Synthesis, characterization, crystal structures and anti-diabetic activity of organotin (IV) complexes with 2-(4-hydroxynaphthylazo)-benzoic acid

Article abstract of DOI:10.1016/j.ica.2020.119736

Three new organotin(IV) complexes 1–3 were synthesized by the reaction of 2-(4-hydroxynaphthylazo)-benzoic acid with bis-tributyltin(IV) oxide (1), dibutyltin(IV) oxide (3) or trimethyltin(IV) chloride (2), respectively. The complete characterization of t

Full text of DOI:10.1016/j.ica.2020.119736

InorganicaChimicaActa510(2020)119736
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Inorganica Chimica Acta
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Research paper
Synthesis, characterization, crystal structures and anti-diabetic activity of
organotin (IV) complexes with 2-(4-hydroxynaphthylazo)-benzoic acid
⁎
Paresh Debnatha, Keisham Surjit Singha, , Thokchom Sonia Devib, 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:
Three new organotin(IV) complexes 1–3 were synthesized by the reaction of 2-(4-hydroxynaphthylazo)-benzoic
acid with bis-tributyltin(IV) oxide (1), dibutyltin(IV) oxide (3) or trimethyltin(IV) chloride (2), respectively. The
complete characterization of the complexes was accomplished by elemental analysis, IR and multinuclear [1H-
and 13C- and 119Sn-] NMR spectroscopy. The crystal structures of all the complexes were elucidated with the help
of X-ray single crystal diffraction analysis. The geometry around tin atoms in 1 and 2 was trigonal bipyramidal
geometry where the equatorial plane was occupied by the three alkyl groups (Bu or Me) and the axial positions
in 1 were being occupied by carboxylate and phenoxide oxygen atoms giving rise to a polymeric structure. In 2, a
hydroxy oxygen atom bridges two tin atoms occupying axial position while the other axial positions in each tin
atom were being occupied by a carboxylate oxygen atom or oxygen atom of a water molecule respectively
thereby completing trigonal bipyramidal geometry. The structure of 3 was a di-nuclear complex with six-co-
ordinate distorted skew trapezoidal and seven-coordinate pentagonal bipyramidal geometry around the tin
atoms, respectively. In the dinuclear structure, one of the tin atoms is coordinated by a terminal azo-ligand while
the other tin atom is coordinated by two terminal azo-ligands. In addition to this, another azo-carboxylate ligand
bridges the two atoms in the dinuclear structure. The NMR study showed that in the solution the complexes 1
and 2 adopted four-coordinate geometry, while in 3 there is five-coordinate structure. Complexes 1 and 3 were
screened for their antidiabetic activities against α-glucosidase enzyme and results of the assay found that
compound 3 exhibited significant inhibition activity.
Organotin(IV) complexes
Spectroscopy
Crystal structures
Antidiabetic activities
1. Introduction
that exhibited as polymeric, monomeric, dimeric and macrocylic
structures [15–20]. Organotin(IV) azo-carboxylates are also important
Organotin compounds particularly organotin(IV) carboxylates were
found to exhibit important biological activities such as biocides, cyto-
toxicity, antimicrobial, anti-proliferation, antitumor, anticancer, anti-
tuberculosis and anti-inflammatory [1–8]. Organotin(IV) carboxylates
also displayed a wide range of intriguing structural diversity for in-
stance they showed monomers, dimers, tetramers, hexameric, cyclic
drum, ladder, 1D-, 2D- macro-cyclic structures etc. [8–13]. Moreover,
diorganotin(IV) carboxylates [R = Me, n-Bu and Ph] with pyridine
dicarboxylic acid exhibited interesting macro-cyclic hybrids having
porous solid- state structures [14]. In addition to this, considerable
attention have also been given to organotin(IV) compounds with azo-
carboxylates because of their various intriguing molecular structures
for their potential biological applications as anti-cancer, cytotoxicity
and toxicity on mosquito larvae [16–19]. Furthermore, in recent years
our research group has been working on organotin compounds with
some functionalized azo-carboxylates derived from 2- and 4- amino
benzoic acids and their molecular structures and biological properties
were studied [21–23]. The compounds were found to exhibit en-
couraging antimicrobial and antidiabetic activity and in some cases,
they showed activity even better than standard drug compounds. In
addition, more recently we have also studied chemistry of organotin
compounds with some azo-carboxylates derived from 2- or 4-amino
benzoic acids and naphthalen-2-ol [24–25]. The molecular structures of
these organotin compounds were determined by X-ray crystallography.
