Metal-organic frameworks with mechanically interlocked pillars: Controlling ring dynamics in the solid-state via a reversible phase change

Article abstract of DOI:10.1021/ja502238a

Metal-organic framework (MOF) materials have been prepared that contain a mechanically interlocked molecule (MIM) as the pillaring strut between two periodic Zn-carboxylate layers. The MIM linker is a [2]rotaxane with a [24]crown-6 (24C6) macrocycle and an aniline-based axle with terminal pyridine donor groups. The single-crystal X-ray structures of MOFs UWDM-2 (1,4-diazophenyl-dicarboxylate) and UWDM-3 (1,4-biphenyl-dicarboxylate) show that both frameworks are large enough to contain the free volume required for rotation of the interlocked 24C6 macrocycle, but the frameworks are interpenetrated (UWDM-2, three-fold, and UWDM-3, two-fold). In particular, for UWDM-3 the 24C6 rings of the pillaring MIM are positioned directly inside the square openings of neighboring zinc dicarboxylate layers. Variable-temperature (VT) ²H SSNMR demonstrated that the 24C6 macrocycles in UWDM-2 and UWDM-3 can only undergo restricted motions related to ring flexibility or partial rotation but are incapable of undergoing free rotation. VT-powder X-ray diffraction studies showed that upon activation of UWDM-3, by removing solvent, a phase change occurs. The new β-phase of UWDM-3 retained crystallinity, and ²H SSNMR demonstrated that the 24C6 macrocyclic ring of the pillared MIM strut is now free enough to undergo full rotation. Most importantly, the phase change is reversible; the β version of the MOF can be reverted to the original α state by resolvation, thus demonstrating, for the first time, that the dynamics of a MIM inside a solid material can be controlled by a reversible phase change.

Full text of DOI:10.1021/ja502238a

Article
pubs.acs.org/JACS
Metal−Organic Frameworks with Mechanically Interlocked Pillars:
Controlling Ring Dynamics in the Solid-State via a Reversible Phase
Change
Kelong Zhu, V. Nicholas Vukotic, Christopher A. O’Keefe, Robert W. Schurko,* and Stephen J. Loeb*
Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario Canada, N9B 3P4
S
* Supporting Information
ABSTRACT: Metal−organic framework (MOF) materials
have been prepared that contain a mechanically interlocked
molecule (MIM) as the pillaring strut between two periodic
Zn-carboxylate layers. The MIM linker is a [2]rotaxane with a
[24]crown-6 (24C6) macrocycle and an aniline-based axle
with terminal pyridine donor groups. The single-crystal X-ray
structures of MOFs UWDM-2 (1,4-diazophenyl-dicarboxy-
late) and UWDM-3 (1,4-biphenyl-dicarboxylate) show that
both frameworks are large enough to contain the free volume required for rotation of the interlocked 24C6 macrocycle, but the
frameworks are interpenetrated (UWDM-2, three-fold, and UWDM-3, two-fold). In particular, for UWDM-3 the 24C6 rings of
the pillaring MIM are positioned directly inside the square openings of neighboring zinc dicarboxylate layers. Variable-
2
temperature (VT) H SSNMR demonstrated that the 24C6 macrocycles in UWDM-2 and UWDM-3 can only undergo
restricted motions related to ring flexibility or partial rotation but are incapable of undergoing free rotation. VT-powder X-ray
diffraction studies showed that upon activation of UWDM-3, by removing solvent, a phase change occurs. The new β-phase of
UWDM-3 retained crystallinity, and 2H SSNMR demonstrated that the 24C6 macrocyclic ring of the pillared MIM strut is now
free enough to undergo full rotation. Most importantly, the phase change is reversible; the β version of the MOF can be reverted
to the original α state by resolvation, thus demonstrating, for the first time, that the dynamics of a MIM inside a solid material can
be controlled by a reversible phase change.
controlled by an external perturbation; in this case via a
reversible phase change of the material.
1. INTRODUCTION
Molecular switches based on mechanically interlocked mole-
cules (MIMs) function well in controlled solution environ-
ments,1 but in this medium their motion is incoherent, and the
molecules widely dispersed. In an effort to achieve a higher
level of molecular organization and create functional materials
employing interlocked components, it has been suggested that
MIMs could be organized inside the pores of metal−organic
framework (MOF) materials.2 To this end, we recently
reported a prototype MOF, UWDM-1, which contains a
large and flexible [24]crown-6 (24C6) macrocycle interlocked
onto a rigid aromatic framework and demonstrated that upon
activation of the material, free volume is created inside the
pores of the material, and thermally driven rotation of the
macrocycle can be observed directly by 2H SSNMR.3 We
believe the next steps in this chemistry are to learn how the
internal structure of the MOF affects the motion exhibited by
the interlocked macrocycle, and more importantly, how to
achieve control over the motion.
