Synthesis, Structures, and Antibacterial Activities of Two Methanol Coordinated Zinc Complexes with Tetradentate Bis-Schiff Bases

Article abstract of DOI:10.1134/S1070328417100025

Two methanol coordinated zinc(II) complexes, [Zn(L¹)(MeOH)] (I) and [Zn(L²)(MeOH)] (II), where L¹ and L² are the dianionic form of N,N'-bis(5-fluorosalicylidene)ethane-1,2-diamine and N,N'-bis(5-fluorosalicylidene)propane-1,2-diamine, respectively, have been synthesized and characterized by physical chemical methods and single crystal X-ray diffraction (CIF files CCDC nos. 950907 (I) and 950908 (II)). Crystallographic data for I: triclinic, P1 ˉ , a = 8.8765(8), b = 9.6577(9), c = 10.5117(9) ?, α = 114.590(2)°, β = 91.648(3)°, γ = 97.114(3)°, V = 809.87(13) ?³, Z = 2, R₁ = 0.0307, wR₂ = 0.0698. Crystallographic data for II: orthorhombic, Pca2₁, a = 22.946(2), b = 7.6942(7), c = 9.6234(8) ?, V = 1699.0(2) ?³, Z = 4, R₁ = 0.0320, wR₂ = 0.0676. X-ray crystal structural study indicated that the coordination environment around each zinc(II) atom in the complexes is a five-coordinated distorted pyramid in which the apical position is occupied by a methanol oxygen atom, and the basal plane is defined by the nitrogen and oxygen donor atoms of the Schiff base ligand. The antibacterial activities of the complexes were assayed.

Full text of DOI:10.1134/S1070328417100025

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2017, Vol. 43, No. 10, pp. 663–669. © Pleiades Publishing, Ltd., 2017.
Synthesis, Structures, and Antibacterial Activities of Two Methanol
Coordinated Zinc Complexes with Tetradentate Bis-Schiff Bases1
L. F. Cai
Key Laboratory of Surface and Interface Science of Henan, School of Material and Chemical Engineering, Zhengzhou
University of Light Industry, Zhengzhou, 450002 P.R. China
e-mail: cailifang2016@126.com
Received November 5, 2016
1
2
Abstract—Two methanol coordinated zinc(II) complexes, [Zn(L )(MeOH)] (I) and [Zn(L )(MeOH)] (II),
1
2
where L and L are the dianionic form of N,N'-bis(5-fluorosalicylidene)ethane-1,2-diamine and N,N'-
bis(5-fluorosalicylidene)propane-1,2-diamine, respectively, have been synthesized and characterized by
physical chemical methods and single crystal X-ray diffraction (CIF files CCDC nos. 950907 (I) and
9
1
50908 (II)). Crystallographic data for I: triclinic, P1, a = 8.8765(8), b = 9.6577(9), c = 10.5117(9) Å, α =
3
14.590(2)°, β = 91.648(3)°, γ = 97.114(3)°, V = 809.87(13) Å , Z = 2, R = 0.0307, wR = 0.0698. Crystallo-
1 2
3
graphic data for II: orthorhombic, Pca2 , a = 22.946(2), b = 7.6942(7), c = 9.6234(8) Å, V = 1699.0(2) Å ,
1
Z = 4, R = 0.0320, wR = 0.0676. X-ray crystal structural study indicated that the coordination environment
1
2
around each zinc(II) atom in the complexes is a five-coordinated distorted pyramid in which the apical posi-
tion is occupied by a methanol oxygen atom, and the basal plane is defined by the nitrogen and oxygen donor
atoms of the Schiff base ligand. The antibacterial activities of the complexes were assayed.