⁎ Corresponding authors.
E-mail addresses: surjitkeisham@yahoo.co.in (K.S. Singh), waldemar.maniukiewicz@p.lodz.pl (W. Maniukiewicz).
https://doi.org/10.1016/j.ica.2020.119736
Received 15 April 2020; Received in revised form 3 May 2020; Accepted 3 May 2020
Availableonline06May2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
8
The geometry around tin atoms in triorganotin complexes showed five
coordinate distorted trigonal bipyramidal geometry [24] while crystal
structure analysis of dibutyltin(IV) compounds with these ligand sys-
tems revealed structures ranges from five coordinate distorted trigonal
bipyramidal geometry or six coordinate distorted octahedral or skew
trapezoidal structures [25]. Therefore, as part of our ongoing research
work on organotin chemistry with these functionalized azo-carboxylate
ligands, now we are interested to explore further into the chemistry of
organotin(IV) complexes with azo-carboxylate derived from 2-amino
benzoic acid and naphthalen-1-ol. Herein, we employed naphthalen-1-
ol (α-naphthol) as coupling moiety which introduces hydroxy group at
4-position in the naphthalene ring in the ligand framework. By em-
ploying this azo-carboxylate ligand for the synthesis of organotin (IV)
compounds, we expect to get different structures from those organotin
compounds [24] where hydroxy group is present at 2-position (naph-
thalen-2-ol) in the naphthalene ring of azo-carboxylate ligand frame-
work which consequently would influence their biological activities.
Thus bearing all these in mind, in this present report, we have syn-
thesized three organotin(IV) compounds by reacting bis-tributyltin(IV)
or dibutyltin(IV) oxide or trimethyltin(IV) chloride with this azo-car-
boxylate ligand functionalized at 4-position with hydroxy group in the
naphthalene ring. The newly synthesized compounds were character-
ized with the help of elemental analysis, IR and multinuclear (1H–13C-
and 119Sn)-NMR spectroscopy. The structures of all organotin com-
pounds were determined by X-ray crystallography and the results of
study are discussed herein. Anti-diabetic activity of complexes 1 and 3
and the ligand was investigated using alpha- glucosidase enzyme and
compared with standard drug and results of the assay are discussed in
this contribution.
5'
7
6
9
4'
3'
6'
10
N
1'
N
2'
1
5
2
4
HO
HO
O
3
2-(4-hydroxynaphthylazo) benzoic acid (H2L)
Scheme 1. The structural formula and numbering scheme for the ligand H2L.
with distilled water for several hours till it became neutral and finally
the product was dried on water bath. The azo ligand was then re-
crystallized from methanol to get the pure deep red crystalline product
of the ligand. Yield: 7.9 g, 74%; m.p.: 232–234 °C. Anal. calcd. for
C17H12N2O3: C, 69.86; H, 4.14; N, 9.58%. Found: C, 69.54; H, 4.21; N,
9.43%.IR (KBr, cm−1): 3416 ν(OH), 1673 ν(COO)asy, 1455 ν(N]N),
1187 ν(CeO). 1H NMR (DMSO‑d6, 400.13 MHz) δH: 12.9 [s, 1H, OH],
8.49 [d, 1H, H-9, J = 7.6 Hz], 8.06 [d, 1H, H-6, J = 7.6 Hz], 7.92 [m,
́
2H, H-2, H-3], 7.72 [m, 2H, H-7, H-8], 7.64 [t, 1H, H-4′, J = 7.6 Hz],
́
́
7.55 [t, 1H, H-5, J = 7.6 Hz], 7.18 [m, 1H, H-6 ], 6.79 [d, 1H, H-3,
J = 8.8 Hz ] ppm; Signal for –COOH proton could not be detected due
to rapid solvent exchange. 13C NMR (DMSO‑d6, 100.62 MHz) δC: 179.9
́
[COO], 169.8 [C-4],146.1 [C-1′], 134.5 [C-1], 134.1 [C-5 ], 133.8 [C-
10], 131.3 [C-3ʹ], 130.8 [C-4ʹ], 128.4 [C-6], 115.0 [C-3]; other signals:
127.5, 124.6, 122.0, ppm. The structural formula and numbering
scheme for the H2L is shown in Scheme 1.
