Ligand-controlled assembly of cobalt(II) metal-organic complexes from different semirigid bis(imidazole) derivatives and aromatic monocarboxylates: Electrochemical behaviors and fluorescent properties

Article abstract of DOI:10.1016/j.ica.2012.09.033

To explore the influence of the conformation and substitute group of organic ligands on constructing coordination polymers, we synthesized four Co(II) metal-organic complexes, namely, [Co(bix)(BBA)₂]₂ (1), [Co(bix)(DNBA)₂] (2), [Co(dmpbbbm)(BBA)₂] (3) and [Co(dmpbbbm)(DNBA)(Cl)] (4) (bix = 1,4-bis(imidazol-1-ylmethyl)benzene, dmpbbbm = 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)benzene, HBBA = 4-bromobenzoic acid, HDNBA = 3,5-dinitrobenzoic acid) under hydrothermal conditions. Complex 1 is a dinuclear cluster and is finally extended into a one-dimensional (1D) supramolecular chain though H-bonding interactions. Complex 2 shows 1D right-handed helical chains. Complex 3 possesses meso-helical chains with left- and right-handed helix, which is further extended into a two-dimensional (2D) supramolecular wave-like network through π-π stacking interactions. Complex 4 exhibits a meso-helical chain with left- and right-handed helical loops in one single strand, which is ultimately packed into a three-dimensional (3D) supramolecular framework through π-π stacking interactions. In addition, the thermal stability, the fluorescent properties and the electrochemical behaviors of the title complexes at room temperature have been investigated.

Full text of DOI:10.1016/j.ica.2012.09.033

Inorganica Chimica Acta 394 (2013) 715–722
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Inorganica Chimica Acta
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Ligand-controlled assembly of cobalt(II) metal–organic complexes from
different semirigid bis(imidazole) derivatives and aromatic monocarboxylates:
Electrochemical behaviors and fluorescent properties
⇑
Lian-Li Liu, Jing-Jing Huang, Xiu-Li Wang , Guo-Cheng Liu, Song Yang, Hong-Yan Lin
Department of Chemistry, Bohai University, Liaoning Province Silicon Materials Engineering Technology Research Centre, Jinzhou 121000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
To explore the influence of the conformation and substitute group of organic ligands on constructing
coordination polymers, we synthesized four Co(II) metal–organic complexes, namely, [Co(bix)(BBA)2]2
(1), [Co(bix)(DNBA)2] (2), [Co(dmpbbbm)(BBA)2] (3) and [Co(dmpbbbm)(DNBA)(Cl)] (4) (bix = 1,4-
bis(imidazol-1-ylmethyl)benzene, dmpbbbm = 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)benzene,
HBBA = 4-bromobenzoic acid, HDNBA = 3,5-dinitrobenzoic acid) under hydrothermal conditions. Com-
plex 1 is a dinuclear cluster and is finally extended into a one-dimensional (1D) supramolecular chain
though H-bonding interactions. Complex 2 shows 1D right-handed helical chains. Complex 3 possesses
meso-helical chains with left- and right-handed helix, which is further extended into a two-dimensional
Received 19 June 2012
Received in revised form 24 September
2012
Accepted 27 September 2012
Available online 12 October 2012
Keywords:
Conformation of ligands
Crystal structure
Semirigid bis(imidazole) derivative
Electrochemical behaviors
Fluorescent properties
(2D) supramolecular wave-like network through
helical chain with left- and right-handed helical loops in one single strand, which is ultimately packed
into a three-dimensional (3D) supramolecular framework through stacking interactions. In addition,
p–p stacking interactions. Complex 4 exhibits a meso-
p–p
the thermal stability, the fluorescent properties and the electrochemical behaviors of the title complexes
at room temperature have been investigated.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
ethylbenzimidazol-1-ylmethyl)benzene (dmpbbbm) as N-hetero-
cyclic ligands. In our previous research, we usually use flexible
As the foundation of the genetic code, helical structures have
received much attention in coordination chemistry not only for
their ubiquitous appearance in nature, but also for their promis-
ing applications in multidisciplinary areas, such as biomimetic
chemistry and structural biology [1–6]. Sometimes, such helical
structures were considered as the primary structures, which
can be further organized to supramolecular structures by coordi-
nation or/and hydrogen bonds or other weaker supramolecular
double imidazole derivative ligands [15–18]. Comparing with
those ligands, dmpbbbm or bix not only possesses two freely rotat-
ing imidazole or 5,6-dimethylbenzimidazole arms, but also has a
rigid spacer of phenyl ring, which may generate two flexures and
favor the formation of helical motifs [19]. In addition, the aromatic
ring system may provide potential supramolecular recognition
sites for p–p stacking interactions. The two N-donor ligands were
chosen here to investigate the effect of conformations and substi-
tuting group of ligand on the structures of complexes. To the best
of our knowledge, dmpbbbm ligand used in the complexes has not
been reported up to now.
