ZnII coordination polymers constructed with malonate and bipyridyl ligands: Photoluminescence and heterogeneous catalytic reactivity

Article abstract of DOI:10.1016/j.poly.2013.01.027

Four three-dimensional Zn^II frameworks containing malonates and various bipyridyl pillars (4,4′-bipyridine (1), 1,2-bis(4-pyridyl)ethane (2), 1,2-bis(4-pyridyl)ethylene (3) and 1,3-bis(4-pyridyl)propane (4)) have been prepared, and their structures were determined by X-ray crystallography. In all four frameworks, malonate ligands bridge three Zn ions to form two-dimensional layers, and these layers are connected by bipyridyl pillars to form three-dimensional frameworks with the general formula [{Zn(H₂O)(μ- malonate)}₂(μ-bipyridyl)] and which also contain water or acetonitrile solvates. Reactivity studies of compounds 1-4 showed that they catalyzed a heterogeneous transesterification reaction of phenyl acetate and could be reused without any significant loss of activity through five runs. Emissions were observed at 347-365 nm (λ_ex = 313 nm) for 2 and 363 nm (λ_ex = 328 nm) for 3, while compounds 1 and 4 showed no luminescence properties. The thermal stabilities of these complexes were also examined.

Full text of DOI:10.1016/j.poly.2013.01.027

Polyhedron 53 (2013) 166–171
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
II
Zn coordination polymers constructed with malonate and bipyridyl ligands:
Photoluminescence and heterogeneous catalytic reactivity
a
a
a
a
a
a,
⇑
Min Young Hyun , In Hong Hwang , Myoung Mi Lee , Hyun Kim , Kyung Beom Kim , Cheal Kim
,
Ha-Yeong Kim , Youngmee Kim , Sung-Jin Kim b
b
b,
⇑
a
Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Four three-dimensional ZnII frameworks containing malonates and various bipyridyl pillars (4,4 -bipyri-
dine (1), 1,2-bis(4-pyridyl)ethane (2), 1,2-bis(4-pyridyl)ethylene (3) and 1,3-bis(4-pyridyl)propane (4))
have been prepared, and their structures were determined by X-ray crystallography. In all four
frameworks, malonate ligands bridge three Zn ions to form two-dimensional layers, and these layers
are connected by bipyridyl pillars to form three-dimensional frameworks with the general formula
0
Article history:
Received 23 October 2012
Accepted 19 January 2013
Available online 4 February 2013
Keywords:
[
2 2
{Zn(H O)(l-malonate)} (l-bipyridyl)] and which also contain water or acetonitrile solvates. Reactivity
II
Zn coordination polymers
studies of compounds 1–4 showed that they catalyzed a heterogeneous transesterification reaction of
phenyl acetate and could be reused without any significant loss of activity through five runs. Emissions
were observed at 347–365 nm (kex = 313 nm) for 2 and 363 nm (kex = 328 nm) for 3, while compounds 1
and 4 showed no luminescence properties. The thermal stabilities of these complexes were also examined.
Ó 2013 Elsevier Ltd. All rights reserved.
Malonate
Photoluminescence
Heterogeneous catalysts
Transesterification
1
. Introduction
As an extension of our previous work, we have employed mal-
onate as the flexible
a
,
x
-alkane-dicarboxylate with four bipyridyls
0
0
Metal–organic frameworks (MOFs) are valuable materials for a
wide range of applications, such as selective gas sorption [1–4],
heterogeneous catalysis [5–10], separation [11–15], drug delivery
as the pillars (4,4 -bipyridine (4,4 -bpy), 1,2-bis(4-pyridyl)ethane
(bpa), 1,2-bis(4-pyridyl)ethylene (bpe) and 1,3-bis(4-pyridyl)pro-
pane (bpp)) to prepare four new functional coordination polymers
with intriguing structures and potential applications, particularly
in catalysis. All four compounds possess three-dimensional (3-D)
[
16] and biological imaging [17]. MOFs have been constructed by
metals and ligands with carboxylate moieties. Dicarboxylates have
been used in a variety of metal–organic frameworks (MOFs) with
several metal ions, and their structures have shown various
dimensionalities with different coordination modes and pore sizes.
