Formation of a Polythreaded, Metal–Organic Framework Utilizing an Interlocked Hexadentate, Carboxylate Linker

Article abstract of DOI:10.1002/ejic.201600311

A unique hexacarboxylate linker was prepared on the basis of an interlocked [2]rotaxane motif. The linker contains four carboxylate groups attached to a rigid, H-shaped axle and two carboxylate units appended to a crown ether wheel. The resulting Zn^II-based metal–organic framework material has a unique lattice, in which three independent 3-periodic frameworks (threefold interpenetration) are interconnected only by virtue of the threading of their individual components in the rotaxane linker.

Full text of DOI:10.1002/ejic.201600311

DOI: 10.1002/ejic.201600311
Full Paper
Polythreaded MOFs
Formation of a Polythreaded, Metal–Organic Framework
Utilizing an Interlocked Hexadentate, Carboxylate Linker
Ghazale Gholami,[a] Kelong Zhu,[a] Jas S. Ward,[b] Paul E. Kruger,[b] and Stephen J. Loeb*[a]
Abstract: A unique hexacarboxylate linker was prepared on unique lattice, in which three independent 3-periodic frame-
the basis of an interlocked [2]rotaxane motif. The linker contains
four carboxylate groups attached to a rigid, H-shaped axle and
two carboxylate units appended to a crown ether wheel. The
resulting ZnII-based metal–organic framework material has a
works (threefold interpenetration) are interconnected only by
virtue of the threading of their individual components in the
rotaxane linker.
Introduction
be interpenetrated only by virtue of the mechanically inter-
locked nature of the ligand core – a MIM node. Herein, we
describe the first example of a metal–carboxylate-derived MOF
that combines the interpenetration of frameworks owing to an
interlocked ligand node with the classical interpenetration of
frameworks that result from the filling of a void in a lattice.[14]
The MOF structure is thus best described as being polythreaded
by virtue of this unique combination of two types of interpene-
tration, that is, through linker (mechanically interlocked) and
lattice (n-fold interpenetration).[15]
The incorporation of mechanically interlocked molecules
(MIMs)[1] such as rotaxanes, catenanes, and knots into polymeric
materials[2] has become an important area of study to deter-
mine the effect that these unique molecular connections might
have on the mechanical[3] (rheology, elasticity), electronic[4] (fer-
roelectric, piezoelectric), optical[5] (nonlinear optics), or host–
guest[6] (absorption, storage) properties of the resulting materi-
als. Most recently, there has been interest in the incorporation
of MIMs into metal–organic framework (MOF) materials as a
method to organize molecular switches and potentially molecu-
lar machines in the solid state.[7] In particular, novel MOFs have
been prepared with rotaxanes as linkers and large amplitude
rotational and translational motion (shuttling) characterized by
solid-state NMR spectroscopy.[8] These types of materials, often
called metal–organic rotaxane frameworks (MORFs),[9] were first
prepared by simply adding donor atoms to the ends of a linear
axle that could thread through a macrocyclic wheel such as a
crown ether or a cucurbituril.[10] This methodology led to exam-
ples of 1-, 2- and 3-periodic structures containing [2]rotaxane
axles as linkers between metal nodes with macrocyclic wheels
threaded onto the framework skeleton.[11] This was followed by
a few examples in which the framework was constructed by
utilizing donors attached to the wheels.[12]
Results and Discussion
A hexadentate carboxylic acid ligand was constructed by using
a previously successful molecular design for a [2]rotaxane that
combines a rigid H-shaped axle containing two benzimidazole
recognition sites and a single macrocyclic wheel in the form of
a crown ether.[16] Four of the carboxylic acid groups are at-
tached to the ends of the H-shaped axle and the other two are
appended to the crown ether wheel. Scheme 1 describes the
preparation of the [2]rotaxane linker in two steps from known
precursors by capping a preformed [2]pseudorotaxane [1⊂2]+
with 3 followed by hydrolysis of resulting hexaester 4 to yield
hexaacid L.