2. Experimental
2.2.2. Synthesis of Bu3SnHL (1)
2.1. Materials and methods
Tributyltin(IV) compound 1 was synthesized by the reaction of bis-
tributyltin(IV) oxide with 2-(4-hydroxynaphthylazo) benzoic acid
[H2L] in (M: L = 1:1) molar ratio in refluxing condition by using Dean-
Stark moisture trap apparatus. In this process, 2-(4-hydro-
xynaphthylazo) benzoic acid (0.147 g, 0.503 mmol) was added to
50 mL anhydrous toluene in a round bottom flask on an oil bath fitted
with the Dean-Stark apparatus and was refluxed for 30 min to get a red
colored suspension. Then, bis-tributyltin (IV) oxide (0.3 g, 0.503 mmol)
was added to the suspended solution with continuous stirring. Stirring
was continued for 6 h and the water produced during reaction was
trapped by the Dean-Stark receiver. The red colored solution was then
filtered and filtrate was kept for about 7 days to get dark red crystals of
the desired complex 1. Yield: 0.47 g, 88%; m.p.: 110–112 °C. Anal.
calcd. for C29H38N2O3Sn: C, 49.92; H, 6.59; N, 4.82%. Found: C, 49.30;
H, 6.35; N, 4.55%. IR (KBr, cm−1): 3440 ν(NeH str.), 2921 ν(CeH str.
of Bu), 1622 ν(COO)asym, 1447 ν(N]N), 1351 ν(COO)sym 1149 ν(CeO),
676 ν(Sn-C), 495 ν(Sn-O).1H NMR (CDCl3, 400.13 MHz) δH, Ligand
skeleton: 13.08 [s, 1H, OH], 8.48 [d, 1H, H-9, J = 8.4 Hz], 8.20 [d, 1H,
H-6, J = 8.0 Hz], 8.06 [d, 1H, H-3′, J = 6.8 Hz], 7.99 [d, 1H, H-8,
J = 8.4 Hz], 7.73[d, 1H, H-2, J = 10.4 Hz], 7.65 [t, 1H, H-7,
J = 7.6 Hz], 7.55 [t, 1H, H-4′, J = 7.6 Hz ], 7.49[t, 1H, H-5′,
J = 7.6 Hz], 7.01[t, 1H, H-6′, J = 7.6 Hz], 6.74[d, 1H, H-3,
J = 10.4 Hz]; Sn-nBu Skeleton: 1.71[m,6H, H-α], 1.40 [m, 12H, H-β &
H-γ], 0.94 [t, 9H, H-δ] ppm. 13C NMR (CDCl3, 100.62 MHz) δC: 185.1
Dibutyltin(IV) oxide, bis-tributyltin(IV) oxide, tributyltin(IV)
chloride, trimethyltin(IV) chloride, 2-amino benzoic acid, naphthalen-
1-ol and triethyl amine were obtained from MERCK by purchase and
were used without further purification. All solvents used for the reac-
tion were dried following standard procedures. The elemental (Carbon,
Hydrogen and Nitrogen) analyses were obtained from Perkin Elmer
2400 series II instrument. For recording IR spectra (4000–400 cm−1
using KBr discs) Shimadzu FT-IR-8400S spectrophotometer was em-
ployed. The multinuclear (1H, 13C and 119Sn) NMR spectra were re-
corded on Bruker AMX 400 spectrometer measured at 400.13 for 1H,
100.62 for 13C and 149.18 MHz for 119Sn-NMR using Me4Si 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. Synthesis
2.2.1. Synthesis of 2-(4-hydroxynaphthylazo) benzoic acid (H2L)
The azo-carboxylate ligand 2-(4-hydroxynaphthylazo) benzoic acid
(H2L) was prepared by usual diazo-coupling reaction of 2-amino ben-
zoic acid with 1-naphthol following our analogous procedure [24,25].
