Syntheses and characterizations of four metal coordination polymers constructed by the pyridine-3,5-dicarboxylate ligand

Article abstract of DOI:10.1016/j.poly.2007.10.020

A series of metal-organic frameworks, namely [Ni(PDB)(H₂O)]_n (1), [Pb(PDB)(H₂O)] · (H₂O) (2), [Co₂(PDB)₂(bpy)₂(H₂O)₄] · 4H₂O (3) and [Co₂(PDB)₂(phen)₂]_n (4) (H₂PDB = pyridine-3,5-dicarboxylic acid, bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline), have been synthesized based on pyridine-3,5-dicarboxylate acid and two neutral chelate ligands, with different metal ions such as Ni^II, Co^II and Pb^II, under hydrothermal conditions. The framework structures of these polymeric complexes have been determined by the X-ray single crystal diffraction technique. In the four complexes, the pyridine-3,5-dicarboxylate acid ligand exhibits diverse coordination modes, which play an important role in the construction of metal-organic frameworks. The thermal analyses of these four complexes have been measured and discussed. In addition, complex 2 shows strong phosphorescent emission at room temperature and the magnetic measurement of the polymer of 4 reveals a typical antiferromagnetic exchange.

Full text of DOI:10.1016/j.poly.2007.10.020

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 583–592
www.elsevier.com/locate/poly
Syntheses and characterizations of four metal coordination
polymers constructed by the pyridine-3,5-dicarboxylate ligand
*
Ya-Hui Zhao, Zhong-Min Su , Yao-Mei Fu, Kui-Zhan Shao, Peng Li, Yong Wang,
Xiang-Rong Hao, Dong-Xia Zhu, Si-Dong Liu
Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China
Received 9 June 2007; accepted 8 October 2007
Available online 21 December 2007
Abstract
A series of metal-organic frameworks, namely [Ni(PDB)(H2O)]n (1), [Pb(PDB)(H2O)] Æ (H2O) (2), [Co2(PDB)2(bpy)2(H2O)4] Æ 4H2O
(3) and [Co2(PDB)2(phen)2]n (4) (H2PDB = pyridine-3,5-dicarboxylic acid, bpy = 2,20-bipyridine, phen = 1,10-phenanthroline), have
been synthesized based on pyridine-3,5-dicarboxylate acid and two neutral chelate ligands, with different metal ions such as NiII, CoII
and PbII, under hydrothermal conditions. The framework structures of these polymeric complexes have been determined by the X-
ray single crystal diffraction technique. In the four complexes, the pyridine-3,5-dicarboxylate acid ligand exhibits diverse coordination
modes, which play an important role in the construction of metal-organic frameworks. The thermal analyses of these four complexes
have been measured and discussed. In addition, complex 2 shows strong phosphorescent emission at room temperature and the magnetic
measurement of the polymer of 4 reveals a typical antiferromagnetic exchange.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Coordination polymer; Pyridine-3,5-dicarboxylate acid; Crystal structure; Phosphorescence; Magnetic property
1. Introduction
in the preparation of such metal-organic compounds [4].
Pyridyl dicarboxylic acids containing N- and O-donors
are also particularly attractive as excellent building blocks
with charge and multi-connecting ability [5]. In particular,
pyridine-2,4-dicarboxylate acid, pyridine-3,4-dicarboxylate
acid, pyridine-2,5-dicarboxylate acid and pyridine-2,6-
dicarboxylate acid have been widely used by Kitagawa,
Wang, Cao, Zhu and their co-workers in the construction
of high-dimensional structures with large pores or undu-
lated layers [5–8]. In contrast, pyridine-3,5-dicarboxylic
acid [9], as a member of the multicarboxylate ligands con-
taining N-donors, is rarely used. In this context, the major-
ity of this research focuses on the synthesis, structural
elements and properties of complexes constructed from dif-
ferent divalent metal ions and pyridine-3,5-dicarboxylic
acid as well as two chelate bpy-liked ligands, and the utility
of these versatile ligands in the fabrication of functional
organic–inorganic hybrid materials is contemplated. In this
paper, four new coordination complexes, namely,
The construction of metal-organic framework (MOF)
structures is receiving increasing attention because of the
potential applications of novel functional materials such
as magnetism, gas storage, molecular sieves, size- and
shape-selective catalysis and electric conductivity [1,2].
