From one to three: A serine derivate manipulated homochiral metal-organic framework

Article abstract of DOI:10.1039/b823323c

A porous homochiral MOF was constructed from a serine derivate ligand bridged and chelated copper atom subunit, which can expand one chiral center into multitopic homochiral centers to systematically tune the heterogeneous catalytic properties. The Royal

Full text of DOI:10.1039/b823323c

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From one to three: a serine derivate manipulated homochiral
metal-organic frameworkw
Miao Wang, Ming-Hua Xie, Chuan-De Wu* and Yan-Guang Wang*
Received (in Cambridge, UK) 5th January 2009, Accepted 20th February 2009
First published as an Advance Article on the web 12th March 2009
DOI: 10.1039/b823323c
A porous homochiral MOF was constructed from a serine
derivate ligand bridged and chelated copper atom subunit, which
can expand one chiral center into multitopic homochiral centers
to systematically tune the heterogeneous catalytic properties.
new 2D homochiral solid [Cu2L2Cl2]ꢀH2O (1), which presents
interesting heterogeneous catalytic properties.
The chiral ligand L was synthesized according to a literature
method,11 while blue crystals of 1 were formed from a mixture
of L and CuCl2ꢀ2H2O in a mixed solvent of H2O and EtOH.z
Compound 1 is not soluble in water and common organic
solvents, such as MeOH, EtOH, THF and DMF etc. It is very
stable in basified solution, but the framework structure can be
destroyed by adding strong acidified HCl solution. Single
X-ray structural analysis has revealed that 1 crystallizes in
the monoclinic P21 space group.y There are two copper(II)
atoms, two L ligands, two chlorides and one water molecule
guest in the asymmetric unit (Fig. 1). The Cu atoms comprise
two kinds of coordination geometries. Every Cu atom coordinates
to one carboxyl group (Cu–O = 1.957(3)–1.973(3) A), one
amino group (Cu–N = 1.981(3)–1.989(3) A) and one hydroxyl
group (Cu–O = 2.392(3)–2.576(3) A) from one L ligand, with
one pyridyl group (Cu–N = 2.016(3)–2.023(3) A) from the
neighboring L ligand. Additionally, one crystallographically
independent copper atom coordinates to one chloride
In the past decades, the generation of metal-organic frame-
works (MOFs) has gained considerable interest because of
their variety of topological architectures and diverse fascinating
functionalities for potential applications.1–7 The systematic
tunable organic linkers and the various geometrical metal
nodes are two key components for the formation of desired
functional MOFs. Under suitable reaction conditions, the
prescribed architectures of MOFs comprising periodically
decorated functionalities can be generated, which might act
as promising candidates for special applications.
In our work on the synthesis of functionalized homochiral
MOFs, we are particularly interested in the synthesis of
coordination solids based on multitopic linkers derived from
amino acids because of their biological functional properties
and highly selective substrate-binding abilities.8–10 According
to the coordination preferences of transition metal ions, we
have modified the serine molecule with a pyridyl unit to
generate an effective bridging linker, (S)-3-hydroxy-2-(pyridin-
4-ylmethylamino)propanoic acid (L, Scheme 1). Basically,
the additional pyridyl group together with the carboxylate
group of L can bridge transition metal ions to form extended
solid-state materials, while the remaining unit can act as a
functional group for special applications. Most important of
all, a second homochiral center can be generated when the
amino group is coordinated to the metal ion, which is a
subsequent induction of the neighboring homochiral carbon
center. Furthermore, the metal ion can be chelated by the
amino group and the carboxylate group together with two
chiral centers, which act as the third chiral center in some
cases. Herein we wish to report our recent elaboration on a
(Cu–Cl
= 2.263(1) A) in square-pyramidal geometry,
while another copper atom coordinates to two chlorides
(Cu–Cl = 2.263(1)–3.001(1) A) to generate distorted octahedral
geometry. Subsequently, each L ligand chelates one copper atom
through hydroxyl, carboxyl and amino groups and further
bridges a neighboring copper atom via the pyridyl group to
extend into 1D polymeric chains running along two different
orientations. The chains are further coupled by one of the two
chlorides to form a thick bilayer framework structure (Fig. 2).
Two striking features of 1 should be noted. In the bilayer
framework structure of 1, the two-directional chains stack on
each other in such a way that each Cu(II) center is located ca.
4.791 A below the gaps of the next chain (Fig. 2b and c).
Another interesting feature of 1 is that the nitrogen atom of
the amino group is induced by the neighboring chiral carbon
center to generate the second homochiral center, which is
stabilized via coordinating to the copper atom. Consequently,
Scheme 1
Department of Chemistry, Zhejiang University, Hangzhou 310027,
PR China. E-mail: orgwyg@zju.edu.cn; cdwu@zju.edu.cn;
Fax: (+86)-571-87951895; Tel: (+86)-571-87953740
w Electronic supplementary information (ESI) available: Additional
experiments and figures of 1. CCDC 709104. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b823323c
Fig. 1 ORTEP representation of the symmetry expanded local
structure for 1 (25% probability ellipsoids).
ꢁc
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2396 | Chem. Commun., 2009, 2396–2398

