Synthesis, spectroscopic characterization, DFT calculations, and antimicrobial activities of N-arylsalicylaldiminate derivatives of diorganotin(IV)

Article abstract of DOI:10.1080/00958972.2019.1689563

Equimolar reaction of di-n-butyltin(IV) complexes of bidentate Schiff bases of the type [(n-Bu)₂Sn(sb)Cl] with sodium salt of mono-functional bidentate ligands in THF-benzene solution afforded structurally interesting complexes of the type [(n-

Full text of DOI:10.1080/00958972.2019.1689563

Journal of Coordination Chemistry
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Synthesis, spectroscopic characterization, DFT
calculations, and antimicrobial activities of N-
arylsalicylaldiminate derivatives of diorganotin(IV)
Raj Kumar Dubey, Avadhesh Pratap Singh, Nitesh Jaiswal, Divya Pratap
Singh, Meenakshi, Ajeet Kumar Kushwaha, Manoj Kumar & Afreen Anjum
To cite this article: Raj Kumar Dubey, Avadhesh Pratap Singh, Nitesh Jaiswal, Divya
Pratap Singh, Meenakshi, Ajeet Kumar Kushwaha, Manoj Kumar & Afreen Anjum (2019)
Synthesis, spectroscopic characterization, DFT calculations, and antimicrobial activities of N-
arylsalicylaldiminate derivatives of diorganotin(IV), Journal of Coordination Chemistry, 72:19-21,
3371-3384, DOI: 10.1080/00958972.2019.1689563
To link to this article: https://doi.org/10.1080/00958972.2019.1689563
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JOURNAL OF COORDINATION CHEMISTRY
2019, VOL. 72, NOS. 19–21, 3371–3384
https://doi.org/10.1080/00958972.2019.1689563
Synthesis, spectroscopic characterization, DFT
calculations, and antimicrobial activities of
N-arylsalicylaldiminate derivatives of diorganotin(IV)
Raj Kumar Dubeya†, Avadhesh Pratap Singha,b, Nitesh Jaiswala,c,
Divya Pratap Singhd, Meenakshia, Ajeet Kumar Kushwahaa, Manoj Kumara and
Afreen Anjuma
aSynthetic Inorganic and Metallo-Organic Research Laboratory, Department of Chemistry, University
b
of Allahabad, Allahabad, India; Department of Chemistry, Kamla Nehru Institute of Physical and
c
Social Sciences, Sultanpur, India; Department of Chemistry, Prof. Rajendra Singh (Rajju Bhaiya)
Institute of Physical Sciences for Study & Research, VBS Purvanchal University, Jaunpur, India;
dDepartment of Chemistry, Rajkiya Engineering College, Deogaon, Azamgarh, India
ABSTRACT
ARTICLE HISTORY
Received 14 August 2018
Accepted 19 October 2019
Equimolar reaction of di-n-butyltin(IV) complexes of bidentate Schiff
bases of the type [(n-Bu)2Sn(sb)Cl] with sodium salt of mono-functional
bidentate ligands in THF-benzene solution afforded structurally interest-
ing complexes of the type [(n-Bu)2Sn(sb)(L)] (1–6) [where sbH¼ Schiff
bases: N-salicylidene-2-aminopyridine (sapH) I, N-salicylidene-2-methyla-
minobenzene (o-smabH) II, and N-salicylidene-4-methylaminobenzene
(p-smabH) III; LH¼ mono-functional bidentate ligands, acetylacetone
(acacH), ethanolamine (eaH)]. All these colored solid complexes were
soluble in common organic solvents and characterized by elemental (C,
H, N, and Sn) analysis, spectroscopic techniques [IR, (1H, 13C, and 119Sn)
NMR] and mass spectrometry. Thermogravimetric analysis of complexes
shows thermal behavior and stability of complexes. Computational
studies of the synthesized Schiff bases and their organotin(IV) com-
plexes were carried out using DFT which validate the structure of com-
plexes proposed on the basis of spectroscopic data. The mixed-ligand
complexes of diorganotin(IV) and Schiff bases were screened for their
antibacterial and antifungal activities.
KEYWORDS
Schiff bases; organotin(IV)
complexes; 119Sn NMR;
DFT; biological activity
CONTACT Avadhesh Pratap Singh
avadheshchem@ymail.com; dpsingh_011@yahoo.co.in
Department of
Chemistry, Kamla Nehru Institute of Physical and Social Sciences, Sultanpur-228118, India
†Deceased November 2018
Supplemental data for this article can be accessed here.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

