Synthesis, spectroscopic characterization, crystal structure, and anti-bacterial activity of diorganotin(IV) complexes with 5-bromo-2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone

Article abstract of DOI:10.1080/00958972.2018.1468888

Three new diorganotin(IV) complexes, [Me₂Sn(BDET] (2), [Bu₂Sn(BDET)] (3), and [Ph₂Sn(BDET)] (4), were synthesized by reacting R₂SnCl₂ (R?=?Me, Bu, and Ph) with 5-bromo-2-hydroxybenzaldehyde-N(4)-ethyl

Full text of DOI:10.1080/00958972.2018.1468888

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, spectroscopic characterization,
crystal structure and anti-bacterial activity
of diorganotin(IV) complexes with 5-
bromo-2-hydroxybenzaldehyde-N(4)-
ethylthiosemicarbazone
M. A. Salam, Mahbubul Alam, Sohug Sarker & Mohammed M. Rahman
To cite this article: M. A. Salam, Mahbubul Alam, Sohug Sarker & Mohammed M. Rahman
(2018): Synthesis, spectroscopic characterization, crystal structure and anti-bacterial activity of
diorganotin(IV) complexes with 5-bromo-2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1468888
To link to this article: https://doi.org/10.1080/00958972.2018.1468888
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Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2018.1468888
Synthesis, spectroscopic characterization, crystal structure and anti-bacterial
activity of diorganotin(IV) complexes with 5-bromo-2-hydroxybenzaldehyde-
N(4)-ethylthiosemicarbazone
M. A. SALAM*a, MD. MAHBUBUL ALAMb, MD. SOHUG SARKERc and MOHAMMED M. RAHMANd
aBangladesh Petroleum Exploration and Production Co. Ltd. (BAPEX), 4, Karwan Bazar, BAPEX Bhabon,
Dhaka-1215, Bangladesh
bDepartment of Chemistry, Shahjalal University of Science and Technology, Sylhet-3114, Bangladesh
cDepartment of Chemistry, Netrakona Government College, Netrakona-2400, Bangladesh
dChemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, P.O. Box 20803,
Saudi Arabia
Three new diorganotin(IV) complexes, [Me2Sn(BDET] (2), [Bu2Sn(BDET)] (3) and
[Ph2Sn(BDET)] (4), were synthesized by reacting R2SnCl2 (R = Me, Bu and Ph) with 5-bromo-
2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone [H2BDET, (1)] in the presence of KOH in
absolute methanol. The newly synthesized complexes were characterized by elemental analysis,
molar conductivity, UV-Vis, FT-IR, 1H, 13C and 119Sn NMR spectroscopies. The molecular
structure of 4 was confirmed by X-ray crystallography. X-ray crystallography revealed that the
doubly deprotonated O,N,S-tridentate thiosemicarbazone coordinates to tin(IV), resulting in a
distorted trigonal bipyramidal geometry. Their 1H, 13C and 119Sn NMR spectra support a five-
coordinate tin(IV) in solution for all complexes, in accord with the solid state X-ray structure
determined for 4. Compounds 1-4 were evaluated for their antibacterial activities against
Staphylococcus aureus, Enterobacter aerogenes, Escherichia coli and Salmonella typhi. The
results exhibited that 2-4 were active with comparable potency compared to the standard drug.
Antibacterial studies also indicated that the complexes have potential for biological evaluation.
Keywords: Synthesis; Organotin(IV) complexes; Spectral characterization; Crystal structure;
Antibacterial activity
*Corresponding author. Emails: salambpx@yahoo.com; salam@bapex.com.bd
1