Rigid, aromatic dicarboxylates [2–7,11,13–15] or flexible cyclohex-
anedicarboxylates [18,19] have been primarily selected as the
frameworks with the general formula [{Zn(H
2 2
O)(l-malonate)}
(l-bipyridyl)] and contain water or acetonitrile solvates. We report
here the syntheses, structures, thermal stabilities and photolumi-
nescence properties of the four novel coordination polymers as
well as their heterogeneous catalytic activities in transesterifica-
tion reactions.
dicarboxylate ligands in coordination polymers. Flexible
a,x-
alkane-dicarboxylates can also be suitable ligands for coordination
polymers with different topologies. Recently, we reported two
Cu-MOFs with flexible glutarate and bipyridyl ligands, which
possessed very similar pore shapes with different pore dimensions
2
. Experimental
[
2 2 2
20]. Both MOFs exhibited good selectivity for CO over N and H ,
2
.1. Materials
and MOF 1 appears to be an efficient, mild and easily recyclable
heterogeneous catalyst for the transesterification of esters [20].
In contrast to metal complexes with aromatic dicarboxylates, few
metal complexes with flexible
been reported in the literature [20–26].
Malonic acid, 1,2-bis(4-pyridyl)ethane, trans-1,2-bis(4-pyri-
0
0
dyl)ethylene, 4,4 -trimethylene dipyridine, 4,4 -bipyridine, aceto-
nitrile, methanol, 4-nitrophenyl acetate, phenyl acetate, 4-
methylphenyl acetate, 4-nitrophenyl benzoate, phenyl benzoate,
a,x-alkane dicarboxylates have
4
-chlorophenyl benzoate, 4-methylphenyl benzoate, vinyl acetate,
methylacetate, methylbenzoate, methyl acetate and Zn(NO ,6H
were purchased from Aldrich and were used as received.
⇑
Corresponding author. Tel.: +82 2 3277 4164; fax: +82 2 3277 3419.
E-mail addresses: chealkim@snut.ac.kr (C. Kim), ymeekim@ewha.ac.kr (Y. Kim).
)
3 2
2
O
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.027

M.Y. Hyun et al. / Polyhedron 53 (2013) 166–171
167
2
.2. Instrumentation
X-ray analysis were obtained after two weeks. The yield was
7.2 mg (82.5%) for compound 4. The purity of a bulk sample of
4 was determined by powder XRD (see Fig. S4 in Supporting infor-
6
The elemental analyses for carbon, nitrogen and hydrogen were
ꢁ1
carried out using a vario MACRO (Elemental Analysensysteme,
Germany) in the Laboratory Center of Seoul National University
of Science and Technology, Korea. IR spectra were measured on a
BIO RAD FTS 135 spectrometer with the samples pressed into
KBr pellets. Thermogravimetric analyses (TGA) were carried out
on a Shimadzu TA50 integration thermal analyzer. Product analy-
ses for the transesterification reactions were performed on either
a Hewlett-Packard 5890 II Plus gas chromatograph interfaced with
a Hewlett-Packard Model 5989B mass spectrometer or on a Donam
Systems 6200 gas chromatograph equipped with an FID detector
with a 30-m capillary column (Hewlett-Packard, HP-1, HP-5 and
Ultra 2). The emission/excitation spectra were recorded on a Per-
kin–Elmer LS45 fluorescence spectrometer.
mation). IR (KBr)
1024(w), 869(w), 809(m), 695(s), 602(w). Anal. calcd. for C23
(651.23), 4: C, 42.42; H, 4.34; N, 8.61. Found: C, 41.88;
H, 4.30; N, 8.72%.
v(cm ): 1575(s), 1447(m), 1339(s), 1223(w),
H
28
4 2
N O10Zn
2.7. Catalytic activity of compounds 1–4
Catalytic reaction conditions: Phenyl acetate (0.05 mmol) was
dissolved in methanol (1 mL), and compounds 1–4 (5.0 mg,
ꢁ3
ꢁ3
8
1
.5 ꢂ 10 mmol for 1; 5.0 mg, 15.7 ꢂ 10 mmol for 2; 5.0 mg,
ꢁ3 ꢁ3
5.7 ꢂ 10 mmol for 3; 5.0 mg, 7.7 ꢂ 10 mmol for 4) were
added and shaken at 50 °C (450 rpm). The reaction conversions
were monitored by GC/Mass analysis of 20 L aliquots withdrawn
l
periodically from the reaction mixture. All of these reactions were
run at least three times and the average conversion yields shown
are based on the formation of methyl acetate.