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R. K. DUBEY ET AL.
1. Introduction
Metal complexes are versatile molecules due to the inherent characteristics of both
metal center and ligand, providing for a wide range of applications. The N-, O-, and S-
donor ligands categorized under Schiff bases are playing a vital role in reconnoitering
new dimensions of coordination chemistry of tin complexes [1–3]. Tin complexes of
Schiff base derivatives are getting momentous attention due to its important applica-
tions. Instead of being toxic in bulk, tin complexes at molecular level in trace amounts
play a very crucial role in biological systems [4–8]. Recently, the chemistry of
organotin(IV) complexes of Schiff bases has also shown significant potential for their
antitumor, antimicrobial, antinematicidal, antiinsecticidal, and antiinflammatory activ-
ities [9, 10]. Beside this, organotin(IV) compounds have been paid reasonable attention
currently due to their biocompatibility with DNA and cells at molecular level [11–15].
In view of the structural variety and the potential applications of organotin com-
pounds in medicinal chemistry and biotechnology, it was considered worthwhile to syn-
thesize the mixed-ligand organotin(IV) complexes with salicylaldehyde-derived Schiff
bases to investigate the interaction of these compounds with biological systems. In con-
tinuation of our studies on synthesis and characterization of tin(IV) and organotin(IV)
complexes, we have recently reported the synthesis, structural, and DNA cleavage prop-
erties of a series of diorganotin(IV) complexes [16]. Furthermore, studies on organotin(IV)
derivatives containing O,N- and O,O-bidentate ligands having possibility of structural
diversity and our pioneering work [17, 18] on tin(IV) and organotin(IV) chemistry has
earlier focused mainly on the use of N-arylsalicylaldimine Schiff bases as effective and
versatile coordinating ligands. This encouraged us to synthesize mixed-ligand complexes
of organotin(IV), which may lead to complexes with different properties.
We have selected the N-arylsalicylaldimine Schiff bases and mono-functional biden-
tate ligands as the suitable chelating unit [19], which result in attractive molecular
architecture with dibutylorganotin(IV) chloride through their versatile bonding nature
either g1 or g2. We report in this paper for the first time the reaction of Schiff base
complexes of di-n-butyltin(IV) dichloride of the type [(n-Bu)2Sn(sb)Cl] containing chlor-
ide as reactive species with acetyl acetone (acacH) and ethanolamine (eaH) and
resulted complexes were characterized by various spectroscopic techniques. The com-
putational calculation of complexes as well as ligands was also performed. The synthe-
sized mixed-ligand complexes of diorganotin(IV) and Schiff bases were investigated for
their subsequent applications in antibacterial and antifungal activities.
2. Experimental
2.1. Materials and physical measurements
All the chemicals used throughout the present course of experimental work were of AR
grade. The solvents used were of reagent grade and purified by standard procedures
[20]. Acetylacetone and ethanolamine from Merck were distilled prior to use. Di-n-
butyltin(IV) dichloride, o-toluidine, 2-aminopyridine, and salicylaldehyde from Sigma
Aldrich were used as they were received. Tin was estimated gravimetrically [21] as SnO2,
after decomposition with HNO3. A Euro-E 3000 elemental analyzer was used for elemental

JOURNAL OF COORDINATION CHEMISTRY
3373
analysis. A Perkin-Elmer 100 FT-IR spectrometer (4000–400 cmꢀ1) was used for recording
infrared spectra of complexes. 1H and 13C NMR spectra were recorded on a Bruker DRX-
300 spectrometer and chemical shifts were given in ppm relative to Me4Si for carbon and
hydrogen in CDCl3. 119Sn NMR spectra were recorded on a JEOL-500 spectrometer and
chemical shifts were given in ppm relative to Me4Sn in CDCl3. ESI-MS spectra were
recorded on Agilent 6520 Q-Tof LCMS, MS/MS spectrometers, respectively. A TGA
Q500 instrument was used for thermal analysis of complexes. The DFT calculations for
geometry optimization were performed with the help of Gaussian 09 quantum
mechanical software, employing a widely used hybrid exchange-correlation functional
B3LYP and LANL2DZ basis set to describe all the atoms including Sn metal [21].
2.2. In vitro microbial activity measurement
The antimicrobial test of the Schiff bases and mixed-ligand organotin(IV) complexes (1–6)
was carried out using the agar well diffusion method [22]. Complexes 1–6 were tested for
the inhibitory effect on growth of different gram bacteria such as Escherichia coli,
Pseudomonas aeruginosa (as Gram-negative bacteria), and Staphylococcus aureus,
Staphylococcus epidermidis (as Gram-positive bacteria). The concentration of reference,
ligands, and diorganotin(IV) complexes is 1 mg/mL in DMSO for antibacterial studies. The
antifungal activity of the synthesized organotin complexes has been tested against
Aspergillus niger, Candida albicans, Trichophyton rubrum, and Microsporum. The concentra-
tion of reference, ligands and, diorganotin(IV) complexes is 100lg/mL in DMSO for antifun-
gal studies. Nutrient agar media was prepared and autoclaved. Each bacterial/fungal
species was inoculated by pour plate method (i.e. 20lL/15 cm3 of media) to get a confluent
growth. The media were poured in sterile and autoclaved petriplates and allowed to solid-
ify for 1 h. When the media were solidified wells were made in it and marked as 1 (positive
control i.e. tertracycline with conc. 50lL/cm3), 2 (negative control i.e. distilled water), 3 and
4 (different extracts with conc. 400mg/cm3). 30 lL of each sample (i.e. controls and
extracts) was loaded in respective wells. The plates were then incubated at 37ꢁC overnight.
After 24h, the antibacterial/antifungal activities were expressed in terms of the diameter of
the zone of inhibition (in mm) of each bacterial species by different samples.
2.3. Synthesis of Schiff bases
The Schiff base ligands (LH) were synthesized according to the procedure described in
our earlier publications [10, 18]. A solution of salicylaldehyde (1.54 g, 12.61 mmol) in
methanol was added to a methanolic solution of 2-aminopridine (1.18 g, 12.61 mmol)
with constant stirring. The resulting reaction mixture was refluxed for 4 h and left over-
night. Yellow crystals were obtained which were recrystallized from methanol.
2.4. Synthesis of complexes
The precursor of the reaction, [n-Bu2Sn(sb)Cl], was synthesized by the interactions of
di-n-butyltin(IV) dichloride with sodium salts of Schiff bases in equimolar ratio. The
reaction of [n-Bu2Sn(sb)Cl] with mono-functional bidentate ligands (LH) in equimolar

