Effect of organic bicarboxylates on the structures of cobalt(II) coordination polymers derived from a semi-rigid bis(benzimidazole) derivative

Article abstract of DOI:10.1007/s11243-013-9692-x

Two new Co(II) coordination polymers, [Co(L)(glu)] (1) and [Co(L)(npht)]·H₂O (2) (H₂glu = glutaric acid, H₂npht = 3-nitrophthalic acid, L = 1,4-bis(5,6-dimethylbenzimidazol- 1-ylmethyl)benzene), have been hydrothermally synthesized by self-assembly of cobalt chloride with a semi-rigid bis(benzimidazole) derivative and different organic bicarboxylic acids. Single crystal X-ray diffraction analysis reveals that complex 1 is a one-dimensional tube-like coordination polymer containing one helical [Co-L] and two linear [Co-glu] chains. In complex 2, two npht ligands connect two Co(II) atoms to form a binuclear [Co(npht)]₂ subunit, which is further linked by L ligands with two kinds of conformations to form a 3-D CdSO₄-like framework. In addition, the electrochemical behaviors of the title complexes in bulk-modified carbon paste electrodes, and their thermal stabilities and fluorescent properties were investigated in this paper.

Full text of DOI:10.1007/s11243-013-9692-x

Transition Met Chem (2013) 38:359–365
DOI 10.1007/s11243-013-9692-x
Effect of organic bicarboxylates on the structures of cobalt(II)
coordination polymers derived from a semi-rigid
bis(benzimidazole) derivative
•
Guo-Cheng Liu Jing-Jing Huang
•
•
Ju-Wen Zhang Xiu-Li Wang Hong-Yan Lin
•
Received: 12 November 2012 / Accepted: 4 January 2013 / Published online: 20 March 2013
ꢀ Springer Science+Business Media Dordrecht 2013
Abstract Two new Co(II) coordination polymers, [Co(L)-
(glu)] (1) and [Co(L)(npht)]ꢀH2O (2) (H2glu = glutaric
acid, H2npht = 3-nitrophthalic acid, L = 1,4-bis(5,6-
dimethylbenzimidazol-1-ylmethyl)benzene), have been hydro-
thermally synthesized by self-assembly of cobalt chloride with
a semi-rigid bis(benzimidazole) derivative and different
organic bicarboxylic acids. Single crystal X-ray diffraction
analysis reveals that complex 1 is a one-dimensional tube-like
coordination polymer containing one helical [Co-L] and two
linear [Co-glu] chains. In complex 2, two npht ligands connect
two Co(II) atoms to form a binuclear [Co(npht)]2 subunit,
which is further linked by L ligands with two kinds of confor-
mations to form a 3-D CdSO4-like framework. In addition, the
electrochemical behaviors of the title complexes in bulk-
modified carbon paste electrodes, and their thermal stabilities
and fluorescent properties were investigated in this paper.
their structural diversities, but also owing to their potential
applications in fluorescence, magnetism and electrochem-
istry [1–3]. However, it is still a challenge to predict the
final architectures of the desired crystalline products,
because many factors such as organic ligands, pH and
temperature may have a great influence on the self-
assembly process [4–6]. One of the key factors to generate
target complexes is the careful selection of organic ligands
containing appropriate coordination sites linked by proper
spacers [7]. Recently, some bis(benzimidazole) deriva-
tives, such as 1,1-(1,4-butanediyl)bis-1H-benzimidazole)
[8], a,a0-bis(benzimidazole-1-yl)-para-xylene [9], 1,4-
bis(2-methylbenzimidazol-1-ylmethyl)benzene [10] and 4,
40-bis(benzimidazole-1-yl)biphenyl [11] with remarkable
coordination ability and versatile conformations, have
attracted great interest within coordination chemistry. As is
known, bis(benzimidazole) derivatives can be divided into
three types: flexible, semi-rigid and rigid ligands [4–12].
The coordination behavior of flexible and rigid bis(benz-
imidazole) derivatives in transition metal complexes was
widely investigated by Ma, Hou, Bu and our group [4–13].
However, the reports on transition metal coordination
polymers derived from semi-rigid bis(benzimidazole)
derivatives are relatively limited [14, 15]. On the other
hand, as an important family of multidentate O-donor
ligands, organic bicarboxylates seem to be excellent
building blocks with charge, versatile coordination modes
as well as the remarkable stability for constructing high-
dimensional networks with interesting properties [1–3].
