Synthesis, spectral, theoretical studies and in vitro antimicrobial activities of novel diphenyltin(IV) complexes of Schiff bases derived from phenacylamine

Article abstract of DOI:10.1002/aoc.3160

A series of new diphenyltin(IV) complexes of the type Ph₂SnL (L¹: N-phenacyl-5-bromosalicylideneimine, Ph₂SnL ¹; L²: N-phenacyl-3,5-dichlorosalicylideneimine, Ph ₂SnL²; L³: N-phenacyl-4- methoxysalicylideneimine, Ph₂SnL³) were synthesized and characterized by elemental analysis, IR, ¹H, ¹³C, ¹¹⁹Sn NMR spectroscopy and mass spectrometry techniques. The C-Sn-C angles in the complexes were calculated using equations with the ¹J(^117/119Sn-¹³C) values from ¹³C NMR spectra. The possible structures, NMR and electronic properties of the studied molecules were calculated through density functional theory and results compared with experimental data. All the complexes were found to be mildly active against several microorganisms and some fungi. Copyright 2014 John Wiley & Sons, Ltd. Structures of a new series penta-coordinated diphenyltin(IV) complexes of Schiff bases derived from phenacylamine, elucidated with both experimental methods including IR, ¹H, ¹³C, ¹¹⁹Sn NMR, mass spectroscopic techniques and theoretical methods using Density Functional Theory approach. Antimicrobial activities of all complexes also investigated against several microorganizms and some fungi. Copyright

Full text of DOI:10.1002/aoc.3160

Full Paper
Received: 1 December 2013
Revised: 6 April 2014
Accepted: 6 April 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3160
Synthesis, spectral, theoretical studies and
in vitro antimicrobial activities of novel
diphenyltin(IV) complexes of Schiff bases
derived from phenacylamine
Gökhan Şirikcia*, Nilgün Ancına, Selma Gül Öztaşa, Gülgün Yenişehirlib
and Nurşen Altuntaş Öztaşc
A series of new diphenyltin(IV) complexes of the type Ph2SnL (L1: N-phenacyl-5-bromosalicylideneimine, Ph2SnL1; L2:
N-phenacyl-3,5-dichlorosalicylideneimine, Ph2SnL2; L3: N–phenacyl-4-methoxysalicylideneimine, Ph2SnL3) were synthesized
and characterized by elemental analysis, IR, 1H, 13C, 119Sn NMR spectroscopy and mass spectrometry techniques. The
1
C―Sn―C angles in the complexes were calculated using equations with the J(117/119Sn―13C) values from 13C NMR spectra.
The possible structures, NMR and electronic properties of the studied molecules were calculated through density functional
theory and results compared with experimental data. All the complexes were found to be mildly active against several
microorganisms and some fungi. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: diphenyltin(IV); phenacylamine; Schiff base; antimicrobial activity; quantum chemical calculation
and CDCl3 solvents on a Bruker 400 MHz Ultrashield NMR spectrome-
Introduction
ter with TMS as internal standard. 119Sn NMR spectra were recorded
The synthesis of organotin(IV) complexes derived from Schiff ba-
ses has been extensively studied.[1–7] Interest is growing widely
as a result of their antimicrobial and antitumor activities.[8–16]
These types of compounds have also found application in
homogeneous catalysis[17,18] and nonlinear optics.[6,19]
In this paper, we report the synthesis and structural analysis of
diorganotin(IV) complexes containing tridentate Schiff base anions
derived from 5-bromosalicylaldehyde, 3,5-dichlorosalicylaldehyde,
4-methoxysalicylaldehyde and phenacylamine. The complexes
were synthesized starting from Ph2SnCl2 and the Schiff base (H2L)
in methanol in the presence of sodium methoxide. The reactions
are summarized in Fig. 1. All the diphenyltin(IV) complexes
prepared were stable under atmospheric conditions.
The fully optimized equilibrium geometries of Schiff bases and
their diphenyltin(IV) complexes were obtained through density
functional theory (DFT). A satisfactory correlation was established
between theory and experiments.
Antibacterial and antifungal activities of the novel Schiff bases and
complexes were also investigated. As the complexes were insoluble
in water, solutions in DMSO were used in the activity studies.
on a Jeol ECX-400 NMR spectrometer. IR spectra were recorded on a
Mattson-1000 FT-IR spectrophotometer using KBr pellets, in the range
4000–400 cmꢀ1. Bands were located by means of a microprocessor.
API-ES mass spectra were recorded on a Waters 2695 Alliance
Micromass ZQ LC-MS spectrometer. Chemical analysis of C, H and N
were determined with a LECO CHNS-932 elemental analyzer.
