Tin(IV) Schiff base complexes: Synthesis, thermodynamic and anti bacterial investigation, experimental and theoretical studies

Article abstract of DOI:10.4314/bcse.v33i1.8

A series of five coordinated diorganotin(IV) unsymmetrical Schiff base complexes have been synthesized. The structure determination and characterization of these complexes were made on the basis of UV-Vis, IR, (¹H and ¹¹⁹Sn) NMR spec

Full text of DOI:10.4314/bcse.v33i1.8

Bull. Chem. Soc. Ethiop. 2019, 33(1), 77-90.
 2019 Chemical Society of Ethiopia and The Authors
DOI: https://dx.doi.org/10.4314/bcse.v33i1.8
ISSN 1011-3924
Printed in Ethiopia
TIN(IV) SCHIFF BASE COMPLEXES: SYNTHESIS, THERMODYNAMIC AND ANTI
BACTERIAL INVESTIGATION, EXPERIMENTAL AND THEORETICAL STUDIES
Sheida Esmaielzadeh1* and Mohammad Sharif-Mohammadi2
1Department of Chemistry, Darab branch, Islamic Azad University, Darab, Iran
2Department of Chemistry, Firozabad branch, Islamic Azad University, Firoozabad, Iran
(Received December 29, 2017; Revised November 10, 2018; Accepted November 14, 2018)
ABSTRACT. A series of five coordinated diorganotin(IV) unsymmetrical Schiff base complexes have been
synthesized. The structure determination and characterization of these complexes were made on the basis of UV-
Vis, IR, (1H and 119Sn) NMR spectroscopy as well as elemental analysis. The binding site of the ligand was
identified by IR spectroscopic measurement. Computational analyses at the level of DFT were performed to study
its electronic and molecular structures. The molecular geometry, infrared vibrational frequencies, HOMO-LUMO
energy gap, dipole moment, Mulliken charges, HF energies were calculated. The theoretical results were
consistent with the experimental data reported. The Schiff base ligands and synthesized tin(IV) complexes were
screened for their in vitro growth inhibiting activity against different strains of bacteria. Results indicated that the
complexes exhibited good antibacterial activities than the ligands. Also the thermodynamic formation constants of
the Schiff bases as donors with Me2SnCl2 as an acceptor were measured using UV-Vis spectrophotometric
titration for 1:1 complex formation at constant ionic strength (I = 0.1 M NaClO4) and at 25 ºC.
KEY WORDS: Tin(IV) complexes, Thermodynamic parameter, Biological activity, Computational analyses
INTRODUCTION
The discovery of several new organotin species and new applications has led to a renewed
interest in organotin complexes. A large number of organotin compound are used in several
areas such as pharmaceuticals, pesticides, stabilizers, fire retardants, miticides, molluscicides,
marine antifouling paints, surface disinfectants and wood preservatives and many more [1-4].
On the other hand, Schiff base ligands received instant and enduring popularity because they
have played a seminal role in the development of modern coordination chemistry, as well as
they can also be found at key points in the development of inorganic chemistry, catalysis,
pigment and dyes, medicinal imaging, optical materials and thin films [5-8]. Schiff base ligands
containing hetero atoms such as N, O, and S show a broad biological activity and are of special
interest because of their variety of ways in which they are interacted to metal ions. Schiff base in
neutral and deprotonated forms react with organotin(IV) halides and the complexes extensively
studied because they have some characteristics properties like manifestations of novel
structures, thermal stability, high synthesis flexibility, medicinal utility, industrial applications
and relevant biological properties [9-12]. In addition to their special antimicrobial activities,
organotin(IV) compound with Schiff bases present an interesting variety of structural
possibilities, so that a remarkable diversity in structure may be observed even when only a small
change in the chemistry occurs [13].
The present study will entail a description of synthesis and characterization of some tin(IV)
complexes with Schiff bases containing sulfur, nitrogen and oxygen atoms, and illustration of
the geometrical structure by using spectral analysis. By comparing the spectral and
thermodynamic properties of Schiff base ligands and their complexes, we aimed to investigate
the effects of different electronic and steric behaviors. Moreover, the density functional theory
computational calculations were done for tin(IV) Schiff base complexes for comparing with
experimental results. Also, the present work involves antibacterial activities against gram-
positive and gram-negative bacterial for the Schiff base ligand and their complexes.
__________
*Corresponding author. E-mail: esmaielzadehsheida@yahoo.com
This work is licensed under the Creative Commons Attribution 4.0 International License

