Syntheses, crystal structures and antibacterial activities of azido-bridged cobalt(III) complexes with Schiff bases

Article abstract of DOI:10.1007/s11243-010-9388-4

Two end-on azido-bridged Co(III) complexes, [Co₃(L1) ₂(μ1,1-N₃)₄(N₃) ₂(OH₂)(OCH₂CH₃)]·0.5H ₂O (1) and [Co₂(L2)₂(μ1,1-N

Full text of DOI:10.1007/s11243-010-9388-4

Transition Met Chem (2010) 35:745–749
DOI 10.1007/s11243-010-9388-4
Syntheses, crystal structures and antibacterial activities
of azido-bridged cobalt(III) complexes with Schiff bases
•
Yang Zhu Wen-Hui Li
Received: 18 May 2010 / Accepted: 18 June 2010 / Published online: 6 July 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Two end-on azido-bridged Co(III) complexes,
[Co3(L1)2(l1,1-N3)4(N3)2(OH2)(OCH2CH3)]Á0.5H2O (1) and
[Co2(L2)2(l1,1-N3)2(N3)2] (2), where L1 and L2 are the
deprotonated form of 5-methoxy-2-[(2-morpholin-4-ylethy-
limino)methyl]phenol and 2-ethoxy-6-[(2-isopropylamin-
oethylimino)methyl]phenol, respectively, were prepared
and structurally characterized by physicochemical and
spectroscopic methods and single crystal X-ray determi-
nation. Complex (1) is a trinuclear Co compound, while
complex (2) is a centrosymmetric dinuclear Co compound.
In both complexes, the Co atoms are in octahedral coor-
dination. The preliminary biological tests show that the
complexes have excellent antibacterial activity.
Crystallographic Database (version 5.31 with addenda up
to February 26, 2010) [13] revealed that there are only
fourteen end-on azido-bridged polynuclear cobalt com-
plexes [14–27]. In the present paper, two new azido-bridged
cobalt(III) complexes with Schiff bases 5-methoxy-
2-[(2-morpholin-4-ylethylimino)methyl]phenol (HL1) and
2-ethoxy-6-[(2-isopropylaminoethylimino)methyl]phenol
(HL2) (Scheme 1) have been reported. The preliminary
antibacterial activity of both complexes was investigated.
Experimental
All chemicals and solvents were of analytical reagent grade
and were purchased from Beijing Chemical Reagent
Company. Microanalyses (C, H, N) were performed using
a Perkin-Elmer 2400 elemental analyzer. Infrared spectra
were carried out using the JASCO FT-IR model 420
spectrophotometer with KBr disks in the region 4,000–
Introduction
Polynuclear complexes are of considerable interest in
recent years. A number of bridging groups have been used
to construct polynuclear complexes [1–4]. Pseudohalides,
especially azide, are versatile polyatomic bridging units
that can bind metal ions via end-on (EO, l1,1), end-to-end
(EE, l1,1), and many other modes [5–8]. Multidentate
Schiff base ligands have generally been used to construct
the polynuclear complexes with azide bridges [9–12].
Recently, a large number of azido-bridged copper(II) and
nickel(II) complexes with Schiff bases have been reported;
however, the end-on azido-bridged polynuclear cobalt
complexes are very few. Thus, a search in the Cambridge
200 cm-1
.
Caution! Azido complexes of transition metal atoms
containing organic ligands are potentially explosive. Only a
small amount of material should be prepared, and it should
be handled with care.
Synthesis of HL1 and HL2
Both Schiff bases were prepared by a similar procedure.