0
2
.3. Synthesis of [{Zn(H
2
O)(
l
-malonate)}
2
(
l
-4,4 -bpy)]ꢀ(H
2 3
O)(CH CN)
(1)
A solution of 13.1 mg (0.125 mmol) of malonic acid and 37.9 mg
2
.8. X-ray crystallography
(
0.125 mmol) of Zn(NO
ered with a 4 mL acetonitrile solution of 4,4 -bipyridine (39.8 mg,
3
)
2
,6H
2
O in 4 mL of H
2
O was carefully lay-
0
The X-ray diffraction data for all four compounds were collected
0
.25 mmol). Crystals of compound 1 suitable for X-ray analysis
on a Bruker SMART APX diffractometer equipped with a mono-
chromator in an Mo K (k = 0.71073 Å) incident beam. Each crystal
were obtained after two weeks. The yield was 54.3 mg (74.1%)
for compound 1. The purity of a bulk sample of 1 was determined
by powder XRD (see Fig. S1 in Supporting information). IR (KBr)
a
was mounted on a glass fiber. The CCD data were integrated and
scaled using the Bruker-SAINT software package, and the structure
was solved and refined using SHEXTL V6.12 [27]. All hydrogen atoms
were placed in calculated positions. The crystallographic data for
compounds 1–4 are listed in Table 1. Selected bond distances
and angles are listed in Table 2.
ꢁ1
v
8
(
(cm ): 1563(s), 1446(m), 1347(s), 1218(w), 1074(w), 870(w),
13(m), 698(m), 596(m). Anal. calcd. for
586.12), 1: C, 36.88; H, 3.62; N, 7.17%. Found: C, 36.35; H, 3,56;
N, 6.93%.
18 21 3 2
C H N O11Zn
2.4. Synthesis of [{Zn(H
2 2 3
O)(l-malonate)} (l-bpa)]ꢀ2(CH CN) (2)
3
. Results and discussion
A solution of 13.1 mg (0.125 mmol) of malonic acid and 37.9 mg
3
.1. Syntheses and general characterization
(
0.125 mmol) of Zn(NO ,6H O in 4 mL of H O was carefully lay-
ered with a 4 mL acetonitrile solution of 1,2-bis(4-pyridyl)ethane
47.5 mg, 0.25 mmol). Crystals of compound 2 suitable for X-ray
analysis were obtained after one week. The yield was 26.7 mg
67.0%) for compound 2. The purity of a bulk sample of 2 was
determined by powder XRD (see Fig. S2 in Supporting informa-
3
)
2
2
2
The structures of zinc(II)–malonate complexes containing vari-
(
ous bipyridyls have been investigated. The infrared spectra of 1–4
are fully consistent with their formulations. Very strong, slightly
(
ꢁ1
broadened bands at ꢃ1600 and ꢃ1400 cm indicate asymmetric
ꢁ
1
and symmetric C@O stretching modes of the ligated malonate moi-
tion). IR (KBr)
022(w), 873(w), 826(m), 694(m), 602(m), 543(m). Anal. calcd.
for C11 Zn (318.60), 2: C, 41.47; H, 4.12; N, 8.79. Found:
C, 41.89; H, 4.13; N, 9.02%.
v(cm ): 1557(s), 1444(m), 1337(s), 1224(w),
eties [28–30]. The separation (d) between the
d = )– )) suggests that compounds 1–4 may pos-
sess a bridging coordination. The absence of any bands in the area
2
m(CO ) frequencies
1
(
2 2
masym(CO msym(CO
13 2 5
H N O
ꢁ1
of ꢃ1710 cm indicates full deprotonation of all the carboxylate
groups in 1–4 [28–30], which is consistent with the results of the
X-ray analysis.