78
Sheida Esmaielzadeh and Mohammad Sharif-Mohammadi
EXPERIMENTAL
Materials and physical measurements
All chemicals and reagents were commercially available and were grade quality. All the
reagents were used as received. The infrared spectra of the solid compounds were recorded on a
Shimadzu FTIR 8300 spectrophotometer in the range 4000-200 cm-1 as KBr discs. The melting
points were determined in open capillaries with electronic melting point apparatus. C, H, N, S
1
analyses were carried on a Termo Fininngan-Flash-1200. The H and 119Sn NMR spectral data
were obtained on Bruker Avance DPZ 500 MHz spectrometer using TMS and SnMe4 as
references and chemical shift are expressed in ppm. The conductivity were performed using
Jenway 4310 conductivity meter and a diptype cell with a platinized electrode in DMF having
10-3 M solutions of the metal complexes at room temperature. UV-Vis spectra were recorded in
DFT with T80 UV-Vis spectrophotometers.
Preparation of ligands and corresponding complexes
The unsymmetrical Schiff base ligands was synthesized by stirring at room temperature a 1:1
molar ratio mixture of methyl-2-(1-methyl-2'-amino-ethane)amino-1-cyclopentenedithio-
carboxylate [HcdMeen] and the substituted salicylaldehyde in methanolic solution following a
reported method [14-16]. The resultant yellow powder was recrystallized from methanol/
chloroform 2:1 (v:v). To a stirred solution of methyl-2-{[1-methyl-2-(2-hydroxy-5-methoxy-
phenyl)methylidynenitrilo]ethyl}amino-1-cyclopentenedithiocarboxylate [H2cd5OMesalMeen],
methyl-2-{[1-methyl-2-(2-hydroxyphenyl)methyllidynenitrilo]ethyl}-amino-1-cyclopentenedi-
thiocarboxylate, [H2cdsalMeen], methyl-2-{[1-methyl-2-(2-hydroxy-5-bromophenyl)methyl-
idynenitrilo]ethyl}amino-1-cyclopentenedithiocarboxylate, [H2cd5-BrsalMeen], methyl-2-{[1-
methyl-2-(2-hydroxy-5-chlorophenyl)methylidynenitrilo]ethyl}-amino-1-cyclopentene-dithio-
carboxylate, and [H2cd5-ClsalMeen] methyl-2-{[1-methyl-2-(2-hydroxy-5-nitrophenyl)methyl-
idynenitrilo]ethyl}amino-1-cyclopentenedithiocarboxylate, [H2cd5-NO2salMeen] (1 mmol) in
15 mL of chloroform/methanol 2:1(v:v), a solution of Me2SnCl2 (1 mmol) in 15 mL of methanol
was added slowly at r.t. The mixture was then stirred for 4-6 h. During this period, the yellow
solid precipitate formed was filtered and washed with petroleum ether (Figure 1).
Me
MeS
S
NH NH2
S
H
Me
N
MeS
NH
S
N
Cl
Me
Me2SnCl2
H
O
N
+
HO
A
Sn
Cl
Me
MeS
R2C=O
OH
Me
A
A
A= H, H2L1
A= OMe, H2L2
A= Br, H2L3
A= Cl, H2L4
A= NO2, H2L5
A= H
[Me2SnCl2.H2L1]
A= OMe [Me2SnCl2.H2L2]
A= H, OMe, Br, Cl, NO2
A= Br
A= Cl
[Me2SnCl2.H2L3]
[Me2SnCl2.H2L4]
A= NO2 [Me2SnCl2.H2L5]
Figure 1. Schematic representation of the Sn(IV) complexes.
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Tin(IV) schiff base complexes: synthesis, thermodynamic and antibacterial investigation
79
Thermodynamic studies of complex formation
The formation constants, Kf, of the Sn(IV) complexes were determined by spectrophotometric
titration of the ligands H2cdXsalMeen with various concentration of the solution of
dimethyltin(IV)dichloride at constant ionic strength (0.1 M NaClO4) at 25 oC. The interaction of
NaClO4 with the ligands was negligible. In a typical measurement, 2.5 mL of the ligand solution
(10-5 M) in DMF was transferred into the thermostated cell compartment of UV-Visible
instrument, and was titrated by the dimethyltin(IV)dichloride solution (10-5-10-4 M) in DMF.
The titration was performed by adding aliquots of the tin ion with a Hamilton μL syringe to the
ligand. The UV-Vis spectra were recorded in the range 290-600 nm about 5 min after each
addition. The formed product showed different absorption from the free ligand, while the metal
ion solution shows no absorption at those wavelengths. As an example, the variation of the
electronic spectra for [H2L5] titrated with various concentration of dimethyltin(IV)dichloride at
o
25 C in DMF. The same procedure was followed for other systems. The electronic spectra of
the formed complexes at the end of titration were the same as the electronic spectra of the
separately synthesized complexes.
Determination of bacteriological activity
Bioactivities were investigated using agar-well diffusion method [17]. The synthesized Schiff
base ligands and their tin(IV) complexes were screened for their biological activities by using
two bacteria namely Staphylococcus aureus and Escherichia coli by the reported method. The
bacteria were subcultured in agar medium. Recommended concentration (100 μL) of the test
sample 1 mg/mL in DMSO was introduced in the respective wells. The Petri dishes were
incubated immediately for 24 hours at 37 ºC. Activity was determined by measuring the
diameter of the zone showing complete inhibition (mm). Growth inhibition was compared with
standard drugs. In order to clarify the role of DMSO on the biological screening, separate
studies were carried out with solvent DMSO only and it showed no activity against any
microbial strains.
Theoretical methods
The quantum chemical calculations were performed using Gaussian 03 package [18]. The local
minimum energy, Mulliken charges, dipole moment, HOMO-LUMO energy and its band gap of
the optimized structures for Sn(IV) complexes was computed at Becke three parameter hybrid
function (B3LYP) using LANL2DZ and 6-311G** basis sets for all atoms including tin [19]. By
the use of LANL2DZ basis set, computational time and convergence difficulties were
considerably reduced. LANL2DZ basis set uses pseudopotential, the results are very close to the
experimental data. This does not necessarily substantiate preference of LANL2DZ basis set over
6-311G**. Many descriptors were used to recognize and correlate some physical and chemical
properties. The calculated vibrational spectra of the optimized complexes were assigned based
on vibrational mode analysis with the aid of the DFT method.
RESULTS AND DISCUSSION
Elemental analysis
The stoichiometry of the tin(IV) complexes were confirmed by their elemental analysis.
Experimental and calculated elemental compositions of the complexes are given in Table 1. The
analytical data are in good agreement with the proposed structures of the complexes (Figure 1).
The metal/ligand ratio was found to be 1:1 has been arrived at by estimating the carbon,
hydrogen, nitrogen and sulfur contents of the complexes. Solutions of these complexes no react
with silver nitrate indicating the existence of chloride in inside coordination sphere of the tin
ion.
Bull. Chem. Soc. Ethiop. 2019, 33(1)