4-Methoxysalicylaldehyde (0.152 g, 1 mmol) dissolved in
ethanol (50 mL) was added to a stirred ethanol solution
(30 mL) of either 4-(2-aminoethyl)morpholine (0.130 g,
1 mmol) for HL1 or N-isopropylethane-1,2-diamine
(0.102 g, 1 mmol) for HL2. Then, the reaction mixtures
were refluxed for 2 h on a water bath, and the solvent was
evaporated to give yellow products. Yield: 97% for HL1
Y. Zhu Á W.-H. Li (&)
College of Chemical Engineering and Pharmacy,
Jingchu University of Technology, Jingmen,
Hubei 448000, People’s Republic of China
e-mail: liwh066@163.com
123

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Transition Met Chem (2010) 35:745–749
H
Table 1 Crystal data for the complexes
O
N
N
N
Complex
(1)
(2)
N
OH
O
OH
Chemical formula
Fw
C60H92Co6N44O17
2055.36
C28H42Co2N16O4
784.64
O
Crystal shape/color
Crystal size (mm)
T (K)
Block/brown
Block/brown
Scheme 1 The Schiff bases HL1 (left) and HL2 (right)
0.25 9 0.23 9 0.23 0.20 9 0.20 9 0.19
298(2)
298(2)
˚
and 99% for HL2. Anal. Calc. for C14H20N2O3 (HL1): C,
63.6; H, 7.6; N 10.6. Found: C, 63.5; H, 7.7; N, 10.7%.
Anal. Calc. for C14H22N2O2 (HL2): C, 67.2; H, 8.9; N 11.2.
Found: C, 66.8; H, 8.9; N, 11.4%.
k (MoKa) (A)
0.71073
0.71073
Monoclinic
P21/c
Crystal system
Space group
Orthorhombic
P212121
˚
a (A)
11.7731(18)
12.1664(18)
30.269(5)
9.6783(8)
12.3853(9)
16.9865(12)
119.550(4)
1771.3(2)
2
˚
b (A)
Syntheses of [Co3(L1)2(l1,1-N3)4(N3)2(OH2)-
(OCH2CH3)]Á0.5H2O (1) and [Co2(L2)2-
(l1,1-N3)2(N3)2] (2)
˚
c (A)
b (o)
3
V (A )
˚
4335.6(12)
2
Z
l (MoKa) (cm-1
)
1.210
0.995
The complexes (1) and (2) were prepared by the following
general method. A hot ethanol solution (10 mL) of
Co(CH3COO)2Á4H2O (0.025 g, 0.1 mmol) was added to
a hot ethanol solution (10 mL) of the Schiff base
(0.1 mmol). The resulting brown solution was stirred for
10 min at reflux. To the solution, 0.013 g (0.2 mmol) NaN3
was added and the mixture refluxed for 20 min. Micro-
crystalline powders of the complexes were separated out,
which were filtered off and washed with water and meth-
anol. Single crystals suitable for X-ray diffraction were
formed by slow evaporation of the ethanol solutions of the
complexes in air for a few days. Yield: 32% for (1) and
45% for (2). Anal. Calc. for C60H92Co6N44O17 (1): C, 35.1;
H, 4.5; N 30.0. Found: C, 35.4; H, 4.7; N, 29.8%. Anal.
Calc. for C28H42Co2N16O4 (2): C, 42.9; H, 5.4; N 28.6.
Found: C, 42.7; H, 5.3; N, 28.6%.
T (min)
0.7519
0.7683
1.574
0.8258
T (max)
Dc (g cm-3
0.8412
)
1.471
Reflections/parameters 9,432/591
3,822/232
2,549
Unique reflections
Goodness of fit on F2
Rint
5,768
0.931
1.008
0.0664
0.0512
0.1041
0.0972
0.1218
0.0381
R1 [I C 2r(I)]
wR2 [I C 2r(I)]
R1 (all data)
0.0382
0.0738
0.0746
wR2 (all data)
0.0871
Other hydrogen atoms were placed in calculated positions
and constrained to ride on their parent atoms. The analysis
was performed with the aid of the SHELXS-97 and
SHELXL-97 suite of codes [28, 29]. The crystallographic
data for the complexes are summarized in Table 1. Selec-
ted bond lengths and angles are given in Table 2. Crys-
tallographic data for the complexes have been deposited
with the Cambridge Crystallographic Data Center (CCDC
776830 for (1) and 776831 for (2)).