Keywords: tetradentate Schiff base, crystal structure, zinc complexes, supramolecular self-assembly, antibac-
terial activity
DOI: 10.1134/S1070328417100025
INTRODUCTION
substituted Schiff bases have been reported to possess
potential antibacterial activities [13]. In the present
work, two rarely seen methanol coordinated zinc(II)
Zinc is an important element for biological pro-
cesses of human beings [1–3]. However, the mecha-
nism of action of zinc in physiology and pathology are
poorly understood. Zinc is also an essential cofactor in
six classes of enzymes as well as in several families of
regulatory proteins [4]. Its importance in DNA syn-
1
2
complexes, [Zn(L )(MeOH)] (I) and [Zn(L )-
MeOH)] (II), derived from the fluoro-substituted bis-
Schiff bases N,N'-bis(5-fluorosalicylidene)ethane-
1
1
,2-diamine (H L ) and N,N'-bis(5-fluorosali-
2
2
thesis, control of gene expression, and induction of cylidene)propane-1,2-diamine (H L ), have been
2
cell apoptosis is becoming better understood [5]. bis- synthesized, characterized, and investigated for their
Schiff bases derived from benzaldehydes with antibacterial activities.
diamines are important ligands in the coordination of
various metal atoms. Zinc complexes with Schiff bases
have been received particular interest for their biolog-
N
N
ical effects, especially antibacterial and antitumor
activities [6–8]. Zinc is readily five- or six-coordi-
nated, generating square pyramidal, trigonal bipyra-
midal, or octahedral geometry. For the bis-Schiff base
zinc complexes with square pyramidal geometries, the
basal planes are usually coordinated by the donor
atoms of the Schiff base ligands, and the apical posi-
tions are occupied by anions such as nitrate, chloride,
or neutralized ligands such as pyridine or water mole-
cules [9–12]. However, methanol, as a widely used
solvent for the preparation of such complexes, has sel-
F
OH HO
F
1
2
H L (R = H), H L (R = CH )
2
2
3
EXPERIMENTAL
Material and measurements. All chemical reagents
dom coordinated to zinc atoms. In addition, fluoro- and solvents were of analytical grade and were
obtained from Sigma-Aldrich. Methanol was dried
1
The article is published in the original.
over molecular sieves (4 N) prior to use. Elemental
663

6
64
CAI
analyses were performed on a Perkin-Elmer 2400 II tances restrained to 0.85(1) Å. The remaining H atoms
elemental analyzer. Infrared spectra were recorded on were calculated at idealized positions and refined with
a Perkin-Elmer RX I FT-IR spectrophotometer with the riding models. Crystallographic data for the com-
KBr discs.
Synthesis of the Schiff bases H L and H L were
plexes are summarized in Table 1.
Supplementary material for structures has been
carried out by refluxing hot ethanolic solution deposited with the Cambridge Crystallographic Data
1
2
2
2
(
30 mL) of 5-fluorosalicylaldehyde (0.02 mol) with Centre (CCDC nos. 950907 (I) and 950908 (II);
ethane-1,2-diamine (0.01 mol) and propane-1,2- deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.
diamine (0.01 mol), respectively, for about 1 h. The ac.uk).
precipitates formed during reflux were filtered,
Antibacterial activity test. The in vitro activity test
was carried out using the growth inhibitory zone (well
method) [15]. The potency of components was deter-
mined against the Gram-positive bacteria: S. agalac-
tiae and S. aureus and the Gram-negative bacteria:
K. pneumoniae and P. aeruginosa. Microorganisms
were cultured on Muller–Hinton agar medium. The
inhibitory activity was compared with that of standard
antibiotics, such as gentamicine (10 μg). After drilling
wells on medium using a 6-mm cork borer, 100 μL of
solution from different compounds were poured into
each well. The plates were incubated at 37°C over-
night. The diameter of the inhibition zone was mea-
sured to the nearest. Each test was carried out in trip-
licate and the average was calculated for inhibition
zone diameters. A blank containing only methanol
showed no inhibition in a preliminary test. The
macro-dilution broth susceptibility assay was used for
the evaluation of minimal inhibitory concentration
(MIC). By including 1 mL Muller-Hinton broth in
each test, and then adding 1 mL extract with concen-
tration 100 mg/mL in the first tube, we made serial
dilution of this extract from first tube to last tube. Bac-
terial suspension prepared to match the turbidity of
washed with cold EtOH, and recrystallized from hot
1
1
2
EtOH · H L . The yields of H L and H L were 87 and
2
2
2
9
2%, respectively.
