Homochiral metal-organic frameworks for heterogeneous asymmetric catalysis

Article abstract of DOI:10.1021/ja101208s

Homochiral crystallizations of two enantiomeric metal-organic frameworks (MOFs) Ce-MDIP1 and Ce-MDIP2 were achieved by using l- or d-BCIP as chiral inductions, respectively, where the chiralities were characterized by solid state CD spectra. Ce-MDIPs exhibit excellent catalytic activity and high enantioselectivity for the asymmetric cyanosilylation of aromatic aldehydes; the homochiral Cd-TBT MOF having l-PYI as a chiral adduct exhibits stereochemical catalysis toward the Aldol reactions.

Full text of DOI:10.1021/ja101208s

Published on Web 09/29/2010
Homochiral Metal-Organic Frameworks for Heterogeneous Asymmetric
Catalysis
Dongbin Dang, Pengyan Wu, Cheng He, Zhong Xie, and Chunying Duan*
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian, 116012, China
Received February 10, 2010; E-mail: cyduan@dlut.edu.cn
Table 1. Results for the Catalytic Cyanosilylation of Carbonyl
Substrates in the Presence of Ce-MDIPsa
Abstract: Homochiral crystallizations of two enantiomeric metal-
organic frameworks (MOFs) Ce-MDIP1 and Ce-MDIP2 were
achieved by using L- or D-BCIP as chiral inductions, respectively,
where the chiralities were characterized by solid state CD spectra.
Ce-MDIPs exhibit excellent catalytic activity and high enanti-
oselectivity for the asymmetric cyanosilylation of aromatic alde-
hydes; the homochiral Cd-TBT MOF having L-PYI as a chiral
adduct exhibits stereochemical catalysis toward the Aldol reactions.
ee (%)b
Entry
Ar
Ce-MDIP1a
Ce-MDIP2
1
2
3
4
phenyl
4-methoxyphenyl
1-naphthyl
93
91
98
94
97
>98
>98
2-naphthyl
>98
Recently, there has been increasing interest in creating homo-
chiral porous metal-organic frameworks (MOFs), because chiral
MOFs are, beyond the more fundamental search for new topologies,
highly attractive for asymmetric catalysis and other applications
requiring noncentrosymmetric crystal structures such as nonlinear
optical devices.1-3 Among the essential approaches that exist for
the preparation of chiral MOFs,4 self-resolution crystallization offers
great advantages as it does not use chiral components.5 In the case
of the presence of an additional chiral source, such a homochiral
crystallization has the potential to result in a given handedness.6
In several unusual discoveries, the absolute configuration of the
bulk crystals is controllable through the chiral induction effect.7,8
However, this approach has not been successfully used in the
preparation of chiral MOFs for enantioselective catalysis yet,
because chirality control from run to run is generally quite difficult7
and the design of highly enantioselective MOF catalysts Via remote
influence by chiral environments of open channels still remains a
big challenge.9,10
L-Proline and its derivatives are well-known asymmetric orga-
nocatalysts, accelerating a variety of enantioselective organic
reactions, including C-C bond forming Aldol and Michael reactions
under homogeneous reaction conditions.11,12 By using L- and
D-pyrrolidine-2-yl-imidazole (PYI)13 as a chiral adduct, we perform
the homochiral crystallization of the two enantiomorphs
Ce-MDIP1 and Ce-MDIP2, having one-dimensional channels
and coordinatively unsaturated metal sites, resepectively. Asym-
metric cyanosilylations of aromatic aldehydes were dispalyed to
