Controlling self-assembly of zinc(II)-benzoate coordination complexes with 1,4-bis(4-pyridyl)ethane by varying solvent and ligand-to-metal ratio: Their catalytic activities

Article abstract of DOI:10.1016/j.poly.2008.11.048

Four new coordination polymers formed by zinc-benzoate with the 1,2-bis(4-pyridyl)ethane (bpe) bridging ligand have been prepared and characterized. Zinc-benzoates can be rationally tuned to form four different structures with a bridging bpe ligand by con

Full text of DOI:10.1016/j.poly.2008.11.048

Polyhedron 28 (2009) 553–561
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Controlling self-assembly of zinc(II)-benzoate coordination complexes with
1,4-bis(4-pyridyl)ethane by varying solvent and ligand-to-metal ratio: Their
catalytic activities
Han Kwak a, Sun Hwa Lee a, Soo Hyun Kim a, Young Min Lee a, Byeong Kwon Park a, Yu Jin Lee a, Je Yeol Jun a,
b,
b
Cheal Kim a, , Sung-Jin Kim , Youngmee Kim
*
*
a Department of Fine Chemistry, Eco-Product and Materials Education Center, Seoul National University of Technology,
172 Kongnung 2-Dong Nowon-Gu, Seoul 139-743, Republic of Korea
b Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new coordination polymers formed by zinc-benzoate with the 1,2-bis(4-pyridyl)ethane (bpe) bridg-
ing ligand have been prepared and characterized. Zinc-benzoates can be rationally tuned to form four dif-
ferent structures with a bridging bpe ligand by controlling ligand-to-zinc-benzoate molar ratios and by
using different solvent systems, and reveal three coordination polymers having similar one-dimensional
characteristics but having different mono-, di-, trinuclear nodes (1–3), and a dinuclear ring type molecule
(4). This work reveals that the ligand-to-metal ratio and solvent play very important roles in the forma-
tion of different coordination structures. We have also shown that the compounds 1–4 catalyzed effi-
ciently the transesterification of a variety of esters. The complex 3 showed the most efficient reactivity
and is the best among the catalytic efficiencies reported previously with zinc-containing coordination
and polymeric compounds. The substrates with the electron-withdrawing substituents have undergone
faster transesterification than those with the electron-donating ones. In addition, the scope of the appli-
cation of 1–4 as transesterification catalysts has been expanded to now include ethanol and propanol,
suggesting that this catalytic system can be potentially useful for preparing various esters by transeste-
rification. Moreover, the transesterification reaction mechanism was discussed by 1H NMR study.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 1 October 2008
Accepted 19 November 2008
Available online 2 January 2009
Keywords:
Zinc-benzoate
Coordination polymers
Transesterification
Catalytic activity
1. Introduction
ganic spacers over the past few years, there is still little under-
standing of how ligand-to-metal stoichiometry and solvent
The design and construction of metal-organic coordination poly-
mers has recently attracted much attention not only due to their
structural and topological novelty [1–8], but also for their potential
applications as functional materials such as gas storage [9–15], ion-
exchange [16,17], catalysis [18–27], magnetism [28–30], and
molecular sensing [31]. The rational design of these coordination
polymers is highly affected by several factors such as the coordina-
tion geometry of metal ions [32,33], the structure of organic ligands
[34–36], the solvent system [37,38], pH value [39–41], temperature
[42,43], the counteranion [44,45], and the ligand-to-metal ratio
[46,47]. Therefore, an investigation for understanding the relation-
ships between the structures of complexes and those factors is at
the center of controlling coordination network assembly.
system among several factors can be used to control the type of
polymeric structures. While these two factors are relatively nota-
ble in copper- and silver-containing coordination polymers [48–
53], their influence on the formation of coordination polymers,
especially, with zinc ion is much less well understood and system-
atic studies of their role are very scarce [54–56].
Quite recently, we have reacted zinc-benzoate with pyrazine
and its derivatives, 2,3-dimethylpyrazine, 2,5-dimethylpyrazine,
2,6-dimethylpyrazine, and quinoxaline, and characterized five
new Zn(II)-benzoate coordination architectures, from dinuclear,
trinuclear and pentanuclear complexes to one-dimensional and
two-dimensional coordination polymers [57]. These results have
shown that the substituents of pyrazine are very important roles
for construction of various zinc-benzoate complexes. Interestingly,
the compounds catalyzed efficiently the transesterification of a
variety of esters.
Though numerous coordination polymers and macrocycles have
been generated from transition metal nodes with a variety of or-
As an extension of our previous work, in this study, we have
investigated the complex formation of zinc-benzoate with another
bridging ligand 1,2-bis(4-pyridyl)ethane (bpe) in order to further
* Corresponding authors. Tel.: +82 2 970 6693; fax: +82 2 973 9149 (C. Kim), tel.:
+82 2 3277 3589; fax: +82 2 3277 3419 (S.-J. Kim).
E-mail addresses: chealkim@snut.ac.kr (C. Kim), sjkim@ewha.ac.kr (S.-J. Kim).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.11.048

554
H. Kwak et al. / Polyhedron 28 (2009) 553–561
prepare functional supramolecular complexes with intriguing
structures and potential applications especially in catalysis and
to systematically investigate the influence of the concentration
and solvent effects less studied on the assembly processes of the
structure of zinc-carboxylate containing coordination polymers.
