Design and synthesis of squaramide-based MOFs as efficient MOF-supported hydrogen-bonding organocatalysts

Article abstract of DOI:10.1039/c6cc03190k

Herein, we utilize a new, squaramide-based ligand, combined with a postsynthetic exchange (PSE) synthetic approach to prepare a series of Cu(ii)-squaramide MOFs that are active catalysts for the Friedel-Crafts reaction.

Full text of DOI:10.1039/c6cc03190k

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Design and Synthesis of Squaramide-Based MOFs as
Efficient MOF-supported Hydrogen-Bonding
Organocatalysts
Cite this: DOI: 10.1039/x0xx00000x
Xiaoping Zhang,§,a,b Zhenjie Zhang,§,a Jake Boissonnaulta and Seth M. Cohen*,a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
framework. The percentage of squaramide ligand incorporation was
varied from 0ꢀ100%. The authors reported that MOFs with 100%
squaramide ligand incorporation showed low catalytic activity,
similar to those MOFs with 100% unfunctionalized bpdc ligand (e.g.
no catalytic sites). MOFs with 50% squaramide ligand exhibited the
highest catalytic activity, which was attributed to an optimal balance
of catalytic sites and sufficient retention of MOF porosity. Herein,
an alternative approach presented for squaramideꢀbased MOF
catalysts is described. A new squaramide ligand with four
carboxylate ligating groups (5,5’ꢀ((3,4ꢀdioxocylcobutꢀ1ꢀeneꢀ1,2ꢀ
diyl)bis(azanediyl) diisophthalic acid, H4dbda, Fig. 1) has been
designed and synthesized. Using H4dbda, we developed a stable Cuꢀ
MOF, [Cu2(dbda)(CH3OH)2] (Cu(dbda)), through a postsynthetic
exchange (PSE) reaction from its Zn(II) analogue, [Zn2(dbda)(H2O)2]
(Zn(dbda)). The FriedelꢀCrafts reaction, a prototypical reaction for
hydrogenꢀbonding organocatalyst,30ꢀ34 was selected to evaluate the
Herein, we utilize a new, squaramide-based ligand, combined
with a postsynthetic exchange (PSE) synthetic approach to
prepare a series of Cu(II)-squaramide MOFs that are active
catalysts for the Friedel-Craft reaction.
Among various organocatalysts, those based on the thiourea core are
broadly utilized in the field of Hꢀbond donor catalysis.1 Recently,
bifunctional squaramide moieties have emerged as powerful
hydrogenꢀbonding groups for Lewis acid catalysis.2ꢀ8 Squaramides
are fourꢀmembered ring systems, which are derived from squaric
acid and possess hydrogenꢀbond accepting and donating
functionality through their carbonyl and N−H groups, respectively.
The squaramide functionality possesses features such as ditopic
binding, structural rigidity, high N−H acidity, and ease of
preparation.9 Squaramide compounds have been shown to be
competent for biomimetic transport,5, 10, 11 molecular recognition,12
ion sensing,13, 14 and organocatalysis.15, 16 The high propensity for
catalytic performance of these MOFs (Fig. 1).
Cu(dbda)
demonstrated high catalytic efficiency for the FriedelꢀCrafts reaction
between indole and substituted βꢀnitrostyrenes while Zn(dbda) was
nonꢀcatalytic due to its poor stability. The Cu(dbda) system allows
for 100% incorporation of the H4dbda squaramide ligand without a
significant loss of MOF porosity.
hydrogenꢀbonding is driven via
a concomitant increase in
aromaticity on the squaramide ring.17 Although the increased acidity
of squaramides can be useful in hydrogenꢀbond donating
organocatalysis, the strong hydrogenꢀbonding also drives selfꢀ
association/aggregation of squaramides that impedes catalysis.18 To
prevent selfꢀassociation and enhance the catalytic performance of
squaramides, one strategy is to immobilize these groups within
porous materials such as metalꢀorganic frameworks (MOFs).19, 20
MOFs are a class of porous, crystalline materials composed of
inorganic nodes and organic linkers. They have uniform 3ꢀ
dimensional structures of high surface area, large pores sizes, low
22
density, and have potential applications in gas sorption,21,
molecular
recognition,23
proton
conductivity,24
and
organocatalysis.25ꢀ29 Along with their modular synthesis and tunable
porosity, MOFs constitute attractive candidates as platforms for
heterogeneous catalysis.