1. Introduction
Thiosemicarbazone and its substituted derivatives have been investigated due to their flexible
synthesis, chelating capability and various biological applications [1-4]. Thiosemicarbazones
continue to hold an important position as chelating agents in metal coordination chemistry [5-8].
In particular, a number of transition metal complexes of thiosemicarbazone derivatives
containing ONS donors have been studied during the past few years [9-12]. Thiosemicarbazone
derivatives and their metal complexes are known for their range of biological as well as
pharmacological applications [13, 14]. Most of the previous reports have been carried out for
thiosemicarbazones with transition metal ions [15-17]. Tin complexes have various potential
applications such as antibacterial, antifungal, biocides, cytotoxic agents and wood preservatives
[18-20]. Organotin(IV) complexes with thiosemicarbazone derivatives have received attention
due to their structural features, antimicrobial and antitumor activities [21, 22]. Recently, Li et al.
reported the structural and biological activities of diorganotin(IV) complexes with
N(4)-phenylthiosemicarbazones derived from 2-benzoylpyridine and 2-acetylpyrazine [23]. The
results showed that thiosemicarbazones and their complexes exhibited remarkable biological
activities. Important series of organotin(IV) complexes of thiosemicarbazone derivatives have
prepared and investigated for their structural and biological applications [24-27]. Organotin(IV)
complexes with ONS-donor thiosemicarbazone ligands have structural diversities and various
applications depending on the coordination number and mode of the ligands about the tin(IV),
with numerous studies on their structural interactions [28-30]. Khandani et al. reported the
synthesis, crystal structures, and biological activities of diorganotin(IV) complexes with
3-methoxysalicylaldehyde thiosemicarbazone, which showed significant antibacterial and
antitumor activities [31]. Synthesis, structural and biological properties of substituted
thiosemicarbazones and their organotin(IV) complexes have become an important research field.
From literature survey, diorganotin(IV) complexes of ONS-donor thiosemicarbazones are still
scarce [32, 33]. This study reports the synthesis, spectroscopic characterization and antibacterial
activity of three new diorganotin(IV) complexes with 5-bromo-2-hydroxybenzaldehyde-N(4)-
ethylthiosemicarbazone. X-ray crystal structure of diphenyltin(IV) complex is also described.
2

2. Experimental
2.1. Materials and methods
All reagents were purchased from Fluka, Aldrich, and Sigma. All solvents were received as
reagent grade and used without purification. Melting points were measured by the Stuart
Scientific SMP1 melting point apparatus. UV–Vis spectra were recorded in DMSO with a Perkin
Elmer Lambda 25 UV–Vis spectrophotometer. Infrared (IR) spectra were recorded with a Perkin
Elmer System 2000 spectrophotometer using KBr pellets from 4000–400 cm−1 at room
temperature. 1H, 13C, and 119Sn NMR spectra were recorded on Bruker 500 and 400 MHz NMR
spectrophotometers relative to SiMe4 and SnMe4 in DMSO. Elemental analyses were conducted
by the Perkin Elmer 2400 Series-11 CHN analyzer. Molar conductivity measurements were
carried out with a Jenway 4510 conductivity meter using DMSO. X-ray crystallographic data
were recorded on a Bruker SMART APEXII CCD area-detector diffractometer using graphite
monochromated MoKα radiation ( = 0.71073 Å) at 100 K. The data were collected and reduced
using APEX2 and SAINT programs. The structures were solved by direct methods and refined
by full-matrix least-squares on F2 using the SHELXTL program [34]. All non-H atoms were
anisotropically refined. The molecular graphics were created using SHELXTL-97.
2.2. Synthesis of 5-bromo-2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone [H2BDET, (1)]
A solution of 4-ethyl-3-thiosemicarbazide (0.48 g, 4.0 mmol) in 10 mL methanol was treated
with 10 mL methanolic solution of 5-bromo-2-hydroxybenzaldehyde (0.80 g, 4.0 mmol). The
resulting reaction mixture was stirred and refluxed for 4 h (scheme 1) and then filtered. The solid
was washed with methanol and dried in desiccators over anhydrous silica gel. Yield: 0.88 g,
68%: M.p: 178-180 °C: UV–Vis (DMSO)max/nm: 256, 301, 347: FT-IR (KBr, cm-1) νmax: 3303
(s, OH), 3251, 3145 (s, NH), 1610 (m, C=N), 1551 (s, Caro-O), 988 (m, N-N), 1351, 859 (w,
C-S). 1H NMR (DMSO-d6, ppm): 11.30 (s, 1H, OH), 10.31 (s, 1H, N-NH), 8.58 (s, 1H, CS-NH),
8.27 (s, 1H, CH=N), 8.08 (d, 1H, J = 7.6, PhC3-H), 7.34 (d, 1H, J = 7.2, PhC4-H), 7.10 (s, 1H,
PhC6-H), 3.56 (m, 2H, CH2), 1.41 (t, 3H, CH3). 13C NMR (DMSO-d6, ppm): 181.3 (C=S), 155.4
(C=N), 137.5, 132.4, 128.5, 124.3, 120.2, 112.0 (Ph-C), 30.5 (CH2), 18.4 (CH3). Anal. Calc. for
C10H12BrN3OS: C, 39.75; H, 4.00; N, 13.91. Found: C, 39.94; H, 4.16; N, 14.07%.
3

