Substituent effects of pyrazine on construction of crystal structures of Zn(II)-benzoate complexes and their catalytic activities (dinuclear, trinuclear, and pentanuclear to 1-D and 2-D)

Article abstract of DOI:10.1016/j.poly.2008.08.010

Several substituted pyrazine ligands (2,3-dimethylpyrazine, 2,6-dimethylpyrazine, 2,5-dimethylpyrazine, quinoxaline) as well as simple pyrazine have been employed to investigate how the bridging pyrazine ligand influences on construction of Zn-benzoate complexes. Simple pyrazine and 2,5-dimethlpyrazine are used as bridging ligands to form two-dimensional and one-dimensional polymeric compounds, respectively. The other quinoxaline and two dimethyl-substituted pyrazine ligands are used only as terminal ligands to form dinuclear, trinuclear, and pentanuclear complexes. This result indicates that the substituents of pyrazine are very important roles for construction of Zn-benzoate complexes. Interestingly, the compounds 1-5 catalyzed efficiently the transesterification of a variety of esters, and among them, the pentanuclear complex 3 showed the most efficient reactivity. The substrates with the electron-withdrawing substituents have undergone faster transesterification, while those with the electron-donating ones have shown slow reaction. In addition, p-nitrophenyl acetate and p-nitrophenyl benzoate, known to be problematic substrates for the transesterification reaction, were also converted quantitatively to the corresponding products. Selectivity test of primary over secondary alcohol protection in the presence of 3 has provided, exclusively, the primary acetate, propyl acetate, suggesting that this catalytic system can be potentially useful in selecting for primary alcohols.

Full text of DOI:10.1016/j.poly.2008.08.010

Polyhedron 27 (2008) 3484–3492
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Substituent effects of pyrazine on construction of crystal structures
of Zn(II)-benzoate complexes and their catalytic activities (dinuclear,
trinuclear, and pentanuclear to 1-D and 2-D)
Han Kwak a, Sun Hwa Lee a, Soo Hyun Kim a, Young Min Lee a, Byeong Kwon Park a, Eun Yong Lee a,
b,
b
Yu Jin Lee a, Cheal Kim a, , Sung-Jin Kim , Youngmee Kim
*
*
a Department of Fine Chemistry, Eco-Product and Materials Education Center, Seoul National University of Technology, 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:
Several substituted pyrazine ligands (2,3-dimethylpyrazine, 2,6-dimethylpyrazine, 2,5-dimethylpyrazine,
quinoxaline) as well as simple pyrazine have been employed to investigate how the bridging pyrazine
ligand influences on construction of Zn-benzoate complexes. Simple pyrazine and 2,5-dimethlpyrazine
are used as bridging ligands to form two-dimensional and one-dimensional polymeric compounds,
respectively. The other quinoxaline and two dimethyl-substituted pyrazine ligands are used only as ter-
minal ligands to form dinuclear, trinuclear, and pentanuclear complexes. This result indicates that the
substituents of pyrazine are very important roles for construction of Zn-benzoate complexes. Interest-
ingly, the compounds 1–5 catalyzed efficiently the transesterification of a variety of esters, and among
them, the pentanuclear complex 3 showed the most efficient reactivity. The substrates with the elec-
tron-withdrawing substituents have undergone faster transesterification, while those with the elec-
tron-donating ones have shown slow reaction. In addition, p-nitrophenyl acetate and p-nitrophenyl
benzoate, known to be problematic substrates for the transesterification reaction, were also converted
quantitatively to the corresponding products. Selectivity test of primary over secondary alcohol protec-
tion in the presence of 3 has provided, exclusively, the primary acetate, propyl acetate, suggesting that
this catalytic system can be potentially useful in selecting for primary alcohols.
Received 12 February 2008
Accepted 8 August 2008
Available online 1 October 2008
Keywords:
Zinc benzoate
Pyrazine
Transesterification
Catalytic activity
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
is still little understanding concerning the factors that determine
their synthesis and the resulting structure, and the attainment of
Coordination polymers through self-assembly have attracted
considerable attention in recent years, not only due to their struc-
tural and topological novelty [1,2], but also for their potential
applications as functional materials such as ion exchange [3], gas
storage [4], molecular sensing [5], and catalysis [6]. While the con-
trol of structure and topology of assemblies is one of the major
goals in coordination polymer chemistry, many efforts have been
devoted to rational design of specific coordination polymers
including one-, two-, and three-dimensional structures. Impor-
tantly, the structure of coordination polymers is highly influenced
by many factors such as the coordination geometry of metal ions
[7], the structure of organic ligands [8], the solvent system [9],
pH value [10], temperature [11], the counteranion [12], and the ra-
tio of ligands to metal ions [13]. In some cases, a subtle change in
any of these factors can lead to new complexes with different
structural topologies and different functions. So far, however, there
rational control to give rise to desired topologies and specific prop-
erties still remains a great challenge.