Here, we report the syntheses, characterization and crystal struc-
tures of four zinc coordination complexes [Zn2(O2CPh)4(H2O)2][Z-
1032(m), 939(w), 842(w), 812(m), 715(s), 683(m), 558(w),
524(m), 431(w).
Synthesis of [(Zn3(O2CPh)6)(l-bpe)(Zn2(O2CPh)4)]n (3 ): 38.0 mg
(0.125 mmol) of Zn(NO3)2 ꢀ 6H2O and 36.0 mg (0.25 mmol) of
C6H5COONH4 were dissolved in 4 ml water and carefully layered
by 4 ml acetone solution of 1,2-bis(4-pyridyl)ethane (26.6 mg,
0.25 mmol). The yield was 58.3 mg (24.5%) for compound 3. Suit-
able crystals of compound 3 for X-ray analysis were obtained in
a week. Anal. Calc. for C94H74N4O20Zn5 (1906.42), 3: C, 59.22; H,
3.92; N, 2.94. Found: C, 59.51; H, 3.81; N, 2.72%. IR (KBr):
n(O2CPh)2-(bpe)]n (1), [Zn3(O2CPh)6(bpe)]n (2), [(Zn3(O2CPh)6)(l-
bpe)(Zn2(O2CPh)4)]n (3), and [Zn2(O2CPh)4(bpe)2] (4) from the
reaction of zinc-benzoate and bpe which were obtained by tuning
solvent mixtures and ligand-to-zinc-benzoate ratios. These com-
plexes contain mononuclear nodes 1, trinuclear nodes 2, and trinu-
clear alternating with dinuclear nodes 3 and a discrete dinuclear
complex 4, respectively. These results illustrate how both stoichi-
ometry and solvent play a vital role in affecting the product forma-
tion, their structures and topology. We have also reported that the
compounds 1–4 catalyzed efficiently the transesterification of a
variety of esters with different alcohols, and discussed about the
reaction mechanism of the transesterification reaction based on
1H NMR study.
m
(cmꢁ1) = 3077(brm), 1619(s), 1573(s), 1438(m), 1364(s),
1229(m), 1173(m), 1069(m), 1032(m), 940(w), 841(m), 811(m),
719(s), 684(m), 558(w), 524(m), 439(w).
Synthesis of [Zn2(O2CPh)4(bpe)2] (4 ): 38.0 mg (0.125 mmol) of
Zn(NO3)2 ꢀ 6H2O and 36.0 mg (0.25 mmol) of C6H5COONH4 were
dissolved in 4 ml water and carefully layered by 4 ml methanol
solution of 1,2-bis(4-pyridyl)ethane (23.3 mg, 0.125 mmol). The
yield was 78.0 mg (63.4%) for compound 4. Suitable crystals of
compound 4 for X-ray analysis were obtained in five days. The
structure 4 was also obtained with the ratios of bpe to zinc-benzo-
ate (2:1 and 4:1) in methanol/water and (3:1 and 4:1) in ethanol/
water system, respectively. Anal. Calc. for C52H44N4O8Zn2 (983.65),
4: C, 63.49; H, 4.52; N, 5.70. Found: C, 63.70; H, 4.49; N, 5.47%.
2. Experimental
IR(KBr):
m
(cmꢁ1) = 3078(m), 2958(w), 1612(s), 1572(s), 1429(m),
Materials: Acetone, methanol, ethanol, methylene chloride, pro-
panol, 2-propanol, para-substituted phenyl acetate, para-substi-
tuted phenyl benzoate, methylacetate, methylbenzoate, 1,2-bis(4-
pyridyl)ethane (bpe), ammonium benzoate, and zinc nitrate hexa-
hydrate were purchased from Aldrich and were used as received.
4-Fluorophenyl acetate and 4-nitrophenyl benzoate were obtained
from Lancaster.
Instrumentation: Elemental analysis for carbon, nitrogen, and
hydrogen was carried out by using an EA1108 (Carlo Erba Instru-
ment, Italy) in the Organic Chemistry Research Center of Sogang
University, Korea. IR spectra were measured on a BIO RAD FTS
135 spectrometer as KBr pellets. Product analysis for the transeste-
rification reaction was performed on either a Hewlett–Packard
5890 II Plus gas chromatograph interfaced with Hewlett–Packard
Model 5989B mass spectrometer or a Donam Systems 6200 gas
chromatograph equipped with a FID detector using 30-m capillary
column (Hewlett–Packard, HP-1, HP-5, and Ultra 2). 1H NMR spec-
tra were recorded on a Varian Mercury 400 spectrometer. Chemical
shifts (d) are reported in ppm, relative to TMS (0.00).
1367(s), 1227(m), 1171(w), 1066(m), 1032(m), 939(w), 842(m),
812(m), 715(s), 683(m), 558(m), 524(m), 431(m).
Catalytic activity of compounds 1–4 : Catalytic homogeneous
reaction conditions: Ester (0.05 mmol) was dissolved in an appro-
priate solvent (1 ml), methanol or propanol, and the catalyst
(1.3 mg, 1.10 ꢂ 10ꢁ3 mmol for 1, 1.9 mg, 1.01 ꢂ 10ꢁ3 mmol for 2,
1.0 mg, 1.00 ꢂ 10ꢁ3 mmol for 3, and 0.986 mg, 1.00 ꢂ 10ꢁ3 mmol
for 4, respectively) were added and shaken in an incubator at
50 °C (450 rpm). Reaction conversion was monitored by GC/Mass
analysis of 20 ll aliquots withdrawn periodically from the reaction
mixture. All reactions were run at least three times and the average
conversion yields are presented.
