Incorporation of an A1/A2-difunctionalized pillar[5]arene into a metal-organic framework

Article abstract of DOI:10.1021/ja3082523

An efficient synthetic route to an A1/A2-difunctionalized pillar[5]arene containing resolvable planar chirality has been developed and the arene employed as a strut in the synthesis of P5A-MOF-1, which has been demonstrated by X-ray powder diffraction analysis-supported by modeling-to be isoreticular with MOF-5. This metal-organic framework has an active domain that expresses good and selective uptake of neutral and positively charged electron-poor aromatic guests, which effect color changes of the cubic crystals from faint yellow to deep orange, arising from charge transfer between the guests and active domain of P5A-MOF-1.

Full text of DOI:10.1021/ja3082523

Communication
pubs.acs.org/JACS
Incorporation of an A1/A2-Difunctionalized Pillar[5]arene into a
Metal−Organic Framework
Nathan L. Strutt,† David Fairen-Jimenez,‡ Julien Iehl,† Marianne B. Lalonde,† Randall Q. Snurr,‡
Omar K. Farha,† Joseph T. Hupp,† and J. Fraser Stoddart*,†
‡
†Department of Chemistry and Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois
60208, United States
S
* Supporting Information
Scheme 1. Synthesis of A1/A2-Difunctionalized
Pillar[5]arene Organic Strut
a
ABSTRACT: An efficient synthetic route to an A1/A2-
difunctionalized pillar[5]arene containing resolvable planar
chirality has been developed and the arene employed as a
strut in the synthesis of P5A-MOF-1, which has been
demonstrated by X-ray powder diffraction analysis
supported by modelingto be isoreticular with MOF-5.
This metal−organic framework has an active domain that
expresses good and selective uptake of neutral and
positively charged electron-poor aromatic guests, which
effect color changes of the cubic crystals from faint yellow
to deep orange, arising from charge transfer between the
guests and active domain of P5A-MOF-1.
acrocycles such as cyclodextrins,1 crown ethers,2
3
cucurbiturils,4 and cyclophanes5 have
Mcalixarenes,
become an integral part of host−guest chemistry.6 A relatively
new class of macrocycles to enter the field, the pillararenes,7 are
analogues of calixarenes composed of five, six, or seven
hydroquinone rings linked through their para-positions by
methylene bridges. Since pillar[5]arene was first introduced as a
novel macrocycle by Ogoshi and co-workers in 2008,7a the
chemistry of the pillararenes has been developed steadily, and
they have been shown to have applications in liquid crystals,7q
artificial transmembrane channels,7p nanoparticle formation,7o
and sensing.7j Here we report the synthesis of an A1/A2-
difunctionalized8 pillar[5]arene that undergoes cross-coupling
reactions to create a rigid strut which is then incorporated into a
metal−organic framework9 (MOF) having an active domain10
containing docking sites for electron-poor guests.
MOFs with organic struts incorporating macrocycles have
been used recently10 to prepare extended frameworks with active
domains, which, as a result of highly favorable and specific
noncovalent interactions, play host to a well-ordered distribution
of guest molecules. In 2009, we described10a the use of a π-
electron-rich BPP34C10-functionalized organic strut in the
synthesis of MOF-1001 which was shown to soak up the π-
electron-poor guest, methyl viologen. MOFs containing active
domains show promise in the fields of chromatographic
separation11 and sensing,12 thereby making designer organic
struts containing novel macrocycles attractive synthetic targets.