Therefore, the introduction of semi-rigid 5,6-dimethyl-
benzimidazole ligands into Co(II)-carboxylates networks
may be an effective synthetic strategy to construct new
functional materials.
Keywords Cobalt(II) complexes ꢀ Semi-rigid
bis(benzimidazole) derivative ꢀ Organic bicarboxylates ꢀ
Electrochemical behavior ꢀ Fluorescent properties
Introduction
The design and preparation of cobalt metal–organic com-
plexes has been greatly developed, not only because of
G.-C. Liu ꢀ J.-J. Huang ꢀ J.-W. Zhang (&) ꢀ X.-L. Wang (&) ꢀ
H.-Y. Lin
Department of Chemistry, Liaoning Province Silicon Materials
Engineering Technology Research Centre, Bohai University,
Jinzhou 121000, People’s Republic of China
e-mail: juwenzhang@yahoo.cn
As an extension of our previous studies [16, 17], in this
X.-L. Wang
e-mail: wangxiuli@bhu.edu.cn
work, we selected
a
semi-rigid bis(benzimidazole)
123

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Transition Met Chem (2013) 38:359–365
derivative 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)ben-
zene (L) as the main ligand and two organic bicarboxylic
acids [flexible glutaric acid (H2glu) and rigid 3-nitroph-
thalic acid (H2npht)] as the auxiliary ligands to investigate
the influence of bicarboxylate anions on the structures of
their cobalt coordination polymers, based on the following
considerations: (1) as a multidentate flexible O-donor
ligand, H2glu with multi –CH2– groups for free twisting
may induce diverse metal-carboxylate coordination
frameworks [18], (2) as a multidentate rigid O-donor
ligand, H2npht with a rigid phenyl ring spacer and two
adjacent carboxyl groups may favor the formation of
metal-carboxylate subunits [19], (3) comparing with flex-
ible 5,6-dimethylbenzimidazole ligands [16, 17], L not
only possesses excellent coordination ability and two freely
rotating –CH2– groups, but also has a rigid phenyl ring
spacer, which may extend the metal-carboxylate subunits
into high-dimensional networks.
H 5.5, N 9.6. Found: C 63.9, H 5.6, N 9.7 %. IR (KBr,
cm-1): 3,157(m), 3,105(m), 2,983(m), 2,927(m), 1,645(m),
1,606(s), 1,510(m), 1,460(m), 1,429(m), 1,383(s), 1,330(m),
1,251(w), 1,211(w), 1,143(w), 1,074(w), 918(w), 812(m),
719(s), 665(m).
Synthesis of [Co(L)(npht)]•H2O (2)
The synthetic procedure for 2 is the same as that for 1 except
that H2npht (0.1 mmol) was used instead of H2glu. Red block
crystals of 2 were obtained (yield: 34 % based on Co). Anal.
Calcd. for C34H31CoN5O7: C 60.0, H 4.6, N 10.3. Found: C
60.1, H 4.5, N 10.2 %. IR (KBr, cm-1): 3,419(m), 3,018(w),
2,937(w), 2,889(w), 1,647(s), 1,612(m), 1,539(m), 1,458(m),
1,384(s), 1,348(m), 1,209(m), 1,135(m), 1,089(m), 1,047(m),
948(m), 896(w), 819(w), 746(s), 657(m).
X-ray crystallographic study
In this paper, we report two new metal–organic coor-
dination polymers [Co(L)(glu)] (1) and [Co(L)(npht)]ꢀH2O
(2). The influence of the bicarboxylates on the coordination
frameworks is presented and discussed. Furthermore, the
electrochemical behaviors of complexes 1 and 2 in bulk-
modified carbon paste electrodes (1-CPE and 2-CPE), and
the thermal stabilities and fluorescent properties of the
complexes were studied.
Crystallographic data for the title complexes were collected
on a Bruker Smart 1000 CCD diffractometer with Mo Ka
˚
(k = 0.71073 A) by x scan mode in the range of
1.72ꢁ B h B 28.31ꢁ for 1 and 1.67ꢁ B h B 25.00ꢁ for 2.