80
Sheida Esmaielzadeh and Mohammad Sharif-Mohammadi
Molar conductance
The tin(IV) complexes under investigation are quite stable at ambient temperature. They are
sparingly soluble in donor solvents such as DMF and DMSO. The molar conductance of these
complexes (Table 1) indicating that it is essentially a non-electrolyte in this solvent [20]. The
very low conductance for these complexes is strong evidence that the Schiff base is coordinated
to the tin atom as an unnegatively charged ligand and that the two chloro ligands are also
coordinated to the tin ion.
Infrared spectra
The IR spectra give enough information to elucidate the nature of bonding of the ligand to the
metal ions. The IR spectra of free Schiff base were compared with the spectra of the tin(IV)
complexes in order to study the binding mode of the Schiff base to Sn(IV) ion in the new
complexes. In Table 2 the main infrared bands and their assignments are shown. In this spectra
of free ligand and new complexes no band is observed in the region 3500-3600 cm-1 attributable
to the stretching vibration of the free phenolic OH group indicating that the ring formed by the
intramolecular hydrogen bond in the ligand which, is retained in complexes. Weak broad
absorption bands appearing in the range of 2800-2900 cm-1 for the free Schiff base ligands are
assigned to (O-H) and (N-H) overlapping with the (C-H) [16]. These bands are observed to
shift slightly to higher frequency due to the coordination of phenolic oxygen atom with tin and
changes in hydrogen bonding [21]. The strong bands at 1622-1649 cm-1, which can be attributed
to (C=N) stretching frequency on Schiff base ligands [16]. This band showed a major shift to
higher frequency (1638-1659 cm-1) is suggesting the complex formation and the proton transfer
from phenolic oxygen atom to the imine nitrogen atom [22]. In the spectra of tin(IV) complexes
a sharp peak at 1558-1567 cm-1 due to the (C=O) providing evidence of participation of the
phenolic oxygen in the tin-ligand bonding [23]. The aromatic (C=C) stretching frequency
occurs at 1479-1497 cm-1 [24]. The characteristic sulfur vibration of (C-S) appearing at 763-
798 cm-1 [16].
Table 1. Physical properties and analytical data of the synthesized tin(IV) complexes.
Empirical formula
Formula Yield ΛM (Ω-1 m.p.
weight (%) cm2mol-1) (°C)
Anal. found
(calc.) (%)
Complexes
C
H
N
S
[Me2SnCl2.H2L1]
554.15 49
20
20
15
17
21
170 41.07(41.18) 5.21(5.09) 5.11(5.06) 11.64(11.57)
173 41.22(41.12) 5.46(5.18) 4.66(4.80) 10.75(10.98)
161 35.93(36.05) 4.21(4.30) 4.697(4.43) 10.36(10.13)
168 39.13(38.77) 4.54(4.62) 5.05(4.76) 11.03(10.89)
157 38.39(38.09) 4.68(4.54) 7.15(7.01) 10.51(10.70)
C19H28N2Cl2OS2Sn
[Me2SnCl2.H2L2] C20H30N2Cl2O2S2Sn 584.18 53
[Me2SnCl2.H2L3] C19H27N2Cl2OS2BrSn
633.05 49
[Me2SnCl2.H2L4] C19H27N2Cl3OS2Sn
588.60 51
[Me2SnCl2.H2L5] C19H27N3Cl2O3S2Sn 599.15 47
Table 2. Selected experimental and theoretical IR frequencies (cm-1) of Sn(IV) Schiff base complexes.
Complexes

(Sn-O)
m

(C-S)