2. RESULTS AND DISCUSSION
MIM Pillaring Ligand. Scheme 1 outlines the synthesis of
the pillaring ligand LMIM used in the construction of these
hybrid linker MOFs. Ring closing metathesis4 was used to
create a 24-membered macrocycle around an anilinium-based
axle with 4-bromo substituted xylene groups as the stoppers. A
standard Suzuki coupling protocol was then used to replace the
bromine atoms with 4-pyridyl groups in good yield. Finally, the
double bond of the macrocycle (E and Z isomers) could be
hydrogenated (or deuterated) employing H2 (D2) gas and Pd/
C. The structure of the neutral LMIM was determined by single-
crystal X-ray diffraction (XRD) and is shown in Figure 1. The
length of this new bipyridine ligand is 17.3 Å (N···N), while the
diameter around the central macrocycle is ca. 11.2 Å. A single
NH hydrogen-bond (N···O 3.44 Å; NH···O, 160°) is the major
noncovalent interaction remaining after removal of the
templating anilinium charge. The four methyl groups prevent
unthreading of the 24-membered ring, thus ensuring the
Herein, we report a new type of MIM-pillared MOF and
show how changing the internal structure of the MOF can
affect the type of motion (e.g., conformational flexing, partial
rotation, or full rotation) exhibited by the macrocycle. Most
importantly, we demonstrate for the first time that these large
amplitude dynamics associated with a MIM linker can be
Received: March 4, 2014
© XXXX American Chemical Society
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acid. Of the three acid linkers utilized, 4,4′-azodiphenyl-
dicarboxylic acid (ADC), 2,6-naphthyl-dicarboxyclic acid
(NDC), and 4,4′-biphenyldicarboxylic acid (BPDC), good
yields of a MOF were obtained only with the longer acids ADC
and BPDC. It was rationalized that since the shorter NDC
linker would produce a Zn···Zn distance of only 13.0 Å in the
two-periodic layer there would be steric interactions between
macrocycles of adjacent LMIM pillars, and this ultimately
prevented formation of the target pillared MOF. On the
other hand, analogous spacings of 15.2 and 17.0 Å for BPDC
and ADC, respectively, were large enough to accommodate
side-by-side alignment of [2]rotaxane pillars; thus, formation of
a pillared MOF material was successful as designed.
Scheme 1. Synthesis of a MIM Ligand (LMIM) Designed for
Pillaring inside a MOF
a
The two MOFs with bridging dicarboxylate linkers ADC and
BPDC were designated UWDM-2 and UWDM-3 (UWDM =
University of Windsor Dynamic Material), respectively, and
isolated as homogeneous materials in good yield with formulas
[Zn2(BPDC)2(LMIM)](DMF)2 and [Zn2(ADC)2(LMIM)]-
(DMF)13 (see SI for details). Both were analyzed by single-
crystal X-ray analysis, and the structures of the basic
frameworks are shown in Figure 2.
As designed, the framework structures of UWDM-2 and
UWDM-3 consist of two-periodic grids constructed from
Zn(II) paddlewheels and carboxylate ligands with the
bipyridine ligand LMIM pillaring the layers. Although the
structures appear to contain large openings sufficient for
potentially unimpeded motion of the interlocked macrocycles
as desired, both of these structures are interpenetrated;
UWDM-2 is three-fold interpenetrated, while UWDM-3 is
two-fold interpenetrated.
a
(i) Grubbs’ first-generation catalyst, CH3NO2/CH2Cl2 (1:9), 48 h,
reflux; (ii) NEt3/CH3OH; (iii) catalytic Pd(PPh3)4, DMF/toluene, 12
h, 110 °C; and (iv) 10% Pd/C, H2 (or D2), CH3OH, 3 h, RT.