Synthesis of the Ligands
N-Phenacyl-5-bromosalicylideneimine (H2L1)
Phenacylamine hydrochloride (2.0 mmol), 5-bromosalicylaldehyde
(2.0 mmol) and sodium carbonate (2.0 mmol) dissolved in 50 ml
methanol was refluxed for 1 h. The mixture was then filtered to
remove NaCl and other insoluble impurities. The precipitate thus
formed upon cooling in an ice bath was filtered off. The powder
products were crystallized from methanol–dichloromethane (1:1)
mixture. Yellow crystals; m.p. 148–152°C. IR (cmꢀ1): 3468 br
1
υ(O―H); 1696 s υ(C¼O); 1635 s υ(C¼N). H NMR (CDCl3, δ, ppm):
3
3
5.10 (s, 2H, H8), 6.87 (d, 1H, J = 9.4, H3), 7.40 (dd, 1H, J = 9.4,
*
Correspondence to: Gökhan Şirikci, Department of Chemistry, Faculty of
Science, Ankara University, 06100 Ankara, Turkey. E-mail: gsirikci2010@gmail.com
Experimental
a Department of Chemistry, Faculty of Science, Ankara University, 06100
Ankara, Turkey
Materials and Physical Measurements
All chemicals and reagents were of reagent-grade quality. Diphenyltin
dichloride, phenacylamine hydrochloride, 5-bromosalicylaldehyde,
3,5-dichlorosalicylaldehyde, 4-methoxysalicylaldehyde and solvents
were purchased from Aldrich and used without further purification.
1H NMR and 13C NMR spectra were obtained in deuterated DMSO
b Department of Microbiology and Clinical Microbiology, Faculty of Medicine,
Gaziosmanpaşa University, 60100 Tokat, Turkey
c
Department of Chemistry, Faculty of Science, Hacettepe University, 06800
Ankara, Turkey
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

G. Şirikci et al.
sodium methoxide (4.0 mmol) in methanol was added and
the resulting sodium salt solution of the ligand was filtered to
remove any insoluble impurities. To this solution, a solution of
Ph2SnCl2 (2.0 mmol) in 15 ml dry methanol was added slowly and
heated to 50–60°C. The resulting mixture was allowed to cool to
room temperature and kept at room temperature overnight.
The crystals were filtered off and recrystallized from butanol–
dichloromethane (1:1, v/v) mixture. A description of the individual
complexes follows.
[N-Phenacyl-5-bromosalicylideneiminato]diphenyltin(IV) (Ph2SnL1)
Orange crystals, m.p. 192–194°C. IR (cmꢀ1): 1608 s υ(C¼N), 697
1
υ(Sn―C), 564, υ(Sn―O), 492 υ(Sn―N). H NMR (CDCl3, δ, ppm):
6.95 (d, 1H, 3J = 8.9, H3), 7.05 [s, 1H, 3J(117/119Sn―1H) = 50.1, H8],
7.15 (d, 1H, 4J = 2.7, H6), 7.39 (dd, 1H, 3J = 8.9, 4J = 2.6, H4),
7.40–7.50 [m, 9H, meta-H + para-H(SnPh2) + H12, H14 + H13],
7.87–7.94 [m, 6H, J(117/119Sn―1H) = 108.0, ortho(SnPh2) + H11,
3
Figure 1. Numbering, preparation of compounds and keto-enol
3
H15], 8.01 (s, 1H, J(117/119Sn―1H) = 58.9 H7). 13C NMR (CDCl3, δ,
equilibrium.[31]
ppm): 108.12 (C5), 110.79 (C8), 120.00 (C1), 123.96 (C4), 125.62
(C3), 128.23 (C6), 128.54 (C11, C15,C12, C14), 128.84 [3J(117/119Sn―13C)
= 84.2 Hz, meta-C], 129.74 (C13), 130.48 [3J(117/119Sn―13C) = 17.0 Hz,
para-C], 134.45 (C10), 136.57 [3J(117/119Sn―13C) = 54.0 Hz, ortho-C],
4J = 2.5, H4), 7.40 (d, 1H, 4J = 2.5, H6), 7.51 (dd, 2H, 3J = 8.0, 3J = 8.0, H12,
H14), 7.62 (dd, 1H, 3J = 8.0, 4J = 1.6, H13), 8.01 (dd, 2H, 3J = 8.0, 4J = 1.6,
H11, H15), 8.33 (s, 1H, H7), 13.04 [s, 1H, (OH)a]. 13C NMR (CDCl3, δ, ppm):
64.17 (C8), 110.23 (C5), 119.31 (C3), 120.20 (C1), 128.35 (C11, C15),
129.01 (C12, C14), 133.85 (C6), 133.94 (C13), 135.28 (C10), 135.51 (C4),
160.23 (C2), 167.50 (C7), 193.92(C9). Mass spectrum (ESI) {m/z [assign-
ment] (%)}: 317 [M]+ (5.9). Elemental anal.: found C, 56.52; H, 3.72; N,
4.49%; calcd for C15H12NO2Br: C, 56.63; H, 3.80; N, 4.40%.
