Syntheses, crystal structures and antibacterial activities of six cobalt(II) pyrazole carboxylate complexes with helical character

Article abstract of DOI:10.1016/j.poly.2012.01.017

Six new cobalt(II) complexes, [Co_1.5(HL)₃(H ₂O)₃]·H₂O (1), [Co(HL) ₂(2,2′-bipy)] (2), [Co(HL)₂(phen)] (3), [Co(HL) ₂(H₂O)₂]·(4,4′-bipy) (4), [Co(

Full text of DOI:10.1016/j.poly.2012.01.017

Polyhedron 36 (2012) 6–14
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses, crystal structures and antibacterial activities of six cobalt(II)
pyrazole carboxylate complexes with helical character
a
a
b
Yuan Chen a, Chong-Bo Liu a, , Yun-Nan Gong , Jin-Mao Zhong , Hui-Liang Wen
⇑
a School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, PR China
b State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, PR China
a r t i c l e i n f o
a b s t r a c t
Six new cobalt(II) complexes, [Co1.5(HL)3(H2O)3]ꢀH2O (1), [Co(HL)2(2,20-bipy)] (2), [Co(HL)2(phen)] (3),
[Co(HL)2(H2O)2]ꢀ(4,40-bipy) (4), [Co(L)(4,40-bipy)(H2O)]ꢀH2O (5) and [Co(L)(4,40-bipy)0.5(H2O)]ꢀ1.5 H2O
(6), (H2L = 5-chloro-1-phenyl-1H-pyrazole-3,4-dicarboxylic acid; 2,20-bipy = 2,20-bipyridine; phen =
1,10-phenanthroline; 4,40-bipy = 4,40-bipyridine) have been synthesized and characterized. Complexes
1–4 exhibit monomeric structures, but with the help of hydrogen bonding interactions all extend into
3D supramolecular networks with double-, single-, single- and quadruple-stranded helical chains,
respectively. Complex 5 displays a layer structure comprising two kinds of single-stranded helical chains,
and complex 6 also exhibits a layer structure, however, this consists of double-stranded and meso-helical
chains; the 2D structures of complexes 5 and 6 are further assembled into 3D and 2-fold interpenetrating
2D supramolecular networks through hydrogen bond interactions, respectively. The thermal stabilities of
the title complexes have been discussed. Furthermore, the antibacterial activities of the title complexes
against tested bacteria were studied and compared to the activities of free ligands.
Article history:
Received 23 November 2011
Accepted 10 January 2012
Available online 25 January 2012
Keywords:
Helix
Cobalt(II) complexes
Pyrazole carboxylic acid
N-donor co-ligands
Antibacterial activity
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
to construct novel coordination polymers with helix structures in
view of its following characteristics: (1) it has multiple O- and
In the past decade, helical structures have attracted particular
interest not only because of their ubiquitous appearance in nature,
such as in proteins [1], collagens [2], quartz [3], single-walled car-
bon nanotubes [4] and many more natural or artificial fiber-type
derivatives [5], polypeptides [6] and DNA [7], but also owing to
their potential applications in molecular recognition, asymmetric
catalysis, biologic pharmacy, etc. Such helical structures in coordi-
nation chemistry can be constructed by different supramolecular
synthons, through coordination interactions, hydrogen bonds and
N-coordination sites located at appropriate angles which may allow
it to connect metal ions to generate helical chains; (2) the introduc-
tion of a non-coordinated group-phenyl ring not only increases the
possibility of C–Hꢀ ꢀ ꢀO(N,
p) hydrogen bonds and p–p interactions,
but also induces different bridging modes of the pyrazole carboxylic
acid due to geometrical constrains, in addition, the phenyl and
pyrazole rings are severely twisted across the C–C single bond, all
of which help to result in distinctive helical structures; (3) it has
two carboxylate groups that allow various acidity-dependent coor-
dination modes, and changes of pH will provide the potential for the
formation of different structures, and further different helix chains.
To the best of our knowledge, complexes with the H2L ligand have
not been previously reported.
p–p stack interactions, etc. Up until now, a lot of single- [8], dou-
ble- [9] or multi-stranded helices [10] constructed through coordi-
nation interactions have been generated by self-assembly
processes, however, helical structures constructed via hydrogen
bondings and other supramolecular interactions are still rare [11].
Such helical structures can be constructed using enantiopure
[12], racemic [13] or achiral molecules [14] as building blocks.
Right- and left-handed helices are often formed in equal amounts
within a single crystal to produce a meso compound when racemic
or achiral molecules are used as building blocks.
