Synthesis, structural characterization and antimicrobial activities of triorganotin(IV)azo-carboxylates derived from ortho/para-amino benzoic acids and β-naphthol

Article abstract of DOI:10.1016/j.ica.2019.119172

Triorganotin (IV) complexes 1–3 were synthesized by the reaction of azo-carboxylic acid viz. 2/4-(2-hydroxynaphthylazo)benzoic acids with appropriate triorganotin(IV) chlorides [R = Me (compounds 1 and 2) and Bu (compound 3)] in presence of triethyl amine

Full text of DOI:10.1016/j.ica.2019.119172

InorganicaChimicaActa498(2019)119172
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Research paper
Synthesis, structural characterization and antimicrobial activities of
triorganotin(IV)azo-carboxylates derived from ortho/para-amino benzoic
acids and β-naphthol
Paresh Debnatha, Ankit Dasa, Keisham Surjit Singha,⁎, Tage Yamab, S.Sureshkumar Singhb,
Ray J. Butcherc, Lesław Sierońd, Waldemar Maniukiewiczd,⁎
a Department of Chemistry, National Institute of Technology Agartala, Jirania, Tripura (west) 799046, India
b Department of Forestry, North Eastern Regional Institute of Science And Technology, Nirjuli, Arunachal Pradesh 791109, India
c Department of Chemistry, Howard University, 525 College Street, NW, Washington DC 20059, USA
d Institute of General and Ecological Chemistry, Lodz University of Technology, 90-924 Lodz, Zeromskiego 116, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
Tri-organotin(IV) azo-carboxylates
NMR spectroscopy
X-ray crystal structure
Antimicrobial activity
Triorganotin (IV) complexes 1–3 were synthesized by the reaction of azo-carboxylic acid viz. 2/4-(2-hydro-
xynaphthylazo)benzoic acids with appropriate triorganotin(IV) chlorides [R = Me (compounds 1 and 2) and Bu
(compound 3)] in presence of triethyl amine base. The complexes were characterized by elemental analysis, IR
and multinuclear (1H and 13C- and 119Sn)-NMR spectroscopy. The structures and mode of coordination around
tin ions in the complexes were determined by single crystal X-ray crystallography. The complexes exhibit tri-
gonal bipyramidal geometry around tin atoms where the base of the equatorial plane is being occupied by the
three alkyl groups [Me or Bu] while the axial positions are occupied by carboxylate oxygen atoms in 2 and in
case of 1 and 3, by carboxylate and phenoxide oxygen atoms, respectively. It has been found that, the crystal
structure of 1 or 3 is a cyclic dimeric, whereas 2 exhibits a polymeric structure. The 119Sn NMR studies show that
all complexes have four-coordinate structures indicating dissociation of these complexes in solution state. The
complexes were also screened for their antimicrobial activity and the compound 3 was found to exhibit effective
antimicrobial activity.
1. Introduction
these compounds were studied. In complex described above [17], both
carboxylate group of the ligands coordinated to the adjacent tin atoms
Organotin (IV) carboxylates have been explored by several research
groups for the last few decades because of their intriguing diverse
molecular structures such as monomeric, dimeric, polymeric, tetra-
meric, oligomeric ladder, macro-cyclic, cluster, cage, 1D and 2D mo-
lecular structures etc. [1–10]. Further, organotin(IV) carboxylates have
been also studied for their potential biological activities, for instance
they have been found to be effective as antitumor, antibacterial, anti-
fungal, cytotoxic, insecticidal, anti-proliferative and anti-tuberculosis
etc. [9–14]. Likewise, organotin(IV) complexes with azo-carboxylates
have also been studied in great details owing to their structural di-
versity [15–18] and promising biological properties [15–17]. Recently,
we have explored the chemistry of organotin (IV) complexes with azo-
dicarboxylic acids derived from salicylic acid and ortho- and para-
amino benzoic acids [19–20]. The biological properties such as anti-
microbial and antidiabetic activities as well molecular structures for
exclusively in bridging bidentate fashion giving rise to a polymeric
structure with trigonal bipyramidal geometry around tin atoms. The
compound showed significant antimicrobial activity. In addition, more
recently our group have reported molecular structures and anti-diabetic
effects of triorganotin(IV) azo-carboxylates derived from amino benzoic
acids and resorcinol [21]. These compounds showed promising anti-
diabetic activities even higher than the standard compound acarbose
and were probably the first organotin(IV) azo carboxylates that re-
vealed this type of activity. Thus, in our recent investigation on orga-
notin(IV) chemistry with azo-carboxylate ligands derived from ortho-
and para- amino benzoic acids, we have been exploring organotin(IV)
complexes for quite some time with functionalized azo carboxylates by
taking different diazo-coupling moieties viz. salicylic acid [19,20] and
resorcinol [21]. In this work our objective is to increase the number of
binding sites of the carboxylate ligands which would coordinate to the
⁎ Corresponding authors.
E-mail addresses: keisham.chem@nita.ac.in (K.S. Singh), waldemar.maniukiewicz@p.lodz.pl (W. Maniukiewicz).
https://doi.org/10.1016/j.ica.2019.119172
Received 26 August 2019; Received in revised form 23 September 2019; Accepted 23 September 2019
Availableonline24September2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

P. Debnath, et al.
InorganicaChimicaActa498(2019)119172
metal atom. Therefore, as a continuous effort for our research group in
the design of various novel molecular structures which would exhibit
potential biological activities, we are interested in further research on
the chemistry of organotin(IV) complexes with different azo-carbox-
ylates derived from ortho and para amino-benzoic acids. Thus, in this
present contribution we report a new series of triorganotin (IV) com-
plexes [R = Me (1 and 2); and Bu (3)] with azo- carboxylates obtained
from diazo coupling of amino benzoic acids and β-naphthol. The syn-
thesized complexes were characterized by elemental analysis, IR,
multinuclear (1H, 13C, 119Sn) NMR spectroscopy. Structures of all
compounds were determined by X-ray crystallography. In addition, we
have studied anti-microbial activities of the complexes and compared
them with the standard antibiotics.
129.40 [C-10], 129.08 [C-5], 128.15 [C-6], 126.94 [C-8], 126.64 [C-9],
124.28 [C-7], 122.01 [C-6′], 116.86 [C-3], 116.12 [C-2′].