H
S
Br
O
H2N
+
NH CH2-CH3
N
H
OH
4-ethyl-3-thiosemicarbazide
5-bromo-2-hydroxybenzaldehyde
Abs. MeOH
-H2O
Refluxed 4 h
H
N
Br
NH CH2-CH3
N
S
OH
Scheme 1. Synthesis of 5-bromo-2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone
[H2BDET, (1)].
2.3. Synthesis of [Me2Sn(BDET)] (2)
A methanolic solution of Me2SnCl2 (0.219 g, 1.0 mmol) was added dropwise to a methanolic
solution (10 mL) of 5-bromo-2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone
[H2BDET, (1)] (0.302 g, 1.0 mmol) and KOH (0.11 g, 2.0 mmol). The resulting reaction mixture
was stirred and refluxed for 5 h (scheme 2) and then filtered. The solid was washed with
methanol and dried in desiccators over anhydrous silica gel. The yellow solids were purified by
recrystallization from methanol and dried in vacuo over silica gel. Yield: 0.39 g, 74%: M.p: 222-
224 °C: Molar conductivity (1×10-3 mol L-1; DMSO) Ω-1 cm2 mol-1: 13.22: UV–Vis
(DMSO)max/nm: 250, 288, 342, 410: FT-IR (KBr, cm-1) νmax: 3171 (s, NH), 1590 (m, C=N),
1522 (s, Caro-O), 1045 (m, N-N), 1328, 837 (w, C-S), 588 (w, Sn-C), 548 (w, Sn-O), 450 (w,
Sn-N). 1H NMR (DMSO-d6, ppm): 8.55 (s, 1H, CS-NH), 8.24 (s, 1H, CH=N), 8.15 (d, 1H, J =
7.7, PhC3-H), 7.48 (d, 1H, J = 7.3, PhC4-H), 7.18 (s, 1H, PhC6-H), 3.54 (m, 2H, CH2), 1.42
(t, 3H, CH3) 1.05 (m, 6H, 2JSn-H = 75.2 Hz, Sn-(CH3)2). 13C NMR (DMSO-d6, ppm): 169.3
(C=S), 162.4 (C=N), 142.2, 138.1, 132.3, 129.3, 125.4, 122.5 (Ph-C), 30.8 (CH2), 18.5 (CH3),
14.3 (1JSn-C = 541.0 Hz, Sn-(CH3)2). 119Sn NMR (DMSO-d6, ppm): –158.15. Anal. Calc. for
C12H16BrN3OSSn: C, 32.10; H, 3.59; N, 9.36%. Found: C, 32.26; H, 3.75; N, 9.49%.
4

6
H
N
Br
H
CH2-CH3
R2SnCl2
N
+
5
4
1
N
2
S
3
OH
Abs. MeOH
KOH
Refluxed, 5 h
N2 atmosphere
6
Br
H
N
CH2-CH3
N
N
5
4
1
2
3
S
O
Sn
R
R
2
3
4
Where, R = Me ( ), R = n-Bu ( ), R = Ph ( )
Scheme 2. Reaction scheme for the synthesis of diorganotin(IV) complexes (2-4).
The other diorganotin(IV) complexes (3 and 4) were synthesized following the same
procedure by using the appropriate diorganotin(IV) chloride(s) (scheme 2).
2.4. Synthesis of [Bu2Sn(BDET)] (3)
Yield: 0.52 g, 80%: M.p: 233-235 °C: Molar conductivity (1×10-3 mol L-1; DMSO) Ω-1 cm2
mol-1: 10.81: UV–Vis (DMSO)max/nm: 260, 274, 333, 409: FT-IR (KBr, cm-1) νmax: 3183
(s, NH), 1587 (m, C=N), 1531 (s, Caro-O), 1030 (m, N-N), 1333, 840 (w, C-S), 582 (w, Sn-C),
544 (w, Sn-O), 448 (w, Sn-N). 1H NMR (DMSO-d6, ppm): 8.56 (s, 1H, CS-NH), 8.21 (s, 1H,
CH=N), 8.14 (d, 1H, J = 7.5, PhC3-H), 7.50 (d, 1H, J = 7.4, PhC4-H), 7.22 (s, 1H, PhC6-H),
3.57 (m, 2H, CH2), 1.43 (t, 3H, CH3), 1.33-1.30 (t, 2H, J = 7.4 Hz, Sn-CH2-CH2-CH2-CH3),
1.26-1.21 (m, 2H, Sn-CH2-CH2-CH2-CH3), 1.13-1.10 (m, 2H, Sn-CH2-CH2-CH2-CH3), 1.03-0.93
(t, 3H, J = 7.3 Hz, Sn-CH2-CH2-CH2-CH3). 13C NMR (DMSO-d6, ppm): 170.8 (C=S), 165.2
(C=N), 144.4, 135.6, 130.1, 122.5, 118.8, 112.9 (Ph-C), 31.0 (CH2), 28.4, 24.19, 22.7, 20.2
(1JSn-C = 535.1 Hz, Sn-Bu), 18.1 (CH3). 119Sn NMR (DMSO-d6, ppm): –146.53. Anal. Calc. for
C18H28BrN3OSSn: C, 40.55; H, 5.29; N, 7.88%. Found: C, 40.43; H, 5.41; N, 7.96%.
5