interactions, such as
p–p stacking interactions [7–9]. It is impor-
tant to rationally design the organic ligands and to choose metal
salts with suitable coordination geometry in the self-assembly
synthesis of such species [10,11]. However, the structure of coor-
dination polymers is influenced by substitute group and confor-
mation of organic ligands, molar ratio of reactants, and pH value
of the solution, etc., which makes the controllable preparation of
the target metal organic complexes still be a great challenge
[12–14].
Taking all of the above discussion into account, by introducing
two aromatic monocarboxylate coligands, that is, 4-bromobenzoic
acid (HBBA) and 3,5-dinitrobenzoic acid (HDNBA), four Co(II)
coordination polymers with different structures, namely [Co(bix)
(BBA)2]2 (1), [Co(bix)(DNBA)2] (2), [Co(dmpbbbm)(BBA)2] (3), and
[Co(dmpbbbm)(DNBA)(Cl)] (4) have been obtained. The effect of
conformations and substituting group of ligands on the ultimate
framework have been represented and discussed. Moreover, the
thermal stability, photoluminescence properties along with
electrochemical properties of 1–4 have also been investigated in
detail.
With this understanding, we select two semirigid ligands 1,4-
bis(imidazol-1-ylmethyl)benzene (bix) and 1,4-bis(5,6-dim-
⇑
Corresponding author. Tel.: +86 416 3400158.
E-mail address: wangxiuli@bhu.edu.cn (X.-L. Wang).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.09.033

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L.-L. Liu et al. / Inorganica Chimica Acta 394 (2013) 715–722
tar and pestle for approximately 40 min to achieve an even mixture,
2. Experimental
and then 0.2 mL paraffin oil was added and stirred with a glass rod.
The homogenized mixture was pack into a glass tube (3 mm inner
diameter) to a length of 6 mm. The electrical contact was established
with the copper stick and the surface of the 1-CPE was wiped with
weighingpaper[22]. A similarprocedurewas used for thepreparation
of 2-CPE, 3-CPE and 4-CPE and bare CPE without any complexes.
2.1. Materials and measurements
All chemicals used for synthesis were of reagent grade and used
without further purification. The dmpbbbm and bix ligands were
prepared according to the literature method [20]. Elemental anal-
yses were performed on a Perkin-Elmer 240CHN analyzer. Thermo-
gravimetric analyses (TGA) were carried out using a Pyris Diamond
thermal analyzer. The luminescence spectra were measured on a
HITACHI F-4500 Fluorescence Spectrophotometer. The electro-
chemical properties were performed on a CHI 440 Electrochemical
Quartz Crystal Microbalance. A conventional three-electrode cell
was used at room temperature. The 1-, 2-, 3- and 4-CPE were used
as working electrodes. An SCE and a platinum wire were used as
reference and auxiliary electrodes, respectively.
2.4. X-ray crystallographic study
The diffraction data of the four complexes were collected on a
Bruker Smart 1000 CCD area detector diffractometer with Mo K
a
radiation (k = 0.71073 Å for 1–4) by using an –2h scan mode.
x
The crystal structures were solved by the direct method and re-
fined with a full-matrix least-squares technique based on F2 with
the SHELXL-97 software package [23]. All the non-hydrogen atoms
were refined with anisotropic thermal parameters on F2. The
hydrogen atoms of ligands were placed in geometrically idealized
positions and refined isotropically with thermal factors. In the
complex 4, the O1, O2, C1, O5, O6, N5 of the DNBA anions were re-
fined as disordered atoms. Distances restraints (DFIX) were applied
in the unreasonable C32 N6 O4 O7 positions of complex 4. The de-
tailed crystallographic data and structure processing parameters
for complexes 1–4 are listed in Table 1. Selected bond lengths
and angles of complexes 1–4 are summarized in Table S1 of the
Supporting Information.