2.2.2. Synthesis of 4-(2-hydroxynaphthylazo) benzoic acid (H2L2)
Analogous synthetic procedure was followed as in case of H2L1
where para-amino benzoic acid was used in place of ortho-amino ben-
zoic acid. A red crystalline product was obtained. Yield: 8.1 g, 76.05%;
m.p.: 290–295 °C. Anal. calcd. for C17H12N2O3: C, 69.86; H, 4.14; N,
9.58%. Found: C, 69.8; H, 4.17; N, 10.15%. UV–visible (DMF) λmax
(nm): 251, 375, 483. IR (KBr, cm−1): 3372 ν(OH),1716 ν(COO)asy
,
1448 ν(N]N), 1257 ν(CeO). 1H NMR (DMSO‑d6, 400.13 MHz) δH:
15.85 [s, 1H, OH], 8.36 [d, 1H, H-9, J = 8.4 Hz], 8.04 [d, 2H, H-3′, H-
5′, J = 8.4 Hz], 7.86 [d, 1H, H-4, J = 9.6 Hz], 7.78 [d, 2H, H-2′, H-6′,
J = 8.4 Hz], 7.67 [d, 1H, H-6, J = 7.6 Hz], 7.56 [t, 1H, H-8,
J = 7.6 Hz], 7.43 [t, 1H, H-7, J = 7.6 Hz], 6.72 [d, 1H, H-3, J = 9.6 Hz
] ppm; Signal due to –COOH could not be detected due to solvent ex-
change. 13C NMR (DMSO‑d6, 100.62 MHz) δC: 175.95 [COO], 166.66
[C-2],146.38 [C-1′], 142.08 [C-1], 132.57 [C-4], 130.95 [C-2′, C-6′],
130.01 [C-4′], 129.34 [C-10], 129.02 [C-5], 127.92 [C-8, C-9], 126.63
[C-7], 125.37 [C-6], 121.59 [C-3], 117.11 [C-3′, C-5′]. The ligand
skeleton and numbering scheme for ligands H2L1 and H2L2 are shown in
Scheme 1.
2. Experimental
2.1. Materials and methods
Tri-n-butyltin(IV) chloride, trimethyltin(IV) chloride, o-amino ben-
zoic acid, p-amino benzoic, and triethyl amine were obtained from
MERCK by purchase and were used without further purification.
Solvents were dried and purified following the standard procedures.
Carbon, hydrogen and nitrogen analyses were performed on a Perkin
Elmer 2400 series II instrument. UV–Visible spectra of the ligands and
the complexes were recorded on UV-1800 Shimadzu spectrophotometer
in DMF in the range 200–800 nm while the IR spectra were obtained
from Shimadzu FT-IR-8400S spectrophotometer in the range of
4000–400 cm−1 using KBr discs. 1H, 13C and 119Sn NMR spectra were
recorded on Bruker AMX 400 spectrometer measured at 400.13, 100.62
and 149.18 MHz respectively. Me4Si was used as reference for 1H- and
13C- chemical shifts set at 0.00 ppm while Me4Sn set at 0.00 ppm was
employed as reference for 119Sn- chemical shifts.
2.2.3. Synthesis of Me3SnHL1 (1)
Trimethyltin (IV) compound 1 was synthesized by the reaction of 2-
(2-hydroxynaphthylazo) benzoic acid (H2L1) with tri-methyl tin (IV)
chloride using triethylamine as base under reflux condition with (1:1)
molar ratio. In this procedure, the ligand (0.733 g, 2.059 mmol) was
dissolved in 30 mL anhydrous methanol in a round bottom flask and
then triethylamine base (0.2537 g, 2.509 mmol) was added dropwise
and was refluxed on an oil bath for about 30 min with an equipped
water cooled condenser. Trimethyltin (IV) chloride (0.5 g, 2.509 mmol)
was then added to the above solution with continuous stirring and was
again refluxed for about 6 h. The reaction mixture was then filtered and
the precipitate containing Et3N.HCl was filtered off, the filtrate was
then collected and evaporated to dryness and further purification was
done by hexane to get the pure red crystalline product. The crystalline
product was then recrystallized in anhydrous methanol to obtain pure
red crystals of complex 1. Yield: 0.49 g, 71%; m.p.: 226–227 °C.
Anal.Calcd. for C40H40N4O6Sn2: C, 52.78; H, 4.43; N, 6.16%. Found: C,
52.67; H, 4.32; N, 6.13%. UV–visible (DMF) λmax (nm): 287, 366, 478.