1
For C H N F O (H L )
16
14
2
2
2
2
Anal. calcd., %:
Found, %:
C, 63.2;
C, 63.0;
H, 4.6;
H, 4.7;
N, 9.2.
N, 9.1.
2
For C H N F O (H L )
17
16
2
2
2
2
Anal. calc., %:
Found, %:
C, 64.1;
C, 64.3;
H, 5.1;
H, 5.0;
N, 8.8.
N, 8.7.
Synthesis of the complexes. A methanolic solution
(
10 mL) of Schiff base (0.1 mmol) was mixed with a
methanolic solution (5 mL) of ZnCl (0.1 mmol), and
2
refluxed in a water bath for 1 h. The separated complex
was filtered, washed thoroughly with water, methanol,
ether, and finally dried in a vacuum over fused CaCl .
2
The yields of I and II were 65 and 57%, respectively.
For C H N F O Zn (I)
17
16
2
2
3
Anal. calc., %: C, 51.1;
Found, %: C, 50.8;
For C H N F O Zn (II)
H, 4.0;
H, 4.1;
N, 7.0.
N, 7.1.
0
.5 Mcfarland turbidity standards. Matching this tur-
bidity provides a bacterial inoculum concentration of
8
18
18
2
2
3
1.5 ×10 cfu/mL. Then 1 mL of bacterial suspension
Anal. calc., %: C, 52.3;
Found, %: C, 52.1;
H, 4.4;
H, 4.3;
N, 6.8.
N, 6.6.
was added to each test tube. After incubation at 37°C
for 24 h, the last tube was determined as the MIC
without turbidity.
A small amount of these complexes was recrystal-
lized from methanol, affording colorless single crystals
suitable for X-ray analysis.
RESULTS AND DISCUSSION
Two rarely seen methanol coordinated zinc(II)
complexes were synthesized by reaction of Schiff bases
with zinc chloride in methanol. In fact, we have tried
a lot of reaction systems for the preparation of these
complexes, yet, the same products were obtained. The
replacement of ZnCl by ZnBr or ZnI , as well as the
Single crystal X-ray diffraction. X-ray data for the
complexes were collected on a Bruker SMART
APEXII diffractometer equipped with graphite-
monochromated MoK radiation (λ = 0.71073 Å). A
α
preliminary orientation matrix and cell parameters
were determined from three sets of ω scans at different
starting angles. Data frames were obtained at scan
2
2
2
addition of ammonium thiocyanate or sodium azide,
can not interfere with the coordination of the metha-
nol solvent.
–1
intervals of 0.5° with an exposure time of 10 s frame .
The reflection data were corrected for Lorentz and
polarization factors. Absorption corrections were car-
The structures of both complexes are very similar
ried out using SADABS. The structures were solved by to each other (Fig. 1). The Zn atom in each complex
direct methods and refined by full-matrix least- shows a distorted square pyramidal coordination
squares analysis using anisotropic thermal parameters geometry, with the two phenoxy oxygen atoms and two
for non-H atoms with the SHELXTL program [14]. imino nitrogen atoms defining the basal coordination
The methanol H atoms were located from difference sites, while an O-coordinating methanol molecule
Fourier maps and refined isotropically, with O–H dis- occupies the apical position. The Zn atom deviates
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SYNTHESIS, STRUCTURES, AND ANTIBACTERIAL ACTIVITIES
665
Table 1. Crystallographic data and structure refinements for complexes I and II
Value
Parameter
I
II
Formula weight
Crystal size, mm
Temperature, °C
Crystal system
Space group
a, Å
b, Å
c, Å
α, deg
β, deg
399.69
0.23 × 0.23 × 0.20
298(2)
413.71
0.27 × 0.23 × 0.22
298(2)
Orthorhombic
Pca21
Triclinic
P1
8.8765(8)
9.6577(9)
10.5117(9)
114.590(2)
91.648(3)
97.114(3)
22.946(2)
7.6942(7)
9.6234(8)
90
90
90
γ, deg
V, Å3
809.87(13)
1699.0(2)
Z
2
4
ρcalcd, g cm–3
1.639
1.617
μ(MoK ), mm–1
1.557
1.487
α
F(000)
408
7947
2597
3010
230
1
848
12201
2697
3081
240
Number of measured reflections
Unique reflections
Number of observations (I > 2σ(I))
Independent parameters
Number of restraints
R , wR (I > 2σ(I))*
2
0.0307, 0.0698
0.0398, 0.0741
0.0320, 0.0676
0.0408, 0.0707
1.034
1
2
R , wR (all data)*
1
2
Goodness of fit of F2
1.054
Largest difference peak and hole, e Å–3
0.272 and –0.222
0.424 and –0.359
2
2 2
2 2 1/2
*
R = ∑||F | – |F ||/∑|F |, wR = [∑w(F – F ) /∑w(F ) ]
.