We have developed a synthetic protocol (Scheme 1) to obtain
an A1/A2-difunctionalized8 pillar[5]arene organic strut, starting
from 1, which is made through the co-cyclization of 1,4-
dimethoxybenzene and 1,4-bis(3-bromopropoxy)benzene, sim-
a
In the X-ray crystal structure of 3, C is gray, O is red, H is white; alkyl
H atoms are omitted for clarity.
ilar to our previously reported7j reaction for preparing
monofunctionalized pillar[5]arene. Compound 1 undergoes
Received: August 20, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja3082523 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 1. Solid-state structure of 5 (C is gray, O is red) which displays
planar chirality and does not racemize between the Rp-5 (left) and Sp-5
(right) enantiomers. A DMF molecule and hydrogens have been
removed from the structure to aid visual clarity.
elimination to give the diallyl ether 2, which was deprotected
using standard conditions13 to give the A1/A2-dihydroxy-
pillar[5]arene 3. Single crystals of 3, suitable for X-ray
crystallography, were grown; the solid-state structure14 (Scheme
1) of 3 shows that the hydroquinone unit is oriented in a
direction opposite to that adopted by the 1,4-dimethoxybenzene
units to support two intramolecular hydrogen bonds.
Compound 3, when treated with triflic anhydride, affords the
ditriflate 4. Ogoshi and co-workers7c showed that a pertriflated
pillar[5]arene can undergo 10 Pd-catalyzed cross-couplings to
give a highly conjugated pillar[5]arene. In similar fashion, 4 can
be converted into a rigid strut 5 by means of a Pd-catalyzed
Suzuki reaction with 4-(methoxycarbonyl)phenylboronic acid,
followed by saponification of the intermediate diester. The solid-
state structure of 5 (Figure 1) was elucidated by single-crystal X-
ray analysis15 using crystals grown from diffusion of MeOH into a
solution of 5 in DMF. The analysis indicates the presence of
enantiomers in the unit cell. In keeping with its molecular C2
symmetry, the H NMR spectrum (see SI) of 5 displays two
pairs of doublets for the two homotopic pairs of constitutionally
hetereotopic methylene groupswhere in each case the protons
are diastereotopic,17 given the fact that 5 is conformationally
rigidand a singlet for the remaining constitutionally
hetereotopic methylene group lying on the C2 axis, which
renders its methylene protons homotopic. The conformational
rigidity of the pillar[5]arene-based strut 5 means that it exists as
(potentially resolvable18,19) enantiomers, Rp and Sp (Figure 1)
due to the molecule’s planar chirality.20 Compound 5
demonstrates that only two bulky monosubstituted phenyl
rings are required at the A1/A2 positions on a pillar[5]arene to
impart resolvable planar chirality21 upon its constitution.
The pillar[5]arene-based strut 5 has been used to synthesize a
MOF (Figure 2a) with Zn4O secondary building units (SBUs)
which is isoreticular to MOF-5.9b P5A-MOF-1 was prepared in a
conventional manner by heating a mixture of 5 and Zn-
(NO3)2·6H2O in DMF at 100 °C over 24 h. The crystals (Figure
3a) of P5A-MOF-1 are cubic and transparent. Powder X-ray
diffraction (PXRD) confirmed their crystallinity (Figure 2b),
while thermal gravimetric analysis (TGA) was performed to
determine their thermal stability: a one-step TGA profile shows
that P5A-MOF-1 is stable to 450 °C. Crystals of P5A-MOF-1
were activated using supercritical CO2, and a NLDFT surface
area of 300 m2 g−1 was obtained from a CO2 isotherm (see SI).
Single-crystal X-ray data obtained for P5A-MOF-1 were not
well enough resolved to discern the solid-state structure of the
extended framework as a result of disorder within the MOF itself.
Figure 2. (a) Model of P5A-MOF-1 (pillar[5]arene macrocycles are
red, terphenylene moieties are black, zinc SBUs are yellow). (b)
Experimental PXRD pattern for P5A-MOF-1 (black), calculated PXRD
patterns for P5A-MOF-1 in a P1 space group (red) and a Pm3m space
group (green), and PXRD pattern for IRMOF-16 (blue). See SI for full
PXRD of P5A-MOF-1.