IR (KBr, cm−1): 3449 ν(OH), 2920 ν(CeH str. of Sn-CH3), 1632
ν(COO)asym, 1476 ν(N]N), 1443 ν(COO)sym 1185 ν(CeO), 664 ν(Sn-C),
490 ν(Sn-O). 1H NMR (CDCl3, 400.13 MHz) δH, Ligand skeleton: 8.43
[d, 1H, H-9, J = 8.0 Hz], 8.28 [d, 1H, H-3′, J = 8.4 Hz], 8.12 [dd, 1H,
H-6, J = 8 Hz and 1.6 Hz], 7.60–7.56 [m, 2H, H-4, H-4′], 7.51–7.46 [m,
2.2. Synthesis
2.2.1. Synthesis of 2-(2-hydroxynaphthylazo) benzoic acid (H2L1)
The azo-carboxylate ligand 2-(2-hydroxynaphthylazo) benzoic acid
[H2L1] was prepared by diazo-coupling reaction of o-amino benzoic
acid with β-naphthol following analogous reported procedure
[19,22,23]. o-Amino benzoic acid (5 g, 36.45 mmol) was mixed with
12 mL concentrated HCl and 40 mL water; the mixture was then di-
gested in a water bath till the solution became clear. The digested so-
lution was then kept overnight in refrigerator and then diazotized at
0–5 °C with ice cold aqueous solution of NaNO2 (2.51 g, in 20 mL water)
for about 1 h. It was then added to the alkaline solution (10% NaOH
solution, 5 g in 50 mL water) of β-naphthol (5.256 g, 36.45 mmol) at
about 0–2 °C with vigorous stirring. A deep red color appeared im-
mediately and the stirring was continued for 3 h. The reaction was kept
in refrigerator for overnight again and then kept at room temperature
for 2–3 h. It was then acidified with dilute acetic acid to get the red
precipitate of azo ligand. The product was then filtered, washed with
distilled water till the filtrate became neutral and finally the product
was dried on water bath. The solid mass was then recrystallized from
methanol to get desired pure red crystalline product of H2L1. Yield:
7.4 g, 69.4%; m.p.: 260–262 °C. Anal. calcd. for C17H12N2O3: C, 69.86;
H, 4.14; N, 9.58%. Found: C, 69.79; H, 4.19; N, 9.60%. UV–visible
(DMF) λmax (nm): 252, 316, 477. IR (KBr, cm−1): 3365 ν(OH), 1710
́
2H, H-6, H-8 ], 7.36 [t, 1H, H-5′, J = 7.6 Hz ], 7.16[t, 1H, H-7,
J = 7.6 Hz], 6.67[d, 1H, H-3 J = 9.6 Hz]; Sn-CH3Skeleton: 0.74 [s, 9H,
ν(COO)asy
,
1448 ν(N]N), 1255 ν(CeO). 1H NMR (DMSO‑d6,
400.13 MHz) δH: 16.11 [s, 1H, OH], 8.11 [d, 1H, H-9, J = 8.0 Hz], 8.04
[d, 1H, H-3′, J = 8.4 Hz], 7.78 [d, 1H, H-6, J = 7.6 Hz], 7.57 [d, 1H, H-
́
4, J = 9.6 Hz], 7.48 [t, 1H, H-4′, J = 7.6 Hz], 7.38 [d, 1H, H-6
J = 7.6 Hz], 7.30 [t, 1H, H-8, J = 7.6 Hz], 7.17 [t, 1H, H-5′, J = 7.6 Hz
], 7.04[t, 1H, H-7, J = 7.6 Hz], 6.38[d, 1H, H-3 J = 9.6 Hz] ppm; Signal
due to –COOH could not be detected due to solvent exchange. 13C NMR
(DMSO‑d6, 100.62 MHz) δC: 178.94 [COO], 167.43 [C-2],143.9 [C-1′],
142.46 [C-1], 134.29 [C-4], 131.36 [C-4′], 133.17 [C-5′], 130.40[C-3′],
Scheme 1. The ligand skeleton and numbering scheme of ligands H2L1 and
H2L2.
2

P. Debnath, et al.
InorganicaChimicaActa498(2019)119172
2.3. Crystallographic data collection and structure refinement
(Sn-CH3)] 2J [119Sn-1H (57.2 Hz)] ppm. 13C NMR (CDCl3, 100.62 MHz)
δC, Ligand skeleton: 179.3 [COO], 171.60 [C-2],144.2 [C-1′], 141.82
[C-1], 134.30 [C-4], 133.27 [C-4′], 132.52 [C-5′], 129.10[C-3′], 128.74
[C-10], 128.42 [C-5], 127.49 [C-6], 126.47 [C-8], 125.87 [C-9], 124.06
[C-7], 122.37 [C-6′], 119.5 [C-3], 116.34 [C-2′]; Sn-CH3Skeleton:
Single crystal X-ray diffraction data were collected by the ω-scan
technique using MoKα (λ = 0.71073 Å) radiation. The 1 crystal was
studied at 100 K using a RIGAKU XtaLAB Synergy, Dualflex, Pilatus
300 K diffractometer [24] with Photon Jet micro-focus X-ray Source
and 2, 3, H2L2 at room temperature using a Bruker AXS Smart APEX-II
CCD diffractometer [25]. Data collection, cell refinement, data reduc-
tion and absorption correction were carried out using CrysAlis PRO
software [24] for 1 with the SMART and SAINT-PLUS [26] for 2, 3,
H2L2. The crystal structures were solved by using direct methods with
the SHELXT 2018/2 program [27]. Atomic scattering factors were
taken from the International Tables for X-ray Crystallography. Posi-
tional parameters of non-H-atoms were refined by a full-matrix least-
squares method on F2 with anisotropic thermal parameters by using the
SHELXL 2018/3 program [28]. All hydrogen atoms were placed in
calculated positions (CeH = 0.93–0.98 Å) and included as riding con-
tributions with isotropic displacement parameters set to 1.2–1.5 times
the Ueq of the parent atom. In 3, a model of disorder of n-butyl groups
was proposed. Crystal data and structure refinement parameters are
shown in Table S1, while selected geometric parameters are collected in
Tables 2, 3 and S2.
−1.709 [Sn-CH3] ppm. 119Sn NMR (CDCl3, 149.18 MHz):
+
139.63 ppm. The other triorganotin (IV) complexes were prepared fol-
lowing the analogous procedure which was described above by reacting
appropriate triorganotin (IV) chlorides and azo ligands.
2.2.4. Synthesis of Me3SnHL2 (2)
The tri-methyl tin (IV) compound 2 was synthesized by reacting the
ligand 4-(2-hydroxynaphthylazo) benzoic acid (H2L2) with tri-methyl
tin (IV) chloride using triethylamine as a base under refluxing condition
with (1:1) molar ratio to produce red crystalline compound. Yield:
0.57 g, 83.2%; m.p.: 221–222 °C. Anal.Calcd. for C20H20N2O3Sn: C,
52.78; H, 4.43; N, 6.16%. Found: C, 52.64; H, 4.27; N, 5.95%.
UV–visible (DMF) λmax (nm): 305, 366, 484. IR (KBr, cm−1): 3443
ν(OH), 2923 ν(CeH str. of Sn-CH3), 1595 ν(COO)asy, 1498 ν(N]N),
1440 ν(COO)sym 1160 ν(CeO), 671 ν(Sn-C), 485 ν(Sn-O). 1H NMR
(CDCl3, 400.13 MHz) δH, Ligand skeleton: 8.47 [d, 1H, H-9,
J = 8.4 Hz], 8.14 [d, 2H, H-3′, H-5′, J = 7.2 Hz], 7.68–7.65 [m, 3H, H-
4, H-2′, H-6′], 7.56–7.52 [m, 2H, H-6, H-8], 7.38 [t, 1H, H-7,
J = 7.6 Hz], 6.75 [d, 1H, H-3, J = 9.6 Hz ]; Sn-CH3Skeleton: 0.66 [s, 9H
(Sn-CH3)] 2J [119Sn-1H (57.2 Hz)] ppm. 13C NMR (CDCl3, 100.62 MHz)
δC: 176.95 [COO–], 170.97 [C-2],146.43 [C-1′], 141.85 [C-1], 133.50
[C-4], 131.87 [C-2′, C-6′], 130.88 [C-4′], 129.45 [C-10], 129.32 [C-5],
128.85 [C-8], 128.27 [C-9], 126.53 [C-7], 125.93 [C-6], 122.10 [C-3],
2.4. Antimicrobial assay
The antimicrobial screening of the compounds 1–3 and ligands were
performed following modified Kirby-Bauer-Disc- Diffusion Assay [29].