To 5 g (36.45 mmol) 2-aminobenzoic acid, 12 mL concentrated HCl and
40 mL water was added and then digested on a water bath for about 1 h
till the solution became clear. It was then kept in refrigerator for
overnight 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. The diazotized so-
lution was then added to 10% NaOH (5 g in 50 mL water) solution of 1-
naphthol (5.256 g, 36.45 mmol) at about 0–2 °C with vigorous stirring
to form a deep red colored solution and it was continued under stirring
condition for about 3 h. The reaction mixture was again kept in re-
frigerator for overnight. After 2–3 h at room temperature, the reaction
mixture was acidified with dilute acetic acid to form the red precipitate
of the desired azo-ligand. The final product was then filtered, washed
́
[COO], 173.1 [C-4],145.5 [C-1′], 135.9 [C-1], 134.1 [C-5 ], 133.4 [C-
10], 132.5 [C-3ʹ], 132.1 [C-4ʹ], 130.2 [C-6], 129.6 [C-8], 127.8 [C-7],
126.0 [C-2′], 124.5 [C-2], 122.9 [C-5], 121.0 [C-9], 114.6 [C-6′], 114.0
[C-3]; Sn-nBu Skeleton: 27.9 [C-β], 27.0 [C- γ] 3J [119Sn-13C(63.3 Hz)],
17.0 [C-α] 1J [119Sn-13C(344.1 Hz)], 13.7 [C-δ] ppm. 119Sn NMR
(CDCl3, 149.18 MHz): + 127.19 ppm.
2.2.3. Synthesis of [(CH3)3Sn]2(OH)(H2O)HL (2)
The compound 2 was synthesized by the reaction of trimethyltin(IV)
2

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
chloride with 2-(4-hydroxynaphthylazo) benzoic acid using triethyla-
mine as base under refluxing condition with (M:L = 2:1) molar ratio. In
this procedure, 2-(4-hydroxynaphthylazo) benzoic acid (0.219 g,
0.752 mmol) was dissolved in 30 mL anhydrous methanol in a round
bottom flask in presence of triethylamine base (0.152 g, 1.505 mmol)
and was refluxed on an oil bath for about 30 min with an equipped
water cooled condenser. Then, trimethyltin(IV) chloride (0.3 g,
1.5055 mmol) was then added to the reaction mixture with continuous
stirring. It was again refluxed for about 6 h and then filtered. The
precipitate containing Et3N.HCl was filtered off and the filtrate was
then collected, evaporated to dryness. The final product was washed by
hexane to get the pure deep red crystalline product. The small crys-
talline product obtained after purification was then recrystallized from
anhydrous methanol to obtain pure red crystals of the complex 2. Yield:
0.672 g, 68%; m.p.: 216–217 °C. Anal.calcd. for C23H32N2O5Sn2: C,
42.24; H, 4.93; N, 4.28%. Found: C, 42.67; H, 4.32; N, 4.13%. IR (KBr,
cm−1): 3439 ν(NH), 2923 ν(CeH str. of Sn-CH3), 1639 ν(COO)asy, 1450
ν(N]N), 1352 ν(COO)sym 1160 ν(CeO), 681 ν(Sn-C), 522 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-6, J = 7.6 Hz], 8.12 [d, 1H, H-3,
J = 8.0 Hz], 7.59 [m, 2H, H-7, H-8], 7.48[m, 2H, H-2 , H-4′], 7.37 [m,
Dualflex, Pilatus 300 K diffractometer [26] with Photon Jet micro-focus
X-ray Source while the crystals 2 and 3 were measured using a Bruker
AXS Smart APEX-II CCD diffractometer [27]. Data collection, cell re-
finement, data reduction and absorption correction were carried out
using CrysAlis PRO software [26] for H2L and 1. Whereas the SMART
and SAINT-PLUS [28] programs were used for 2 and 3, respectively.
