Chiral metal-organic frameworks bearing free carboxylic acids for organocatalyst encapsulation

Article abstract of DOI:10.1002/anie.201408896

Two chiral carboxylic acid functionalized microand mesoporous metal-organic frameworks (MOFs) are constructed by the stepwise assembly of triple-stranded hepta-metallic helicates with six carboxylic acid groups. The meso-porous MOF with permanent porosity

Full text of DOI:10.1002/anie.201408896

Angewandte
Chemie
DOI: 10.1002/anie.201408896
Heterogeneous Catalysis
Chiral Metal–Organic Frameworks Bearing Free Carboxylic Acids for
Organocatalyst Encapsulation**
Yan Liu,* Xiaobing Xi, Chengcheng Ye, Tengfei Gong, Zhiwei Yang, and Yong Cui*
[
11]
Abstract: Two chiral carboxylic acid functionalized micro-
and mesoporous metal–organic frameworks (MOFs) are
constructed by the stepwise assembly of triple-stranded hepta-
metallic helicates with six carboxylic acid groups. The meso-
porous MOF with permanent porosity functions as a host for
encapsulation of an enantiopure organic amine catalyst by
combining carboxylic acids and chiral amines in situ through
acid–base interactions. The organocatalyst-loaded framework
is shown to be an efficient and recyclable heterogeneous
catalyst for the asymmetric direct aldol reactions with signifi-
cantly enhanced stereoselectivity in relative to the homoge-
neous organocatalyst.
enzyme-catalyzed transformations.
In particular, acid–
base assembly of chiral amines is one of the most efficient
bifunctional enamine catalysts. The acids utilized in these
cases were essential units that dramatically impacted the
[
12]
[
13]
catalytic activity and stereoselectivity. Based on the acid–
base principle, it should be effective for supporting chiral
amine in MOFs with channels decorated with free carboxylic
acids, which may play a dual role as catalyst anchors and
modulators for activity and stereoselectivity. However, MOFs
with struts containing free carboxylic acids remain rare. We
have recently described the assembly of a pyridyl-function-
alized cluster helicate into MOFs, but which lack binding
[
14]
[
15]
functional groups.
Herein we report the synthesis of
M
etal–organic frameworks (MOFs) provide a unique
a carboxylic acid functionalized triple-stranded helicate to
build micro- and mesoporous MOFs having free carboxylic
groups. Encapsulation of a chiral amine in the mesoporous
MOF led to a recyclable heterogeneous catalyst for the
asymmetric direct aldol reactions with significantly enhanced
stereoselectivity.
opportunity to design and synthesize functional crystalline
porous materials for diverse applications from organic struts
[1,2]
and metal ions.
modularity, high surface areas and tunable porosity, MOFs
have been targeted as particularly attractive supports for
molecular catalysts, and especially for asymmetric reactions
that cannot be realized with traditional porous inorganic and
organic materials.
Owing to their hybrid composition,
[
3]
The ligand H L-(2MOM) (MOM = methoxymethyl) was
4
synthesized in 80% yield by the Schiff-base condensation of
5-tert-butyl-3-(4-carboxylic acid)salicylaldehyde and enantio-
pure 3,3’-diamino-5,5’,6,6’-tetramethyl-2,2’-methoxymethyl-
[
4–6]
Chiral MOF catalysts are typically made
from metal ions and functionalized privileged chiral ligands
such as BINOL and salen,
[
7,8]
but it is challenging for this
1,1’-biphenyl. Treatment of H L-(2MOM) and Zn-
4
direct approach to build MOFs from organic catalysts because
they are often the ligating functionality of choice for frame-
(ClO ) ·6H O in a 1:3 molar ratio in DMF and MeOH at
4
2
2
808C afforded [Zn (H L) (OMe) ]·H O (1) in 75% yield
7
2
3
2
2
[
6,9]
work construction. Alternatively, the introduction of chiral
auxiliaries with reactive binding sites inside MOFs may be
(Scheme 1). Heating 1 and Cd(NO ) ·4H O and Zn-
3 2 2
(NO ) ·6H O (1:2 molar ratio) in DMF and pyridine at
3
2
2
[
10]
achieved by post-synthetic modification. In this approach,
the synthesis of MOFs with desired functionalities, stability,
and pore sizes seems to be within reach. Nonetheless, there
are only several reports of MOFs that displayed enantiopure
1008C afforded [Cd {Zn L(HL) (OH) }(Py) (H O)]·DMF (2)
2 7 2 2 2 2
and [Zn O] [Zn L (H L)(OH) ]·3H O (3), respectively (see
4
2/3
7
2
2
2
2
the Supporting Information). Compounds 2 and 3 are stable
in air and insoluble in water and common organic solvents.