Three bacterial species (Escherichia coli, Pseudomonas auriginosa and
Staphylococcus aureus) and three species of fungi (Fusarium verticilloides,
Fusarium chlamydosporum and Fusarium oxysporum) were employed for
the assay. Prior to the assay all the test organisms were revived by
streaking on respective agar media plates. For preparing bacterial in-
oculums, fresh culture was inoculated in nutrient broth and incubated
for 24 h at 37 °C and diluted up to 10-4 using sterile water. For pre-
paring fungal inoculums, 7-days old fungal culture was used, 5 mL of
sterile distilled water was added to the vials containing the fungal
cultures. The colonies were gently scraped out along with water and the
suspension was vortexed at a minimum speed and used directly as in-
oculums. Filter paper discs of 6 mm diameter were prepared from
Whatman No.1 and sterilized by autoclaving prior to the assay. A thin
layer of bacterial culture was prepared using an L-spreader on the sur-
face of Tomato Juice Agar (TJA) medium plates by inoculating 150 µL
of bacterial cell suspension. For fungal culture, Rojo Congo Agar (RCA)
medium plates were inoculated with 150 µL of fungal spore suspension
(diluted to 10-5). A manual rotary plate spreader was used for a proper
and uniform layer of bacteria and fungi on their respective media
plates. The plates were allowed to dry for about 1 min inside the la-
minar air flow chamber. Filter paper disc previously saturated with test
sample solutions (1 mg/mL) for screening test and for quantitative
assay, different doses (25, 50, 100, 200, 500 µg/mL) were placed on the
surface of the bacterial/fungal culture. Standard antibiotic discs of
Streptomycin (1 mg/mL) and Fluconazole (1 mg/mL) were used against
the test bacteria and fungi for comparison with the test samples. All
tests were performed in triplicate and mean values are presented. The
plates were incubated for 24 h at 37 °C for bacteria and 72 h at 30 °C for
fungi. The bacterial and fungal inhibition zones (mm) were recorded for
all samples as well as for the standard antibiotics.
116.98 [C-3′, C-5′]; Sn-CH3 skeleton: −4.124 [Sn-CH3], 1J [119Sn-13
C
(197.5 Hz)] ppm. 119Sn NMR (CDCl3, 149.18 MHz): +139.72 ppm.
2.2.5. Synthesis of Bu3Sn HL1 (3)
Tri-butyl tin (IV) compound 3 was synthesized by reacting the li-
gand 2-(2-hydroxynaphthylazo) benzoic acid (H2L1) with tri–butyl tin
(IV) chloride using triethylamine base under refluxing condition in
(1:1) molar ratio to obtain deep red crystalline product. Yield: 0.64 g,
60.3%; m.p.: 71–72 °C. Anal.Calcd. for: C116H152N8O12Sn4: C, 59.92; H,
6.59; N, 4.82%. Found: C, 60.23; H, 6.71; N, 4.78%. UV–visible (DMF)
λmax (nm): 287, 366, 480. IR (KBr, cm−1): 2951 ν(CeH str. of nBu),
1626 ν(COO)asym, 1435 ν(N]N), 1343 ν(COO)sym 1151 ν(CeO), 665
ν(Sn-C), 497 ν(Sn-O).1H NMR (CDCl3, 400.13 MHz) δH, Ligand ske-
leton: 8.43 [d, 1H, H-9, J = 8.0 Hz], 8.27 [d, 1H, H-3′, J = 8.4 Hz],
8.10 [dd, 1H, H-6, J = 8.0 Hz and 1.2 Hz], 7.59–7.55 [m, 2H, H-4, H-
́
4′], 7.51–7.46 [m, 2H, H-6, H-8], 7.35 [t, 1H, H-5′, J = 7.6 Hz ], 7.17[t,
1H, H-7, J = 7.6 Hz], 6.66[d, 1H, H-3 J = 9.6 Hz]; Sn-nBu Skeleton:
1.71[m,6H, H-α], 1.48 [m, 6H, H-β], 1.39 [m, 6H, H-γ], 0.92 [t, 9H, H-
δ] ppm. 13C NMR (CDCl3, 100.62 MHz) δC: 179.50 [COO], 170.85 [C-
2],144.22 [C-1′], 141.64 [C-1], 134.34 [C-4], 133.15 [C-4′], 132.49 [C-
5′], 130.95 [C-3′], 129.06 [C-10], 128.72 [C-5], 128.45 [C-6], 127.50
[C-8], 126.44 [C-9], 124.01 [C-7], 122.36 [C-6′], 119.45 [C-3], 116.34
[C-2′] Sn-nBu Skeleton: 27.91 [C-β] 2J [119Sn-13C(19 Hz)], 27.09 [C- γ]
3J [119Sn-13C(65.1 Hz)], 16.99 [C-α] 1J [119Sn-13C(342.7 Hz)], 13.68
[C-δ] ppm. 119Sn NMR (CDCl3, 149.18 MHz): +126.53 ppm. The
numbering scheme of Sn-Bu skeletal in the given complex is shown
below (Scheme 2).
3. Results and discussion
3.1. Synthesis
Triorganotin (IV) complexes 1–3 were synthesized by reacting 2/4-
(2-hydroxynaphthylazo) benzoic acids (H2L1-2) with appropriate trior-
ganotin (IV) chlorides [R = Me (1 and 2), Bu (3)] using anhydrous
methanol in presence triethylamine base in 1:1 stoichiometric molar
Scheme 2. The numbering scheme in SnBu skeletal.
3

P. Debnath, et al.
InorganicaChimicaActa498(2019)119172
determined from the study of IR stretching frequency of carboxylate
group [29]. In general, carboxylate exhibits two types of bands, strong
asymmetric stretching band, [νasym(COO)] observed near
1580–1650 cm−1 and
a
weaker symmetric stretching band,
[νsym(COO)] near 1320–1450 cm−1
.