Drawings depicting the interpenetration of the lattices for
UWDM-2 and UWDM-3 are in Figure 3. For UWDM-2, the
24C6 ring is positioned to one corner and slightly above the
square grid defined by the Zn-carboxylate grid. For UWDM-3,
two-fold interpretation actually places the macrocyclic 24C6
ring directly inside the Zn-carboxylate square frameworkakin
to a “round peg in a square hole”. It is quite clear from
examination of these solid-state structures that in both cases the
24C6 macrocycles are tightly packed on account of being
positioned close to the square grid framework and the lack of
any large internal voids due to interpenetrations.
Unit cells for UWDM-2 and UWDM-3 were calculated
(PLATON)9 to contain void spaces of 17 and 50% occupied by
8 and 104 molecules of DMF, respectively. Thermal gravimetric
analysis showed gradual weight loss attributable to DMF
removal and eventual decomposition of the MOF frameworks
at 370 and 250 °C, respectively (see SI for details).
Figure 1. A ball-and-stick representation of the single-crystal X-ray
structure of the bipyridine, pillaring ligand LMIM showing a single
NH···O interaction between the axle and wheel components. Color
key: red = oxygen, blue = nitrogen, black = carbon; gold bonds = axle,
silver bonds = wheel.
The stability of UWDM-2 was verified by powder X-ray
diffraction (PXRD) which showed very little variation when the
material was dried under vacuum (Figure 4). Interestingly,
however, VT-PXRD for UWDM-3 demonstrated that this
material (α-UWDM-3) goes through a gradual phase change
over the temperature range of 100−125 °C (Figure 5),
resulting in a second, stable crystalline phase (β-UWDM-3).
The new β-UWDM-3 phase can be attributed to desolvation
and tilting of the pillaring strut which brings the carboxylate
layers closer together and reduces the volume accessible for
solvent molecules. This type of deformation is a known
structural feature for a number of similar pillared/layered
MOFs.10 Unfortunately, attempts to index this new phase
(DICVOL, DASH)11 and unambiguously determine cell
parameters for β-UWDM-3 were not successful due to the
broadness of the PXRD patterns. This α to β phase change can
[2]rotaxane remains a permanently interlocked entity through-
out synthesis and postsynthetic modification of the MOF.
MOF Materials. It has been well established by Kim,5
Hupp,6 and others7 that linear bipyridine ligands can be used as
pillaring struts between two-periodic layers of zinc carboxylate
paddlewheel structures to form robust MOFs. In fact, varying
the complexity of this pillaring ligand has been a popular
strategy for the incorporation of functional groups into MOFs.8
Following the same solvo-thermal protocols, ligand LMIM was
combined with Zn(NO3)2·(H2O)x and 2 equiv of a dicarboxylic
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Figure 3. Ball-and-stick representations of the single-crystal X-ray
structures of UWDM-2 (top) and UWDM-3 (bottom) showing the
interpenetrated lattice frameworks (UWDM-2 is three-fold inter-
penetrated, and UWDM-3 is two-fold interpenetrated). Space filling
models show the position of the macrocycle relative to the square
framework grid. Color key: blue = aniline axle, red = 24C6 wheel,
yellow = carboxylate linkers, green = zinc atoms.
Figure 2. Ball-and-stick representations of the single-crystal X-ray
structures of UWDM-2 (top) containing ADC linkers and UWDM-3
(bottom) containing BPDC linkers, showing a portion of one of the
interpenetrated lattice frameworks. Color key: blue = aniline axle, red
= 24C6 wheel, yellow = carboxylate linkers, green = zinc atoms.
Figure 4. PXRD patterns for UWDM-2: (a) simulated from single-
crystal XRD data; (b) as-synthesized material; and (c) exchanged with
CH2Cl2 and dried in vacuum at RT for 6 h.
also be initiated by solvent exchange with CH2Cl2 and gentle
drying of the sample. Most importantly, the phase change can
be reversed by re-exposure of the desolvated sample to DMF as
shown in Figure 5.
in temperature. Three motional regimes are defined on the
2
basis of the typical range of magnitudes of the H quadrupolar
Dynamics in the Solid State. In order to characterize the
mobility of the macrocyclic 24C6 rings inside these MOFs,
UWDM-2 and UWDM-3 were prepared containing deuterium
labels via the addition of D2 to the double bond created by
coupling constant for terminal C−D bonds (i.e., 100−180
kHz): the slow motion limit (SML), where rates are <103 Hz;
the intermediate motion regime (IMR), where rates are
between 104 and 107 Hz; and the fast motion limit (FML),
where rates are >107 Hz. Motions in the SML and FML can
effectively be modeled as time averages of the three principal
12
2
RCM and the resulting materials probed by H SSNMR. 2H
SSNMR powder patterns are extremely sensitive to molecular-
level dynamical motions and act as exquisite probes of changes
in the types of motions and their rates that arise from variation
2
components of the H electric field gradient (EFG) tensor,
whereas motions in the IMR require modeling with an
C
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Figure 5. PXRD patterns for UWDM-3 at various temperatures: (a)
simulated from single-crystal XRD data; as-synthesized material at (b)
25, (c) 50, (d) 100, and (e) 125 °C; (f) after exchange of solvent from
DMF to CH2Cl2 and then drying under vacuum at room temperature,
(g) sample (f) at 25 °C after soaking in DMF for 12 h.