The formulations were supported by elemental analysis and
single-crystal X-ray diffraction (XRD) analysis. The phase
purity of the bulk samples was confirmed by their powder
XRD patterns.
[
6]
organocatalysis.
Asymmetric organocatalysis has emerged as a powerful
synthetic method that is complementary to metal- and
Compound 1 crystallizes in the chiral trigonal space group
[
*] Prof. Y. Liu, Dr. X. Xi, C. Ye, T. Gong, Z. Yang, Prof. Y. Cui
School of Chemistry and Chemical Engineering and
State Key Laboratory of Metal Matrix Composites
Shanghai Jiao Tong University, Shanghai 200240 (China)
E-mail: liuy@sjtu.edu.cn
[
16]
R3 and adopts a heptanuclear helical structure. The MOM
groups were removed from the ligands upon complexation
with Zn ions, and each L binds to two Zn ions through two
tridentate N + 2O donors and to another two Zn through two
biphenolate oxygen atoms. Seven Zn ions thus formed two
Zn O distorted cubanes by sharing one Zn ion. Each of the
yongcui@sjtu.edu.cn
Prof. Y. Cui
4
4
Collaborative Innovation Center of Chemical Science and
Engineering
Tianjin 300072 (China)
six outer Zn ions is square-pyramidally coordinated by three
O and one N atoms from two L ligands and one OMe anion,
while the central Zn ion is octahedrally coordinated to three
O and three N atoms from three L ligands. The cluster can be
viewed as an M-configured triple-stranded helicate that has
[
**] This work was supported by the NSFC (21025103, 21371119,
2
2
1431004, and 21401128), “973” Program (2014CB932102 and
012CB8217), and SSTC-12XD1406300 and 14YF1401300.
perfect D point-group symmetry, with one crystallographic
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408896.
3
À
C3 axis running through two m - OMe units and three
3
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adjacent helicates are linked by the Cd1 ions to form a left-
handed 2 helix with a pitch of 1.75150(19) nm, a pair of which
1
associate in parallel forming a tube with an opening of about
0
.24 ꢀ 0.34 nm. Four such tubes are linked by the Cd2 to give
a 1D channel of about 0.20 ꢀ 0.54 nm with the CO H groups
2
pointing to Cd centers. The helicates are thus linked by two
kinds of Cd ions to form a chiral 3D network functionalized
with carboxylic groups.
Compound 3 crystallizes in the chiral hexagonal space
group P6 22. Again, the Zn helicate acts as a tetradentate
3
7
ligand and binds to four newly generated well-known [Zn (m -
4
4
O)] clusters using four of its six carboxylate groups in
a bidentate fashion. In the Zn O core, the Zn ions are each
4
tetrahedrally coordinated to one m -O anion and three
4
carboxylate oxygen atoms from three Zn7 helicates. As
a result, each Zn7 helicate binds to four Zn O via four
4
bidentate carboxylate groups, whereas each Zn O connects
4
six Zn helicates to form a (4,6)-connected 3D framework
7
with 1D chiral hexagonal channels along the a-axis, which are
periodically decorated with pairs of six uncoordinated CO H
2
groups that are related by the crystallographic six-fold axis
(Figure 2). The 1D channel can also be viewed as being
Scheme 1. Synthesis of helicate 1. MOM=methoxymethyl.
composed of a number of cylindrical cages with D symmetry,
6
each of which is enclosed by twelve Zn helicates and six
7
crystallographic C axes bisecting three opposite L edges.
Zn O clusters. The cage has a height of about 1.4 nm and
2
4
Complex CÀH···O interactions and hydrophobic interactions
direct packing of helicates along the ab plane forming a 2D
framework. Such 2D grids stack on top of each other along
the c-axis to form a 3D supramolecular structure with an
opening size of about 1.7 nm ꢀ 1.6 nm along the [211]
direction.
a maximum inner width of about 2.36 nm (considering van der
Waals radii). The hexagonal apertures that surround by six
free carboxylic groups on the top and bottom faces have
a diagonal distance of about 1.6 nm ꢀ 1.4 nm.
Compound 2 crystallizes in the chiral orthorhombic space
group I2 2 2 . The Zn cluster adopts almost the same helicate
1
1
1
7
À
structure as in 1, but with the m - OMe bridges replaced by m -
3
3
À
OH anions. The helicate acts as a tetradentate ligand,
binding to four Cd ions using four of its six carboxylate groups
in a chelating fashion (Figure 1). Of the two independent Cd
ions, the Cd1 is octahedrally coordinated by two pyridine
molecules and two bidentate carboxylate groups from two
different Zn helicates, and the Cd2 is heptahedrally coordi-
7
nated by one water, two pyridine, and two bidentate
carboxylate groups from two Zn helicates. Along the a-axis,
7
Figure 1. Space-filling and stick model of 2 viewed along the c-axis,
with different channel sizes owing to different distributions of the
ligands.
