Anion effects on the crystal structures of ZnII complexes containing 2,2′-bipyridine: Their photoluminescence and catalytic activities

Article abstract of DOI:10.1016/j.poly.2011.03.040

Six new structures of Zn^II complexes containing 2,2′-bipyridine (2,2′-bpy) ligands have been determined. Halide anions were used as simple coordinating ligands to form a tetrahedral environment around the Zn^II ion. Nitrate and benzoate anions were used as chelating ligands to form an octahedral environment. For non-coordinating ClO4- and bridging SO42- anions, three 2,2′-bpy ligands chelate Zn^II to form an octahedral environment, and both ClO4- and SO42- anions occupy empty positions between the metal cations for charge balance. Polymeric structures of the Zn^II-2,2′-bpy complexes are produced by hydrogen bonding. [Zn(2,2′-bpy)(O₂CC ₆H₅)₂] catalyzed efficiently the transesterification reaction of esters with methanol under the neutral conditions, while the remaining compounds showed very slow conversions.

Full text of DOI:10.1016/j.poly.2011.03.040

Polyhedron 30 (2011) 1555–1564
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Anion effects on the crystal structures of ZnII complexes containing 2,20-bipyridine:
Their photoluminescence and catalytic activities
b
Geun Hee Eom a, Hyun Min Park a, Min Young Hyun a, Seung Pyo Jang a, Cheal Kim a, , Jun Ho Lee ,
⇑
Suk Joong Lee b, , Sung-Jin Kim c, , Youngmee Kim c
⇑
⇑
a Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea
b Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-701, Republic of Korea
c 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
Six new structures of ZnII complexes containing 2,20-bipyridine (2,20-bpy) ligands have been determined.
Halide anions were used as simple coordinating ligands to form a tetrahedral environment around the
ZnII ion. Nitrate and benzoate anions were used as chelating ligands to form an octahedral environment.
For non-coordinating ClO4ꢀ and bridging SO42ꢀ anions, three 2,20-bpy ligands chelate ZnII to form an octa-
hedral environment, and both ClO4 and SO4 anions occupy empty positions between the metal cations
for charge balance. Polymeric structures of the ZnII–2,20-bpy complexes are produced by hydrogen bond-
ing. [Zn(2,20-bpy)(O2CC6H5)2] catalyzed efficiently the transesterification reaction of esters with metha-
nol under the neutral conditions, while the remaining compounds showed very slow conversions.
Ó 2011 Elsevier Ltd. All rights reserved.
Article history:
Received 25 January 2011
Accepted 10 March 2011
Available online 31 March 2011
ꢀ
2ꢀ
Keywords:
ZnII complexes
Photoluminescence
2,20-Bipyridine
Catalyst
Transesterification
1. Introduction
2,6-bis(N0-1,2,4-triazolyl)pyridine (btp) as an angular ligand
[11,67,68], 1,3-bis(4-yridyl)propane (bpp) as a flexible ligand [69]
Metal–organic coordination polymers are of current interest in
the fields of supramolecular chemistry and crystal engineering
[1–5]. This interesting topic has shown their intriguing variety of
topologies and structural diversity, and their potential applications
as functional materials, such as in gas storage [6,7], photolumines-
cence [8,9], catalysis [10–13], magnetism [14] and molecular
sensing [15,16]. Many efforts have been devoted to the study of
fundamental structural aspects to understand and control several
factors that affect the self-assembly of supramolecular architec-
tures. These factors are the coordination geometry of metal ions
[17,18], structure of organic ligands [19–22], the hydrogen
and 2,20-dipyridiylamine (Hdpa) as a chelating ligand [70,49]. Espe-
cially, the chelating ligand 2,20-dipyridylamine (Hdpa) showed the
formation of mononuclear ZnII complexes by the reaction with zinc
salts [35,70], and then these ZnII complexes could be used as build-
ing blocks for the construction of polymeric compounds through
weak non-classical C/N–Hꢁ ꢁ ꢁX and C/N–Hꢁ ꢁ ꢁO hydrogen bonding.
As a part of our continued interest (a) for a comparison of the
syntheses, structures and reactivities of ZnII complexes with
chelating Hdpa ligands, (b) in the aim to fully understand the anion
effect of our earlier work, and (c) in order to find efficient catalysts
capable of carrying out various catalytic reactions under mild
reaction conditions, we have chosen an another chelating ligand,
2,2-bipyridine (2,20-bpy), with zinc salts containing different coun-
bonds[23–32], p–p stacking [4], counter-anions [33–36], pH value
[37,38], temperature [39,40], ligand-to-metal ratio [41–44], sol-
vent system [45–47] and template effect [48].
ter-anions (Clꢀ, Brꢀ, NO32ꢀ, C6H5CO2ꢀ, ClO4 or SO42ꢀ). We report
ꢀ
Among them, the anion effect has been realized as a very impor-
tant role in the self-assembled construction [49]. Many efforts have
been devoted to the investigation of new synthetic strategies, with
the use of rigid, angular and flexible spacer ligands [50–66]. In our
previous attempt to investigate the design and control of the self-
assembly of an organic/inorganic supramolecular motif, we have
chosen 1,2-trans-(4-pyridyl)ethene (bpe) as a rigid ligand [56],
here the syntheses, structures and photoluminescence of six new
structures of ZnII complexes containing 2,20-bipyridine ligands,
and their catalytic activities for transesterification reactions have
been also investigated.