(C=C)
s

(C=O)
s

(C=N)
vs

(C-H)
m
br, m
763
790
766
798
774
767
785
769
802
779
Experimental [Me2SnCl2.H2L1]
591
584
587
587
590
590
589
585
586
593
1480
1485
1475
1477
1473
1563
1558
1567
1565
1559
1567
1561
1573
1562
1565
1638
1659
1638
1641
1657
1642
1663
1645
1645
1661
2867
2925
2917
2971
2944
2872
2927
1921
2973
2949
[Me2SnCl2.H2L2]
frequencies
[Me2SnCl2.H2L3]
[Me2SnCl2.H2L4]
[Me2SnCl2.H2L5]
Calculated
frequencies
[Me2SnCl2.H2L1]
[Me2SnCl2.H2L2]
[Me2SnCl2.H2L3]
[Me2SnCl2.H2L4]
[Me2SnCl2.H2L5]
1495, 1475
1750,1560,1477
1492, 1483, 1476
1492,1481, 1476
1490, 1470, 1474
vs: very strong; s: strong; m: medium; w: weak; br: broad.
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Tin(IV) schiff base complexes: synthesis, thermodynamic and antibacterial investigation
81
New bands in the range of 580-590 cm-1 which are not present in the free Schiff base are due
to (Sn-O) vibration [25] and the appearance of the vibrations support the involvement of the
oxygen atoms of phenolic group complexation with the tin ion under investigations. Two sharp
peaks at around 1300 and 1550 cm-1 are typical of nitro group in [Me2SnCl2.H2cd5NO2salMeen]
[16].
1H and 119Sn NMR spectra
The NMR spectrum is recorded to confirm the binding sites during the complexation. The NMR
data are given in Table 3. The chemical shift observed for the –OH and –NH protons in free
Schiff base ligands (δ 12.45-14.06 ppm) and (δ 12.30-12.38 ppm) was observed in all
complexes. The deshielding of this group and shifted down field in complexes may be due to
bonding of the oxygen to the tin(IV) ion which lead to decrease of the density of electrons on
the hydroxyl group. The same results were confirmed by the IR spectroscopy. The signal at 8-9
ppm were assigned to imine proton (HC=N) is not flanked by satellites, this is an indicating that
the N atom is not coordinated to tin(IV) [26]. The lack of down field shift in the position of the
signal attributable to S-CH3 (δ 2.55-2.57 ppm in free ligand) indicates no participitation of the
–C=S group in binding [27]. The ligand shows multiplet signal in the region δ 6.83-8.23 ppm
for the aromatic protons and these values are remains almost same position in the spectra of
1
Sn(IV) complexes. H NMR spectra of Schiff base ligands and their complexes show one peak
at chemical shift ca. 1.39-1.41, this singlet peak with three proton integration has been assigned
to the methyl group on diamine bridge. The high field regions in the spectra of dimethyl tin
show signal at 1.15 ppm for all complexes and these are due to the methyl groups in the
organotin fragment. These signals have satellites due to coupling with tin (2J 119Sn-H 71.0 and
77.1 Hz). The data describe above are all consistent with those observed for the other five
coordinated diorganotin complexes containing Schiff base ligands [28].
The value of chemical shift 119Sn spectra expresses the coordination number of the nucleus
in the related metal complexes. In general, 119Sn chemical shifts move to lower frequency with
increasing coordination number of the nuclei. In order to confirm the geometry of the
complexes, 119Sn NMR spectra were recorded. The spectra in each complex show only a sharp
singlet, indicating the formation of a single species. The 119Sn NMR spectra of all titled
complexes [Me2SnCl2.H2L1-5] give sharp signals at -166.65, -163.55, -161.43, -161.76, -168.93
ppm, respectively, which is indicative of five coordinated environment around the tin atom.
Thus on the basis of the above evidences it is suggested that the geometry of the resulting tin
complexes be characterized as trigonal bipyramidal [21, 29, 30]. The proposed structures of
newly synthesized complexes are shown in Figure 1.
Table 3. 1H and 119 NMR spectral data of prepared complexes (δ/ppm).
H2L1
[Me2SnCl2.H2L1] H2L2
[Me2SnCl2.H2L2] H2L3
[Me2SnCl2.H2L3]
1.41 (d, 3H, 1.15 (s, 6H, Sn- 1.39 (d, 3H, 1.15 (s, 6H, Sn- 1.39 (d, 3H, 1.15 (s, 6H, Sn-
Me), 2.56 (s, 3H, Me), 1.41 (3H, d, Me), 2.56 (s, 3H, Me), 1.39 (3H, d, Me), 2.57 (s, Me), 1.39 (3H, d,
SCH3),
23.54- Me), 2.56 (3H, s, SCH3),
3.52- Me), 2.56 (3H, s, 3H,
SCH3), Me), 2.56 (3H, s,
1H NMR 3.59 (m, 3H, SCH3),
data
2.61 3.55 (m, 3H, SCH3), 3.52-3.55 3.49-3.88 (m, SCH3), 3.50-3.86
Hen), 6.88 (dd, 3.54-3.59 (3H, Hen), 3.77 (s, 3H, (3H, m, Hen), 3H, Hen), 6.83 (3H, m, Hen), 6.85
1H, H5), 6.96 (d, m, Hen), 6.87 OCH3), 6.73 (s, 3.77 (3H, s, (d, 1H, H3), (1H, d, H3), 7.18-
1H, H3), 7.22- (1H, dd, H5), 1H, H6), 6.86 (d, OMe), 6.77 (1H, 7.34 (s, 1H, 7.27 (1H, m,
7.31 (m, 2H, 6.95 (1H, d, H3), 1H, H4), 7.24 (d, s, H6), 6.83 (1H, H6), 7.40 (d, H4,6), 8.32 (1H, s,
H4,6), 8.38 (s, 7.23-7.33 (2H, 1H, H3), 8.28 (s, d, H4), 7.21 (1H, 1H, H4), 8.27 CH=N),
12.39
1H,
CH=N), m, H4,6), 8.38 1H,
CH=N), d, H3), 8.11 (1H, (s,
1H, (1H, br, NH)
12.38 (br, 1H, (1H, s, CH=N), 12.37 (br, 1H, s, CH=N), 12.42 CH=N), 12.33 12.97 (1H, br,
NH), 12.93 (br, 12.41 (1H, br, NH), 12.45 (br, (1H, br, NH), (br, 1H, NH), OH)
1H, OH);
NH), 13.05 (1H, 1H, OH);
br, OH)
12.55 (1H, br, 12.93 (br, 1H,
OH) OH)
119Sn
-166.65
-163.55
-161.43
NMR data
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82
Sheida Esmaielzadeh and Mohammad Sharif-Mohammadi
Table 3. Continues
5
H2L4
[Me2SnCl2.H2L4]
H2L5
[Me2SnCl2 .H2L]
1.39 (d, 3H, Me), 1.15 (s, 6H, Sn-Me), 1.43 (d, 3H, Me), 2.55 1.15 (s, 6H, Sn-Me),
2.56 (s, 3H, 1.39 (3H, d, Me), (s, 3H, SCH3), 3.61- 1.43 (3H, d, Me), 2.55
SCH3), 3.50-3.86 2.57 (3H, s, SCH3), 3.66 (m, 3H, Hen), 6.99 (3H, s, SCH3), 3.61-
1H NMR (m, 3H, Hen), 3.49-3.88 (3H, m, (d, 1H, H3), 8.20-8.23 3.66 (3H, m, Hen), 6.97
data
6.88 (d, 1H, H3), Hen), 6.80 (1H, d, (m, 2H, H4,6), 8.46 (s, (1H, d, H3), 8.18-8.20
7.20-7.28
(m, H3), 7.32 (1H, s, H6), 1H, CH=N), 12.30 (br, (2H, m, H4,6), 8.46 (1H,
1H, H4,6), 8.32 (s, 7.38 (1H, d, H4), 8.27 1H, NH), 14.06 (br, s, CH=N), 12.37 (1H,
1H,
CH=N), (1H, s, CH=N), 12.38 1H, OH)
br, NH), 14.14 (1H, br,
OH);
12.32 (br, 1H, (1H, br, NH), 13.01
NH), 12.91 (br, (1H, br, OH)
1H, OH)
119Sn
-161.76
-168.93
NMR data