However, rational design and synthesis of MOFs with
unique structures and functions is still hard work. There-
fore, the rational exploitation of the knowledge of the coor-
dination geometry of a metal and ligand structure
continues to be an important synthetic strategy widely used
in the construction of metal-organic framework structures
[3]. Organic aromatic polycarboxylate ligands as an impor-
tant family of multidentate O-donors have been employed
*
Corresponding author. Tel.: +86 431 85099108; fax: +86 431
85684009.
E-mail address: zmsu@nenu.edu.cn (Z.-M. Su).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.10.020

584
Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
[Ni(PDB)(H2O)]
(1),
[Pb(PDB)(H2O)] Æ (H2O)
(2),
bipyridine (0.031 g, 0.02 mmol), NaOH (0.024 g,
[Co2(PDB)2(bpy)2(H2O)4] Æ 4H2O (3) and [Co2(PDB)2-
(phen)2] (4) (H2PDB = pyridine-3,5-dicarboxylic acid,
bpy = 2,20-bipyridine, phen = 1,10-phenanthroline), were
prepared and investigated to determine the influence of
metal ions and chelate ligands on the formation of such
metal-organic frameworks. In addition, the photolumines-
cent property of complex 2 and the magnetic property of
complex 4 were measured and discussed.
0.6 mmol) and H2O (10 ml) was heated at 150 ꢁC for 3
days in a 25 ml Teflon-lined stainless steel vessel under
autogenous pressure. After the reaction mixture was
slowly cooled down to room temperature, pink block
crystals were produced in a yield of 70%. Elem. Anal.
Calc. for complex C34H38Co2N6O16 (3) (904.56): C,
45.15; H, 4.20; N, 9.29. Found: C, 45.10; H, 4.27; N,
9.31%. IR (cmꢀ1, KBr): 3420 (br), 1608 (vs), 1566 (s),
1489 (w), 1478 (w), 1438 (m), 1397 (m), 1375 (s), 1282
(w), 1151 (w), 1125 (w), 932 (w), 771 (s), 736 (mm), 695
(m), 529 (m), 439 (m).
2. Experimental
2.1. Materials and analyses
2.2.4. [Co2(PDB)2(phen)2] (4)
All chemicals were commercially purchased and used
without further purification. Elemental analyses (C, H
and N) were performed on a Perkin–Elmer 2400CHN Ele-
mental Analyzer. FT-IR spectra were recorded in the range
400–4000 cmꢀ1 on an Alpha Centaurt FT/IR spectropho-
tometer as KBr pellets. The emission/excitation spectra
were recorded on a Varian Cary Eclipse spectrometer.
TG analyses were performed on a Perkin–Elmer TGA7
instrument in flowing N2 with a heating rate of 10 ꢁC
minꢀ1. Variable-temperature magnetic susceptibility data
were obtained on a SQUID magnetometer (Quantum
Design, MPMS-5) in the temperature range 5–300 K with
an applied field of 1000 Oe.
An identical procedure to 3 was followed to prepare 4
except 2,20-bipyridine was replaced by 1,10-phenanthroline
(0.031 g, 0.02 mmol), yield 50%. Elem. Anal. Calc. for com-
plex C38H22Co2N6O8 (4) (808.48): C, 56.45; H, 2.72; N,
10.39. Found: C, 56.38; H, 2.81; N, 10.32%. IR (cmꢀ1
,
KBr): 3420 (br), 3056 (m), 2359 (w), 1663 (s), 1610 (s),
1576 (s), 1516 (m), 1496 (w), 1442 (m), 1404 (s), 1354 (s),
1311 (s), 1284 (s), 1226 (w), 1144 (w), 1125 (m), 1104 (w),
1047 (w), 1031 (w), 936 (w), 918 (w), 866 (w), 846 (s), 818
(w), 771 (s), 725 (s), 715 (m), 696 (m), 641(w), 610(w),
564 (w), 438 (w).