2. Experimental
2.1. Materials
⇑
Corresponding authors. Tel.: +82 2 970 6693, fax: +82 2 973 9149 (C. Kim), fax:
+82 2 3290 3145 (S.J. Lee); tel.: +82 2 3277 2350, fax: +82 2 3277 3419 (S.-J. Kim).
E-mail addresses: chealkim@seoultech.ac.kr (C. Kim), slee1@korea.ac.kr (S.J. Lee),
sjkim@ewha.ac.kr (S.-J. Kim).
Methanol, ethanol, pentane, methylene chloride, acetone, para-
substituted phenyl acetate, para-substituted phenyl benzoate,
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.03.040

1556
G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
Table 1
Crystallographic data for compounds 1–6.
1
2
3
4
5
6
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Space group
Unit cell dimensions
a (Å)
C
10H8Cl2N2Zn
C10H8Br2N2Zn
381.37
293(2)
0.71073
C2/c
C20H16N6O6Zn
501.76
170(2)
0.71073
P21/n
C24H18N2O4Zn
463.77
293(2)
0.71073
P21/n
C30H24Cl2N6O8Zn
732.82
293(2)
C30H39N6O11.5SZn
765.10
173(2)
0.71073
C2/c
292.45
170(2)
0.71073
0.71073
ꢀ
ꢀ
P1
P1
7.5521(8)
8.8244(9)
9.0551(9)
112.488(2)
94.305(2)
95.437(2)
550.98(10)
2
10.724(2)
14.743(3)
7.8700(16)
90.00
97.46(3)
90.00
1233.7(4)
4
2.053
11.2282(12)
12.2245(13)
15.2117(16)
90.00
100.854(2)
90.00
2050.6(4)
4
1.625
7.599(4)
7.9890(16)
10.979(2)
18.666(4)
100.89(3)
90.73(3)
98.73(3)
1587.7(5)
2
22.6082(16)
13.6295(9)
24.3347(17)
90.00
115.0970(10)
90.00
6790.5(8)
8
1.497
b (Å)
c (Å)
16.649(9)
17.357(10)
90.00
101.864(14)
90.00
2149(2)
4
1.433
1.176
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (Mg/m3)
Absorption coefficient(mmꢀ1
Crystal size (mm)
Reflections collected
1.763
2.677
1.533
1.002
)
8.436
1.251
0.854
0.10 ꢂ 0.08 ꢂ 0.08
0.20 ꢂ 0.09 ꢂ 0.08
0.20 ꢂ 0.15 ꢂ 0.10
0.15 ꢂ 0.10 ꢂ 0.10
0.25 ꢂ 0.20 ꢂ 0.15
0.20 ꢂ 0.03 ꢂ 0.03
3087
2976
11 037
11 819
8967
18 737
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
2105 (0.0669)
2105/0/137
1.037
1178 (0.0446)
1178/0/70
1.007
4011 (0.0599)
4011/0/298
0.892
4219 (0.1227)
4219/0/280
0.818
6100 (0.0595)
6100/0/422
0.927
6662 (0.0825)
6662/13/486
0.955
Final R indices [I > 2
r
(I)]
R1 = 0.0353,
wR2 = 0.0858
R1 = 0.0381,
wR2 = 0.0864
0.948 and ꢀ0.809
R1 = 0.0625,
wR2 = 0.1708
R1 = 0.0922,
wR2 = 0.2273
0.974 and ꢀ1.200
R1 = 0.0318,
wR2 = 0.0620
R1 = 0.0439,
wR2 = 0.0633
0.343 and ꢀ0.525
R1 = 0.0353,
wR2 = 0.0548
R1 = 0.1321,
wR2 = 0.0639
0.230 and ꢀ0.212
R1 = 0.0610,
wR2 = 0.1579
R1 = 0.0846,
wR2 = 0.1655
0.787 and ꢀ0.895
R1 = 0.0407,
wR2 = 0.0918
R1 = 0.0548,
wR2 = 0.0946
0.529 and ꢀ0.825
R indices (all data)
Largest difference in peak and
hole (e Åꢀ3
)
Scheme 1. Structures of ZnII compounds containing 2,20-bpy ligands.
methylacetate, methylbenzoate, 2,20-bipyridine, zinc chloride, zinc
bromide, zinc sulfate, zinc nitrate, Zn(OTf)2, ammonium benzoate
and zinc perchlorate were purchased from Aldrich and were used
as received. 4-Fluorophenyl acetate and 4-nitrophenyl benzoate
were obtained from Lancaster.
