Syntheses, structures, fluorescent properties and natural bond orbital analyses of metal-organic complexes based on 5,6-substituted 1,10-phenanthroline derivatives

Article abstract of DOI:10.1016/j.poly.2013.04.054

Six new metal-organic coordination complexes, [Cd(MEDPQ)(2,5-pydc)] _n (1), [Cd₂(MOPIP)₂(2,5-pydc)₂] _n (2), [Cu₂(MOPIP)₂(2,5-pydc) ₂]·H₂O (3), [Mn(MOPIP)₂(2,5-pydc)] ·3H₂O (4), [Cd(MOPIP)₂(2,6-pydc)]·3H ₂O (5) and [Cd₂(MOPIP)₂(2,3-pydc) ₂·H₂O]_n (6) (MEDPQ = 2-methyldipyrido[3,2-f:2′,3′-h]quinoxaline, MOPIP = 2-(4-methoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline, 2,5-H₂pydc = pyridine-2,5-dicarboxylic acid, 2,6-H₂pydc = pyridine-2,6- dicarboxylic acid, and 2,3-H₂pydc = pyridine-2,3-dicarboxylic acid), have been prepared through hydrothermal reactions. The transformation of coordination modes of transition metal ions and organic carboxylate ligands has a crucial influence on the multinuclear structures of these series. In compounds 1 and 2, binuclear Cd(II) subunits occur, which are connected by 2,5-pydc anions to construct an undulating network. By selecting the different transition metal ions, we get the compounds 3 and 4. For the compound 3, the Cu(II) centers are linked by 2,5-pydc ligands to form a binuclear dimer. Compound 4 shows a mononuclear compound. For complexes 5 and 6, by changing the dicarboxylate ligands, Cd(II) adopts a heptacoordinated mode to form a mononuclear compound and chain framework. In addition, fluorescent properties of 2-4 have been reported.

Full text of DOI:10.1016/j.poly.2013.04.054

Polyhedron 59 (2013) 115–123
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses, structures, fluorescent properties and natural bond orbital
analyses of metal–organic complexes based on 5,6-substituted
1,10-phenanthroline derivatives
b
Lei Wang a, Liang Ni a, , Jia Yao
⇑
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China
b Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Six new metal–organic coordination complexes, [Cd(MEDPQ)(2,5-pydc)]n (1), [Cd2(MOPIP)2(2,5-pydc)2]n
(2), [Cu2(MOPIP)2(2,5-pydc)2]ꢀH2O (3), [Mn(MOPIP)2(2,5-pydc)]ꢀ3H2O (4), [Cd(MOPIP)2(2,6-pydc)]ꢀ3H2O
(5) and [Cd2(MOPIP)2(2,3-pydc)2ꢀH2O]n (6) (MEDPQ = 2-methyldipyrido[3,2-f:20,30-h]quinoxaline,
MOPIP = 2-(4-methoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline, 2,5-H2pydc = pyridine-2,5-dicar-
boxylic acid, 2,6-H2pydc = pyridine-2,6-dicarboxylic acid, and 2,3-H2pydc = pyridine-2,3-dicarboxylic
acid), have been prepared through hydrothermal reactions. The transformation of coordination modes
of transition metal ions and organic carboxylate ligands has a crucial influence on the multinuclear struc-
tures of these series. In compounds 1 and 2, binuclear Cd(II) subunits occur, which are connected by 2,5-
pydc anions to construct an undulating network. By selecting the different transition metal ions, we get
the compounds 3 and 4. For the compound 3, the Cu(II) centers are linked by 2,5-pydc ligands to form a
binuclear dimer. Compound 4 shows a mononuclear compound. For complexes 5 and 6, by changing the
dicarboxylate ligands, Cd(II) adopts a heptacoordinated mode to form a mononuclear compound and
chain framework. In addition, fluorescent properties of 2–4 have been reported.
Received 26 February 2013
Accepted 29 April 2013
Available online 9 May 2013
Keywords:
Metal–organic complex
Fluorescent property
NBO analysis
Structural diversity
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
On the other hand, the rigid multicarboxylate-containing li-
gands with aromatic rings have been used to control, and adjust
The development of new materials based on transition-metal
ions and organic ligands, especially coordination polymers, has
been greatly developed in the past decades. These coordination
polymers can be rationally synthesized and tuned by a suitable
choice of metal centers and organic ligands to control their frame-
work structure and functionality [1]. Many metal complexes of
1,10-phenanthroline (1,10-phen) and its derivatives often show
fascinating chemical and physical properties in the area of coordi-
nation chemistry, analytical chemistry, material chemistry, metal-
loenzymes, probes of nucleic acids, and redox processes, because of
the topologies of these transition metal complexes. Compared with
other dicarboxylic acids, the pyridine dicarboxylic acids have five
donor atoms and it mainly coordinates to metal ion via the nitro-
gen and oxygen atom of carboxyl group, acting as a bridging linker
[9]. Thus, the pyridine dicarboxylic acids may connect metal ions in
different directions, generating multidimensional architectures.