Moreover, if the value of
Δν[νasym(COO)- νsym(COO)] is below 200 cm−1 indicates bi-dentate
mode of coordination while if the value of Δν is greater than 200 cm−1
is an indicative of the presence of mono-dentate mode of coordination
of the carboxylate ligands to the tin in the complexes [20,32]. In
complexes 1 and 3, the characteristic asymmetric stretching frequency
bands were observed at 1632 cm−1 and 1626 cm−1 while the sym-
metric stretching frequency bands were observed at 1345 cm−1 in and
1343 cm−1 and the value of Δν was found to be 287 cm−1 and
283 cm−1 respectively. Thus it can be concluded that in both the
complexes 1 and 3, carboxylate ligand coordinate in mono-dentate
mode of coordination to the Sn-centre [17]. Furthermore, in case of
complex 2, the similar asymmetric IR stretching frequency band was
observed at 1625 cm−1 while the symmetric stretching frequency band
was observed at 1440 cm−1 and the value of Δν was found to be
185 cm−1. This implies that the complex 2 is in bi-dentate bridging
mode of coordination of carboxylate ligand to the Sn-centre [19,32].
The mode of coordination of carboxylate ligands in these complexes
were found to be in consistent with the crystal structures of the com-
plexes (vide infra). The coordination mode of the carboxylate ligands in
the complexes are shown in Scheme 5.
Scheme 3. Reaction scheme for the synthesis of compounds 1 and 3.
3.2.3. Multinuclear (1H, 13C and 119Sn) NMR spectroscopy
Scheme 4. Reaction scheme for the synthesis of compound 2.
1H, 13C NMR spectral data of the ligands (H2L1 and H2L2) were
recorded in DMSO‑d6 while 1H, 13C and 119Sn NMR spectral data for the
complexes 1–3 were recorded in CDCl3 and the data are provided in the
experimental section while some spectra for the ligands and complexes
are also provided as the Electronic Supplementary Information (ESI).
The complete assignments of 1H, 13C NMR spectra of the ligands were
accomplished by examining their chemical shift values, integration
values, multiplicity patterns and also by interpretation of their corre-
lated spectroscopy (1H–1H COSY). These data was also examined by
comparing the proton NMR spectral data of similar type of azo-ligands
derived from either o/p-amino benzoic acids or β-naphthol reported
earlier [17,22,33–35]. The assignments of the chemical shift position of
the protons for the complexes were achieved by inferring the conclu-
sions which were drawn from the ligand assignments owing to the si-
milarity of their spectral data. The aromatic protons of the azo ligands
H2L1 and H2L2 were observed in the range 6.38–8.37 ppm whereas the
aromatic protons in all the complexes 1–3 were observed in the range
6.66–8.47 ppm. In complex 1 and 2, a singlet peak was observed at
0.74 ppm and 0.66 ppm respectively which can be assigned to tin-me-
thyl protons. In complex 3, a triplet and three multiplet peaks were
observed at 0.92 ppm and 1.39–1.71 ppm due to methyl and three
methylene protons of tri-butyltin moiety. The 13C NMR spectra of the
azo ligands H2L1 and H2L2 showed δ(COO) signals at 178.94 and
175.95 ppm respectively, but in case of all the complexes 1–3, the
δ(COO) signals were observed in the range 176–179 ppm. The 13C NMR
signals for aromatic ring carbons of the free azo ligands H2L1 and H2L2
were observed at the range 116.12–178.94 ppm whereas in case of
complexes 1–3, the corresponding values were found in the range
116.34–179.50 ppm. The number of 13C NMR signals observed in the
ligands and the complexes were found to be in good agreement with the
ratio. The complexes were obtained in good yield and were found to be
soluble in all common organic solvents. The reaction schemes for the
synthesis of complexes 1 and 3 are shown in Scheme 3 and for complex
2 in Scheme 4.
3.2. Spectroscopic characterization
3.2.1. UV-spectroscopy
The UV–visible spectra of the ligands (H2L1-2) and compounds (1–3)
were recorded in DMSO and chloroform solution (10-4 M) respectively
at room temperature. The electronic spectra of H2L1 and H2L2 showed
UV absorption peaks at 251–252 and 315–316 nm which may be as-
signed to π → π* transition of aromatic ring and n → π* transition of
carboxylic acid group respectively [19] . The UV–visible spectra for
compounds 1–3 exhibited three absorption bands at 287–306, 366 and
478–487 nm respectively. After coordination, π → π* and n → π* tran-
sitions were observed at slightly higher wavelength in the complexes.
The new bands observed at 478–487 nm indicate ligand to metal charge
transfer [19,30]. The slight increase of the λmax value from the ligand to
the organotin (IV) complexes indicates the coordination of the ligand to
tin atom.
3.2.2. IR spectroscopy
The IR spectroscopic data for azo-carboxylate ligands H2L1-2 and
compounds 1–3 are given in the experimental section. The OeH
stretching absorption band for the free azo-carboxylate ligands (H2L1
and H2L2) was found in the region 3300–3400 cm−1. The asymmetric
[νasym(COO)] stretching frequencies of the ligands H2L1 and H2L2 were
observed at 1710 and 1716 cm−1 respectively and in the complexes,
these bands were shifted to lower frequencies at 1625–1632 cm−1. The
shifting of the absorption bands of these complexes relative to the free
carboxylate ligands indicates the carboxylate coordination to Sn-atom
[16]. The IR absorption bands observed at 530–680 cm−1 and
450–500 cm−1 in the complexes are assigned for Sn-C and Sn-O re-
spectively [21,31]. Further, useful information about mode of co-
ordination of carboxylate group with the Sn-centre can also be
Scheme 5. Coordination mode of carboxylate ligands in complexes 1–3.
4

P. Debnath, et al.
InorganicaChimicaActa498(2019)119172
Scheme 6. Azo and hydrazone tautomeric forms of azo-carboxylate ligands
H2L1 and H2L2.
expected number of signals. The azo-carboxylate ligands H2L1 and H2L2
which contain an OH group at the ortho-position to the azo linkage can
lead to tautomerize to give two tautomeric structures, azo and hy-
drazone forms. In solution state these two isomeric forms remain in
equilibrium as shown in Scheme 6.