Figure 2. a) The mesoporous cage and b) the 3D porous network in 3
viewed along the c-axis.
2
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ESI-MS showed that helicate 1 was stable in DMF, as
evidenced by a prominent signal at m/z = 2549.5 that is due to
+
[
Zn (OH) L +H] (Supporting Information, Figure S17).
7
2
3
Both UV/Vis and CD spectra of 1 are almost the same at
room temperature and 808C in DMF, indicating the good
stability of the helical structure and its optical activity during
MOF crystallization. The key factor for the formation of the
Scheme 2. Synthesis of Ap@3 by post-modification. The hexagonal
cage in 3 is shown as a hexagonal prism.
CO H-functionalized MOFs may be due to steric crowding
2
around the CO H positions in the Zn7 helicate, which
2
prevented some of the carboxylic acid groups from partic-
was not detrimental to the crystal structure. Ap@3 gave
[
14e]
2
À1
ipating in the metal coordination.
Solid-state CD spectra
a decreased BET surface area (354.6 m g ) compared with 3.
However, the addition of 2 into a solution of Ap led to the
framework decomposition.
of 1–3 made from R and S enantiomers of H L-(2MOM) are
2
mirror images of each other, indicative of their enantiomeric
nature. TGA revealed that the solvent molecules could be
removed from 1–3 in the 80–1508C range. PXRD showed that
they retain their structural integrity and crystallinity upon
guest removal (Supporting Information, Figures S4–S6).
Calculations using PLATON indicate that 1–3 have 49.3,
After optimization of reaction conditions, Ap@3 was
found to be an active catalyst for the direct aldol reaction of
both acetone and cyclohexanone with nitro-substituted aro-
matic aldehydes in a ketone/water (1:0.05 molar ratio)
mixture. Especially, 10 mol% loading of (S/S)-Ap@3 cata-
lyzes the reaction of acetone with 4-nitrobenzaldehyde and 3-
nitrobenzaldehyde to afford the products in 80 and 74% ee
and 77 and 73% yield of isolated product, respectively, at
room temperature after 48 h. The catalytic reaction also
worked well with cyclohexanone, affording the products in 74
and 66% ee, 72 and 68% yield, and 3.3:1.0 and 2.0:1.0 anti/syn
ratio, respectively. A control experiment with MOF 3
afforded about 10% conversion of 4-nitrobenzaldehyde but
with less than 5% ee after 48 h, presumably catalyzed by the
free carboxylic acids. The reaction between 4-nitrobenzalde-
hyde and acetone catalyzed by either (S/R)-Ap@3 or a mix-
ture of (S/S)-Ap@3 and (S/R)-Ap@3 (a 1:1 molar ratio) gave
the R over the S enantiomer (ca. 80% ee) as well, suggesting
that the solid catalyst relies on the intrinsic chiral nature of
the organocatalyst Ap to exert stereocontrol.
5
0.2, and 68.5% of total volume occupied by solvent
[17]
molecules, respectively. The N sorption measurements at
2
7
7 K showed that the apohost 3 exhibit a pseudo-type-II
2
À1
sorption behavior with a BET surface area of 1015.5 m g .
The pore size distribution calculated using nonlocal density
functional theory is centered at about 2.3 nm, consistent with
the result of single-crystal X-ray diffraction (Supporting
Information, Figure S11). In contrast, 1 and 2 only exhibit
surface sorption. Interestingly, 3 could readily adsorb 3.2
Rhodamin 6G molecules (ca. 1.4 nm ꢀ 1.6 nm in size) and 1.1
Brilliant Blue R-250 molecules (1.8 nm ꢀ 2.2 nm in size) per
formula unit in MeOH. The inclusion solids exhibited similar
PXRD patterns to the pristine sample, but a structural
distortion occurred (Supporting Information, Figure S7),
suggesting that the structural integrity and open mesochan-
nels of 3 are maintained in solution. To our knowledge, 2 and
To study the confinement effect of a MOF on the organic
catalyst, the activity of Ap was assessed. At 10 mol% catalyst
loadings, Ap afforded the desired aldol products in 48–64%
ee and 71–79% yield of isolated product (Table 1, entries 2, 4,
3
are the first two examples of chiral MOFs containing free
CO H groups that are beneficial for enantioselective recog-
2
[
14]
nition.
[18]
Pyrrolidine derivatives are widely used as organocatalysts
for a variety of organic transformations, such as the aldol
6, and 8) in the presence of benzoic acid. The catalysis by
Ap was significantly accelerated by benzoic acid, as it may
[
18]
reaction.