All the structures were solved by direct methods using the
SHELXS program of the SHELXTL package and refined
by full-matrix least-squares methods with SHELXL [20,
21]. Metal atoms in each complex were located from the E-
maps, and other non-hydrogen atoms were located in suc-
cessive difference Fourier syntheses and refined with
anisotropic thermal parameters on F2. The hydrogen atoms
of the organic ligands were placed in geometrically ideal-
ized positions and refined isotropically. In complex 2, the
hydrogen atoms of water molecules were added by dif-
ference Fourier maps, the O1 and O3 positions of npht
ligand were refined as disordered. The details of crystal-
lographic information for complexes 1 and 2 are summa-
rized in Table 1. Selected bond lengths and angles of the
title complexes are listed in Table 2. CCDC 900521 for 1
and 900522 for 2 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Experimental section
Materials and methods
All reagents employed were commercially available and
used as received without further purification. Ligand L was
synthesized according to the literature [16, 17]. FT-IR
spectra (KBr pellets) were taken on a Varian FT-IR 640
Spectrometer, and the elemental analyses (C, H and N) were
carried out on a Perkin-Elmer 240C elemental analyzer.
A CHI 440 electrochemical workstation was used for the
electrochemical experiments. 1-CPE and 2-CPE were used
as working electrodes. Thermogravimetric analysis was
carried out with a Pyris Diamond TG–DTA instrument, and
the luminescence spectra for the samples were measured on
a Hitachi F-4500 Fluorescence Spectrophotometer.
Preparation of 1-CPE
Synthesis of [Co(L)(glu)] (1)
1-CPE was fabricated as follows: 0.7 g graphite powder
and 0.04 g complex 1 (0.045 g for complex 2) were mixed
and ground together by agate mortar and pestle for ca. 1 h,
and then, 0.2 mL paraffin oil was added with stirring until a
homogenized mixture was formed. The mixture was
packed into a glass tube with an inner diameter of 3 mm
and length of 6 mm. The electrical contact was established
with a copper stick, and the surface of the 1-CPE was
A mixture of CoCl2ꢀ6H2O (0.1 mmol), H2glu (0.1 mmol), L
(0.1 mmol), H2O (12 mL) and NaOH (0.2 mmol) was stir-
red for 20 min in air, then transferred and sealed in a 25 mL
Teflon reactor, which was heated at 150 ꢁC for 72 h leading
to the formation of red block crystals of 1 (yield: 28 %
based on Co). Anal. Calcd. for C31H32CoN4O4: C 63.8,
123

Transition Met Chem (2013) 38:359–365
361
polished by weighing paper to achieve a mirror finish
before use. The same procedures were used for prepara-
tions of a bare CPE without Co(II) complex and 2-CPE.
Table 1 Crystal data and structure refinement for complexes 1 and 2
Empirical formula
C31H32CoN4O4
C34H31CoN5O7
Fw
583.54
Monoclinic
P21/n
680.57
Monoclinic
C2/c
Crystal system
Space group
Results and discussion
˚
a (A)
12.8953(8)
9.7077(6)
23.0556(15)
90
19.129(4)
16.040(4)
21.314(5)
90
˚
b (A)
Description of the structures of complexes 1–2
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
Single crystal X-ray analysis shows that complex 1 is a
three-dimensional (3-D) supramolecular network con-
structed from one-dimensional (1-D) tube-like coordination
polymers linked by hydrogen bonding interactions. In 1,
each Co(II) atom is tetra-coordinated by two oxygen atoms
from two carboxyl groups of two different glu ligands and
two nitrogen atoms belonging to two L ligands (Fig. 1).
The bond distances around Co(II) atoms are 1.9472(18)
97.808(2)
90
100.506(5)
90
3
˚
V (A )
2,859.4(3)
4
6,430(2)
8
Z
Dcalc (g/cm3)
l/mm-1
F(000)
1.355
1.406
0.643
0.590
1,220
2,824
˚
˚
and 1.9495(18) A for Co–O, 2.018(2) and 2.0326(19) A for
Co–N, which are near to the reported Co(II) complexes
[16]. The N–Co–N angle is 106.65(8)ꢁ and the O–Co–O
angle is 112.49(8)ꢁ. The N–Co–O angles are in the range of
92.82(8)–118.17(8)ꢁ. The Co(II) coordination polyhedron
is slightly distorted, which may be caused by the steric
effect of the ligands.