To incorporate squaramide catalysts into MOFs, a feasible design
approach is to employ squaramide compounds as ligand struts for
MOFs. Hupp, Farha, and Mirkin developed a mixedꢀligand MOF
catalyst, UiOꢀ67ꢀSquar/bpdc,20 which was constructed using a
combination of unfunctionalized and squaramideꢀappended biphenyl
dicarboxylate (bpdc) ligands. In this system, the squaramide group
was attached as a ‘dangling’ substituent off of the bpdc ligand
Fig. 1 (a) Structure of the H4dbda ligand. (b) Structure of H4tptc ligand.
(c) Frediel−Crafts reaction between indole and substituted βꢀnitrostyrenes.
The H4dbda ligand was synthesized using a modified literature
procedure.35 Both 1H NMR and MS data confirmed the composition
of the ligand (Fig. S1, S2). Combining H4dbda and Zn(NO3)2 in a
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DOI: 10.1039/C6CC03190K
mixture of DMF and EtOH at 80 ºC for 24 h afforded paleꢀyellow
crystals of Zn(dbda). Singleꢀcrystal Xꢀray diffraction (XRD)
structure determination revealed that Zn(dbda) crystallized in the Rꢀ
3m space group with unit cell parameters a = b = 18.8687(9) Å, c =
38.3555(17) Å, α = β = 90o, γ = 120o (Table S1). As shown in Fig. 2,
Zn(dbda) is built by [Zn2(COO)4] paddlewheel secondary building
units (SBUs). The 1,3ꢀbenzenedicarboxylate (1,3ꢀbdc) moieties in
dbda4ꢀ are linked by [Zn2(COO)4] SBUs to form a 2ꢀdimensional
Kagomé lattice that are connected via the squaramide core to form
an overall 3ꢀdimensional framework with nbo topology (Fig. S3, S4).
There are large channels with approximate dimensions of 13×5 Å
along the crystallographic cꢀaxis. N2 sorption of Zn(dbda) showed
essentially no uptake (Fig. 3), which is attributed to a loss of
Zn(dbda) crystallinity upon thermal activation (60 ºC, 10 h) as
evidenced by powder Xꢀray diffraction (PXRD, Fig. S5). Zn(dbda)
was found to be stable in common organic solvents such as CHCl3
for 24 h at room temperature, but not stable in water (Fig. S6, S7).
Despite these stability limitations, the catalytic performance of
Zn(dbda) in the Friedel−Crafts reaction was examined. Indole (0.15
mmol) and βꢀnitrostyrene (0.1 mmol) were chosen as test substrates,
and the reaction was performed at 50 ºC in CHCl3 with 5 mol%
loading of the Zndbda catalyst (based on an empirical formula of
Zn4(dbda)2(H2O)4); however, no products were observed after 24 h
(Fig. S8). PXRD of Zn(dbda) after 24 h revealed that the MOF had
lost crystallinity under the reaction conditions (Fig. S9).
Fig. 3 N2 gas sorption of Cu(dbda)x(tptc)1-x (x = 1, 0.75, 0.49, 0.18, 0).
Adsorption and desorption branches are shown with filled and empty
symbols, respectively.
The model reaction between indole (0.15 mmol) and βꢀ
nitrostyrene (0.10 mmol, Fig. 1) was examined with 5 mol% loading
of Cu(dbda) (based
on an
empirical
formula
of
Cu4(dbda)2(CH3OH)4) as a catalyst. Under identical conditions to
those reported for UiOꢀ67ꢀSquar/bpdc (CD2Cl2, 25 ºC, 24 h),
Cu(dbda) gave a yield of only 32%, compared to 78% for UiOꢀ67ꢀ
Squar/bpdc.20 Therefore, reaction conditions were optimized for
Cu(dbda) including the use of different solvents (toluene, CH3CN,
CHCl3) and temperatures (25, 35, and 50 ºC) (Table S4, Fig. S14). It
was found that CHCl3 gave the best results, and activity could be
increased with increasing temperature from ~60% (25 ºC), to ~78%
(35 ºC), to >99% (50 ºC). Control reactions either without catalyst
or with free H4dbda ligand (not as part of a MOF) were also
performed under the same optimized reaction conditions. Without
catalyst, no product was observed. The catalytic activity of free
H4dbda was much lower than the MOF, giving a yield of only 33%
after 24 h (Fig. S15). The improved activity of Cu(dbda) suggests
that the MOF prevents squaramide selfꢀaggregation, which likely
causes the sluggish activity of the free H4dbda ligand. A timeꢀ
dependent study was performed from 30 min to 24 h (Fig. S16). The
conversion of βꢀnitrostyrene was ~60% (turnover frequency, TOF =
3.0×10ꢀ3 sꢀ1) after 1 h, with 95% conversion after 8 h (TOF = 6.6×10ꢀ
4 sꢀ1), and complete consumption of the starting material after 24 h.