2.2. Instrumentation
Elemental analysis for carbon, nitrogen and hydrogen was
carried out using a Vario MACRO (Elemental Analysensysteme,
Germany) in the Laboratory Center of Seoul National University

G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
1557
of Science and Technology, Korea. IR spectra were measured on a
BIO RAD FTS 135 spectrometer as KBr pellets. 1H NMR spectra were
recorded on a Varian Mercury 400 spectrometer. Product analysis
for the transesterification reaction was performed on either a Hew-
lett–Packard 5890 II Plus gas chromatograph interfaced with Hew-
lett–Packard Model 5989B mass spectrometer or a Donam Systems
6200 gas chromatograph equipped with a FID detector using a 30-
m capillary column (Hewlett–Packard, HP-1, HP-5 and Ultra 2). The
emission/excitation spectra were recorded on a Perkin–Elmer LS45
fluorescence spectrometer.
39.4 mg (0.25 mmol) of 2,20-bpy ligand were dissolved in 4 mL
methylene chloride and carefully layered by 4 mL acetone solution
of ZnBr2ꢁ2H2O (33.0 mg, 0.125 mmol). Suitable crystals of
compound 2 for X-ray analysis were obtained in a month. The yield
was 21.5 mg (45.2%) for compound 2. 1H NMR (DMSO, 400 MHz) d:
8.60 (d, 4H, pyridyl-H), 8.13 (s, 2H, pyridyl-H), 7.61 (s, 2H, pyridyl-
H). IR (KBr)
m
(cmꢀ1): 3401(brm), 3073(w), 1647(w), 1596(s),
1472(s), 1440(s), 1315(s), 1251(w), 1158(m), 1100(w), 1062(w),
Table 2
Selected bond lengths (Å) and angles (°) for compounds 1–6.
2.3. Synthesis of [Zn(2,20-bpy)X2] (X = Cl for 1, Br for 2)
1
Zn(1)–N(2)
Zn(1)–Cl(1)
N(2)–Zn(1)–N(1)
N(1)–Zn(1)–Cl(1)
N(1)–Zn(1)–Cl(2)
2.056(2)
2.2045(8)
80.45(9)
118.58(7)
102.91(6)
Zn(1)–N(1)
Zn(1)–Cl(2)
N(2)–Zn(1)–Cl(1)
N(2)–Zn(1)–Cl(2)
Cl(1)–Zn(1)–Cl(2)
2.061(2)
39.4 mg (0.25 mmol) of 2,20-bpy ligand were dissolved in 4 mL
methylene chloride and carefully layered by 4 mL methanol solu-
tion of ZnCl2 (17.4 mg, 0.125 mmol). Suitable crystals of compound
1 for X-ray analysis were obtained in 3 weeks. The yield was 19 mg
(52.0%) for compound 1. 1H NMR (DMSO, 400 MHz) d: 8.65 and
8.58 (d, 4H, pyridyl-H), 8.18 and 7.66 (t, 4H, pyridyl-H). IR (KBr)
2.2171(8)
112.46(7)
120.04(7)
116.84(3)
2
Zn(1)–N(1)#1
Zn(1)–Br(1)#1
N(1)#1–Zn(1)–N(1)
N(1)–Zn(1)–Br(1)#1
N(1)–Zn(1)–Br(1)
2.038(8)
2.3504(13)
80.3(4)
120.71(19)
107.4(2)
Zn(1)–N(1)
Zn(1)–Br(1)
N(1)#1–Zn(1)–Br(1)#1
N(1)#1–Zn(1)–Br(1)
Br(1)#1–Zn(1)–Br(1)
2.038(8)
2.3504(13)
107.4(2)
120.71(19)
116.05(8)
m
(cmꢀ1): 3105(s), 2981(m), 1606(s), 1597(s), 1565(s), 1491(s),
1473(s), 1442(s), 1319(s), 1251(s), 1157(s), 1099(s), 1067(s),
1029(m), 905(s), 767(m), 731(s), 650(m). Anal. Calc. for
10H8Cl2N2Zn (292.45), 1: C, 41.07; H, 2.76; N, 9.58. Found: C,
41.42; H, 2.57; N, 9.83%.
