Syntheses, crystal structures and antibacterial activitiesof 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

� 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)]�0.5H2O (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.

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

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–

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 � 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

Transition Met Chem (2010) 35:745–749

DOI 10.1007/s11243-010-9388-4

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%.

Syntheses of [Co3(L1)2(l1,1-N3)4(N3)2(OH2)-

(OCH2CH3)]�0.5H2O (1) and [Co2(L2)2-

(l1,1-N3)2(N3)2] (2)

The complexes (1) and (2) were prepared by the following

general method. A hot ethanol solution (10 mL) of

Co(CH3COO)2�4H2O (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%.

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 H���H distances

restrained to 0.85(1), 0.90(1) and 1.37(2) A, respectively.

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)).

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

Table 1 Crystal data for the complexes

Complex (1) (2)

Chemical formula C60H92Co6N44O17 C28H42Co2N16O4

Fw 2055.36 784.64

Crystal shape/color Block/brown Block/brown

Crystal size (mm) 0.25 9 0.23 9 0.23 0.20 9 0.20 9 0.19

T (K) 298(2) 298(2)

k (MoKa) (A) 0.71073 0.71073

Crystal system Orthorhombic Monoclinic

Space group P212121 P21/c

a (A) 11.7731(18) 9.6783(8)

b (A) 12.1664(18) 12.3853(9)

c (A) 30.269(5) 16.9865(12)

b (o) 119.550(4)

V (A3) 4335.6(12) 1771.3(2)

Z 2 2

l (MoKa) (cm-1) 1.210 0.995

T (min) 0.7519 0.8258

T (max) 0.7683 0.8412

Dc (g cm-3) 1.574 1.471

Reflections/parameters 9,432/591 3,822/232

Unique reflections 5,768 2,549

Goodness of fit on F2 0.931 1.008

Rint 0.0664 0.0381

R1 [I C 2r(I)] 0.0512 0.0382

wR2 [I C 2r(I)] 0.1041 0.0738

R1 (all data) 0.0972 0.0746

wR2 (all data) 0.1218 0.0871

O OH

NN

O

OH

NNH

O

Scheme 1 The Schiff bases HL1 (left) and HL2 (right)

746 Transition Met Chem (2010) 35:745–749

123

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.

Results and discussion

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

Table 2 Selected bond lengths (A) and angles (�) for the complexes

(1)

Bond lengths

Co1–O7 2.126(4) Co1–O8 2.117(4)

Co1–N5 2.165(5) Co1–N8 2.120(4)

Co1–N11 2.175(4) Co1–N14 2.126(4)

Co2–N3 1.883(5) Co2–O4 1.913(4)

Co2–N20 1.924(5) Co2–N14 1.983(4)

Co2–N11 2.018(5) Co2–N4 2.056(5)

Co3–N1 1.889(5) Co3–O1 1.904(4)

Co3–N17 1.956(5) Co3–N8 1.978(4)

Co3–N5 1.985(4) Co3–N2 2.045(4)

Bond angles

O8–Co1–N8 170.4(2) O8–Co1–N14 87.6(2)

N8–Co1–N14 99.5(2) O8–Co1–O7 88.9(2)

N8–Co1–O7 85.5(2) N14–Co1–O7 166.8(2)

O8–Co1–N5 98.7(2) N8–Co1–N5 73.6(2)

N14–Co1–N5 102.8(2) O7–Co1–N5 90.3(2)

O8–Co1–N11 88.8(2) N8–Co1–N11 99.4(2)

N14–Co1–N11 73.7(2) O7–Co1–N11 93.5(2)

N5–Co1–N11 171.7(2) N3–Co2–O4 93.9(2)

N3–Co2–N20 92.0(2) O4–Co2–N20 90.2(2)

N3–Co2–N14 173.7(2) O4–Co2–N14 87.7(2)

N20–Co2–N14 94.1(2) N3–Co2–N11 93.7(2)

O4–Co2–N11 89.1(2) N20–Co2–N11 174.4(2)

N14–Co2–N11 80.2(2) N3–Co2–N4 86.6(2)

O4–Co2–N4 176.9(2) N20–Co2–N4 86.7(2)

N14–Co2–N4 92.2(2) N11–Co2–N4 93.9(2)

N1–Co3–O1 94.1(2) N1–Co3–N17 92.7(2)

O1–Co3–N17 89.8(2) N1–Co3–N8 175.8(2)

O1–Co3–N8 87.8(2) N17–Co3–N8 91.1(2)

N1–Co3–N5 95.6(2) O1–Co3–N5 87.5(2)

N17–Co3–N5 171.4(2) N8–Co3–N5 80.7(2)

N1–Co3–N2 86.0(2) O1–Co3–N2 178.6(2)

N17–Co3–N2 88.7(2) N8–Co3–N2 92.2(2)

N5–Co3–N2 93.9(2)

(2)

Bond lengths

Co1–N1 1.875(2) Co1–O1 1.892(2)

Co1–N6 1.919(2) Co1–N3A 1.979(2)

Co1–N3 2.002(2) Co1–N2 2.033(2)

Bond angles

N1–Co1–O1 94.7(1) N1–Co1–N6 92.5(1)

O1–Co1–N6 89.9(1) N1–Co1–N3A 172.9(1)

O1–Co1–N3A 85.9(1) N6–Co1–N3A 94.5(1)

N1–Co1–N3 95.4(1) O1–Co1–N3 87.6(1)

N6–Co1–N3 171.8(1) N3–Co1–N3A 77.5(1)

N1–Co1–N2 86.2(1) O1–Co1–N2 178.0(1)

N6–Co1–N2 91.8(1) N3A–Co1–N2 93.0(1)

N3–Co1–N2 90.5(1)

Symmetry code for A: 1-x, 1-y, -zFig. 1 Molecular structure of (1) with 30% probability thermal

ellipsoids. Hydrogen atoms are omitted

Transition Met Chem (2010) 35:745–749 747

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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 Co1���Co3 and

Co1���Co2 distances in the complex are 3.212(2) and

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 Co1���Co1A [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

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].

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].

Acknowledgments The authors greatly acknowledge Jingchu Uni-

versity of Technology for financial support.

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

Table 3 The MIC values (lg mL-1) of the antibacterial activity

B. subtilis E. coli P. fluorescens S. aureus

HL1 15.0 54.5 63.0 31.0

HL2 27.0 [100 [100 45.0

(1) 4.0 12.0 21.3 7.0

(2) 8.5 18.5 37.5 8.7

NaN3 12.5 27.0 [100 15.5

Penicillin 1.7 [100 72.0 2.1

748 Transition Met Chem (2010) 35:745–749

123

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