Figure 6. Experimental (left) and simulated (right) 2H SSNMR
powder patterns as a function of temperature for UWDM-2.
Illustrations (below) of the motions of the 24C6 macrocyclic ring
relative to the framework axle: (a) the SML where no motion is
occurring on the NMR time scale, (b) the FML for jumps between
two sites 72° apart, and (c) partial rotation of the ring over 225° in 45°
steps.
exchange matrix and generally result in powder patterns of
increasing complexity that vary with both changes in temper-
ature and echo spacing. Details of the simulations including
tables of relevant rates can be found in the SI.
considerably, but at the high-temperature limit (424 K) begin
to broaden again. Ratcliffe et al. observed a similar
phenomenon for solid complexes of [18]crown-6.13 That this
hindered motion (i.e., two-site jumps and partial rotation
motion) does not occur at rates in the FML is consistent with
the constrained nature of the ring in this system.
For UWDM-2, which utilizes the longest ADC linker, three-
fold interpenetration occurs which severely restricts the
possible motion of the 24C6 macrocycle. Indeed, any large
amplitude ring flexing would likely result in significant,
unfavorable interactions between the rings. The static spectrum
acquired at 171 K was simulated as a single site using
quadrupolar parameters that are typical for (i.e., CQ = 165 kHz
and ηQ = 0.0) this type of C−D alkyl linakge. At this
temperature, motion of the crown ethers is too slow to affect
Although the single-crystal X-ray structure of α-UWDM-3
showed that this framework is only two-fold interpenetrated,
the 24C6 ring is confined within a square of the zinc-
carboxylate grid. It was therefore postulated that interactions
between the ring and the framework would also hinder the
motion of the ring in this material. The experimental and
2
the H powder patterns, i.e., the slow motion limit. Increasing
the temperature to ca. 255 K caused variations in the powder
pattern shapes which indicate that motions were occurring at a
rate of ∼10−100 kHz, well within the IMR. The spectra were
simulated using a two-site jump of the CD2 group with an angle
of 72° (Figure 6b) combined with a partial rotation motion,
which is rotation of the macrocycle through 225° in 45° steps,
such that there is a single hydrogen-bonding contact between
the pillar and macrocycle (Figure 6c). This motion is described
as a partial rotation, since the macrocycle is not free to make a
complete rotation about the pillar. The sites are connected such
that successive jumps can occur in either direction, but only to
adjacent oxygen atoms on the ring and not through the alkyl
portion of the macrocycle. The rate of the two-site jump
motion is lower than that of the partial ring rotation, as the
conformational changes induced in the ring from the latter
result in steric interactions between rings. The patterns narrow
2
simulated VT H SSNMR spectra for α-UWDM-3 are shown
in Figure 7. The low-temperature spectrum collected at 234 K
was simulated with a single set of quadrupolar parameters (vide
supra). This SML spectrum was obtained at a higher
temperature than that previously observed for UWDM-1,
suggesting that the motions of the 24C6 ring in α-UWDM-3
are indeed hindered by comparison.
Increasing the temperature to 247 K produced a distinct
spectrum that could be simulated as a two-site jump of the CD2
groups between sites separated by an angle of 75° (Figure 7b).
At 255 K, the width of the spectrum is less than half that of the
low-temperature spectrum, indicating that the lone two-site
jump model can no longer be used. Collecting spectra at this
temperature with different pulse spacings also produced notable
changes in the powder pattern; thus, it can be concluded that
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Figure 7. Experimental (left) and simulated (right) 2H SSNMR
powder patterns as a function of temperature for α-UWDM-3.