Symmetry transformations used to generate equivalent atoms: #1 ꢀx + 1, y,
C
ꢀz + ½
3
Zn(1)–N(21)
Zn(1)–N(12)
Zn(1)–O(32)
2.0647(17)
2.1067(16)
2.1989(16)
106.47(7)
79.03(7)
104.15(7)
151.83(6)
89.57(6)
Zn(1)–N(11)
Zn(1)–N(22)
Zn(1)–O(33)
N(21)–Zn(1)–N(12)
N(21)–Zn(1)–N(22)
N(12)–Zn(1)–N(22)
N(11)–Zn(1)–O(32)
N(22)–Zn(1)–O(32)
N(11)–Zn(1)–O(33)
N(22)–Zn(1)–O(33)
2.0724(17)
2.1100(16)
2.2788(15)
101.80(7)
78.86(7)
176.48(7)
100.93(6)
88.34(6)
N(21)–Zn(1)–N(11)
N(11)–Zn(1)–N(12)
N(11)–Zn(1)–N(22)
N(21)–Zn(1)–O(32)
N(12)–Zn(1)–O(32)
N(21)–Zn(1)–O(33)
N(12)–Zn(1)–O(33)
O(32)–Zn(1)–O(33)
95.89(6)
90.76(6)
57.87(6)
156.84(6)
85.73(6)
4
Zn(1)–O(22)
2.020(2)
Zn(1)–O(12)
2.078(2)
Zn(1)–N(1)
2.098(3)
Zn(1)–N(2)
2.099(3)
Zn(1)–O(11)
2.289(3)
Zn(1)–O(21)
2.366(3)
O(22)–Zn(1)–O(12)
O(12)–Zn(1)–N(1)
O(12)–Zn(1)–N(2)
O(22)–Zn(1)–O(11)
N(1)–Zn(1)–O(11)
O(22)–Zn(1)–O(21)
N(1)–Zn(1)–O(21)
O(11)–Zn(1)–O(21)
146.79(10)
98.67(11)
101.57(10)
95.99(10)
157.11(10)
59.43(8)
O(22)–Zn(1)–N(1)
O(22)–Zn(1)–N(2)
N(1)–Zn(1)–N(2)
O(12)–Zn(1)–O(11)
N(2)–Zn(1)–O(11)
O(12)–Zn(1)–O(21)
N(2)–Zn(1)–O(21)
106.89(10)
104.21(10)
77.91(14)
60.41(9)
96.33(11)
96.76(9)
Fig. 1. Structure of 1. Displacement ellipsoids are shown at the 50% probability
level.
98.05(12)
93.90(9)
161.62(10)
5
N(11)–Zn(1)
2.118(3)
N(12)–Zn(1)
2.240(3)
N(21)–Zn(1)
2.207(4)
N(22)–Zn(1)
2.140(4)
N(31)–Zn(1)
2.141(3)
N(32)–Zn(1)
2.110(4)
N(32)–Zn(1)–N(11)
N(11)–Zn(1)–N(22)
N(11)–Zn(1)–N(31)
N(32)–Zn(1)–N(21)
N(22)–Zn(1)–N(21)
N(32)–Zn(1)–N(12)
N(22)–Zn(1)–N(12)
N(21)–Zn(1)–N(12)
94.56(13)
164.84(14)
101.43(13)
171.01(13)
75.65(15)
102.20(14)
92.06(13)
83.40(13)
N(32)–Zn(1)–N(22)
N(32)–Zn(1)–N(31)
N(22)–Zn(1)–N(31)
N(11)–Zn(1)–N(21)
N(31)–Zn(1)–N(21)
N(11)–Zn(1)–N(12)
N(31)–Zn(1)–N(12)
96.95(14)
76.86(15)
90.80(13)
93.64(14)
97.89(14)
75.83(13)
177.08(13)
6
Zn(1)–N(22)
Zn(1)–N(12)
Zn(1)–N(11)
2.128(2)
2.150(2)
2.154(2)
96.24(8)
96.11(8)
76.33(8)
96.37(8)
76.52(8)
76.45(8)
92.41(8)
94.81(8)
Zn(1)–N(31)
Zn(1)–N(32)
Zn(1)–N(21)
N(22)–Zn(1)–N(12)
N(22)–Zn(1)–N(32)
N(12)–Zn(1)–N(32)
N(31)–Zn(1)–N(11)
N(32)–Zn(1)–N(11)
N(31)–Zn(1)–N(21)
N(32)–Zn(1)–N(21)
2.149(2)
2.152(2)
2.175(2)
166.38(8)
92.99(8)
95.48(8)
96.58(8)
168.85(8)
167.10(8)
93.26(8)
N(22)–Zn(1)–N(31)
N(31)–Zn(1)–N(12)
N(31)–Zn(1)–N(32)
N(22)–Zn(1)–N(11)
N(12)–Zn(1)–N(11)
N(22)–Zn(1)–N(21)
N(12)–Zn(1)–N(21)
N(11)–Zn(1)–N(21)
Fig. 2. Structure of 2. Displacement ellipsoids are shown at the 50% probability
level.

1558
G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
903(w), 774(s), 731(s), 525(s). Anal. Calc. for C10H8Br2N2Zn
(381.37), 2: C, 31.49; H, 2.12; N, 7.35. Found: C, 31.37; H, 2.10;
N, 7.57%.
crystals of compound 3 for X-ray analysis were obtained in a week.
The yield was 26.9 mg (41.9%) for compound 3. 1H NMR (DMSO,
400 MHz) d: 8.71 and 8.31 (m, 6H, pyridyl-H), 7.75 (m, 2H, pyri-
dyl-H). IR (KBr)
m
(cmꢀ1): 3065(w), 1608(m), 1599(m), 1576(w),
1474(s), 1443(s), 1340(s), 1319(s), 1276(s), 1158(m), 1103(w),
1060(w), 1024(s), 969(w), 830(w), 762(s), 734(s), 651(s), 635(s).