Carboxylates constitute an important class of ligands in the
formation of coordination polymers [14]. These ligands are capable
of binding a metal in either a monodentate, bidentate or bridging
mode that produce both mono- and polynuclear molecular and
polymeric structures [15]. Moreover, zinc-carboxylate complexes
are of interest for their relevance to biological systems [16] and have
recently been employed as components in porous metal–organic
framework materials [17]. In addition to mononuclear species
[18], dinuclear [19], trinuclear [19], tetranuclear [20], heptanuclear
[21] and compounds of higher nuclearity [22] have been reported.
These types of polynuclear units can be used as building blocks to
form polymeric compounds.
There are a few reports that simple substitutions on ligands can
have profound effects on coordination geometry [8,23]. However,
such an effect is seldom systematically considered in the assembly
of zinc-carboxylate containing coordination polymers. Therefore, a
systematic investigation for the understanding of the relationships
between the structures of the coordination polymers and the nature
* 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), ymeekim@ewha.ac.kr (Y. Kim).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.08.010

H. Kwak et al. / Polyhedron 27 (2008) 3484–3492
3485
of ligands, as well as other factors, is still important and necessary to
be studied.
In order to prepare functional supramolecular complexes with
azine ligand (27.0
l
L, 0.25 mmol). The yield was 60.0 mg (42.2%)
for compound 2. Slow evaporation of the diffused mixture pro-
vided suitable crystals of 2 for X-ray crystallography in a month.
Anal. Calc. for C54H46N4O12Zn3 (1139.06), 2: C, 56.94; H, 4.08; N,
4.92. Found: C, 56.54; H, 4.10; N, 4.73%. IR (KBr):
intriguing structures and potential applications especially in cataly-
sis and to systematically investigate the substituent effect of an or-
ganic ligand on the structure of zinc-carboxylate containing
coordination polymers, in this work, we have reacted Zn-benzoate
with pyrazine (pyz) and its derivatives, 2,3-dimethylpyrazine (2,3-
Me2pyz), 2,5-dimethylpyrazine (2,5-Me2pyz), 2,6-dimethylpyr-
azine (2,6-Me2pyz), and quinoxaline, and report herein five new
Zn(II)-benzoate coordination architectures, 1 (Zn2(O2CPh)4 (quinox-
m
(cmꢁ1) = 3061(s), 3030(w), 1973(w), 1919(w), 1829(w), 1638(br
s), 1572(br s), 1421(br s), 1258(w), 1178(s), 1071(s), 1024(s),
978(m), 942(m), 884(w), 862(s), 841(s), 819(m), 723(s), 680(s),
596(w), 466(s), 438(m).
2.5. Synthesis of pentanuclear zinc-benzoate with 2,6-
dimethylpyrazine ligand (Zn5(l3-OH)2(O2CPh)8(2,6-Me2pyz)2) (3)
aline)2), 2 (Zn3(O2CPh)6(2,3-Me2pyz)2), 3 (Zn5(
(2,6-Me2pyz)2), (1-D, [Zn2(O2CPh)4(2,5-Me2pyz)]n),
[Zn(O2CPh)( 2-O2CPh)( 2-pyz)0.5]n). Assembly of zinc(II)-benzoate
l3-OH)2 (O2CPh)8
4
5
(2-D,
l
l
Thirty-eight milligrams (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 acetone solution of 2,6-
dimethylpyrazine ligand (28.0 mg, 0.25 mmol). The yield was
32.0 mg (16.6%) for compound 3. Slow evaporation of the diffused
mixture provided suitable crystals of 3 for X-ray crystallography in
a month. Anal. Calc. for C68H58N4O18Zn5 (1546.03), 3: C, 52.82; H,
3.79; N, 3.62. Found: C, 52.55; H, 4.00; N, 3.88%. IR (KBr):
with different types of pyrazine shows different structures, from
dinuclear, trinuclear and pentanuclear complexes to one-dimen-
sional and two-dimensional coordination polymers. Moreover,
compounds 1–5 catalyzed efficiently the transesterification of a
variety of esters.
2. Experimental
m
(cmꢁ1) = 3440(br), 3066(m), 1603(s), 1565(s), 1416(br s),
1309(w), 1259(w), 1174(m), 1070(m), 1025(m), 942(w), 842(m),
719(s), 685(s), 576(w), 472(s).
2.1. Materials
Acetone, methanol, ethanol, propanol, 2-propanol, para-substi-
tuted phenyl acetate, para-substituted phenyl benzoate, methylac-
etate, methylbenzoate, quinoxaline, 2,3-dimethylpyrazine, 2,6-
dimethylpyrazine, 2,5-dimethylpyrazine, pyrazine, ammonium
benzoate, and zinc nitrate were purchased from Aldrich and were
used as received. 4-Fluorophenyl acetate and 4-nitrophenyl benzo-
ate were obtained from Lancaster.