As metal ions, Cd, Cu and Mn, with different radius, electronic
properties and flexible coordination environment, exhibits variable
coordination number and geometry with ligands [10], which in-
spire chemists’ extensive interests in their fluorescence, magnetic
and biological activities [11]. Thus, we tried to study the influence
of transition metal coordination nature on the assembly of multi-
nuclear subunits in compounds.
Herein, we report six new compounds with multinuclear struc-
tures, [Cd(MEDPQ)(2,5-pydc)]n (1), [Cd2(MOPIP)2(2,5-pydc)2]n (2),
[Cu2(MOPIP)2(2,5-pydc)2]ꢀH2O (3), [Mn(MOPIP)2(2,5-pydc)]ꢀ3H2O
(4), [Cd(MOPIP)2(2,6-pydc)]ꢀ3H2O (5) and [Cd2(MOPIP)2(2,3-pydc)2
ꢀH2O]n (6) (MEDPQ = 2-methyldipyrido[3,2-f:20,30-h]quinoxaline,
MOPIP = 2-(4-methoxyphenyl)-1H-imidazo[4,5-f][1,10]phenan-
throline, 2,5-H2pydc = pyridine-2,5-dicarboxylic acid, 2,6-H2
pydc = pyridine-2,6-dicarboxylic acid, and 2,3-H2pydc = pyridine-
2,3-dicarboxylic acid). The influence of 5,6-substituted 1,10-phen
their fine
p-conjugated character and strong chelating abilities
[2,3]. Up to now, many kinds of 5,6-substituted 1,10-phen deriva-
tives have been used as terminal or bridging ligands to prepare me-
tal complexes [4–8]. In our paper, we chose two 5,6-substituted
1,10-phen derivatives 2-methyldipyrido[3,2-f:20,30-h]quinoxaline
and 2-(4-methoxyphenyl)-1H-imidazo[4,5-f][1,10]-phenanthro-
line (Scheme 1). Compared with 1,10-phen, an extended p-system
of the two ligands contain potential supramolecular interactions
for molecular recognition or assembly processes.
⇑
Corresponding author. Tel.: +86 51188791800.
E-mail address: niliang@ujs.edu.cn (L. Ni).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.054

116
L. Wang et al. / Polyhedron 59 (2013) 115–123
CH3
calculation were all from the initial X-ray structures of the com-
O
plexes, which were used for the geometry optimization. All the
subsequent calculations were performed based on the optimized
geometries. Natural bond orbital (NBO) analyses were performed
by applying 6-31+G(d) basis set for C, H, O, N atoms, and the effec-
tive core potential basis set Lanl2dz for metal atoms.
The molecular orbitals of 2–4 were plotted using the GAUSSIAN 03
view program.
H
3C
N
N
NH
N
2.3. Synthesis of the complexes 1–6
[Cd(MEDPQ)(2,5-pydc)]n (1):
A
mixture of Cd(OAc)2ꢀ2H2O
N
N
N
N
(0.133 g, 0.5 mmol), 2,5-H2pydc (0.083 g, 0.5 mmol), MEDPQ
(0.124 g, 0.5 mmol), NaOH (0.004 g, 0.1 mmol) and H2O (18 mL)
was placed in a 25 mL Teflon-lined stainless steel vessel under
autogenous pressure at 165 °C for 5 days. After cooling to room
temperature, red block crystals of 1 were collected by filtration
and washed with distilled water in 60% yield (based on Cd). Anal.
Calc. for C22H13N5O4Cd (1): C, 50.45; H, 2.50; N, 13.37. Found: C,
MEDPQ
MOPIP
Scheme 1. Structures of the MEDPQ and MOPIP ligands used in this work.
derivative, the coordination nature of transition-metal ions and the
organic carboxylate linkers on the assembly of multinuclear sub-
units is discussed.
50.47; H, 2.51; N, 13.35%. IR (KBr):
(s), 1521 (m), 1474 (m), 1383 (s), 1334 (s), 1275 (m), 1167 (m),
1035 (w), 825 (m), 763 (m), 551 (w), 440 (w) cmꢁ1
m = 3439 (m), 3053 (m), 1628
.
2. Experimental
[Cd2(MOPIP)2(2,5-pydc)2]n (2): Complex 2 was also synthesized
by using a method similar to that described for the synthesis of 1
except that MOPIP (0.163 g, 0.5 mmol) was used instead of MEDPQ
(0.124 g, 0.5 mmol). Yellow block crystals of 2 were collected by
filtration and washed with distilled water in 52% yield (based on
Cd). Anal. Calc. for C54H34N10O10Cd2 (2): C, 53.70; H, 2.84; N,
2.1. General materials and methods
The neutral chelating ligands MEDPQ and MOPIP were synthe-
sized according to the previous literature method [12]. Other re-
agents and solvents for synthesis were purchased from
commercial sources and used as received. Transmission mode FT-
IR spectra were obtained as KBr pellets between 400 and
4000 cmꢁ1 using a Nicolet Nexus 470 infrared spectrometer. Ele-
mental analysis was carried out with a Perkin-Elmer 240C ana-
11.60. Found: C, 53.71; H, 2.82; N, 11.62%. IR (KBr):
m = 3422 (m),
3192 (m), 1612 (s), 1524 (m), 1482 (s), 1455 (m), 1386 (s), 1342
(s), 1255 (s), 1180 (m), 1074 (m), 822 (m), 769 (s), 509 (w), 477
(w) cmꢁ1
.