NMR experiments for the structural determination of the complex 4
before and after the transesterification reaction: 1H NMR spectra of
ammonium benzoate and bpe were obtained in CD3OD at room
temperature. For ammonium benzoate, 1H NMR (CD3OD,
400 MHz): d 7.91–7.94 (m, 2H, aromatic-ortho-H), d 7.30–7.44 (m,
3H, aromatic-meta-H and aromatic-para-H). For bpe, 1H NMR
(CD3OD, 400 MHz): d 8.38–8.40 (d, 2H, aromatic-meta-H), d 7.27–
7.29 (d, 2H, aromatic-ortho-H), d 3.032 (s, 4H, CH2). 1H NMR spec-
trum of the complex 4 was obtained in CD3OD; 1H NMR (CD3OD,
400 MHz): d 8.02–8.05 (m, 2H, aromatic-ortho-H for benzoate), d
7.45–7.50 (q, 1H, aromatic-para-H for benzoate), d 7.37–7.42 (q,
2H, aromatic-meta-H for benzoate), d 8.40–8.43 (d, 2H, aromatic-
meta-H for bpe), d 7.29–7.32 (d, 2H, aromatic-ortho-H for bpe), d
3.045 (s, 4H, CH2 for bpe). 1H NMR spectrum of the complex 4
was again obtained after the reaction mixture of 4 and phenyl ace-
tate in CD3OD was incubated for 1 day at 50 °C. The transesterifica-
tion reaction of phenyl acetate was found to be complete by GC
and 1H NMR. 1H NMR (CD3OD, 400 MHz): 8.02–8.05 (m, 2H, aro-
matic-ortho-H for benzoate), d 7.45–7.50 (q, 1H, aromatic-para-H
for benzoate), d 7.37–7.41 (q, 2H, aromatic-meta-H for benzoate),
d 8.41–8.43 (d, 2H, aromatic-meta-H for bpe), d 7.29–7.31 (d, 2H,
aromatic-ortho-H for bpe), d 3.045 (s, 4H, CH2 for bpe).
Synthesis of [Zn2(O2CPh)4(H2O)2][Zn(O2CPh)2(bpe)]n (1): 38.0 mg
(0.125 mmol) of Zn(NO3)2 ꢀ 6H2O and 36.0 mg (0.25 mmol) of
C6H5COONH4 were dissolved in 4 ml water and carefully layered
by 4 ml acetone solution of 1,2-bis(4-pyridyl)ethane (11.6 mg,
0.0625 mmol). The yield was 34.5 mg (21.2%) for compound 1. Suit-
able crystals of compound 1 for X-ray analysis were obtained in a
week. The structure 1 was also obtained with the same ratio of
bpe to zinc-benzoate in methylene chloride (4 ml) /methanol
(4 ml) system. Anal. Calc. for C66H56N4O13Zn3 (1309.26), 1: C,
60.54; H, 4.32; N, 4.28. Found: C, 60.45; H, 4.30; N, 4.55%. IR (KBr):
m
(cmꢁ1) = 3422(br), 3064(w), 2362(m), 1619(s), 1572(m), 1364(s),
1229(w), 1070(w), 1030(w), 940(w), 720(s), 684(w), 550(m).
Synthesis of [Zn3(O2CPh)6(bpe)]n (2 ): 38.0 mg (0.125 mmol) of
Zn(NO3)2 ꢀ 6H2O and 36.0 mg (0.25 mmol) of C6H5COONH4 were
dissolved in 4 ml methanol and carefully layered by 4 ml methy-
lene chloride solution of 1,2-bis(4-pyridyl)ethane (23.3 mg,
0.125 mmol). The yield was 53.7 mg (38.9%) for compound 2. Suit-
able crystals of compound 2 for X-ray analysis were obtained in 5
days. The structure 2 was also obtained with the ratio of bpe to
zinc-benzoate (0.5:1) in ethanol/water system. Anal. Calc. for
C54H42N2O12Zn3 (1107.01), 2: C, 58.58; H, 3.83; N, 2.53. Found: C,
X-raycrystallography: The X-ray diffraction data for all four com-
pounds were collected on a Bruker SMART AXS diffractometer
equipped with a monochromater n the Mo Ka (k = 0.71073 Å) inci-
dent beam. Each crystal was mounted on a glass iber. The CCD data
were integrated and scaled using the Bruker-SAINT software pack-
age, and the structure was solved and refined using SHEXTL V6.12
[58]. All hydrogen atoms were placed in the calculated positions.
The crystallographic data for compounds 1–4 are listed in Table 1.
58.64; H, 3.93; N, 2.65%. IR (KBr):
2362(w), 1613(s), 1572(m), 1445(m), 1367(s), 1227(m), 1066(m),
m
(cmꢁ1) = 3078(w), 2958(w),

H. Kwak et al. / Polyhedron 28 (2009) 553–561
555
Table 1
Crystallographic data for compounds 1, 2, 3, and 4.