2.6. Synthesis of one-dimensional polymeric compound containing
zinc-benzoate with 2,5-dimethylpyrazine ligand (1-D,
[Zn2(O2CPh)4(2,5-Me2pyz)]n) (4)
Thirty-eight milligrams (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 acetone solution of 2,5-
dimethylpyrazine ligand (27.0 lL, 0.25 mmol). The yield was
2.2. Instrumentation
75.0 mg (72.8%) for compound 4. Slow evaporation of the diffused
mixture provided suitable crystals of 4 for X-ray crystallography in
a month. Anal. Calc. for C40H36N4O8Zn2 (831.47), 4: C, 57.78; H,
4.37; N, 6.74. Found: C, 58.02; H, 4.11; N, 6.35%. IR (KBr):
Elemental analysis for carbon, nitrogen, and hydrogen was car-
ried out by using an EA1108 (Carlo Erba Instrument, 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 transesterification reaction
was performed on either a Hewlett–Packard 5890 II Plus gas chro-
matograph interfaced with Hewlett–Packard Model 5989B mass
m
(cmꢁ1) = 3741(w), 3431(br w), 3064(w), 2360(w), 1637(s),
1573(w), 1499(w), 1406(s), 1171(w), 1082(w), 1049(w), 1027(w),
840(w), 727(s), 715(s), 680(m), 464(w), 429(w).
spectrometer or
a Donam Systems 6200 gas chromatograph
2.7. Synthesis of two-dimensional polymeric compound containing
equipped with a FID detector using 30-m capillary column (Hew-
lett–Packard, HP-1, HP-5, and Ultra 2).
zinc-benzoate with simple pyrazine ligand (2-D, [Zn(O2CPh)(l2
-
O2CPh)( 2-pyz)0.5]n) (5)
l
Thirty-eight milligrams (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 pyrazine ligand
(20.0 mg, 0.25 mmol). The yield was 43.0 mg (24.7%) for compound
5. Slow evaporation of the diffused mixture provided suitable crys-
tals of 5 for X-ray crystallography in a month. Anal. Calc. for
C64H48N4O16Zn4 (1390.54), 5: C, 55.28; H, 3.49; N, 4.03. Found: C,
54.96; H, 3.40; N, 3.98%. IR (KBr):
1598(s), 1555(s), 1428(s), 1398(m), 1157(w), 1121(m), 1067(s),
1024(w), 859(w), 828(m), 715(s), 688(s), 558(w), 461(s), 424(m).
2.3. Synthesis of dinuclear zinc-benzoate with quinoxaline ligand
(Zn2(O2CPh)4(quinoxaline)2) (1)
Thirty-eight milligrams (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 quinoxaline li-
gand (29.0 lL, 0.25 mmol). The yield was 55.0 mg (50.5%) for com-
m
(cmꢁ1) = 3127(w), 3070(w),
pound 1. Suitable crystals of compound 1 for X-ray analysis were
obtained in a month. Anal. Calc. for C44H32N4O8Zn2 (875.48), 1: C,
60.36; H, 3.69; N, 6.40. Found: C, 60.01; H, 3.62; N, 6.70%. IR
(KBr):
m
(cmꢁ1) = 3434(br), 3068(w), 2369(m), 1641(s), 1574(w),
1505(w), 1402(s), 1068(w), 1039(w), 967(w), 866(w), 840(w),
2.8. Catalytic activity of compounds 1–5
764(m), 717(s), 680(m), 464(m).
Catalytic homogeneous reaction conditions: Ester (0.05 mmol)
was dissolved in an appropriate solvent (1 mL), methanol or propa-
nol, and the catalyst (1.0 mg, 1.14 ꢂ 10ꢁ3 mmol for 1, 1.0 mg,
0.88 ꢂ 10ꢁ3 mmol for 2, 1.0 mg, 0.65 ꢂ 10ꢁ3 mmol for 3, 1.0 mg,
1.20 ꢂ 10ꢁ3 mmol for 4, and 1.0 mg, 0.72 ꢂ 10ꢁ3 mmol for 5,
respectively) was added and shaken in an incubator at 50 °C
(450 rpm). Reaction conversion was monitored by GC/Mass analy-
2.4. Synthesis of trinuclear zinc-benzoate with 2,3-dimethylpyrazine
ligand (Zn3(O2CPh)6(2,3-Me2pyz)2) (2)
Thirty-eight milligrams (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 2,3-dimethylpyr-

3486
H. Kwak et al. / Polyhedron 27 (2008) 3484–3492
sis of 20
l
L aliquots withdrawn periodically from the reaction
3.2. Dinuclear zinc-benzoate with quinoxaline ligand
mixture. All reactions were run at least three times and the average
conversion yields are presented. We had the same results under N2
instead of air, suggesting that air does not effect the transesterifi-
cation reaction.
