Inorganic Chemistry Communications
“Click” post-synthetic modification of metal−organic frameworks for
asymmetric aldol catalysis
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Wenting Zhu, Cheng He, Xiao Wu, Chunying Duan
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of dimethyl 5-(prop-2-ynyloxy)isophthalic acid, meso-alpha, and beta-di (4-pyridyl) glycol with
ZnCl2 in a methanol aqueous solution affords MOF Zn−DPPG. The enantiomeric Zn−Fun1 and Zn−Fun2 were ob-
tained through a strategy of in situ click reactions of Zn–DPPG with two opposite chiral adducts. With the L- or
D-proline chiral functionality, the two Zn−Funs exhibited remarkable catalytic activities in the relative asym-
metric aldol reactions, leading to the formation of opposite enantiomorphs, respectively.
© 2013 Published by Elsevier B.V.
Received 24 September 2013
Accepted 25 October 2013
Available online 6 November 2013
Keywords:
Click
Metal–organic framework
Postsynthesis
Aldol catalysis
Metal–organic frameworks had received intense interest due to
their tunable structural features, high surface areas, and well defined
pore architectures [1–4]. The research of MOFs with functional proper-
ties had led to intense studies because they possess advantageous cu-
mulative attributes emerging from both metal and ligand components
[5–8]. The structural and functional diversity of these coordination poly-
mers has brought these materials wide attention in recent years. As
good candidates for heterogeneous asymmetric catalysts, MOFs had
the tenability, versatility and original flexibility, of which place them-
selves at the frontier between zeolites and enzymes [9–12].
In our previous work, we described the generation of two enantio-
meric solids Zn–MOF1 and Zn–MOF2, through an in situ click reaction
between the external alkynes of a preassembled achiral frameworks
and the azide groups on the specially enantioselective catalytic adduct,
for the application on asymmetric aldol reactions [13]. In the present
work, the MOFs Zn-DPPG was obtained. Single crystal structural analy-
sis demonstrated the Zn–DPPG was crystallized in a space group Pbca
with one zinc ion, one depronated L1 ligand, one L2 adduct molecule
and two water molecules in an asymmetric unit [14]. As shown in
Fig. 1a, each zinc (II) atom was coordinated by two oxygen from two dif-
ferent L1 ligands and two nitrogen donors from two different L2 linkers.
The zinc (II) ions were connected by depronated DPYI2− ligands alter-
natively to form a one-dimensional infinite chain. Adjacent chains
were linked together by L2 ligands to consolidate a wave-like two-
dimensional sheet structure with angles of N(1)–Zn(1)–N(2) being
114°. The presence of large void in the two-dimensional network
makes the adjacent layers interpenetrate each other to form a 2-fold
parallel interpenetrating framework (Fig. 1b). They are stacked parallel
to get a pseudo 3d network structure through the strong π⋯π interac-
tions between the benzene rings in adjacent L1 ligands. The alkyne moi-
eties were well-positioned in the channels as clickable sites.
The block crystals of Zn-DPPG and L-AMP were mixed at 1:10 molar
ratio in the presence of Cu(CH3CN)4ClO4 at 323 K in CH3OH. After isola-
tion of the MOFs with the removal of unreacted azide and solvents by
repeated washing and drying in vacuum, the alkyne stretching band at
3282 cm−1 and 2121 cm−1 decreased and completely disappeared
after 24 h monitored by IR spectra [15]. Clear proof of click reaction hap-
pened was obtained by 1H NMR, samples of Zn–DPPG were dissolved in
dilute DCl/DMSO-d6 solution and the signals of alkyne in 3.63 ppm is
disappearance and the corresponding triazole derivative was formed
as the unique product was assigned by 1H NMR in 8.44 ppm (Fig. 2)
[16]. These results suggested the progressing of the click reaction in
the Zn–DPPG network (the post-synthetic yield is about 90%). More-
over, most of the clicked Zn–DPPG crystals were still kept their shape
with the main peaks in the XRD pattern maintained after the click reac-
tion. These results suggested that the click reaction proceeded without
dissolution and reconstruction of MOF in CH3OH.
The catalytic activities of Zn−Fun1 in asymmetric aldol reactions
between various aromatic aldehydes and cyclohexanone were
assessed (Table 1). First, Zn−Fun1 catalyzed an aldol reaction be-
tween 4-nitrobenzaldehyde and cyclohexanone with a decent yield
(85%) and a fairly high enantioselectivity (ee 77%). A control experi-
ment with unmodified Zn–DPPG showed less than 10% conversion
and almost undectable ee value (possibly catalyzed by the mild Lewis
acidic zinc centers). Interestingly, at the same reaction conditions
when performed under homogeneous conditions with L-AMP resulted
in much lower ee value (26%). The better enantioselectivity in the
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