Furthermore, N-donor co-ligands, such as 2,20-bipyridine, 4,40-
bipyridine and 1,10-phenanthroline, not only own the features of
flexible and angular ditopic ligands [15], but also can act as
hydrogen-bonded donors and acceptors to form rich directional
hydrogen bonds, which further encourage the constitution of
hydrogen-bonded helical structures. Herein, we report six new co-
balt(II) complexes: [Co1.5(HL)3(H2O)3]ꢀH2O (1), [Co(HL)2(2,20-bipy)]
(2), [Co(HL)2(phen)] (3), [Co(HL)2(H2O)2]ꢀ(4,40-bipy) (4),
[Co(L) (4,40-bipy)(H2O)]ꢀH2O (5) and [Co(L)(4,40-bipy)0.5
(H2O)]ꢀ1.5H2O (6), which display diverse helical characters through
coordination and hydrogen bond interactions.
In this paper, we have reported the synthesis of the new racemic
ligand 5-chloro-1-phenyl-1H-pyrazole-3,4-dicarboxylic acid (H2L)
⇑
Corresponding author.
E-mail address: cbliu@nchu.edu.cn (C.-B. Liu).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2012.01.017

Y. Chen et al. / Polyhedron 36 (2012) 6–14
7
2.2.6. [Co(L)(4,40-bipy)0.5(H2O)]ꢀ1.5H2O (6)
2. Experimental
The synthesis of 6 was similar with that of 4, except that the
reaction temperature was decreased to 120 °C, 0.015 g isonicotinic
acid was added and NaOH was increased to 0.30 mL. Red stick crys-
tals were obtained. Yield: 22.7%. Anal. Calc. for C16H14ClCoN3O6.5: C,
43.02; H, 3.13; N, 9.41. Found: C, 43.45; H, 2.83; N, 9.87%. IR data
2.1. Materials and instrumentation
All reagents, except the H2L ligand, were commercially available
and used without further purification. 1H NMR spectra were re-
corded at 400 MHz on a Bruker WH400 DS spectrometer. The ele-
mental analysis of C, H and N were carried out with a Flash EA 1112
Elementar analyzer. The IR spectra were recorded with a Nicolet
Avatar 360 FT-IR spectrometer using the KBr pellet technique.
Thermal stability studies were carried out on a Pyris diamond
TGA/DTA thermal analyzer.
(KBr pellet, m
/cmꢁ1): 3139 m, 1606 s, 1515 m, 1396 s, 1215 w,
1027 w, 975 w, 890 w, 817 m, 720 w, 630 w.
2.3. X-ray crystallographic study
The X-ray single-crystal data of complexes 1–6 were recorded
on a Brucker APEX II area detector diffractometer with a graph-
ite-monochromated MoKa radiation (k = 0.71073 Å). Semi-empiri-
2.2. Synthesis of the complexes
cal absorption corrections were applied to the title complexes
using the SADABS program [16]. The structures were solved by direct
methods [17] and refined by full-matrix least squares on F2 using
SHELXL-97 [18]. All non-hydrogen atoms were refined anisotropi-
cally. The carboxyl and water H atoms were located from differ-
ence Fourier maps, the other hydrogen atoms were placed in
geometrically calculated positions. Experimental details for X-ray
data collection of 1–6 are presented in Table 1, and selected bond
lengths and angles are listed in Table 2.
2.2.1. [Co1.5(HL)3(H2O)3]ꢀH2O (1)
A mixture of CoCl2ꢀ6H2O (0.024 g, 0.1 mmol), H2L (0.04 g,
0.15 mmol), H2O (8 mL), EtOH (2 mL) and NaOH (0.07 mL,
0.65 mol Lꢁ1) was sealed in a Teflon-lined stainless reactor (23
mL) and heated at 170 °C for 96 h under autogenous pressure. Then
cooling to room temperature, the remaining solution was filtered
and evaporated slowly at room temperature. Red block crystals
were obtained after 4 days. Yield: 35.5%. Anal. Calc. for
C
33H26Cl3Co1.5N6O: C, 41.40; H, 2.72; N, 8.78. Found: C, 41.75; H,
2.4. Antibacterial testing
2.48; N, 9.01%. IR data (KBr pellet,
m
/cmꢁ1): 3468 s, 1705 s, 1570
m, 1525 m, 1403 s, 1385 m, 1296 m, 1027 m, 884 w, 768 s, 675 m.
The in vitro antibacterial activities of the H2L ligand and com-
plexes 1–6 were tested against gram positive bacteria, Staphylococ-
cus aureus, Candida albicans and Bacillus subtilis, and gram negative
bacteria, Escherichia coli and Pseudomonas aeruginosa, by the min-
imum inhibitory concentration (MIC) method. The compounds
were dissolved in DMF with 2-fold serial dilutions from 200 to
2.2.2. [Co(HL)2(2,20-bipy)] (2)
A mixture of CoCl2ꢀ6H2O (0.024 g, 0.1 mmol), H2L (0.027 g,
0.1 mmol), 2,20-bipyridine (0.016 g, 0.1 mmol), H2O (10 mL) and
NaOH (0.15 mL, 0.65 mol Lꢁ1) was sealed in a Teflon-lined stainless
reactor (23 mL) and heated at 150 °C for 72 h under autogenous
pressure. After cooling to room temperature, red block crystals
were obtained. Yield: 55.5%. Anal. Calc. for C32H20Cl2CoN6O8: C,
51.49; H, 2.68; N, 11.26. Found: C, 51.65; H, 2.35; N, 11.71%. IR data
6.25 lg/mL. Sterile micro tubes were filled with 1 mL of serial
2-fold dilutions of the compounds. A growth tube (broth plus inoc-
ulum) and a sterility control tube (broth only) were included each
time. The tubes were incubated at 37 °C for 24 h. MICs were
defined as the lowest concentrations of the compounds which
inhibited the growth of microorganisms, DMF was inactive under
the applied conditions.