1
o
c
o
2
o
c
o
from the best coordination plane defined by the atoms described as severely distorted square pyramids,
N(1), N(2), O(1) and O(2) by 0.428(1) Å for I and instead of trigonal bipyramids.
0
.396(1) Å for II in direction of the apical methanol
In the crystal structure of I, two complex molecules
oxygen atom. The coordinate bond lengths and angles
are linked together by O–H···O hydrogen bonds
(
Table 2) are comparable to each other, and also in line
(
Table 3) to form a dimer (Fig. 2). In the crystal struc-
with the corresponding values found in related five-
coordinated zinc(II) salen complexes [16, 17].
ture of II, the complex molecules are linked together
by O–H···O hydrogen bonds (Table 3), to form 1D
chains running along the z axis (Fig. 3).
The question arises as to whether the coordination
polyhedron around the five-coordinated zinc atoms
can be described as a distorted square pyramid or a
distorted trigonal bipyramid. Further information can
be obtained by determining the structural index τ
which represents the relative amount of trigonality
The infrared spectra of the bis-Schiff bases and
their complexes provide information about the metal-
ligand bonding. The medium and broad absorptions in
–1
the 3200–3500 cm region for the Schiff bases and
the complexes substantiate the presence of O–H
–1
(
square pyramid, τ = 0; trigonal bipyramid, τ = 1); τ = groups. Strong absorption at about 1655 cm in the
(
β – α)/60°, α and β being the two largest angles spectra of the bis-Schiff bases is assigned to the
around the central atom [18]. The values of τ are 0.454 azomethine group, ν(C=N) [19]. The band undergoes
–1
for I and 0.289 for II. Therefore, the coordination negative shift of 12–19 cm in the complexes, which
geometries of the zinc atoms in the complexes are best can be attributed to donation of the nitrogen atom
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CAI
a)
(
C(9)
C(8)
C(10)
N(2)
C(7)
C(11)
C(16)
C(15)
C(6)
C(1)
C(5)
N(1)
O(1)
F(2)
F(1)
C(2)
Zn(1)
O(3)
C(14)
C(12)
C(4)
C(3)
O(2)
C(13)
C(17)
(b)
C(10)
C(8)
N(1)
C(9)
C(11)
C(7)
C(17)
C(12)
C(6)
N(2)
F(2)
C(16)
C(1)
C(5)
C(13)
Zn(1)
C(15)
O(2)
F(1)
C(14)
C(2)
C(4)
O(1)
C(3)
O(3)
C(18)
Fig. 1. ORTEP diagrams of complexes I (a) and II (b) with 30% thermal ellipsoids for all non-hydrogen atoms.
lone pair of the azomethine group to the metal ion.
Antibacterial activities of the Schiff bases and the
This conclusion is further supported by the presence of two complexes as well as the reference drug gentami-
–1
a medium band in the 510–590 cm region for the cine are summarized in Table 4 (zone of growth inhi-
complexes, which is absent in the spectra of the bis- bition) and Table 5 (minimal inhibitory concentra-
Schiff bases. This band can be assigned to ν(Zn–N) tions). The results indicate strong activity of the free
[
20]. The presence of a single band for ν(C=N) in the Schiff bases against Gram-positive bacteria, S. agalac-
spectra of the complexes indicates participation of tiae and S. aureus, and moderate activities of them
both the azomethine nitrogen atoms in coordination toward Gram-negative bacteria, K. pneumoniae and
bond formation with the metal ion. The phenolic P. aeruginosa. The two Schiff base ligands were found
ν(C–O) in the free ligand exhibits a strong band at
1
to be equal activities against the bacteria, indicating
–1
255 cm . In the complexes, the band appears at the methyl substitute group is not in action. In com-
–1
about 1288 cm . The weak bands in low wave num- parison, the zinc complexes have stronger activities
–1
bers of about 430–480 cm for the complexes may be against all the bacteria than the free Schiff bases. The
assigned to ν(Zn–O) [20], and provides further evi- increased activity of the complexes may be explained
dence for coordination through the deprotonated phe- on the basis of chelation theory; chelation reduces the
nolic oxygen atoms.