The presence of free
CO H groups in 2 and 3 has
[a]
2
Table 1: Direct aldol reactions catalyzed by Ap@3 and Ap.
prompted the inclusion of a pyrroli-
dine catalyst for catalysis. After
many attempts, it was found that
(
S)-2-(dimethylaminomethyl) pyr-
[
b]
1
2
[c]
[d]
[e,f]
[g]
rolidine, (S)-Ap, could be entrap- Entry
ped by 3 by solution adsorption
Catalyst
Ar
R /R
Yield [%]
anti/syn
ee [%]
[g]
1
Ap@3
4-NO Ph
4-NO Ph
3-NO Ph
3-NO
4-NO Ph
4-NO Ph
3-NO Ph
3-NO Ph
H/H
H/H
H/H
H/H
77(76)
–
–
–
–
80 (80)
64
74
2
(
Scheme 2). The adduct Ap@3 was
[h]
2
3
4
5
6
7
8
Ap
79
73
76
72
75
68
71
2
achieved by soaking the evacuated
MOF in a dilute anhydrous THF
solution of Ap for two days at 08C.
After this treatment, the crystals
remained transparent but with ap-
parent fracturing. The formation of
a 1:1 host–guest complex was sug-
gested by GC, TGA, and elemental
analysis. The PXRD pattern was
nearly identical to that of the parent
Ap@3
2
[
h]
Ap
Ap@3
Ph
56
2
[
i]
À(CH ) À
3.3/1.0
2.9/1.0
2.0/1.0
1.9/1.0
74
48
66
50
2
2 3
[
h]
[i]
i]
[i]
Ap
À(CH ) À
2
2 3
[
Ap@3
À(CH ) À
2
2 3
[h]
Ap
À(CH ) À
2
2 3
[
(
a] For reaction details see the Experimental Section in the Supporting Information. [b] (S/S)-Ap@3 or
S)-Ap was used as the catalyst unless otherwise noted. [c] The yield of the isolated product based on
aldehyde. [d] Determined by H NMR spectroscopy from the crude reaction mixture. [e] Determined by
HPLC. [f] The absolute configuration (R) was assigned by comparing the retention time with that of the
standard sample. [g] (S/R)-Ap@3 was used as the catalyst. [h] One equivalent of benzoic acid was used.
1
MOF, indicating that the adsorption [i] Value represents the major isomer.
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provide a proton to accelerate the formation of enamine.
Therefore, the ee values observed for Ap@3 are significantly
higher than those for the Ap homogeneous control, whereas
the yields and the diastereoselectivity are comparable to
those for the homogeneous control, although the heteroge-
neous reactions required longer time because of their slow
mass diffusion. A variety of solid-supported chiral amine
catalysts have been developed for the aldol reactions, but they
are typically less effective than their homogeneous ana-
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[
To study whether the catalysis by Ap@3 occurred
predominantly within the pores or just on the surface,
a sterically more demanding substrate 5-formyl-1,3-phenyl-
enebis(3,5-di-tert-butylbenzoate) was subjected to the aldol
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and acetone after filtration through a regular filter did not
afford any obvious product, suggesting the heterogeneous
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Ap@3 could be readily recovered by centrifugation and
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crystallinity after three runs, although the structure became
slightly distorted (Supporting Information, Figure S8). GC
and ICP-OES analyses indicated no leaching of Ap and zinc
ions ( ꢀ 0.005%), respectively, from the framework per cycle.
In conclusion, we have described the synthesis of two
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groups. After encapsulating an organic amine, the mesopo-
rous framework was shown to be an efficient and recyclable
heterogeneous catalyst for the asymmetric direct aldol
reactions that exhibited markedly improved catalytic perfor-
mance relative to its homogeneous counterpart. The acid–
base procedure for guest inclusion enables the use of the
native catalysts without modification of the ancillary ligands
and/or MOF linkers, and, taking advantage of this strategy,
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Received: September 8, 2014
Published online: && &&, &&&&
Keywords: aldol reactions · heterogeneous catalysis · metal–
.
organic frameworks · organocatalysis · porous materials
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Heterogeneous Catalysis
Two chiral carboxylic acid functionalized
micro- and mesoporous metal–organic
frameworks (MOFs) are constructed. The
mesoporous MOF functions as a host for
encapsulation of an enantiopure organic
amine by acid–base interactions. The
organocatalyst-loaded MOF is an efficient
and recyclable heterogeneous catalyst for
asymmetric direct aldol reactions, with
significantly enhanced stereoselectivity
relative to the homogeneous organo-
catalyst.
Y. Liu,* X. Xi, C. Ye, T. Gong, Z. Yang,
Y. Cui*
&&&& — &&&&
Chiral Metal–Organic Frameworks
Bearing Free Carboxylic Acids for
Organocatalyst Encapsulation
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!