1
2
3
4
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Space group
a (Å)
C66H56N4O13Zn3
1309.26
293(2)
C54H42N2O12Zn3
1107.01
293(2)
C94H74N4O20Zn5
1906.42
293(2)
C52H44N4O8Zn2
983.65
293(2)
0.71073
C2/c
19.617(5)
12.185(3)
21.739(6)
90.000
115.665(4)
90.000
4684(2)
4
1.395
1.083
0.71073
0.71073
0.71073
ꢀ
ꢀ
ꢀ
P1
P1
P1
11.9043(14)
14.5166(17)
19.116(2)
77.967(2)
73.248(2)
79.207(2)
3065.0(6)
2
10.2901(8)
11.0667(8)
10.1724(6)
10.8612(7)
b (Å)
c (Å)
13.1179(10)
76.1630(10)
67.4350(10)
64.9150(10)
1244.13(16)
1
22.0757(14)
81.6880(10)
83.8290(10)
66.2990(10)
2206.6(2)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
1
Dcalc. (Mg/m3)
1.419
1.232
1.478
1.500
1.435
1.412
Absorption coefficient (mmꢁ1
Crystal size (mm3)
Reflections collected
)
0.30 ꢂ 0.25 ꢂ 0.20
0.15 ꢂ 0.05 ꢂ 0.05
0.10 ꢂ 0.05 ꢂ 0.05
0.15 ꢂ 0.10 ꢂ 0.08
19307
6994
12504
12342
Independent reflections [Rint
Data/restraints/parameters
Goodness-of-fit on F2
]
13650 (0.0482)
13650/0/781
0.935
4776 (0.0179)
4776/0/322
1.044
8479 (0.0204)
8479/0/556
0.972
4597 (0.0469)
4597/0/298
1.012
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0423, wR2 = 0.1112
R1 = 0.0661, wR2 = 0.1157
1.930 and ꢁ0.546
R1 = 0.0312, wR2 = 0.0823
R1 = 0.0400, wR2 = 0.0852
0.307 and ꢁ0.430
R1 = 0.0371, wR2 = 0.0865
R1 = 0.0556, wR2 = 0.0920
0.367 and ꢁ0.375
R1 = 0.0566, wR2 = 0.1451
R1 = 0.0809, wR2 = 0.1629
1.603 and ꢁ0.744
Largest difference between peak and hole (e Åꢁ3
)
H2O system and of 1:1 in the methylene chloride/methanol system,
respectively, and compound 3 was obtained with the molar ratio of
2:1 in the acetone/H2O system. When we use the methanol/H2O
system, only compound 4 was obtained with any molar ratios of
bpe-to-zinc-benzoate (1:1, 2:1, and 4:1). 4 was also obtained with
the molar ratios of both 3:1 and 4:1 in the ethanol/H2O system, and
4 could not be obtained from the acetone/H2O system. These re-
sults indicate that the formation of compounds 1–4 depends pri-
marily on both the bpe ligand-to-zinc-benzoate ratios and the
solvent systems.
The structure of compound (1): An X-ray structural analysis re-
vealed that compound 1 consists of mononuclear Zn(O2CC6H5)2
units bridged by bpe (bpe = 1,2-bis(4-pyridyl)ethane) ligand to
form a one-dimensional zigzag chain (Fig. 1a). Zn(II) ion is coordi-
nated by two monodentate benzoate ligands, two bridging bpe
ligands with a tetrahedral geometry. The Zn–Obenzoate bond dis-
tances are 1.932(2) and 1.964(1) Å that are similar to those found
3. Results and discussion
To investigate the effect of solvent and ligand-to-metal ratio on
the structures of zinc-containing coordination polymers, we have
reacted Zn(II)-benzoate with 1,2-bis(4-pyridyl)ethane (bpe) by
changing the solvent systems and ligand-to-zinc-benzoate ratios.
The coordination complexes 1–4 were prepared cleanly as single
phase crystalline products, respectively. The infrared spectra of
1–4 were fully consistent with their formulations. Asymmetric
and symmetric C@O stretching modes of the ligated benzoate moi-
eties were evidenced by very strong, slightly broadened bands at
ꢃ1600 cmꢁ1 and ꢃ1400 cmꢁ1 [59–61]. The absence of any bands
in the area of ꢃ1710 cmꢁ1 indicates full deprotonation of all car-
boxylate groups in 1–4 [59–61].
The reaction of zinc-benzoate with the bpe ligand revealed four
different structures as a function of ligand concentration in the dif-
ferent solvent systems. The relationship between the molar ratio of
bpe-to-zinc-benzoate and the solvent system is shown in Table 2.
Compound 1 was obtained with the molar ratio of bpe-to-zinc-
benzoate of 0.5:1 both in the acetone/H2O system and in the meth-
ylene chloride/methanol system. Compound 2 was obtained with
the molar ratios of bpe-to-zinc-benzoate of 0.5:1 in the ethanol/
in [Zn(O2CCH3)2(
l
-bpe)] ꢀ 2H2O (1.922(3) and 1.949(3) Å) [62]
and [Zn(O2CCH3)2(py)2] (1.941(4) and 1.945(2) Å, py = pyri-
dine)[63] and the Zn–Nbpe bond distances range from 2.036(2) to
2.075(2) Å which are typical in Zn complexes (ranging from
2.010(3) to 2.044(3) Å) [62,63]. The packing diagram of the one-
Table 2
Relationship between bpe-to-zinc-benzoate molar ratios and solvent systems.
bpe: zinc-benzoate
Acetone/H2Oa
Ethanol/H2Ob
Methanol/H2Oc
Methylene chloride/methanold
i
0.5:1
1:1
2:1
3:1
4:1
1e
–
2f
–
–
4
4
–
1
2
–
–
–
4h
4
–
3g
–
–
4
a
Zinc-benzoate (Zn(NO3)2 ꢀ 6H2O + C6H5COONH4) was dissolved in water and layered by an acetone solution of bpe.