2.2. Preparation of the complexes 1–4
2.2.1. [Co(bix)(BBA)2]2 (1)
A mixture of CoCl2ꢀ6H2O (0.0238 g, 0.1 mmol), bix (0.0242 g,
0.1 mmol) and HBBA (0.04 g, 0.2 mmol) was dissolved in 10 mL
H2O. The pH value was adjusted about 7 with 0.1 M NaOH solution.
The resulting mixture was sealed in a 25 mL Teflon reactor and
then heated at 150 °C for 3 days. After the reactor was cooled to
room temperature at a rate of 5 °C hꢁ1, purple block-shaped crys-
tals of 1 were gained, washed with distilled water, and dried in
air. For complex 1, yield: 37% (based on Co). Elemental Anal. Calc.
for C56H44Br4Co2N8O8: C, 48.23; H, 3.18; N, 8.04. Found: C, 48.17;
3. Results and discussion
H, 3.13; N, 8.16%. IR (KBr),
m
/cmꢁ1: 3419 w, 3174 s, 3008 m,
3.1. Description of crystal structures
1591 s, 1550 s, 1384 s, 1164 m, 771 m, 655 w.
3.1.1. [Co(bix)(BBA)2]2 (1)
The X-ray crystallographic study shows that complex 1 crystal-
lizes in the asymmetrical triclinic space group P1, and exhibits a
2.2.2. [Co(bix)(DNBA)2] (2)
Similar procedure was used to synthesize the purple block crys-
tals of complex 2, except that HDNBA (0.042 g, 0.2 mmol) was used
instead of HBBA for preparation of 1. Yield: 54% (based on Co). Ele-
mental Anal. Calc. for C28H20CoN8O12: C, 46.74; H, 2.80; N, 15.58.
Found: C, 46.59; H, 2.85; N, 15.47%. IR (KBr),
3089 m, 1624 s, 1577 m, 1539 s, 1461 m, 1444 w, 1344 s, 1292
ꢀ
dinuclear structure. As shown in Fig. 1(a), there are one crystallo-
graphically independent Co(II) atom, one bix ligand and two BBA li-
gands in the asymmetric unit of 1. The Co(II) atom is coordinated by
two oxygen atoms (O1, O3) from two BBA ligands and two nitrogen
atoms (N1, N4A) from two bix ligands, showing a distorted tetrahe-
dralgeometry. TheCo–O bond distances are 1.958(3)and 1.996(3) Å,
while the Co–N bond length are between 2.016(3) and 2.031(4) Å.
The bond angles of O–Co–O are 110.88(15)°, while the bond angles
of N–Co–N are 114.90(16)°. The bond angles of N–Co–O are in the
range of 95.58(15)–116.34(15)°.
The carboxylic groups of BBA ligands coordinate to Co(II) atoms,
adopting a monodentate coordination mode. The bix ligand acts as
bidentate ligand to bridge two Co(II) atoms, which exhibits cis-con-
figuration with a Ndonorꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsion angle of 81.82°. The
dihedral angle between the pairs of imidazole rings is 55.22°. Two
bix ligands connect neighboring Co(II) atoms to form a dinuclear
cluster with the distance of 10.820 Å between Co(II) atoms. In addi-
tion, the structure is further extended by C–Hꢀ ꢀ ꢀO hydrogen bonding
interactions between the carboxylic oxygen atoms from BBA anions
and the C–H from the adjacent imidazole rings [C(12)–H(12A)
ꢀ ꢀ ꢀO(1): 3.271 Å] to afford a one-dimensional (1D) supramolecular
chain. Zheng and co-workers have obtained a similar ring in
m
/cmꢁ1: 3373 w,
w, 1091 m, 792 w, 727 s, 655w.
2.2.3. [Co(dmpbbbm)(BBA)2] (3)
Similar procedure was used to synthesize the purple block crys-
tals of complex 3, except that dmpbbbm (0.039 g, 0.1 mmol) was
used instead of bix for preparation of 1. Yield: 42% (based on Co).