2.3. X-ray crystallography analysis
2.2. Syntheses of the complexes
Crystal data, data collection and refinement details for
1–4 are given in Table 1. X-ray diffraction measurements
were performed using a Bruker APXII CCD diffractometer
2.2.1. [Ni(PDB)(H2O)] (1)
˚
A mixture of NiCl2 Æ 6H2O (0.072 g, 0.3 mmol), pyridine
3,5-dicarboxylate acid (0.05 g, 0.3 mmol), NaOH (0.024 g,
0.6 mmol) and H2O (10 ml) was heated at 150 ꢁC for 3 days
in a 25 ml Teflon-lined stainless steel vessel under autoge-
nous pressure. After the reaction mixture was slowly
cooled down to room temperature, green block crystals
were produced in a yield of 60%. Elem. Anal. Calc. for
complex C7H5NiNO5 (1) (241.83): C, 34.77; H, 2.07; N,
with graphite monochromated Mo Ka (k = 0.71073 A)
radiation. An empirical absorption correction was applied
using the SADABS computer program [10]. The coordinates
of the heavy atoms were determined by direct methods
with the SHELXS program [11]. The coordinates and aniso-
tropic thermal displacement parameters for all non-hydro-
gen atoms were refined by the least squares methods based
on F2 using the SHELXL97 program [12]. The H atoms
attached to C atoms were positioned geometrically, with
Uiso values derived from Ueq values of the corresponding
C atom. The hydrogen atoms from water molecules were
directly located from successive Fourier differences synthe-
ses and were included in the calculation with estimated iso-
tropic displacement parameters without refinement, with
5.79. Found: C, 34.80; H, 2.11; N, 5.71%. IR (cmꢀ1
KBr): 3398 (br), 1607 (s), 1560 (s), 1432 (m), 1377 (s),
1302 (m), 1124 (m), 1035 (w), 936 (w), 835 (m), 775 (m),
697 (m), 564 (sm), 437 (m).
,
2.2.2. [Pb(PDB)(H2O)] Æ (H2O) (2)
˚
An identical procedure to 1 was followed to prepare 2
except NiCl2 Æ 6 H2O was replaced by Pb(NO3)2 (0.099 g,
0.3 mmol), yield 65%. Elem. Anal. Calc. for complex
C7H7PbNO6 (2) (408.33): C, 20.59; H, 1.71; N, 3.43.
Found: C, 20.50; H, 1.75; N, 3.40%. IR (cmꢀ1, KBr):
3396 (br), 3080 (m), 1603 (s), 1557 (s), 1361 (vs), 1261
(m), 1147 (ms), 1208 (w), 958 (w), 911 (w), 821 (m), 767
(m), 725 (s), 681 (m), 669 (w), 573 (w), 515 (w), 430 (w).
O–H distances restrained (DFIX) to 0.85(2) A, in order
to ensure a chemically reasonable geometry for these
molecules.
3. Results and discussion
3.1. Description of the structures
Crystallographic and structural refinement data for the
compounds are given in Table 1. Selected bond distances
and angles for 1, 2, 3 and 4 are shown in Tables 2–5,
respectively.
2.2.3. [Co2(PDB)2(bpy)2(H2O)4] Æ 4H2O (3)
A mixture of Co(OAc)2 Æ 4H2O (0.075 g, 0.3 mmol),
pyridine 3,5-dicarboxylate acid (0.05 g, 0.3 mmol), 2,20-

Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
585
Table 1
Crystal data and structure refinement for 1–4
1
2
3
4
Empirical formula
Formula weight
T(K)
C7H5NiNO5
C7H7PbNO6
408.33
293(2)
0.71069
monoclinic
P21/n
10.3826(6)
6.8387(4)
12.7710(8)
90
92.277(1)
90
906.07(9)
4
2.993
18.630
744
C34H38Co2N6O16
904.56
293(2)
0.71069
triclinic
C38H22Co2N6O8
808.48
293(2)
0.71069
monoclinic
P21/c
9.677(5)
31.194(5)
11.181(5)
90
108.069(5)
90
3029(2)
4
1.674
1.103
1640
241.83
293(2)
0.71069
monoclinic
P21/c
7.751(5)
15.049(5)
6.435(5)
90
108.440(5)
90
712.1(8)
4
˚
k(A)
Crystal system
Space group
ꢀ
P1
˚
a (A)
9.542(5)
12.351(5)
16.654(5)
84.766(5)
73.737(5)
86.728(5)
1875.3(14)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (g cmꢀ3
)
2.256
2.716
488
1.602
0.968
932
l (mmꢀ1
F(000)
)
Reflections collected
Independent reflections [Rint
Data/restraints/parameters
Goodness-of-fit on F2
R1a
4335
1654
1654/0/135
1.039
0.0290
0.0752
5039
2132
2132/0/152
1.004
0.0222
0.0492
11709
8435
8435/24/587
0.985
0.0366
0.1148
19926
7628
7628/0/487
0.940
0.0393
0.1064
]
wRb2
P
P
a
R1 = ||F0| ꢀ | Fc||/ |F0 |.