Rint
0.0568
0.0464
0.1136
1.008
0.1032
0.0813
0.2457
1.025
Ra1 [I [ 2r(I)]
wRb2 (all data)
GOF
-3
˚
Dqmax (e A
)
0.716
2.017
-3
)
˚
Dqmin (e A
–0.318
–0.591
a
R1 = RjjFojꢁjFcjj=RjFoj
In complex 1, two carboxylic groups of glu adopt
monodentate coordination. As shown in Fig. 2a, the Co(II)
atoms are linked by glu ligands to form a linear (Co–glu)
chain extended along the b axis. The distance of Co1ꢀꢀꢀCo1A
h
i
h
i
1=2
ꢀ
ꢁ
ꢀ
ꢁ
2
2
b
wR2 = R w Fo2 ꢁ Fc2 =R w Fo2
˚
linked by glu is 9.7077(8) A. In addition, owing to the cis–cis
conformation of ligand L and the coordination character of
the Co(II) atom in 1, a 1-D helical chain based on Co(II)
atoms and L ligands is obtained (Fig. 2a, c). As can be
seen from Fig. 2b, two adjacent parallel chains are further
connected by L ligands to generate a tube-like structure with
˚
Table 2 Selected bond distances (A) and angles (ꢁ) for complexes 1
and 2
1
Co(1)–O(3)A
Co(1)–N(1)
1.9472(18) Co(1)–O(1)
1.9495(18)
2.0326(19)
113.70(8)
˚
the dimensions of about 6 9 12 A. The distance of
2.018(2)
Co(1)–N(4)B
˚
Co1ꢀꢀꢀCo1B connected by L is 11.5576(7) A. Finally, the
O(3)A–Co(1)–
O(1)
112.49(8)
O(3)A–Co(1)–N(1)
tube-like polymers are extended into a 3-D supramolecular
framework by hydrogen bonding interactions between the
O(1)–Co(1)–N(1)
111.14(8)
92.82(8)
O(3)A–Co(1)–
N(4)B
118.17(8)
106.65(8)
O(1)–Co(1)–N(4)B
N(1)–Co(1)–N(4)B
2
Co(1)–N(1)
2.064(6)
2.075(14)
2.311(10)
126.7(4)
97.9(2)
Co(1)–N(3)
Co(1)–O(2)
2.017(6)
1.994(6)
Co(1)–O(1)
Co(1)–O(3)
O(2)A–Co(1)–N(3)
N(3)–Co(1)–N(1)
N(3)–Co(1)–O(1)
O(2)A–Co(1)–O(3)
N(1)–Co(1)–O(3)
O(2)A–Co(1)–N(1)
O(2)A–Co(1)–O(1)
N(1)–Co(1)–O(1)
N(3)–Co(1)–O(3)
O(1)–Co(1)–O(3)
97.0(2)
118.6(6)
94.9(5)
87.2(3)
79.7(6)
110.7(5)
82.8(3)
173.6(4)
Symmetry code: A x, 1 ? y, z, B –x ? 1/2, y - 1/2, –z ? 3/2
Symmetry code: A –x ? 1/2, -y - 1/2, -z ? 1
Fig. 1 The coordination environment of Co(II) atom in 1
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Transition Met Chem (2013) 38:359–365
carboxyl O atoms of glu and the C atoms from L ligands
˚
[C(18)–H(18A)ꢀꢀꢀO(3) = 3.135(4) A, 141ꢁ (1/2 ? x, –1/2 –
˚
y, 1/2 ? z); C(10)–H(10B)ꢀꢀꢀO(2) = 3.209(4) A, 169ꢁ (1 -
x, -y, 1 - z)] (Fig. 3).
As is well known, many kinds of 1-D coordination
polymer chains have been synthesized in the past decade,
such as linear, helical and zigzag single chain, ladder and
ribbon double chain, etc. [1, 2]. Chen and coworkers have
obtained the first zipper-like double-strand helical chain
[Cu2(ipa)2(phen)2H2O] (ipa = isophthalate, phen = 1,10-
phenanthroline) [22]. Robson’s group has prepared a 2-D
polyrotaxane network based on 1-D lantern-like coordina-
tion polymers Ag2(bix)3(NO3)2 (bix = 1,4-Bis(imidazol-1-
ylmethyl)benzene) [23]. Complex 1 exhibits a tube-like
structure composed of linear and helical subunits, which is
different from the reported complexes.