The crystal structures were solved by using direct methods with the
SHELXT 2018/2 program [29]. Atomic scattering factors were taken
from the International Tables for X-ray Crystallography. Positional
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 [30]. All hydrogen atoms were placed in calculated
positions (CeH = 0.93–0.98 Å) and included as riding contributions
with isotropic displacement parameters set to 1.2–1.5 times the Ueq of
the parent atom. Despite many attempts to crystallize, the quality of
crystals used in the analysis was not very good, therefore there were
some problems with refining the structures. In structure 1, a disorder in
one of the butyl groups was observed. Thus the ellipsoids of some C
atoms in the disordered region were odd-shaped. The SIMU restraints
have been applied in the refinement and a slight improvements in shape
o ellipsoids was obtained. The disorder ratio was 0.731(9):0.269(9). In
structure 2 all attempts to model the disorder in water molecule in
order to find proper positions of hydrogen atoms failed. So, we decided
to bypass these hydrogen atoms from the model of structure during the
refinement. In addition, for 2 and ligand salt data twin refinement was
required. In 1 and 3 contribution of highly disordered solvent (pre-
sumably toluene and/or water), which could not be modeled, was re-
moved by SQUEEZE routine using PLATON [31]. For structure 1, the
three voids of about 350 Å3 each containing 59 electrons were found.
Whereas for structure 3 the one void volume was 520 Å3 and contained
125 electrons. Crystal data and structure refinement parameters are
shown in Table 1, while selected geometric parameters are collected in
Table 2. Hydrogen bonding parameters are presented in Table 3.
1H, H-5′], 7.16 [t, 1H, H-6′,
J = 8.0 Hz], 6.67[d, 1H, H-3,
J = 10.0 Hz]; Sn-CH3Skeleton: 0.75 [s, 9H, (Sn-CH3)] 2J [119Sn-1H
(57.21 Hz)] ppm. 13C NMR (CDCl3, 100.62 MHz) δC, Ligand skeleton:
179.2 [COO], 173.7 [C-4],144.2 [C-1′], 141.8 [C-1], 134.3 [C-5′],
133.7 [C-10], 133.3 [C-3′], 132.5 [C-4ʹ], 129.1 [C-6], 128.7 [C-8],
128.4 [C-7], 127.5 [C-2′], 126.5 [C-2], 125.6 [C-5], 124.1 [C-9], 122.4
[C-6′], 116.3 [C-3]; Sn-CH3 Skeleton: −1.7 [Sn-CH3] ppm. 119Sn NMR
(CDCl3, 149.18 MHz): + 139.21 ppm.
2.2.4. Synthesis of [Bu2Sn(HL)2]2 (3)
Dibutyltin(IV) compound 3 was synthesized following analogous
procedure as in case of compound 1 where dibutyltin(IV) oxide (0.3 g,
1.20 mmol) was used instead of bis-tributyltin(IV) oxide with 2-(4-hy-
droxynaphthylazo) benzoic acid (0.704 g, 2.41 mmol) in (M:L = 1:2)
molar ratio fixed with the Dean-Stark apparatus under refluxing condi-
tion to get deep red compound 3. Yield: 0.67 g, 84%; m.p.: 211–212 °C.
Anal.calcd. for C84H80N8O12Sn2: C, 61.86; H, 4.94; N, 6.87%. Found: C,
61.73; H, 5.1; N, 6.81%. IR (KBr, cm−1): 3139 ν(NeH str.), 2921 ν(CeH
str. of Bu), 1627 ν(COO)asym, 1475 ν(N]N), 1440 ν(COO)sym 1178
ν(CeO), 675 ν(Sn-C), 501 ν(Sn-O).1H NMR (CDCl3, 400.13 MHz) δH,
Ligand skeleton: 12.5 [s, 1H, OH], 8.48 [d, 1H, H-9, J = 8.0 Hz], 8.17