Fig. 1. View of the H2L2 molecule showing the atom-labeling scheme.
Displacement ellipsoids are drawn at 50% probability level.
However, due to rapid exchange process at room temperature we
could not observe different resonances for the two tautomers in solu-
tion. For the identification of position of the azo-hydrazone equili-
brium, we have used the 13C NMR chemical shift values of naphthalene
ring carbon, C-2 for the estimation of the relative tautomeric population
as calculated in case of 1- phenylazo-2-naphthol derivatives [35,36]. A
strong electron withdrawing group present in the para position of
phenyl ring can shift the equilibrium almost completely to the hy-
drazone form with a C-2 chemical shift of approximately 180 ppm [33],
while the corresponding value for azo form was found by additivity rule
to be approximately 147 ppm [36,37]. Due to the structural similarity
of our azo-carboxylate ligands H2L1 and H2L2 with the azo ligands with
β-naphthol reported earlier [32], we calculated equilibrium constants
for the tautomeric proton exchange reaction using the following Eq. (1)
[34].
+139.72 ppm respectively. For complex 3 the sharp signal of 119Sn
resonance was observed at +126.53 ppm. These resonances of the
complexes 1–3 suggest the four coordinate tetrahedral geometry in
solution state [21,38–41].
3.3. X-ray crystal structure description
3.3.1. Crystal structure of H2L2
The ORTEP image of the free acid (H2L2), belonging to the C2/c
space group, is shown in Fig. 1. The molecule is approximately planar
with no twist about the central C-N]N-C bonds linking the aromatic
rings, as indicated by the torsion angles N1-N2-C5-C6 = 0.8(5)° and
N2-N1-C8-C9 = −0.2(5)°. This can be rationalized by the formation of
intramolecular hydrogen bond N2-H2A…O3, which additionally stif-
fens the entire molecule. The carboxylic acid group is coplanar with its
parent phenyl ring [O(1)-C(1)-C(2)-C(3) = −178.7(3)°]. The car-
boxylic acid H-atom forms an intramolecular hydrogen bond, O1-
H1A…O2#1 [#1:1-x,-1-y,1-z], with the O-atom from neighboring car-
boxylic acid group (Table 2). This causes, that the crystal packing (Fig.
S7) features centrosymmetrically related dimers associate via the eight-
membered carboxylic acid dimer synthon {…HOC(=O)}2, which can
be described by graph set notation as R22 (8) [43].
[Hydrazone]
[azo]
δC2 − 147 ppm
180 ppm − δC2
Kazo−hydrazone
=
=
(1)
The equilibrium constant of the ligands calculated by using Eq. (1)
are listed in Table 1.
For complexes, the geometry and coordination number around tin
centers in solution state can be determined using 2J (119/117Sn-1H) and
nJ(119Sn-13C) coupling constants [17,19,38–40]. In complexes 1 and 2,
the 2J (119Sn-1H) coupling constant values were found to be 57.2 Hz
indicating 4-coordinate tetrahedral geometry of Sn-centre in both the
complexes [17,19,38–41]. Furthermore, four coordinate tetrahedral
geometry of the complexes were also determined by measuring the nJ
3.3.2. Crystal structure of complex 1
The molecular structure of 1 is shown in Fig. 2. Selected bond dis-
tances and angles are listed in Table 3. The complex 1 is a cyclic cen-
trosymmetric dimmer. The geometry around the five-coordinated Sn
atom is distorted trigonal bipyramidal, in which three methyl groups
occupying the equatorial positions with almost identical bond distances
(
119Sn-13C) coupling constant values. It is generally observed that four
coordinate tributyltin compounds exhibit couplings 1J (119Sn-13C) in
the range of 325–390 Hz and five coordinated complexes in the range of
430–540 Hz [17,19,38–41]. In complex 3, the 1J (119Sn-13C), 2J (119Sn-
13C) and 3J (119Sn-13C) coupling satellites were observed at 342.7 Hz,
19 Hz and 65.1 Hz respectively which indicate the four coordinate
tetrahedral geometry of the tin centre for the complex in solution state
[19,21,38]. In addition, using Lockhart and Mander’s equation [39]
{θ(C-Sn-C) = 0.0161 | 2J (Sn-H) |2 – 1.32 | 2J (Sn-H) | + 133.4}, the
angle between C-Sn-C in complexes 1 and 2 were calculated and found
to be 110.56° for both the complexes indicating the four coordinated
tetrahedral geometry [21,38]. The geometry of the complexes was
further confirmed by 119Sn NMR spectral study. The 119Sn NMR spectra
for the complexes 1 and 2 were observed at +139.63 ppm and
Table 2
Hydrogen bonding parameters for the ligand (H2L2) and the triorganotin(IV)
complexes 1, 2 and 3.
Compound
D–H…A
d(D–H)
d(H…A)
d(D…A)
< (DHA)
H2L2
1
O(1)-H(1A)…O(2)#1
N(2)-H(2A)…O(3)
N(1)-H(1).O(2)
0.82
1.84
2.642(3)
2.547(4)
2.693(2)
2.650(2)
3.189(2)
3.382(1)
2.555(1)
2.694(2)
2.691(2)
2.717(2)
2.657(2)
2.706(2)
2.662(2)
2.690(2)
2.672(2)
164.2
139(3)
130(2)
130(2)
146.0
151.7
146(2)
129.5
128.3
128.6
129.9
128.4
129.9
127.2
130.1
1.02(4)
0.84(2)
0.84(2)
0.98
0.93
0.85(2)
0.86
0.86
0.86
0.86
0.86
1.68(4)
2.08(2)
2.03(2)
2.36
2.53
1.81(1)
2.06
2.07
2.10
2.02
2.09
N(1)-H(1).O(3)
C(2)-H(2C)…O(2)#2
C(4)-H(14)…O(3)#3
N(1)-H(1A)…O(3)
N(2)-H(2A)…O(1)
N(2)-H(2A)…O(2)
N(4)-H(4A)…O(4)
N(4)-H(4A)…O(5)
N(6)-H(6)…O(7)
N(6)-H(6)…O(8)
N(8)-H(8)…O(10)
N(8)-H(8)…O(11)
2
3
Table 1
The equilibrium constants of azo ligands H2L1 and H2L2 in DMSO‑d6 calculated
from 13C NMR chemical shifts of C-2 using Eq. (1).