Therefore, it is clear from these results that the data obtained from the elemental analysis, IR
and NMR spectral measurements are in agreement with each other.
UV-Vis absorption spectra
The electronic absorption spectra of all the synthesized ligands and their complexes at very low
concentrations (~10-4 M range) are recorded in DMF solution. The ligands exhibit the band at
312-314 and 395-398 nm can be assigned to intra ligand to π→π* and n→π* transition [16].
Complex formation with dimethyltin(IV)dichloride results changes of the spectra took place in
the UV-Vis region (250-600 nm) of π→π* and n→π* absorption band upon this interaction.
After coordination of Schiff base to Me2SnCl2, the original peaks of the Schiff base ligands
changed and have a red shift. A new peak is appeared in 460-470 nm regions. It seems that this
is an LMCT from nO to Sn(IV) via coordination of Schiff base ligand to dimethyltindichloride
[31].
The formation constants and the thermodynamic free energy interpretations
To interpret the steric and the electronic parameters of the ligands on the formation constants
and the thermodynamic free energy of the complexation, the interaction between five ligands
(H2L1-5) and dimethyltindichloride (Me2SnCl2) was carried out by UV-Vis absorption
spectroscopy through titration of the ligands with various concentrations of the Sn(IV) ion at 25
oC. The complex formation constants, Kf, were calculated using SQUAD computer program
[32], designed to calculate the best values for the formation constants of the proposed equation
model (Eq. (1)) by employing a non-linear, least-squares approach.
Me2SnCl2 + H2LX → [Me2SnCl2. H2LX]
(X = 1-5)
(1)
Also, the free energy change, ∆G°, of the complexes were determined by ∆G° = -RT ln Kf, at 25
oC (Table 4). As the results show, In the para substituted Schiff base ligands, the formation
constants varies as can be expected according to the electronic effects of the substituents at
position 5. Thus, the formation constants decreases according to the sequence OMe > H > Br >
Cl > NO2. This is somewhat in agreement with decreasing electron releasing character of the
substituents in the same direction, which result in a decrease in basicity of the phenolic oxygen
groups of the ligand, and consequently decreased tendency toward complex formation. The
withdrawing functional groups make the Schiff base as a poor donor ligand and decrease the
formation constants while the electron donor group increasing the formation constants because
they leads to increase the donor ability of Schiff base ligands. Therefore, the ligands having NO2
group, [H2L5], have the smallest formation constants while the ligands with OMe group, [H2L2],
have the highest because OMe is a donating group. Therefore, in the stabilization of the
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Tin(IV) schiff base complexes: synthesis, thermodynamic and antibacterial investigation
83
complexes, the donation power of the Schiff base is important, and hence their formation
constants, Kf, with donation are higher.
Table 4. The formation constants and the free energy values of tin(IV) complexes at 25 oC in DMF.
Complexes
log Kf
∆G° (kJmol-1)
-42.54(0.27)
-43.51(0.32)
-41.12(0.22)
-40.89(0.29)
-39.52(0.14)
[Me2SnCl2.H2L1]
[Me2SnCl2.H2L2]
[Me2SnCl2.H2L3]
[Me2SnCl2.H2L4]
[Me2SnCl2.H2L5]
7.47(0.11)a
7.63(0.13)
7.21(0.09)
7.17(0.12)
6.93(0.06)
aThe numbers in parentheses are the standard deviations.
Antibacterial activity results
The free Schiff base ligands and their corresponding tin(IV) complexes were tasted against the
selected bacteria E.coli (G-) and S. aureus (G+). The measured zones of inhibition against of
various microorganisms are summarized in Table 5. All the tested compounds showed good
biological activity against microorganism. It is found that the complexes have higher
antimicrobial activity than the free ligands. This can be explained on the Tweedy’s chelation
theory [33]. The lipid membrane that surrounds the cell favors the passage of only lipid soluble
materials due to which lipophilicity is an important factor which controls the antimicrobial
activity. On chelation, 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 the positive charge of the metal ion with the
donor groups. Further, it increases the delocalization of p-electron over the whole chelate ring
and hence enhances the liposolubility of the complexes. This increased liposolubility enhances
the penetration of the complexes into the lipid membrance, the lipophilic group to derive the
compound through the semipermeable of the cell, and blocks the metal binding sites in the
enzymes of the microorganism.
Table 5. The Growth inhibition zone of the Schiff base ligands and their Sn(IV) complexes.
Bacteria Standard
(Strepto-
Zone of inhibition in nm (25 μg/disc)
H2L1 [Me2SnCl2. H2L2 [Me2SnCl2. H2L3 [Me2SnCl2. H2L4 [Me2SnCl2. H2L5 [Me2SnCl2.
H2L1]
17
H2L2]
19
H2L3]
15
H2L4]
16
H2L5]
16
mycin)
S. Aureus
E. Coli
22
24
11
10
13
12
10
8
12
10
11
12
15
16
18
19
15
Computational details
The molecular properties of the complex structures under investigation were determined by
density functional theory (DFT). The important structural analysis referring to bonding
distances, bonding angles and dihedral angles parameters for all optimized structures of Sn(IV)
complexes in the ground state configuration are gathered in Table 6. The fully optimized
molecular structures of the tin(IV) complexes with atomic numbering are shown in Figure 2.
The structure shows that the complex is monomer in which the tin atom adopts a five-
coordinate geometry being ligated to two chloride ions, two carbon atoms of methyl group and
O atom of the H2L1-5 ligands. The Schiff base is coordinated to the tin atom as a neutral
chelating agent in it is without deprotonated phenolic form via the hydroxyl oxygen atom. The
two Sn-Cl42 and Sn-Cl43 bond lengths are similar and the calculated value is longer than Sn-
C44, Sn-C45 and Sn-O38 (0.25-0.63 Å).
Bull. Chem. Soc. Ethiop. 2019, 33(1)