16
1
We believe this disorder arises principally from the rotational
freedom of pillar[5]arenes around every terphenylene linker in
the extended structure and a random distribution of “enantio-
meric” pillar[5]arenes associated with their planar chirality.
The extended structure of P5A-MOF-1 was modeled (see SI)
using non-interpenetrated IRMOF-169e as the backbone and
incorporating pillar[5]arenes with randomly distributed chir-
alities and orientations with respect to the terphenylene linkers.
The geometry of the predicted structure was optimized to give a
cubic unit cell with dimensions of a = b = c = 42.980 Å and a space
group of P1. The simulated PXRD pattern of the modeled
structure matches closely with the experimental one for P5A-
MOF-1 (Figure 2b). An alternative approach to modeling the
extended structure with a Pm3m space group was also pursued to
determine if a model with higher symmetry might also fit the
experimental MOF data. In this alternative model, each organic
strut, ordered throughout the 3D framework, contains four
pillar[5]arene rings in the shape of both “enantiomers” in two
different orientations, each with a 0.25 occupancy disorder.
Although the cubic cell dimensions of the model are identical
with those of the first model, the simulated PXRD pattern
presents extra peaks (Figure 2b), including a sharp 001 reflection.
B
dx.doi.org/10.1021/ja3082523 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Mole Ratio of Guest to Organic Strut in P5A-MOF-1
and IRMOF-16-OPX, Obtained from 1H NMR Spectra after
a
Guest Uptake and Digestion of MOF
P5A-MOF-1
IRMOF-16-OPX
Single-Guest Uptake Experiments
G1+
G22+
G3
0.755
0.366
0.293
0.121
0.125
0.084
Two-Guest Uptake Experiments
G1+ + G22+
G1+ + G3
G22+ + G3
0.091 G1+/0.176 G22+
0.090 G1+/0.127 G3
0.310 G22+/0.100 G3
0.089 G1+/0.069 G22+
0.027 G1+/0.032 G3
0.068 G22+/0.014 G3
a
Uptake with a single guest was performed with a saturated solution of
the guest in Me2CO. Uptake with two guests was performed in a
Me2CO solution with each guest at 40.0 mM. Ka values of guest with 5
1
determined by H NMR titration in CD3COCD3: G1+, 43.2
2.9
M−1; G22+, 170 50 M−1; G3, 66.2 1.9 M−1.
has been shown7a to be too large to reside inside the cavity of
pillar[5]arene, but small enough to pass through the pores of
either MOF, was similar for both P5A-MOF-1 and IRMOF-16-
OPX (see SI).
Figure 3. (Top) Optical microscopy images of P5A-MOF-1 (a) with no
guest (scale bar, 200 μm), (b) after uptake of G2·2PF6 (scale bar, 100
μm, and (c) after uptake of G3 (scale bar, 100 μm). (Bottom) Electron-
poor compounds used in guest uptake studies with P5A-MOF-1: N-
hexylpyridinium cation (G1+), N,N′-dihexyl-4,4-bipyridinium dication
(G22+), and 1,4-dinitrobenzene (G3).
In a final experiment, P5A-MOF-1 was suspended in Me2CO
with equimolar concentrations (40.0 mM) of two different guests
to determine if there is preferential uptake of one guest over the
other. While we envisioned that the observed guest-to-MOF
ratios would depend to some extent on the Ka values of the guests
with 5 in solution, other factors, including the sizes and diffusion
rates of the guests, might also be significant. P5A-MOF-1
showed almost twice the uptake of G2·2PF6 compared to G1·PF6
(Table 1), reflecting the larger Ka value for the former than the
latter in binding 5. Under identical conditions, the uptake by
IRMOF-16-OPX of these two guests is very similar, as expected.
Comparable results, which reflect ratios of Ka values, can be
observed when P5A-MOF-1 is exposed to equimolar combina-
tions of the other guests (Table 1).