2.5. Synthesis of [Ph2Sn(BDET)] (4)
Yield: 0.56 g, 76%: M.p: 244-246 °C: Molar conductivity (1×10-3 mol L-1; DMSO) Ω-1 cm2
mol-1: 8.95: UV–Vis (DMSO)max/nm: 257, 292, 345, 418: FT-IR (KBr, cm-1) νmax: 3165 (s, NH),
1593 (m, C=N), 1537 (s, Caro-O), 1038 (m, N-N), 1301, 819 (w, C-S), 589 (w, Sn-C), 546
(w, Sn-O), 459 (w, Sn-N). 1H NMR (DMSO-d6, ppm): 8.54 (s, 1H, CS-NH), 8.25 (s, 1H,
CH=N), 8.10-7.20 (m, 13H, Ph-H), 3.55 (m, 2H, CH2), 1.44 (t, 3H, CH3). 13C NMR (DMSO-d6,
ppm): 168.4 (C=S), 160.0 (C=N), 148.1-123.7 (Ph-C, 1JSn-C = 524.8 Hz, Sn-Ph), 30.5 (CH2), 18.8
(CH3) 119Sn NMR (DMSO-d6, ppm): –162.38. Anal. Calc. for C22H20BrN3OSSn: C, 46.11; H,
3.52; N, 7.33%. Found: C, 46.27; H, 3.68; N, 7.49%.
2.6. Antibacterial test
The synthesized compounds 1-4 were screened in vitro for their antibacterial activities against
Staphylococcus aureus (ATCC 6538), Enterobacter aerogenes (ATCC 13048), Escherichia coli
(ATCC 15224) and Salmonella typhi (ATCC 10749) using the agar well diffusion method [35].
Ciprofloxacin was used as the reference drug. The bacteria from stock culture were lightly
inoculated into the Mueller Hinton Broth (MHB) and allowed to grow overnight at 37 °C in an
ambient air incubator. The culture was diluted with new MHB in order to get an absorbance
value of 2.0×106 colony forming units (CFU/mL) or 0.168 at wavelength of 550 nm in the
spectrophotometer. Sterile cotton swab was dipped into the broth culture and inoculated on the
Mueller Hinton Agar (MHA). Sterile paper discs with 6 mm diameter were placed on the agar in
equal distance. The recommended concentration of the test sample (2 mg/mL in DMSO) was
introduced individually to each of the discs. The agar plates were incubated immediately at 37 °C
for 24 h. For each plate, DMSO mixture and reference antibacterial drug like doxycycline served
as negative and positive controls, respectively. The activity was confirmed by measuring the
diameter of zones showing complete inhibition (mm). Growth inhibition was calculated with
reference to the positive control.
3. Results and discussion
3.1. Synthesis
5-Bromo-2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone (H2BDET) was synthesized by
condensation of 5-bromo-2-hydroxybenzaldehyde and 4-ethyl-3-thiosemicarbazide (scheme 1).
6