Anal. Calc. for C20H16N6O6Zn (501.76), 3: C, 47.87; H, 3.22; N,
16.75. Found: C, 47.41; H, 3.18; N, 16.66%.
2.4. Synthesis of [Zn(2,20-bpy)2(NO3)](NO3) (3)
38.0 mg (0.125 mmol) of Zn(NO3)2ꢁ6H2O and the 2,20-bpy ligand
(39.4 mg, 0.25 mmol) were stirred in 2 mL methanol for 6 h at
room temperature and carefully layered by 6 mL pentane. Suitable
2.5. Synthesis of [Zn(2,20-bpy)(O2CC6H5)2] (4)
38.0 mg (0.125 mmol) of Zn(NO3)2ꢁ6H2O and 34.8 mg
(0.25 mmol) of ammonium benzoate were dissolved in 4 mL H2O
and carefully layered by an ethanol solution of 2,20-bpy ligand
(39.4 mg, 0.25 mmol). Suitable crystals of compound 4 for X-ray
analysis were obtained in 3 weeks. The yield was 16.7 mg (28.7%)
for compound 4. 1H NMR (pyridine-d5, 400 MHz) d: 8.76 (d, 2H,
pyridyl-H), 8.70 (d, 2H, pyridyl-H), 8.65 (d, 2H, pyridyl-H), 7.77
(d, 2H, benzoate-H), 7.47 (m, 6H, benzoate-H), 7.25 (d, 2H, benzo-
ate-H). IR (KBr)
m
(cmꢀ1): 3072(w), 1595(s), 1538(s), 1474(m),
1442(m), 1401(s), 1317(m), 1252(w), 1172(w), 1074(w),
1025(m), 950(w), 856(m), 769(s), 723(s), 686(s), 633(m), 512(s).
Anal. Calc. for C24H18N2O4Zn (463.77), 4: C, 62.15; H, 3.92; N,
6.04. Found: C, 61.87; H, 4.21; N, 6.37%.
2.6. Synthesis of [Zn(2,20-bpy)3](ClO4)2 (5)
47.5 mg (0.125 mmol) of Zn(ClO4)2ꢁ6H2O were dissolved in
4 mL H2O and carefully layered with 4 mL methanol solution of
the 2,20-bpy ligand (39.4 mg, 0.25 mmol). Suitable crystals of com-
pound 5 for X-ray analysis were obtained in 2 months. The yield
was 29.3 mg (48.1%) for compound 5. 1H NMR (DMSO, 400 MHz)
d: 8.77 and 8.31 (m, 6H, pyridyl-H), 7.75 (m, 2H, pyridyl-H). IR
(KBr)
m
(cmꢀ1): 3117(w), 1601(m), 1578(w), 1568(w), 1477(m),
1443(s), 1322(w), 1251(w), 1162(w), 1075(s), 1025(s), 923(m),
903(w), 763(s), 733(s), 652(m), 619(s), 519(s). Anal. Calc. for
Fig. 3. Structure of 3. Displacement ellipsoids are shown at the 50% probability
level.
C
30H24Cl2N6O8Zn (732.82), 5: C, 49.17; H, 3.31; N, 11.47. Found:
C, 48.92; H, 3.17; N, 11.21%.
Fig. 4. Structure of 4. Displacement ellipsoids are shown at the 50% probability level.

G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
1559
2.7. Synthesis of [Zn(2,20-bpy)3](SO4) (6)
3.2. Crystal structure of [Zn(2,20-bpy)2(NO3)](NO3) (3)
23.2 mg (0.08 mmol) of ZnSO4 were dissolved in 4 mL methanol
and carefully layered with 4 mL acetone solution of the 2,20-bpy li-
gand (25.2 mg, 0.16 mmol). Suitable crystals of compound 6 for X-
ray analysis were obtained in a week. The yield was 20.7 mg
(62.1%) for compound 6. 1H NMR (D2O, 400 MHz) d: 8.54 (m, 3H,
pyridyl-H), 8.27 (m, 3H, pyridyl-H), 7.66 (m, 2H, pyridyl-H). IR
Two 2,20-bpy ligands and a nitrate chelate the ZnII ion to
form a distorted octahedral environment, and there is a nitrate
counter-anion to make the charge balance (Fig. 3). The Zn–N
(KBr)
m
(cmꢀ1): 3212(brm), 3075(m), 1647(w), 1596(m), 1474(s),
1472(w), 1439(m), 1315(w), 1253(w), 1087(s), 1055(s), 018(s),
976(m), 915(m), 763(s), 734(s), 614(s), 521(s). Anal. Calc. for
C30H39N6O11.5SZn (765.10), 7: C, 47.09; H, 5.15; N, 10.99. Found:
C, 47.35; H, 5.01; N, 11.23%.