(Zn2(O2CPh)4(quinoxaline)2) (1)
Asymmetric unit contains half of whole molecule, and there is
an inversion center in the middle of Zn–Zn bond. Symmetric oper-
ation (ꢁx + 2, ꢁy + 1, ꢁz + 1) produces a paddle-wheel type dinu-
clear zinc-benzoate complex 1 (Fig. 1). The paddle-wheel type
dinuclear complex is constructed by four bridging benzoate groups
and two terminal quinoxaline ligands. Thus each Zn atom is NO4
donor penta-coordinated to form square pyramidal geometry.
The similar paddle-wheel type of structures from zinc-benzoate
or zinc-acetate have been reported in literature [19,26]. Zn–O bond
distances range from 2.0191(15) to 2.0537(17) Å, Zn–N bond dis-
tance is 2.0792(18) Å which is typical in Zn(II) complexes, and
Zn–Zn distance is 2.9900(7) Å (Table 2).
2.9. X-ray crystallography
The X-ray diffraction data for all five compounds were collected
on a Bruker SMART APX diffractometer equipped with a mono-
chromator in the Mo Ka (k = 0.71073 Å) incident beam. Each crys-
tal 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 [24]. All hydrogen atoms
were placed in the calculated positions. The crystallographic data
for compounds 1–5 are listed in Table 1.
3.3. Trinuclear zinc-benzoate with 2,3-dimethylpyrazine ligand
(Zn3(O2CPh)6(2,3-Me2pyz)2) (2)
3. Results and discussion
Asymmetric unit contains half of whole molecule, and there is a
crystallographic inversion center on the central Zn atom. Symmet-
ric operation (ꢁx + 2, ꢁy + 2, ꢁz + 1) produces a trinuclear zinc-
benzoate complex 2 constructed by six bridging benzoate ligands
and two axial 2,3-dimethylpyrazine ligands (Fig. 2) which is simi-
lar to the previously described trinuclear zinc-benzoate with axial
pyridine ligands [19b]. Out of three Zn centers the central Zn is all
O-donor hexa-coordinated from six benzoate groups to form a dis-
torted octahedral geometry. The other two Zn centers are symmet-
ric to each other, and each has a terminal 2,3-dimethylpyrazine
ligand and four oxygen atoms from benzoate ligands. Thus each
Zn2 center has NO4 coordination environment with a distorted tri-
gonal bipyramid geometry. The coordination modes of benzoate li-
gands show two kinds of bridging modes [15]: one mode is a
typical bridging mode, and the other is the unusual coordination
mode that chelates to one Zn2 atom and, at the same time, bridges
the other Zn1 atom as shown in Fig. 2. This unusual coordination
mode of carboxylate group provides a big distortion around Zn2. In
the typical bridging carboxylate coordination mode, Zn1–Obridging
distances are 2.0362(15) and 2.0894(16) Å, and Zn2–Obridging
3.1. Synthesis and spectral characterization
To investigate the effect of structure and substituent of organic
ligands on the structure of their metal complexes, four pyrazine
derivative ligands, quinoxaline, 2,3-dimethylpyrazine, 2,6-dimeth-
ylpyrazine, 2,5-dimethylpyrazine and simple pyrazine, were used
to react with Zn(II)-benzoate. The coordination complexes 1–5
were prepared cleanly as single phase crystalline products and
shows dinuclear 1, trinuclear 2, pentanuclear complexes 3, one-
4 and two-dimensional 5 polymeric compounds, respectively. All
five compounds are shown in Scheme 1. The infrared spectra of
1–5 were fully consistent with their formulations [25]. Sharp med-
ium intensity bands in the range of ꢃ1300 cmꢁ1 to ꢃ700 cmꢁ1
were ascribed to stretching modes of the pyrazine ring moiety.
Asymmetric and symmetric C@O stretching modes of the ligated
benzoate moieties were evidenced by very strong, slightly broad-
ened bands at ꢃ1600 cmꢁ1 and ꢃ1400 cmꢁ1. The absence of any
bands in the area of ꢃ1710 cmꢁ1 indicates full deprotonation of
all carboxylate groups in 1–5.