1
136.68 [1J(117Sn―13C) = 970.0 Hz, J(119Sn―13C) = 1029.0 Hz, ipso-C],
153.06 (C7), 161.43 (C9), 165.79 (C2). 119Sn NMR (CDCl3, δ,
ppm):¼327.50. Mass spectrum (ESI) {m/z [assignment] (%)}: 590 [M
+H]+ (120Sn) (13.3), 588 [M+H]+ (118Sn) (8.5), 586 [M+H]+ (116Sn) (3.1),
512 [MꢀC6H5]+
(
120Sn) (100.0), 510 [MꢀC6H5]+
(
118Sn) (61.0), 508
H2L2 and H2L3 were synthesized and purified in a similar
method as for H2L1.
[MꢀC6H5]+
(
116Sn) (26.1), 435 [Mꢀ2C6H5]+
(
120Sn) (24.0), 433
[Mꢀ2C6H5]+
(
118Sn) (15.9), 431 [Mꢀ2C6H5]+ (116Sn) (6.0), . Elemental
anal.: found C, 54.97; H, 3.51; N, 2.40%; calcd for C27H20NO2BrSn: C,
55.05; H, 3.42; N, 2.38%.
N-Phenacyl-3,5-dichlorosalicylideneimine (H2L2)
Yellow crystals, m.p. 141°C. IR (cmꢀ1): 3469 br υ(O―H); 1689 s
υ(C¼O); 1650 s υ(C¼N). 1H NMR (CDCl3, δ, ppm): 5.09 (s, 2H,
[N-Phenacyl-3,5-dichlorosalicylideneiminato]diphenyltin(IV) (Ph2SnL2)
Orange crystals, m.p. 230°C. IR (cmꢀ1): 1606 s υ(C¼N), 699
4
4
H8), 7.19 (d, 1H, J = 2.4, H6), 7.41 (d, 1H, J = 2.4, H4), 7.51 (dd,
1
3
3
3
4
υ(Sn―C), 552, υ(Sn―O), 497 υ(Sn―N). H NMR (CDCl3, δ, ppm):
2H, J = 7.7, J = 7.7, H12, H14), 7.62 (dd, 1H, J = 7.7, J = 1.6,
6.94 (d, 1H, 4J = 2.0, H6), 7.09 (s, 1H, 3J(117/119Sn―1H) = 58.0,
H8), 7.39–7.49 (m, 10H, meta-H + para-H (SnPh2) + H4 + H12,
H14 + H13), 7.86–8.10 [m, 6H, 3J(117/119Sn―1H) = 93,0, ortho
(SnPh2) + H11, H15], 8.00 (s, 1H, 3J(117/119Sn―1H) = 47.1, H7).
13C NMR (CDCl3, δ, ppm): 110.99 (C8), 111.63 (C1), 123.89 (C5),
125.72 (C11, C15) ,128.58 (C3), 128.63 (C12, C14), 128.98 [3J(117/
119Sn―13C) = 84.2 Hz, meta-C], 129.24 (C13), 129.93 (C6), 130.02
[4J(117/119Sn―13C) = 17.6 Hz, para-C], 132.93 (C4), 134.45 (C10),
136.57 [2J(117/119Sn―13C) = 54,8 Hz, ortho-C], 136.68 [1J(117Sn―13C)
3
4
H13), 7.99 (dd, 2H, J = 7.7, J = 1.6, H11, H15), 8.32 (s, 1H, H7),
15.10 [s, 1H, (OH)a]. 13C NMR (CDCl3, δ, ppm): 63.39 (C8), 119.73
(C1), 122.99 (C3), 128.34 (C11, C15), 129.09 (C12, C14), 129.43 (C4),
131.17 (C5), 132.68 (C6), 134.09 (C13), 136.58 (C10), 156.42 (C2),
167.20 (C7), 193.28(C9). Mass spectrum (ESI) {m/z [assignment]
(%)}: 307 [M]+ (10.2). Elemental anal.: found C, 58.27; H, 3.40;
N, 4.41%; calcd for C15H11NO2Cl2: C, 58.46; H, 3.60; N, 4.55%.