We reasoned that it should also be possible to create a [2]-
rotaxane linker with donor atoms attached to both the axle and
the wheel.[13] This is a particularly attractive idea, because it
might be possible for the axle and wheel to be involved in the
propagation of completely independent frameworks that would
1
The H NMR spectrum of hexaester, [2]rotaxane 4 is shown
in Figure 1; although similar data was obtained for hexaacid, L,
the low solubility of this compound limited the utility of these
spectra for full characterization and signal assignment (see the
Supporting Information). In the neutral forms of these [2]rotax-
anes, there is very little interaction between the axle and wheel
components. Only two weak NH···O interactions between the
benzimidazole NH groups and the crown ether O atoms remain
from the charge-assisted, noncovalent interaction used to tem-
plate the formation of this species. Therefore, although the axle
is quite rigid and the macrocycle is permanently locked to the
axle, there are a wide variety of possible orientations for the
carboxylate groups of the two interlocked components owing
[a] Department of Chemistry and Biochemistry, University of Windsor,
Windsor, Ontario N9B 3P4, Canada
E-mail: loeb@uwindsor.ca
www1.uwindsor.ca/people/loeb/
[b] MacDiarmid Institute for Advanced Materials and Nanotechnology,
Department of Chemistry, University of Canterbury,
Private Bag 4800, Christchurch 8041, New Zealand
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600311.
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two independent centrosymmetric molecules). This is consist-
ent with the chemical shift data observed in solution by
1H NMR spectroscopy. Although X-ray structures previously de-
termined for similar H-shaped rotaxanes most often reveal that
a single macrocyclic wheel such as dibenzo[24]crown-8 ether
prefers to occupy one end of the axle, interacting with only a
single benzimidazole recognition site,[16,8c] there is precedent
for larger macrocycles such as dibenzo[30]crown-10 (DB30C10)
or bis-meta-phenylene[26]crown-8 (BMP26C8) to interact simul-
taneously with both NH groups in this fashion.[17] Most impor-
tantly, it is clear from the solid-state structure that the six carb-
oxylic acid groups are well separated and each could potentially
act as an independent donor group to a metal centered sec-
ondary building unit (SBU) upon deprotonation, that is, result
in the formation of a MOF.
The reaction of L with zinc nitrate in a solvent mixture of
DMF/1,4-dioxane/H2O (1:1:1) at 90 °C for 48 h produced a solid
material in the form of yellow crystals (58 % yield). The material
was subsequently analyzed by single-crystal X-ray diffraction
and was found to have the formula {[NH2Me2]2[Zn2-
(H2O)2(L)](H2O)1.25}. Figure 3 (a) shows the repeating unit of the
material, in which all of the carboxylate ligands of L are coordi-
nated to a SBU consisting of a single ZnII ion. The combination
of two independent ZnII ions and a linker with six negative
charges results in a formally anionic framework. This negative
Scheme 1. Synthesis of interlocked, hexadentate carboxylic acid ligand L.
Labels are assignments for the resonances observed in the 1H NMR spectrum
shown in Figure 1.
to the large amplitude and rotational and translational degrees
of freedom available for this molecule.
Figure 1. 1H NMR spectrum of hexaester 4 with resonance assignments corre-
sponding to the labeling shown in Scheme 1.
The single-crystal X-ray structure of linker L was determined
from crystals with formula L·(DMSO)7 (see Figure 2). The two
molecules of L in the asymmetric unit both have a crystallo-
graphically imposed center of symmetry and differ only in how
they interact with the DMSO solvent molecules. The central
phenyl ring and the two benzimidazole groups of the H-shaped
axle lie in approximately the same plane, whereas the macrocy-
cle is positioned over the central phenyl ring and bridges the
two benzimidazole groups to form two NH···O hydrogen bonds
(N···O, 2.90 Å; N–H···O 155° and N···O, 2.92 Å; N–H···O 167° for
Figure 2. Crystal structure of L·(DMSO)7. (Top) View of the H-shaped axle and
orientation of the four COOH groups. (Bottom) View emphasizing the crown
ether wheel and positions of two other COOH groups. Color key: C black, N
blue, O red, H white, axle gold, wheel silver, H-bonds dashed lines. DMSO
and H atoms not involved in H-bonding are omitted for clarity. Only one of
the molecules in the asymmetric unit is shown.
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charge is balanced by the presence of two cations in the form in which the interpenetrating lattices simply fill void spaces
of dimethylammonium ions, [NH2Me2]+, produced during the within the structure, each one of these three lattices must pass
solvothermal synthesis from the decomposition of DMF.[18] Fig- through the other two by virtue of the interlocked nature of
ure 3 (c) shows the detailed coordination spheres of the two the linker axle and wheel. Figure 4 (b, c) shows how a second
independent ZnII centers. Each ZnII ion has approximately tetra- and then a third lattice interpenetrates through this linker-
hedral geometry and coordinates to three different carboxylate based, mechanical bonding motif. The result is a tightly packed
groups: two from the axle (A), opposite ends of a single triphen- material with an anionic framework, in which the only sizeable
ylene strut, and one from the macrocyclic wheel (W), as well as voids are filled with the charge-balancing [NH2Me2]+ cations
a single water molecule.