Zinc-benzoate (Zn(NO3)2 ꢀ 6H2O + C6H5COONH4) was dissolved in water and layered by an ethanol solution of bpe.
Zinc-benzoate (Zn(NO3)2 ꢀ 6H2O + C6H5COONH4) was dissolved in water and layered by a methanol solution of bpe.
Zinc-benzoate (Zn(NO3)2 ꢀ 6H2O + C6H5COONH4) was dissolved in methanol and layered by a methylene chloride solution of bpe.
b
c
d
e
A
one-dimensional zigzag chain containing Zn(O2CPh)2 units bridged by bpe ligands with dinuclear paddle-wheel Zn2(O2CPh)4(H2O)2, [Zn2(O2CPh)4(H2O)2]-
[Zn(O2CPh)2(bpe)]n.
f
A one-dimensional chain containing trinuclear Zn3(O2CPh)6 units bridged by bpe ligands, [Zn3(O2CPh)6(bpe)]n.
g
A
one-dimensional chain containing trinuclear Zn3(O2CPh)6 units and dinuclear Zn2(O2CPh)4 units bridged by bpe ligands alternatively, [(Zn3(O2CPh)6)
(
l-bpe)(Zn2(O2CPh)4)]n.
h
i
A dinuclear ring molecule containing two Zn(O2CPh)2 bridged by two bpe ligands, [Zn2(O2CPh)4(bpe)2].
We could not obtain any crystals suitable for X-ray analysis under these conditions.

556
H. Kwak et al. / Polyhedron 28 (2009) 553–561
Fig. 1. (a) Structure of one-dimensional zigzag chain coexisting with dinuclear paddle-wheel Zn complex, 1. (b) The packing diagram of the one-dimensional zigzag chains
along an axis. All hydrogen atoms were omitted for clarity.
dimensional zigzag chains is shown in Fig 1b. There coexist dinu-
clear paddle-wheel type Zn2(O2CC6H5)4 molecules with water axial
ligands (Fig. 1a). Each Zn atom in this unit has distorted square-
pyramidal environment, with four oxygen atoms from benzoates,
which form the equatorial plane, and one water oxygen atom.
The Zn–Obenzoate bond distances range from 2.042(2) to
2.062(2) Å longer than those of the 1-D zigzag chain, but similar
to those of the paddle-wheel type complexes [64–67], and the
Zn–Owater bond distance is 1.986(1) Å (Table 3). The ZnꢀꢀꢀZn dis-
tance is 2.983(2) Å similar to those found in Zn2(MeCH@CHCO2)4
(quinoline)2] (2.976 Å) [64], [Zn2(O2CPh)4(py)2] (2.927(4) Å) [65],
[Zn(bdc)ꢀ(DMF)(H2O)] (2.940(3) Å) [66], and [Zn2(bdc)2(dabco)]
(2.909(1) Å, bdc = 1,4-benzenedicaboxylate and dabco = 1,4-diaza-
bicyclo[2.2.2]octane) [67]. There is a hydrogen bond interaction
between oxygen atom of a benzoate ligand in a zigzag chain and
the axial water hydrogen atom of a dinuclear unit (O22benzo-
ateꢀꢀꢀH15A–O15water 1.657(2) Å). Importantly, this is the first exam-
ple of coexistence of 1-D and Zn dinuclear paddle-wheel type, to
our best knowledge.
The structure of compound (2): An X-ray structural analysis re-
vealed that compound 2 consists of trinuclear Zn3(O2CC6H5)6 units
bridged by bpe ligand to form a one-dimensional chain (Fig. 2a). A
trinuclear Zn(II)-benzoate unit is constructed by six bridging ben-
zoate ligands and similar to the previously described trinuclear
zinc-carboxylate with axial N- or O-donor ligands [57,65,68–70].
There is an inversion center on the central Zn atom. Out of three
Zn centers the central Zn is all O-donor hexa-coordinated from
six benzoate groups to form a distorted octahedral geometry. The
other two Zn centers are symmetric to each other, and each has
a bridging bpe ligand and four oxygen atoms from benzoate li-
gands. Thus each Zn1 center has NO4 coordination environment
with distorted trigonal bipyramid geometry. The coordination
modes of benzoate ligands show two kinds of bridging modes
[71–73]: one mode is a typical bridging mode, and the other is

H. Kwak et al. / Polyhedron 28 (2009) 553–561
557
Table 3
Selected bond distances for compounds 1, 2, 3, and 4.
1
2
3
4
Zn–O (Å)
Zn–N (Å)
Zn–Zn (Å)
1.932(2)–2.062(2)
2.036(2)–2.075(2)
2.9827(7) (paddle-wheel type unit)
1.9628(16)–2.2640(19)
2.0384(18)
3.370(2)
1.9607(19)–2.373(2)
2.035(2)
2.9541(5) (paddle-wheel type unit), 3.347(2) (trinuclear unit)
1.950(3), 1.951(3)
2.039(3), 2.064(3)
8.502(14)
Fig. 2. (a) Structure of one-dimensional chain 2 containing trinuclear Zn units bridged by bpe ligands. (b) The packing diagram of the one-dimensional zigzag chains along an
axis. All hydrogen atoms were omitted for clarity.
the unusual coordination mode, a monatomic coordination mode,
that chelates to one Zn2 atom and, at the same time, bridges the
other Zn1 atom as shown in Fig. 2a. This unusual coordination
mode of carboxylate group provides a big distortion around Zn1.