N-Phenacyl-4-methoxysalicylideneimine (H2L3)
1
= 980.0 Hz, J(119Sn―13C) = 1024.0 Hz ipso-C], 152.24 (C7), 159.50
Yellow crystals, m.p. 139–140°C. IR (cmꢀ1): 3469 br υ(O―H); 1699
(C9), 162.17 (C2). 119Sn NMR (CDCl3, δ, ppm): ꢀ324.40. Mass spectrum
(ESI) {m/z [assignment] (%)}: 580 [M+H]+ (120Sn) (22.0), 578 [M+H]+
1
s υ(C¼O); 1637 s υ(C¼N). H NMR (CDCl3, δ, ppm): 3.77 (s, 3H,
3
OCH3), 5.24 (s, 2H, H8), 6.35 (s, 1H, H3), 6.40 (d, 1H, J = 8.6, H5),
118Sn) (13.1), 576[M+H]+ (116Sn) (6.2), 502 [MꢀC6H5] +
(
120Sn) (100),
7.31 (d, 1H, 3J = 8.6, H6), 7.59 (dd, 2H, 3J = 7.7, 3J = 7.7 H12,
(
500 [MꢀC6H5]+
(
118Sn) (66.8), 498 [MꢀC6H5]+
(
116Sn) (28.2), 425
(
118Sn) (1.2), 421
3
3
H14), 7.69 (d, 1H, J = 7.7, H13), 8.02 (d, 2H, J = 7.7, H11, H15),
8.42 (s, 1H, H7), 13.88 [s, 1H, (OH)a]. 13C NMR (CDCl3, δ, ppm):
55.18 (OCH3), 61.57 (C8), 101.17 (C3), 105.96 (C5), 116.11 (C1),
128.03 (C11, C15), 128.87 (C12, C14), 132.86 (C6), 133.45
(C13), 136.02 (C10), 163.70 (C4), 166.36 (C2), 167.28 (C7), 194.00
(C9). Mass spectrum (ESI) {m/z [assignment] (%)}: 269 M+ (43.1).
Elemental anal.: found C, 71.11; H, 5.40; N, 5.33%; calcd for
C16H15NO3: C, 71.36; H, 5.61; N, 5.20%.
[Mꢀ2C6H5]+
(
120Sn) (2.0), 423 [Mꢀ2C6H5]+
[Mꢀ2C6H5]+ (116Sn) (0.5). Elemental anal.: found C, 56.10; H, 3.27; N,
2.47%; calcd for C27H19NO2Cl2Sn: C, 56.00; H, 3.31; N, 2.42%.
[N-Phenacyl-4-methoxysalicylideneiminato]diphenyltin(IV) (Ph2SnL3)
Reddish-orange crystals, m.p. 195–198°C. IR (cmꢀ1): 1608 s
υ(C¼N), 686 υ(Sn―C), 544, υ(Sn―O), 499 υ(Sn―N). 1H NMR
4
(CDCl3, δ, ppm): 3.83 (s, 3H, OCH3), 6.26 (dd, 1H, 3J = 8.7,
J = 2.3, H5), 6.48 (d, 1H, 4J = 2.3, H3), 6.90 (d, 1H, J = 8.7, H6), 6.97
3
Preparation of the Complexes
(s, 1H, 3J(117/119Sn―1H) = 62.8, H8), 7.32–7.39 [m, 6H, meta-H + para-
H Sn (Ph2Sn)], 7.39–7.49 (m, 3H, H12, H14 + H13), 7.81–8.0 [m, 4H, 3
J(117/119Sn―1H) = 96.0, ortho(SnPh2)], 7.85–7.90 (m, 2H, H11, H15), 8.00
Tin(IV) complexes of the Schiff base ligands were prepared from
the diphenyltin(IV) dichloride and the Schiff base as follows. To
a solution of Schiff base (2.0 mmol) in 15 ml dry methanol,
(s, 1H, J(117/119Sn―1H) = 51.4, H7). 13C NMR (CDCl3, δ, ppm): 55.47
3
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Novel diphenyltin(IV) complexes with mild antimicrobial activities
(OCH3), 104.22 (C3), 106.96 (C5), 110.52 (C8),112.82 (C1), 125.22 (C11,
C15), 128.39 (C12, C14), 128.69 [3J(117/119Sn―13C) = 88.0 Hz, meta-C],
128.94 (C13), 130.22 [4J(117/119Sn―13C) = 19.8 Hz, para-C], 134.75 (C6),
on test performance. Routine quality control testing of commer-
cially prepared Mueller–Hinton agar and blood agar were checked
with Enterococcus faecalis ATCC 29212.
1
136.29 [1J(117Sn―13C) = 987.0 Hz, J(119Sn―13C) = 1021.0 Hz ipso-C],
Antifungal activity
136.60 [2J(117/119Sn―13C) = 53.7 Hz, ortho-C], 140.63 (C10), 155.00 (C7),
158.62 (C9). 165.52 (C4), 169.56 (C2). 119Sn NMR (CDCl3, δ, ppm):
ꢀ325.79. Mass spectrum (ESI) {m/z [assignment] (%)}:542 [M+H]+
The in vitro antifungal activities of compounds were also evaluated
against Aspergillus niger, Aspergillus fumigatus, Aspergillus flavus and
Candida albicans (clinical isolates). Each Aspergillus strain was inocu-
lated on to potato dextrose agar (Oxoid) at 35°C for 7 days. Spore sus-
pensions were prepared in sterile saline and adjusted to a turbidity
equivalent to that of 0.5 McFarland, corresponding to approximately
106 colony-forming units mLꢀ1, then inoculated on to the agar
surface using cotton swabs. The agar plates were prepared
by using RPMI-1640 medium (Sigma, St Louis, MO, USA)
supplemented with 1.5% agar and 2% glucose and buffered to
a pH of 7.0 with 0.165 mol Lꢀ1 MOPS (3-[N-morpholino]
propanesulfonic acid) (Sigma). For Candida albicans, Mueller–Hinton
agar supplemented with 2% glucose was used. Sterile filter paper
discs (diameter 6 mm) (Oxoid) soaked with 25 ml of compounds
were placed on the agar plate surface. Inhibition zone diameters
were measured following 24 h incubation at 35°C in ambient air.