Table 1
Crystallogrphic data for compounds 1, 2, 3, 4, and 5
1
2
3
4
5
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Space group
a (Å)
C44H32N4O8Zn2
875.48
293(2)
C54H46N4O12Zn3
1139.06
170(2)
C68H58N4O18Zn5
1546.03
170(2)
0.71073
P2/c
11.644(12)
11.639(13)
23.67(3)
90.000
95.231(19)
90.000
C40H36N4O8Zn2
831.47
170(2)
C64H48N4O16Zn4
1390.54
170(2)
0.71073
P21/c
14.197(3)
6.2166(12)
16.306(3)
90.000
95.557(3)
90.000
1432.4(5)
1
1.612
0.71073
0.71073
0.71073
ꢀ
ꢀ
ꢀ
P1
P1
P1
10.146(3)
10.338(3)
10.444(3)
65.677(4)
72.974(4)
81.781(4)
954.2(4)
10.7629(13)
11.3112(13)
11.7684(14)
61.846(2)
78.843(2)
80.246(2)
1234.3(3)
1
9.868(3)
b (Å)
c (Å)
10.012(3)
11.865(3)
65.386(4)
89.490(5)
63.081(4)
926.6(4)
a
(°)
b (°)
c
(°)
Volume (Å3)
3194(6)
2
1.607
1.927
Z
1
1
Dcalc (Mg/m3)
1.524
1.319
1.532
1.515
1.490
1.353
Absorption coefficient (mmꢁ1
Crystal size (mm)
Reflections collected
)
1.732
0.20 ꢂ 0.08 ꢂ 0.08
0.05 ꢂ 0.05 ꢂ 0.05
0.10 ꢂ 0.05 ꢂ 0.05
0.25 ꢂ 0.10 ꢂ 0.03
0.25 ꢂ 0.10 ꢂ 0.08
5351
6947
17173
5202
7587
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
3659 (0.0162)
3659/0/262
1.099
4737 (0.0163)
4737/0/333
1.037
6264 (0.0512)
6264/0/435
1.056
3539 (0.0282)
3539/0/246
0.946
2818 (0.0289)
2818/0/199
1.059
Final R indices [I > 2
r
(I)]
R1 = 0.0315,
wR2 = 0.0827
R1 = 0.0378,
wR2 = 0.0848
0.226 and ꢁ0.371
R1 = 0.0306,
wR2 = 0.0784
R1 = 0.0392,
wR2 = 0.0811
0.317 and ꢁ0.513
R1 = 0.0391,
wR2 = 0.1007
R1 = 0.0584,
wR2 = 0.1117
0.550 and ꢁ0.418
R1 = 0.0439,
wR2 = 0.0921
R1 = 0.0631,
wR2 = 0.0966
0.598 and ꢁ0.300
R1 = 0.0327,
wR2 = 0.0815
R1 = 0.0421,
wR2 = 0.0841
0.487 and ꢁ0.260
R indices (all data)
Largest difference in peak and hole (e Åꢁ3
)

H. Kwak et al. / Polyhedron 27 (2008) 3484–3492
3487
Scheme 1.
distances are 1.9532(16) and 1.9453(16) Å. In the unusual coordi-
nation mode, Zn1–Obridging distance is 2.1744(16) Å which is a little
longer than typical one, and Zn2–Ochelating distances are 2.1804(17)
and 2.1798(18) Å. Zn–N bond distance is 2.075(2) Å which is typi-
cal in Zn complexes. The Zn–Zn distance is 3.362(4) Å which is
much longer than that of dinuclear one (Table 2).
3.4. Pentanuclear zinc-benzoate with 2,6-dimethylpyrazine ligand
(Zn5(l3-OH)2(O2CPh)8(2,6-Me2pyz)2) (3)
Asymmetric unit contains half of whole molecule, and there is a
C2 axis through the middle Zn atom. Symmetric operation (ꢁx + 1,
y, ꢁz + 1/2) produces a pentanuclear zinc-benzoate complex 3 con-
structed by six bridging benzoate ligands, two l3-hydoxyl ligands,
two terminal 2,6-dimethylpyrazine ligands, and two terminal
monodentate dangling benzoate ligands (Fig. 3). The central Zn1
ion is all O-donor hexa-coordinated from four benzoate ligands
and two hydroxyl ligands to form a distorted octahedral geometry,
and another two Zn2 ions are all O-donor tetra-coordinated from
two bridging benzoate ligands, a dangling (monodentate)-benzo-
ate ligand and a hydroxyl ligand to form a distorted tetrahedral
geometry, and another two Zn3 ions are NO3-donor tetra-coordi-
nated from two bridging benzoate ligands, a hydroxyl ligand and
a 2,6-dimethylpyrazine ligand to form a distorted tetrahedral
geometry. Importantly, this Zn-containing pentanuclear complex
is unprecedented, to our best knowledge. The coordination modes
of benzoate ligands are two types, bridging and monodentate (dan-
gling) modes as shown in Fig. 3. A hydroxyl ligand bridges three Zn
atoms. Zn–Obenzoate bond distances range from 1.933(3) to
Fig. 1. Structure of dinuclear Zn complex 1. Displacement ellipsoids are shown at
the 30% probability level.