(KBr pellet, m
/cmꢁ1): 3140 s, 1708 m, 1618 m, 1554 m, 1400 s, 1328
w, 1261 w, 1074 m, 968 w, 764 m, 697 w.
2.2.3. [Co(HL)2(phen)] (3)
The synthesis of 3 was similar with that of 2, only that 0.019 g
1,10-phenanthroline was used instead of 0.016 g 2,20-bipyridine.
Purple block crystals were obtained. Yield: 43.5%. Anal. Calc. for
3. Results and discussion
3.1. Synthesis and characterization
C
34H20Cl2CoN6O8: C, 53.00; H, 2.60; N, 10.91. Found: C, 53.35; H,
2.32; N, 11.25%. IR data (KBr pellet,
m
/cmꢁ1): 3117 s, 1704 m,
The H2L ligand was prepared according to the literature [19–
22], and the synthesis route of H2L ligand is shown in Scheme 1.
Yield ca. 85%. M.p. 239-240 °C. Anal. Calc. for C11H7N2ClO4: C,
49.55; H, 2.65; N, 10.51. Found: C, 49.83; H, 2.36; N, 10.95%. IR
1605 m, 1547 s, 1403 s, 1336 s, 1247 m, 1065 m, 957 s, 768 m,
701 m.
2.2.4. [Co(HL)2(H2O)2]ꢀ(4,40-bipy) (4)
(KBr pellet, m
/cmꢁ1): 3451 m, 1698 vs, 1589 m, 1515 s, 1467 m,
1431 s, 1185 m. 1H NMR (400 MHz, DMSO d6): d 7.40–7.74 (m,
The synthesis of 4 was similar with that of 2, only that 0.016 g
4,40-bipyridine was used instead of 0.016 g 2,20-bipyridine. Red
block crystals were obtained. Yield: 56.5%. Anal. Calc. for
5H); 12.56–12.62 (d, 2H).
C
32H24Cl2CoN6O10: C, 49.16; H, 3.07; N, 10.74. Found: C, 49.43; H,
3.2. Crystal structure of [Co1.5(HL)3(H2O)3]ꢀH2O (1)
2.85; N, 10.96%. IR data (KBr pellet,
m
/cmꢁ1): 3168 m, 1708 m,
1557 m, 1514 m, 1400 s, 1328 w, 1258 w, 1012 m, 871 m, 778 s,
698 m.
Single crystal X-ray diffraction analysis reveals that complex 1
has two crystallographically independent Co(II) ions separated by
a distance of 9.652 Å in an asymmetric unit, as shown in Fig. 1a.
Each Co(II) ion displays a distorted N2O4 octahedral geometry,
and is coordinated to two nitrogen and two oxygen atoms from
two HLꢁ ligands, and two water oxygen atoms. The coordinated
water molecules around Co1 are in cis-positions, while those
around Co2 are in trans-positions, and Co2 is located at a symmetry
center while Co1 is not, therefore the angle of the pyrazole rings of
the two HLꢁ ligands coordinated to Co1 is 67.1°, while the N2–
Co2–N2 angle is 180°. The Co1–O and Co2–O, Co1–N and Co2–N
2.2.5. [Co(L)(4,40-bipy)(H2O)]ꢀH2O (5)
The synthesis of 5 was similar with that of 4, except that the
reaction temperature was decreased to 90 °C, 0.025 g CuSO4ꢀ5H2O
was added and NaOH was increased to 0.30 mL. Purple block crys-
tals were obtained. Yield: 18.5%. Anal. Calc. for C21H17ClCoN4O6: C,
48.90; H, 3.30; N, 10.87. Found: C, 49.25; H, 3.16; N, 11.02%. IR data
(KBr pellet,
1222 w, 1106 m, 810 m, 705 w, 629 w.
m
/cmꢁ1): 3423 m, 1604 s, 1513 m, 1423 m, 1360 m,

8
Y. Chen et al. / Polyhedron 36 (2012) 6–14
Table 1
Crystal data for complexes 1–6.