Fluconazol (25 μg per disc) and amphotericin B (100 U per disc)
were used as positive control discs for fungi.
(
120Sn) (24.0), 540 [M+H]+ (118Sn) (18.6), 538 [M+H]+ (116Sn) (10.8), 464
[MꢀC6H5]+ (120Sn) (1.0), 462 [MꢀC6H5]+ (118Sn) (0.8), 460 [MꢀC6H5]+
(
116Sn) (0.4), 387 [M-2C6H5]+ (0.1). Elemental anal.: found C, 62.33; H,
4.20; N, 2.65%; calcd for C28H23NO3Sn: C, 62.26; H, 4.29; N, 2.59%.
Computational Details
All calculations in this study were carried out using the Gaussian
09 software package,[20] and all molecules were characterized by
complete optimization of the molecular geometries in solution
with DFT using B3LYP hybrid functional including Becke’s three-
parameter (B3) gradient corrected exchange functional,[21]
Lee, Yang and Parr (LYP)[22] and VWN correlation functionals
[functional III],[23] as implemented in Gaussian 09.[24] All calcula-
tions were done without any symmetry constraints. Minimum
energy geometries of the Schiff base ligands were obtained using
6-311++G(d,p) basis set starting from previously optimized
structures at semi-empirical Parametric Model 6[25] level of the-
ory. Diorganotin(IV) complex structures were optimized with
mixed-basis set approach using 6-311++G(d,p) basis set for C,
H, N, O, X (Cl, Br) atoms and DGDZVP basis set for Sn atom. All
stationary points were confirmed as true minima without any
negative frequency through analytical vibrational analysis.
Cheeseman et al.’s GIAO-DFT[26] approach, which can utilize
hybrid functionals, used for NMR calculations and solvent effects,
are included using the default polarizable continuum model
(IEF-PCM) of the software (Solvent: Chloroform). Molecular
visualizations are represented with GaussView software.[27]
Results and Discussion
NMR Spectra
1
The H NMR spectra of all the ligands display a single signal for
the azomethine proton (H-7) at 8.32–8.42 ppm. The H-7 NMR
signal of tin(IV) complexes shifted downfield to the range 8.01–
8.00 ppm, reflecting the more electropositive nature of the tin
atom relative to hydrogen. In the 1H NMR spectra of the ligands,
the signals at 13.04–15.10 ppm were assigned to the phenolic
proton on the salicylaldehyde moiety (OH) (Fig. 1). The signal
disappears in the 1H NMR spectra of the corresponding Sn(IV)
complexes, indicating the engagement of phenolic O atoms in
complexation. The singlet signal at 5.09–5.24 ppm in 1H NMR
spectra indicates the presence of keto form ―CH2 (H8) in solution.
The signal shifted to the range 6.97–7.09 ppm in the 1H NMR spec-
tra of the corresponding Sn(IV) complexes. For the diphenyltin(IV)
complexes detection of 3J (117/119Sn―1H) coupling (47.1 and 58.9
Antimicrobial Studies
Antibacterial activity
Antibacterial activity of the compounds were investigated against
standard strains of Gram-negative bacteria (Escherichia coli ATCC
(American Type Culture Collection) 35218, E. coli ATCC 25922,
Enterobacter cloacae ATCC 23355, Serratia marcescens ATCC 8100,
Pseudomonas aeruginosa ATCC 27853) and Gram-positive bacteria
(Enterococcus faecalis ATCC 29212, Staphylococcus epidermidis ATCC
12228, Staphylococcus aureus ATCC 25923) by Kirby Bauer disc dif-
fusion method.[28] Each bacteria was inoculated onto blood agar
plates (Oxoid Ltd, Basingstoke, UK) containing 5% sheep blood
and then incubated aerobically at 37°C for 18–24 h. The subsequent
growth was used for preparation of bacterial suspension. The tur-
bidity of bacterial suspensions was adjusted with sterile saline to
the 0.5 McFarland standards (approximately 1–2 × 108 colony-
forming units mlꢀ1). Each bacterial suspension was inoculated on
to Mueller–Hinton agar plates (Oxoid) using cotton swabs. All com-
pounds were dissolved at a concentration of 200 mg Lꢀ1 in DMSO.