Fig. 3 View of the 3-D supramolecular framework derived from the
1-D coordination polymers by hydrogen bonding interactions
The structure of 2 is a 3-D CdSO4-like framework based
on [Co(npht)]2 bimetallic nodes and L linkers. The asym-
metric unit of 2 consists of one npht, one L ligand, one
Co(II) atom and one lattice water molecule. As displayed
in Fig. 4, each Co(II) atom adopts the distorted pyramid
coordination geometry of [CoN2O3], which is defined by
two nitrogen atoms of L ligands and three oxygen atoms
from two npht ligands. The Co–O bond lengths range from
˚
1.994(6) to 2.311(10) A, and the Co–N bond lengths are
˚
2.017(6) and 2.064(6) A, respectively.
Fig. 4 The coordination environment of Co(II) atom in complex 2
In complex 2, the carboxyl groups of npht adopt
monodentate and bridging coordination fashions. The
bridging carboxyl groups of npht connect neighboring
crystallographically equivalent Co1 and Co1A atoms to
form a binuclear [Co(npht)]2 unit with non-bonding
˚
CoꢀꢀꢀCo distance of 4.632(16) A. The L ligands exhibit two
kinds of conformations (La and Lb). The binuclear units are
bridged by La ligands with cis–trans conformation to form
1D [Co2–La] subunit-A. The distance between the cores of
˚
binuclear units is 15.563 A. Lb ligands possessing trans–
trans conformation connect the binuclear units to generate
1-D [Co2–Lb] subunit-B with different directions. The
˚
corresponding distance is 12.482 A. Thus, taking the
binuclear unit [Co(npht)]2 as the 4-connected node and
considering the two kinds of L ligands as spacers (Fig. 5a),
the structure of complex 2 can be described as a 3D
CdSO4-like network (Fig. 5b). To our knowledge, though
some CdSO4-like topology nets have been reported [1–3,
24], this is the first example with CdSO4-type topology
derived from 5,6-dimethylbenzimidazole derivative.
By tuning the lengths and flexibility of organic bicarb-
oxylate ligands under the same conditions, the title com-
plexes exhibit completely different architectures. When a
long and flexible bicarboxylate glu is used in 1, the Co(II)
atoms adopt four-coordinated mode and ligand L exhibits
Fig. 2 View of the 1-D coordination polymer containing two [Co–
glu] chains and one [Co–L] helix (a); view of the 1-D channel (b) and
the schematic view of the coordination polymer (c)
123

Transition Met Chem (2013) 38:359–365
363
H2npht ligand is used, the coordination number of the
Co(II) atom increases from 4 in 1 to 5 in 2. The L ligands
show trans–trans and cis–trans conformations with the
˚
terminal NꢀꢀꢀN distances of 9.8980(86) and 9.1423(28) A,
respectively. The adjacent Co(II) centers are bridged by
two npht anions to form a bimetallic unit, which is finally
connected by L ligands to give a 3-D framework with
CdSO4-type topology. All of these may be attributed to the
different structures and coordination characters of bicarb-
oxylates in 1 and 2.
IR spectra of complexes 1–2
The main features in the IR spectra of the title complexes
concern the carboxylates, water molecules and N-containing
ligands. There is no absorption peak between 1,730 and
1,690 cm-1, indicating that all carboxyl groups of the
organic moieties are deprotonated [25]. The strong bands at
about 1,645 and 719 cm-1 for 1, 1,647 and 746 cm-1 for 2
can be attributed to the m(C–N) stretching of the N-hetero-
cyclic rings of the L ligand. The weak absorption peaks of –
CH2– and –CH3 groups in the two complexes appear at
around 2,983 and 2,927 cm-1 for 1, 2,937 and 2,889 cm-1
for 2 [26]. The characteristic bands at 1,606 and 1,383 cm-1
for 1, 1,612 and 1,384 cm-1 for 2, respectively, are due to
the vibrations of the carboxyl groups [27]. For 2, the strong
broad bands center at around 3,000–3,600 cm-1, relating to
the O–H stretching vibration modes of water molecules [28].
The absorption bands at about 1,348 and 1,539 cm-1 arise
from the nitro-groups of npht in 2 [29].