and a small pocket close to one of the benzimidazole NH
groups is occupied by hydrogen-bonded water molecules. Cal-
culations (PLATON) show a total void space of <2 %.[19]
Figure 3. Crystal structure of MOF {[NH2Me2]2[Zn2(H2O)2(L)]}. (a) The inter-
locked core of the MOF showing all bonds to ZnII; coordinated H2O not
shown. (b) The core unit emphasizing the rotation of the wheel relative to
the axle. (c) The coordination spheres of the two unique ZnII ions. A = axle,
W = wheel. Color key: C black, N blue, O red, Zn1 teal, Zn2 cyan, axle gold,
wheel silver. Cations, solvent, and hydrogen atoms are omitted for clarity.
The coordination vectors of the four axle carboxylate groups
are essentially coplanar (Figure 3, a) with those of the macrocy-
cle, which are oriented in opposite directions (≈180° apart), off-
set by ca. 25° owing to rotation of the ring relative to the H-axle
plane (Figure 3, b). There are no significant bonding interactions
between the axle and wheel; the closest NH···O interaction that
could be construed as hydrogen bonding shows a N···O dis-
tance of 4.04 Å with a N–H···O angle of 159°. Thus, the position-
ing of the macrocycle with respect to the wheel is almost cer-
tainly dictated by optimizing coordination to the ZnII ions and
packing the molecular components rather than the noncova-
lent interactions between axle and wheel observed for the
structure of the free ligand.
Figure 4. Crystal structure of the MOF {[NH2Me2]2[Zn2(H2O)2(L)]}. a) A single
2-periodic, framework (red) with alternating axles and wheels. b) Two of the
frameworks (red and blue) interpenetrated through the linker. c) Three of the
frameworks (red, blue and green) interpenetrated through the linker. d) A
space-filling model of the three interpenetrated frameworks. Color key:
frameworks red, blue, green, Zn dark blue. For a), b), and c) hydrogen atoms
have been omitted for clarity.
The overall framework is best described as arising from the
threefold interpenetration of individual lattices constructed
Within one of these single lattices (see Figure 4, a, red) the
from alternating axles and wheels, as shown in part (a) of Fig- various Zn···Zn distances are, for the H-shaped axle: (1) on the
ure 4 (red). However, unlike classical interpenetration for MOFs, same triphenylene strut, 19.2 and 19.3 Å; (2) between different
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struts, 10.4 and 15.8 Å; (3) diagonally, 22.8 and 23.5 Å; (4) be- Experimental Section
tween two ends of a single macrocycle, 19.0 Å; and (5) between
an axle and a wheel, 5.6 and 8.2 Å.
General Methods: 2,1,3-Benzothiadiazole was purchased from TCI
Chemicals, Co. 4-Ethoxycarbonylphenylboronic acid, triethylene gly-
col di(p-toluenesulfonate), and 3,5-dihydroxybenzoic acid were pur-
chased from Sigma–Aldrich, Co., and were used as received. Com-
pounds 1,[8c] 2,[22] and 3[8c] (see Scheme 1) were prepared accord-
ing to literature methods. Deuterated solvents were obtained from
Cambridge Isotope Laboratories and were used as received. Sol-
vents were dried by using an Innovative Technologies Solvent Purifi-
cation System. 1H NMR and 13C NMR solution experiments were
performed with either a Bruker Avance 300 or 500 instrument at
298 K unless otherwise indicated. Chemical shifts are quoted in
ppm relative to tetramethylsilane by using the residual solvent sig-
nal as a reference standard. PXRD were recorded with a Bruker D8
Discover diffractometer equipped with a GADDS 2D-detector and
operated at 40 kV and 40 mA. Cu-Kα radiation (λ = 1.54187 Å) was
used. Elemental compositions were determined with a Perkin–Elmer
2400 Series II Elemental Analyzer. Thermal gravimetric analyses
were conducted with a Mettler Toledo TGA SDTA 851e instrument.
Helium (99.99 %) was used to purge the system with a flow rate of
The thermal stability of the MOF material was evaluated by
variable-temperature powder X-ray diffraction (VT-PXRD), which
showed that over the temperature range of 25 to 200 °C there
was essentially no loss of crystallinity (see Figure 5). These ob-
servations are consistent with the nonporous nature of the ma-
terial. Notably, many materials that contain interpenetrated lat-
tices undergo phase changes or rapid loss of crystallinity owing
to the relative motion of one lattice with respect to the others.