Sterile filter paper discs (diameter 6 mm) (Oxoid) soaked with 25 ml
of compounds were placed on the agar plate surface. After incuba-
tion at 37°C for 24 h, inhibition zone diameters surrounding the
each disc was measured and recorded in millimeters. Gentamicin
(10 μg per disc) was used as a positive control disc for bacteria. A
sterile distilled-water soaked disc was used as a negative control.
The solvent, DMSO, was also tested alone to establish its influence
3
Hz) with H7 hydrogen, and J (117/119Sn―1H) coupling (50.1 and
62.8 Hz) with H8 indicates the coordination of enolic oxygen and
nitrogen atoms of the ligand with tin. The extent of coupling is
comparable with the literature values.[5,13,29] The other protons in
the phenyl rings are observed in their normal δ range.
3C-NMR spectral data also confirm the proposed structures.
The considerable shifts in the positions of carbon atoms adjacent
to the imine nitrogen (C7), phenolic oxygen (C2) and enolic
oxygen (C9) and C8 support the proposed coordination sites in
the complexes. The shifts in the positions of carbon atoms
adjacent to the coordinating atoms clearly indicate the bonding of
the imine nitrogen and the two oxygen atoms to the central metal
atom. For the Ph2SnL1, Ph2SnL2 and Ph2SnL3, the 1J (117/119Sn―13C)
couplings are detected at 970, 980 and 987 Hz, respectively.
The magnitude of the coupling constants agrees well with those pre-
viously reported for analogous five-coordinate derivatives.[6] Using
1
the equation J (117/119Sn―13C) = (15.91 0.72)θ ꢀ (1164 84),
1
the C―Sn―C angle in solution can be estimated from the J (117/
119Sn―13C) coupling constants.[30] Calculated values are 138.1°,
137.8° and 137.6° for Ph2SnL1, Ph2SnL2 and Ph2SnL3, respectively.
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

G. Şirikci et al.
In the 119Sn NMR spectra, the observed δ (119Sn) values for Ph2SnL1,
Ph2SnL2 and Ph2SnL3 are ꢀ327.50, ꢀ324.40 and ꢀ325.79 ppm,
respectively. These 119Sn chemical shifts are in the range reported
(90–310 Hz) for five-coordinate tin compounds..[6,13–15,31] These δ
Table 1. Calculated properties of the compounds
Compound
E (a.u.)a
ZPVEb
ΔE(Lumo-Homo) Dipole
(kcal molꢀ1
)
(eV)
moment
(Debye)
(
119Sn) values compare well with other diorganotin(IV) complexes
Schiff bases (enol-imine forms)
containing ONO donor atoms and strongly indicate penta-coordi-
nate nuclei. It is well known that δ (119Sn) values depend on the
coordination number of the tin center and the ligand bite angle.[8]
H2L1
H2L2
H2L3
ꢀ3358.3801
145.56
145.93
171.87
3.44
3.42
3.47
3.06
5.22
5.51
ꢀ1704.0784
ꢀ899.3982
Schiff bases (keto-imine forms)
IR Spectra
H2L1
H2L2
H2L3
ꢀ3358.3838
142.63
139.69
171.69
4.23
4.32
4.18
6.55
8.30
6.73
The infrared spectrum of the ligands is consistent with the keto
form [I] (Fig. 1). Stretching vibration bands of C¼O and C¼N were
observed between 1704 and 1686 cmꢀ1 and between 1643–1633
cmꢀ1, respectively. The differences are very significant between
the IR spectra of the ligands and those of complexes in that that
the stretching vibration bands of the C¼O and phenolic O―H
groups disappear from the spectra of the complexes. This clearly
reflects the deprotonation of the phenolic and enolic oxygen
atoms on the ligand upon complexation. In the complexes the
υ(C¼N) band, occurring between 1608 and 1605 cmꢀ1, shifts
towards lower frequencies with respect to that of the free
Schiff bases, confirming the coordination of the azomethine ni-
trogen to the organotin moiety. Also Sn―C, Sn―O and Sn―N
bands were observed, with ranges of 686–699, 544–564 and
492–499 cmꢀ1 , respectively. This is in agreement with the liter-
ature values.[9]
ꢀ1704.0822
ꢀ899.4019
Diphenyltin(IV) complexes
Ph2SnL1
Ph2SnL2
Ph2SnL3
ꢀ9845.4065
246.49
240.87
3.04
3.02
3.13
2.44
4.44
5.00
ꢀ8191.1067
c
ꢀ7386.4249
—
aSum of electronic energies in atomic units.
bScaling factor 0.9877 was applied to ZVPE (zero point vibrational
energy) value results of Schiff base ligands that were proposed
for use with similar basis sets.[33] No scaling factor was applied to
ZVPE values of organotin(IV) complexes.
cValue could not obtained owing to incomplete analytical frequency
calculation, because of scratch space limitation of 32-bit Gaussian 09W.