3374
R. K. DUBEY ET AL.
ratio afforded structurally interesting mixed-ligand complexes of type [(n-Bu)2Sn(sb)(L)]
(1–6). Similar procedure was used for the preparation of 1–6, therefore, general pre-
parative details are given for 1: To a THF (ꢂ20 mL) solution of [n-Bu2Sn(sap)Cl] (1.92 g,
4.12 mmol), sodium salt of mono-functional bidentate ligands [prepared by the reac-
tion of equimolar amounts of sodium metal (0.094 g, 4.12 mmol) and a mono-
functional bidentate ligand, acetyacetone (0.412 g, 4.12 mmol) in THF (ꢂ30 mL)] in 1:1
molar ratios was added dropwise with constant stirring. The reaction mixture was
refluxed for ꢂ5 h. The precipitated NaCl was removed by filtration and the solvent
was removed by distillation. The solid products were dried under reduced pressure
and recrystallized from a mixture of THF/n-hexane (20:80) at ꢀ20 ꢁC.
2.4.1. [(n-Bu)2Sn(sap)(acac)] (1)
Yield: 69%, brown solid, m.p.135–138 ꢁC, MW: 529.26; Elem. Anal. for C25H36N2O3Sn,
Calcd: C, 56.82; H, 6.96; N, 5.41; Sn, 22.53. Found: C, 56.52; H, 6.83; N, 5.12; Sn, 22.34%. 1H
NMR (CDCl3), d (ppm): 9.69 (1H, s, CH ¼ N); 8.09 (1H, d, Py–H); 6.72–7.81 (7H, m, Ar, Py);
1.28–1.61 (12H, m, Sn–(CH2)3–); 0.93 (6H, t, CH3); 3.51 (3H, s, O¼C–CH3); 2.36 (3H, br,
–O–C–CH3); 5.39 (1H, ¼CH–). 13C NMR (CDCl3) d (ppm): 159.2 (1C, CO); 157.9 (s, CN); 148.7,
135.2, 134.9, 132.5, 131.9, 130.1, 125.2, 120.5, 115.9, 114.7 (10C, Ar–C, Py–C); 180.6
(–C¼O); 189.9 (–C–O); 81.6 (1C, –CH¼); 25.6, 16.9 (2C, –CH3); 32.8, 26.3, 21.9, 13.8 (8C,
Sn–Bun). 119Sn NMR (CDCl3) d (ppm): ꢀ244.5. IR (cmꢀ1): 1610 v(C¼N), 1579, 1525 v(C¼O);
683, 656 v(Sn–C); 575, 552, 533 v(Sn–O); 429 v(Sn–N). ESI-MS (m/z): 531.1.
2.4.2. [(n-Bu)2Sn(sap)(ea)] (2)
Yield: 76%, brown solid, m.p. 120–121 ꢁC, MW: 490.23; Elem. Anal. for C22H33N3O2Sn, Calcd:
1
C, 53.90; H, 6.79; N, 8.57; Sn, 24.22. Found: C, 53.69; H, 6.48; N, 8.45; Sn, 24.10%. H NMR
(CDCl3) d (ppm): 9.89 (1H, s, CH ¼ N); 7.99 (1H, d, Py–H); 7.68–6.64 (7H, m, Ar, Py); 1.26–1.78
(12H, m, Sn–(CH2)3–); 0.85 (6H, t, CH3); 3.66 (2H, t, O–CH2–); 2.66 (2H,br, N–CH2); 2.52 (1H,
NH2). 13C NMR (CDCl3) d (ppm): 157.8 (s, CO); 156.7 (s, CN); 147.4, 137.9, 136.9, 134.4, 132.4,
130.2, 128.3, 126.3, 121.1, 115.6 (10C, Ar–C, Py–C); 49.0 (1C, –O–CH2); 31.8 (1C, –N–CH2);
27.1, 24.1, 21.5, 13.0 (8C, Sn–Bun). 119Sn NMR (CDCl3) d (ppm): ꢀ247.0. IR (cmꢀ1): 1614
v(C¼N); 677, 640 v(Sn–C); 525, 552 v(Sn–O); 450, 435 v(Sn–N). ESI-MS (m/z): 490.1.
2.4.3. [(n-Bu)2Sn(o-smab)(acac)] (3)
Yield: 69%, brown solid, m.p.127–129 ꢁC, MW: 544.31; Elem. Anal. for C27H39NO3Sn,
Calcd: C, 59.58; H, 7.22; N, 2.57; Sn, 21.81. Found: C, 59.38; H, 7.06; N, 2.36; Sn, 21.57%.
1H NMR (CDCl3) d (ppm): 9.49 (1H, s, CH¼N); 8.81–7.72 (8H, m, Ar–H); 1.52–1.31 (12H,
m, Sn–(CH2)3–); 0.90 (6H, t, CH3); 2.21 (3H, s, Ar–CH3); 1.51 (3H, s, O¼C–CH3); 2.26 (3H,
s, –O–C–CH3); 5.39 (1H, ¼CH–). 13C NMR (CDCl3) d (ppm): 163.2 (s, CO); 159.5 (s, CN);
147.8, 141.6, 139.2, 136.2, 133.9, 130.2, 128.5, 124.8, 119.9, 115.1, 113.5 (11C, Ar–C);
182.1 (–C ¼ O); 190.9 (–C–O); 80.1 (1C, d, –CH¼); 24.9, 16.8 (2C, q, –CH3); 31.8, 25.1,
22.1, 14.5 (8C, Sn–Bun). 119Sn NMR (CDCl3) d (ppm): ꢀ243.8. IR (cmꢀ1): 1608 v(C ¼ N);
1590, 1535 v(C¼O); 675, 680 v(Sn–C); 529, 557, 575 v(Sn–O); 447 v(Sn–N). ESI-MS (m/
z): 543.6.