X-ray structure determination
X-ray measurements were performed using a Bruker Smart
1000 CCD diffractometer with graphite monochromated
˚
Mo Ka radiation (k = 0.71073 A) using the x-scan tech-
nique. Determination of the Laue class, orientation matrix,
and cell dimensions were performed according to the
established procedures where Lorentz polarization and
absorption corrections were applied. Absorption correc-
tions were applied by fitting a pseudoellipsoid to the
w-scan data of selected strong reflections over a wide range
of 2h angles. The positions of almost all non-hydrogen
atoms were located with direct methods. Subsequent
Fourier syntheses were used to locate the remaining non-
hydrogen atoms. All non-hydrogen atoms were refined
anisotropically. Water H atoms in (1) and amino H atoms
in (2) were located from difference Fourier maps and
refined isotropically, with O–H, N–H and HÁÁÁH distances
Antibacterial tests
The bacterial subcultures for Bacillus subtilis (B. subtilis),
Escherichia coli (E. coli), Pseudomonas fluorescens (P.
fluorescens) and Staphylococcus aureus (S. aureus) were
obtained from the Dalian Medical University. A standard
inoculum was introduced onto the surface of sterile agar
plates, and a sterile glass spreader was used for even dis-
tribution of the inoculum. The disks measuring 7.0 mm in
diameter were sterilized by dry heat at 140 °C for 1 h. The
sterile disks previously soaked in a known concentration
(5,000 lg mL-3 in DMSO) of the compound were placed
˚
restrained to 0.85(1), 0.90(1) and 1.37(2) A, respectively.
123

Transition Met Chem (2010) 35:745–749
747
˚
in nutrient agar medium. Solvent and growth controls were
maintained. The plates were inverted and incubated for
24 h at 37 °C. Penicillin was used as a standard. The
inhibition zones were measured and compared with the
control. Minimum inhibitory concentration (MIC) was
determined by broth dilution technique. The nutrient broth,
which contained logarithmic serially twofold diluted
amounts of the compound and the control, was inoculated
with *1 9 106 CFU cm-3 of actively dividing bacterial
cells. The cultures were incubated for 24 h at 37 °C, and the
growth was monitored visually and spectrophotometrically.
Table 2 Selected bond lengths (A) and angles (°) for the complexes
(1)
Bond lengths
Co1–O7
2.126(4)
2.165(5)
2.175(4)
1.883(5)
1.924(5)
2.018(5)
1.889(5)
1.956(5)
1.985(4)
Co1–O8
Co1–N8
Co1–N14
Co2–O4
Co2–N14
Co2–N4
Co3–O1
Co3–N8
Co3–N2
2.117(4)
2.120(4)
2.126(4)
1.913(4)
1.983(4)
2.056(5)
1.904(4)
1.978(4)
2.045(4)
Co1–N5
Co1–N11
Co2–N3
Co2–N20
Co2–N11
Co3–N1
Co3–N17
Co3–N5
Bond angles
O8–Co1–N8
N8–Co1–N14
N8–Co1–O7
O8–Co1–N5
N14–Co1–N5
O8–Co1–N11
N14–Co1–N11
N5–Co1–N11
N3–Co2–N20
N3–Co2–N14
N20–Co2–N14
O4–Co2–N11
N14–Co2–N11
O4–Co2–N4
N14–Co2–N4
N1–Co3–O1
O1–Co3–N17
O1–Co3–N8
N1–Co3–N5
N17–Co3–N5
N1–Co3–N2
N17–Co3–N2
N5–Co3–N2
(2)
Results and discussion
170.4(2)
99.5(2)
85.5(2)
98.7(2)
102.8(2)
88.8(2)
73.7(2)
171.7(2)
92.0(2)
173.7(2)
94.1(2)
89.1(2)
80.2(2)
176.9(2)
92.2(2)
94.1(2)
89.8(2)
87.8(2)
95.6(2)
171.4(2)
86.0(2)
88.7(2)
93.9(2)
O8–Co1–N14
O8–Co1–O7
N14–Co1–O7
N8–Co1–N5
O7–Co1–N5
N8–Co1–N11
O7–Co1–N11
N3–Co2–O4
O4–Co2–N20
O4–Co2–N14
N3–Co2–N11
N20–Co2–N11
N3–Co2–N4
N20–Co2–N4
N11–Co2–N4
N1–Co3–N17
N1–Co3–N8
N17–Co3–N8
O1–Co3–N5
N8–Co3–N5
O1–Co3–N2
N8–Co3–N2
87.6(2)
88.9(2)
166.8(2)
73.6(2)
90.3(2)
99.4(2)
93.5(2)
93.9(2)
90.2(2)
87.7(2)
93.7(2)
174.4(2)
86.6(2)
86.7(2)
93.9(2)
92.7(2)
175.8(2)
91.1(2)
87.5(2)
80.7(2)
178.6(2)
92.2(2)
The complexes were synthesized by the reaction of the
Schiff bases, cobalt acetate and sodium azide in ethanol in
a 1:1:2 mol proportion at reflux. Both Schiff bases act as
tridentate ligands. The azide bridges cobalt atoms through
the end-on bridging mode. The chemical formulae of the
complexes have been confirmed by elemental analyses and
X-ray single crystal structure determination.