The packing diagram is shown in Fig. 2b. In the typical bridging
carboxylate coordination mode, Zn1–Obridging distances are
1.963(1) and 1.968(1) Å, and Zn2–Obridging distances are 2.014(1)
and 2.098(1) Å. In the unusual coordination mode, Zn2–Obridging
distance is 2.207(2) Å which is a little longer than typical one,
and Zn1–Ochelating distances are 2.103(1) and 2.264(1) Å. Zn–N
bond distance is 2.038(1) Å which is typical in Zn complexes (Table
3) [68–70]. The ZnꢀꢀꢀZn distance is 3.370(2) Å that is much longer
than that of dinuclear one in the compound 1, and that can be com-
parable to those found in [Zn3(MeCH@CHCO2)6(quinoline)2]
(3.26 Å) [68], [Zn3(O2CPh)6(2,3-diMepz)2] (3.362(4) Å, 2,3-diM-
epz = 2,3-dimethylpyrazine) [57], {[Zn3(1,4-BDC)3(DEF)2] ꢀ DEF}1
(3.228(4) and 3.528(4) Å, 1,4-BDC = 1,4-benzene dicarboxylate
and DEF = diethylformamide) [70].
The structure of compound (3): An X-ray structural analysis re-
vealed that compound 3 consists of dinuclear paddle-wheel type
Zn2(O2CC6H5)4 units and trinuclear Zn3(O2CC6H5)6 units bridged
by bpe ligands alternatively to form a one-dimensional chain
(Fig. 3a). Trinuclear Zn3(O2CC6H5)6 units are similar to that of com-
pound 2, and dinuclear paddle-wheel type Zn2(O2CC6H5)4 units are
similar to that of the dinuclear one in compound 1. While the tri-
nuclear zinc-carboxylate units and the dinuclear paddle-wheel
zinc-carboxylate units of structures from zinc-carboxylates have
been often reported in literature [63,64–70], this arrangement in
which bipyridyl ligands bridge dinuclear units and trinuclear units
alternatively is unprecedented, to our best knowledge. On the
other hand, it is worthwhile to mention that in Cu2(maa)4/bpp sys-
tems (Hmaa = 2-methylacrylic acid, bpp = 1,3-bis(4-pyridyl)pro-

558
H. Kwak et al. / Polyhedron 28 (2009) 553–561
Fig. 3. (a) Structure of one-dimensional chain 3 containing dinuclear and trinuclear units bridged by bpe ligands alternatively. (b) The packing diagram of the one-
dimensional zigzag chains along an axis. All hydrogen atoms were omitted for clarity.
pane), dinuclear and mononuclear alternating paddle-wheel 1-D
instead of trinuclear and dinuclear alternating 1-D has been ob-
served [48]. However, we could not obtain the dinuclear and
mononuclear alternating polymer systems in our Zn systems. The
packing diagram is shown in Fig. 3b. In the dinuclear unit, the
Zn–Obridging bond distances range from 2.0240(19) to 2.054(2) Å
similar to those of the dinuclear unit in the compound 1, and the
Zn–N bond distance is 2.035(2) Å which is typical in Zn complexes
[64–67]. In the trinucelar unit, the Zn–Obridging bond distances of
the typical bridging mode range from 1.9607(19) to 2.0903(19) Å,
the Zn2–Ochelating bond distances of the unusual coordination mode
are 2.0491(17) and 2.373(2) Å, and the Zn1–Obridging bond distance
of the unusual mode is 2.2428(17) Å which is a little longer than
that of compound 2. The Zn–N bond distance of the unusual mode
is 2.035(2) Å which is typical in Zn complexes (Table 3) [68–70].
The ZnꢀꢀꢀZn distances of dinuclear and trinuclear units are
2.9541(5) and 3.347(2) Å, respectively, which are shortened from
dinuclear unit of 1 and trinuclear unit of 2.
Fig. 4. Structure of dinuclear molecule 4.
not participate to form an one-dimensional chain. If the concentra-
tion of bpe ligand increases such as 1:1 molar ratio in acetone/
MeOH solvent system, compound 2 was obtained and revealed tri-
nuclear units connected by bpe ligands to form one-dimensional
chains without the paddle-wheel type Zn2 units. In ethanol/H2O
solvent system, 2 was also obtained in 0.5:1 bpe-to-zinc-benzoate
molar ratio. If the concentration of bpe ligands increases more such
as 2:1 molar ratio in acetone/H2O system, compound 3 was ob-
tained and revealed trinuclear and dinuclear units connected by
bpe ligands alternatively, that is the first example in zinc-carboxyl-
ate systems. If the concentration of bpe ligands increases much
more such as 4:1 molar ratio in EtOH/H2O system, compound 4
was obtained and revealed the dinuclear ring type molecule which
is a totally different structure from the previous structures. In
MeOH/H2O solvent system, importantly, only 4 was obtained with-
out respect to the molar ratio of bpe-to-zinc-benzoate.