2
.5. Synthesis of [{Zn(H
2 2 3
O)(l-malonate)} (l-bpe)]ꢀ2(CH CN) (3)
A solution of 13.1 mg (0.125 mmol) of malonic acid and 37.9 mg
II
(
0.125 mmol) of Zn(NO ,6H O in 4 mL of H O was carefully lay-
ered with a 4 mL acetonitrile solution of trans-1,2-bis(4-pyri-
dyl)ethylene (47.0 mg, 0.25 mmol). Crystals of compound
3
)
2
2
2
3.2. Structures of MOFs of Zn containing malonate and various
bipyridyl ligands
3
suitable for X-ray analysis were obtained after 10 days. The yield
was 28.6 mg (72.4%) for compound 3. The purity of a bulk sample
of 3 was determined by powder XRD (see Fig. S3 in Supporting
The X-ray structure analyses of all four compounds revealed
three-dimensional (3-D) frameworks with the general formula
[{Zn(H O)(l-malonate)} (l-bipyridyl)]. The malonate ligands
2 2
ꢁ1
information). IR (KBr)
224(w), 1022(w), 979(w), 904(w), 817(m), 701(m), 550(m). Anal.
calcd. for C11 Zn (317.6) 3: C, 41.60; H, 3.82; N, 8.82. Found:
C, 42.05; H, 3.75; N, 8.53%.
v
(cm ): 1571(s), 1443(m), 1345(s),
bridge three Zn ions to form two-dimensional layers, and these lay-
0
1
ers are connected by bipyridyl pillars (4,4 -bpy, bpa, bpe and bpp)
12
H N
2
O
5
to form 3-D frameworks (Fig. 1 for 1, Fig. S5 for 2, Fig. S6 for 3 and
Fig. S7 for 4). The coordination mode of each carboxylate of a mal-
II
onate is bridging, and each malonate bridges three Zn ions. The 3-
2
.6. Synthesis of [{Zn(H
2
O)(
l
-malonate)}
2
(
l
-bpp)]ꢀ2(CH
3
CN) (4)
D frameworks contain one-dimensional (1-D) channels occupied
by water and acetonitrile solvate molecules in 1 and by acetonitrile
solvate molecules in 2, 3 and 4. The pore dimensions for 1 are esti-
mated by the atom-to-atom closest distances (6.69 ꢂ 3.25 Å), and
those for 2 are estimated by the distance between parallel pyridyl
groups and the distance between the two planes determined by
A solution of 13.1 mg (0.125 mmol) of malonic acid and 37.9 mg
(
0.125 mmol) of Zn(NO
3
)
2
,6H
2
O in 4 mL of H
2
O was carefully lay-
0
ered with a 4 mL acetonitrile solution of 4,4 -trimethylene dipyri-
dine (50.6 mg, 0.25 mmol). Crystals of compound 4 suitable for

1
68
M.Y. Hyun et al. / Polyhedron 53 (2013) 166–171
Table 1
Crystallographic data for compounds 1–4.