P
P
b
2
wR2 = [w(F0 ꢀ Fc2)2]/ [w(F02)2]1/2
.
Table 2
Selected bond lengths (A) and angles (ꢁ) for compound 1
Table 4
˚
˚
Selected bond lengths (A) and angles (ꢁ) for compound 3
Ni(1)–O(4)
Ni(1)–O(3)
Ni(1)–O(5)
2.005(2)
2.094(3)
2.120(3)
Ni(1)–N(2)
Ni(1)–O(1)
Ni(1)–O(2)
2.064(2)
2.115(2)
2.143(2)
Co(1)–O(1)
Co(1)–O(2W)
Co(1)–O(1W)
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(6)
2.039(2)
2.075(3)
2.136(3)
2.102(3)
2.128(3)
2.186(3)
Co(2)–O(8)
Co(2)–O(3W)
Co(2)–O(4W)
Co(2)–N(4)
Co(2)–N(3)
Co(2)–N(5)a
2.069(2)
2.079(3)
2.093(3)
2.123(3)
2.147(3)
2.196(3)
O(4)–Ni(1)–N(2)
N(2)–Ni(1)–O(3)
N(2)–Ni(1)–O(1)
O(4)–Ni(1)–O(5)
O(3)–Ni(1)–O(5)
O(4)–Ni(1)–O(2)
O(3)–Ni(1)–O(2)
O(5)–Ni(1)–O(2)
109.67(8)
89.0(1)
O(4)–Ni(1)–O(3)
O(4)–Ni(1)–O(1)
O(3)–Ni(1)–O(1)
N(2)–Ni(1)–O(5)
O(1)–Ni(1)–O(5)
N(2)–Ni(1)–O(2)
O(1)–Ni(1)–O(2)
87.39(9)
92.35(7)
89.7(1)
91.35(8)
90.07(8)
96.17(8)
61.70(7)
157.85(8)
92.37(7)
179.64(9)
153.80(7)
88.88(8)
91.22(7)
O(1)–Co(1)–(1W)
N(1)–Co(1)–O(1W)
O(1)–Co(1)–N(1)
O(1)–Co(1)–N(2)
N(1)–Co(1)–N(2)
N(2)–Co(1)–O(1W)
O(2W)–Co(1)–N(6)
90.5(1)
176.5(1)
93.0(1)
169.8(1)
76.9(1)
99.6(1)
172.9(1)
O(3W)–Co(2)–N(3)
O(8)–Co(2)–N(4)
N(4)–Co(2)–N(3)
O(8)–Co(2)–O(3W)
O(3W)–Co(2)–N(4)
O(8)–Co(2)–N(3)
O(4W)–Co(2)–N(5)a
94.5(1)
93.6(1)
77.0(1)
94.8(1)
171.6(1)
169.3(1)
175.0(1)
Symmetry transformations used to generate equivalent atoms: (a) x, y + 1,
z.