2.8. Catalytic activity of compounds 1–6
Catalytic reaction conditions: The ester (0.05 mmol) was
dissolved in methanol (1 mL), and the compounds 1–6 (1.0 mg,
3.42 ꢂ 10ꢀ3 mmol for 1, 1.0 mg, 2.62 ꢂ 10ꢀ3 mmol for 2, 1.0 mg,
2.00 ꢂ 10ꢀ3 mmol for 3, 1.0 mg, 2.16 ꢂ 10ꢀ3 mmol for 4, 1.0 mg,
1.36 ꢂ 10ꢀ3 mmol for 5 and 1.0 mg, 1.31 ꢂ 10ꢀ3 mmol for 6) were
added and shaken at 50 °C (450 rpm). Reaction conversion was
monitored by GC/Mass analysis of 20 lL aliquots withdrawn peri-
odically from the reaction mixture. All reactions were run at least
three times and the average conversion yields are presented.
2.9. Crystallography
The diffraction data for all six compounds were collected on a
Bruker SMART AXS diffractometer equipped with a monochroma-
tor in the Mo K
a (k = 0.71073 Å) incident beam. The crystal 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 [71]. Hydrogen atoms were
ꢀ
located in calculated positions. One of ClO4 anions in 5 has two
sets of chlorine atoms with 56%:44% occupancies and was refined
isotropically. All hydrogen atoms except hydrogen atoms of water
molecules were located in calculated positions. The crystallo-
graphic data for compounds 1–6 are listed in Table 1.
3. Results and discussion
In order to understand the anion effects on the construction of
crystal structures with the rigid chelating 2,20-bpy ligand, we syn-
thesized six new compounds, 1–6. All six structures have been
determined by X-ray crystallography. The structures of all six ZnII
compounds containing the 2,20-bpy ligand are shown in Scheme 1.
3.1. Crystal structures of [Zn(2,20-bpy)X2] (X = Cl for 1, Br for 2)
The asymmetric unit of 1 consists of a whole molecule, with one
2,20-bpy ligand chelating the ZnII ion and two halides coordinating
to the ZnII ion to form a distorted tetrahedral environment (Fig. 1).
The asymmetric unit of 2 consists of half of a molecule, and there is
a twofold axis through the C5–C5 (ꢀx + 1, y, ꢀz + 1/2) bond and the
ZnII ion (Fig. 2). The geometry of the ZnII ion in 2 is also distorted
tetrahedral. The 2,20-bpy ligands of both compounds are nearly
planar. The Zn–N bond distances of both compounds range from
2.038(8) to 2.061(2) Å. The Zn–Cl bond distances are 2.2045(8)
and 2.2171(8) Å, and the Zn–Br bond distance is 2.3504(13) Å.
The N–Zn–N angles of both compounds range from 80.3(4) to
80.45(9)°, and the X–Zn–X angles are 116.84(3)° for Cl and
116.05(8)° for Br (Table 2). Crystal packing is consolidated by in-
ter-molecular C–Hꢁ ꢁ ꢁX non-covalent interactions (Fig. S1 for 1
and S2 for 2, Table S1).
Fig. 5. (a) Structure of 5. Displacement ellipsoids are shown at the 50% probability
level. (b) The unit cell containing
D and K optical isomers.

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G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
bond distances range from 2.0647(17) to 2.1100(16) Å, and Zn–
Onitrate distances are 2.1989(16) and 2.2788(15) Å (Table 2).
The N–Zn–N angles of 78.86(7)° and 79.03(7)° in this octahedral
compound are much smaller than those of tetrahedral com-
pounds 1 and 2 since it has bigger tilting angles between the
two pyridyl rings (12.5° and 15.1°). Inter-molecular C–Hꢁ ꢁ ꢁOnitrate
hydrogen bonding interactions stabilized the crystal structure
(Fig. S3, Table S1).
2.099(3) Å, and Zn–Obenzoate distances range from 2.020(2) to
2.366(3) Å. The N–Zn–N angle is 77.91(4)° (Table 2). The inter-
molecular C–Hꢁ ꢁ ꢁObenzoate hydrogen bonding interactions provide
a one-dimensional chain (Fig. S4 and Table S1).
3.4. Synthesis of [Zn(2,20-bpy)3](ClO4)2 (5) and [Zn(2,20-bpy)3](SO4)
(6)
Both compounds 5 and 6 consist a ZnII ion chelated by three
ꢀ
3.3. Crystal structure of [Zn(2,20-bpy)(O2CC6H5)2] (4)
2,20-bpy ligands, with two ClO4 counter-anions for 5 (Fig. 5(a))
2ꢀ
or one SO4 anion for 6 (Fig. 6(b)). Both compounds contain
D
Two benzoate ligands and a 2,20-bpy ligand chelate the ZnII ion
to form a distorted octahedral environment (Fig. 4). The 2,20-bpy li-
gand is nearly planar. The Zn–N bond distances are 2.098(3) and
and K optical isomers in the unit cell, and thus they are optically
inactive (Fig. 5(b) for 5 and Fig. 6(b) for 6). The Zn–N bond dis-
tances of both compounds range from 2.110(4) to 2.240(3) Å, and
Fig. 6. (a) Structure of 6. Displacement ellipsoids are shown at the 50% probability level. (b) The unit cell containing
D and K optical isomers.