[Cu2(MOPIP)2(2,5-pydc)2]ꢀH2O (3): Complex 3 was also synthe-
sized by using a method similar to that described for the synthesis
of 2 except that Cu(OAc)2ꢀH2O (0.1 g, 0.5 mmol) was used instead
of Cd(OAc)2ꢀ2H2O (0.133 g, 0.5 mmol). Green block crystals of 3
were collected by filtration and washed with distilled water in
78% yield (based on Cu). Anal. Calc. for C54H36N10O11Cu2 (3): C,
57.50; H, 3.22; N, 12.42. Found: C, 57.52; H, 3.20; N, 12.41%. IR
lyzer. Thermogravimetric analysis (TG) was performed on
a
Germany Netzsch STA449C at a heating rate of 10 °C ꢀminꢁ1 in
nitrogen. Fluorescence measurement was carried out at room tem-
perature with a Cary Eclipse spectrometer.
2.2. Computational procedures
(KBr):
m = 3438 (m), 3064 (m), 1611 (s), 1522 (s), 1482 (s), 1456
All computations were performed using GAUSSIAN 03 program
package [13] for complexes 2–4. The molecular structures for the
(s), 1344 (s) 1252 (m), 1182 (m), 1079 (m), 819 (m), 769 (s), 507
(w), 473 (w) cmꢁ1
.
Table 1
Crystal data and structure refinements for compounds 1–6.
Parameters
1
2
3
4
5
6
Empirical formula
Formula weight (g molꢁ1
Crystal system
Space group
a (Å)
C
22H13N5O4Cd
C54H34N10O10Cd2
1207.73
C54H36N10O11Cu2
1128.03
C47H33N9O8Mn
906.76
monoclinic
P21/c
10.8045(7)
17.8202(12)
21.5161(14)
90
97.638(1)
90
4105.9(5)
4
1.467
1868
0.391
0.0492
1.081
C47H31N9O9Cd
978.22
monoclinic
P21/c
17.442(3)
14.925(3)
17.640 (3)
90
114.36(3)
90
4183.3(13)
4
1.553
1984
0.594
0.0290
1.060
C54H34N10O11Cd2
1223.73
)
523.78
monoclinic
P21/c
11.142(2)
10.256(2)
18.895(6)
90
111.17(3)
90
2013.5(8)
4
1.728
1040
1.126
0.0201
1.148
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
9.4713(19)
12.967(3)
19.570(4)
89.69(3)
89.95(3)
71.40(3)
2277.9(8)
2
1.761
1208
1.012
0.0266
1.026
8.6466(17)
10.411(2)
13. 278(3)
106.84(3)
102.57(3)
95.93(3)
1098.8(4)
1
1.705
576
1.052
0.0356
1.052
12.189(2)
13.987(3)
14.323(3)
88.28(3)
86.95(3)
74.87(3)
2353.6(8)
2
1.727
1224
0.982
0.1154
1.066
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cm3)
F (000)
Absorption coefficient (mmꢁ1
Rint
)
Goodness-of-fit (GOF) on F2
R1a[I > 2
r
(I)]
0.0474
0.1077
0.0335
0.0715
0.0514
0.1156
0.0619
0.1785
0.0395
0.0862
0.1154
0.3315
wR2b[I > 2
r(I)]
a
R1
wR2 = {
=
R
||Fo| ꢁ |Fc||/
R|Fo|.
b
R
w(|Fo|2 ꢁ |Fc|2)2/
R .
[w(|Fo|2)]2}1/2

L. Wang et al. / Polyhedron 59 (2013) 115–123
117
Fig. 1. (a) View of the coordination environment of Cd(II) in complex 1; thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for
clarity. (b) View of the linkage of the dinuclear core with four adjacent cores (H atoms and part of the aromatic rings are omitted for clarity). (c) The four-connected rhombic
grid network of 1 along bc plan. (d) The supramolecular architecture by hydrogen bonding interactions (green broken lines represent hydrogen bonding interactions). (Color
online.)
[Mn(MOPIP)2(2,5-pydc)]ꢀ3H2O (4): Complex 4 was also synthe-
sized by using a method similar to that described for the synthesis
of 2 except that MnCl2ꢀ4H2O (0.099 g, 0.5 mmol) was used instead
of Cd(OAc)2ꢀ2H2O (0.133 g, 0.5 mmol). Yellow block crystals of 4
were collected by filtration and washed with distilled water in
55% yield (based on Mn). Anal. Calc. for C47H33N9O8Mn (4): C,
62.19; H, 3.78; N, 13.89. Found: C, 62.21; H, 3.77; N, 13.88%. IR
2.4. X-ray crystallography
Single-crystal X-ray data were collected at room temperature
with a Rigaku Saturn 724 CCD diffractometer equipped with a
graphite-monochromatic Mo K
a radiation (k = 0.71073 Å) by using
an scan mode at 293(2) K. The structures were solved by
u-x
direct methods with SHELXS-97 program [14] and refined by
SHELXL-97 [15] using full-matrix least-squares techniques on F2.