Table 3
˚
Selected bond lengths (A) and angles (ꢁ) for compound 2
Pb(1)–O(1W)
Pb(1)–O(5)
Pb(1)–N(1)
2.392(4)
2.492(3)
2.824(3)
Pb(1)–O(2)
Pb(1)–O(1)
2.463(3)
2.685(3)
equatorial plane, and the other carboxyl oxygen atom
(O5C) and the oxygen atom (O3) from a coordinated water
molecule occupy the axial positions (O3–Ni–O5C =
O(1W)–Pb(1)–O(2)
O(2)–Pb(1)–O(5)
O(2)–Pb(1)–O(1)
77.9(1)
83.1(1)
50.7(1)
O(1W)–Pb(1)–O(5)
O(1W)–Pb(1)–O(1)
O(5)–Pb(1)–O(1)
74.8(1)
74.4(1)
128.5(1)
˚
179.64(2)ꢁ), the Ni–O (2.005(2)–2.143(2) A) and Ni–N
˚
(2.064(2) A) bond distances are similar to those found in
related Ni(II) complexes [13] (see Figs. 2a and 2b).
3.1.1. [Ni(PDB)(H2O)] (1)
The coordination modes of PDB in this work are sum-
marized in Scheme 1. As can be seen in Scheme 1c, one car-
boxylate bridges two metal atoms in a carboxylate O,O0
mode, and the other links to the third Ni atom in biden-
tate–chelate mode. The pyridyl N atom bonds to the fourth
Ni center, and thus, each pydc ligand acts as a tetra-con-
nector. Interestingly, one carboxylate group is roughly
coplanar with the adjacent pyridine ring and the other is
Single crystal X-ray diffraction analysis shows that the
asymmetric unit of compound 1 contains one NiII ion,
one PDB ligand and one coordinated water molecule. As
shown in Fig. 1, each NiII ion adopts distorted octahedral
coordination geometry supplied by one N atom and five O
atoms. Three carboxylate oxygen atoms (O1,O2,O4) and
˚
the nitrogen atom (N2) (Ni1–N2 2.064(2) A) comprise the

586
Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
Table 5
˚
Selected bond lengths (A) and angles (ꢁ) for compound 4
Co(1)–O(1)
Co(1)–O(3)a
Co(1)–O(7)b
Co(1)–N(5)
Co(1)–N(1)c
Co(1)–N(6)
2.076(2)
2.089(2)
2.107(2)
2.141(3)
2.163(3)
2.166(3)
Co(2)–O(3)
Co(2)–O(5)
Co(2)–O(8)d
Co(2)–O(1)e
Co(2)–N(2)
Co(2)–N(3)
2.226(2)
1.985(2)
2.079(2)
2.082(2)
2.137(3)
2.102(3)
N(5)–Co(1)–N(6)
O(7)b–Co(1)–N(6)
O(3)a–Co(1)–O(7)b
O(3)a–Co(1)–N(5)
O(3)a–Co(1)–N(6)
O(7)b–Co(1)–N(5)
O(1)–Co(1)–N(1)c
77.0(1)
165.6(1)
84.23(8)
171.38(9)
109.6(1)
88.87(9)
174.75(9)
N(3)–Co(2)–N(2)
O(8)d–Co(2)–N(2)
O(8)d–Co(2)–O(1)e
O(1)e–Co(2)–N(2)
O(8)d–Co(2)–N(3)
O(1)e–Co(2)–N(3)
O(5)–Co(2)–O(3)
78.0(1)
89.7(1)
88.08(9)
176.77(9)
163.3(1)
103.7(1)
165.60(9)
Symmetry transformations used to generate equivalent atoms: (a) x + 1, y,
z; (b) x + 1, ꢀy + 1/2, z ꢀ 1/2; (c) x + 1, y, z ꢀ 1; (d) x, ꢀy + 1/2, z ꢀ 1/2;
(e) x ꢀ 1, y, z.
Fig. 3. ORTEP diagram showing the coordination environment of the Pb
atom in 2. Symmetry code A: x + 1, y, z.
deviated from the plane with a dihedral angle of 21.55ꢁ,
nipping two Ni atoms located in adjacent single layers.
Such a regular crossing-plane coordination fashion is still
rare in the system of multi-carboxylates, and it plays an
important role in the construction of the 3D metal-organic
framework (see Fig. 4b). Moreover, the hydrogen bondings
between the water molecule and carboxylic O atoms
˚
(O3ꢁ ꢁ ꢁO1i = 2.708(3) A, i: 1 ꢀ x, 1 ꢀ y, 1 ꢀ z; O3ꢁ ꢁ ꢁO5ii =
˚
3.143(4) A, ii: x, y, 1 + z) further stabilize the framework.