84
Sheida Esmaielzadeh and Mohammad Sharif-Mohammadi
Figure 2. The optimized structures of the [SnMe2Cl2 .H2L5].
Table 6. Part of the data of bond distances (Å), bond angles and dihedral angles (º) obtained through
B3LYP/LANL2DZ level of theory, labels for atoms can be found in Figure 2.
Bond length (Å) [Me2SnCl2.H2L1] [Me2SnCl2.H2L2] [Me2SnCl2.H2L3] [Me2SnCl2.H2L4] [Me2SnCl2.H2L5]
Sn-Cl43
Sn-Cl42
Sn-O38
Sn-C44
Sn-C45
O38-H52
S7-H20
N24-C31
N13-C4
2.652
2.673
2.039
2.120
2.119
2.047
2.233
1.319
1.357
2.656
2.669
2.035
2.120
2.119
2.019
2.233
1.434
1.335
2.649
2.668
2.046
2.119
2.119
2.050
2.231
1.318
1.358
2.652
2.664
2.046
2.119
2.119
2.046
2.230
1.318
1.357
2.643
2.665
2.063
2.118
2.118
2.077
2.226
1.315
1.359
Bond angle (º)
Cl42-Sn-Cl43
O38-Sn-Cl43
O38-Sn-Cl42
Cl43-Sn-C44
Cl42-Sn-C45
O38-Sn-C45
O38-Sn-C44
Dihedral angle (º)
O38-C33-C32-C31
N13-C4-C3-C6
179.04
90.97
80.07
92.70
89.46
119.87
119.98
179.73
90.75
90.99
90.86
89.06
120.04
120.11
179.88
90.15
89.74
93.13
89.12
120.11
120.08
179.97
90.26
89.25
93.25
89.75
120.13
120.12
179.81
90.57
89.25
93.52
89.20
120.15
120.23
-0.06
0.13
-0.11
-0.03
-0.17
0.04
0.41
0.28
-0.11
0.11
The DFT estimated bond lengths are generally overestimated. The coordination geometry
can be views as a trigonal bipyramidal with the two C atoms and O atom remains in the
equatorial position and two Cl atoms in the axial position. In view of the bond angle,
Cl42-Sn-Cl43 angles make approximately a 180º angle and the calculated C45-Sn-C44,
Bull. Chem. Soc. Ethiop. 2019, 33(1)