All non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were generated theoretically onto the specific
atoms and refined isotropically with fixed thermal factors. The
H-atoms of water molecules have not been localized. The detailed
crystallographic data and structure refinement parameters for the
compounds 1–6 are summarized in Table 1. Selected bond lengths
and angles are listed in Table S1 (Supplementary Data). The crystal
structure representation of complexes 1–6 were made by the soft-
ware DIAMOND [16].
(KBr):
(s), 1254 (m), 1179 (m), 1076 (m), 836 (m), 738 (s), 514 (w), 478
(w) cmꢁ1
m = 3421 (m), 3072 (m), 1609 (s), 1524 (s), 1482 (s), 1354
.
[Cd(MOPIP)2(2,6-pydc)]ꢀ3H2O (5): Complex 5 was also synthe-
sized by using a method similar to that described for the synthesis
of 2 except that 2,6-H2pydc was used instead of 2,5-H2pydc. Yellow
block crystals of 5 were collected by filtration and washed with
distilled water in 62% yield (based on Cd). Anal. Calc. for C47H31N9-
O9Cd (5): C, 57.71; H, 3.19; N, 12.89. Found: C, 57.73; H, 3.18; N,
12.86%. IR (KBr):
1485 (s), 1456 (m), 1427 (s), 1360 (s), 1256 (s), 1178 (m), 1075
(m), 841 (m), 734 (s), 512 (w), 480 (w) cmꢁ1
m = 3419 (m), 3088 (m), 1614 (s), 1525 (m),
.
[Cd2(MOPIP)2(2,3-pydc)2ꢀH2O]n (6): Complex 6 was also synthe-
sized by using a method similar to that described for the synthesis
of 2 except that 2,3-H2pydc was used instead of 2,5-H2pydc. Yellow
block crystals of 6 were collected by filtration and washed with
distilled water in 68% yield (based on Cd). Anal. Calc. for C54H34N10-
O11Cd2 (6): C, 53.00; H, 2.80; N, 11.45. Found: C, 53.01; H, 2.78; N,
3. Results and discussion
3.1. Structural analysis of complexes 1–6
3.1.1. Crystal structure of [Cd(MEDPQ)(2,5-pydc)]n (1)
Single-crystal X-ray diffraction analysis reveals that the asym-
metric unit of complex 1 consists of one Cd(II) ion, one MEDPQ
ligand and one 2,5-pydc anion. As shown in Fig. 1a, each Cd(II) is
surrounded by a distorted octahedral geometry consisting of two
11.46%. IR (KBr):
(s), 1454 (m), 1398 (s), 1358 (s), 1251 (s), 1185 (m), 1073 (m), 831
(m), 740 (s), 534 (w), 448 (w) cmꢁ1
m = 3440 (m), 3093 (m), 1607 (s), 1524 (m), 1482
.

118
L. Wang et al. / Polyhedron 59 (2013) 115–123
O
O
O
O
O
O
O
O
O
O
O
O
N
M
N
M
M
N
M
M
M
M
M
M
M
(c)
(b)
(a)
O
M
M
O
O
O
O
N
N
M
O
N
M
O
M
O
M
M
O
M
M
O
O
O
(d)
(e)
(f)
Scheme 2. Coordination modes of carboxylic groups in complexes 1–6.
nitrogen atoms from one MEDPQ chelating ligand, one nitrogen
atom from 2,5-pydc anion and three oxygen atoms belonging to
three carboxyl groups from three separated 2,5-pydc anions. The
Cd–O distances are longer than those found in Cd–pydc complex
[17].
Both carboxyl groups present in 2,5-pydc ligands are coordi-
nated in a monodentate mode (Scheme 2a), and bridging Cd(II) cen-
ters into a dinuclear cluster with Cdꢀ ꢀ ꢀCd distances of 3.731 Å. The
dinuclear cluster may be viewed as the basic unit, which is con-
nected to the same four clusters by four carboxyl groups of 2,5-pydc
ligands (Fig. 1b), giving rise to a layer (Fig. 1c). Considering the
dinuclear cluster as nodes and keeping the 2,5-pydc ligands as spac-
ers, we can see that the layer is composed of a four-connected
rhombic grid topology. It should be pointed out that the coordina-
tion mode of 2,5-pydc anion is very different from the related com-
plexes [Cd2(OH)2(2,4-pydc)] [17] and [Cd(2,3-pydc)(H2O)2]n [18].