The new complexes 2-4 were synthesized by reaction of R2SnCl2 (R = Me, Bu and Ph) with
H2BDET (1) in absolute methanol in the presence of KOH in 1:2:1 molar ratio
(metal:base:ligand) (scheme 2). All these new diorganotin(IV) complexes were yellow solids, air
stable and soluble in DMSO, DMF, CHCl3, CH2Cl2, and THF at RT. The low molar
conductivities of the complexes (8.5-13.5 Ω-1cm2mol-1) in DMSO show the non-electrolytic
behavior of the complexes [36]. All the complexes were characterized by various physico-
chemical techniques. The physical properties and analytical data of 1-4 are given in section 2.
The molecular structure of 4 has been determined by X-ray diffraction studies. H2BDET (1)
coordinated to tin(IV) center in all complexes as dinegative ONS-tridentate chelating agent.
3.2. UV–Vis spectra
The electronic spectra of the H2BDET (1) and its complexes (2-4) were carried out in DMSO at
RT (figure 1). Electronic spectra of H2BDET (1) showed three broad bands at 256, 301 and 347
nm. The absorption band at 256 nm may be due to -* (aromatic ring) transitions. Bands at 301
and 347 nm were assigned to n-* transitions of the C=N chromophore and thiolate group,
respectively. A little shift of these absorption bands in the complexes may be due to the intra-
ligand transitions. Furthermore, a new sharp band was observed at 418-410 nm in the spectra of
the complexes (2-4), which can be assigned as ligand to metal charge transfer (LMCT)
transitions [37]. There is significant shifting in the chromophore due to coordination with tin
[38]. The appearance of these peaks depends on the electron donor ability of the R groups
attached to the Sn atom. Therefore, the shift in the bands indicates coordination between ligand
and tin(IV).
3.3. Infrared spectra
The IR spectra of H2BDET (1) show a strong band at 3303 cm-1 due to (OH) and a strong band
at 3251 cm-1 due to (NH). These bands are not observed in 2-4, indicating coordination with
tin(IV) after deprotonation of H2BDET. A medium band at 1610 cm-1 due to the (C=N) in free
1 is shifted 23-17 cm-1 to lower frequencies on complexation, indicating coordination of
azomethine nitrogen to tin(IV) [38]. A medium intensity band at 1551 cm-1 may be due to the
(Caro–O) of free 1. This band is slightly shifted to the lower frequencies in spectra of 2-4,
indicating coordination of phenolic oxygen to tin(IV). The band at 988 cm-1 in free 1 due to the
7

(N-N) shifted to higher frequencies after complexation, further indicating coordination through
the azomethine nitrogen [39]. Absorption in the spectra of free 1 at 1351 and 859 cm-1 are due to
(C=S). The lowering of these absorptions in spectra of 2-4 indicate that the thiolate sulphur
bonded to tin(IV) [40]. The appearance of some new bands in spectra of 2-4 at 589-582, 548-544
and 459-448 cm-1 are possibly due to (Sn-C), (Sn-O) and (Sn-N), respectively, support for
the bonding of oxygen and nitrogen to tin(IV) [41]. Therefore, IR spectra support the proposed
coordination in the organotin(IV) complexes which is confirmed by the X-ray crystal studies.
3.4. 1H, 13C and 119Sn NMR spectra
NMR spectral studies of 1 and 2-4 were carried out in DMSO and interpreted according to the
atom labelling in scheme 2. 1 exhibits sharp OH and NH-N proton signals at 11.30 and 10.31
ppm, respectively. The absence of both signals in the diorganotin(IV) complexes indicates
deprotonation and coordination of phenolic oxygen and thiolate sulphur of the ligand. 1 shows a
CH=N proton signal at 8.27 ppm, which shifts slightly downfield in 2-4 due to participation of
the azomethine nitrogen in bonding with tin(IV). The signal due to aromatic protons of 1 is
shifted slightly downfield in the complexes, indicating phenolic oxygen is coordinated to tin(IV).
Complex 2 exhibits a sharp singlet at 1.05 ppm for Sn(Me)2 protons. The 2J [119Sn-1H] coupling
satellite for this complex is 75.2 Hz, supporting five-coordinate tin(IV), comparable to published
reports [41, 42]. Substituting the value of 2J[119Sn-1H] in the Lockhart–Manders equations (θ =
0.0161 [2J]2 – 1.32[2J] + 133.4) affords 125.18° for Me-Sn-Me angle in 2, which also supports
the five-coordinate tin(IV) [43]. 1 shows a multiplet at 3.56 ppm for (-CH2) and a triplet at 1.41
ppm for (-CH3) protons and these are at almost the same chemical shifts in spectra of the
diorganotin(IV) complexes. Complex 3 shows four resonances at 1.33-1.30 ppm (t, 2H, J = 7.4
Hz, -CH2), 1.26-1.21 ppm (m, 2H, –CH2), 1.13-1.10 ppm (m, 2H, –CH2) and 1.03–0.93 ppm (t,
2H, J = 7.3 Hz, CH3) due to the Sn-Bu protons.
The 13C NMR spectra of 1 and 2-4 were recorded in DMSO and given in Section 2.
Free 1 exhibited resonances at 181.37 and 155.40 ppm due to (C=S) and (C=N) groups,
respectively. The carbon signals of the (C-S) group shifted upfield to 170.85-168.42 ppm
whereas chemical shifts of (C=N) carbon shifted downfield to 165.27–160.08 ppm in all the
complexes, clearly indicating coordination of thiolate sulphur and azomethine nitrogen to
tin(IV). The δ value of aromatic ring carbon was observed to be slightly downfield in the
8

