Copper(II) Complexes With Bis-Schiff Bases: Synthesis, Crystal Structures, and Antibacterial Activities

Article abstract of DOI:10.1080/15533174.2015.1137069

Reaction of copper perchlorate with the tridentate Schiff bases N,N’-bis(5-methoxysalicylidene)-1,2-ethanediamine (H₂L¹) and N,N’-bis(5-chlorosalicylidene)-1,3-propanediamine (H₂L²), afforded a dinuclear complex

Full text of DOI:10.1080/15533174.2015.1137069

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Copper(II) Complexes with Bis-Schiff Bases:
Synthesis, Crystal Structures and Antibacterial
Activities
Xin-Li Zhang
To cite this article: Xin-Li Zhang (2016): Copper(II) Complexes with Bis-Schiff Bases: Synthesis,
Crystal Structures and Antibacterial Activities, Synthesis and Reactivity in Inorganic, Metal-
Organic, and Nano-Metal Chemistry, DOI: 10.1080/15533174.2015.1137069
To link to this article: http://dx.doi.org/10.1080/15533174.2015.1137069
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Published online: 24 May 2016.
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Copper(II) Complexes with Bis-Schiff Bases: Synthesis, Crystal Structures and
Antibacterial Activities
XIN-LI ZHANG
College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji
721013, P. R. China
E-mail: baojizhangxinli@163.com
Abstract
Reaction
of
copper
perchlorate
with
the
tridentate
(H2L1)
Schiff
bases
and
N,N’-bis(5-methoxysalicylidene)-1,2-ethanediamine
N,N’-bis(5-chlorosalicylidene)-1,3-propanediamine (H2L2), afforded a dinuclear complex,
[Cu2(L1)2]·2CH3OH (1), and a mononuclear complex, [CuL2] (2). The complexes were
characterized by single-crystal X-ray diffraction, FT-IR, and elemental analysis. The Cu atom in
complex 1 is in a square pyramidal coordination, with two phenolate O and two imine N atoms
of L1 ligand defining the basal plane, and with one phenolate O atom of another L1 ligand
occupying the apical position. The Cu atom in complex 2 is coordinated by two phenolate O and
two imine N atoms of L2 ligand, forming a tetrahedrally distorted square planar geometry. The
free Schiff bases and the complexes were assayed for antibacterial activities. Both complexes are
more active against the bacteria than the free Schiff bases. Complex 2 has the MIC value of 0.10
μg mL–1 against Bacillus subtilis.
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Keywords
bis-Schiff base, copper complex, crystal structure, antibacterial activity
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Introduction
The chemistry and the biological potential of Schiff base ligands and their metal complexes have
been investigated extensively. Schiff bases are reported to possess various biological activities,
such as antibacterial[1, 2] and antitumour activities.[3, 4] Metal complexes with Schiff bases as
ligands have played important role in the development of coordination chemistry due to their
preparative accessibility, structural variety, and biological properties.[5–7] Interest is still high as
few have found their way into application as therapeutic drugs, health, skin care products and in
paint dye manufacturing.[8] Copper complexes with Schiff bases have interesting biological
properties.[9,10] Bis-Schiff bases are readily coordinate to transition metal atoms. We report herein
the synthesis and characterization of a new dinuclear complex, [Cu2(L1)2]·2CH3OH (1), and a
mononuclear complex, [CuL2] (2), where L1 and L2 are the anions of bis-Schiff bases
N,N’-bis(5-methoxysalicylidene)-1,2-ethanediamine
(H2L1)
and
N,N’-bis(5-chlorosalicylidene)-1,3-propanediamine (H2L2; Scheme 1). Their antibacterial
activities against Bacillus subtilis, Staphylococcus aureus, Escherichia coli, and Pseudomonas
fluorescens were evaluated.
Experimental
Materials and Measurements
5-Methoxysalicylaldehyde,
5-chlorosalicylaldehyde,
ethane-1,2-diamine
and
propane-1,3-diamine were purchased from Sigma-Aldrich. All other chemicals were commercial
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products and used without further purification. Copper perchlorate were prepared from
perchloric acid with copper carbonate in distilled water. C, H and N elemental analysis was
performed on a Vario EL-III analyzer. Infrared spectra were recorded as KBr pellet on a Nicolet
Avatar 360 spectrophotometer in the range 4000–400 cm-1. Molar conductivity was determined
in methanol at room temperature on a DDS-11A conductometer. 1H NMR spectra were recorded
on a Bruker spectrometer at 300 MHz.