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3375
2.4.4. [(n-Bu)2Sn(o-smab)(ea)] (4)
Yield: 79%, Yellow solid, m.p. 119–121 ꢁC, MW: 503.26; Elem. Anal. for C24H36N2O2Sn, Calcd:
1
C, 57.28; H, 7.21; N, 5.57; Sn, 23.59. Found: C, 57.09; H, 7.01; N, 5.35; Sn, 23.10%. H NMR
(CDCl3) d (ppm): 9.61 (1H, s, CH¼ N; 7.95–6.85 (8H, m, Ar–H); 1.65–1.21 (12H, m, Sn–(CH2)3–);
0.88 (6H, t, CH3); 2.34 (3H, s, Ar–CH3); 3.36 (2H, t, O–CH2–); 2.48 (2H, t, N–CH2); 2.63(1H, NH2).
13C NMR (CDCl3) d (ppm): 163.2 (1C, CO) 158.3 (s, –C5N); 146.8, 139.1, 135.2, 133.8, 130.9,
129.0, 125.1, 124.9, 121.8, 118.5, 114.9 (11C, Ar–C); 46.9 (1C, –O–CH2); 33.1 (1C, –N–CH2);
29.0, 23.5, 19.5, 13.2 (8C, Sn–Bun). 119Sn NMR (CDCl3) d (ppm): ꢀ248.4. IR (cmꢀ1): 1607
v(C ¼ N); 642, 675 v(Sn–C); 530, 548 v(Sn–O); 454, 429 v(Sn–N). ESI-MS (m/z): 502.9.
2.4.5. [(n-Bu)2Sn(p-smab)(acac)] (5)
Yield: 74%, dark yellow solid, m.p. 115–118 ꢁC, MW: 544.31; Elem. Anal. for C25H38N2O2Sn,
Calcd: C, 59.58; H, 7.22; N, 2.57; Sn, 21.81. Found: C, 59.12; H, 7.09; N, 2.41; Sn, 21.13%. 1H
NMR (CDCl3) d (ppm): 9.48 (1H, s, CH ¼ N); 8.14–7.15 (8H, m, Ar–H); 1.59–1.21 (12H, m,
Sn–(CH2)3–); 0.90 (6H, t, CH3); 2.29 (3H, s, Ar–CH3); 3.55 (2H, t, O–CH2–); 2.49 (2H, t, N–CH2);
2.45 (1H, NH2). 13C NMR (CDCl3) d (ppm): 163.0 (s, CO); 158.7 (1C, –C¼N); 148.0, 141.5,
138.7, 135.4, 132.5, 129.4, 124.5, 120.7, 116.9, 115.6, 113.1 (11C, Ar–C); 188.9 (–C–O); 81.5
(1C, –CH¼); 23.9, 16.2 (2C, –CH3); 29.1, 24.5, 21.7, 14.8 (8C, Sn–Bun). 119Sn NMR (CDCl3) d
(ppm): ꢀ245.1. IR (cmꢀ1): 1610 v(C¼N); 1568, 1531 v(C¼O); 651, 675 v(Sn–C); 511, 529,
552 v(Sn–O); 424 v(Sn–N). ESI-MS (m/z): 543.9.
2.4.6. [(n-Bu)2Sn(p-smab)(ea)] (6)
Yield: 71%, brown solid, m.p. 130–132 ꢁC, MW: 503.26; Elem. Anal. for C24H36N2O2Sn,
Calcd: C, 57.28; H, 7.21; N, 5.57; Sn, 23.59 Found: C, 57.10; H, 1.36; N, 7.45; Sn, 23.22%.
1H NMR (CDCl3) d (ppm): 9.62 (1H, s, CH ¼ N); 7.65–6.95 (8H, m, Ar); 1.69–1.30 (12H, m,
Sn–(CH2)3–); 0.88 (6H, t, CH3); 2.27 (3H, s, Ar–CH3); 3.51 (2H, t, O–CH2–); 2.56 (2H, br,
N–CH2); 2.39 (1H, NH2). 13C NMR (CDCl3) d (ppm): 159.2 (1C, CO); 157.9 (1C, –C¼N);
146.8, 136.1, 135.9, 134.7, 131.3, 130.4, 123.3, 121.1, 114.9, 115.6 (11C, Ar–C); 45.3 (1C,
–O–CH2); 32.5 (1C, –N–CH2); 27.7, 23.1, 20.9, 13.8 (8C, Sn–Bun). 19Sn NMR (CDCl3) d
(ppm): ꢀ246.9. IR (cmꢀ1): 1612 v(C¼N); 651, 678 v(Sn–C); 559, 532 v(Sn–O); 449, 429
v(Sn–N). ESI-MS (m/z): 502.2.
3. Results and discussion
Complexes 1–6 have been synthesized by the reaction of N-arylsalicylaldiminate deriv-
atives of di-n-butyltin(IV) chloride with sodium salts of mono-functional bidentate
ligands in 1:1 ratio in THF-C6H6 mixture (Scheme 1). All these complexes are colored
solids, soluble in polar solvents (e.g. methanol, ethanol, THF, DMF, and DMSO).
3.1. Infrared spectroscopy
The IR spectra of mixed-ligand complexes of di-n-butyltin(IV) dichloride provide useful
information for assigning the coordination mode of Schiff bases and monofunctional
bidentate ligands to the tin atom. Salient characteristic of IR spectra is the disappear-
ance of stretching vibration bands in the region 3430–3458 cmꢀ1 due to phenolic