[m, 2H, H-2 & H-3′], 8.04 [d, 1H, H-6, J = 8.0 Hz], 7.67 [m, 3H, H-7, H-
8 & H-4′], 7.51[t, 1H, H-5′, J = 7.6 Hz], 7.11 [t, 1H, H-6′, J = 7.6 Hz],
6.74 [d, 1H, H-3, J = 10 Hz]; Sn-nBu Skeleton: 1.95[m,2H, H-α], 1.82
[m, 2H, H-β], 1.50 [m, 2H, H-γ], 0.93 [t, 3H, H-δ] ppm. 13C NMR (CDCl3,
100.62 MHz) δC: 184.9 [COO], 176.7 [C-4],145.8 [C-1′], 135.6 [C-1],
2.4. Anti-diabetic assay
Anti-diabetic activities of compounds 1–3 and standard drug acar-
bose were screened following the 96-well microplate-based α-glucosi-
dase assay as described by Kumar et al.[32] and also using our earlier
antidiabetic assay [25]. In this method, 1 mg of each compound was
dissolved in 20 µL DMSO (D) and Ethanol (E) and diluted to 1000 µL in
a 2 mL Eppendorf tube with sterile water (Milli-Q) and these samples
were employed for the enzyme assay. Studies of the α-glucosidase en-
zyme inhibition assay were performed in a reaction volume of 75 μL
using 96-well microplate. In this method, 25 μL each of sample solution
and α-glucosidase enzyme (0.5U) were gently mixed and pre-incubated
at 37 °C 1 °C for 10 min. Then, 25 μL of the substrate (0.5 mM, p-
nitrophenyl α-D glucopyranoside, PNPG) was added to the reaction
mixture and incubated again at 37 °C 1 °C for 30 min. After this, by
́
134.1 [C-5 ], 132.8 [C-10], 132.3 [C-3ʹ], 130.2 [C-4ʹ], 129.0 [C-6], 128.2
[C-8], 126.1 [C-7], 125.3 [C-2′], 123.9 [C-2], 123.0 [C-5], 121.3 [C-9],
114.5 [C-6′], 112.4 [C-3]; Sn-nBu Skeleton: 26.9 [C-β], 26.4 [C- γ], 21.4
[C-α], 13.6 [C-δ] ppm. 119Sn NMR (CDCl3, 149.18 MHz): −133.39 ppm.
The numbering scheme of Sn-Bu skeletal in the tributyltin(IV) and di-
butyltin(IV) complexes is shown below
adding 100 μL of 0.2 M sodium carbonate solution, the reaction was
terminated. The amount of p-nitrophenol released from PNPG (yellow
colour) was then quantified on a 96-well microplate at 405 nm using
UV visible spectrophotometer (Mustikan GO, Thermo-Scientific, Fin-
land). The α-glucosidase inhibitor drug (Acarbose, ACB) was employed
as standard reference. All reactions were performed in five replicates.
The percentage of α-glucosidase inhibition activity was calculated with
the help of the formula i.e. α-glucosidase inhibition activity
(%) = [(Control OD-Sample OD) / Control OD] x100) where Control
OD = OD of the control reaction without inhibitor-Blank OD; Sample
OD = Sample OD-Sample blank OD]. Similarly, the experiment for
dose–effect analysis of antidiabetic activity was performed using in vitro
α-glucosidase assay in a reaction volume of 75 μL using 96-well mi-
croplate as described above. A sample solution of 5, 10, 15, 20 and
25 μL (~67, 133, 266 and 333 μg/mL) were mixed with 25 μL of the α-
2.3. Crystallographic data collection and structure refinement
Single crystal X-ray diffraction data were collected by the ω-scan
technique using MoKα (λ = 0.71073 Å) radiation. The H2L salt and 1
crystals were studied at 100 K using a RIGAKU XtaLAB Synergy,
3

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
Table 1
Crystal data and structure refinement parameters for the H2L salt and complexes 1, 2 and 3.