Complex
1
2
3
4
5
6
Empirical formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
C
33H26Cl3Co1.5N6O16 C32H20Cl2CoN6O8 C34H20Cl2CoN6O8 C32H24Cl2Co N6O10 C21H17ClCoN4O6 C16H14ClCoN3O6.5
957.34
296(2)
triclinic
746.37
296(2)
monoclinic
C2/c
770.39
296(2)
monoclinic
C2/c
782.40
296(2)
triclinic
515.77
296(2)
orthorhombic
Pccn
446.68
296(2)
orthorhombic
Iba2
ꢀ
ꢀ
P1
P1
9.417(17)
12.558(2)
16.502(3)
78.612(2)
77.473(2)
82.095(2)
1858.4(6)
2
10.131(10)
20.339(19)
15.127(14)
90
100.596(10)
90
10.086(2)
21.345(5)
14.980(4)
90
99.331(2)
90
7.838(5)
9.659(6)
11.719(7)
107.538(10)
97.404(10)
93.455(10)
834.3(9)
1
9.853(3)
20.303(6)
22.592(7)
90
90
90
10.009(8)
47.603(4)
7.488(6)
90
90
90
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
3063.8(5)
4
3182.6(13)
4
4519.0(2)
8
3567.5(5)
8
Z
h range (°)
l
2.23–25.50
0.977
2.28–27.48
0.800
2.26–25.50
0.773
2.22–25.50
0.742
2.30–24.47
0.923
2.41–25.49
1.155
(mmꢁ1
)
F(000)
971
1.711
1.011
14246
6849
0.0273
1516
1.618
0.997
13600
3524
0.0313
1564
1.608
1.016
9320
2969
0.0425
399
1.557
1.020
6451
3096
0.0171
2104
1.516
1.021
18512
3726
0.1729
1816
1.663
1.051
13353
3311
0.0346
Dcalc (Mg mꢁ3
)
Goodness-of-fit (GOF) on F2 (e Åꢁ3
No. of data collected
No. of unique data
Rint
)
R1, wR2 [I > 2
R1, wR2 (all data)
Largest difference in peak and hole (e Åꢁ3
r
(I)]
0.0382, 0.0764
0.0649, 0.0903
0.525, ꢁ0.281
0.0385, 0.0663
0.0598, 0.0731
0.260, ꢁ0.230
0.0406, 0.0812
0.0748, 0.0946
0.229, ꢁ0.258
0.0273, 0.0672
0.0296, 0.0689
0.187, ꢁ0.356
0.0911, 0.1820
0.1756, 0.2218
1.453, ꢁ1.695
0.0300, 0.0670
0.0348, 0.0691
0.661, ꢁ0.196
)
Table 2
through O14–H14Aꢀ ꢀ ꢀO10 and O14–H14Bꢀ ꢀ ꢀO9 hydrogen bonds,
generating double-stranded helical chains, and the left- and
right-handed helices are united by Co1 and O14 atoms (Fig. 2b).
The pitch of the helix running along the b axis is the same as its
length (12.56 Å). The helical chains are further joined into a layer
structure via O15–H15Aꢀ ꢀ ꢀO7 hydrogen bonds, in which another
kind of double-stranded helical chain can be observed. The
repeating unit can be described as (–Co1–HLbꢀ ꢀ ꢀO15–Co1–O14–
H14Bꢀ ꢀ ꢀO9–Co1–HLcꢀ ꢀ ꢀO14ꢀ ꢀ ꢀHLbꢀ ꢀ ꢀO15–Co1–O14ꢀ ꢀ ꢀHLc–Co1)n,
and the pitch of the helix running along the b-axis is twice the
length of the b-axis. Two helical chains with opposite chirality
are joined via O14–H14Bꢀ ꢀ ꢀO9 hydrogen bonding, as shown in
Fig. 2c. As shown in Fig. 2d, these zig-zag chains and layers are
connected into a 3D supramolecular network via O5-H5Aꢀ ꢀ ꢀO6
hydrogen bonds along the b axis.
Selected bond lengths (Å) for complexes 1–6.