Synthesis of H2L1
5-Methoxysalicylaldehyde (3.04 g, 0.02 mol) dissolved in methanol (30 mL) was added to
ethane-1,2-diamine (0.060 g, 0.01 mol) in methanol (30 mL). The reaction mixture was heated
under reflux for 1 h and then cooled to room temperature followed by concentrating the resulting
mixture to give yellow solid product. Yield 95%. For C18H20N2O4: Anal. calcd, %: C, 65.84; H,
6.14; N, 8.53. Found, %: C, 65.67; H, 6.73; N, 8.60. 1H NMR (300 MHz, d6-DMSO): δ 11.32 (s,
2H, OH), 8.52 (s, 2H, CH=N), 7.37 (s, 2H, ArH), 6.95-6.83 (m, 4H, ArH), 3.79 (s, 6H, CH3).
Synthesis of H2L2
5-Chlorosalicylaldehyde (3.12 g, 0.02 mol) dissolved in methanol (30 mL) was added to
propane-1,3-diamine (0.074 g, 0.01 mol) in methanol (30 mL). The reaction mixture was heated
under reflux for 1 h and then cooled to room temperature followed by concentrating the resulting
mixture to give yellow solid product. Yield 97%. For C17H16Cl2N2O2: Anal. calcd, %: C, 58.13;
H, 4.59; N, 7.98. Found, %: C, 58.27; H, 4.68; N, 7.81. 1H NMR (300 MHz, d6-DMSO): δ 11.77
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(s, 2H, OH), 8.53 (s, 2H, CH=N), 7.56 (s, 2H, ArH), 7.33 (d, 2H, ArH), 6.98 (d, 2H, ArH), 3.72
(t, 4H, NCH2), 1.98 (m, 2H, CH2).
Synthesis of [Cu2(L1)2]·2CH3OH (1)
Copper perchlorate (0.37 g, 1 mmol) in methanol (20 mL) was added to H2L1 (0.33 g, 1 mmol) in
methanol (20 mL), and the resultant blue reaction mixture was stirred at room temperature for 1
h to give a clear solution. Blue single crystals were obtained by slow evaporation of the solution
in air. Yield 45%. For C38H44Cu2N4O10: Anal. calcd, %: C, 54.08; H, 5.26; N, 6.64. Found, %: C,
53.82; H, 5.41; N, 6.55.
Synthesis of [CuL2] (2)
The blue single crystals of complex 2 were prepared according to a similar procedure as that for
1, with H2L2 (0.35 g, 1 mmol) instead of H2L1. Yield 53%. For C17H14Cl2CuN2O2: Anal. calcd, %:
C, 49.47; H, 3.42; N, 6.79. Found, %: C, 49.62; H, 3.35; N, 6.70.
X-ray Crystallography
Crystal structure determination of the complexes were carried out on a Bruker APEX II CCD
area diffractometer equipped with graphite-monochromatized MoKα (λ = 0.71073 Å) radiation.
Structures of both complexes were solved by direct methods and refined on F2 by full-matrix
least-squares methods using SHELX-97.[11] All the non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included at geometrically calculated positions and refined
using a riding model. The crystal data, experimental details, refinement results and details of
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structure determinations are listed in Table 1. Selected bond lengths and bond angles are listed in
Table 2.
Results and Discussion
The Schiff bases H2L1 and H2L2 were prepared in quantitative yields in methanol. The
compounds are yellow solid product. The elemental analyses are in good agreement with the
chemical formulae proposed for the compounds. Complexes 1 and 2 were prepared by the
reaction of copper perchlorate with H2L1 and H2L2, respectively, in methanol (Scheme 2). Both
the Schiff bases and the copper complexes are stable in air at room temperature, and soluble in
common polar organic solvents, such as DMSO, DMF, methanol, ethanol, and acetonitrile. The
molar conductance values of the complexes measured in methanol at concentrations of 10–3
mol·L–1 at 298 K are 18 and 27 Ω–1 cm2 mol–1, indicating the non-electrolytic nature.[12]
IR Spectra
The IR spectra of the free Schiff bases H2L1 and H2L2 showed weak bands due to the phenolic
groups centered at about 3430 cm–1, which are assigned to the νOH vibrations. The bands of the
phenolic groups are absent in the IR spectra of the copper complexes, indicating the hydroxyl
groups are deprotonated during coordination. The intense bands at 1283 cm–1 for H2L1 and 1287
cm–1 for H2L2 assigned to phenolic C–O linkage shifting toward to higher wave numbers of 1330
cm–1 for 1 and 1337 cm–1 for 2, confirming the involvement of the deprotonated phenolic groups
in bond formation with the copper atoms.[13] The strong bands in the free Schiff bases at 1645
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cm–1 for H2L1 and 1651 cm–1 for H2L2 underwent negative shift of about 10–15 cm–1 in the
complexes, confirming the coordination of the azomethine N atoms to the copper atoms.[14] The
weak absorption bands in the 400–600 cm–1 region are assigned to the vibrations of Cu–N and
Cu–O coordinate bonds.