3376
R. K. DUBEY ET AL.
(OH) group of Schiff bases which is consistent with metallation of phenolic hydrogen
on complexation [15]. The strong and sharp bands characteristic of ꢀ(C ¼ N) in the
region 1634–1616 cmꢀ1 observed in the free Schiff base ligands were shifted to lower
frequencies (ꢂ31–11 cmꢀ1) in complexes [18].
The disappearance of a band at 1680 cmꢀ1 is due to m(C¼O) group of free acetylacetone
and appearance of bands at 1583 8 cmꢀ1 and 1530 10 cmꢀ1 are assigned to v(C …__ C)
coupled with v(C …__ O) and v(C …__ O) coupled with v(C …__ C), respectively, that is consistent
with the involvement of C ¼ O group in the chelate formation in 1, 3, and 5 [23, 24].
Free ethanolamine in the liquid state gives a strong band at 3340 cmꢀ1, which is
attributed to the ꢀ(O–H) mode while the two other bands appearing at 3290 and
3160 cmꢀ1 are assigned to the asymmetric and symmetric N–H stretching modes,
respectively. The disappearance of a band at 3340 cmꢀ1 and shifting of N–H stretching
frequencies in the lower frequency region in 2, 4, and 6 clearly indicates coordination
of tin to the oxygen and nitrogen atom of the NH2 group [25, 26].
The new strong to medium absorption bands appear in the range 510 60 cmꢀ1
,
which should be assigned to the ꢀ(Sn–O) stretching mode according to the literature
[22–24]. The results prove that the hydroxyl group is deprotonated and coordinated to
the central tin atom. Further appearance of medium intensity bands 430 30 cmꢀ1
and strong to medium absorption bands 690 15 cmꢀ1 were attributed to ꢀ(Sn–N)
and ꢀ(Sn–C), respectively [22–26] (representative IR spectrum of 2 in Supplementary
material Figure S1).
1
3.2. H NMR spectral studies
1H NMR spectra of 1–6 exhibit no signal due to phenolic O–H of Schiff bases in the
region 13.25–12.75 ppm due to metallation of phenolic O–H. In the spectra of com-
plexes slightly downward chemical shift values in the region 9.89–9.48 ppm for
–CH¼N– protons may arise due to participation of azomethine nitrogen in chelation
compared to that of the parent Schiff bases in the region 9.23–8.23 ppm [19].
Complexes 1, 3, and 5 show signals in the region 2.07–1.94 ppm and 5.50–5.48 ppm
due to methyl and methine protons of acetylacetonate moiety, indicating the biden-
tate bonding mode of acetylacetonate [24]. The absence of a signal at 5.41 ppm corre-
sponding to O–H group of ethanolamine supports its coordination to tin atom via
deprotonation in 2, 4, and 6 [26]. In 2, 4, and 6, signals due to NH2 group were
observed in the region 2.54–2.53 ppm. The resonance signals in the range
3.63–3.59 ppm and 2.56–2.51 ppm were assigned to CH2–O and CH2–N protons of
ethanolamine, respectively [25], in 2, 4, and 6. The characteristic aromatic signatures in
1–6 were observed in the range 7.17–8.06 ppm. Additionally, 3–6 displayed a sharp
1
signal at 1.19 ppm assigned to CH3 protons (representative H NMR spectrum of 2 in
Supplementary material Figure S2).
3.3. 13C NMR spectral studies
13C NMR spectra of 1–6 exhibit signals due to azomethine carbon in the region
160.5–158.9 ppm, shifted downfield compared to that of parent Schiff bases