1
Co(1)–O(9)
Co(1)–O(14)
Co(1)–N(4)
Co(2)–O(4)
Co(2)–N(2)
2.134(2)
2.043(2)
2.171(2)
2.084(2)
2.166(2)
Co(1)–O(13)
Co(1)–O(15)
Co(1)–N(6)
Co(2)–O(5)
2.074(2)
2.052(2)
2.166(3)
2.051(2)
2
Co(1)–O(4)
Co(1)–N(3)
2.038 (2)
2.097(2)
Co(1)–N(1)
Co(1)–N(1)
Co(1)–O(5)
2.229(2)
2.098(2)
2.044(1)
3
Co(1)–O(4)
Co(1)–N(3)
2.035(2)
2.228(2)
4
Co(1)–O(4)
Co(1)–N(2)
2.080(1)
2.212(1)
5
3.3. Crystal structure of [Co(HL)2(2,20-bipy)] (2) and [Co(HL)2(phen)] (3)
Co(1)–O(4)
Co(1)–N(5)
Co(2)–O(2)
Co(2)–N(3)
2.051(6)
2.186(1)
2.032(7)
2.146(1)
Co(1)–O(5)
Co(1)–N(6)#1
Co(2)–O(3)
Co(2)–N(4)#2
2.086(7)
2.108(1)
2.056(6)
2.196(1)
As shown in Fig. 1b and c, the coordination environments of the
Co(II) ions in complexes 2 and 3 are similar. Each Co(II) ion in com-
plexes 2 and 3 lies on an inversion center and coordinates to six
atoms: two carboxylate oxygen and two nitrogen atoms from
two HLꢁ ligands, and another two nitrogen atoms from one 2,20-
bipy or one phen molecule. In complexes 2 and 3, each H2L ligand
loses one proton and adopts the same coordination mode as in 1. In
2, the [Co(HL)2(2,20-bipy)] monomers are connected through C–
Hꢀ ꢀ ꢀO hydrogen bonds between the C–H group of a phenyl ring
of HLꢁ and an uncoordinated carboxylate group, giving left- and
right-handed helical chains (Fig. 3a and b). The pitch of the helix
running along the a axis is the same as its length (10.13 Å). As
shown in Fig. 4a, the helical chains are further extended into a
6
Co(1)–O(2)#3
Co(1)–O(4)
Co(1)–N(3)
2.033(2)
2.050(2)
2.106(2)
Co(1)–O(3)#3
Co(1)–O(5)
2.078(2)
2.019(2)
Symmetry operations: #1 x, ꢁy + 1/2, z + 1/2; #2 x, ꢁy + 1/2, z ꢁ 1/2; #3 ꢁx + 1, y,
z ꢁ 1/2.
bond lengths are slightly different (Table 2); the Co1–O distances
are in the range 2.043(2) to 2.134(2) Å and the Co1–N distances
are 2.166(3) and 2.171(2) Å, while the Co2–O distances are
2.051(2) and 2.084(2) Å and the Co2–N distances are 2.165(2) Å.
In complex 1, each H2L ligand loses one proton, and coordinates
to one Co(II) ion in an N,O-chelating mode (see Scheme 2a), how-
ever the HL ligands exhibit three orientations in the crystal. For
convenience the HL anions having the nitrogen atoms labeled N2,
N4 and N6 (Fig. 1a) are named as HLa, HLb and HLc, respectively.
In complex 1, the [Co2(HL)2(H2O)2] monomers are joined into a
zig-zag chain through O5–H5Bꢀ ꢀ ꢀO1 hydrogen bonds (Fig. 2a); on
the other hand, the [Co1(HL)2(H2O)2] monomers are connected
3D supramolecular network via C–Hꢀ ꢀ ꢀ
the C–H groups of phenyl rings of HLꢁ and pyrazole rings (the
angle is 126°). Similar
p interactions between
Hꢀ ꢀ ꢀ
p
distance is 3.02 Å and the C–Hꢀ ꢀ ꢀ
p
to 2, the [Co(HL)2(phen)] monomers in 3 are linked through C–
Hꢀ ꢀ ꢀO hydrogen bonds between the C–H group of phen and a coor-
dinated carboxylate group, giving left- and right-handed helical
chains (Fig. 3c and d). Again, the pitch of the helix running along
the a axis is the same as its length (10.09 Å). As shown in Fig. 4b,
the helical chains are further stacked into a 3D supramolecular

Y. Chen et al. / Polyhedron 36 (2012) 6–14
9
Scheme 1. The synthesis route for the H2L ligand.
Fig. 1. (a–f) Coordination environment of the Co(II) ions in complexes 1–6 with 30% thermal ellipsoids.

10
Y. Chen et al. / Polyhedron 36 (2012) 6–14
Scheme 2. Coordination modes of HL and L ligands.
Fig. 3. View of the helical chains with left-handed (a) and right-handed (b) helices
of complex 2, and left-handed (c) and right-handed (d) helices of complex 3.
Fig. 2. (a) The 1D zig-zag chain of 1 formed by [Co2(HL)2] units along the b axis. (b
and c) View of two kinds of double-stranded helical chains along the b axis in the 2D
layer structure constructed by [Co1(HL)2] monomers with the help of hydrogen
bonds in complex 1. (d) The 3D supramolecular structure of 1 along the b axis, the
phenyl rings, chlorine atoms and lattice water molecules are omitted for clarity.
and two right-handed helices (Fig. 5a). The left- and right-handed
helical chains are united at the C3 atoms, and the pitch of the helix
running along the a axis is two times the length of the a axis
(15.68 Å). Moreover, the quadruple-stranded helical chains are fur-
ther joined into a 3D supramolecular network through O–Hꢀ ꢀ ꢀO
hydrogen bonds, as shown in Fig. 5b. As is known, the formation
of single- and double-stranded helical chains are usual, however,
triple-, quadruple-stranded as well as circular and cylindrical heli-
cal structures are rather rare, and complex 4 is a new example of a
quadruple-stranded helix structure [23–26].
network via C–Hꢀ ꢀ ꢀ
p interactions between the C–H groups of phe-
nyl rings of HLꢁ and pyrazole rings (the Hꢀ ꢀ ꢀ
p distance is 3.02 Å
and the C–Hꢀ ꢀ ꢀ
p angle is 130°).