Table 2
Selected bond distances for compounds 1, 2, 3, 4, and 5
1
2
3
4
5
Zn–O (Å)
2.0191(15)–
2.0537(17)
2.0792(18)
2.9900(7)
1.9454(16)–
2.1804(17)
2.075(2)
Zn–O(benzoate) 1.933(3)– 2.083(3)
Zn–O(hydroxyl) 1.941(3)–2.111(3)
2.013(3)
2.017(2)–
2.063(2)
2.083(2)
Zn–O(bridging benzoate) 1.9627(17), 1.9924(17)
Zn–O(chelating benzoate) 2.0068(18), 2.3407(18)
2.085(2)
N/A
Zn–N (Å)
Zn–Zn (Å)
3.362(4)
3.143(2)–3.397(4)
2.9525(10)

3488
H. Kwak et al. / Polyhedron 27 (2008) 3484–3492
Fig. 2. Structure of trinuclear Zn complex 2. Displacement ellipsoids are shown at the 30% probability level.
and Zn–N bond distance is 2.083(2) Å which is typical in Zn(II)
complexes. Zn–Zn distance is 2.9525(10) Å (Table 2), that is shorter
than that of the discrete dinuclear complex 1. The Zn0–Zn–N bond
angle (175.25(1)°) shows a nearly linear chain structure. Interest-
ingly, this compound shows an one-dimensional structure unlike
1, 2, and 3. The coordination mode of benzoate shows only bridging
type as shown in Fig. 4. In case of similar one-dimensional com-
pound [Zn2(bpy)(OAc)4]n, acetate bridged dinuclear Zn2 unit is
not a paddle-wheel type, and each Zn(II) ion has different coordi-
nation geometry: Zn1 has an octahedral geometry and Zn2 has a
trigonal bipyramid geometry [27].
3.6. Two-dimensional polymeric compound containing zinc-benzoate
with simple pyrazine ligand (2-D, [Zn(O2CPh)(l2-O2CPh)(l2-pyz)0.5]n)
(5)
Asymmetric unit contains a Zn atom, two benzoate ligands, and
half pyrazine, and symmetry operations (ꢁx + 2, ꢁy + 3, ꢁz + 2),
(ꢁx + 2, y + 1/2, ꢁz + 3/2), and (ꢁx + 2, y ꢁ 1/2, ꢁz + 3/2) produce
a two-dimensional polymeric compound (Fig. 5). Mononuclear
Zn(O2CPh) units are bridged by bridging benzoate and pyrazine li-
gands to form a two-dimensional sheet. Thus, the Zn atom has
NO4-donor penta-coordinated environment. The coordination
modes of benzoate ligands show two types, bridging and chelating
modes as shown in Fig. 5. Zn–Obridging bond distances are
1.9627(17) and 1.9924(17) Å, and Zn–Ochelating bond distances are
2.0068(18) and 2.3407(18) Å. Zn–N bond distance is 2.085(2) Å
which is typical in Zn complexes (Table 2). Pyrazine does not have
any substituents on its own ring, indicating no steric effect. There-
fore, pyrazine may produce 2-D structure. However, in case of
using acetate ligands instead of benzoate ligands, an unusual hep-
Fig. 3. Structure of pentanuclear Zn complex 3. Displacement ellipsoids are shown
at the 30% probability level. All hydrogen atoms were omitted for clarity.
2.083(3) Å, Zn–Ohydroxyl bond distances range from 1.941(3) to
2.111(3) Å, and Zn–N bond distance is 2.013(3) Å which is typical
in Zn complexes. The Zn1–Zn2, Zn1–Zn3, and Zn2–Zn3 distances
are 3.295(21), 3.397(4), 3.143(2) Å, respectively (Table 2).
tanuclear complex Zn7(l4-O)2(OAc)10(pyz)2 has been made by
reaction of Zn(OAc)2 ꢀ 2H2O with pyrazine (pyz) in refluxing etha-
nol [21]. The structure of the heptanuclear complex revealed a cen-
tral Zn core in which two pseudo-tetrahedral Zn4 units are joined
at a common vertex, and two pyrazine molecules are bound as ter-
minal ligands. Furthermore, Cu2(O2CPh)4-pyz system showed a lin-
ear one-dimensional [28]. The structure of Cu2(O2CPh)4-pyz
showed the linear geometry with the chain skeleton bridged by
pyrazine in axial direction of the paddle-wheel type Cu2 core
which is similar to our one-dimensional compound 4 containing
3.5. One-dimensional polymeric compound containing zinc-benzoate
with 2,5-dimethylpyrazine ligand (1-D, [Zn2(O2CPh)4(2,5-Me2pyz)]n)
(4)
The crystal structure of 4 is shown in Fig. 4. The paddle-wheel
type units are bridged by 2,5-dimethylpyrazine to form a one-
dimensional polymeric compound. The paddle-wheel type units
are similar to those of compound 1. Zn–O bond distances range
from 2.017(2) to 2.063(2) Å, which are almost close to those in 1,

H. Kwak et al. / Polyhedron 27 (2008) 3484–3492
3489
Fig. 4. Structure of one-dimensional Zn complex 4 containing paddle-type dinuclear Zn units. Displacement ellipsoids are shown at the 30% probability level. All hydrogen
atoms were omitted for clarity.