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Scheme 2
isolated yield (Scheme 2). Despite the excellent yield reached,
however, we failed to observe any enantioenriched isomer
from the product. The supernatant from solid 1 was inactive
for the reaction, which confirmed the heterogeneous behavior
of the catalyst system.
To further test the heterogeneous catalytic behavior of 1,
addition of Grignard reagent to a,b-unsaturated ketones was
used to assess catalytic ability. Solid 1 catalyzed the addition
of cyclohexylmagnesium chloride to (E)-4-phenylbut-3-en-2-one
to afford the 1,2-addition product with excellent conversion
(97%) and enantiomeric excess (ee) of 65% (Table 1, entry 1).
Additional experiments showed that ether solvent media was
slightly better than THF, while low reaction temperature can
improve the ee value but depress the conversion (entries 2–3).
Solid 1 also catalyzed 1,2-addition of Grignard reagent to a
range of other ketones (aldehyde) to give high conversions
with the highest ee value up to 99% (entries 5–7). However, the
–NO2 substituted substrate only generated the corresponding
product with very low conversion (o10%), which might be a
consequence of the weakened electronic density induced by the
–NO2 group (entry 8).
Fig. 2 (a) Perspective view of the lamellar framework of 1 (one
orientation chains are highlighted in orange); (b) and (c) side and
top views of the geometric relationship between two directional
chain frameworks (ball-and-stick and space-filling representations,
respectively) within a thick layer; (d) packing diagram of 1 viewed
down the a axis, showing the cavities that accommodated water
molecular guests (space-filling spheres).
the double homochiral center-chelated copper atom is induced
into the third homochiral center.
All thick lamellar frameworks are stacked together via
supermolecular interactions to form a 3D porous framework
with 1D chiral channels (5.123 ꢂ 2.866 A2, van der Waals radii
taken into account). The cavities are occupied by water
molecule guests, which were confirmed by thermogravimetric
analysis (TGA) and PLATON calculations (Fig. 2d).12 The
supermolecular interactions are strong edge. . .p interactions
between the uncoordinated carboxyl group and the pyridyl
ring with the shortest O. . .C distances of 3.056(5) A,
hydrogen bonding between the chloride and the amino group
(Cl. . .N = 3.583(4)–3.683(4) A), and hydrogen bonding
between the hydroxyl group and the uncoordinated carboxyl
group (O. . .O = 2.737(4)–2.742(4) A). The clathrated water
molecular guests contact to the hydroxyl groups of the host
framework through relatively weak hydrogen bonding
(O. . .O = 3.053(8) A), which can be easily removed by heating
solid 1 in vacuum. TGA indicates that a weight loss of 3.2%
occurred between 20 and 90 1C, corresponding to the loss of
solvent molecules (expected 3.0%). There was almost no
weight loss up to 191 1C. Above 191 1C, compound 1 began
to lose coordinated L ligand and to decompose. After a sample
of 1 was ground and heated at 110 1C for 5 h, an XRPD of the
resultant powder showed a sharp diffraction pattern similar to
that of the pristine sample. This result indicates that the
neutral chiral porous framework was maintained after the
removal of solvent molecules.
Solid
1 saturated supernatant cannot promote the
transformation under otherwise identical conditions. To check
whether the basic Grignard reagent can destroy the framework
structure, and subsequently utilize the released free ligand
or copper ions to catalyze the reaction, we have added
a,b-unsaturated ketone into the filtrate from a mixture of
Grignard regent and solid 1 in THF under N2 atmosphere.
After 12 h under identical reaction conditions, we cannot
detect trace conversion from a,b-unsaturated ketone. All these
experiments proved the heterogeneous nature of the
Table 1 Compound 1 catalyzed 1,2-addition of a,b-unsaturated
ketones (aldehyde)a
Entry
R1
R2
Catalyst
Solvent
Conv.%
ee%b
1
2
3
4
5
6
7
8
H
H
H
H
Me
Me
Me
Me
H
Me
Me
Me
1
1
1
L
1
1
1
1
THF
Et2O
THF
THF
THF
THF
THF
THF
97
98
48
84
88
93
94
o10
65
67
88c
51
The interesting topological framework structure comprising
multitopic homochiral centers prompted us to study the
heterogeneous catalytic properties of material 1, as copper(II)
compounds are promising catalysts in many important
organic transformations, such as cross-coupling reaction,
oxidation, Biginelli reaction, Michael addition, etc.13 To evaluate
the heterogeneous catalytic properties of 1, we first probed the
Biginelli catalytic reaction of benzaldehyde, urea and ethyl
acetoacetate in the presence of solid 1 in CH2Cl2 at 40 1C for
12 h to give corresponding dihydropyrimidinone in 90%
H
55
Me
Cl
NO2
499
97
—
a
Solid 1 (0.01 mmol), substrate (0.1 mmol) and Grignard reagent
(0.12 mmol) were mixed in THF or Et2O (2 ml) under N2 at 0 1C,
which was warmed to room temperature over 12 h. Conversion%
b
and ee% were determined by GC on a Supelco b-Dex 120 column.
c
The reaction temperature is ꢃ80 1C.
ꢁc
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Chem. Commun., 2009, 2396–2398 | 2397