3.4. Crystal structure of [Co(HL)2(H2O)2]ꢀ(4,40-bipy) (4)
Single crystal X-ray diffraction analysis reveals that the asym-
metric unit of complex 4 consists of one Co(II) ion, two HLꢁ ligands,
two coordinated water molecules and one free 4,40-bipy molecule.
As shown in Fig. 1d, the Co(II) ion lies on an inversion center and is
six-coordinated by two carboxylate oxygen and two nitrogen
atoms from two HLꢁ ligands, and two water oxygen atoms; the
4,40-bipy molecule also lies on an inversion center. In complex 4,
one HL ligand bridges one Co(II) ion, adopting the same coordina-
tion mode as in 1–3. In 4, there exist abundant hydrogen bonds: (a)
O–Hꢀ ꢀ ꢀO hydrogen bonds between coordinated water molecules
and uncoordinated carboxylate oxygen atoms; (b) O–Hꢀ ꢀ ꢀN hydro-
gen bonds between coordinated water molecules and the nitrogen
atoms of 4,40-bipy; (c) C–Hꢀ ꢀ ꢀO hydrogen bonds between the C–H
groups of 4,40-bipy and the uncoordinated carboxylate oxygen
atoms. It is worthwhile noting that the [Co(HL)2(H2O)2]ꢀ(4,40-bipy)
monomers are linked via O–Hꢀ ꢀ ꢀO (the Oꢀ ꢀ ꢀO distances range from
2.49 to 2.80 Å), O–Hꢀ ꢀ ꢀN (with an Oꢀ ꢀ ꢀN distance of 2.68 Å) and C–
Hꢀ ꢀ ꢀO (with a Cꢀ ꢀ ꢀO distance of 3.46 Å) hydrogen bonds generating
quadruple-stranded helical chains consisting of two left-handed
3.5. Crystal structure of [Co(L)(4,40-bipy)(H2O)]ꢀH2O (5)
Different from complexes 1–4, the two carboxylate groups of
the H2L ligand are deprotonated in complex 5, one carboxylate
group adopts a l1
a
- :g
g1 0-bridging mode and the other one employs
l2- :g
g1
1-bridging mode, and each L ligand adopts a tridentate
coordination mode and joins two Co2+ ions, as shown in Scheme
2b. As shown in Fig. 1e, the asymmetric unit of complex 5 contains
two distinct Co(II) centers separated by 5.02 Å, two L2ꢁ ligands,
two 4,40-bipy molecules, two coordinated and two free water mol-
ecules. Co1 is six-coordinated by four oxygen and two nitrogen
atoms from two L2ꢁ ligands, two water molecules and two 4,40-
bipy molecules, while Co2 is six-coordinated by four oxygen and
two nitrogen atoms from two L2ꢁ ligands and two 4,40-bipy mole-
cules. In complex 5, two kinds of single-stranded helical chains can
be observed. One is formed through the l2
ylate group linking Co(II) ions in a syn-anti conformation, the pitch
- :g
g1 1-bridging carbox-

Y. Chen et al. / Polyhedron 36 (2012) 6–14
11
Fig. 4. View of the 3D supramolecular networks along the b axis for 2 (a) and 3 (b), 2,20-bipyridine and Cl atoms in 2 and Cl atoms in 3 have been omitted for clarity.
a 3D supramolecular network via O–Hꢀ ꢀ ꢀO (the Oꢀ ꢀ ꢀO distances
range from 2.71 to 2.73 Å) and O–Hꢀ ꢀ ꢀN (with an Oꢀ ꢀ ꢀN distance
of 3.11 Å) hydrogen bonds between water molecules and carboxyl-
ate oxygen or pyrazolyl nitrogen atoms.
3.6. Crystal structure of [Co(L)(4,40-bipy)0.5(H2O)]ꢀ1.5H2O (6)
Single crystal X-ray diffraction analysis reveal that there is only
one crystallographically independent Co(II) ion in complex 6, as
shown in Fig. 1f. The Co(II) ion is five-coordinated by three carbox-
ylate oxygen atoms from two L2ꢁ ligands, one nitrogen atom from
one 4,40-bipy molecule and one water oxygen atom.
