КООРДИНАЦИОННАЯ ХИМИЯ, 2010, том 36, № 7, с. 517-522
SYNTHESIS AND CRYSTAL STRUCTURES OF SCHIFF BASE ZINC(III) COMPLEXES WITH ANTIBACTERIAL ACTIVITY
© 2010 J. Zhang*, F. Pan, H. Cheng, and W. Du
School of Food and Biological Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, P.R. China *E-mail: firstname.lastname@example.org Received November 23, 2009
Two halide-coordinated zinc(II) complexes with the Schiff base 2-ethoxy-6-[(2-piperidin-1-ylethylimi-no)methyl]phenol were synthesized and structurally characterized by elemental analysis (C, H, N), FT-IR spectra, and single-crystal X-ray diffraction. The Zn atom in each complex is four-coordinated in a tetrahe-dral geometry by one phenolic O and one imine N atoms of the Schiff base ligand and by two halide atoms. The antibacterial activity of the complexes and the Schiff bases against the bacteria Staphylococcus aureus, Bacillus anthracis, Pseudomonas aeruginosa, and Streptococcus agalactiae was investigated.
Schiff base compounds are readily synthesized from the condensation reaction between aldehydes and primary amines, which have been widely investigated for their biological activities, such as antibacterial, antifungal, and antitumor [1—5]. The imine N atom and some other donor atoms of the Schiff base compounds easily coordinate to metal atoms, forming versatile complexes [6—8]. Studies show that the antibacterial activities of the metal complexes were increased when compared with the corresponding Schiff bases [9, 10]. In the present work, two isostructural ha-lide-coordinated zinc complexes, ZnLCl2 (I) and ZnLBr2 (II), where L is the Schiff base 2-ethoxy-6-[(2-piperidin- 1-ylethylimino)methyl]phenol, were synthesized and structurally characterized. The antibacterial activity of the complexes and the Schiff base against the bacteria Staphylococcus aureus, Bacillus anthracis, Pseudomonas aeruginosa, and Streptococcus agalactiae was investigated. The structure of L is shown below:
Materials and measurements. 3-Ethoxysalicylalde-hyde and 2-piperidin-1-ylethylamine of AR grade were purchased from Aldrich. Other chemicals were commercially available and used without further purification. Elemental analyses (C, H, N) were carried out using a Vario
El III elemental CHN analyzer. FT-IR spectra were recorded using KBr discs on a Nicolet AVATAR 360 spectrophotometer.
Synthesis of Schiffbase L. A methanolic solution (20 ml) of 3-ethoxysalicylaldehyde (1 mmol, 166 mg) was added to a methanolic solution (20 ml) of 2-piperidin-1-yleth-ylamine (1 mmol, 128 mg) to give an orange solution. The solution was evaporated to give yellow microcrystals, which were washed with methanol and dried in air. The yield was 95%.
anal. calcd, %: C, 69.5; H, 8.8; N, 10.1. Found, %: C, 69.2; H, 8.8; N, 10.3.
Synthesis of complex I. To a methanolic solution (10 ml) of L (0.1 mmol, 27.6 mg) was added a methanolic solution (10 ml) of ZnCl2 (0.1 mmol, 13.7 mg) with stirring, and the mixture was stirred under reflux for 30 min. The clear colorless solution was left at room temperature to give colorless block-shaped single crystals. The yield was 73% based on L.
anal. calcd, %: Found, %:
C, 46.6; C, 47.1;
H, 5.9; H, 6.1;
N, 6.8. N, 6.5.
Synthesis of complex II. To a methanolic solution (10 ml) of L (0.1 mmol, 27.6 mg) was added a methanolic solution (10 ml) of ZnBr2 (0.1 mmol, 22.5 mg) with stirring, and the mixture was stirred under reflux for 30 min. The clear colorless solution was left at room tem-
Table 1. Crystal data and structural refinement details for complexes I and II
Formula weight 412.64 501.56
Color and habit Colorless, block Colorless, block
Crystal system Monoclinic Monoclinic
Space group P2\/c P2\/c
a, Â 7.9403(13) 8.155(3)
b, Â 16.539(3) 16.781(5)
c, Â 14.359(2) 14.418(5)
P, deg 102.337(8) 101.26(2)
V, Â3 1842.2(5) 1935.0(11)
Z 4 4
P calcd g cm-3 1.488 1.722
p, mm-1 1.633 5.408
F(000) 856 1000
Number of unique data 3961 4159
Number of restraints 1 1
Number of parameters 212 213
GOOF on F 2 1.088 0.977
R1, wR2 (I> 2ct(T))* 0.0302, 0.0400 0.0441, 0.0868
R1, wR2 (all data)* 0.0697, 0.0736 0.0917, 0.1071
* R1 = ZiFoi - jFcii/SiFoi, wR2 = EwCFo2 - FCy/EwCO2]1/2.
perature to give colorless block-shaped single crystals. The yield was 61% based on L.
anal. calcd, %: Found, %:
C, 38.3; C, 38.0;
H, 4.8; H, 5.1;
N, 5.6. N, 5.3.