(IV) complexes are comparable to reported experimental bond
lengths in our previous work.[32] However, XRD data are not avail-
able for these diphenyltin(IV) complexes’ calculated Sn―N,
Sn―C, Sn―O1, Sn―O2 bond distances, which are significantly
longer than previously reported for some five-coordinated
diphenyltin(IV) complexes,[13,35–37] but computational ap-
proaches tend to give shorter or longer bond lengths than exper-
imental ones.[38] In this paper a full electron DGDZVP[39] basis set
was used to describe the relatively heavy tin element at a non-
relativistic level of theory to apply a higher electron correlation.
Negletting relativistic effects could cause an increase in relavent
bond lengths.[40] Calculated O1―Sn―O2, N―Sn―O1 and
N―Sn―O2 bond angles are also in good agreement with the ex-
perimental literature data.[13,32,35–37] However, there is no cer-
tain proof, but all these findings concerning geometrical
parameters are very supportive of experimental and theoretical
correlation. C―Sn―C bond angles of optimized structures lie
between 125.54° and 127.12° and these values are lower than
our experimental values, but underestimation of C―Sn―C bond
angles was reported in a similar study.[41]
Theoretical NMR calculations play an important role in assisting
spectrum assignment, analyzing experimental spectrum, elucidating
structures and exploring the nature of reactive intermediates.[42]
Improvements in computer systems and software algorithms are very
beneficial for calculating isotropic shielding constants. Clearly, the
complex nature of some molecules can lead to incorrect structural
propositions even with the assistance of 2D-NMR experiments,[43] so
theoretical calculations could become a powerful tool to overcome
this problem. An essential point in these calculations is error
reduction for better agreement with experimental spectra. On
this matter, one of the most successful approach is empirical
scaling; in other words, the use of linear regression equations
to enhance theoretical prediction accuracy.[44] If there is a reli-
able, adequate pool of experimental and computational data
then this procedure becomes applicable. Since raw data are
Mass Spectra
In the ESI mass spectra of all the compounds, molecular ions and
some of the tin-containing fragments are clearly visible and are
summarized in the Experimental section. Fragment ions, [L]+,
[RSnL]+ and [SnL]+ were also detected in the mass spectra of all
the complexes. Fragmentation patterns are also in agreement
with the literature reports.[32]
The Sn-containing fragments display the natural abundance of
the major Sn isotopes. The experimental isotopic distributions of
all the Sn-containing fragment ions were compared with the
theoretically calculated one and found to be in agreement with
the theoretical relative abundances.
Theoretical Studies
Calculated electronic properties of the compounds are summarized
in Table 1.
Our DFT studies revealed that keto-imine forms of the ligands
are more stable than enol-imine forms. The Boltzmann distribu-
tion equation dictates the ratio of species which varies exponen-
tially depending on the free energy difference and in our case
keto:enol tautomer ratios were found to be over 100:1 for the
Schiff base ligands. This result indicates predomination of keto
forms in solution. Furthermore, calculated HOMO-LUMO gap
values of enol-imine forms are remarkably lower than keto-imine
tautomers, making them softer species.[34] This may be a key
point in reactivity towards complexation of enol tautomers.
Calculated geometric parameters of the compounds are sum-
marized in Table 2 and optimized structures are represented in
Figures 2 and 3. The 6-311++G(d,p) basis set, which contains d,
p type polarization functions as well as diffusion functions, is a
sufficent basis set and led accurate predictions on geometric pa-
rameters for such organic parts of the compounds, so that theo-
retical C¼N, C2―O1 and C9―O2 bond lengths of the organotin
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Novel diphenyltin(IV) complexes with mild antimicrobial activities
Table 2. Calculated geometric parameters of ligands and complexes
Bond lengths (Å)
H2L1
H2L2
H2L3
Ph2SnL1
Ph2SnL2
Ph2SnL3
C―O
C¼O
1.343
1.216
1.279
1.444
—
1.335
1.216
1.279
1.445
—
1.345
1.216
1.283
1.443
—
1.314
—
1.305
—
1.316
—
C N
1.308
1.389
2.125
2.150
2.162
2.229
1.307
1.388
2.131
2.147
2.160
2.232
1,310
1.393
2.124
2.150
2.164
2.217
C―N
Sn―O1
Sn―O2
Sn―C
Sn―N
—
—
—
—
—
—
—
—
—
Bond angles (°)
Ph2SnL1
Ph2SnL2
Ph2SnL3
C―Sn―C
126.53
158.79
83.27
127.12
158.72
82.68
125.54
159.30
83.54
O1―Sn―O2
N―Sn―O1
N―Sn―O2
75.52
75.54
75.76
ligands in solution state. Experimental chemical shift values (δ)
were plotted against calculated isotropic shieldings (σ) to obtain
linear regression equations with slope and intercept values that
were used as scaling factors for our molecular systems. Isotropic
shielding values of heavy atom bonded carbons not included in
equations which are used to calculate 13C NMR chemical shifts, in
order to prevent negative effects over regression quality.[46] In ad-
dition, calculated isotropic shieldings have been averaged when
experimental distinction not available.[42] Resulting equations and
calculated chemical shift values are represented in Table 3, includ-
ing mean absolute deviation (MAD) and root mean square (RMS)
parameters. Results indicate a well-established correlation between
experimental spectra and calculated chemical shifts and MAD; RMS
values are comparable with the published results of a few studies
1
utilizing DFT and Popple-type basis sets to calculate either H or
13C chemical shifts in organic molecules using the linear regression
method.[47–49]
Figure 2. Optimized structures of the Schiff base ligands HL1, HL2 and HL3.