complexes compared to the free ligand, indicating involvement of phenolic oxygen in
coordination. The chemical shifts of (-CH2CH3) carbons were observed at similar position in the
complexes with free 1. The carbons of butyl groups attached to the tin(IV) in 3 were observed at
28.48-20.25 ppm and comparable with other diorganotin(IV) complexes [44]. The 1J(13C-119Sn)
value is a key parameter to determine the possible coordination number of diorganotin(IV)
complexes in solution. The calculated 1J(13C-119Sn) values are 562.33, 548.45 and 552.34 Hz for
[Me2Sn(BDET] (2), [Bu2Sn(BDET)] (3) and [Ph2Sn(BDET)] (4), respectively, indicating five-
coordinate tin(IV) in these complexes [45]. In diorganotin(IV) complexes, the C–Sn–C angle can
be assessed using the Lockhart–Manders equation θ(C-Sn-C) = [|1J(13C-119Sn)| + 875]/11.4. The
observed C–Sn–C angles for the diorganotin(IV) complexes (2-4) were 126.08-125.20°,
consistent with five-coordinate geometry around tin(IV) [46]. The estimated C-Sn-C angles
observed from NMR spectra of 2-4 show a little deviation from Cph-Sn-Cph angle found from
X-ray crystal analysis of 4.
The 119Sn chemical shifts present key information to authenticate the coordination
environment around the central tin atom. The occurrence of sharp signals from -162.38 to
-146.53 ppm in the 119Sn NMR spectra of 2-4 strongly indicates five-coordinate geometry around
tin(IV) [47, 48].
3.5. Crystallographic study of [Ph2Sn(BDET)] (4)
The molecular structure of diphenyltin(IV) complex of 5-bromo-2-hydroxybenzaldehyde-N(4)-
ethylthiosemicarbazone (4) was determined by single crystal X-ray diffraction. The molecular
structure of 4 along with the atomic numbering scheme is shown in figure 2. X-ray
crystallographic data and structure refinement results of 4 are summarized in table 1. Selected
bond lengths (Å) and angles (°) are given in table 2. Compound 4 crystallizes in a triclinic lattice
with space group P-1. X-ray structure of 4 revealed that the doubly deprotonated ligand is
coordinated to tin(IV) via phenolic oxygen, azomethine nitrogen and thiolate sulphur. The
structure shows that tin(IV) is five coordinate in a trigonal bipyramidal geometry. In 4, the ligand
is coordinated to tin(IV) as a tridentate ONS-donor. In the coordination polyhedron one carbon
of each of two phenyl groups (C11 and C17) and azomethine nitrogen (N1) occupy the
equatorial plane whereas the coordinated O1 and S1 occupy the axial positions. The bond angles
may be ascertained as a proof of distorted trigonal bipyramidal geometry, equatorial–equatorial
9