The molecular structure of (1) is shown in Fig. 1. The
compound consists of an end-on azido-bridged trinuclear
cobalt(III) complex molecule and half of a water molecule
of crystallization. The middle Co1 atom is coordinated
by four N atoms from four end-on azido bridges, one water
O atom, and one ethanol O atom, forming an octahedral
coordination. The distortion of the Co1 coordination can be
observed from the small bond angles N5-Co1-N8 and N11-
Co1-N14 of the four-membered chelate rings. Each of the
two outer Co atoms (Co2 and Co3) is coordinated by the
phenolate O, imine N and morpholine N atoms of L1, and
by three N atoms from three azido groups, two of which are
Bond lengths
Co1–N1
1.875(2)
1.919(2)
2.002(2)
Co1–O1
Co1–N3A
Co1–N2
1.892(2)
1.979(2)
2.033(2)
Co1–N6
Co1–N3
Bond angles
N1–Co1–O1
O1–Co1–N6
O1–Co1–N3A
N1–Co1–N3
N6–Co1–N3
N1–Co1–N2
N6–Co1–N2
N3–Co1–N2
94.7(1)
89.9(1)
85.9(1)
95.4(1)
171.8(1)
86.2(1)
91.8(1)
90.5(1)
N1–Co1–N6
N1–Co1–N3A
N6–Co1–N3A
O1–Co1–N3
N3–Co1–N3A
O1–Co1–N2
N3A–Co1–N2
92.5(1)
172.9(1)
94.5(1)
87.6(1)
77.5(1)
178.0(1)
93.0(1)
Fig. 1 Molecular structure of (1) with 30% probability thermal
ellipsoids. Hydrogen atoms are omitted
Symmetry code for A: 1-x, 1-y, -z
123

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Transition Met Chem (2010) 35:745–749
Table 3 The MIC values (lg mL-1) of the antibacterial activity
B. subtilis
E. coli
P. fluorescens
S. aureus
HL1
15.0
27.0
4.0
54.5
[100
12.0
63.0
[100
21.3
31.0
45.0
7.0
HL2
(1)
(2)
8.5
18.5
37.5
8.7
NaN3
Penicillin
12.5
1.7
27.0
[100
72.0
15.5
2.1
[100
1,619 cm-1 for (2), what can be attributed to the coordi-
nation of the azomethine N atoms of the Schiff bases to the
Co atoms. The phenolic groups in the free ligands exhibit
strong bands at about 1,210 cm-1; however, the bands
appear at lower wave numbers (1,183 cm-1 for (1) and
1,181 cm-1 for (2)) in the complexes, indicating the
coordination of the phenolate O atoms. Other weak
absorption bands in the region 470–430 cm-1 for both
complexes can be assigned to the Co–O and Co–N bonds
[30, 31].