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current catalyst system. However, it has been known that
copper compounds only promote 1,4-addition of Grignard
reagent to a,b-unsaturated ketones.13 Our results showed the
transformation underwent an otherwise catalytic mechanism.
To discover which part is responsible for the catalytic process,
reference reactions with L ligand and CuCl2 were carried out.
Free ligand can generate the 1,2-addition product with
conversion of 84% and ee value of 51%, respectively
(Table 1, entry 4), while copper chloride did not work at all.
The reference reactions suggest that the transformation is
a typically organic functionality promoted process.14 The
improved enantioselectivity for 1 is attributed to the homochiral
amino group in solid 1 instead of the achiral center in L ligand.
Our results demonstrated that the incorporation of suitable
synthons in a network can generate a good catalyst. Structural
analysis of 1 revealed that the active sites exist mainly on the
surface and the small size of the pores avoids the accessibility
of substrates to active centers through the channels, which
suggest that the catalytic reaction took place on the catalyst
surface.
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In summary, we have successfully synthesized a new porous
homochiral MOF constructed from serine derivate ligand
bridged copper atoms. We have observed remarkable
asymmetric catalytic activities of the current framework
structure for heterogeneous catalysis. This work established
a new strategy in the rational expandsion of one chiral center
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as
a chiral molecular module for the construction of
porous homochiral MOFs to systematically tune heterogeneous
catalytic properties.
We are grateful for the financial support of the NSF of
China (Grant No. 20673096), Zhejiang Provincial Natural
Science Foundation of China (Grant No. R406209) and the
State Education Ministry.
Notes and references
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ꢁc
This journal is The Royal Society of Chemistry 2009
2398 | Chem. Commun., 2009, 2396–2398