Tin(IV) schiff base complexes: synthesis, thermodynamic and antibacterial investigation
85
C45-Sn-O38 and C44-Sn-O38 angles are 120º respectively (Table 6). On the basis of the results
the tbp structure was confirmed.
The Sn(IV)-Schiff base ligand interaction is not lead to twisting of the ligand compared to
the free ligand. In the optimized structure of the complexes cyclopentene and benzene ring are
in the same plane. Their planes make approximately a 2º dihedral angle to each other. The
calculated O38-C33-C32-C31 and N13-C4-C3-C6 dihedral angles are near to each other. These
molecules are not twisted, so that the 5-X substituents group (5-OMe, 5-H, 5-Br, 5-Cl, 5-NO2) is
in opposite orientation. The aromatic rings are essentially planar, where the bond lengths of
C=C (142.9-143.7 Å) are in the expected range [34]. The two O38-H52 and S7-H20
asymmetrical intramolecular hydrogen bonds with distances 2.046-2.077 and 2.226-2.233 Å
[35]. However, the two C-N bond lengths differ significantly of the two types of C-N bonds, the
bond length for the imine nitrogen (N24=C31) is 0.038-0.044 Å shorter than that for primary
amine nitrogen (N13-C4). These C-N bonds are essentially in the same plane with the plane of
benzene and cyclopentene ring. The frontier orbital for all titled complex is plotted in Figure S2.
The LUMO surface mostly delocalized within the non-metallic atoms, and for the HOMO level,
the Sn atom is overlapped. HOMO which can be thought the outermost orbital containing
electrons tends to give these electrons such as an electron donor. LUMO can be thought the
innermost orbital containing free places to acceptor electron, owing to the interaction between
HOMO and LUMO orbitals, transition of π→π* type was observed with regard to the molecular
orbital theory. As shown in Figure S2 the HOMO is localized over the azomethine group and
cyclopentene ring, while the LUMO is localized over the benzene ring and N13-C4 bond. The
energy difference between HOMO and LUMO orbitals so called as energy gap is an important
stability for structures [36, 37].
The HOMO-LUMO energy gap of the Sn(IV) complexes are shown in Table 7 and revealed
that the energy gap reflect to the chemical activity of the complexes. A large HOMO-LUMO
gap increases stability and decreases chemical reactivity. The results show the following trend in
HOMO-LUMO gap for the complexes:
[Me2SnCl2.H2L2] > [Me2SnCl2.H2L1] > [Me2SnCl2.H2L3] > [Me2SnCl2.H2L4]> [Me2SnCl2.H2L5]
The [Me2SnCl2.H2L2] complex has a large gap between all complexes. A large HOMO-
LUMO gap indicate explain the experimental formation constant values. So, the DFT
calculations support the experimental formation constant. Absolute hardness is half of the
HOMO-LUMO gap. According to the maximum hardness principle, greater hardness (
causes more stability in the molecule [38]. On the basis of the data in Table 7 [Me2SnCl2.H2L2]
complex have a higher hardness and is stable than the other complexes.
)

In order to obtain reliable and stable structures, vibration frequencies were calculated for all
optimized complexes. Selected experimental and calculated FTIR vibration frequencies (cm-1)
of the complexes were listed in Table 2. The calculated C=C stretching frequency is somewhat
different from the experimental data, may be due to the fact that the theoretical data calculated
in gas phase. In [Me2SnCl2.H2L2] predict stretching modes at 1750, 1560 and 1477 cm-1
correspond to C=C bond. C=C bond near the 5-OMe group vibrates at a higher frequency
compared to the vibration of C=C bond on the ring in the farthest position to 5-OMe. The C=C
stretching mode in cyclopentene rings is 1477 cm-1 which occurs at a lower frequency relative to
the C=C bond in the ring involving substituent. On the other hand, in [Me2SnCl2.H2L5]
complexes C=C bond near the 5-NO2 group vibrates at a lower frequency (1470 cm-1) compared
to the vibration of C=C bond on the ring in the farthest position to 5-NO2 (1490 cm-1) and the
C=C stretching mode in cyclopentene rings is appeared at 1474 cm-1. For C-O, C=N and Sn-O
stretching frequency showed the agreement between the experimental and scale calculated
frequencies.
Base on the optimized structure of the title compound at B3LYP/LANL2DZ level of theory,
the atomic charge distributions for all atoms were calculated and the results are listed in Table 8.
Bull. Chem. Soc. Ethiop. 2019, 33(1)