3.1.2. [Pb(PDB)(H2O)] Æ (H2O) (2)
The structural analysis of complex 2 reveals that it is
a 2D layer structure containing (6,3) topology. The Pb
atom is in a square-pyramidal coordinated geometry with
four O atoms and one N atom from three separated PDB
ligands and one aqua ligand (Fig. 3). Three oxygen atoms
˚
˚
(Pb1–O2 = 2.463(3) A, Pb1–O5 = 2.492(3) A, Pb1–O1 =
˚
2.685(3) A) from two carboxylic groups of the PDB ligands
˚
and one pyridyl N atom (Pb1–N1 = 2.824(3) A) are ligated
Fig. 1. ORTEP diagram showing the coordination environment for the Ni
atoms in 1. Symmetry code A: x ꢀ 1, y, z; B: x, ꢀy + 1/2, z + 1/2; C: ꢀx,
ꢀy + 1, ꢀz.
to the PbII ion in the quasi-plane, with another oxygen
atom belonging to
a water molecule (Pb1–O1W =
Fig. 2. Views of the 2D layer structure of 1 viewed down along the (a) c-axis; (b) a-axis.

Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
587
Scheme 1. Coordination modes of the PDB ligand in compounds 1–4.
Fig. 4. (a) 2D layer structure of 2 viewed along the b-axis; (b) undulated along the c-axis; (c) projection of 2 showing the (6,3) net based on the Pb atoms
and PDB ligands.
˚
2.392(4) A) located in the axial position, as displayed in
Fig. 4a. Longer distances are observed between Pb and
zene rings of the BTC ligands, a distorted 2D (6,3) network
sustained by 3-connected nodes is then yielded, as illus-
trated in Fig. 4c. The multipoint hydrogen-bonding inter-
actions among free water molecules, coordinated water
molecules and carboxylic O atoms extend the 2D layer into
a 3D supramolecular net.
˚
O4 (2.781(4) A); some long Pb–O weak bonds have also
been reported in other lead complexes [14].
Different from that of complex 1, each PDB ligand acts
as a tri-connector linking three Pb atoms in complex 2
(Scheme 1b), one of carboxylate groups connects with
one Pb atom through a mono-dentate O atom, the other
bonds with the second Pb atom; the pyridyl N atom binds
to the third Pb atom, consequently resulting in the forma-
tion of a 2D layer. In compound 2, two carboxylate groups
and the pyridine ring of the ligand are both distorted. The
twist angles between the two –CO2 planes and the pyridyl
ring plane of PDB are 9.39ꢁ (O4,O5) and 14.51ꢁ
(O1,O2), so the 2D layer is developed in an undulated layer
pattern parallel to the ab-plane. By treating the Pb atom as
a single node and connecting the nodes according to the
connectivity defined by the geometrical centers of the ben-
3.1.3. [Co2(PDB)2(bpy)2(H2O)4] Æ 4H2O (3)
When the chelate neutral ligand 2,20-bpy is introduced,
and simultaneously replacing the divalent lead salt with a
divalent cobalt salt, complex 3 is obtained. Single-crystal
X-ray diffraction analysis reveals that complex 3 crystal-
ꢀ
lizes in a triclinic space group P1 and there are two crystal-
lographically independent cobalt atoms, two PDB, two
bpy, four aqua ligands and four free water molecules in
the asymmetric unit. The Co atom, in a distorted octahe-
dral geometry (Fig. 5), is bounded by two N atoms from
one bpy molecule, two O atoms from coordinated water

588
Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
Fig. 5. ORTEP diagram showing the coordination environment of two Co atoms in 3. Symmetry code A: x, y + 1, z.