Fig. 5. Structure of two-dimensional Zn complex 5 containing Zn(O2CC6H5) units. All hydrogen atoms were omitted for clarity.
2,5-dimethylpyrazine. These different results might be because of
(1) using different metal ions (Cu(II) versus Zn(II)), (2) using differ-
ent ligands (acetate versus benzoate) and (3) using different reac-
tion conditions from our self-assembly system.
2,6-dimethylpyrazine, 2,5-dimethylpyrazine, and quinoxaline) as
well as simple pyrazine have been employed. Simple pyrazine
and 2,5-dimethlpyrazine are used as bridging ligands to form
two-dimensional and one-dimensional polymeric compounds,
respectively. The other quinoxaline and two dimethyl-substituted
pyrazine ligands are used only as terminal ligands to form dinucle-
ar, trinuclear, and pentanuclear complexes. The structures of com-
plexes 1–5 clearly show that the change of the substituents on the
pyrazine ring are critical in the construction of molecular architec-
ture of Zn-benzoate complexes. The coordination modes of benzo-
ate ligand are also different according to the change of the
3.7. Influence of the substituents of the organic ligand on the structure
of the coordination compounds
To investigate how the change of the substituents of the bridg-
ing pyrazine ligand influences on construction of Zn-benzoate
complexes, substituted pyrazine ligands (2,3-dimethylpyrazine,

3490
H. Kwak et al. / Polyhedron 27 (2008) 3484–3492
Table 3
substituents of pyrazine. Complexes 1 and 4 have only a bridging
coordination mode, and 2 shows two different coordination modes,
unusual chelating and bridging. 3 and 5 have two different coordi-
nation modes (bridging and monodentate (dangling) modes for 3
and bridging and chelating modes for 5). These results suggest that
selection of appropriate substituted groups would be an ideal way
to construct novel metal-pyz coordination polymers.
Transesterification of esters by methanol and propanol in the presence of compounds
1, 2, 3, 4, and 5 at 50 °Ca
Entry Substrate
1 (time/ 2 (time/
3
4 (time/ 5 (time/
days)b
MeOHc
0.21
days)b
MeOHc
0.5
(time/days)b
days)b
days)b
MeOHc
0.13
MeOHc PrOHd MeOHc
1
2
4-Nitrophenyl
acetate
4-Fluorophenyl
acetate
0.04
1
1
4
0.25
3
4
1.5
2
3.8. Catalytic transesterification reactions by the compounds 1–5
3
4
Phenyl acetate
4-Methylphenyl
acetate
2
9
1.5
4
0.37
1
4
8
2
6
2
6
Transesterifications are important transformations in organic
synthesis in industrial as well as in academic laboratories [29].
Although there are many catalysts available for transesterification,
they require harsh reaction conditions such as high reaction tem-
peratures and acidic conditions [30]. Such drawback experienced
with these catalysts derives us to develop new catalysts that oper-
ate under milder conditions. Therefore, as part of our efforts to de-
sign transesterification catalysts based on metal ions that are not
redox active, we have recently reported that Zn-containing coordi-
nation polymers could carry out the catalytic transesterification of
a range of esters with methanol at room temperature under the
mild conditions [31]. While these catalyst systems constitute 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 type of catalysts that show
a high efficiency. Based on the previous results, therefore, com-
pounds 1–5 have been also employed as a homogeneous catalyst
for transesterification reaction in methanol. The ester, p-nitro-
phenyl acetate, was initially used as a substrate (Eq. (1)).
5
6
4-Nitrophenyl
3
3
0.5
2
0.21
1
13
11
3
3
3
4
benzoatee
4-Chlorophenyl
benzoate
Phenyl benzoate
4-Methylphenyl
benzoate
7
8
7
6
4
3
0.83
1
26
25
7
6
7
6
9
Vinyl acetate
0.46
0.5
0.06
3
0.35
0.42
a
All esters were completely converted to the corresponding products, methyl
acetate and methyl benzoate. Reaction conditions: esters; 0.05 mmol, catalyst;
1.14 ꢂ 10ꢁ3 mmol for 1, 0.88 ꢂ 10ꢁ3 mmol for 2, 0.65 ꢂ 10ꢁ3 mmol for 3,
1.20 ꢂ 10ꢁ3 mmol for 4, 0.72 ꢂ 10ꢁ3 mmol for 5, solvent; methanol or propanol
(1 mL). See Section 2 for the detailed reaction conditions.
b
Time necessary for the complete conversion of substrate to product.