Parameters
H2L salt
1
2
3
Empirical formula
Formula weight
Temperature(K)
Crystal system
Space group
a (Å)
C
23H31N3O5
C29H38N2O3Sn
C24H36N2O6Sn2
685.97
C84H80N8O12Sn2
1630.94
429.51
581.32
100(3)
100(3)
100(3)
296(2)
Triclinic
Trigonal
Triclinic
Triclinic
P-1
R3c
P-1
P-1
10.7854(3)
13.5964(3)
15.4929(3
103.894(2)
90.498(2)
91.519(2)
2204.41(9)
4
38.4630(3)
7.5426(4)
12.1092(7)
15.8208(9)
97.935(2)
92.760(2)
107.510(2)
1358.68(13)
2
12.6862(4)
19.2092(7)
19.2904(7)
63.733(2)°
78.236(2)°
88.210(2)°
4117.6(3)
b (Å)
38.4630(3)
c (Å)
10.2363(1)
α (°)
90
β (°)
90
γ(°)
120
Volume(Å3)
Z
13114.8(3)
18
2
Density(Mg/m3)
Absorp. coeff.(mm−1
F(0 0 0)
1.294
1.325
1.677
1.315
)
0.092
0.906
1.877
0.669
920
5400
684
1672
Crystal size (mm3)
Radiation
0.55 × 0.28 × 0.21
MoKα(λ = 0.71073)
0.22 × 0.28 × 0.33
MoKα(λ = 0.71073)
2.56 to 25.02°
−45≤h≤45,
−45≤k≤45,−12≤l≤ 12
131,510
0.08 × 0.25 × 0.29
MoKα(λ = 0.71073)
1.30 to 25.02°
−8≤h≤8,
−14≤k≤14,0≤l≤18
9917
0.10 × 0.10 × 0.05
MoKα(λ = 0.71073)
2.13 to 24.74°
−14≤h≤14, –22≤k≤22,
–22≤l≤22
93,972
Theta range for data collection 2.31 to 25.03°
Index ranges
−12≤h≤12, −16≤k≤16,
−18≤l≤18
Reflection collected
Independent reflections
Goodness of fit on F2
Final R indices[I greater
than 2sigma(I)]
31,156
15,389 [R(int) = 0.0858]
1.061
5150 [R(int) = 0.031]
1.10
4588 [R(int) = 0.0127]
1.15
13,973 [R(int) = 0.0858]
0.98
R1 = 0.0585, wR2 = 0.1771
R1 = 0.0130, wR2 = 0.0342
R1 = 0.0236, wR2 = 0.0785
R1 = 0.0577, wR2 = 0.1352
R indices(all data)
R1 = 0.0665, wR2 = 0.1854
R1 = 0.0131, wR2 = 0.0343
R1 = 0.0242, wR2 = 0.0798
R1 = 0.1284, wR2 = 0.1598
Largest diff. peak and
0.315 and −0.312
0.375 and −0.258
0.636 and −0.584
0.688 and −0.458
hole(eÅ−3
)
glucosidase enzyme (0.5U) and then volume was made up to 75 μL with
assay buffer. The reaction mixture was pre-incubated at 37 °C 1 °C
for 10 min and then 25 μL of the substrate (0.5 mM, PNPG) was added
solution. Amount of p-nitrophenol released from PNPG was quantified
on a 96-well microplate at 405 nm on a UV visible spectrophotometer
(Mustikan GO, Thermo-Scientific, Finland). All reactions were per-
formed in five replications. The percentage of α-glucosidase inhibition
activity was calculated by using the same formula as mentioned above.
to the reaction mixture and incubated at 37 °C
1 °C for 30 min. The
reaction was terminated adding 100 μL of 0.2 M sodium carbonate
Table 2
Selected bond lengths (Å) and bond angles (°) for H2L salt and the tri or diorganotin (IV) complex 1, 2 and 3.
Atoms
H2LBond Length (Å)
Atoms
1Bond Length (Å)
Atoms
2Bond Length (Å)
Atoms
3Bond Length (Å)
N1-N2
O1-C1
O2-C1
O3-C11
N1-C7
N2-C8
N3-N4
O4-C21
O5-C21
O6-C31
1.334(3)
1.273(3)
1.255(3)
1.230(3)
1.389(3)
1.306(4)
1.330(3)
1.268(3)
1.257(3)
1.231(3)
Sn1-O1
2.148(2)
2.537(3)
2.143(3)
2.151(3)
2.134(3)
Sn1-O1
Sn1-O4
Sn2-O4
Sn2-O5
Sn1-C18
Sn1-C19
Sn1-C20
Sn2-C21
Sn2-C22
Sn2-C23
