6 Building Blocks. Cryst. Growth Des. 2007, 7,
2066−2070.
(34) Farha, O. K.; Malliakas, C. D.; Kanatzidis, M. G.; Hupp, J. T.
Control over Catenation in Metal- Organic Frameworks via Rational
Design of the Organic Building Block. J. Am. Chem. Soc. 2010, 132,
950−952.
(35) Spek, A. L. PLATON SQUEEZE: A Tool for the Calculation of
the Disordered Solvent Contribution to the Calculated Structure
Factors. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 9−18.
(36) Dybtsev, D. N.; Chun, H.; Kim, K. Rigid and Flexible: A Highly
Porous Metal-Organic Framework with Unusual Guest-Dependent
Dynamic Behavior. Angew. Chem., Int. Ed. 2004, 43, 5033−5036.
(37) Jiang, X.; Duan, H. B.; Khan, S. I.; Garcia-Garibay, M. A.
Diffusion-Controlled Rotation of Triptycene in a Metal-Organic
Framework (MOF) Sheds Light on the Viscosity of MOF-Confined
Solvent. ACS Cent. Sci. 2016, 2, 608−613.
(38) Heslot, F.; Baumberger, T.; Perrin, B.; Caroli, B.; Caroli, C.
Sliding Friction: Physical Principles and Applications. Phys. Rev. E:
Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1994, 49, 4973−
4988.
(39) Gadzikwa, T.; Zeng, B. S.; Hupp, J. T.; Nguye n, S. T. Ligand-
Elaboration as a Strategy for Engendering Structural Diversity in
Porous Metal-Organic Framework Compounds. Chem. Commun.
2008, 31, 3672−3674.
(10) Suslick, K. S.; Bhyrappa, P.; Chou, J. H.; Kosal, M. E.;
Nakagaki, S.; Smithenry, D. W.; Wilson, S. R. Microporous Porphyrin
Solids. Acc. Chem. Res. 2005, 38, 283−291.
(11) Kosal, M. E.; Chou, J. H.; Wilson, S. R.; Suslick, K. S. A
Functional Zeolite Analogue Assembled from Metalloporphyrins. Nat.
Mater. 2002, 1, 118−121.
́
(12) Morris, W.; Volosskiy, B.; Demir, S.; Gandara, F.; Mcgrier, P.
L.; Furukawa, H.; Cascio, D.; Stoddart, J. F.; Yaghi, O. M. Synthesis,
Structure, and Metalation of Two New Highly Porous Zirconium
Metal−Organic Frameworks. Inorg. Chem. 2012, 51, 6443−6445.
(13) Luo, J.; Wang, J.; Cao, Y.; Yao, S.; Zhang, L.; Huo, Q.; Liu, Y.
Assembly of an Indium-Porphyrin Framework JLU-Liu7: A
Mesoporous Metal-Organic Framework with High Gas Adsorption
and Separation of Light Hydrocarbons. Inorg. Chem. Front. 2017, 4,
139−143.
(14) Wang, K.; Feng, D.; Liu, T. F.; Su, J.; Yuan, S.; Chen, Y. P.;
Bosch, M.; Zou, X.; Zhou, H. C. A Series of Highly Stable
Mesoporous Metalloporphyrin Fe-MOFs. J. Am. Chem. Soc. 2014,
136, 13983−13986.
(15) Yuan, S.; Liu, T. F.; Feng, D.; Tian, J.; Wang, K.; Qin, J.; Zhang,
Q.; Chen, Y. P.; Bosch, M.; Zou, L.; et al. A Single Crystalline
Porphyrinic Titanium Metal-Organic Framework. Chem. Sci. 2015, 6,
3926−3930.
(16) Feng, D.; Jiang, H.-L.; Chen, Y.-P.; Gu, Z.-Y.; Wei, Z.; Zhou,
H.-C. Metal−Organic Frameworks Based on Previously Unknown
Zr8/Hf8 Cubic Clusters. Inorg. Chem. 2013, 52, 12661−12667.
(17) Feng, D.; Gu, Z. Y.; Chen, Y. P.; Park, J.; Wei, Z.; Sun, Y.;
Bosch, M.; Yuan, S.; Zhou, H. C. A Highly Stable Porphyrinic
Zirconium Metal-Organic Framework with Shp-a Topology. J. Am.
Chem. Soc. 2014, 136, 17714−17717.
(18) Feng, D.; Chung, W. C.; Wei, Z.; Gu, Z. Y.; Jiang, H. L.; Chen,
Y. P.; Darensbourg, D. J.; Zhou, H. C. Construction of Ultrastable
Porphyrin Zr Metal-Organic Frameworks through Linker Elimination.
J. Am. Chem. Soc. 2013, 135, 17105−17110.
(19) Jiang, H. L.; Feng, D.; Wang, K.; Gu, Z. Y.; Wei, Z.; Chen, Y.
P.; Zhou, H. C. An Exceptionally Stable, Porphyrinic Zr Metal-
Organic Framework Exhibiting PH-Dependent Fluorescence. J. Am.
Chem. Soc. 2013, 135, 13934−13938.
(20) Fateeva, A.; Devautour-Vinot, S.; Heymans, N.; Devic, T.;
̀
Greneche, J.-M.; Wuttke, S.; Miller, S.; Lago, A.; Serre, C.; De
Weireld, G.; et al. Series of Porous 3D Coordination Polymers Based
on Iron (III) and Porphyrin Derivatives. Chem. Mater. 2011, 23,
4641−4651.
(21) Park, J.; Feng, D.; Yuan, S.; Zhou, H. C. Photochromic Metal-
Organic Frameworks: Reversible Control of Singlet Oxygen
Generation. Angew. Chem., Int. Ed. 2015, 54, 430−435.
(22) Li, J.; Fan, Y.; Ren, Y.; Liao, J.; Qi, C.; Jiang, H. Development
of Isostructural Porphyrin-Salen Chiral Metal-Organic Frameworks
through Postsynthetic Metalation Based on Single-Crystal to Single-
Crystal Transformation. Inorg. Chem. 2018, 57, 1203−1212.
(23) Dolgopolova, E. A.; Williams, D. E.; Greytak, A. B.; Rice, A. M.;
Smith, M. D.; Krause, J. A.; Shustova, N. B. A Bio-Inspired Approach
for Chromophore Communication: Ligand-to-Ligand and Host-to-
Guest Energy Transfer in Hybrid Crystalline Scaffolds. Angew. Chem.,
Int. Ed. 2015, 54, 13639−13643.
(24) Williams, D. E.; Rietman, J. A.; Maier, J. M.; Tan, R.; Greytak,
A. B.; Smith, M. D.; Krause, J. A.; Shustova, N. B. Energy Transfer on
Demand: Photoswitch-Directed Behavior of Metal-Porphyrin Frame-
works. J. Am. Chem. Soc. 2014, 136, 11886−11889.
(25) Farha, O. K.; Shultz, A. M.; Sarjeant, A. A.; Nguyen, S. T.;
Hupp, J. T. Active-Site-Accessible, Porphyrinic Metal-Organic Frame-
work Materials. J. Am. Chem. Soc. 2011, 133, 5652−5655.
(26) Lee, C. Y.; Farha, O. K.; Hong, B. J.; Sarjeant, A. a; Nguyen, S.
T.; Hupp, J. T. Strut-to-Strut Energy Transfer in Bodipy and
Porphyrin-Based MOFs Light-Harvesting Metal-Organic Frameworks
G
DOI: 10.1021/acs.cgd.9b00669
Cryst. Growth Des. XXXX, XXX, XXX−XXX