Crystal Structure Description of Complex 1
The structure of complex 1 is shown in Figure 1. The compound contains a crystallographic
inversion center symmetry. The independent component of the compound contains a dinuclear
complex molecule and two methanol molecules of crystallization. The Cu atom is in a square
pyramidal coordination, with two phenolate O and two imine N atoms of a Schiff base ligand L1
defining the basal plane, and with one phenolate O atom occupying the apical position. The
Cu-N and Cu-O bonds in the complex are within normal values as compared with other similar
copper complexes with Schiff base ligands.[15–17] The cis and trans angles subtended at the Cu
atom in the basal plane of the complex are in the range 84.8(1)–93.1(1)º and 175.8(1)–177.0(1)º,
respectively. The coordinate bond angles among the apical and basal donor atoms are in the
range 85.2(1)–98.2(1)º. Thus, the square pyramidal coordination in the complex is distorted. The
dihedral angle between the two benzene rings of the Schiff base ligand is 3.7(3)º. In the crystal
structure of the complex, the methanol molecules are linked to the complex molecules through
O5–H5···O1i hydrogen bonds [O5–H5 = 0.82 Å, H5···O1i = 1.95 Å, O5···O1i = 2.765(4) Å,
O5–H5···O1i = 174º; symmetry code for i: 1 + x, y, z] (Figure 2).
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Crystal Structure Description of Complex 2
The structure of complex 2 is shown in Figure 3. The Cu atom is coordinated in a square planar
geometry, with two phenolate O and two imine N atoms of a Schiff base ligand. The Cu-N and
Cu-O bonds in the complex are comparable to those of complex 1 and also within normal values
as compared with other similar copper complexes with Schiff bases.[15–17] The cis and trans
angles subtended at the Cu atom are in the range 87.5(1)–97.7(1)º and 153.0(2)–154.1(2)º,
respectively. Thus, the square pyramidal coordination in the complex is tetrahedrally distorted. In
the crystal structure of the complex, the complex molecules are stack along the b axis through
π···π interactions [Cg1···Cg1ii = 4.738(3) Å, Cg1···Cg2ii = 4.998(3) Å, Cg1···Cg3iii = 4.313(3)
Å, Cg2···Cg3iv
=
4.534(3) Å; Cg1, Cg2, Cg3 are the centroids of
C(1)-C(2)-C(3)-C(4)-C(5)-C(6), and
Cu(1)-O(2)-C(13)-C(12)-C(11)-N(2),
C(12)-C(13)-C(14)-C(15)-C(16)-C(17), respectively. Symmetry codes: ii: x, – y, –1/2 + z; iii: x, –
y, 1/2 + z; iv: 1/2 + x, 1/2 – y, 1/2 + z] (Figure 4).
Antibacterial Activity
The free Schiff bases and the two complexes were screened for antibacterial activity against
Bacillus subtilis, Staphylococcus aureus, Escherichia coli, and Pseudomonas fluorescens by the
MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) method. The minimum
inhibitory concentrations (MICs) of the compounds against the bacteria are presented in Table 3.
Penicillin was used as a reference. The results revealed that the free Schiff base H2L1 showed
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weak activity against Bacillus subtilis, Staphylococcus aureus and Escherichia coli, and no
activity against Pseudomonas fluorescens. The free Schiff base H2L2 showed medium activity
against Bacillus subtilis and Staphylococcus aureus, and weak activity against Escherichia coli
and Pseudomonas fluorescens. It is clear that the activities of the two complexes are much
effective than the free Schiff bases. Complexes 1 and 2 have strong activities against Bacillus
subtilis, Staphylococcus aureus and Escherichia coli, and weak activity against Pseudomonas
fluorescens. It should be noted that complex 2 has the MIC value of 0.10 μg mL–1 against
Bacillus subtilis, which is much lower than the reference drug. This phenomenon is in
accordance with the conclusion that compounds bearing halide groups have more potent
antibacterial activities.[18]
Conclusion
In the present work, a dinuclear copper(II) complex and a mononuclear copper(II) complex were
prepared and characterized by single-crystal X-ray diffraction. The biological assay revealed that
both complexes are more active against the bacteria than the free Schiff bases. The mononuclear
copper(II) complex with the bis-Schiff base N,N’-bis(5-chlorosalicylidene)-1,3-propanediamine
has the MIC value of 0.10 μg mL–1 against Bacillus subtilis.