validate the enantioselectively catalytic performance of the MOFs.
Solvothermal reaction of methylenediisophthalic acid (H4MDIP)
and Ce(NO3)3 ·6H2O in the presence of L-N-tert-butoxy-carbonyl-
2-(imidazole)-1-pyrrolidine (L-BCIP) in water gave compound Ce-
MDIP1 in a high yield (60%). Elemental analysis along with
powder X-ray analysis of the compound indicated the pure phase
of its bulky sample. Circular dichroism (CD) studies of the bulk
samples of Ce-MDIP1 showed clearly positive cotton effects at
about 254 nm and negative cotton effects at about 322 nm. The
whole spectrum was quite different from those of the chiral adducts
L-BCIP and PYI. Compound Ce-MDIP1 crystallized in a chiral
space group P21 with one cerium ion, one HMDIP ligand, and
one water molecule included in an asymmetric unit. Molecules of
a Reaction conditions: (CH3)3SiCN, 1.2 mmol; aldehyde, 0.5 mmol;
Ce-MDIP catalysts, 0.01 mmol (2 mol %); room temperature under N2
for 24 h to ensure high conversion (>95%) of the reaction. b The
enantiomeric excess (ee) was determined by HPLC analysis.
the chiral adduct were not found in the crystals. Each cerium ion
was coordinated by five oxygen atoms corresponding to five
monodentate carboxyl groups, two oxygen atoms from one bidentate
carboxyl group, and one water molecule. The HMDIP ligands
linked the metal ions through the carboxyl groups forming a
noninterpenetrating 3D network containing chiral channels with a
10.5 × 6.0 Å2 cross section along the a-axis. The cerium ions having
removable coordinated water molecules were well-positioned within
the channels. They were able to interact with potential guest
molecules that enter the framework channels, suggesting
Ce-MDIP1 had the potential to functionalize as an actively
heterogeneous catalyst.14
The asymmetric cyanosilylation experiments were employed with
a 1:2.4 mol ratio of the selected aromatic aldehyde and cyanotri-
methylsilane in CH3CN at room temperature through a heteroge-
neous manner. As shown in Table 1, the loading of only a 2 mol
% ratio of Ce-MDIP1 (0.01 mmol) catalyst leads to the almost
complete conversion of (S)-2-phenyl-2-(trimethylsilyloxy)acetoni-
trile corresponding to benzaldehyde and excellent stereochemical
performance with an ee value up to 91%.15 Ce-MDIP1 thus represents
the first example of MOF-based heterogeneous asymmetric catalysts
for that important reaction, providing a convenient route to canhydrins.
Also, the catalysis efficiency of Ce-MDIP1 is comparable to the best
prior result for metal-organic framework catalysts (Supporting
Information Table S1).16 Noticeably, the removal of Ce-MDIP1 by
filtration after only 4 h completely shut down the reaction, affording
only 10% additional conversion upon stirring for another 7 h. These
experiments unambiguously demonstrate that Ce-MDIP1 is a true
heterogeneous asymmetric catalyst. The solids of Ce-MDIP1 can
be easily isolated from the reaction suspension by simple filtration
alone and can be reused at least three times with a slight decrease in
the reactivity and enantioselectivity. The index of XRD patterns of
the Ce-MDIP1 bulky sample filtered off from the catalytic reaction
evidences the maintenance of the crystallinity. Infrared spectroscopy
of the catalyst impregnated with a dichloromethane solution of
9
10.1021/ja101208s  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 14321–14323 14321