Additionally, the layers are packed through weak C–Hꢀ ꢀ ꢀO hydro-
gen bonds originating from the phen ring H9A atom of MEDPQ li-
gand and the coordinated carboxyl O2 atom of 2,5-pydc anion
into a supramolecular network (Fig. 1d).
3.1.2. Crystal structure of [Cd2(MOPIP)2(2,5-pydc)2]n (2)
When another 5,6-substituted 1,10-phen derivative MOPIP was
used in the presence of 2,5-H2pydc, complexes 2 was obtained.
Complex 2 possesses a 2D polymeric network structure and is con-
nected into a 3D network by supramolecular interactions. The
Fig. 2. (a) View of the coordination environment of Cd(II) in complex 2; thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for
clarity. (b) The undulating network of 2 along ac plan. (c) The supramolecular architecture by hydrogen bonding interactions (green broken lines represent hydrogen bonding
interactions). (Color online.)

L. Wang et al. / Polyhedron 59 (2013) 115–123
119
asymmetric unit of complex 2 consists of two Cd(II) ions, two
MOPIP ligands and two 2,5-pydc anions. As depicted in Fig. 2a,
Cd1 is hexacoordinated by two nitrogen atoms from one MOPIP
chelating ligand, one nitrogen atoms from 2,5-pydc and three
oxygen atoms belonging to three carboxyl groups from three
separated 2,5-pydc anions. The coordination environment of Cd2
is the same as that of Cd1, as shown in Fig. 2a.
interactions are present between the adjacent dinuclear Cu units
and assemble the neighboring layer into a supramolecular struc-
ture (Fig. 3b). To the best of our knowledge, this unique layer Cu
cluster has not been reported so far, although other 3D Cu coordi-
nation polymer including 3,4-pydc as bridging ligands have been
reported [20].
Two distorted octahedrons, which are separately formed by Cd1
3.1.4. Crystal structure of [Mn(MOPIP)2(2,5-pydc)]ꢀ3H2O (4)
and Cd2, are bridged by two
l2-O monodentate carboxyl groups
Complex 4 exhibits a mononuclear molecule structure. The
asymmetric unit of complex 4 consists of one Mn(II) ions, two MO-
PIP ligands, one 2,5-pydc anions and three water molecules. As de-
picted in Fig. 4a, each Mn(II) ion is hexacoordinated and exhibits a
octahedral environment supplied by four nitrogen atoms from the
MOPIP ligand, one nitrogen atom from one 2,5-pydc anion, and one
oxygen atom from the same 2,5-pydc anion.
(Scheme 2a) from two different 2,5-pydc ligands to generate a dou-
ble cadmium cluster. Thus each double cadmium cluster is linked
by four 2,5-pydc ligands, to lead to an undulating network along
the ac plane (Fig. 2b). In addition, unlike 1, the MOPIP ligands in
complex 2 act as hydrogen bonding donors to the coordinated oxy-
gen atoms of 2,5-pydc anions, and exhibits one kind of hydrogen
bonding interactions (N–Hꢀ ꢀ ꢀO interactions). The other kind of
hydrogen bond is weak C–Hꢀ ꢀ ꢀO interactions. Inter-network
N–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO hydrogen bonds assemble the neighboring
undulating network into a supramolecular structure (Fig. 2c). In
The 2,5-pydc anions in complex 4 only act as terminal ligands to
connect Mn(II) ions (Scheme 2c), which result to a mononuclear
structure. The centroid-to-centroid distances in the range of
3.5097(1) to 3.7244(2) Å (The slippage values are from 0.8306 to
1.4682 Å.) between a pair of MOPIP ligands in adjacent mononu-
clear units are observed, showing significant intra-molecular
addition, the larger
p-conjugated planes of MOPIP ligands generate
face-to-face stacking interactions between the terminal ben-
p
ꢀ ꢀ ꢀ
p
zene units and phen rings of MOPIP ligands in the supramolecular
structure of 2 with a centroid-to-centroid distance of 3.7426(1) Å
(Fig. S1). The slippage value of the terminal benzene units and
face-to-face
ture may be viewed as the basic subunit which is connected by
stacking interactions, giving rise to a chain (Fig. 4b). In com-
pꢀ ꢀ ꢀ
p stacking interactions. The mononuclear struc-
pꢀ ꢀ ꢀp
phen rings is 1.1433 Å. Significant
p interactions also play a signif-
plex 4, inter-chain N–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO hydrogen bonds are pres-
ent and assemble the neighboring chain into a supramolecular
structure (Fig. S3).
icant role in the stabilization of the supramolecular structure.
3.1.3. Crystal structure of [Cu2(MOPIP)2(2,5-pydc)2]ꢀH2O (3)
In order to investigate the effect of the metal centers on their
complex structures, two metals Cu(II) and Mn(II) were selected
to react with 2,5-H2pydc and MOPIP under the same synthetic con-
ditions. Two new complexes [Cu2(MOPIP)2(2,5-pydc)2]ꢀH2O and
[Mn(MOPIP)2(2,5-pydc)]ꢀ3H2O were obtained. The asymmetric
unit of compound 3 is shown in Fig. 3a. There are two Cu(II) ions,
two MOPIP ligands, two 2,5-pydc anions and one water molecule.