G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
1561
the N–Zn–N angles range from 75.65(15)° to 76.86(15)° (Table 2),
which are much smaller than those of the tetrahedral compounds
due to the bigger tilting angles between pyridyl rings (5.1° and
8.9°), to form an octahedral geometry with three 2,20-bpy ligands.
There are inter-molecular C–Hꢁ ꢁ ꢁOperchlorate hydrogen bonding
interactions in 5 (Fig. S5 and Table S1). In 6 there are many water
molecules, and hydrogen bonds between those water molecules
generate hydrogen-bonded water layers between the molecule
layers. The hydrogen-bonded water layers are then connected by
hydrogen bonds between water hydrogen atoms and sulfate oxy-
gen atoms. There are also inter-molecular C–Hꢁ ꢁ ꢁOsulfate and C–
Hꢁ ꢁ ꢁOwater hydrogen bonding interactions (Fig. S6 and Table S1).
Hdpa has been used as a chelating ligand to mostly form mono-
nuclear ZnII complexes [35], and then these ZnII complexes can be
used as building blocks for one-, two- or three-dimensional struc-
tures through weak hydrogen bonding and
p–p interactions. To
further investigate anion effects for the corresponding ZnII com-
plexes, another chelating ligand, 2,2-bipyridine (2,20-bpy), has
Scheme 2. Structures according to anions.

1562
G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
Table 3
effects, also play a very important role in the construction of crystal
structures in both the ZnII–Hdpa and ZnII–2,20-bpy systems.
Transesterification of esters by methanol in the presence of compound 4 at 50 °C.a
Entry
Substrate
4 (time/days)b
3.5. Catalytic transesterification reactions by compounds 1–6
1
2
3
4
5
6
7
8
9
4-Nitrophenyl acetate
4-Fluorophenyl acetate
Phenyl acetate
4-Methylphenyl acetate
4-Nitrophenyl benzoatec
4-Chlorophenyl benzoate
Phenyl benzoate
0.13
0.71
0.46
0.38
0.71
1.0
0.75
1.0
0.13
As part of our efforts to develop transesterification catalysts
based on metal ions that are not redox active, we have previously
investigated the catalytic transesterification of several zinc com-
plexes containing a chelating Hdpa ligand at room temperature
under mild conditions. Importantly, they have shown efficient
catalytic activities on the transesterification reaction of a range
of esters with methanol [35,70]. Our interest for a comparison of
the reactivities of compounds 1–6 with ZnII complexes having a
chelating Hdpa ligand led us to attempt the transesterification
reaction using compounds 1–6. Treatment of 4-nitrophenyl acetate
and methanol in the presence of 1–6 at 50 °C produced methyl ace-
tate quantitatively within 5 days for 1, 8 days for 2, 7 days for 3,
0.13 days for 4, 8 days for 5 and 11 days for 6, under neutral condi-
tions (Eq. (1)). Only a little transesterification occurred without
catalysts 1–6 over the same time period1
4-Methylphenyl benzoate
Vinyl acetate
a
All esters were completely converted to the corresponding products, methyl
acetate and methyl benzoate. Reaction conditions: esters; 0.05 mmol, catalyst;
1.0 mg, 2.16 ꢂ 10ꢀ3 mmol for 4, solvent; methanol (1 mL). See Section 2 for the
detailed reaction conditions.
Time necessary for the complete conversion of substrate to product.
The solvent was a mixture of CH3OH/CH2Cl2 (1/1) because of low solubility of
the substrate in CH3OH.
b
c
Ph
O
O
O
O
catalysts 1-6
HOCH3
..
O
O2N
OCCH3
CH3COCH3
+
O2N
OH +
Zn
(2,2-bpy)
Ph
O
O
ð1Þ
CH3COPh
Ph
Compound 4, with two labile benzoate ligands, was most effi-
O
O
O
(2,2-bpy) Zn
Ph
cient and the reactivity of 4 is the second best among the catalytic
systems reported previously in Zn-containing coordination and
polymeric compounds, to the best of our knowledge [35,49,68–
70]. With these promising results, we have examined further the
transesterification of other esters by 4 and the results are shown
in Table 3. The substrates p-nitrophenyl acetate and benzoate, with
an electron-withdrawing substituent at the para-position, undergo
faster transesterification (entries 1 and 5). Also, para-substituted
phenyl acetate groups (entries 1–3) display better reactivity than
para-substituted phenyl benzoate groups (entries 4–8). Moreover,
vinyl acetate was converted effectively to the product methyl
acetate within 0.13 days by 4. These results suggest that com-
pound 4 could be an efficient catalyst for ester synthesis by
transesterification.