C O M M U N I C A T I O N S
benzaldehyde exhibited one broad C-O stretch at 1685 cm-1. The
red shift from 1692 cm-1(free aldehyde) suggested the absorbance of
benzaldehyde and possible activation of the substrate in the channels
of Ce-MDIP1.
Similarly, a chirality induction effect was observed with D-BCIP,
which resulted in the formation of another enantiomorph,
Ce-MDIP2. Ce-MDIP2 also crystallized in the chiral space group
P21 with cell dimensions similar to those of Ce-MDIP1. The CD
spectrum of bulk crystals of Ce-MDIP2 exhibited Cotton effects
exactly opposite to those of Ce-MDIP1 (Figure 1c), indicating
that the homochiral crystallization is controllable through chiral
induction. Without the chirality induction effect, such an observation
is highly unlikely.
Figure 2. Schematic representation of the construction of compound
Cd-TBT. The catalytically active units were drawn in the space-filling
diagram. The framework was drawn in a ball/stick diagram. The green,
red, blue, and gray balls represent the Cd, O, N, and C atoms, respectively.
Table 2. Aldol Reactions between Aldehydes and Cyclohexanonea
Cd-TBT
L-PYI
Yield
(%)b
ee
(%)c
Yield
(%)b
ee
(%)c
entry
Ar
1
2
3
4
2-nitrophenyl
3-nitrophenyl
4-nitrophenyl
1-naphthyl
42
77
97
8
60
61
58
n.d.
92
93
98
6
25
25
21
n.d.
a Reactions conditions: 25 °C for 10 days in a 1:1 solution of
methanol and water using 0.5 mmol of aldehyde and 5 mmol of
cyclohexanones with about 0.025 mmol (5 mol %) of catalyst. b Isolated
yield based on aldehydes. c Value represents the major isomer. n.d. )
not determined.
Figure 1. Crystal structure of Ce-MDIP1 showing the positions of the
removable coordinative molecules and the metal ions exposed within the
1D channels (a) and (b); the cyan balls show the removable coordinated
water molecule, and the red, gray, and green spheres represent the oxygen,
carbon, and cerium atoms, respectively. H atoms and lattice water molecules
are omitted for clarity. (c) CD spectra of bulk crystals of Ce-MDIP1
(yellow line) and Ce-MDIP2 (cyan line), respectively, showing the
opposite Cotton effects of the two compounds.
were assessed and shown in Table 2. The same sequence of catalytic
activities corresponding to Cd-TBT in a heterogeneous manner
and free PYI in a homogeneous manner suggested that the aldol
reactions were mainly prompted by the chiral adduct L-BCIP, rather
than the structural motifs of the MOF. Also, structural simulations
of the substrates demonstrated that the rhombus channel of 4.0 ×
6.5 Å2 was not large enough to encapsulate molecules of PYI and
the substrates. In this case, the better enantioselection in the case
of the heterogeneous catalyst than that of the corresponding PYI
and its derivatives in the homogeneous manner13 may partially
originate from the restricted movement of the substrates on the
MOF’s exterior coupled with multiple chiral inductions.
To further investigate the catalytic performance of the MOFs
assembled with the chiral adduct L-BCIP, compound Cd-TBT was
produced by a hydrothermal reaction using cadmium perchlorate,
1,3,5-tris(4-carboxyphenyl)benzene and L-BCIP as the original
reactants. IR spectroscopy of Cd-TBT showed the N-H stretching
band at 2974 cm-1, revealing the incorporation of PYI in the
MOF.17 CD studies of the Cd-TBT bulk samples showed the
homochirality with a positive cotton effect centered at 385 nm.
Single crystal structural analysis revealed that Cd-TBT crystallizes
in the chiral space group P21. It comprised two structurally similar
parts that interpenetrate each other. In each part, the pseudotetra-
hedral Cd2+ ion was coordinated by three bidentate chelating
carboxylate groups from three HTBT bridging ligands and a
nitrogen atom of an imidazole ring as a terminal one. The Cd2+
ion and the HTBT anion bridge each acted as a three-connected
node to produce a 2D honeycomb network. The rhombus channels
with dimensions of 4.0 × 6.5 Å2 (Figure 2) are constructed through
tumbling one 2D sheet to another in an interpenetration manner.
Molecules of PYI locate above or beneath the 2D interpenetration
layer with the potential catalytic active sites N-H of pyrrolidine
exposed.17 Catalysis activities of Cd-TBT in asymmetric aldol
reactions between various aromatic aldehydes and cyclohexanone
Acknowledgment. This work is supported by NSFC
(20871025).
Supporting Information Available: Crystal data (CIF) experimental
details and the general procedure of catalysis reactions. This material
is available free of charge via the Internet at http://pubs.acs.org.
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