Cu1 lies on a general position and has a pentacoordinated square
pyramidal geometry as exemplified by its tau parameter of
3.1.5. Crystal structure of [Cd(MOPIP)2(2,6-pydc)]ꢀ3H2O (5)
To evaluate the effect of the position of carboxyl groups on the
framework formation of complex, we selected 2,6-H2pydc and 2,3-
H2pydc to react with cadmium salt in the presence of the same
MOPIP ligand. The complexes 5 and 6 with different structures,
compared to complex 2, were obtained. Complex 5 exists as a neu-
tral mononuclear compound. The asymmetric unit of complex 5
consists of one Cd(II) ions, two MOPIP ligands, one 2,6-pydc anions
and three water molecules. As depicted in Fig. 5a, Each Cd(II)) cen-
ter is heptacoordinated (Scheme 2d) in a distorted pentagonal
bipyramidal geometry consisting of four nitrogen donors from
the MOPIP ligand, one nitrogen donor from the 2,6-pydc ring and
two oxygen atoms from two carboxyl groups of one 2,6-pydc
ligand.
s
= 0.085 (where the s range from 0 to 1 represents the geometric
distortions from a perfect square pyramid to a trigonalbipyramid,
respectively) [19]. The geometry is comprised of three nitrogen
atoms from one MOPIP chelating ligand and one 2,5-pydc anion,
and two oxygen atoms belonging to two carboxyl groups from
two separated 2,5-pydc. Cu2 has
environment.
Two crystallographically independent 2,5-pydc anions in the
complex 3 are coordinated to two Cu(II) ions through four oxygen
atoms of the carboxyl groups and two nitrogen atoms (Scheme 2b)
to form a dinuclear structure. In the adjacent nuclears, face-to-face
a
similar coordination
The independent mononuclear units are packing through face-
to-face
troid distance of 3.7903(5) Å (The slippage value of two phen rings
is 1.2078 Å.), and edge-to-face interactions between terminal
ꢀ ꢀ ꢀ
p
ꢀ ꢀ ꢀ
p interactions between phen rings at a centroid-to-cen-
p
p
benzene units at a centroid-to-face distance of 3.5955(7) Å, which
result in the formation of a layer structure as presented in Fig. 5b.
Significant hydrogen bonding interactions are also implicated for
the reinforcement and overall stability of the crystal lattice. Com-
plex 5 exhibits strong N–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀN hydrogen
bonds between MOPIP rings and 2,6-pydc molecules (Fig. S4). PLA-
TON analysis [21] shows that total potential solvent accessible void
volume is 69.5 Å3. The percentage of the total cell volume is 1.7%.
p p stacking interactions between the imidazole and phen ring of
ꢀ ꢀ ꢀ
MOPIP ligands are found with the centroid-to-centroid separations
of 3.7635(8) and 3.4823(9) Å. The slippage values of two rings are
1.897 and 0.9948 Å. The resulting discrete dinuclear structure is
further cross-linked via the
a layer (Fig. S2). Moreover, the weak C–Hꢀ ꢀ ꢀO hydrogen bonding
p
ꢀ ꢀ ꢀ
p stacking interactions to generate
Fig. 3. (a) View of the coordination environment of Cu(II) in complex 3; thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for
clarity. (b) The supramolecular architecture by hydrogen bonding interactions (green broken lines represent hydrogen bonding interactions). (Color online.)

120
L. Wang et al. / Polyhedron 59 (2013) 115–123
Fig. 4. (a) View of the coordination environment of Mn(II) in complex 4; thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for
clarity. (b) The chain structure by stacking interactions (blue broken lines represent ꢀ ꢀ ꢀ stacking interactions). (Color online.)
ꢀ ꢀ ꢀ
p
p
p p
Fig. 5. (a) View of the coordination environment of Cd(II) in complex 5; thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for
clarity. (b) The layer structure by stacking interactions (blue broken lines represent ꢀ ꢀ ꢀ stacking interactions). (Color online.)
ꢀ ꢀ ꢀ
p
p
p p
3.1.6. Crystal structure of [Cd2(MOPIP)2(2,3-pydc)2ꢀH2O]n (6)
3.2. Effect of the center ions and N-donor ligands on the structures of
the complexes
Complex 6 was crystallized in triclinic crystal system. The
asymmetric unit of compound 6 consist of two Cu(II) ions, two MO-
PIP ligands, two 2,3-pydc anions and one coordinated water mole-
cule. Of the two Cd(II) centers, Cd(1) is heptacoordinated by two
nitrogen atoms from the MOPIP ring, one nitrogen donor from
the 2,3-pydc ring, three oxygen atoms from two carboxyl groups
of two different 2,3-pydc ligands and one oxygen atoms from the
coordinated water molecule. It exhibited a distorted pentagonal
bipyramidal geometry. The second Cd(II) ion, Cd(2), is surrounded
by two nitrogen atoms from the MOPIP ring, one nitrogen donor
from the 2,3-pydc ring and three oxygen atoms from two carboxyl
groups of two different 2,3-pydc molecules in a distorted octahe-
dral geometry, as depicted in Fig. 6a.