Biological Activity
In this work, the in vitro antibacterial and antifungal activities of
several Schiff bases and their diphenyltin(IV) complexes were
investigated and the results are listed in Table 4.
The data obtained indicate that H2L2 had moderate activity
against Gram-positive bacteria Staphylococcus aureus ATCC
25923, S. epidermidis ATCC 12228 and also against the fungus
Candida albicans. In general, complexes exhibit antibacterial
properties higher than those of the corresponding ligands. With
regard to the literature, organotin(IV) complexes with Schiff
bases often have the higher antibacterial activity than their
ligands.[3,50]
All the complexes and ligands were also screened against the
Gram-negative bacteria, E. coli ATCC 25922, E. coli ATCC 35218,
Enertobacter cloacae ATCC 23355, Serratia marcescens ATCC 8100
and Pseudomonas aeruginosa ATCC 27853, but no antibacterial
activity was observed. Apparently the complexes are only toxic
towards Gram-positive strains. The reason probably lies in the
difference between the structures of the cell walls. The relatively
Figure 3. Optimized structures of the organotin(IV) complexes Ph2SnL1,
Ph2SnL2 and Ph2SnL3.
known to be contain some systematic errors, linear regression is
known to be a one-step solution for reducing them.[42,45] In this
work we have successfully applied this method to derive scaling
factors for organotin complexes of ONO donor-type Schiff base
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
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G. Şirikci et al.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Novel diphenyltin(IV) complexes with mild antimicrobial activities
Table 4. Antibacterial and antifungal activities of compounds
Microorganism
Inhibiton zone (mm)
Ph2SnL2
H2L1
H2L2
H2L3
Ph2SnL1
Ph2SnL3
G
A
F
S. aureus ATCC 25923
S. epidermidis ATCC 12228
E. faecalis ATCC 29212
E. coli ATCC 25922
E. coli ATCC 35218
E. cloacae ATCC 23355
S. marcescens ATCC 8100
P. aeruginosa ATCC 27853
A. niger
—
—
—
—
—
—
—
—
—
—
—
—
17
17
—
—
—
—
—
—
—
—
—
16
—
—
—
—
—
—
—
—
—
—
—
—
20
19
10
—
—
—
—
—
16
18
18
15
17
17
—
—
—
—
—
—
11
15
12
10
20
18
13
—
—
—
—
—
15
18
18
15
22
25
20
25
20
22
24
18
—
—
—
—
23
20
25
23
—
—
—
25
A. flavus
A. fumigatus
C. albicans
G, gentamicin (10 μg per disc); A, amphotericin B (100 U per disc); F, fluconazol (25 μg per disc).
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more complex walls of Gram-negative cells may prevent the
diffusion of chemicals into the cytoplasm of the organisms, which
may not be the case for Gram-positive cells. These antibacterial
and antifungal activity results are in accordance with those found
for other similar diorganotin(IV) complexes.[2,3]
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Conclusion
N-Phenacylsalicylideneimine derivatives and their diphenyltin(IV)
complexes were prepared and characterized by elemental analy-
ses, NMR spectroscopy, IR spectroscopy and mass spectrometry.
The Schiff base ligands remain predominantly as the keto form
[I] in solution. In the presence of diphenyltin(IV) chloride they
convert to the enolate form, deprotonate and coordinate with
the tin atom as uninegatively charged ONO tridentate chelating
agents with the elimination of HCl. Five-coordinate geometry is
proposed for the organotin(IV) complexes since 119Sn NMR values
indicate that and close agreement between experimental and
theoretical NMR chemical shifts support that reproduced struc-
tures correlate well with the experimental structures. The results
of antibacterial screening of the complexes indicate mild inhibi-
tory effects against Gram-positive bacteria.
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We wish to thank Ankara University Research Fund (project nos.
11B4240006 and 13H4240003) for their financial support and
Professor Mürşide Tüzün for his helpful suggestions
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