Fig. 2 Molecular structure of (2) with 30% probability thermal
ellipsoids. Atoms labeled with the suffix A are at the symmetry
position 1-x, 1-y, -z
end-on azido bridges, while the third is a terminal azido
group, forming an octahedral coordination. The octahedral
coordination of the Co2 and Co3 atoms are also distorted,
as evidenced by the bond angles. The Co1ÁÁÁCo3 and
Co1ÁÁÁCo2 distances in the complex are 3.212(2) and
The MIC values for the preliminary antibacterial activ-
ities of the Schiff bases, the two complexes, the sodium
azide, and the Penicillin (as a standard compound) are
listed in Table 3. The data indicate stronger activity of the
two complexes against the bacteria than the Schiff bases,
and even stronger than the cytotoxic sodium azide. Both
complexes show high activity against B. subtilis, E. coli
and S. aureus, and moderate activity against P. fluorescens.
The moderate antibacterial activity of the Schiff bases may
arise from the presence of imine groups, which imports in
elucidating the mechanism of transformation reaction in
biological systems, and also from the presence of the
hydroxyl groups, which may play an important role in the
antibacterial activity [32, 33]. The increased activity of
the complexes can be explained on the basis of Overtone’s
concept [34] and Tweedy’s Chelation theory [35] that the
metal complexes have a better activity than the free
ligands. On chelating, the polarity of the metal ion will be
reduced to a greater extent due to the overlap of the ligand
orbital and partial sharing of positive charge of the metal
ion with donor groups. Further, it increases the delocal-
ization of p-electrons over the whole chelate ring and
enhances the lipophilicity of the complex. This increased
lipophilicity enhances the penetration of the complexes
into lipid membranes and blocks the metal binding sites on
enzymes of microorganisms. The results in this work are
accord with those reported previously that the complexes
have more antibacterial activity than the corresponding free
ligands [36, 37].
˚
3.227(2) A, respectively.
The molecular structure of (2) is shown in Fig. 2. The
compound possesses crystallographic inversion center
symmetry, with the inversion center located at the midpoint
of the two Co atoms. Each Co atom is coordinated by the
phenolate O, imine N and amine N atoms of L2, and by
three N atoms from three azido groups, two of which are
end-on azido bridges, while the third is a terminal azido
group, forming an octahedral coordination. The octahedral
coordination of the Co atoms is distorted, as evidenced by
the bond angles. The Co1ÁÁÁCo1A [symmetry code for A:
˚
1-x, 1-y, -z] separation is 3.105(2) A, which is compa-
rable to those in complex (1).
In both complexes, the coordinate bond lengths and
angles are comparable to each other. The complex (1) is a
trinuclear molecule, while the complex (2) is a dinuclear
molecule, which might be caused by the larger hindrance
effect of the morpholine ring in L1 than that of the iso-
propyl group in L2.
Infrared spectra data were assigned on comparing with
the free Schiff bases and the complexes. The most signif-
icant observation is the appearance of strong sharp stret-
ches at 2,054–2,072 cm-1 for both complexes. These
absorption bands correspond to the stretching vibration of
the azide groups. The strong absorption bands at
1,645 cm-1 in the spectrum of HL1 and at 1,641 cm-1 in
the spectrum of HL2 are assigned to the azomethine
groups, m(C=N). The bands are shifted to lower wave
numbers in the two complexes, 1,623 cm-1 for (1) and
Acknowledgments The authors greatly acknowledge Jingchu Uni-
versity of Technology for financial support.