˚
molecules (Co–O(water) 2.075(3)–2.136(3) A), one pyridyl
N atom and one carboxylic O atom from two PDB
ligands.
ligands and two PDB ligands. As shown in Fig. 7, the
two crystallographically unique cobalt atoms exhibit two
different coordination geometries. The Co1 site shows a
distorted-octahedral geometry, being chelated by two N
atoms from one phen ligand. The remaining four coordina-
tion sites are occupied by one N atom and three oxygen
atoms from three pydc ligands. The Co1–O bond lengths
Each PDB ligand acts as a bridge to connect with
between CoII centers through one carboxyl O atom and
the pyridyl N atom (Scheme 1a), and thus a one-dimen-
sional zig-zag chain is generated, as shown in Fig. 6.
Two types of supramolecular interactions, pꢁ ꢁ ꢁp stacking
˚
are in the range 2.076(2)–2.107(2) A and the Co1–N bond
˚
˚
interactions between bpys of adjacent layer (3.375 A)
lengths are 2.141(3) and 2.166(3) A. The Co2 ion is also
and multipoint hydrogen-bonding interactions between
lattice water molecules, and coordinated water molecules,
as well as uncoordinated carboxylate O atoms, contribute
to the formation of the three-dimensional supramolecular
architecture.
six-coordinated in a more distorted octahedral symmetry
with a N2O4 donor set. The equatorial plane is occupied
by two N-donors from a phen molecule and two O-donors
from carboxylate groups of two PDB ligands, and the axial
positions are completed by two oxygen atoms from another
two PDB ligands. The Co2–O bond lengths are in the range
˚
3.1.4. [Co2(PDB)2(phen)2] (4)
1.985(2)–2.226(2) A and Co2–N bond lengths are 2.102(3)
˚
When another chelate ligand, phen, is used, we obtained
complex 4 under similar reaction conditions. The asymme-
try unit of complex 4 contains two Co ions, two phen
and 2.137(3) A.
In complex 4, PDB ligands adopt two coordination
modes different from those in complexes 1–3: one acts as
Fig. 6. View of the 1D zigzag chain highlighting the p–p interactions between pyridine rings in adjacent chains in 3.

Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
589
Fig. 7. ORTEP diagram showing the coordination environment for the two Co atoms in 4. Symmetry code A: x + 1, y, z; B: x + 1, ꢀy + 1/2, z ꢀ 1/2;
C: x + 1, y, z ꢀ 1; D: x, ꢀy + 1/2, z ꢀ 1/2; E: x ꢀ 1, y, z.
a tetra-connector to link four Co atoms through two
monodentate carboxylic O atoms from two carboxylic
groups in the l-O mode, containing an uncoordinated pyr-
idyl N atom; the other, also being a tetra-connector, bonds
with four Co atoms through one pyridyl N atom, one
monodentate carboxylic O atom from one carboxylate
group and two bis-monodentate carboxylic O atom from
the other carboxylate group, as depicted in Scheme 1d–e.
A pair of l-O atoms link to two adjacent Co atoms,
generating a [Co2O2] unit with a Co1ꢁ ꢁ ꢁCo2 separation
connects with three [Co2O2] units in two layers, conse-
quently resulting in the formation of a bilayer structure
(see Fig. 9).
3.2. Properties
3.2.1. Luminescent properties
Luminescent compounds are of great current interest
because of their various applications in chemical sensors,
photochemistry and electroluminescent (EL) displays [16].
Recently, the luminescent properties of complexes contain-
ing PbII have been reported [17]. In this work, we investi-
gated the photoluminescent property of compound 2 in
the solid-state at room temperature. The free pyridine-
3,5-dicarboxylic acid ligand only shows a fluorescent emis-
sion band at kmax = 378 nm under 320 nm excitation (see
Fig. 1S), whereas, the complex exhibits an emission peak
located at about 475 nm (upon excitation at 330 nm), and
undergoes a red shift by 97 nm compared to the acid
ligand, as shown in Fig. 10. Hence, the emissions of com-
pound 2 in this work might be assigned to be derived from
a ligand to metal charge transition (LMCT). The low-
energy emissions associated with large Stokes shifts for
complex 2 have been commonly observed for other s2-
metal complexes [18]. The heavy-atom effect of lead(II)
increases the spin–orbit coupling and facilitates intersystem
crossing, so the phosphorescence is fast enough to success-
fully compete with radiationless deactivations [18d]. The
observation indicates that complex 2 may be an excellent
candidate for potential photoactive materials.