Illustrations (below) of the motions of the 24C6 macrocyclic ring
relative to the framework axle: (a) the SML where no motion is
occurring on the NMR time scale; (b) the FML for jumps between
two sites 75° apart; (c) partial rotation of the ring over 225° in 45°
steps; and (d) large amplitude jumps of the alkyl portion of the ring
through 60° between positions above and below the square of the
framework.
Figure 8. Experimental (left) and simulated (right) 2H SSNMR
powder patterns as a function of temperature for β-UWDM-3.
Illustrations (below) of the motions of the 24C6 macrocyclic ring
relative to the framework axle: (a) the SML where no motion is
occurring on the NMR time scale; (b) the FML for jumps between
two sites 75° apart; (c) partial rotation of the ring over 225° in 45°
steps; and (d) partial rotation of the ring over 225° in 45° steps
combined with jumps over the alkyl portion of the ring resulting in full
rotation.
the rates of the new motions are within the IMR. The spectrum
was simulated by considering the two-site jump of the CD2
groups and the onset of partial rotation, as described above for
UWDM-2.
simulated using the familiar two-site jump of the CD2 groups
through an angle of 75° (Figure 8b). Increasing the
temperature further induces the onset of partial rotation
motion through 225° in 45° increments (Figure 8c), in addition
to the aforementioned two-site jump motion. The spectrum at
318 K was simulated with rates in the FML for this combined
motion. The temperature at which this mode of motion occurs
is similar to that of UWDM-1 (324 K), again validating the
congruous motions in these two systems.
The spectra acquired at higher temperatures could not be
simulated using the combined two-site jump and partial
rotation model, nor could they be simulated with free rotation
of the crown ether ring about an axis of Cn ≥ 3 symmetry. The
simulation of the high temperature spectra was achieved by
considering a combination of motions best described as full
rotation (i.e., an intermediate case between partial rotation and
free rotation) combined with the two-site jump. Specifically,
eight sites separated by 45° rotations were used: six
corresponding to the hydrogen-bonding oxygen atom positions
and two corresponding to the alkyl portion of the ring (Figure
8d). A rate matrix was constructed (see SI) where jumps
between the oxygen atom positions are occurring at rates in the
FML (i.e., >107 Hz) and jumps through the alkyl positions
occur at a significantly slower rate in the IMR (i.e., 50−1000
kHz). The spectra acquired at 381 and 402 K were simulated
Further increasing the temperature produced significantly
narrower patterns that could not be accounted for with only a
combination of two-site jumps and partial ring rotation. The 2H
NMR spectra acquired at 318 and 339 K were simulated by
including a third mode of motion: large amplitude jumps of the
deuterons through an angle of 60° about an axis in the plane of
the ring (Figure 7d). This motion was attributed to the flexible
alkyl portion of the ring moving 60° between positions above
and below the constraining square of the Zn-carboxylate layer
without significantly altering the length of the NH···O
hydrogen bonds, in order to minimize short-range interactions
between the macrocyclic ring and the surrounding framework.
For β-UWDM-3, it is postulated that the macrocycle no
longer sits within the square of the Zn-carboxylate framework
and that the 24C6 ring is more mobile in this less constrained
2
environment. This notion is supported by the VT H SSNMR
spectra which are strikingly similar to those previously observed
for UWDM-1 (see SI for a detailed comparison). The static
spectrum at 183 K was simulated as a single site, and increasing
the temperature to 208 K resulted in no significant change in
the powder pattern (Figure 8a). The spectrum acquired at 234
K indicates the onset of motion, and the spectrum at 255 K was
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using this model, and excellent agreement with the
experimental data was obtained.
Ontario Innovation Trust, and the University of Windsor for
support of the XRD and solid-state NMR facilities at the
University of Windsor. Additional support was provided to
S.J.L. through the NSERC Canada Research Chair (CRC)
program and to R.W.S. through an Early Researcher Award
(ERA) from the Province of Ontario.
3. CONCLUSIONS
It has been demonstrated that the dynamic motion of a
macrocyclic wheel inside a MOF can be controlled by a
reversible phase change in the framework of the material; this
concept is illustrated in Figure 9. It is not surprising that the
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details describing the synthesis and character-
ization of all new compounds including solution NMR
assignments, full characterization details for all new materials,
and the details of all SSNMR experiments and simulations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
loeb@uwindsor.ca
■
schurko@uwindsor.ca
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
S.J.L. and R.W.S. are grateful for the awarding of NSERC of
Canada Discovery grants in support of this research. S.J.L. and
R.W.S. also thank the Canadian Foundation for Innovation, the
F
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