0.86
0.86
0.86
2.03
2.08
2.04
Azo ligands
δC2/ppm (in DMSO‑d6)
Kazo-hydrazone
H2L1
H2L2
167.43
166.66
1.625
1.47
Symmetry code: (#1) 1-x,-1-y,1-z; (#2) 1-x,1-y,1-z ; (#3) 2-x,-y,1-z.
5

P. Debnath, et al.
InorganicaChimicaActa498(2019)119172
Fig. 2. The molecular structure of 1 Displacement ellipsoids drawn at 50%
probability level.
[mean bond length Sn-C = 2.121(2)Å] and the axial positions being
filled by a carboxylate oxygen and a phenoxide oxygen of an adjacent
molecule. The Sn–C bond lengths are consistent with those reported in
the literature [42–44]. The C-Sn-C bond angles (C1-Sn1-C2, C1-Sn1-C3,
C2-Sn1-C3) are 126.63(9)°, 118.86(9)° and 110.79(8)°, respectively and
their sum is 356.28(9)°. Whereas, two oxygen atoms are bonded
asymmetrically to Sn atom with significantly different Sn-O distances of
2.127(1)Å and 2.616(2)Å respectively. Longer weak Sn-O bond should
rather be considered as secondary interactions but nevertheless with a
physical meaning. The literature search showed that for apical Sn-O
bonds with monodentate ligands, in pentacoordinated compound of Sn
such bonds are reported [45]. The O1-Sn1-O3#2 [#2: 1-x,1-y,1-z], bond
angle is 174.62(5)°. The asymmetrical arrangement of oxygen atoms
causes significant deformations of the Sn coordination sphere. The de-
formation of the Sn coordination sphere can be explained by the Berry
pseudorotation mechanism [46] and characterized quantitatively by
parameter τ defined by Addison et al. [47], (τ = 0.80; cf. the τ values
for the idealized geometries are τ = 0, square planar, τ = 1, trigonal
bipyramidal). An intramolecular NeH…O hydrogen bonds [N1–H1…
O2 and N1–H1…O3] help to establish near planar conformation of
molecule (Table 2). Packing diagrams of complex 1 are shown in Fig.
S8a-b.
Fig. 3. The 1-D infinite zigzag chain of 2. Displacement ellipsoids are drawn at
50% probability level.
Sn-O1 = 2.177(8)Å and Sn-O2#2 = 2.352(7)Å respectively. The two O
atoms are to a lesser extent bonded asymmetrically to tin atom then in
1. Like in 1 the coordination geometry of the tin center is a slightly
distorted trigonal bipyramid, thus the Sn atom is five-coordinated with
the two oxygen atoms occupying the axial sites [the O–Sn–O axial angle
is O1–Sn1–O2#2 = 175.3(3)o, #2: 3/2-x,-1/2 + y,3/2-z ]. Three Sn-
methyl groups define the equatorial plane, and the sum of the trigonal
C–Sn–C angles is 359.0° which indicates that the three methyl groups
and tin atoms are nearly coplanar. The value of parameter τ = 0.90
confirms the 5-coordinated distorted trigonal bipyramidal geometry of
Sn.
The molecules additionally stabilize an intramolecular NeH…O
hydrogen bonds (Table 2). The molecular packing of compound 2 is
shown in Fig. S9.
3.3.4. Crystal structure of complex 3
In crystals of compound 3 (Fig. 4), which crystallizes in P21 space
group, there are two symmetry independent molecules in the asym-
metric unit. Selected bond lengths and bond angles are presented in
Table 3. In contrast to 1, both molecules form non-centrosymmetric
dimmers. The atomic coordinates of those two independent molecules
have been thoroughly tested without success for higher symmetry by
means of the PLATON program [48]. Like in 1 and 2, the geometry
around the each Sn atoms is distorted trigonal bipyramidal. The para-
meter τ varies from 0.73 to 0.82 for Sn centers, respectively.
3.3.3. Crystal structure of complex 2
The molecular structure of complex 2 is shown in Fig. 3. Selected
bond lengths and bond angles are presented in Table 3. The compound
2 forms in solid state an infinite zig-zag 1-D chain structure connected
by bridging carboxylate coordination to tin ions. The bond distances are
Table 3
Selected bond lengths (Å) and bond angles (°) for the organotin (IV) complexes.
Atoms
1 Bond Length (Å)
Atoms
2 Bond Length (Å)
Atoms
3 Bond Length (Å)
Sn(1)-O(1)
Sn(1)-O(3)#1
Sn(1)-C(1)
Sn(1)-C(2)
Sn(1)-C(3)
2.1273(13)
2.6161(15)
2.118(2)
2.111(2)
2.133(2)
Sn(1)-O(1)
Sn(1)-O(2)#2
Sn(1)-C(18)
Sn(1)-C(19)
Sn(1)-C(20)
2.178(7)
2.353(7)
2.236(9)
2.117(12)
2.121(12)
Sn(1)-O(3)
Sn(1)-O(4)
Sn(1)-C(18)
Sn(1)-C(22)
Sn(1)-C(26)
2.116(11)
2.721(11)
2.18(2)
2.22(3)
2.18(2)
O(1)-Sn(1)-C(1)
O(1)-Sn(1)-C(2)
O(1)-Sn(1)-C(3)
O(1)-Sn(1)-O(3)#1
C(1)–Sn(1)–C(2)
C(1)-Sn(1)-C(3)
C(2)-Sn(1)-C(3)
96.04(7)
103.05(7)
89.41(7)
174.62(5)
126.63(9)
118.86(9)
110.79(8)
O(1)-Sn(1)-C(18)
O(1)-Sn(1)-C(19)
O(1)-Sn(1)-C(20)
O(1)-Sn(1)-O(2)#2
C(18)-Sn(1)-C(19)
C(18)-Sn(1)-C(20)
C(19)-Sn(1)-C(20)
99.2(3)
86.3(4)
94.0(4)
175.3(3)
119.6(4)
121.4(4)
118.0(5)
O(3)-Sn(1) -C(18)
O(3)-Sn(1)-C(22)
O(3)-Sn(1)-C(26)
O(3)-Sn(1)-O(4)
C(18)-Sn(1)-C(22)
C(18)-Sn(1)-C(26)
C(22)-Sn(1)-C(26)
100.2(6)
90.8(6)
98.3(8)
174.8(4)
115.7(9)
127.0(9)
113.3(10)
Symmetry code: (#1) 1-x,-1-y,1-z; (#2) 3/2-x,-1/2 + y,3/2-z.