It is well-known that different metal centers, bearing different
stereochemically active lone-pair electrons and ionic radius, can
adopt variable coordination numbers and versatile coordination
modes such as hemidirected and holodirected geometries. These
decisive factors can be successful to construct complexes with dif-
ferent structures and dimensions [4b]. Two kinds of coordination
numbers are observed in them: hexacoordinated for 2 and 4, and
five-coordinated for 3. On the other hand, in complexes 2 and 4,
the coordination sphere of the central ion Cd(II) and Mn(II) is holo-
directed, whereas in complexes 3, the coordination sphere of Cu(II)
ion can be regarded as hemidirected and shows the presence of a
stereochemically active lone pair of electrons (Fig. S6). As a result,
the structural differences of 2–4 imply that the number of the
coordination sites and the coordination geometry of the different
center ions have an important influence on the formation of their
structures. Moreover, the selection of metal centers with dissimilar
coordination preferences can influence the coordination modes of
the carboxylate anions and framework dimensionality of these
coordination polymers. This can presumably be attributed to the
metal-controlled assembly. For example, complexes 3 and 4 show
a binuclear dimer and a mononuclear structure; whereas the 2,5-
pydc anion has a different bridging mode, which results complex
2 shows an undulating network.
Thus, there are two crystallographically independent Cd(II) ions
along with the MOPIP groups and two 2,3-pydc anions in the
asymmetric unit of complex 6. The two 2,3-pydc anions exhibit dif-
ferences in their connectivity with Cd(II) ions into a infinite chain
with a loop (Fig. 6b). Out of two carboxylates present in one pyri-
dine-2,3-dicarboxylic acid, one carboxylate group is coordinated in
a chelating bidentate or bridging dimonodentate mode, whereas
the other is coordinated in a monodentate mode as depicted in
Scheme 2e and 2f. In addition, inter-chain strong N–Hꢀ ꢀ ꢀO and C–
Hꢀ ꢀ ꢀO hydrogen bonding interactions between MOPIP rings and
2,3-pydc molecules are present and assemble the neighboring
chain into a layer (Fig. 6c). There exist weak face-to-face
pꢀ ꢀ ꢀp
stacking interactions (the centroid-to-face distance is
3.5971(10) Å) among phen rings of MOPIP ligands from neighbor-
ing chains, which further stabilize the layer of complex 6 (Fig. S5).
As important phen derivatives, the 5,6-substituted 1,10-phen li-
gands of MEDPQ and MOPIP both contain an extended
p-system,
an pyrazine (or imidazole) ring and additional coordination sites,

L. Wang et al. / Polyhedron 59 (2013) 115–123
121
Fig. 6. (a) View of the coordination environment of Cd(II) in complex 6; thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for
clarity. (b) The chain with a loop of complex 6. (c) The layer structure by hydrogen bonding interactions (green broken lines represent hydrogen bonding interactions). (Color
online.)
capable of acting as hydrogen bond acceptors/donors or of forming
coordination interactions to some metal ions [5d]. Obviously, the
coordination behavior of MEDPQ and MOPIP ligand is similar to
1,10-phen in complexes 1–6, which exhibits a strongly chelating
coordination mode. Comparing MEDPQ and MOPIP, the differences
are the imidazole ring and additional benzene ring, which may al-
low the MOPIP ligand have more chance to assemble higher
dimensional complexes in the presence of the bigger N-donor li-
gand. Complex 1 shows the hydrogen bonding interactions origi-
nating from H atom of the phen ring and the O atom of
carboxylate anion. It is different from MOPIP; the imidazole ring
in the MOPIP ligand is a good hydrogen bonding donor. On the
formation of such coordination architectures in the presence of
the same MOPIP ligand and cadmium ions.
In contrast, the 2,5-H2pydc, 2,6-H2pydc and 2,3-H2pydc possess
different steric hindrance, and the two carboxylate groups have
180, 120 and 60 °C angles, respectively. The 2,5-H2pydc is a very
flexible ligand, and the two carboxyl groups bridge adjacent Cd2
units into an undulating network in 2. Different from 2,5-H2pydc,
the carboxyl groups of 2,3-H2pydc adopt a chelating bidentate
mode or bridging dimonodentate coordination mode, which re-
sults in a chain in 6. When the rigid ligand 2,6-H2pydc was em-
ployed, a mononuclear compound 5 was obtained. Obviously, as
far as complexes 2, 5 and 6, the distinct positions of the organic
carboxyl groups result in the different coordination modes of the
carboxylate ligand, which further determine the final structures
of complexes.
other hand, it is noteworthy that the
pꢀ ꢀ ꢀp stacking interactions
are found in complexes 2–6 and play the most important roles in
the assembly of supramolecular structures. So, the MOPIP ligands
have more important function in the formation of the supramolec-
ular framework.