Thermal analyses of complexes 1–2
Fig. 5 View of the bimetallic 4-connected node [Co(npht)]2 (a), the
3-D framework of 2 (npht ligands were omitted for clarity) (b) and
CdSO4-like topology (c)
Thermal stabilities of the title complexes were investigated
by thermogravimetric analyses (TGA) with a heating rate
of 10 ꢁCꢀmin-1 in the temperature range of 30–700 ꢁC
(Fig. 6). The TG curve of 1 indicates that the network is
stable from room temperature to about 350 ꢁC, after which
Fig. 6 The TG curves of complexes 1 and 2
only one cis–cis conformation with the terminal NꢀꢀꢀN
˚
distance of 9.6315(28) A. Finally, the isolated Co(II) cen-
Fig. 7 Cyclic voltammograms of a the bare CPE, b 1-CPE and c 2-
CPE in 1 M Na2SO4 solution in the potential range of -100 to
950 mV. Scan rate 60 mVꢀs-1
ters of complex 1 are linked by L ligands and glu anions to
form a 1-D tube-like polymer. When a short and rigid
123

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Transition Met Chem (2013) 38:359–365
emission band can be observed for free ligand L
(kex = 320 nm, kem = 395 nm). Comparing with the
emission of ligand L (p* ? p transition), the emissions of
the title complexes exhibit red shift of ca. 4 nm for 1 and
ca. 41 nm for 2, respectively. The carboxylate ligands
show very weak p* ? n transitions and contribute little to
the photoluminescence of the title complexes [31]. Thus,
the emissions of the two polymers may be assigned to
intraligand charge transitions of L. The red shifts are
probably caused by the coordination of L to the metal
centers [32]. The varying degrees of red shift may be
attributed to different coordination environments of L, and
the different structures caused by different organic bi-
carboxylates in this work [33].
Fig. 8 Emission spectra of L ligand and complexes 1–2
significant decomposition of the framework occurs. The
complete decomposition is finished at 650 ꢁC with defined
decomposition product CoO (calcd. 12.8 %; found
12.7 %). For 2, the release of water molecules occurs at
about 200 ꢁC (calcd. 2.6 %; found 2.5 %). Comparing with
1, complex 2 is more stable up to 425 ꢁC, where the col-
lapse of the coordination framework starts a significant
weight loss. The framework of 2 keeps decomposing until
600 ꢁC to form CoO as the final product (calcd. 11.0 %;
found 11.1 %). The different decomposition temperature
for the frameworks may be due to the difference of the
structures and the bicarboxylates in the complexes [30].
Conclusions
In summary, we have synthesized two new Co(II) coordi-
nation polymers constructed from a semi-rigid bis(benz-
imidazole) derivative and two organic bicarboxylates. The
bicarboxylates with different coordination characters
influence the coordination behaviors of Co(II) atoms and
the conformations of L ligands, leading to the formation of
a 1-D tube-like coordination polymer of 1 and 3-D CdSO4-
like framework of 2. Moreover, the remarkable thermal
stability and blue/green fluorescent properties of the title
complexes and the electrochemical behavior of 2 may
make them candidates for potential photoactive and elec-
trochemical materials.
Electrochemical behaviors of the 1-CPE and 2-CPE
The title complexes are insoluble in water and common
organic solvents, so 1-CPE and 2-CPE were prepared to
investigate their cyclic voltammetry behaviors, which is an
inexpensive alternative [17]. Figure 7 shows the cyclic
voltammograms at a bare CPE, 1-CPE and 2-CPE in 1 M
Na2SO4 solution at room temperature. As shown in Fig. 7,
there is no redox peak at the bare CPE in the potential
range -100 to ?950 mV, while an obvious quasi-revers-
ible redox couple was observed at the 1-CPE and 2-CPE.
The mean peak potentials E1/2 = (Epa ? Epc)/2 are 359
and ?365 mV for 1-CPE and 2-CPE, respectively
(60 mVꢀs-1), which could be assigned to the redox of
Co(III)/Co(II) [16]. The subtle differences of the mean
peak potentials between 1-CPE and 2-CPE might be
attributed to the effect of the different structures of the title
complexes.
Acknowledgments This work was supported by the NCET-09-
0853, the National Natural Science Foundation of China (No.
21171025 and 21201021), the Natural Science Foundation of Lia-
oning Province (No. 201102003) and Program of Innovative Research
Team in University of Liaoning Province (LT2012020).
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Photoluminescent properties of L ligand and the title
complexes
The emission spectra of 1 and 2 in the solid state at ambient
temperature were measured (Fig. 8). Complex 1 shows
blue photoluminescence with an emission maximum at
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