Elemental Anal. Calc. for C40H34Br2CoN4O4: C, 56.29; H, 4.02; N,
6.56. Found: C, 56.42; H, 3.87; N, 6.71%. IR (KBr), m
/cmꢁ1: 3423
w, 3168 s, 3012 m, 1589 s, 1546 m, 1400 s, 1275 m, 1194 w, 857
w, 736 m.
2.2.4. [Co(dmpbbbm)(DNBA)(Cl)] (4)
The same synthetic method as that for 1 was used except that
bix was replaced by dmpbbbm (0.039 g, 0.1 mmol) and HBBA
was replaced by HDNBA (0.042 g, 0.2 mmol). Blue block-shaped
crystals of 4 were gained. Yield: 40% (based on Co). Elemental Anal.
Calc. For C33H29ClCoN6O6: C, 56.62; H, 4.18; N, 12.01. Found: C,
(L = 4,40-(hexafluoroisopropylid-
56.38; H, 4.24; N, 12.27%. IR (KBr),
m
/cmꢁ1: 3427 w, 3109 m,
{[Zn(L)(bix)]3(H2O)(DMF)2}n
ene)bis(benzoic acid)) complex [24]. The bix ligand adopts similar
cis-conformation with the dihedral angles between imidazole and
phenyl are 71.791(4)° and 86.333(4)°, which is a little different from
75.18° and 84.68° in 1.
1624 s, 1541 m, 1382 m, 1344 s, 1278 m, 1218 m, 1068 w, 858
w, 727 s.
2.3. Preparation of the 1-, 2-, 3- and 4-CPEs
Complex 1-modified carbon-paste electrode (1-CPE) was fabri-
cated according to the literature method [21]: 0.04 g complex 1 and
0.6 g graphite powder were mixed and ground together by agate mor-
3.1.2. [Co(bix)(DNBA)2] (2)
Single crystal X-ray diffraction analysis reveals that 2 crystal-
lizes in the monoclinic space group C2, exhibits a 1D right-handed

L.-L. Liu et al. / Inorganica Chimica Acta 394 (2013) 715–722
717
Table 1
Crystal data and structure refinement details for complexes 1–4.
Complex
1
2
3
4
Formula
Formula wt.
Crystal system
Space group
a (Å)
C
56H44Br4Co2N8O8
C28H20CoN8O12
719.45
monoclinic
C2
22.432(4)
6.2562(12)
11.100(2)
90
99.851(3)
90
1534.8(5)
2
C40H34Br2CoN4O4
853.44
monoclinic
C2/c
25.0246(15)
6.2508(4)
22.7773(13)
90
95.951(2)
90
3543.7(4)
4
C33H29ClCoN6O6O6
700.00
monoclinic
C2/c
19.1278(11)
16.8327(10)
21.3003(12)
90
99.3350(10)
90
6767.3(7)
8
1394.45
triclinic
P1
ꢀ
10.076(2)
12.750(3)
13.189(3)
64.037(3)
80.602(3)
68.594(3)
1418.4(6)
1
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Åꢁ3
)
Z
D (g cmꢁ3
)
1.633
3.462
1.557
0.637
1.600
2.788
1.374
0.638
l
(mmꢁ1
)
F(000)
694.0
734
1724
2888
hmax (°)
24.99
26.47
27.70
28.33
Total data collected
Unique data reflections
9622
4999
4461
1736
28212
4152
30123
8431
Rint
R1 [I>2
wR2 (all data)
0.0214
0.0559
0.1839
1.003
1.521
ꢁ1.514
0.0198
0.0688
0.2012
1.008
0.557
ꢁ0.308
0.0252
0.0400
0.1086
1.022
1.324
ꢁ1.140
0.0395
0.0591
0.1952
1.047
1.112
ꢁ0.452
a
r(I)]
b
GOF
D
D
qmax (e Åꢁ3
)
)
qmin (e Åꢁ3
a
R1
=
R
(||Fo| ꢁ |Fc||)/
R|Fo|.
b
wR2 = [w(|Fo|2 ꢁ |Fc|2)2/(w|Fo|2)2]1/2
.