This result indicates that the solvent and the ligand-to-metal ra-
tio are simultaneously playing a controlling role over the structure.
Further, solvent seems to have more effect than the ligand-to-me-
tal ratio, because in a certain case only one class of complexes was
obtained (e.g., only 4 in MeOH/H2O system), although the ratios of
bpe-to-zinc-benzoate increase from 1 to 2 to 4. Interestingly, the
solvents used in this work were not involved in the structures of
The structure of compound (4): An X-ray structural analysis re-
vealed that compound 4 consists of two Zn(O2CC6H5)2 bridged by
two bpe ligands to form a dinuclear molecule (Fig. 4). The Zn(II)
ion has a distorted tetrahedral environment with two monoden-
tate benzoate ligands and two bridging bpe ligands. The bpe ligand
adopts a gauche conformation to form a ring type molecule. The
Zn–O bond distances are 1.950(1) and 1.951(1) Å which are a little
shorter than those of compounds 1, 2, and 3, and the Zn–N bond
distances are 2.039(2) and 2.064(6) Å which are typical in Zn com-
plexes (Table 3). The ZnꢀꢀꢀZn distance is 8.502(14) Å.
Structural diversity of zinc polymeric compounds by tuning ligand-
to-metal ratio and solvent system: Zinc-benzoates can be rationally
tuned to form four different structures with a bridging bpe ligand
by controlling ligand-to-zinc-benzoate molar ratios and by using
different solvent systems, and reveal three coordination polymers
having similar one-dimensional characteristics but having differ-
ent mono-, di-, trinuclear nodes, and a dinuclear ring type mole-
cule. In case of the low concentration of the bpe ligand such as
the bpe-to-zinc-benzoate molar ratio of 0.5:1 in both acetone/
H2O and CH2Cl2/MeOH solvent systems, compound 1 was obtained
and revealed unprecedented one-dimensional zigzag chains coex-
isting with the paddle-wheel type dinuclear Zn2 units which would

H. Kwak et al. / Polyhedron 28 (2009) 553–561
559
Table 4
Transesterification of esters by methanol and propanol in the presence of compounds 1, 2, 3, and 4 at 50 °C.a
Entry
Substrate
1
2
3
4
MeOHc
PrOHd
MeOHc
PrOHd
MeOHc
PrOHd
MeOHc
PrOHd
(time/days)b
(time/days)b
(time/days)b
(time/days)b
(time/days)b
(time/days)b
(time/days)b
(time/days)b
1
2
3
4
5
6
7
8
9
4-Nitrophenyl acetate
4-Fluorophenyl acetate
Phenyl acetate
4-Methylphenyl acetate
4-Nitrophenyl benzoate
4-Chlorophenyl benzoate
Phenyl benzoate
4-Methylphenyl benzoate
Vinyl acetate
0.7
2.5
1.5
5.5
2.5
2
5
5
1.5
2.5
11
5
18
6
17
24
24
2.5
0.9
4.5
1.5
4.5
3.5
2.5
5
1.5
9
7
18
10
17
33
12
3.5
0.3
2
0.5
4
0.3
0.5
2
0.5
11
2
14
9
5
7
17
0.5
0.6
3
1
3
0.7
2
2
4
0.7
1
15
4
17
18
10
17
35
1
e
5
0.9
2
0.3
a
All esters were completely converted to the corresponding products, methyl acetate and methyl benzoate. Reaction conditions: esters; 0.05 mmol, catalyst;
1.10 ꢂ 10ꢁ3 mmol for 1, 1.01 ꢂ 10ꢁ3 mmol for 2, 1.00 ꢂ 10ꢁ3 mmol for 3, 1.00 ꢂ 10ꢁ3 mmol for 4, solvent; methanol or propanol (1 ml). See Section 2 for the detailed
reaction conditions.
Time necessary for the complete conversion of substrate to product.
Transesterification of esters was carried out in methanol (MeOH).
b
c
d
e
Transesterification of esters was carried out in propanol (PrOH).
The solvent was a mixture of MeOH/CH2Cl2 or PrOH/CH2Cl2 (1/1) because of low solubility of substrate in MeOH or PrOH.
1–4 which means that the coordinating solvents such as H2O,
our best knowledge [77–84]. Since we have established that 1–4
represent excellent catalysts for the tansesterification reaction of
p-nitrophenyl acetate with methanol, we have investigated the
transesterification of various p-substituted phenyl acetates and
benzoates. As shown in Table 4, the substrates with the electron-
withdrawing substituents have undergone faster transesterifica-
tion (entries 1 and 5) than those with the electron-donating ones
(entries 4 and 8). In addition, vinyl acetate as a precursor widely
used for ester synthesis [85–87] was converted efficiently to the
product methyl acetate by 1–4 within 0.3–1.5 day (entry 9). This
result suggests that this catalytic system can be useful for prepar-
ing various esters by transesterification. Moreover, it is worthwhile
that 4-nitrophenyl acetate and 4-nitrophenyl benzoate with nitro
substituents at the para-position, which are known to be problem-
atic substrates for the transesterification reaction due to undesir-
able side reactions such as isomerization or polymerization [88],
were also converted quantitatively to the corresponding products.