Fig. 2 Coordination environment of Zn(dbda) (left). Packing of Zn(dbda)
along the crystallographic cꢀaxis (right). Color code: O, red; N, blue; Zn,
cyan; C, gray.
Literature reports suggest that [Zn2(COO)4] SBUs may possess
low chemical stability.36 However, there are reports of Zn(II) SBUs
that can undergo PSE with Cu(II) cations and the resulting Cu(II)ꢀ
based SBUs showing greater chemical stability.37 PSE of Zn(dbda)
with Cu(NO3)2 produced a new compound Cu(dbda) (Fig. S10);
attempts to prepare Cu(dbda) directly from H4dbda and various Cu
salts were unsuccessful under a variety of reaction conditions. Both
EDX (~99% Cu) and ICPꢀOES (~99% Cu) showed that the Zn(II)
ions were essentially completely replaced by Cu(II) by this PSE
process (Fig. S11, Table S2). XRD of Cu(dbda) revealed that
Cu(dbda) exhibited the same structure as Zn(dbda), with only
slight differences in cell parameters: a = b = 18.2623(6) Å, c =
39.6543(13) Å. Cu(dbda) possesses a slightly smaller unit cell (V =
11453.3(7) Å3) than Zn(dbda) (V = 11826.1(10) Å3) (Table S3) due
to the shorter CuꢀO bonds. For Zn(dbda), the average distance of
ZnꢀO (carboxylate) bonds is 2.036 Å, while for Cu(dbda), the
average CuꢀO (carboxylate) distance is 1.947 Å. Moreover, the
metalꢀmetal distance in the paddlewheel SBUs decreased from 2.999
Å (ZnꢀZn) to 2.629 Å (CuꢀCu). The permanent porosity of Cu(dbda)
was measured by N2 absorption at 77 K, which showed a typical
type I isotherm,38 giving a BET surface area of 1516±66 m2/g and a
total pore volume of 0.662 cm3/g (Fig. 3). The PXRD pattern of
Cu(dbda) remains intact upon immersion in organic solvents for 90
h, suggesting improved stability consistent with the gas sorption data;
however, Cu(dbda) was unstable in water (Fig. S12, S13).
Table 1. FriedelꢀCrafts reaction between indole and βꢀnitrostyrenes to
produce 3ꢀ(2ꢀnitroꢀ1ꢀphenylethyl)ꢀ1Hꢀindoles.
entry
Ra
catalyst
Temp
(ºC)
Time
(h)
Yield
(%)
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
none
50
50
50
50
25
35
50
50
50
50
50
50
50
50
50
50
50
50
24
24
24
24
24
24
1
0
H4dbdaa
32
28
0
b
Cu(NO3)2
Zn(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
Cu(dbda)
60
78
60
95
>99
97
91
92
90
74
61
99
95
95
8
9
24
24
24
24
24
24
24
24
24
24
10
11
12
13
14
15
16
17
18
Cl
F
Br
CH3
OCH3
NO2
H
Cu(dbda)0.75(tptc)0.25
Cu(dbda)0.49(tptc)0.51
UiOꢀ67ꢀSquar/bpdcc
H
H
2 | J. Name., 2012, 00, 1-3
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DOI: 1C0.O10M39M/CU6CNCI0C3A19T0IOK N
19
20
21
H
H
H
UiOꢀ67ꢀUrea/bpdcc
Cu(dbda)0.18(tptc)0.82
Cu(tptc)
50
50
50
24
24
24
79
88
38
based on the empirical formula Cu4(dbda)2x(tptc)2ꢀ2x(H2O)4, x = 0.75,
0.49, 0.18 ). Using Cu(tptc) (5 mol% loading, no squaramide sites)
as a catalyst gave a low yield of ~39% (Fig. S28); the residual
activity may arise from the Lewis acid Cu(II) sites, as Cu(II)ꢀ
complexes have been reported to show catalytic activity for the
Conditions: Indole (0.15 mmol), βꢀnitrostyrenes (0.10 mmol), and catalyst (5
mol%) in CHCl3 (1 mL). Yields monitored by 1H NMR. See supporting
information for details. a Catalyst loading was 10 mol%. b Catalyst loading
was 20 mol%. c Conditions: Indole (0.02 mmol), βꢀnitrostyrene (0.014
mmol), and catalyst (10 mol%) in toluene (0.7 mL), see Ref. 20 for details.