Fig. 1. (a) The coordination environment of Co(II) in complex 1, showing a distorted tetrahedral geometry. The hydrogen atoms were omitted for clarity. (b) The dinuclear
unit is extended into 1D supramolecular structure by H-bonding interactions.
helical chain. As illustrated in Fig. 2(a), the asymmetric unit of 2
possesses one Co(II) atom, one bix ligand and two symmetry-re-
lated DNBA anions. Each Co(II) atom is coordinated by two nitro-
gen atoms (N1, N1A) from two bix ligands and four oxygen
atoms (O1, O2, O1A, O2A) from two carboxylic groups of different
DNBA anions, showing an octahedral geometry. The lengths of Co–
O and Co–N are 2.169(7)–2.295(8) Å, and 2.034(6) Å, respectively.
The bond angles of N–Co–O are in the range of 90.1(3)–142.4(2)°,
while the bond angles of O–Co–O are from 55.3(3)° to 165.5(5)°.
The bond angle of N–Co–N is 106.4(3)°.
In complex 2, the deprotonated carboxylic groups of DNBA an-
ions adopt chelating coordination mode to coordinate with Co(II)
atoms. Semirigid bix ligand adopts cis-conformation with a Ndonor-
ꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsion angle of 82.80°, which connect adjacent

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L.-L. Liu et al. / Inorganica Chimica Acta 394 (2013) 715–722
Fig. 2. (a) The coordination environment of Co(II) in complex 2, showing a distorted octahedral geometry. The hydrogen atoms were omitted for clarity. (b) View of the 1D
right-handed helical chain of complex 2.
Co(II) atoms to form a 1D right-handed helical chain with a period
of 11.10 Å. The dihedral angle between the pairs of imidazole rings
is 63.97°. The bix ligand was also employed by Ma and Yang’s
group in synthesizing [Cd(L)(bix)] (L = (R)-2-(40-(400-carboxyben-
zyloxy)phenoxy)propanoic acid). However, the bix ligand dis-
played trans-conformation to form a left-handed helical chain
with a period of 21.8367(7) Å [25].
3.1.4. [Co(dmpbbbm)(DNBA)(Cl)] (4)
The single crystal X-ray diffraction analysis reveals that 4 also
exhibits a 1D meso-helical chain and crystallizes in the monoclinic
system, space group C2/c. The asymmetric unit of 4 contains a tet-
rahedral Co(II) atom (Fig. 4), which is provided by two nitrogen
atoms (N1, N3) from two dmpbbbm ligands and one oxygen atom
(O1) from the carboxylic group of DNBA anion and one chlorine an-
ion at the axial sites. The lengths of Co–O, Co–N and Co–Cl are
1.934(9), 2.003(3)–2.007(2) and 2.2288(10) Å, respectively. The
bond angles of N–Co–O are from 112.0(3)° to 116.8(3)°, while the
bond angle of N–Co–N is 101.76(10)°.
3.1.3. [Co(dmpbbbm)(BBA)2] (3)
Single crystal X-ray diffraction analysis shows that 3 crystallizes
in the monoclinic space group C2/c, and shows 1D meso-helical
chains. As depicted in Fig. 3(a), the asymmetric unit of 3 consists
of one Co(II) atom, one dmpbbbm ligand and two BBA ligands.
The Co(II) atom shows a distorted octahedral geometry, which
is six-coordinated by four oxygen atoms (O1, O2, O1A, O2A) from
two carboxylate groups and two nitrogen atoms (N1, N1A)
from two dmpbbbm ligands. The Co–O bond lengths vary from
2.174(2) to 2.235(3) Å, while the Co–N bond length is 2.070(2) Å.
The bond angles of N–Co–O are in the range of 87.81(9)–
141.70(9)°, while the bond angles of O–Co–O are from 58.50(9)°
to 172.70(16)°. The bond angle of N–Co–N is 105.16(12)°.
In complex 3, the BBA ligand adopts a chelating coordination
mode. The semirigid dmpbbbm ligands connect adjacent Co(II)
atoms to form a 1D meso-helical chain with left- and right-handed
loops in one single strand along the a-axis (Fig. 3(b)). The helical
pitch is 22.777 Å corresponding to the length of the a-axis, and
the Coꢀ ꢀ ꢀCo separation across the dmpbbbm bridge is 13.480 Å.