Furthermore, this promising result led us to test the transeste-
rification with more challenging nucloephiles such as ethanol, 2-
propanol, and propanol, in the presence of the catalysts 1–4. Phe-
nyl acetate reacted very slowly with either ethanol (70 days) or
2-propanol (90 days), respectively, whereas with propanol the
transesterification reaction was complete within 2–7 days in the
presence of 1–4, respectively (entry 3). The complex 3 again shows
the most efficient reactivity. This result is the best among the cat-
alytic efficiencies reported previously with zinc-containing coordi-
nation and polymeric compounds, to our best knowledge [77–84].
Various esters were also examined with propanol as a nucleophile
in the presence of 1–4. The substrates with the electron-withdraw-
ing substituents have undergone faster conversion to the corre-
methanol, and ethanol, act as the ‘‘templating” role rather than
the ‘‘coordination” one [38]. Therefore, the solvent used in this
preparation should be considered as an innocent one instead of a
potential participant in any coordination polymer formation.
In addition, the structural diversity of compounds 1–4 shows
that the dinuclear paddle-wheel Zn(II) units can be rationally con-
trolled and can serve as mono-, di-, and trinuclear nodes towards a
dipyridyl ligand, respectively, and that four classes of mixed-ligand
complexes can be obtained simply by manipulating the ligand-to-
zinc-benzoate molar ratio and the solvent. This result led us to sug-
gest that the ligand concentration as well as the solvent system can
tune topology in the system of zinc-benzoate with a variety of
bipyridyl bridging ligands.
Catalytic transesterification reactions by the compounds 1–4 :
Transesterifications are important transformations in organic syn-
thesis in industrial as well as in academic laboratories [74–76]. As
part of our efforts to design transesterification catalysts based on
metal ions that are not redox active, therefore, we have recently
synthesized several discrete complexes and coordination polymers
and reported that some of them could carry out efficiently the cat-
alytic transesterification of a range of esters with methanol under
the mild conditions [77–79]. While these catalyst systems consti-
tute a promising class of catalysts that appears to be an efficient,
mild, and easily recyclable method for the alcoholysis of esters,
there is still more demand to develop new types of catalysts that
show better efficiency. With our previous experience, therefore,
compounds 1–4 have been also employed as a catalyst for transe-
sterification reaction in methanol under a homogeneous condition.
The ester, p-nitrophenyl acetate, was initially used as a substrate
(Eq. (1)):
sponding products (entries
1 and 5), while those with the
electron-donating ones have shown slow transesterification reac-
tion (entries 4 and 8).
O
O
catalysts 1-4
HOCH3
..
O2N
OCCH3
CH3COCH3
+
O2N
OH +
Though we do not know, at this moment, about the exact reac-
tive species and the reaction mechanism for the transesterification
reaction by the catalysts, it has been proposed that the mechanism
of metal ion catalyzed transesterification probably involves elec-
trophilic activation of the carbon center of the carbonyl moiety
by binding of the metal to the carbonyl oxygen [89–93]. Therefore,
Lewis acidity of the metal center may be important in catalytic
transesterification. Based on this idea and our previous proposal
on the transesterification reactions [79,94,95], a possible transeste-
rification mechanism in these catalyst systems can be proposed
with the catalyst 4 as shown in Scheme 1. The complex 4 could
ð1Þ
We observed that compounds 1–4 catalyzed the reaction of p-
nitrophenyl acetate with methanol, with quantitative conversion
to methyl acetate in 0.3–0.9 day (see the entry 1 of Table 4) at
50 °C under the neutral conditions. Among them, the complex 3
shows the most efficient reactivity. Importantly, this result is the
second best among the catalytic efficiencies reported previously
with zinc-containing coordination and polymeric compounds, to

560
H. Kwak et al. / Polyhedron 28 (2009) 553–561
Acknowledgements
Financial support from the Korea Research Foundation (2007-
314-C00159), The SEOUL R&BD Program, Environmental Korea
Ministry Environment ‘‘ET-Human resource development Project”,
the Korean Science and Engineering Foundation (R01-2008-000-
20704-0), and the SRC program of the Korea Science and Engineer-
ing Foundation (KOSEF) through the Center for Intelligent Nano-
Bio Materials at Ewha Womans University (Grant R11-2005-008-
00000-0) is gratefully acknowledged.
Appendix A. Supplementary data
Scheme 1. Plausable transesterification mechanism.
CCDC 687836, 687837, 687838, and 687839 contain the supple-
mentary crystallographic data for 1, 2, 3, and 4, respectively. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.poly.2008.11.048.
possibly be a catalytic species because only 4 has been obtained in
methanol/H2O without respect to the ligand-to-metal ratios and
because the transesterification reactions were carried out in meth-
anol. At the first step, the substrate phenyl acetate substitutes a la-
bile ligand benzoate to give the adduct A (Scheme 1). Then, the
nucleophile methanol would attack the carbon atom of carbonyl
moiety of the adduct to produce the product methyl acetate. After
the catalytic reaction, the structure of 4 was proved to be kept,
based on 1H NMR experiments. 1H NMR data of benzoate and
bpe of 4 appear at different positions from those of free benzoate
and free bpe (see Section 2). 1H NMR data of benzoate and bpe of
the complex 4 taken after the transesterification reaction were ex-
actly identical to those of benzoate and bpe of the starting complex
4 and we were not able to observe any free benzoate and bpe sep-
arated from 4 (see Section 2). This result indicates that there is no
structural change of 4 after the transesterification reaction. This
investigation was consistent with the proposed transesterification
mechanism. Further detailed mechanistic studies are currently un-
der investigation.
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