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20. Zhang Y-Z, Wernsdorfer W, Pan F, Wang Z-M, Gao S (2006)
Chem Commun 31:3302
References
21. Gao H, Zhang Z-Z, Jiang P, Li X-R (2006) Transition Met Chem
31:1088
22. Bai S-Q, Gao E-Q, He Z, Fang C-J, Yan C-H (2005) New J Chem
29:935
23. Wang Q-L, Yu L-H, Liao D-Z, Yan S-P, Jiang Z-H, Cheng P
(2003) Helv Chim Acta 86:2441
24. Ray U, Chand B, Mostafa G, Cheng J, Lu T-H, Sinha C (2003)
Polyhedron 22:2587
25. Wang L-Y, Zhao B, Zhang C-X, Liao D-Z, Jiang Z-H, Yan S-P
(2003) Inorg Chem 42:5804
26. Liu T-F, Fu D, Gao S, Zhang Y-Z, Sun H-L, Su G, Liu Y-J (2003)
J Am Chem Soc 125:13976
27. Papaefstathiou GS, Escuer A, Font-Bardia M, Perlepes SP, Solans
X, Vicente R (2002) Polyhedron 21:2027
28. Sheldrick GM (1997) SHELX-97, program for crystal structure
1. Gavrilova AL, Bosnich B (2004) Chem Rev 104:349
2. Ambrosi G, Formica M, Fusi V, Giorgi L, Macedi E, Micheloni
M, Paoli P, Rossi P (2009) Inorg Chem 48:10424
3. Liu HY, Gao F, Niu DZ, Tian LJ (2009) Inorg Chim Acta
362:4179
4. Biyiklioglu Z, Ertem B, Karslioglu S, Gok Y (2007) Transition
Met Chem 32:591
´
´
5. Triki S, Gomez-Garcıa CJ, Ruiz E, Sala-Pala J (2005) Inorg
Chem 44:5501
6. Blanchet-Boiteux C, Mouesca J-M (2000) J Am Chem Soc
122:861
7. Ni LL, You ZL, Zhang L, Wang C, Li K (2010) Transition Met
Chem 35:13
8. Wang QL, Jia XQ, Liao DZ, Yan SP, Cheng P, Yang GM, Ren
HX, Jiang ZH (2006) Transition Met Chem 31:434
9. Adhikary C, Mal D, Okamoto K, Chaudhuri S, Koner S (2006)
Polyhedron 25:2191
10. Mukherjee PS, Dalai S, Mostafa G, Lu T-H, Rentschler E,
Chaudhuri NR (2001) New J Chem 25:1203
11. Koner S, Saha S, Mallah T, Okamoto K (2004) Inorg Chem
43:840
12. Hou P, You Z-L, Zhang L, Ma X-L, Ni L-L (2008) Transition
Met Chem 33:1013
13. Allen FH (2002) Acta Crystallogr B58:380
14. Drew MGB, Esho FS, Nelson SM (1983) Inorg Chim Acta
76:L269
15. De Munno G, Poerio T, Viau G, Julve M, Lloret F, Journaux Y,
Riviere E (1996) Chem Commun 22:2587
16. Drew MGB, Harding CJ, Nelson J (1996) Inorg Chim Acta
246:73
17. Milios CJ, Prescimone A, Sanchez-Benitez J, Parsons S, Murrie M,
Brechin EK (2006) Inorg Chem 45:7053
¨ ¨
solution and refinement. Gottingen University, Gottingen,
Germany
29. Sheldrick GM (1995) SHELXTL version 5. Siemens, Industrial
Automation Inc., Madison
30. Rajasekar M, Sreedaran S, Prabu R, Narayanan V, Jegadeesh R,
Raaman N, Rahiman AK (2010) J Coord Chem 63:136
31. EI-Metwally NM, Gabr IM, Shallaby AM, EI-Asmy AA (2005)
J Coord Chem 58:1145
32. Shi L, Ge HM, Tan SH, Li HQ, Song YC, Zhu HL, Tan RX
(2007) Eur J Med Chem 42:558
33. Lv J, Liu T, Cai S, Wang X, Lu L, Wang Y (2006) J Inorg
Biochem 100:1888
34. Anjaneyula Y, Rao RP (1986) Synth React Inorg Met-Org Nano-
Met Chem 16:257
35. Dharamaraj N, Viswanathamurthi P, Natarajan K (2001) Tran-
sition Met Chem 26:105
˘
˘
36. Kurtoglu M, Purtas¸ F, Toroglu S (2008) Transition Met Chem
33:705
37. Prasad S, Agarwal RK (2007) Transition Met Chem 32:143
18. Liu F-C, Zeng Y-F, Jiao J, Bu X-H, Ribas J, Batten SR (2006)
Inorg Chem 45:2776
19. He Z, Wang Z-M, Gao S, Yan C-H (2006) Inorg Chem 45:6694
123