Transesterification of esters was carried out in methanol (MeOH).
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
c
d
e
solubility of substrate in MeOH or PrOH.
or 2-propanol (75 days), respectively, whereas propanol drove the
reaction to completion within 4 days (the sixth column in Table 3).
Various esters were also examined with propanol as a nucleophile
in the presence of 3. The substrates with the electron-withdrawing
substituents have undergone faster conversion to the correspond-
ing products (entries 1 and 5 of the sixth column), while those with
the electron-donating ones have shown slow transesterification
reaction (entries 4 and 8 of the sixth column).
ð1Þ
We observed that compounds 1–5 catalyzed the reaction of
methanol with phenyl acetate, with quantitative conversion to
methyl acetate in 0.04–0.5 day (1–12 h; see the entry 1 of Table
3) at 50 °C under the homogeneous neutral conditions. Among
them, pentanuclear complex 3 shows the most efficient reactivity.
Importantly, this result is best among the catalytic systems re-
ported previously in Zn-containing coordination and polymeric
compounds, to our best knowledge [31,32]. Once having estab-
lished that 1–5 represent excellent catalysts for the transesterifica-
tion reaction of p-nitrophenyl acetate with methanol, we have
investigated the transesterification of various p-substituted phenyl
acetates and benzoates. The substrates with the electron-with-
drawing substituents have undergone faster transesterification
(entries 1 and 5), while those with the electron-donating ones
have shown slow reaction (entries 4 and 8). In addition, p-nitro-
phenyl acetate and p-nitrophenyl benzoate, known to be problem-
atic substrates for the transesterification reaction due to
undesirable side reactions such as isomerization or polymerization
[33], were also converted quantitatively to the corresponding
products.
Transesterification of esters with alcohol cannot, sometimes,
achieve high conversions since the reaction is reversible. To solve
this problem, therefore, enol esters are usually used as acylating
agents, since the resultant enolate is converted to an aldehyde or
ketone that is unable to participate in the reverse reaction [34]. In
this work, vinyl acetate was used as an acylating agent and, expect-
edly, converted efficiently to the product methyl acetate within
0.06–0.5 day by 1–5 (entry 9), suggesting that this catalytic system
can be useful for preparing various esters by transesterification.
Moreover, this promising result led us to test the transesterifi-
cation with more challenging nucleophiles such as ethanol, 2-pro-
panol, and propanol, in the presence of the most efficient catalyst
3. Phenyl acetate reacted very slowly with either ethanol (70 days)
Protection of a primary alcohol in the presence of a secondary
alcohol is useful in natural product synthesis [34,35]. Therefore,
selectivity of primary over secondary alcohol protection has been
tested with the catalyst 3. In a mixture of propanol and 2-propanol
in the presence of 3 and phenyl acetate, only the propanol reacted,
providing propyl acetate in 99.7% yield while the secondary acetate
formed in <0.3% yield. This reaction clearly demonstrates the po-
tential utility of our catalyst in selecting for primary alcohols.
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 [16]. Therefore, Le-
wis acidity of the metal center may be important in catalytic
transesterification. Based on this idea, a possible transesterification
mechanism in this catalyst system can be proposed. The substrate
phenyl acetate substitutes a labile ligand, maybe benzoate, to give
the adduct –Zn–O@CR1(OR2)R. Then, the nucleophile methanol
would attack the carbon atom of carbonyl moiety of the adduct
to produce the product methyl acetate. Detailed mechanistic stud-
ies are currently under investigation.
4. Conclusions
To investigate the influence of the substituent change of ligands
on the crystal structure, five new coordination polymers con-
structed by self-assembly of the pyrazine and its derivatives and

H. Kwak et al. / Polyhedron 27 (2008) 3484–3492
3491
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azine and 2,5-dimethlpyrazine are used as bridging ligands to form
two-dimensional and one-dimensional polymeric compounds,
respectively. The other quinoxaline and two dimethyl-substituted
pyrazine ligands are used only as terminal ligands to form dinucle-
ar, trinuclear, and pentanuclear complexes. The structural differ-
ences between such complexes reveal that the change of the
substituents of the ligands has a great influence on the complex
structures. This approach may be useful for synthesizing other
complexes with high-dimensional structures. We have also shown
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Acknowledgements
Financial support from Environmental Technology Educational
Innovation Program (2006) of the Ministry of Environment, the Kor-
ea Research Foundation (2006-312-C00569 and 2007-314-
C00159), The SEOUL R & BD Program, and the SRC program of the
Korea Science and Engineering Foundation (KOSEF) through the
Center for Intelligent Nano-Bio Materials at Ewha Womans Univer-
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CCDC 661363, 661364, 661365, 661366 and 661367 contain the
supplementary crystallographic data for 1, 2, 3, 4 and 5. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
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Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
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