Given that this material is threaded by interpenetrating one
coordinating component through another by formation of a
mechanically interlocked linker, this type of lattice motion is
much more restricted, and this presumably results in a more
robust material. All figures were generated by using the Crystal-
Maker software.[21]
30 mL min–1
.
Preparation of 4: A mixture of 1 (100 mg, 0.184 mmol) and 2
(300 mg, 0.552 mmol) was stirred in CHCl3 (30 mL) until a clear
solution was obtained. Then, 3 (74.4 mg, 0.184 mmol) was added,
which was followed by the addition of a small amount of ZrCl4
(4.28 mg, 0.0184 mmol); this reaction needed to be conducted un-
der an atmosphere of air to effect oxidation. After stirring at room
temperature for 2 d, the catalyst was filtered off and the filtrate was
concentrated under vacuum. The residue was dissolved in aceto-
nitrile (15 mL) and filtered. Et3N (0.5 mL) was added, and after
5 min, a precipitate appeared, which was filtered, yield 82 mg, 30 %,
m.p. >250 °C (dec.). 1H NMR (500 MHz, CDCl3): δ = 11.07 (s, 2 H),
8.27 (s, 4 H), 8.25 (d, J = 8.24 Hz, 4 H), 8.18 (m, 8 H), 7.87 (m, 4 H),
7.52 (d, J = 7.50 Hz, 2 H), 7.31 (d, J = 7.31 Hz, 2 H), 6.61 (d, J =
6.61 Hz, 4 H), 5.26 (t, J = 5.26 Hz, 2 H), 4.42 (m, 8 H), 3.74 (m, 8 H),
3.65 (m, 8 H), 3.52 (s, 8 H), 3.42 (s, 6 H), 1.46 (q, J = 1.46 Hz, 12 H)
ppm. 13C NMR (125 MHz, CDCl3): δ = 166.56, 166.03, 165.74, 158.08,
152.27, 143.19, 142.65, 142.08, 132.99, 130.49, 129.91, 129.52,
129.15, 129.04, 128.82, 128.72, 128.50, 128.01, 124.48, 123.12,
121.42, 106.38, 106.02, 70.10, 66.57, 66.17, 60.92, 60.51, 51.35,
14.08 ppm. C84H82N4O20 (1467.59): calcd. C 68.75, H 5.63, N 3.82;
found C 68.04, H 4.49, N 3.63.
Figure 5. VT-PXRD data for the MOF {[NH2Me2]2[Zn2(H2O)2(L)]} (simulated pat-
tern created by using Diamond 4.0 computer software[20]).
Preparation of L: 1
M
NaOH (10 mL) was added to a mixture of 4
Conclusions
(80 mg, 0.054 mmol) in THF (6 mL) and EtOH (10 mL). After the
mixture was stirred at 80 °C for 20 h, the solvent was removed
under reduced pressure. Deionized water (20 mL) was added to
dissolve the residue. Then, HBF4 (48 wt.-% in water) was added to
The hexadentate carboxylate linker L provides a truly unique
coordination environment for constructing a MOF lattice. In par-
ticular, the interlocking of the two components (axle and give a pH value of about 6.5. The precipitate that was formed was
collected by vacuum filtration and was air dried, yield 65 mg, 90 %,
m.p. >250 °C (dec.). H NMR (500 MHz, [D6]DMSO, HBF4 was added
wheel) provides a highly dynamic scaffold and guarantees that
any lattices formed will be threaded. The MOF described herein
is a simple example in which only single ZnII ion SBUs are used.
The threefold interpenetration of the lattices is a direct result
of the interlocked nature of the rotaxane ligand design. This
concept adds another level of complexity to the design of inter-
penetrated and polythreaded lattices and may find applications
in the future design of MOF materials with dynamic and me-
chanical components and the control of bulk materials proper-
ties at the molecular level.