6

P. Debnath, et al.
InorganicaChimicaActa498(2019)119172
Fig. 4. The molecular structure of a dimeric unit of 3. Displacement ellipsoids
are drawn at 20% probability level.
The short Sn-O bonds vary from 2.068(13)Å to 2.117(11)Å and long
Sn-O bonds vary from 2.721(11)Å to 2.814(11)Å. The coordination
sphere is complemented by three disordered n-butyl groups lying in
equatorial positions. As in case of 1 and 2, intra-molecular hydrogen
bonds of the type NeH…O (Table 2) play an important role in stabi-
lizing the structure. The crystal packing of complex 3 is shown in Fig.
S10.
Fig. 5. Microbial growth inhibition properties at different doses of compound 3
(C3) and streptomycin (Str) against S. aureus (A) and F. oxysforum (B). T-bars on
the histogram represent standard deviation of mean. Fluconazole did not show
activity against the fungus F. oxysforum.
quantitative assay test for compound 3 and the standard antibiotics are
also shown in Fig. 5. It has also been observed that, there is linear in-
crease in the antimicrobial activity of the compound 3 with increase in
their concentrations against the bacterium at different doses indicating
concentration dependent activity. The antibiotic streptomycin (Str) also
displayed increase in antimicrobial activity up to 200 µg/mL and de-
creases thereafter with the increase in the dose. However, the anti-
microbial activity of the compound 3 increased significantly from
6.7 mm at 25 µg/mL to 14.3 mm at 500 µg/mL against the bacteria S.
aureus. For fungal, the antifungal activity of the compound 3 increased
significantly from 5.2 mm at 25 µg/mL to 12.6 mm at 200 µg/mL
against F. oxysporum. It is also seen that fluconazole (FZ) did not show
antifungal activity against the fungus F. oxysporum at all different doses.
The picture for antimicrobial assay plates for the antimicrobial ac-
tivity of the compound 3 and antibiotics are also provided in Fig. S6 as
supplementary material. Compound 3 is found to be the most effective
antimicrobial agents against S. aureus and F. oxysporum among the
tested compounds. The higher antimicrobial activity of tributyltin(IV)
compound 3 as compared to its corresponding trimethyltin(IV) com-
pounds 1 and 2 were found to be fully in consistent with the earlier
reports [38,49–51].
3.4. Antimicrobial activity
Antimicrobial activity of the compounds and the ligands were stu-
died by screening them against three bacteria (E. coli, P. auriginosa and
S. aureus) and three fungi (F. verticilloides, F. chlamydosporum and F.
oxysporum). The screening results of the antimicrobial assay of the
synthesized compounds and the ligands along with the standard com-
pounds streptomycin and fluconazole as positive control are shown in
Table 4. From the table, it is seen that only compound 3 exhibited
antimicrobial activity while the ligands and the compounds 1 and 2 did
not show any activity. Therefore, we have performed dose dependant
quantitative assay for only compound 3 against the bacterium (S.
aureus) and the fungus (F. oxysporum) which exhibited significant an-
timicrobial activities in the screening tests along with standard anti-
biotic compounds streptomycin and fluconazole. The results of the
Table 4
Microbial growth inhibition properties of three compounds (1 mg/mL) against
bacteria and fungi along with standard antibiotics.
Sl.No. Test samples compounds/
antibiotics
Microbial growth inhibition zone*
The antimicrobial activity of the compound 3 against the bacteria S.
aureus is found to be comparable with the related triorganotin(IV)
compounds with azo- carboxylate ligands reported earlier [48]. The
antimicrobial activity of compound 3 was found to exhibit higher ac-
tivity than the standard antibiotics streptomycin and fluconazole.
Bacteria
Fungi
EC
PA
SA
FV
FC
FO
1.
2.
3.
4.
5.
6.
7.
1
2
−
−
−
−
−
+
NT
−
−
−
−
−
+
NT
−
−
+++
−
−
−
−
+
−
−
NT
−
−
−
++
−
−
−
−
+++
−
−
3
H2L1
H2L2
4. Conclusions
Streptomycin
Fluconazole
+
NT
NT
−
NT
−
Three triorganotin(IV) complexes 1–3 were synthesized by reacting
azo-carboxylic acid ligands with triorganotin(IV) chlorides using trie-
thylamine base. The complete characterization of the complexes was
accomplished by elemental analysis, UV–Vis, IR and multinuclear NMR
EC = E. cloli, PA = P. auriginosa, SA = S. aureus, FV = F. verticilloides, FC = F.
chlamydosporum and FO = F. oxysporum. [*Inhibition of microbial growth with
more than 5, 10 and 15 mm are marked with ‘+’, ‘++’ and ‘+++’ respec-
tively. ‘−’ = Not active; NT = Not tested].
spectroscopy. Crystal structure of compound
2 exhibit polymeric
7

P. Debnath, et al.
InorganicaChimicaActa498(2019)119172
structure while 1 and 3 showed dimeric structures. The geometry
around tin atom in all the complexes is trigonal bipyramidal geometry
where three alkyl groups occupy the equatorial plane while the axial
positions are being occupied by carboxylate and phenoxide oxygen
atoms. Carboxylate ligands coordinate in bidentate bridging fashion in
2 while monodentate mode in 1 and 3. The X-ray crystal structure
analysis of the compounds reveal that there is intra-molecular hydrogen
bonding between NeH…O in the structures. NMR spectral studies
suggest 4-coordinate geometry of all the complexes in solution state.
Thus, five coordinate structures of the complexes in solid state upon
dissolution undergo dissociation into four coordinate species in solu-
tion. The compounds were also screened for their antimicrobial activity
against few microbes and compared with standard antibiotics strepto-
mycin and fluconazole Compound 3 showed effective antimicrobial
activity against the bacteria S. aureus and fungal species F. oxysporum.
Since compound 3 exhibits higher antimicrobial activity than the
standard antibiotics against few microbes, this compound could be
further investigated for its antimicrobial activity and could be con-
sidered as a potential candidate for antimicrobial therapeutic applica-
tions.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
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Appendix A. Supplementary data
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