The rigid stereochemistry associated with the planar chirality
of the strut 5 means that it should be possible, after resolving 5, to
prepare “enantiomeric” P5A-MOF-1 samples without fear of 5
racemizing during the synthesis (at 100 °C) of the MOF. The
prospect of being able to prepare chiral, enantiomerically pure,
pillar[5]arene-containing MOFs to separate racemic mixtures of
appropriate analytes is being pursued in our laboratories.
Thus, it seems that the lower symmetry model provides a better
match with the experimental data.
We investigated the ability of P5A-MOF-1 to take up guests
(Figure 3). At the outset, however, we evaluated the ability of the
strut 5 to form complexes with three guests:22 the PF6− salts of N-
hexylpyridinium cation (G1+) and N,N′-dihexyl-4,4-bipyridi-
nium dication (G22+), as well as the neutral 1,4-dinitrobenzene
1
(G3). H NMR titrations in CD3COCD3 revealed association
constants (Ka) between 5 and G1·PF6, G2·2PF6, and G3 of 43.2
2.9, 170 50, and 66.2 1.9 M−1, respectively.
Samples of P5A-MOF-1 were suspended in Me2CO prior to
guest uptake experiments to remove excess of DMF from within
the framework. The samples were then introduced into saturated
solutions of G1·PF6, G2·2PF6, and G3 in Me2CO. With G2·2PF6
and G3, the crystals underwent an immediate color change
(Figure 3b,c) from faint yellow to deep orange upon addition of
the guests, most likely because of charge-transfer interactions
between the guests and P5A-MOF-1. The MOF samples were
allowed to take up guests for 12 h before they were washed with
Me2CO and dissolved in DMSO-d6/TFA-d and their 1H NMR
spectra recorded. Integration of appropriate probe protons led to
quantification of the uptake of guests by the MOF (Table 1).
IRMOF-16-OPX, prepared from an oligo-p-xylene (OPX)
derivative23 (S3 in SI) of p-terphenyl-4,4″-dicarboxylic acid,24
was used as a control. Although both P5A-MOF-1 and IRMOF-
16-OPX are isoreticular with IRMOF-16, the latter does not have
an active domain. The guest uptake experiments were performed
under identical conditions for both MOFs.
P5A-MOF-1 takes up G1·PF6, G2·2PF6, and G3 from their
saturated solutions in Me2CO in moderate to high amounts.
Table 1 lists the mole ratios of the guest to the organic strut found
in the MOF. The fact that, under identical conditions, P5A-
MOF-1 takes up a significantly larger amount of each guest than
does IRMOF-16-OPX suggests the active domain of P5A-MOF-
1 is able to interact with guest molecules through favorable
noncovalent bonding interactions. Uptake of adamantane, which
ASSOCIATED CONTENT
* Supporting Information
Experimental details, modeling, and characterization. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
stoddart@northwestern.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Benjamin Moritz for performing the chiral
HPLC studies, Dr. Amy Sarjeant and Dr. Charlotte Stern for
performing the PXRDs and single-crystal X-ray crystallography
data collection, and Dr. Saman Shafie for performing the mass
spectrometry experiments. This work was supported by the Non-
C
dx.doi.org/10.1021/ja3082523 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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Threat Reduction Agency (HDTRA1-10-1-0023) J.T.H. and
O.K.F. gratefully acknowledge financial support from the
Defense Threat Reduction Agency (grant No. HDTRA1-09-1-
0007). N.L.S. thanks the National Science Foundation for a
Graduate Research Fellowship.
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c = 17.7886(2) Å; β = 99.5880(10)°; V = 4466.8(6) Å3; T = 100(2) K, Z
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independent measured reflections, 24 167; R1 = 0.0645 and wR2 =
0.2036 for 7579 independent observed reflections [2θ ≤ 124°, I >
2σ(I)]. CCDC 896923.
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1
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organic solvents.
(24) Grunder, S.; Stoddart, J. F. Chem. Commun. 2012, 48, 3158.
D
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