FriedelꢀCrafts reaction.40
Consistent with this observation,
Cu(NO3)2, gave a 28% conversion under the same reaction
conditions. In contrast, for Cu(dbda)x(tptc)1-x (x = 1, 0.75, 0.49,
0.18, 0), activity increased with increasing dbda4ꢀ content: 39%
An advantage of heterogeneous catalysts is reusability, which was
also investigated for Cu(dbda). Only a small decrease in activity
(from >99% to ~96%) was observed after 5 runs (Fig. S17).
Characterization of Cu(dbda) after the 5th run showed the catalyst
retained its crystallinity, with the PXRD pattern of the recycled
Cu(dbda) in good agreement with the calculated patterns (Fig. S18).
A test was performed to confirm the heterogeneity of the catalysts
(and rule out soluble species) by removing the MOF catalyst by
filtration after 30 min (at which time the yield was ~22%). The
filtrate was then reꢀanalyzed after a total of 24 h showing no
formation of new product and indicating that the catalyst is
heterogeneous and there is no leaching of a catalytic species into
solution (Fig. S16).20 The substrate scope for various substitutedꢀ
nitrostyrene derivatives was also examined to assess the utility of
Cu(dbda) (Fig. S19). Good to excellent yields, ranging from ~61%
for 4ꢀnitroꢀβꢀnitrostyrene (poor solubility in CHCl3) to 97% for 4ꢀ
chloroꢀβꢀnitrostyrene, were obtained (Table 1). Overall, these
results show that Cu(dbda) is an efficient, recyclable, heterogeneous
catalyst for the FriedelꢀCrafts reaction.
(Cu(tptc)),
88%
(Cu(dbda)0.18(tptc)0.82),
95%
(Cu(dbda)0.49(tptc)0.51), 99% (Cu(dbda)0.75(tptc)0.25), and >99%
(Cu(dbda)), respectively. These results verify that the squaramide
group is the governing catalytic functional group in these MOFs.
For a more direct comparison to the prior reports,20 a 2.45 mol%
loading of catalyst Cu(dbda)0.49(tptc)0.51 achieved the same catalytic
performance as the reported UiOꢀ67ꢀSquar/bpdc at 10 mol% loading
(95% yield at 50 ºC for 24 h).20 The result further verified the high
activity of the dbda4ꢀꢀbased MOF.
Taken together, Cu(dbda) has features that compliment and
distinguish it from the previously reported UiOꢀ67ꢀSquar/bpdc
catalyst.20 Cu(dbda) has a different MOF structure type from UiOꢀ
67ꢀSquar/bpdc, wherein the catalytic group is part of the ligand
‘backbone’ rather than a dangling component. Depending on
catalyst design, one can envision scenarios where one or the other
functional group arrangement might be preferable. Also, in contrast
to UiOꢀ67ꢀSquar/bpdc, Cu(dbda) allows for 100% functional ligand
incorporation without loss of surface area. Indeed, using H4tptc as a
complimentary, unfunctionalized ligand the Cu(dbda) system is not
limited with respect to the amount of squaramide ligand that can be
included while maintaining a high degree of porosity and activity.
Both Cu(dbda)x(tptc)1-x and UiOꢀ67ꢀSquar/bpdc catalysts show
good activity under similar conditions, and hence each helps advance
the ability of utilizing squaramides in MOFꢀbased catalytic systems.