The similar left- and right-handed helical chain in one single strand
was obtained by Puddephatt and co-workers based on the semi-
It should be noted that the dmpbbbm ligands show two kinds of
coordination conformations in complex 4. One takes a twisted cis-
conformation with a Ndonorꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsion angle of 94.39°,
which bridges two adjacent Co(II) atoms with Coꢀ ꢀ ꢀCo distance of
12.03 Å. The other shows trans-conformation with a Ndonorꢀ ꢀ ꢀN–
Csp3ꢀ ꢀ ꢀCsp3 torsion angle of 76.10°, which bridges two adjacent
Co(II) atoms with Coꢀ ꢀ ꢀCo distance of 13.45 Å. Hinging at metal
ions, the pitch of the meso-helix is 15.823 Å, corresponding to the
length of a-axis. The dihedral angle between the pairs of benzimid-
azole rings in the cis-conformation ligand is 44.6° in contrast to
that in the tran-conformation ligand of 180°. A similar 1D meso-
helical chain was obtained in complex {[Zn(bmb)(2,6-pydc)]3ꢀ2H2-
O}n
(bmb = 1,4-bis(2-methylbenzimidazol-1-ylmethyl)benzene,
2,6-H2pydc = 2,6-pyridinedicarboxylic acid) by Hou and co-work-
ers [1]. The N-donor ligand only adopts trans-conformation with
a Ndonorꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsion angle of 80.221°. Besides that, the
helical pitch is 18.535 Å in complex {[Zn(bmb)(2,6-pydc)]3ꢀ2H2O}n,
which is longer than that in 4. The deprotonated carboxylic group
of DNBA anion adopts monodentate mode to coordinate with Co(II)
atom. The dmpbbbm bridging neighboring Co(II) atoms afford a
fascinating 1D meso-helical chain with left- and right-handed heli-
cal loops in one single strand (Fig. 5(a)). Viewing from the c-axis,
the projection of the helical chain on the ab plane look likes ‘‘8’’
shape (Fig. 5(b)). Moreover, the discrete meso-helical chains are ex-
tended to three-dimensional (3D) supramolecular network
rigid
N,N-bis(pyridin-3-yl)-1,3-benzenedicarboxamide
ligand
[26]. The dmpbbbm adopts trans-conformation with the Ndonor-
ꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsion angle of 71.03°, which displays two flex-
ures with opposite direction. The extension of the structure into
a two-dimensional (2D) wave-like network is accomplished by
p–p stacking interactions between the aromatic rings from BBA
anions and dmpbbbm ligands with the face-to-face distance of
(Fig. 5(c)) by p–p stacking interactions between the aromatic rings
3.874 Å.

L.-L. Liu et al. / Inorganica Chimica Acta 394 (2013) 715–722
719
Fig. 3. (a) The coordination environment of Co(II) in complex 3, showing a octahedral geometry. The hydrogen atoms were omitted for clarity. (b) View of the 1D left- and
right-handed helical chain of complex 3. (c) The 1D chains are connected to 2D supramolecular network by stacking interactions.
p–p
from DNBA anions and 5,6-dimethylbenzimidazole with the face-
to-face distance of 3.817 Å.
3.2. Effect of the conformation and substitute groups of ligands on
complexes 1–4
The conformation and flexibility of ligands are considered to be
the important factors for the formation of helical features. Semi-
rigid ligands (bix and dmpbbbm), contain a rigid spacer of phenyl
ring and two freely twisting imidazole or 5,6-dimethylbenzimida-
zole arms, respectively. The ligands are principally divided into
two types: the same direction (cis-) and the opposite direction
(trans-) on the basis of the difference in the relative orientations
of two imidazole or 5,6-dimethylbenzimidazole rings in the li-
gands [27]. The two conformations can transform to each other
to suit the coordination environment of the metal cations. The rea-
son may be that the flexible –CH2– group makes the two terminal
groups significantly deviate from co-planarity with a potential
Fig. 4. The coordination environment of Co(II) in complex 4, showing a distorted
tetrahedral geometry. The hydrogen atoms were omitted for clarity.