1
2
3
4
Empirical formula
Formula weight
T (K)
k (Å)
Space group
a (Å)
C
18
H
21
N
3
O
11Zn
2
C
11
H
13
N
2
O
5
Zn
C
11
H
8
N
2
O
5
Zn
C
23 28 4 2
H N O10Zn
586.12
293(2)
0.71073
Pna2
14.610(3)
7.3190(15)
20.564(4)
90.00
90.00
90.00
2198.9(8)
4
1.770
318.60
170(2)
0.71073
313.56
170(2)
0.71073
615.23
170(2)
0.71073
Aba2
26.931(5)
10.336(2)
10.248(2)
90.00
90.00
90.00
2852.6(10)
4
1.516
1.740
1
P2
1
/n
Pnn2
7.2490(14)
23.930(5)
7.4190(15)
90.00
90.37(3)
90.00
1286.9(4)
4
1.644
7.3070(15)
24.960(5)
7.2700(15)
90.00
90.00
90.00
1325.9(5)
4
1.571
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
calc (Mg/m3)
Absorption coefficient (mmꢁ1)
Crystal size (mm)
Reflections collected
2.248
1.926
1.868
0.50 ꢂ 0.12 ꢂ 0.05
0.25 ꢂ 0.20 ꢂ 0.04
0.20 ꢂ 0.12 ꢂ 0.10
0.22 ꢂ 0.18 ꢂ 0.01
11528
6895
6804
7631
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
)
3346 (0.0304)
3346/7/327
1.030
2477 (0.0239)
2477/2/179
1.082
2533 (0.0400)
2533/17/149
1.062
2733 (0.0383)
2733/3/184
1.053
Final R indices [I > 2
R indices (all data)
r
(I)]
R
R
1
= 0.0273, wR
= 0.0372, wR
2
2
= 0.0692
= 0.0739
R
R
–
1
= 0.0321, wR
= 0.0356, wR
2
2
= 0.0753
= 0.0764
R
R
1
= 0.0433, wR
= 0.0570, wR
2
= 0.1066
= 0.1127
R
R
1
= 0.0302, wR
= 0.0364, wR
2
= 0.0660
= 0.0677
1
1
1
2
1
2
Absolute structure parameter
0.40(2)
0.293 and ꢁ0.414
0.52(5)
0.675 and ꢁ0.429
0.021(15)
0.367 and ꢁ0.277
Largest difference peak and hole (e Åꢁ3)
0.593 and ꢁ0.916
Table 2
Selected bond distances (Å) and angles (°) for compounds 1–4.
1
2
3
4
Zn–O
Zn–Owater
Zn–N
2.080(4)–2.127(4)
2.148(6), 2.141(6)
2.153(5)–2.168(5)
173.57(10), 176.67(11)
2.1037(18)–2.1329(18)
2.085(7)–2.112(6)
2.137(3)
2.139(3)
2.101(2)–2.116(2)
2.1113(18)
2.117(6)
–
2.123(2)
2.128(3)
178.01(10)
N–Zn–Owater
178.8(3)
the N11 and C13 atoms (3.85 ꢂ 5.11 Å). The pore dimensions for 3
and 4 are estimated by the atom-to-atom closest distances, which
are 3.87 ꢂ 5.47 Å and 3.63 ꢂ 6.76 Å, respectively (Fig. 2). The sol-
three water molecules (calcd. 9.22%). The third gradual weight loss
of 56.38% in the temperature range 219–401 °C corresponds to the
0
loss of two coordinated malonate ligands and one 4,4 -bipyridine
3
vent-free 3-D frameworks possess 19.9% (438.1 Å/2198.9 Å ),
ligand (calcd. 56.00%). Compound 2 exhibited an 18.50% weight
loss in the temperature range 26–218 °C, which corresponds to
the loss of one acetonitrile molecule and one coordinated water
molecule (calcd. 18.54%; Fig. S9). The second gradual weight loss
of 55.75% in the temperature range 218–472 °C is consistent with
the loss of one coordinated malonate and one-half of a coordinated
1,2-bis(4-pyridyl)ethane ligand (calcd. 55.92%). For compound 3,
an 18.50% weight loss was observed from 66 to 229 °C, correspond-
ing to the loss of one acetonitrile molecule and one water molecule
(calcd. 18.60%) (Fig. S10). The second gradual weight loss of 50.63%
in the temperature range 230–495 °C corresponds to the loss of
one coordinated malonate and one-half of a trans-1,2-bis(4-pyri-
dyl)ethylene ligand (calcd. 50.75%). For compound 4, the first
weight loss in the range 26–200 °C was consistent with the loss
of two acetonitrile and two water molecules (obsd. 18.00%, calcd.