˚
of 3.202 A, in which the distances are in the normal range
˚
compared with those reported (3.076–3.393 A) in the liter-
ature [15]. As shown in Fig. 8, firstly, one kind of PDB
ligand bridges to two [Co2O2] units to form a 2D net.
Interestingly, when viewed along the b-axis, the two-
dimensional layer contains rectangular windows with a
2
˚
dimensions of ca. 9.129 · 10.557 A that are occupied by
phen ligands. Secondly, the other kind of PDB ligand
3.2.2. Magnetic properties
The magnetic property of polymer 4 has been investi-
gated over the temperature range 5–300 K at 1000 Oe.
The temperature dependence of the molar magnetic suscep-
tibility is illustrated in Fig. 11. The vmT value of 4 remains
almost constant from 300 to 115 K (5.659 cm3 K molꢀ1),
and then decreases on further cooling, reaching a value
of 0.793 cm3 K molꢀ1 at 5 K. This behavior indicates a
dominant weak antiferromagnetic interaction between the
CoII ions in the structure. In 4, the Coꢁ ꢁ ꢁCo distance
Fig. 8. View of the 2D net showing the rectangular window in 4.

590
Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
Fig. 9. View of the bilayer structure in 4.
˚
about 4.6 A, which is a very short transmittance distance,
helpful for magnetic coupling. The variation of vMꢀ1 is well
described by the Curie–Weiss law in the experimental tem-
perature range 5–300 K with C = 5.86 cm3 K molꢀ1 and
h = ꢀ 3.69 K.
The experiment data of complex 4 was fitted using the
theoretical expression of the magnetic susceptibility, (Eq.
(1)) which was deduced from the Heisenberg spin Hamilto-
nian H = ꢀJS1 Æ S2, where J is the intradimer interaction
parameter between the Co(II) ions [20].
vdim
¼
2 ꢂ
ð1Þ
2Ng2b
kT
e5x þ 5e3x þ 14
7 þ e6x þ 3e5x þ 5e3x
where x = ꢀJ/kT. Taking into consideration the possibility
of an interdimer interaction in the crystal lattice, the fol-
lowing correction of the molecular field is made:
Fig. 10. Absorption (left) and phosphorescence (right) spectra of complex
2 in the solid state.
vdim
vM
¼
ð2Þ
1 ꢀ vdimðzJ0=N2b2Þ
The best fitting of the experimental magnetic data by
Eq. (2) gives the followingꢀ1parameters: g = 2.52,
J = ꢀ 4.67 cmꢀ1, zJ0 = 0.878 cm
and R = 9.98 · 10ꢀ4
,
P
P
where R = [(vmT)obs ꢀ (vmT)calcd]2/ [(vmT)obs]2. The re-
sults suggest that the interlayer interactions are weak com-
pared with the intralayer interaction. The negative J value
suggests the occurrence of an obvious antiferromagnetic
Co(II)–Co(II) interactions in 4, being in good agreement
with the vmT versus T curve behavior in the temperature
range 300–5 K. It is difficult to estimate the exchange inter-
action accurately within the Co(II) complexes because of
the effect of spin–orbit coupling.
3.2.3. Thermal properties
Thermal gravimetric analyses (TGA) were carried out to
examine the thermal stabilities of 1–4. The results show
that complexes 1–4 are very stable in air at ambient tem-
perature, which make them potential candidates for practi-
cal applications. The samples were heated up in air at 1 atm
pressure with a heating rate of 10 ꢁC minꢀ1. The TGA
curve shows that 1 is stable up to 127 ꢁC. A weight loss
of 7.6% occurred in the temperature range 127–226 ꢁC, pre-
ꢀ1
Fig. 11. Thermal variations of vMT and vM for complex 4.
˚
through two l2-O atoms is 3.202 A. Even small changes in
these parameters could lead to changes in the Coꢁ ꢁ ꢁCo
interaction. For polymer 4, Co–O bridge distances ranging
˚
from 2.076(2) to 2.226(2) A are slightly longer than those in
the literature [19], but the lengths of the pathways are

Y.-H. Zhao et al. / Polyhedron 27 (2008) 583–592
591
sumably due to the removal of a coordinated water mole-
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