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L.-L. Liu et al. / Inorganica Chimica Acta 394 (2013) 715–722
Fig. 5. (a) View of the 1D meso-helical chain of 4. (b) The projection of helical chain look likes ‘‘8’’ along c axis. (c) The meso-helical chains are interconnected to form 3D
supramolecular framework by stacking interactions.
p–p
tendency to form versatile cis- and trans-conformations when bix
or dmpbbbm coordinates with metal cations [28,29]. From the
structural descriptions above, it shows cis-conformation in 1 and
2, and the Ndonorꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsions angles are 81.75° and
82.80°, respectively. The Coꢀ ꢀ ꢀCo distances are 10.82 and 11.10 Å,
respectively. In 3, the dmpbbbm ligand adopt trans-conformation
with Ndonorꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsion angle of 71.03° and the Coꢀ ꢀ ꢀCo
distances of 13.48 Å. Compared with the complexes 1–3, complex 4
takes two kinds of conformations, cis- and trans-conformation,
with the Ndonorꢀ ꢀ ꢀN–Csp3ꢀ ꢀ ꢀCsp3 torsion angles of 94.39° and
76.10°, respectively. The Coꢀ ꢀ ꢀCo distances are 12.03 and
13.446 Å, respectively. As shown in Fig. 6, the Coꢀ ꢀ ꢀCo distances
in complexes 3 and 4 are a little longer than those in 1 and 2, which
may be attributed to the steric hindrance of the substituent groups
of dmpbbbm. Moreover, the dmpbbbm ligand is easier to form
trans-conformation than the bix ligand, which is probably due to
the steric hindrance of the substituent groups of dmpbbbm
increasing the flexures of ligand.
to form a dinuclear structure in 1, while the dmpbbbm act as
bidentate ligands to generate a 1D left- and right-handed helical
chain in 3. Complexes 2 and 4 based on DNBA anion ligand show
different structures, too. In 2, the bix connect adjacent Co(II)
atoms to form a 1D right-handed chain, however, the dmpbbbm
ligands bridging neighboring Co(II) atoms afford a 1D meso-heli-
cal chain in 4. All of these may be ascribed to the different N-het-
erocyclic ligands. The steric hindrance of the substituent groups
of dmpbbbm ligand ultimately results in the different structures.
In addition, it has been demonstrated that the structural diversi-
ties of the complexes are undoubtedly related to the secondary
auxiliary ligand [30]. In complex 1, BBA ligands act as hydrogen
bonding acceptors which make the dinuclear structure extend
into a 1D chain. In 3, BBA ligands play an important role in con-
structing 2D supramolecular wave-like network by
p–p stacking
interactions. When DNBA carboxylic ligand with two nitro groups
is introduced into the synthetic procedure, a 1D right-handed
helical chain 2 decorated with DNBA at one side is constructed
Complexes 1 and 3 exhibit different structures based on the
same carboxylic ligand. The bix bridge two adjacent Co(II) atoms
and a 3D supramolecular network in 4 was obtained by
stacking interactions. So, the introduction of monocarboxylate
p–p
Fig. 6. The conformations of bix and dmpbbbm in complexes 1–4.

L.-L. Liu et al. / Inorganica Chimica Acta 394 (2013) 715–722
721
coligands may effectively adjust the structure features and build
much more various structures.
3.3. Properties
3.3.1. IR spectroscopy
The main features of IR spectra for complexes 1–4 concern the
carboxylate groups of BBA or DNBA and the N-heterocyclic rings
of dmpbbbm or bix ligand. No strong absorption peaks around
1700 cmꢁ1 for –COOH implies that carboxyl groups of organic moi-
eties in the title complexes are completely deprotonated [31,32].
The peaks at 1591, 1384 cmꢁ1 for 1 (1624, 1577 cmꢁ1 for 2;
1589, 1400 cmꢁ1 for 3; 1624, 1382 cmꢁ1 for 4) can be considered
as vibrations of carboxylate groups. The peaks at 1550, 1164,
771 cmꢁ1 for
1
(1461, 1091, 727 cmꢁ1 for 2; 1546, 1275,
736 cmꢁ1 for 3; 1278, 1218, 727 cmꢁ1 for 4) can be attributed to
mC–N of N-heterocyclic rings of bix or dmpbbbm ligands. The pres-
ence of bands at 3174, 3008 cmꢁ1 for 1 (3089 cmꢁ1 for 2; 3168,
3012 cmꢁ1 for 3; 3109 cmꢁ1 for 4) can be considered as mC–C of –
CH3 or –CH2– of bix and dmpbbbm ligands. The peaks at 1539
and 1344 cmꢁ1 for 2 (1541 and 1344 cmꢁ1 for 4) can be attributed
Fig. 8. Emission spectra of two free N-ligands and complexes 1–4 in the solid state
at room temperature.
to the m(–NO2) (see Figs. S1–S4).