polarity of the metal atom mainly because of partial
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SYNTHESIS, STRUCTURES, AND ANTIBACTERIAL ACTIVITIES
Table 2. Selected bond lengths (Å) and angles (deg) for I and II
667
Bond
d, Å
Bond
d, Å
I
II
Zn(1)–O(1)
1.9918(15)
1.9618(16)
2.0773(17)
2.0827(19)
2.0921(19)
Zn(1)–O(1)
1.965(2)
1.992(2)
2.074(3)
2.073(3)
Zn(1)–O(2)
Zn(1)–O(3)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–O(2)
Zn(1)–O(3)
Zn(1)–N(1)
Zn(1)–N(2)
2.094(3)
Angle
ω, deg
Angle
ω, deg
O(2)Zn(1)O(1)
O(2)Zn(1)O(3)
O(1)Zn(1)O(3)
O(2)Zn(1)N(1)
O(1)Zn(1)N(1)
O(3)Zn(1)N(1)
O(2)Zn(1)N(2)
O(1)Zn(1)N(2)
O(3)Zn(1)N(2)
N(1)Zn(1)N(2)
96.61(7)
92.30(7)
108.56(6)
166.58(7)
89.10(7)
97.40(7)
89.24(7)
139.34(7)
111.37(7)
78.61(8)
O(1)Zn(1)O(2)
O(1)Zn(1)N(1)
O(2)Zn(1)N(1)
O(1)Zn(1)O(3)
O(2)Zn(1)O(3)
N(1)Zn(1)O(3)
O(1)Zn(1)N(2)
O(2)Zn(1)N(2)
N(1)Zn(1)N(2)
O(3)Zn(1)N(2)
95.81(9)
89.40(10)
146.67(11)
99.15(11)
104.18(9)
107.43(11)
164.10(12)
89.03(10)
78.45(12)
94.29(13)
Table 3. Geometric parameters of hydrogen bonds for I and II*
Distance, Å
D–H···A
Angle D–H···A, deg
D–H
H···A
D···A
I
O(3)–H(3A)···O(1)i
0
.85(1)
1.82(1)
2.658(2)
2.671(3)
174(3)
169(5)
II
1.83(2)
ii
0.85(1)
O(3)–H(3A)···O(2)
i
ii
*
Symmetry codes: 1 – x, 1 – y, 1/2 + z; 1 – x, 1 – y, 1 – z.
sharing of its positive charge with the donor groups favors its permeation through the lipid layer of the cell
and possible p-electron delocalization within the membrane [21]. The quantitative assays gave MIC val-
–1
whole chelation. Also, chelation increases the lipo- ues in the range 3.125–50 mg mL , that confirmed
philic nature of the central atom which subsequently the above obtained results.
x
z
0
y
Fig. 2. Hydrogen bond (dashed lines) linked dimers of complex I, viewed along the y axis.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 43 No. 10 2017

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x
z
0
y
Fig. 3. Hydrogen bond (dashed lines) linked 1D chains of complex II, viewed along the x axis.
Thus, the present paper reports the synthesis, char- antibacterial materials based on the present complex
acterization and crystal structures of two rarely seen models.
methanol coordinated zinc(II) complexes with flu-
oro-substituted bis-Schiff bases. The complexes show
effective antibacterial activities against Gram-positive
bacteria S. agalactiae and S. aureus and Gram-nega-
tive bacteria K. pneumoniae and P. aeruginosa. Further
work might be carried out to explore more effective
ACKNOWLEDGMENTS
This work was supported by Zhengzhou University
of Light Industry.
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