JOURNAL OF COORDINATION CHEMISTRY
3377
(158.3–156.5 ppm), indicating coordination of azomethine nitrogen to tin. Signals at
163.8–161.1 ppm support formation of Sn–O–C bond and deprotonation of phenolic
group. In 1, 3 and 5, two signals for the carbonyl carbon atoms of the acetylacetonate
moiety are observed in the region 190.31–189.55 ppm and two signals for the methyl
carbon of acetylacetonate have also been observed in the region 25.76–25.38 ppm.
The appearance of two signals each for carbonyl carbon and methyl carbon for 1, 3,
and 5 supports the bidentate chelating nature of the acetylacetonate moiety [24]. In
2, 4, and 6, signals due to the –CH2–O and –CH2–N appear slightly downfield region
in the region 49.0–45.3 and 33.1–31.8 ppm, respectively, compared to free ethanol-
amine at 63.7 and 43.8 ppm, respectively, indicating bidentate coordinating mode of
ethanolamine [27] (representative 13C NMR spectrum of 2 in Supplementary material
Figure S3).
3.4. 119Sn NMR spectral studies
To provide further evidence to establish the structure of the complexes in solution,
119Sn NMR spectra were recorded. It is well known that 119Sn chemical shifts are very
sensitive to changes in the coordination number of tin and to the nature of groups
directly attached to the tin atom. Complexes 1–6 exhibit a single sharp 119Sn reson-
ance (see Experimental Section) in the region ꢀ243.8 to ꢀ248.4 ppm, revealing the
six-coordinate geometry around tin atom [26] (representative 119Sn NMR spectrum of
2 in Supplementary material Figure S4).
3.5. Mass spectral studies
The mass spectra of 1–6 exhibit molecular ion peaks (m/z) which correspond to the
molecular composition of the complexes. In the mass spectra of complexes, most of
the fragments were observed as group of peaks due to various isotopes of tin. One
representative mass spectrum (Supplementary material Figure S5) and its fragmenta-
tion pattern [16–18] (Scheme 2) with m/z for 1, [(n-Bu)2(sap)(acac)], has been sug-
gested. In the spectrum of 1, the molecular peak was observed at m/z 531.1
[calculated mass ¼ 531.27]. The calculated isotopic distribution for molecular ion peak
at m/z 531.2, 485.3, 467.3, and 425.3 for 1 are shown in Supplementary material
Figure S6. The comparison of the calculated (bars) isotopic distribution for molecular
ion peak and other important peaks with the mass spectrum of 1 confirms the
molecular composition of the complex.
3.6. Computational studies
Since we were unable to grow suitable single-crystal for X-ray studies, computational
method has been incorporated to derive properties of the molecule based on a determin-
ation of the electron density of the molecule. DFT calculations were carried out for the
ligands and their complexes in gas phase to fully optimize the ground state structure.
Molecular structure of ligand and organotin(IV) complex with atom labeling scheme is

3378
R. K. DUBEY ET AL.
Scheme 1. Synthetic route for the preparation of mixed-ligand diorganotin complexes.
shown in Figure 1 and selected bond lengths and angles are summarized in Table 1. The
optimized energy, dipole moment and energy band gap values are summarized in Table 2.
The theoretically calculated values of different bond lengths (Sn–O, Sn O, Sn–C,
Sn–N and Sn N) in complexes are in close agreement to the values reported [28, 29]

JOURNAL OF COORDINATION CHEMISTRY
3379
Figure 1. The ground state optimized geometry for ligands (I-III) and complexes (1–6) at B3LYP/
LANL2DZ level.
for diorganotin(IV) complexes of N,O-donor ligands in distorted octahedral system. In
all complexes, the axial positions to the Sn atom are occupied by two n-butyl group
while the four equatorial positions to the Sn atom are engaged with N,O-donor Schiff
base and mono-functional bidentate ligands [11, 29]. In 1, 3, and 5, the presence of
one phenolate and acetylacetonate ion as a result of deprotonation of Schiff base (LH)
and mono-functional bidentate ligands acetylacetone (acacH) indicates covalent bond-
ing via Sn–O(22) and Sn–O(26) and coordinate bonding through Sn N(20) and
Sn O(25). Similarly in 2, 4 and 6, mono-functional bidentate ligand ethanolamine ren-
ders bonding through Sn–O(22), Sn–O(25) covalent and Sn N(20), Sn N(53) coordin-
ate mode. The value of the Sn–O distance, in the range 2.04–2.30 Å and Sn–N distance
in the range 2.47–2.37 Å as well as the other bond distances and angle are in good