Similar to complex 5, the two carboxylate groups of the H2L li-
gand are deprotonated in complex 6, one carboxylate group adopts
a
l1- :g
g1 0-bridging mode, different from complex 5, and the other
one adopts a l2- :g
g1 1-bridging mode that links adjacent Co(II) ions
in an anti–anti conformation, as shown in Scheme 2c, giving rise to
a zig-zag chain structure (chain A). The two carboxylate groups of
the L2ꢁ ligand join Co(II) ions, forming a meso-helical chain (chain
B), the repeating unit of which can be described as (–Co1–O4–C1
1–C9–C8–C10–O2–Co1–O4–C11–C9–C8–C10–O2–)n (Fig. 7a), and
the pitch of the helix running along the c axis is the same as its
length (7.49 Å). Furthermore, the 1D chains are interlinked to pro-
duce a layer structure through 4,40-bipy molecules along the b axis
(Fig. 7a). In addition, the carboxylate group with the l2- : -
g1 g1
bridging mode in the anti–anti conformation and the 4,40-bipy link
Co(II) ions in turn, forming a double-stranded helical chain with
left- and right-handed helices (Fig. 7b), and the pitch of the helix
running along the a axis is twice the length of the a axis. To our
best knowledge, meso-helical and double helical chains observed
in one compound have rarely been reported.
Another interesting feature of 6 is that two identical layers pen-
etrate each other in parallel modes to give a 2-fold interpenetrating
2D network through O–Hꢀ ꢀ ꢀO hydrogen bonds between water mol-
ecules and carboxylate oxygen atoms (Oꢀ ꢀ ꢀO distances range from
2.72 to 3.06 Å). A better insight into the 2-fold interpenetrating 2D
network can be achieved by application of a topological approach,
each Co(II) ion links two L2ꢁ ligands and one 4,40-bipy molecule,
which can be viewed as a three-connected node, and the overall
structure can be simplified to a three-connected 2-fold interpene-
trating net with (63) topology (Fig. 7c).
Fig. 5. (a) View of quadruple-stranded hydrogen-bonded helical chains with two
left-handed (blue and turquoise) and two right-handed (red and green) helices in
complex 4. (b) The 3D supramolecular network of 4 along the a axis (Color online).
is made up of (–Co1–O4–C10–O3–Co2–O3–C10–O4–). The other
one is formed via two carboxylate groups of L ligands linking Co(II)
ions, and the repeating unit can be described as (–Co1–O4–C10–
C9–C8–C11–O2–Co2–O2–C11–C8–C9–C10–O4–)n, as shown in
Fig. 6a. The pitches of the two helices, running along the a-axis,
are the same and equal to the corresponding unit cell length, a
(9.85 Å). Moreover, 4,40-bipy molecules link the 1D chains into a
layer structure along the b axis (Fig. 6a). Topologically, the archi-
tecture of 5 can be reduced to a binodal structure with two kinds
of four-connected nodes (Co1 and Co2 atoms), and the overall
structure can be simplified to a (4,4)-connected network with
(44ꢀ62) topology (Fig. 6b). Finally, adjacent layers are linked into
3.7. Comparison of the structures
It is well known that N-heterocyclic multicarboxylate ligands
are good candidates for the construction of coordination polymers
with specific structures and topologies due to their varied coordina-
tion modes [27–30]. In the syntheses of complexes 5 and 6, the
amount of NaOH was twice that for complexes 1–4. In complexes

12
Y. Chen et al. / Polyhedron 36 (2012) 6–14
Fig. 6. (a) View of the 2D layer structure in complex 5 with two types of helical chains (A and B) along the b axis, the coordinated water molecules are omitted for clarity. (b)
Schematic representation of the (4,4)-connected network with (44ꢀ62) topology. Blue and red spheres represent Co1 and Co2 atoms, respectively (Color online).
Fig. 7. (a) View of the 2D layer structure in complex 6 with zig-zag (chain A), and meso-helical (chain B) chains along the b axis; (b) view of the 2D layer structure in complex
6 with a double-stranded helix along the c axis. (c) Schematic representation of the three-connected 2-fold interpenetrating net with (63) topology.
1–4, the H2L ligand is partly deprotonated, adopts a mono-chelating
mode and one H2L ligand joins one Co2+ ion; while in 5 and 6, the
temperature and higher pH value may help 4,40-bipy coordinate
to metal ions.
H2L ligand is completely deprotonated, shows a
l
3-bridging mode
With the help of coordination bonds and hydrogen-bond inter-
actions, complexes 2 and 3 comprise single-stranded hydrogen-
bonded helical chains, while 1 and 4 contain double-stranded
and quadruple-stranded helical chains, respectively. In complexes
1 and 4, water molecules coordinate to Co2+ ions instead of the
phen and 2,20-bipy molecules in complexes 2 and 3, and rich
hydrogen bonds are formed between the water molecules and
H2L ligands, and water molecules and 4,40-bipy molecules in
complexes 1 and 4, which help to form the double-stranded and
quadruple-stranded helical chains.
Complexes 5 and 6 have similar 2D structures, though Co(II)
ions are bridged by syn–anti carboxylate groups giving helical
chains in 5 and anti–anti carboxylate groups giving zig-zag chains
in 6. Moreover, the L2ꢁ ligand employs its two carboxylate groups
to link Co(II) ions, giving rise to another kind of single-stranded
and one H2L ligand links two Co2+ ions. The carboxylate groups of
the H2L ligand in 5 and 6 are completely deprotonated, as was
confirmed by IR spectral data because no IR bands in the range
1760–1680 cmꢁ1 were observed (see Section 2), as well as by the
results of crystallographic analyses. Furthermore, 2,20-bipy and
phen both act as chelating terminal ligands in 2 and 3, and the
4,40-bipy molecules remain uncoordinated to the Co(II) ions in 4.