O
CH3COCH3
CH3OH
O
O
H3C
C
OPh
+
(A)
Ph
O
O
O
Ph
O
CH3COPh
(2,2-bpy) Zn
O-
O
H3C
C
OPh
OCH3
H+
Scheme 3. Plausible transesterification mechanism.
As it has been proposed that the Lewis acidity of the metal cen-
ter may be important in catalytic transesterification [72,73], a pos-
sible transesterification mechanism in this catalyst system could
be proposed, as shown in Scheme 3. In the first step, the substrate
phenyl acetate substitutes a labile benzoate ligand to give the ad-
duct A. Then, the nucleophile methanol attacks the carbon atom of
carbonyl moiety of the adduct to produce the product methyl
acetate. Further detailed mechanistic studies are currently under
investigation.
been chosen. Various zinc salts (ZnX2 (X = Clꢀ, Brꢀ), Zn(NO3)2,
Zn(O2CC6H5)2, Zn(ClO4)2 and Zn(SO4)) have been adopted in this
system. For each anion, different structures have been obtained
in both ZnII–Hdpa and ZnII–2,20-bpy systems, as shown in Scheme 2.
Halide anions were used as simple coordinating ligands for both li-
gand systems. For halides, the coordination geometry around the
ZnII ion is distorted tetrahedral, and hydrogen bond interactions
provide polymeric structures. Nitrate and benzoate anions were
used as chelating ligands for ZnII ions with very similar structures.
ꢀ
3.6. Photoluminescence property
For non-coordinating ClO4 anions, either three 2,20-bpy ligands or
two Hdpa ligands chelate the ZnII ion along with DMF molecules
2ꢀ
Previous studies have shown that coordination polymers con-
taining cadmium and zinc exhibit photoluminescent properties
[74–76]. Therefore, the emission spectra of ZnII complexes 1–6, to-
gether with that of the ligand 2,20-bpy, were measured in the solid
state at room temperature (see Fig. 7), based on their UV–Vis spec-
tra (Fig. S7). Emissions of compounds 1, 3, 4, 5 and 6 were observed
form an octahedral environment. For bridging SO4 anions, Hdpa
ligands produced a sulfate-bridged polymeric ZnII complex, while
2,20-bpy ligands produce a ZnII cation chelated by three 2,20-bpy li-
II
ꢀ
2ꢀ
gands. For the Zn –2,20-bpy system, both ClO4 and SO4 anions
occupy empty positions between the metal cations for charge bal-
ance. Both chelating ligands Hdpa and 2,20-bpy can provide similar
structures for halides, nitrate and benzoate anions. For perchlorate
and sulfate, both ligand systems provide different structures, as
shown in Scheme 2. Polymeric structures of ZnII–2,20-bpy com-
plexes can be produced by hydrogen bonding interactions, as
shown for the ZnII–Hdpa complexes. These results confirm that
inter-molecular hydrogen bonding interactions, as well as anion
1
We have carried out some control experiments for a comparison of the reactivity
of compound 4 and zinc salts like ZnCl2, ZnBr2, and zinc acetate. Treatment of phenyl
acetate and methanol in the presence of zinc salts at 50 °C produced methyl acetate
quantitatively within 6 days for ZnCl2, 7 days for ZnBr2, 3 days for zinc acetate,
respectively, indicating that compound 4 (0.46 days) shows much more efficient
reactivity than the zinc salts.

G.H. Eom et al. / Polyhedron 30 (2011) 1555–1564
1563
Fig. 7. (a) Emission spectra of complexes 1–6 and the ligand 2,20-bpy at room temperature. (b) Expanded emission spectra of complex 2 and the ligand 2,20-bpy.
at 349 nm (kex = 230 nm) for 1, 400–422 nm (kex = 272 nm) for 3,
350 nm (kex = 272 nm) for 4, 344 nm (kex = 272 nm) for 5, and
366 nm (kex = 272 nm) for 6, but relatively very weak luminescenc-
es were observed under the same experimental conditions for 2
(360 nm, kex = 272 nm) and 2,2-bpy (398 nm, kex = 330 nm) (see in-
set in Fig. 7 for 2 and ligand 2,2-bpy). The emission bands of com-
plexes 1, 4, 5 and 6 are blue-shifted compared to the corresponding
ligand (398 nm for 2,20-bpy), which indicates that the emission
peaks of 1, 4, 5 and 6 may be due to a metal-to-ligand charge trans-
fer (MLCT) and/or ligand-to-metal charge transfer (LMCT). Such an
emission of complex 3 can be tentatively assigned to the intrali-
gand transition of the 2,2-bpy ligand, since a similar emission
was observed for the ligand. In addition, it is noteworthy that 1,
3, 4, 5 and 6 show intense emissions at room temperature, suggest-
ing that they may be good candidates for a potential hybrid inor-
ganic–organic photoactive material [76].
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