3380
R. K. DUBEY ET AL.
Table 1. Selected structural parameters for [(n-Bu)2Sn(sap)(acac)] and [(n-Bu)2Sn(sap)(en)] at
B3LYP/LanL2DZ level of theory.
Bond length (Aꢁ) [(n-Bu)2Sn(sap)(acac)]
Bond angle (ꢁ) [(n-Bu)2Sn(sap)(acac)]
Sn-N20
Sn-O22
Sn-O25
Sn-O26
Sn-C40
2.47795
C43-Sn-N20
88.57330
74.52955
97.86648
94.38568
76.33859
83.67905
132.7432
138.6650
116.3626
116.6345
129.2021
144.8220
105.3988
2.04253
2.30459
2.10504
2.13154
2.13226
1.39970
1.28340
1.30822
1.28497
1.26938
O22-Sn-N20
O22-Sn-C40
C40-Sn-O26
O26-Sn-O25
O25-Sn-C43
Sn-O25-C30
Sn-O26-C31
Sn-C43-C56
Sn-C40-C46
Sn-N20-C19
Sn-O22-C2
Sn-C43
C11-N20
C19¼N20
C2-O22
C31-O26
O25¼C30
Sn-N20-C11
Bond length (Aꢁ) [(n-Bu)2Sn(sap)(ea)]
Bond angle (ꢁ) [(n-Bu)2Sn(sap)(ea)]
Sn-N20
Sn-N53
Sn-O22
Sn-O25
Sn-C27
Sn-C30
2.42890
C27-Sn-C30
C27-Sn-N20
C27-Sn-N20
C27-Sn-N53
C30-Sn-N20
C30-Sn-N53
C27-Sn-O25
C27-Sn-O22
C30-Sn-O25
C30-Sn-O22
O22-Sn-N53
O25-Sn-N53
O25-Sn-N20
O22-Sn-N20
178.2605
91.4404
91.4404
91.9699
86.9436
89.7361
89.4851
88.2459
91.2453
91.91362
73.61931
76.3533
136.5452
73.6462
2.37778
2.25885
2.08783
2.16600
2.17188
Table 2. The computed electronic properties for ligands and complexes.
Complex
Total energy
DM
HOMO
LUMO
HOMO-LUMO gap
sapH (I)
ꢀ648.192
ꢀ1302.408
ꢀ1168.19
ꢀ671.30
1.845
4.650
9.909
2.608
1.5565
2.680
2.458
2.1578
2.3934
ꢀ0.2240
ꢀ0.2794
ꢀ0.2895
ꢀ0.2049
ꢀ0.2065
0.2857
ꢀ0.2198
ꢀ0.2847
ꢀ0.2895
ꢀ0.0799
ꢀ0.0561
ꢀ0.0661
ꢀ0.0501
ꢀ0.0639
ꢀ0.0611
ꢀ0.0694
ꢀ0.0824
ꢀ0.0707
0.1441
0.2233
0.2234
0.1548
0.1417
0.2246
0.1504
0.2023
0.2188
[(n-Bu)2Sn(sap)(acac)] (1)
[(n-Bu)2Sn(sap)(ea)] (2)
o-smabH (II)
[(n-Bu)2Sn(o-smab)(acac)] (3)
[(n-Bu)2Sn(o-smab)(ea)] (4)
p-smabH (III)
ꢀ1334.71
ꢀ1191.16
ꢀ671.47
[(n-Bu)2Sn(p-smab)(acac)] (5)
[(n-Bu)2Sn(p-smab)(ea)] (6)
ꢀ1325.84
ꢀ1191.25
agreement, for 1 and 2, with the values reported for Sn–complexes using X-ray diffrac-
tion [11, 29]. The slight deviations in the values of bond angles from perfect octahe-
dral geometry suggest distorted octahedral geometry around tin [7].
In order to know the stability of the complexes, energy gaps (DE ¼ ELUMOꢀEHOMO
)
of complexes have been calculated. The stability of the complexes is proportional to
the energy band gap [15]. The highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) of ligand I and 1 and 2 are also shown
in Figure 2. The comparison of total energy (E) and energy gap (DE) of ligands and
complexes also suggest stability of complexes. The HOMO-LUMO energy gap of 1 and
2 (0.223 au/6.076 eV and 0.2234 au/6.078 eV, respectively) is higher than corresponding

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3381
Figure 2. Energy diagram of frontier molecular orbitals HOMO and LUMO of ligand I and
complexes 1 and 2 derived from DFT calculations using B3LYP/LANL2DZ level of theory.
ligand I (0.1441 au/3.921 eV). The obtained energy gap (DE) values of optimized
complexes suggest the following order of stability, that is, 4 > 2 > 1 > 6 > 5 > 3.
3.7. Thermal studies
The thermal stability of 1 and 2 was investigated through thermogravimetric analyses
in the temperature range 30–600 ꢁC at a heating rate of 10 ꢁC/min under atmospheric
conditions. The thermograms (Supplementary material Figure S7) of the mixed-ligand
organotin complexes show decomposition during thermal analysis. The first step, at
30–180 ꢁC, involves the loss of two butyl groups with mass loss of 21.2% (calculated
mass loss 21.5%) and 22.9% (calculated mass loss 23.3%) for 1 and 2, respectively.
From 180–260 ꢁC, loss of salicylidimanate moiety corresponds to mass loss of 58.5%
(calculated mass loss 58.8%) and 63.1% (calculated mass loss 63.5%) for 1 and 2,
respectively. Further increase up to 600 ꢁC gives final residue of SnO corresponding to
24.9%, which is in good agreement with calculated value of 25.3% for 1 and 26.5%
compared to calculated value 27.4% for 2.
3.8. In vitro antibacterial and antifungal activity of organotin complexes
The biological activity of the ligands I–III and organotin(IV) complexes 1–6 was carried
out against different strain of bacteria such as E. coli, S. aureus, P. aeruginosa, S.
epidermidis and fungi A. niger, C. albicans, T. rubrum, Microsporum. The diffusion agar
technique was used to assess the antibacterial activity of the synthesized ligands and
mixed-ligand organotin complexes. The antibacterial and antimicrobial activity data
obtained for the ligands and organotin(IV) complexes are summarized in Tables 3
and 4.
Comparison of antimicrobial activities of organotin(IV) complexes 1–6 with the par-
ent Schiff bases I–III showed that the organotin(IV) complexes displayed higher activ-
ity against the same microorganism compared to the parent Schiff base under
identical experimental condition [30].