1
to obtain well-resolved signals): δ = 13.51 (br., 6 H, NH, OH, HBF4),
8.42 (s, 4 H), 8.20 (d, J = 8.20 Hz, 8 H), 8.02 (d, J = 8.02 Hz, 8 H),
7.54 (s, 4 H), 6.55 (s, 4 H), 5.60 (s, 2 H), 3.75 (s, 8 H), 3.60 (s, 16 H)
ppm. 13C NMR (125 MHz, [D6]DMSO): δ = 167.72, 166.96, 158.77,
151.46, 141.78, 140.50, 132.10, 131.44, 130.59, 130.26, 129.78,
129.14, 108.39, 106.84, 105.62, 70.14, 69.48, 66.86 ppm. C74H62N4O20
(1327.30): calcd. C 66.96, H 4.71, N 4.22. Satisfactory elemental anal-
ysis could not be obtained owing to the limited solubility and hy-
1
groscopic nature of the compound. H NMR spectroscopy and sin-
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gle-crystal X-ray diffraction studies are consistent with the proposed
formula and structure. Suitable single crystals were obtained by
slow diffusion of methanol into a DMSO solution of L at room tem-
perature.
Keywords: Metal–organic frameworks · Rotaxanes · Zinc ·
Carboxylate ligands · Interpenetration
Preparation of MOF [NH2Me2]2[Zn2(H2O)2(L)]: Zn(NO3)2·6H2O
(8.92 mg, 0.03 mmol) and ligand L (10 mg, 0.0075 mmol) were
added to a solution of DMF/1,4-dioxane/H2O (1:1:1; 3 mL), to which
HBF4 diethyl ether complex (3 drops) was added. Upon sonication
and heating, a clear solution was obtained, and the sample was
placed in a programmable oven and heated at a constant rate of
1 °C min–1 to 90 °C, kept at that temperature for 48 h, then placed
under vacuum in an oven heated to 100 °C for another 5 h. The
pale yellow crystals were collected and washed with DMF, yield
7 mg, 58 % based on MIM ligand L. The bulk material was deter-
mined to be homogeneous by optical microscopy (see Figure S5,
Supporting Information). All attempts to obtain a satisfactory ele-
mental analysis resulted in low C%. X-ray diffraction data collection,
solution, and refinement details are given in Table 1.
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Table 1. X-ray diffraction data collection, solution, and refinement details.
L·(DMSO)7
{[NH2Me2]2[Zn2(H2O)2(L)](H2O)1.25}
Chemical formula
M [g mol–1
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
C88H104N4O27S7
1874.17
monoclinic
P21/c
15.1666(9)
17.9508(11)
33.565(2)
90
C78H78.5N6O23.25Zn2
1602.70
orthorhombic
Pbca
37.270(8)
10.733(3)
38.171(9)
90
]
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ꢀ [°]
91.437(2)
90
9135.3(10)
4
1.363
2.262
90
90
15269(6)
8
1.394
γ [°]
V [Å3]
Z
Dcalcd. [g cm–3
]
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μ(Cu-Kα) [mm–1
F(000)
]
1.454
6676
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3952
Reflns. collected
Reflns. independ-
ent
121616
16050
39314
8305
Reflns. [I > 2σ(I)]
Rint
Parameters refined 1153
Restraints
R1
wR2
14423
0.042
5208
0.066
1037
1177
0.1515
0.4206
1.512
2.50
21
0.0635
0.2011
1.035
1.31
Goodness-of-fit
Δρ [e Å–3
]
[9] S. J. Loeb, Chem. Commun. 2005, 1511–1518.
CCDC
1458990
[for
L·(DMSO)7]
and
1458989
(for
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{[NH2Me2]2[Zn2(H2O)2(L)](H2O)1.25}) contain the supplementary crys-
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charge from The Cambridge Crystallographic Data Centre.
Supporting Information (see footnote on the first page of this
article): Details of the X-ray diffraction studies, PXRD experiments,
NMR spectra, and additionally a movie.
Acknowledgments
S. J. L. thanks the Natural Sciences and Engineering Research
Council of Canada (NSERC) for support of this research through
the Discovery Grant+ and Canada Research Chair programs.
P. E. K. gratefully acknowledges the Royal Society of New Zea-
land Marsden Fund for financial support.
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Received: March 18, 2016
Published Online: ■
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Polythreaded MOFs
A hexacarboxylate ligand based on an
interlocked [2]rotaxane motif is used
to construct
G. Gholami, K. Zhu, J. S. Ward,
P. E. Kruger, S. J. Loeb* ..................... 1–7
a
ZnII metal–organic
framework material. The structure has
a unique lattice, in which three inde-
pendent 3-periodic frameworks are in-
terpenetrated only by virtue of the
threading of their individual compo-
nents (axle and wheel) in the rotaxane
linker.
Formation of
a
Polythreaded,
Metal–Organic Framework Utilizing
an Interlocked Hexadentate, Carb-
oxylate Linker
DOI: 10.1002/ejic.201600311
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