Acknowledgments
The author thanks to the Shaanxi Key Laboratory of Phytochemistry (grant no. 12JS007) for
financial assistance.
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Table 1. Crystallographic data and structure refinement for the complexes 1 and 2.
Complex
1
C38H44Cu2N4O10
843.85
2
C17H14Cl2CuN2O2
412.74
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
a (Å)
298(2) K
298(2) K
0.71073 Å
Monoclinic
P21/c
0.71073 Å
Monoclinic
Cc
11.1108(9)
7.5550(6)
23.964(2)
114.814(2)
1825.8(2)
2
21.007(2)
8.111(2)
9.399(2)
94.230(2)
1597.1(6)
4
b (Å)
c (Å)
β (°)
V (Å3)
Z
ρ (calcd.) (g cm-3)
μ (mm-1)
1.535
1.717
1.230
1.714
F(000)
876
836
Crystal size (mm3)
0.27 × 0.26 × 0.23
0.19 × 0.18 × 0.18
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θ range for data collection 2.85 – 25.50
1.94 – 25.50
(°)
h/k/l (max, min)
-13, 13/-8, 9/-29,25
15966
-24, 25/-6, 9/-11, 11
Reflections collected
3970
2563
Observed reflections [I > 2931
2σ(I)]
Max.
transmission
Data/restraints/parameters 3392/0/248
Goodness-of-fit on F2
1.046
and
min. 0.7324 and 0.7651
0.7365 and 0.7478
2681/2/217
1.222
Final R1, wR2 indices [I > 0.0406, 0.1066
2σ(I)]
0.0378, 0.1014
R1, wR2 indices (all data)
0.0490, 0.1133
0.0396, 0.1045
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Table 2. Selected bond lengths (Å) and bond angles (°) for the complexes 1 and 2.
1
Cu1-O1
1.898(2)
1.943(2)
2.832(3)
89.4(1)
92.5(1)
177.0(1)
86.6(1)
98.2(1)
Cu1-O2
Cu1-N2
1.904(2)
1.939(2)
Cu1-N1
Cu1-O2A
O1-Cu1-O2
O2-Cu1-N2
O2-Cu1-N1
N1-Cu1-O2A
O1-Cu1-O2A
O1-Cu1-N2
O1-Cu1-N1
N2-Cu1-N1
N2-Cu1-O2A
O2-Cu1-O2A
175.8(1)
93.1(1)
84.8(1)
85.2(1)
94.7(1)
Symmetry operation for generating atoms with the suffix A: – x, – y, – z.
2
Cu1-O1
1.906(5)
1.949(6)
87.47(9)
92.6(2)
Cu1-O2
1.898(5)
1.971(6)
154.1(2)
93.7(2)
Cu1-N1
Cu1-N2
O2-Cu1-O1
O1-Cu1-N1
O1-Cu1-N2
O2-Cu1-N1
O2-Cu1-N2
N1-Cu1-N2
153.0(2)
97.7(1)
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Table 3. Antibacterial activity of the tested material (MIC, μg mL–1).
Compound
Bacillus
subtilis
12.5
Staphylococcus Escherichia
Pseudomonas
aureus
12.5
6.25
3.12
1.56
3.13
coli
25
fluorescens
> 100
50
H2L1
H2L2
3.12
25
0.78
6.25
6.25
> 100
25
1
0.10
25
2
Penicillin
0.78
> 100
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Fig. 1. ORTEP plots (30% probability level) and numbering scheme for 1. Symmetry operation
for generating atoms with the suffix A: – x, – y, – z.
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Fig. 2. Molecular packing structure of 2. Hydrogen bonds are drawn as thin dashed lines.
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Fig. 3. ORTEP plots (30% probability level) and numbering scheme for 2.
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Fig. 4. Molecular packing structure of 2.
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N
N
Cl
Cl
N
N
OMe
MeO
HO
OH
HO
OH
Scheme 1. H2L1 and H2L2.
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N
O
N
O
Cu
OMe
MeO
H2L1
Cu(ClO4)2
+
OMe
O
N
O
N
MeO
Cu
1
N
O
N
O
H2L1
Cu(ClO4)2
+
Cu
Cl
Cl
2
Scheme 2. The synthetic procedure of the complexes.
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