3382
R. K. DUBEY ET AL.
Table 3. Antibacterial activitya (diameter of inhibition zone) of Schiff bases and their mixed-ligand
diorganotin(IV) derivatives.
Inhibition zone diameter (mm)
Bacterium
I
II
0.52
0.41
0.48
III
1
2
3
4
5
6
Rb
E. coli
14 0.32
6
7
6
8
9
7
0.13 17 0.33 19 0.28 15 0.43 17 0.28 14 0.25 16 0.32 22 0.05
0.17 15 0.28 18 0.32 12 0.26 14 0.26 11 0.28 13 0.35 22 0.07
0.14 18 0.36 19 0.36 10 0.37 16 0.51 12 0.29 15 0.38 20 0.08
P. aeruginosa 13 0.40
S. aureus 17 0.26
S. epidermidis 9 0.36 12 0.38 13 0.19 15 0.30 15 0.34 16 0.29 19 0.46 14 0.31 18 0.32 22 0.07
aIn vitro, agar well diffusion method, conc. 1 mg/mL in DMSO.
bR is reference drug, Tetracyclin.
Table 4. Antifungal activitya (diameter of inhibition zone) of Schiff bases and their mixed-ligand
diorganotin(IV) derivatives.
Inhibition zone diameter (mm)
Bacterium
I
II
III
1
2
3
4
5
6
Rb
A. niger
11 0.26 12 0.31 13 0.30 14 0.32 17 0.29 15 0.32 16 0.34 15 0.39 17 0.37 23 0.05
C. albicans 13 0.21
9
0.29
6
8
0.32 15 0.36 18 0.27 13 0.29 17 0.36 13 0.41 15 0.38 23 0.05
0.27 10 0.34 13 0.26 14 0.27 18 0.36 12 0.38 16 0.31 22 0.06
T. rubrum 0.21 12 0.28
7
Microsporum 9 0.22 11 0.21 14 0.26 15 0.31 19 0.26 16 0.25 19 0.32 17 0.40 20 0.19 23 0.05
aIn vitro, agar well diffusion method, conc. 100 lg/mL in DMSO.
bR is reference drug, Fluconazole.
Complexes 1–6 have better antimicrobial activity than the parent Schiff base, which
is due to the more lipophilic nature of complexes than the corresponding Schiff bases.
The increased activity of organotin(IV) complexes can be explained on the basis of the
chelation theory. Due to the polarity of the metal atom will be reduced to greater
extent due to the overlap of the ligand orbital and partial sharing of positive charge
of the metal atom with donor atoms. Furthermore, it also increases the delocalization
of p-electrons over the whole chelate ring and enhance the lipophilicity enhanced the
penetration of the complex into lipid membrane and blocks the metal binding sites
on enzymes of micro-organisms and inhibit their multiplication [31].
4. Conclusion
Synthesis of six-coordinate di-n-butyltin(IV) complexes has been achieved by the reac-
tion of [(n-Bu)2Sn(sb)Cl] with sodium salt of mono-functional bidentate ligands in 1:1
molar ratio and bidentate g2-bonding nature of acetylacetone (acacH), ethanolamine
(eaH) investigated by a variety of physicochemical methods. Molecular composition of
the complexes is confirmed by the mass spectral studies. The DFT data also validate
the results observed from the spectroscopic studies. The results of in vitro antimicro-
bial screening indicate that mixed-ligand n-butyltin(IV) complexes show moderate
activity, which is also higher than that of the Schiff bases.
Acknowledgement
Authors gratefully acknowledge the CSIR and UGC for doctoral fellowship and Head,
Department of Chemistry for providing laboratory facilities. Special thanks to SAIF Chandigarh
for IR, NMR (1H, 13C, and 119Sn) and Q-TOF LC-MS Mass spectra of the complexes and Head,

JOURNAL OF COORDINATION CHEMISTRY
3383
Department of Chemistry, Banaras Hindu University, Varanasi for 119Sn spectra of the complexes
and CDRI, Lucknow for providing elemental analysis.
Disclosure statement
No potential conflict of interest was reported by the authors.
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