X-ray structure determination. Crystals with dimensions 0.20 x 0.20 x 0.17 mm for I and 0.23 x 0.20 x 0.20 mm for II were mounted, and the data were collected at 298(2) K on a Bruker SMART 1000 CCD area-detector with Mo^ radiation (X = 0.71073 A). A total of10475 reflections for I were collected in the range 1.90° < 9 < 27.00° (-9 < h < 9, -18 < k < 21, -17 < l< 18). A total of11119 reflections for II were collected in the range 1.88° < 9 < < 27.00° (-10 < h < 10, -19 < k < 21, -18 < l< 17). Empirical absorption corrections were applied using the
SADABS program . Both structures were solved by direct methods and refined by a full matrix least-squares technique based on F2 using the SHELXL-97 program . All non-hydrogen atoms were refined anisotropical-ly. The H(2) atoms attached to the N(2) atoms in both complexes were located from the difference Fourier maps and refined isotropically with N-H distances restrained to 0.90(1) A. Other hydrogen atoms were refined as riding atoms. Selected crystallographic data and structure determination parameters for complexes I and II are given in Table 1. Selected bond lengths and angles for I and II are listed in Table 2. Atomic coordinates and other structural parameters of the complexes have been deposited with the Cambridge Crystallographic Data Center (nos. 752983 (I) and 752984 (II); email@example.com or http://www.ccdc.cam.ac.uk).
KOOP^HH^HOHHAtf XHMHfl TOM 36 № 7 2010
Fig. 1. Molecular structure of I at 30% probability displacement.
Antibacterial test. The two complexes and the Schiff base were determined against the bacteria Staphylococcus aureus, Bacillus anthracis, Pseudomonas aeruginosa, and Streptococcus agalactiae. Microorganisms were cultured on the Müller— Hinton agar medium. The gentamicin was used as a reference. After drilling wells on the medium using a 6-mm cork borer, 100 ^l of solutions from different compounds were poured into each well. The plates were incubated at 37°C overnight. The diameter of the inhibition zone was measured to the nearest. Each test was carried out in triplicate, and the average was calculated for inhibition zone diameters. A blank containing only methanol showed no inhibition in a preliminary test. The macrodilution broth susceptibility assay was used for the evaluation of a minimal inhibitory concentration (MIC). By including 1 ml of the Müller—Hinton broth in each test and then adding 1 ml of the extract with the concentration 100 mg ml-1 in the first tube, we made serial dilu-
The Schiff base and the two complexes were stable at room temperature in air. The molar conductivities at the 10-3 mol l-1 concentration for the complexes in acetoni-trile were in the expecting range of their formulations as nonelectrolytes .
The molecular structures of isostructural complexes I and II are shown in Figs. 1 and 2, respectively. Each complex is a mononuclear zinc(II) compound with the Zn at-
tion of this extract from the first to last tube. The bacterial suspension was prepared to match the turbidity of 0.5 Mcfarland turbidity standards. Matching this turbidity provides a bacterial inoculum concentration of 1.5 x x 108 cfu ml-1. Then 1 ml of the bacterial suspension was added to each test tube. After incubation at 37°C for 24 h, the last tube was determined as the MIC without turbidity.
RESULTS AND DISCUSSION
The Schiff base L was synthesized by the condensation of 3-ethoxysalicylaldehyde with 2-piperidin-1-yl-ethylamine in a methanolic solution in good yield and with high purity. The two zinc complexes were synthesized by the reaction of equimolar quantities of the Schiff base and zinc halide in methanolic solutions:
om coordinated by one phenolic O and one imine N atom of L and by two halide atoms (Cl for I and Br for II), forming a tetrahedral geometry. The amine N atoms of both complexes are protonated and not coordinated to the Zn atoms.
In both complexes, the Zn—O and Zn—N coordinate bond lengths are comparable to each other and are also comparable to the corresponding values observed in oth-
MeOH 2 Reflux '
(X = Cl for I and Br for II)
KQQPflHH^HQHHAfl XHMHH tom 36 № 7 2010
Table 2. Selected bond lengths and angles for complexes I and II
Bond d, Â Bond d, Â
Zn(1)-O(1) Zn(1)-Cl(1) Zn(1)-O(1) Zn(1)-Br(1) ] 1.9625(14) 2.2190(7) I 1.960(3) 2.3520(9) [ Zn(1)-N(1) Zn(1)-Cl(2) I Zn(1)-N(1) Zn(1)-Br(2) 2.0429(17) 2.2247(7) 2.036(4) 2.3576(10)
Angle w, deg Angle w, deg
O(1)Zn(1)N(1) N(1)Zn(1)Cl(1) N(1)Zn(1)Cl(2) O(1)Zn(1)N(1) N(1)Zn(1)Br(1) N(1)Zn(1)Br(2) ] 93.67(6) 116.73(5) 105.26(5) I 94.25(14) 116.52(11) 105.25(11) [ O(1)Zn(1)Cl(1) O(1)Zn(1)Cl(2) Cl(1)Zn(1)Cl(2) I O(1)Zn(1)Br(1) O(1)Zn(1)Br(2) Br(1)Zn(1)Br(2) 111.90(4) 115.76(4) 112.26(3) 112.76(9) 115.53(9) 111.43(3)
Table 3. Geometric parameters of hydrogen bond for structure I and II*
Contact D H-A Distance, Â Angle D H -A, deg
D-H H-A D-A
N(2)- -H(2)-O(1)i 0.90(1) I 1.90(1) 2.789(2) 169(3)
N(2)- -H(2)-O(2)i 0.90(1) 2.65(3) II 1.953(15) 3.220(2) 122(2)
N(2)- -H(2)-O(1)ii 0.90(1) 2.821(4) 163(3)
N(2)- -H(2)-O(2)ii 0.90(1) 2.62(3) 3.257(5) 129(3)
er Schiff base zinc complexes [14—16]. The Zn—Br bonds are much longer than those of Zn—Cl. The bond angles subtended at the Zn atoms are in the range 93.67(6)°—116.73(5)° for I and 94.25(14)°-116.52(11)° for II, indicating that the tetrahedral coordination o
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