3.4. FT-IR analysis
The FT-IR spectra of the compounds 1–6 exhibit strong charac-
teristic absorptions for the carboxyl groups of the 2,5-pydc, 2,
6-pydc and 2,3-pydc ligands in the asymmetric and symmetric
3.3. Effect of organic carboxylate anions on the structures of the
complexes
vibration regions. The asymmetric stretching vibration
appears in the range of 1628–1607 cmꢁ1, and 1344–1386 cmꢁ1
for the symmetric stretching vibration
s(COOꢁ) [22,23]. The differ-
ence (
= 226–267 cmꢁ1 for 1–6) between s(COOꢁ) and as(COOꢁ)
m
as(COOꢁ)
According to previous reports, the role of organic carboxylate
anions can be illustrated in terms of their differences in shape. Ow-
m
ing to the N-donor ligand with a large conjugated
p
-system, the
D
m
m
selecting of multiple carboxylic acid units have proved to be good
candidates because they can connect metal ions into coordination
polymers exhibiting intriguing structure and dimensional diversi-
ties. In this paper, the structural differences of 2, 5 and 6 indicate
that organic carboxylate anions are also important factors in the
is much more than that of an ionic carboxylate unit, which indi-
cates that carboxyl groups are monodentate coordinated with met-
als, which is in agreement with the crystal structure [24].
Meanwhile, The band corresponding to pyridine ring vibrations
vC@N is observed at lower wave number, 1525–1521 cmꢁ1. This

122
L. Wang et al. / Polyhedron 59 (2013) 115–123
negative shift with respect to the ligand indicates the coordination
of pyridine nitrogen to the metal atom [25]. Weak absorptions ob-
Such frontier orbitals characterize the ligand–ligand and ligand–
metal charges transition for these kinds of compounds.
served at 3192–3053 cmꢁ1 can be attributed to
vC–H of the benzene
ring. The band of 1485–1474 cmꢁ1 is attributed to the skeleton
vibration of the pyridine ring. The signal at 551–507 cmꢁ1 for com-
pounds 1–6 also proves the coordination of metal ions and nitro-
gen atoms. The band at 480–440 cmꢁ1 for 1–6 indicates that the
metal ions are coordinated with oxygen atoms [26]. The broad
bands at around 3440–3419 cmꢁ1 are attributed to the vibrations
of water molecules.
4. Conclusion
Six metal–organic coordination complexes with new topologies
have been obtained under hydrothermal conditions by reactions of
different 5,6-substituted 1,10-phen derivatives and transition me-
tal salts together with organic carboxylate anion ligands. Three
types of coordination structures (0D, 1D, 2D) have been observed
according to the selection of metal centers with different coordina-
tion preferences, as well as the variation of the binding fashions of
organic carboxylate ligands. Moreover, two 5,6-substituted 1,10-
phen ligands (MEDPQ and MOPIP) with the large conjugate system
play crucial roles in the formation of a supramolecular framework
3.5. Fluorescent properties
The solid-state fluorescence spectra of 2–4 at room temperature
were recorded. The free MOPIP ligand also display emission with
two main emission peaks at 380 and 469 nm upon excitation at
by
pꢀ ꢀ ꢀp stacking and hydrogen bonding interactions.
292 nm (Fig. S7), which probably be assigned to the
p ?
p⁄ transi-
tions. The complex 2 exhibits a little red shift with the bands at 382
and 471 nm (kex = 292 nm). The red shift emission peak probably is
related to the intraligand fluorescent emission, and similar red
shifts have been observed before [23,27]. The emission peaks of 3
and 4 (380 and 469 nm (kex = 292 nm)), are mainly from the MOPIP
ligand. Due to the differences of their topological structures, which
result to the emission bands of 2–4 is diverse. The result indicates
that the fluorescence behavior is closely associated with the metal
ions and the ligands coordinated around them.
Acknowledgement
This work is supported by the Research Fund for the Doctoral
Innovative Program of Jiangsu Province (No. CXLX11_0581).
Appendix A. Supplementary data
CCDC 893357, 891047, 891045, 891098, 909209 and 909210
contains the supplementary crystallographic data for compounds
1–6. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.04.054.
3.6. Thermal properties
Thermal stability of the compounds 1 and 2 was performed by
using thermogravimetric (TG) analyzer (Fig. S8). The compound 1
shows a two-step weight loss. The first weight loss of 31.39%
(calcd. 31.52%) occurs from 350 to 450 °C, which corresponds to
the loss of 2,5-pydc ligands. The second weight loss of 46.91%
(calcd. 47.02%) between 620 and 1200 °C is ascribed to the loss
of one MEDPQ ligand. For 2, the framework begins to collapse at
330 °C, which is assigned to the release of 2,5-pydc ligands (obsd
25.93%, calcd. 27.34%), and the departure of MOPIP ligands occur
from 600 to 1308 °C (obsd 50.51%, calcd. 54.04%). After the decom-
position, the final product may be CdO.
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