Therefore, complexes 1–4 exhibit monomeric structures. However,
4,40-bipy plays the role of a bridging ligand in 5 and 6, and
complexes 5 and 6 display 2D structures.
Compared to the synthetic conditions for the preparation of
complex 4, the pH values for complexes 5 and 6 were higher than
that for complex 4, and the reaction temperatures for complexes 5
and 6 were lower than that of complex 4, which indicate a lower

Y. Chen et al. / Polyhedron 36 (2012) 6–14
13
Table 3
could facilitate the ability of a complex to cross a cell membrane
and this can be explained by Tweedy’s chelation theory [31].
Chelation considerably reduces the polarity of the metal ion
because of partial sharing of its positive charge with the donor
groups. Such chelation could enhance the lipophilic character of
the metal atom, which subsequently favors its permeation through
the lipid layers of the cell membrane [32].
Minimal inhibitory concentration (MIC) values of the H2L ligand and the title
complexes against the tested bacteria (lg/mL).
Compound
S. aureus
C. albicans
B. subtilis
E. coli
P. aeruginosa
H2L1
50
100
50
100
12.5
12.5
6.25
12.5
6.25
12.5
100
50
50
100
12.5
12.5
12.5
12.5
12.5
12.5
100
100
50
100
12.5
12.5
12.5
6.25
12.5
12.5
100
100
100
50
12.5
12.5
12.5
6.25
12.5
12.5
50
50
50
50
6.25
12.5
12.5
6.25
12.5
12.5
2,20-Bipy
4,40-Bipy
Phen
1
2
3
4
5
6
4. Conclusion
In summary, six new cobalt(II) pyrazole carboxylate complexes
with diverse helical characters have been successfully isolated
under hydrothermal conditions. Complexes 1–4 exhibit 3D supra-
molecular networks containing double-, single-, single- and qua-
druple-stranded helical chains constructed by hydrogen bonding
interactions, respectively. Complexes 5 and 6 display 3D and 2-fold
interpenetrating 2D supramolecular networks comprising two
kinds of single-stranded helical, and double-stranded and meso-
helical chains constructed via coordination interactions, respec-
tively. The results demonstrate that N-donor co-ligands, hydrogen
bonds and bridging conformations of carboxylate groups have a
significant influence on the helical structures of the title com-
plexes. In addition, the antibacterial activity screening showed that
the complexes have better activities than the corresponding free
ligand against the tested bacteria.
helical chain for complex 5 and a meso-helical chain structure for
complex 6. It may be explained that the flexible carboxylate groups
of the L2ꢁ ligands can form different types of bridging conforma-
tions, resulting in different helical structures in 5 and 6.
Complexes 4 and 6 demonstrate quadruple-stranded and dou-
ble-stranded helical chains with the participation of 4,40-bipy,
and only complex 5 displays two kinds of single-stranded chain
without the participation of 4,40-bipy, while complexes 2 and 3
exhibit single-stranded helical chain structures with the associa-
tion of 2,20-bipy and phen. This shows that 4,40-bipy as a co-ligand
helps to form multi-stranded helical chain structures.
3.8. Thermogravimetric analyses
Acknowledgments
Thermogravimetric analyses (TGA) of complexes 1–6 were car-
ried out to examine their thermal stabilities. The TGA data of 1
exhibited a weight loss of 7.1% from 110 to 195 °C, which corre-
sponds to the loss of one free and three coordinated water mole-
cules (calcd. 7.5%). The residue began to decompose at 235 °C
upon further heating. There are no water molecules in complexes
2 and 3, and they were stable up to about 256 and 245 °C, respec-
tively, and then they began to decompose with continuous weight
loss to above 800 °C. For 4, a weight loss of 4.3% was observed in
the temperature range 108–185 °C, which corresponds to the loss
of two coordinated water molecules (calcd. 4.6%), and a further
weight loss was observed at 185 °C. The TGA data of 5 displayed
a weight loss of 7.3% from 83 to 176 °C corresponding to the loss
of one free and one coordinated water molecules (calcd. 7.0%),
and the decomposition of the residue was observed at about
246 °C. Complex 6 exhibited a weight loss in the temperature
range 78–195 °C, and the total loss was 10.5%, which is in agree-
ment with the departure of one and half free, and one coordinated
water molecules (calcd. 10.1%), and the residue was stable up to
233 °C.
This work was supported by the Natural Science Foundation of
China (Grant No. 20961007), the Aviation Fund (Grant No. 2010Z
F56023) and the Young Scientists Program of Jiangxi Province
(Grant No. 2008DQ00600).
Appendix A. Supplementary data
CCDC 845842–845847 contain the supplementary crystallo-
graphic data for complexes 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.
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