18.14%) (Fig. S11). The second gradual weight loss of 47.88% in
the temperature range 201–800 °C corresponds to the loss of one
3
3
2
(
3.1% (297.9 Å/1286.9 Å ), 29% (384.9 Å/1325.9(5) Å ) and 29.4%
3
837.8 Å/2852.6(10) Å ) void volumes for 1–4, respectively, based
on PLATON analyses [31]. The longer pillar provides the larger void
volume. Various gas sorption analyses for the MOFs were per-
2 2 2
formed using high-purity N , H and CO , but no gas sorption
was observed. The structures of 1–3 indicate 7-connected uninodal
6
8
7
nets with
7
a
Schläfli symbol of 3 ꢀ4 ꢀ5 and
4 indicates a
6
6
9
-connected uninodal net with a Schläfli symbol of 3 ꢀ4 ꢀ5 , by TO-
POS analyses [32]. In the 3-D frameworks, the coordination geom-
II
etry of each Zn ion is distorted octahedral, constructed
equatorially by four oxygen atoms from three malonates and
axially by one water oxygen and one pyridyl nitrogen. The
Zn–OCO2 bond distances range from 2.080(4) to 2.127(4) Å for
1
,
2.1037(18)–2.1329(18) Å for 2, 2.085(7)–2.112(6) Å for
and 2.101(2)–2.116(2) Å for 4. The Zn–Owater bond distances
2.1113(18)–1.148(6) Å for all four compounds) are a little longer
than those of the Zn–OCO2 bonds.
3
(
0
coordinated 4,4 -trimethylene dipyridine and two malonate li-
gands (calcd. 51.95%).
3.3. Thermogravimetric analysis
3.4. Photoluminescence properties
To study the thermal stabilities of these complexes, thermal
gravimetric analyses (TGA) of compounds 1–4 were performed.
The TGA curve of compound 1 showed a weight loss within the
temperature range from 45 to 83 °C of 7.25%, which corresponds
to the removal of one acetonitrile molecule from each complex
Luminescent coordination complexes have attracted great
attention because of their interesting photochemistry and their po-
tential applications as chemical sensors and electroluminescent
displays [33–35]. The synthesis of metal–organic complexes from
conjugated organic spacers and metal centers can be an efficient
method for obtaining new luminescent materials, especially for
(
calcd. 7.00%; Fig. S8). The second gradual weight loss of 9.50% in
the temperature range 84–218 °C is consistent with the loss of

M.Y. Hyun et al. / Polyhedron 53 (2013) 166–171
169
0
2 2
Fig. 1. (a) Crystal structure of [{Zn(H O)(l-malonate)} (l-4,4 bpy)] (1) along the a axis. (b) Crystal structure of 1 along the b axis with a 2-D sheet. All solvent molecules are
omitted for clarity.
d10 systems [36–38]. Therefore, the solid-state emission spectra of
compounds 1–4 and those of the bipyridyl ligands and malonic
acid were measured at room temperature (Fig. 3). Emissions were
observed at 347–365 nm (kex = 313 nm) for 2, 363 nm (kex = 328 -
nm) for 3, 445–470 nm (kex = 229 nm) for bpa, 510 nm (kex = 452 -
nm) for bpe, 455–490 nm (kex = 320 nm) for malonic acid, whereas
compounds 1 and 4 showed no luminescence. The emission bands
of complexes 2 and 3 are blue-shifted compared to the correspond-
ing ligands, which indicates that the emission peaks of 2 and 3 may
be due to metal-to-ligand charge transfers (MLCT) [39,40]. As com-
pounds 1–4 share the same structure type, it is not clear why 1 and
convenience [41–44]. These were then treated with phenyl acetate
and methanol at room temperature under neutral conditions
(Eq. (1)). This reaction quantitatively produced the corresponding
methyl acetate product within 7, 9, 7 and 9 days from 1, 2, 3 and
4, respectively (see the experimental section for the detailed reac-
tion conditions), suggesting that the pore volumes of compounds
1–4 do not correlate with their activities. These results led us to
propose that the transesterification reactions might occur outside
of the frameworks. The phenyl acetate substrate might replace
the labile water molecule of the Zn center outside the MOF to give
the Zn(substrate) adduct; then, the nucleophile methanol may
attack the carbonyl carbon on the adduct to produce the methyl
acetate product. A control reaction without the catalysts showed
trace amounts of the transesterification products in the same
reaction time [45–48], and no induction period was observed for
any of the reactions (Fig. S12).