angles are C11–Sn1–C17 (112.07(6)°, N1–Sn1–C11 (112.77(6)° and N1–Sn1–C17 (135.11(6)°.
The whole sum of equatorial angles is 359.95° showing tin(IV) lies in the plane of the
thiosemicarbazone moiety. These geometric arrangements are in agreement with other
organotin(IV) complexes [32, 49]. The thiolate sulphur (S1) and phenolic oxygen (O1) are
apical. The axial S1-Sn1-O1 angle (160.13(4)°) is evidence of the distorted trigonal bipyramidal
geometry of 4, deviating from 180°. The sum of bond angles of N1-Sn1-O1 (83.59(5)°) and N1-
Sn1-S1 (78.08(4)°) is 161.67°, evidence of distortion from 180°. The deviation from perfect
trigonal bipyramidal geometry is most probably owing to the non flexibility of chelate rings and
large covalent radius of Sn(IV). The tin(IV) atom makes the following non-planar six- and five-
membered chelate rings upon complexation: Sn1-N1-C7-C6-C1-O1 and Sn1–N1-N2-C8-S1. The
bond angles of N1-Sn1-C11 of 112.77°, N1-Sn1-C17 of 135.11° and C11-Sn1-C17 of 112.07°
where N1, C11 and C17 are placed at the three corners of TBP plane. This is may be due to the
repulsion of the two phenyl groups at tin(IV). Hence, the geometric results are strong evidence
for distorted trigonal bipyramidal environment around Sn(IV) moiety. To describe the
geometrical environment around tin we use τ value, the so called trigonal index. The τ value can
be defined as τ = (β–α)/60, where β and α are two largest donor-metal-donor angles around
tin(IV) center [50]. For ideal trigonal bipyramidal geometry τ value should be equal to unity. For
the diphenyltin(IV) complex 4, β = 160.13 and α = 112.07; so τ = 0.80 regarded as a distorted
trigonal-bipyramidal geometry around tin [51]. The bond distance of Sn1–N1 bond (2.2068 Å) is
a little longer than the Sn–N (2.15 Å) [52], indicating strong bonding between azomethine
nitrogen (N1) and Sn1. The observed Sn1-O1 bond distance (2.0710 Å) is similar with Sn-O
(2.10 Å) and comparable to reported organotin(IV) complexes [53, 54]. The Sn1-S1 bond
distance (2.5163 Å) is comparable to reported organotin(IV) complexes and is near to the sum of
the Sn-S covalent radii (2.42 Å) [55, 56]. The bond distances of Sn1-C11 (2.1333 Å) and Sn1-
C17 (2.1426 Å) are comparable with values reported but slightly shorter than the Sn-C (2.17 Å)
covalent radii [57, 58]. The packing diagram of [Ph2Sn(BDET)] (4) is given in figure 3. In the
crystal structure of 4 dimeric structures are formed by N-H···S hydrogen bonding interactions
between two neighboring molecules. Furthermore, the crystal packing structure of 4 is stabilized
by C-H... and ... interactions of the phenyl fragments in successive layers.
10

3.6. Antibacterial activity
The in vitro antibacterial properties of the ligand and diorganotin(IV) complexes were examined
against the strains of the Staphylococcus aureus, Enterobacter aerogenes, Escherichia coli and
Salmonella typhi and the results are reported in table 3. Test compounds are considered active
against the bacteria if the inhibition zone diameter is over 7 mm [59]. The free ligand showed
moderate activity, perhaps due to the presence of NH/OH groups in their structure which impart
biological activity [60]. Metal compounds having halogen groups at different positions of the
aromatic ring show more inhibitory effects [61]. Considering the N(4)-substituted ligand and its
complexes examined, complexes 2, 3 and 4 have more activities than the free ligand. 4 presented
higher inhibition to growth of organisms 28.1 29.8, 27.5 and 26.5 mm against S. Aureus, E. coli,
E. aerogenes and S. typhi, respectively, which represent the highest activity. Complex 2
exhibited second highest activity to four strains of bacteria. Perhaps the presence of bulky phenyl
or methyl groups promote binding to biological molecules via - interactions causing
improvement of their activities. This observation suggested that the activity of the studied
compound is dependent on the group attached to tin(IV) in the order Ph2  Me2  Bu2. The effect
of various compounds may then vary among microorganisms due to difference in the structures
of the cell. Metal chelation with a ligand, when compared with the free ligand improves
biological activities, may favor their permeation through the lipid layer of the cell membrane
[62, 63]. This antibacterial result is corroborated to those reported for other tin compounds,
wherein enhancement of activity has been described due to coordination into bacterial cells
[64-66]. These results also show that efficiency is enhanced by the bromine substituent on the
aromatic ring of the complexes compared with previously published papers [67, 68]. Results
were compared with those of Ciprofloxacin, which is commonly used as a reference in
antibacterial studies because of its high activity against bacteria [68].
4. Conclusion
5-Bromo-2-hydroxybenzaldehyde-N(4)-ethylthiosemicarbazone (H2BDET) is a tridentate ligand
coordinating to tin(IV) through azomethine nitrogen, thionic sulphur, and phenolic oxygen.
UV-Vis, IR, 1H, 13C and 119Sn NMR indicated five-coordinate geometries for these
organotin(IV) complexes. The X-ray crystallographic studies show distorted trigonal bipyramidal
Sn(IV) in 4. Coordination of N(4)-thiosemicarbazone ligand to Sn(IV) results in an enhancement
11