86
Sheida Esmaielzadeh and Mohammad Sharif-Mohammadi
Seen from this data, atomic electronegativity plays important roles for atomic charge
distributions of the non-hydrogen atoms. Namely, between the two connecting atoms, the atom
having bigger electronegativity will carry negative charges, while the atom having smaller
electronegativity will carry positive charges. For example, when a carbon atom is connected
with a nitrogen atom, the carbon atomic charges are positive values and the nitrogen atomic
charges are negative values, since the nitrogen atom has bigger electronegativity than the carbon
atom. However, when a nitrogen atom is joined with an oxygen atom, the nitrogen atom has
positive charges and the oxygen atom has negative charges, since the oxygen atom has bigger
electronegativity than the nitrogen atom. So, for the [Me2SnCl2.H2L5] complexes (Table 8), the
carbon atoms in cyclopentene and phenyl rings all carry negative charges, since the hydrogen
atom has smaller electronegativity than the carbon atom, but C4, C21, C31, C33 and C36 have
positive atomic charges. These carbon atoms were joined to nitrogen (imine and nitro group)
and oxygen (phenolic group) atoms that have bigger electronegativity than the carbon atom. The
N13 and N24 have negatively atomic charges but N54 carry positive charge, because N54 was
joined to two oxygen atoms that have electronegativity than the nitrogen atom. On the other
hand, for the non-hydrogen atom in Table 8, although atoms of N13, N22, N54 and the carbon
atoms in phenyl and cyclopentene rings have negative charge values, steric effect hinders them
to coordinate with Sn(IV) ions further. The phenolic oxygen of the ligand has most negative
atomic charges, respectively and there is none steric effect hindered, so the phenolic oxygen
donates more electron density to the tin atom which has highest positive atomic charge, leading
to rather strong Sn-O bonding. This idea is supported by the IR data.
Table 7. The calculated properties for Sn(IV) complexes at B3LYP/Lan2DZ level of theory.
Complexes
HF energies HOMO/eV LUMO/eV gap/eV
Hardness
0.04784
0.04892
Dipole/ Debye
9.85
[Me2SnCl2.H2L1]
-979.0946
-0.2125
-0.2127
-0.2151
-0.2146
-0.2188
-0.1168
-0.1164
-0.1262
-0.1251
-0.1372
0.09568
0.09638
0.08884
0.08947
0.08159
[Me2SnCl2.H2L2] -1093.5994
[Me2SnCl2.H2L3] -993.4285
9.76
0.04473
0.04442
0.04079
7.76
8.05
5.98
[Me2SnCl2.H2L4]
-991.6471
[Me2SnCl2.H2L5] -1183.5643
Table 8. The computed Mulliken atomic charges (q/e) of the Sn(IV) complexes.
Charge of Charge of Charge of Charge of
Charge of
Atom [Me2SnCl2. Atom [Me2SnCl2. Atom [Me2SnCl2. Atom [Me2SnCl2. Atom [Me2SnCl2.
No.
H2L1]
No.
H2L2]
No.
H2L3]
No.
H2L4]
No.
H2L5]
C1
C2
C3
C4
C5
C6
S7
S8
-0.39089 C1
-0.43132 C2
-0.17114 C3
0.31989 C4
-0.45651 C5
-0.29576 C6
-0.17835 S7
0.24048 S8
-0.73283 C9
0.23828 H10
0.24430 H11
0.24455 H12
-0.63432 N13
0.21479 H14
0.22193 H15
0.22317 H16
0.23268 H17
-0.39088 C1
-0.43135 C2
-0.17162 C3
0.32095 C4
-0.45634 C5
-0.29485 C6
-0.17922 S7
0.23833 S8
-0.73295 C9
0.23815 H10
0.24420 H11
0.24450 H12
-0.63324 N13
0.21486 H14
0.22209 H15
0.22312 H16
0.23274 H17
-0.39085 C1
-0.43139 C2
-0.17043 C3
0.31970 C4
-0.45628 C5
-0.29539 C6
-0.17737 S7
0.24118 S8
-0.73274 C9
0.23867 H10
0.24441 H11
0.24462 H12
-0.63470 N13
0.21498 H14
0.22262 H15
0.22366 H16
0.23313 H17
-0.39091 C1
-0.43130 C2
-0.17093 C3
0.32021 C4
-0.45677 C5
-0.29526 C6
-0.17824 S7
0.24078 S8
-0.73280 C9
0.23859 H10
0.24455 H11
0.24443 H12
-0.63469 N13
0.21483 H14
0.22256 H15
0.22360 H16
0.23297 H17
-0.39064
-0.43152
-0.16879
0.31859
-0.45656
-0.29584
-0.17574
0.24481
-0.73262
0.23922
0.24459
0.24473
-0.63649
0.21533
0.22365
0.22424
0.23353
C9
H10
H11
H12
N13
H14
H15
H16
H17
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Tin(IV) schiff base complexes: synthesis, thermodynamic and antibacterial investigation
87
H18
H19
H20
C21
C22
C23
N24
H25
H26
H27
H28
H29
H30
C31
C32
C33
C34
C35
C36
C37
O38
H39
H40
Sn41
Cl42
Cl43
C44
C45
H46
H47
H48
H49
H50
H51
H52
H53
H54
H55
0.23485 H18
0.21385 H19
0.42768 H20
0.00832 C21
-0.21861 C22
-0.65050 C23
-0.52220 N24
0.20859 H25
0.21670 H26
0.25905 H27
0.23191 H28
0.23666 H29
0.23095 H30
0.19811 C31
-0.17752 C32
0.43794 C33
-0.25877 C34
-0.15174 C35
-0.23880 C36
-0.16987 C37
-0.89355 O38
0.20791 H39
0.21822 H40
2.11270 Sn41
-0.67568 Cl42
-0.63161 Cl43
-1.21620 C44
-1.21656 C45
0.25331 H46
0.25015 H47
0.25488 H48
0.25359 H49
0.25424 H50
0.24934 H51
0.46656 H52
0.25507 H53
0.23104 O54
0.22103 C55
H56
0.23497 H18
0.21361 H19
0.42835 H20
0.00840 C21
-0.21927 C22
-0.65023 C23
-0.51875 N24
0.20876 H25
0.21735 H26
0.26068 H27
0.23208 H28
0.23697 H29
0.23077 H30
0.20050 C31
-0.16343 C32
0.41541 C33
-0.23836 C34
-0.23286 C35
0.30636 C36
-0.20360 C37
-0.91182 O38
0.21037 H39
0.23111 H40
2.11881 Sn41
-0.68109 Cl42
-0.63758 Cl43
-1.21678 C44
-1.21704 C45
0.25304 H46
0.24824 H47
0.25476 H48
0.25328 H49
0.25409 H50
0.24767 H51
0.47034 H52
0.25804 H53
-0.57159 Cl54
-0.26197 H55
0.22222
0.23522 H18
0.21233 H19
0.42814 H20
0.00852 C21
-0.21951 C22
-0.65009 C23
-0.51493 N24
0.20879 H25
0.21848 H26
0.26092 H27
0.23145 H28
0.23685 H29
0.23191 H30
0.19888 C31
-0.16423 C32
0.43415 C33
-0.24049 C34
-0.17078 C35
-0.02224 C36
-0.18880 C37
-0.89643 O38
0.21102 H39
0.23616 H40
2.11314 Sn41
-0.67684 Cl42
-0.63248 Cl43
-1.21547 C44
-1.21588 C45
0.25436 H46
0.24915 H47
0.25568 H48
0.25456 H49
0.25501 H50
0.24850 H51
0.46844 H52
0.26224 H53
-0.04965 Br54
0.25010 H55
0.23528 H18
0.21333 H19
0.42785 H20
0.00906 C21
-0.21907 C22
-0.65003 C23
-0.51578 N24
0.20957 H25
0.21838 H26
0.26040 H27
0.23088 H28
0.23693 H29
0.23165 H30
0.19963 C31
-0.16523 C32
0.43556 C33
-0.24224 C34
-0.16705 C35
-0.13559 C36
-0.18476 C37
-0.89614 O38
0.21112 H39
0.23423 H40
2.11245 Sn41
-0.67636 Cl42
-0.63335 Cl43
-1.21531 C44
-1.21630 C45
0.25422 H46
0.24925 H47
0.25578 H48
0.25468 H49
0.25415 H50
0.24869 H51
0.46861 H52
0.26195 H53
0.06355 N54
0.24837 H55
O56
0.23574
0.21225
0.42760
0.00871
-0.22009
-0.65036
-0.50685
0.20975
0.22074
0.26171
0.22933
0.23749
0.23378
0.20384
-0.17003
0.45503
-0.24620
-0.14433
0.05368
-0.14223
-0.87587
0.21538
0.25461
2.10597
-0.67159
-0.62693
-1.21391
-1.21432
0.25583
0.25005
0.25681
0.25603
0.25609
0.24940
0.46681
0.26449
0.44261
0.26566
-0.36307
-0.38411
H57
0.19393
O57
H58
0.19356
H59
0.23222
In this study, we have synthesized and characterized five new tin(IV) complexes. Physico-
chemical measurements confirm the 1:1 Sn(IV) to ligand stoichiometry of the complexes.
Thermodynamic properties of the prepared complexes have been investigated. The formation
constant of the Schiff bases with dimethyltin(IV)dichloride in DMF decreases according to the
following trend [H2L2] > [H2L1] > [H2L3] > [H2L4] > [H2L5]. Biological activity of the ligands
and their complexes against different bacteria revealed that the complexes have higher
antibacterial activity as compared to the free ligands. We have used density functional theory
(DFT) to compute the electronic and molecular structures. The calculated structural parameters
for complexes are in good agreement with the experimental data, confirming the obtained
geometries for them. The FTIR spectra of the complexes were recorded and the important bands
were identified and compared. They can use for analysis of complexes, help to explain the
Bull. Chem. Soc. Ethiop. 2019, 33(1)

88
Sheida Esmaielzadeh and Mohammad Sharif-Mohammadi
behavior of their vibrational modes. The HOMO-LUMO energy gap of the Sn(IV) complexes
was calculated. The energy gap reflects to the chemical activity of the complexes. A large
HOMO-LUMO gap increases stability and decreases chemical reactivity. The results show the
following trend in HOMO-LUMO gap for the complexes: [Me2SnCl2.H2L2] > [Me2SnCl2.H2L1]
> [Me2SnCl2.H2L3] > [Me2SnCl2.H2L4] > [Me2SnCl2.H2L5].
ACKNOWLEDGEMENTS
We are grateful to Islamic Azad University, Darab branch Council for their financial support.
The authors express their sincere thanks to Dr. Banafsheh Esmaielzade (Department of
Anatomy, Bushehr University of Medical Sciences, Bushehr, Iran) for providing biological data
preparation and to Ghazal Mashhadiagha (Department of Chemistry, Shiraz University, Shiraz,
Iran) for providing the part of the computational method data.
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