4
do not possess luminescence properties. Notably, complex 2
showed more intense emissions compared with complex 3 at room
temperature; this may be a result of the effective increase in rigid-
ity for complex 3, which would reduce the loss of energy by radi-
ationless decay. These results suggest that 2 may be a good
candidate for the development of a hybrid inorganic–organic pho-
toactive material.
O
O
catalysts
OCCH3
+
HOCH
..
OH + CH COCH3
3
3
3.5. Catalytic transesterification reactions by compounds 1–4
ð1Þ
Although an enormous number of MOFs have been reported
Because the potential benefits of such heterogeneous catalysts
include the facilitation of catalyst separation from the reagents
and reaction products, and the simplification of catalyst recycling
[41–44], we have applied our previous experience [41–44] to
examine the recyclability of catalysts 1–4 at room temperature.
After reaction completion, the catalysts were recovered by
filtration and thoroughly washed with methanol for multiple
consecutive runs. The recovered catalysts were re-used with fresh
during the past ten years, their application as heterogeneous cata-
lysts is relatively rare [1–17]. Because the application of MOFs 1–4
as catalysts for various organic reactions is important, we have
examined their activity as heterogeneous catalysts for the
transesterification of esters under mild conditions. Samples of
MOFs 1–4 were ground thoroughly into appropriate particle sizes
in order to maximize the surface area while preserving filtration

1
70
M.Y. Hyun et al. / Polyhedron 53 (2013) 166–171
Fig. 2. The dimensions of 1-D channels of 1 (a), 2 (b), 3 (c) and 4 (d).
dominant reactive species were the heterogeneous catalysts 1–4
and that leached metal ions did not contribute significantly to
catalysis.
4
. Conclusion
Four 3-D ZnII frameworks containing malonates and various
0
bipyridyl pillars (4,4 -bpy (1), bpa (2), bpe (3) and bpp (4)) have
been prepared and their structures determined. In all four frame-
works, malonate ligands bridge three Zn ions to form 2-dimen-
sional layers, and these layers are connected by bipyridyl pillars
to form 3-D frameworks with the general formula [{Zn(H
2
O)
(l
-malonate)} -bipyridyl)] and which also include water or ace-
2
(l
tonitrile solvates. Photoluminescence studies showed emissions
for 2 and 3 at 347–365 nm (kex = 313 nm) and 363 nm (kex = 328 nm),
respectively, whereas compounds 1 and 4 showed no lumines-
cence properties. In particular, the strong emissions from 2 indi-
cate that it may be a good candidate for luminescent material
development. Moreover, MOFs 1–4 appear to be efficient, mild
and easily recyclable transesterification catalysts; they constitute
a promising class of heterogeneous catalysts that can be reused
without significant loss of activity through five reaction cycles. Fur-
thermore, these results may represent an excellent starting point
for the development of new synthetic methods to produce MOFs
as recyclable catalysts.
Fig. 3. Solid-state photoluminescent spectra of complexes 2, 3, free ligands and
acid.
phenyl acetate and were found to exhibit almost the same transe-
sterification catalytic activity. Catalysts 1–4 showed no significant
deterioration of catalytic activity through five cycles (see
Table S1).
We then performed another control reaction to ascertain
whether any of the observed catalyzes may be attributable to lea-
ched metal species from the catalysts, rather than the heteroge-
neous MOF catalysts [41–44,49]. To test whether zinc had been
leached from the catalysts during the transesterification reactions,
each reaction mixture was filtered after the reaction. Then, the
amount of zinc dissolved in each supernatant was measured by
inductively coupled plasma (ICP) spectroscopic analysis and
determined to be <15 ppm. This result strongly suggests that the
Acknowledgements
Financial support from the Converging Research Center Pro-
gram through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2012001725, 2011K000675, and 2012008875) and Basic Science
Research Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and

M.Y. Hyun et al. / Polyhedron 53 (2013) 166–171
171
Technology (20110008018/20120004193) and RP-Grant 2012 of
Ewha Womans University is gratefully acknowledged.
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