In summary, we prepared a squaramide tetracarboxylate ligand,
and developed catalytic squaramide MOFs prepared via metal PSE
from Zn(II) precursor MOFs. The Cu(II) MOFs were more stable
and hence showed good catalytic performance when compared to the
unstable parent Zn(II) MOF, in a FriedelꢀCrafts reaction of indole
To further investigate the catalytic activity of Cu(dbda), a series
of isostructural MOF catalysts, [Cu2(dbda)x(tptc)1ꢀx(CH3OH)2]
(Cu(dbda)x(tptc)1-x, x = 0.75, 0.49, 0.18, 0; H4tptc = pꢀterphenylꢀ
3,5,3
ligand strategy; the singleꢀcrystal structure of Cu(tptc) was
previously reported in 2006.39
The structures of the two
̋,5ꢀ̋ tetracarboxylic acid, Fig. 1), were prepared via a mixedꢀ
tetracarboxylic ligands (H4dbda and H4tptc) are very similar, both
using squareꢀplanar 4ꢀconnected nodes with the distance between 4
and 4' carbon atoms in H4dbda of 11.57 Å versus 11.38 Å in H4tptc.
The synthesis of Cu(dbda)x(tptc)1-x (x = 0.75, 0.49, 0.18) was
achieved using a PSE approach using Zn(dbda)x(tptc)1-x MOFs as
precursors. Both EDX and ICPꢀOES results (Table S2, Fig. S20ꢀS23)
showed the metal content of Cu(II) in all of the Cu(dbda)x(tptc)1-x
(x = 0.75, 0.49, 0.18, 0) was >92%, thus indicating near complete
with βꢀnitrostyrenes.
A
series of isostructural MOFs
Cu(dbda)x(tptc)1-x (x = 0.75, 0.49, 0.18, 0) showed that the catalytic
performance of these MOFs increased with increasing amounts of
the squaramide ligand, demonstrating that squaramide MOFs are
promising MOFꢀsupported heterogeneous organocatalysts. Our
ongoing progress with squaramide MOFs are focusing on their
applications in molecular recognition and ion sensing.
PSE.
The PXRD patterns of the mixedꢀligand MOFs
Cu(dbda)x(tptc)1-x were all in good agreement with calculated
patterns (Fig. S24).
Cu(dbda)x(tptc)1-x (x = 0.75, 0.49, 0.18) determined by H NMR
The ratio of dbda4ꢀ and tptc4ꢀ in
1
(Figure S25) were very close to the expected values (0.75, 0.50, and
This work was financially supported by a grant from the Division
of Chemistry of the National Science Foundation (CHEꢀ1359906)
and China Scholarship Council.
0.25 based on the ratio of starting materials). In addition, a direct
39
solvothermal synthesis was used to prepare Cu(dbda)x(tptc)1-x
.
However, the loading of dbda4ꢀ through solvothermal methods was
much lower than the ratio used in the synthesis, with initial ratios of
dbda4ꢀ of 0.50 and 0.25 giving incorporation of only x = 0.31 and
0.12, respectively. Moreover, Cu(dbda) could not be obtained using
solvothermal methods (Fig. S26, S27). The permanent porosity of
Notes and references
§ These authors contributed equally.
a
Department of Chemistry and Biochemistry, University of California,
Cu(dbda)x(tptc)1-x were also evaluated by N2 sorption isotherms at San Diego, La Jolla, California 92093, United States.
b
77 K, giving the expected typeꢀI isotherms (Fig. 3). The BET
surface areas of Cu(dbda)x(tptc)1-x (x = 0.75, 0.49, 0.18) were
1884±61 m2/g, 1946±124 m2/g, and 1887±45 m2/g, respectively,
with total pore volumes of Cu(dbda)x(tptc)1-x (x = 0.75, 0.49, 0.18)
of 0.775 cm3/g, 0.803 cm3/g, and 0.775 cm3/g, respectively. These
results show that the porosity of the Cu(dbda)x(tptc)1-x fall in the
expected range between those of the singleꢀligand MOFs, Cu(dbda)
(1516 m2/g) and Cu(tptc) (1942 m2/g).
Department of Chemistry, Key Laboratory of Advanced Energy
Materials Chemistry (MOE), Collaborative Innovation Center of
Chemical Science and Engineering, Nankai University, Tianjin 300071,
P. R. China.
Electronic Supplementary Information (ESI) available: [Experminetal
procedures, FTIR, TGA, PXRD, 1H NMR, and crystallographic data.
CCDC 1472314 and 1472315 ]. See DOI: 10.1039/c000000x/
Cu(dbda)x(tptc)1-x was employed as catalyst for the Friedelꢀ
Crafts reaction between indole (0.15 mmol) and βꢀnitrostyrene (0.1
mmol), carried out at 50 ºC in CHCl3 for 24 h (5 mol% loading,
1.
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