3.3.2. Thermal stability analysis
To characterize the complexes 1–4 more fully in terms of ther-
mal stability, their thermal behaviors were investigated by ther-
mogravimetric analysis (TGA). The TG curves of complexes 1–4
exhibit only one obvious weight loss step (Fig. 7). The framework
of complexes 1 and 2 began to collapse from 290 °C, while the
complexes 3 and 4 are more stable up to 340 °C, where the decom-
position of the framework starts. Pyrolysis of the residual sub-
stance with a series of consecutive weight losses (88.63% for 1,
88.84% for 2, 91.86% for 3, 90.04% for 4) stops at 680 °C for 1,
660 °C for 2, 700 °C for 3 and 720 °C for 4, which implies the
decomposition of organic ligands bix or dmpbbbm and BBA or
DNBA (calculated: 89.24% for 1, 89.58% for 2, 91.21% for 3,
89.29% for 4). The remaining residue weight (10.76% for 1,
10.42% for 2, 8.79% for 3, 10.71% for 4) is presumed to be CoO (cal-
culated: 10.06% for 1, 10.97% for 2, 9.49% for 3, 10.24% for 4).
Fig. 9. Cyclic voltammograms of the 2-CPE in 1 M H2SO4 aqueous solution at
different scan rates (from inner to outer: the bare CPE, 20, 40, 60, 80, 100, 120, 140,
160, 180, 200, 250, 300, 350, 400, 450, 500 mV sꢁ1). The inset shows the plots of the
anodic and cathodic peak currents against scan rates.
3.3.3. Fluorescence properties of complexes 1–4
Luminescent complexes are attracting more and more atten-
tion due to their various potential applications [33,34], such as
chemical sensors, white light-emitting diodes (LEDs), and
electroluminescent materials (OLEDs) for displays [35,36]. Hence,
the solid state photoluminescent properties of complexes 1–4,
free bix and dmpbbbm ligands were investigated at room tem-
perature. As illustrated in Fig. 8, complexes 1 and 2 exhibit in-
tense emission bands with
a maximum at 415 nm upon
excitation at 268 nm and a maximum at 427 nm upon excitation
at 260 nm, respectively. The fluorescent emission peaks of 1 and
2 can be probably assigned to the intraligand
p
⁄ꢁ
p
charge tran-
sitions of bix (kem = 432 nm, kex = 340 nm) due to their similar
emission peaks. The fluorescent emission peaks can be observed
at 446 nm (kex = 265 nm) for 3 and 456 nm (kex = 275 nm) for 4,
which are different from that of (kem = 395 nm, kex = 320 nm)
dmpbbbm ligand. The significant red-shift should be attributed
to the metal–ligand coordination interactions. Compared with
the complexes 1 and 3, the significant red-shift in 2 and 4
may be attributed to the coordination interactions of DNBA an-
ions and metal ions [37].
3.3.4. Electrochemical properties of complexes 1–4
In order to study the redox properties of the Co(II) complexes,
the 1–4 bulk-modified carbon paste electrode (1-, 2-, 3- and 4-
CPE) becomes the optimal choice due to their insolubility in water
Fig. 7. The TG curves of complexes 1–4.

722
L.-L. Liu et al. / Inorganica Chimica Acta 394 (2013) 715–722
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (No. 21171025), and the Natural Science Founda-
tion of Liaoning Province (Nos. 201102003 and 2009402007).
Appendix A. Supplementary material
CCDC 882813, 882814, 882815 and 882816 contain the supple-
mentary crystallographic data for complexes 1–4. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.ica.2012.09.033.
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In conclusion, by introducing bix, dmpbbbm and aromatic
monocarboxylic acids as mixed ligands, four Co(II) complexes
have been successfully synthesized. Complexes 1–4 show diverse
crystal architectures from dinuclear unit to 1D chain, which are
ultimately extended into 1D to 3D supramolecular networks via
p–p stacking interactions or H-bonding interactions. Our study
demonstrates that the conformation and substitute groups of li-
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in the formation of 1D helix chain. Moreover, the fluorescent
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