of their antibacterial activities. All complexes show antibacterial activities against four bacterial
strains in the order 4  2  3  1. The study showed all complexes were active but found to be
less active compared to the standard drug. 4 may represent a new strategy to prepare antibacterial
agents. Further investigations must be done to design new metal-based drugs in the future.
Supplementary data
CCDC reference number 1400811 contains the supplementary crystallographic data for
[Ph2Sn(BDET)] (4). This data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.ac.uk/data_request/cif or from the Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223-336-
033; or E-mail: deposit@ccdc.cam.ac.uk.
Acknowledgement
The authors thank Bangladesh Petroleum Exploration and Production Co., Ltd., Dhaka,
Bangladesh, for supporting this work.
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Graphical abstract
16

Table 1. Crystal data and structure refinement parameters for
[Ph2Sn(BDET)] (4).
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C22H20BrN3OSSn
573.07
100(2)
0.71073
Triclinic
P-1
Unit cell dimensions
a (Å)
8.5020(3)
b (Å)
11.2097(5)
12.3125(5)
71.8201(10)
78.5679(11)
72.1086(10)
1054.13(7)
2
c (Å)
 (°)
β (°)
 (°)
V (Å3)
Z
Calculated density (mg/m3)
Radiation type  (Å)
F(000)
1.805
M0K/α
564
Crystal size (mm)
Crystal color
0.320 × 0.208 × 0.134
Yellow
Scan range θ (°)
Absorption coefficient (µ) (mm-1)
Max. and min. trans.
Goodness-of-fit (GOF) on F2
Data / restraints / parameters
Final R indices [I> 2σ(I)]
R indices (all data)
2.26-30.26
3.225
0.425 and 0.672
1.034
6251 / 0 / 267
R1 = 0.0210, wR2 = 0.0466
R1 = 0.0262, wR2 = 0.0489
17

Table 2. Selected bond lengths (Å) and angles (°) of 4.
Bond lengths (Å)
Sn1-S1
2.5162(5)
O1-C1
N1-N2
N1-C7
N2-C8
N3-C8
N3-C9
C1-C2
1.323(2)
1.388(2)
1.304(2)
1.310(2)
1.345(2)
1.451(2)
1.406(3)
Sn1-O1
2.0710(14)
2.2068(14)
2.1333(18)
2.1427(18)
1.8978(18)
1.7467(17)
Sn1-N1
Sn1-C11
Sn1-C17
Br1-C4
S1-C8
Bond angles (°)
S1-Sn1-O1
N1-Sn1-C11
S1-Sn1-C11
S1-Sn1-C17
O1-Sn1-N1
O1-Sn1-C11
O1-Sn1-C17
S1-Sn1-N1
160.13(4)
112.77(6)
99.33(5)
96.95(5)
83.59(5)
94.76(6)
90.60(6)
78.09(4)
C11-Sn1-C17
Sn1-S1-C8
Sn1-O1-C1
Sn1-N1-N2
Sn1-N1-C7
N2-N1-C7
N1-N2-C8
C8-N3-C9
112.07(6)
96.17(6)
132.63(12)
121.92(11)
125.52(12)
112.53(14)
116.34(14)
122.73(17)
Table 3. Antibacterial activitiesa of free 1 and 2-4 (inhibition zone
in mm).
Bacterium
Samples
S. aureus
E. coli
11.4
24.7
22.8
29.8
34.5
E. aerogenes
10.1
S. typhi
9.2
1
2
3
4
R
13.2
26.4
26.7
25.4
24.3
26.5
33.8
20.6
25.6
28.1
27.5
33.9
31.8
a Concentration used: 2 mg/ml of DMSO, R = standard drug:
ciprofloxacin
18

Figure 1. Electronic spectra of 1-4 in DMSO (1×10-4 M).
Figure 2. Molecular structure of [Ph2Sn(BDET)] (4) showing displacement ellipsoids at the 50%
probability level.
19

Figure 3. The packing diagram of [Ph2Sn(BDET)] (4) in the crystal lattice, viewed along the c
axis.
20