2.275(5)
2.188(5)
2.136(5)
2.402(5)
2.138(8)
2.134(8)
2.139(8)
2.132(7)
2.120(8)
2.123(8)
Sn1-O1
Sn1-O2
Sn1-O4
Sn1-O5
Sn1-O9
Sn2-O7
Sn2-O8
Sn2-O10
Sn2-O11
Sn1-C35
2.479(5)
2.165(4)
2.506(4)
2.188(4)
2.690(5)
2.089(4)
2.627(5)
2.104(4)
2.467(5)
2.074(8)
Sn1-O3#1
Sn1-C18
Sn1-C22
Sn1-C26
N3-C27
N4-C28
1.389(3)
1.307(3)
Sn1-C39
Sn2-C77
2.096(7)
2.096(9)
Sn2 -C81
Atoms
2.095(8)
3
Atoms
H2L
Atoms
1
Atoms
2
Bond Angle (°)
119.6(2)
120.0(2)
121.0(2)
126.1(2)
116.2(2)
119.5(2)
120.5(2)
120.3(2)
126.0(2)
116.0(2)
Bond Angle (°)
171.20(8)
123.84(11)
92.02(10)
100.91(10)
116.49(11)
115.56(12)
81.90(10)
80.18(10)
87.54(9)
Bond Angle (°)
176.31(19)
89.1(2)
Bond Angle (°)
55.36(13)
85.77(15)
79.07(14)
55.57(13)
84.15(15)
54.34(19)
80.24(18)
156.5(3)
135.5(3)
N2-N1-C7
O1-Sn1-O3#1
C22-Sn1-C26
O1-Sn1-C18
O1-Sn1-O4
O1-Sn1-O2
O1-Sn1-O9
O2-Sn1-O5
O4-Sn1-O5
O4-Sn1-O9
O7-Sn2-O8
O7-Sn2-O10
C35-Sn1-C39
C77-Sn2-C81
N1-N2-C8
O1-Sn1-C18
O1-Sn1-C19
O1 -Sn1-C20
C18-Sn1-C19
O4-Sn2-O5
N1-C7-C6
90.4(3)
N2-C8-C9
O1-Sn1-C22
85.1(2)
N2-C8-C17
N4-N3-C27
N3-N4-C28
N3-C27-C26
N4-C28-C29
N4-C28-C37
C18-Sn1-C22
C18-Sn1-C26
O3#1-Sn1-C18
O3#1-Sn1-C26
O3#1-Sn1-C22
123.7(3)
179.5(2)
93.3(2)
O4-Sn2-C21
O4-Sn2-C22
O4-Sn2-C23
C22-Sn2-C23
95.0(3)
95.9(3)
120.8(3)
(#1)
1/3-x + y,5/3-x,2/3 + z;
4

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
Table 3
carboxylate ligand as monodentate or bidentate in complexes in solid
state [15,23,24,33,34]. The asymmetric [νasy(COO)] stretching ab-
sorption band of the ligand was observed at 1673 cm−1 and in the
complexes 1–3, this band was shifted to lower frequency range at
1622–1639 cm−1 which indicates the carboxylate coordination to Sn-
atom in the complexes [16,17,23]. Usually, carboxylate displays two
type of bands, one strong asymmetric stretching band [νasy(COO)] at
1580–1650 cm−1 and a weaker symmetric stretching band [νsym(COO)]
near 1320–1450 cm−1 [15,23,24,33,34]. Moreover, Δν[νasy(COO)- νsym
(COO)] value can be used for determining the coordination behavior of
carboxylate ligand. If this value is below 200 cm−1, then it indicates a
Hydrogen bonding parameters for the H2L salt and tri- or diorganotin(IV)
complex 1, 2 and 3.
Compound D-H…..A
d(D-H) d(H….A) d(D…A)
< (DHA)
H2L salt
N(1)-H(1 N)…O(2)
0.88
0.88
1.97
.98
2.650(3) 133
N(3)-H(3 N)…O(5)
N(5)-H(5 N)…O(1)
N(6)-H(6 N)…O(4)
O(7)-H(7A)…O(8)I
O(7)-H(7B)…O(1)
O(8)-H(8A)…O(7)
O(8)-H(8B)…O(5)
2.648(3) 132
2.655(3) 174
2.693(3) 171
2.803(3) 169
2.807(3) 173
2.791(3) 166
2.837(3) 177
2.792(3) 170
2.779(4) 164
2.845(3) 170
3.498(3) 169
3.481(3) 165
2.600(3) 145(3)
1.00
1.00
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.95
0.95
1.66
1.70
1.96
1.96
1.96
1.96
1.95
1.95
2.00
2.56
2.55
bi-dentate mode of coordination while if Δν is greater than 200 cm−1
,
then the presence of mono-dentate mode of coordination of the car-
boxylate ligand to the tin atom can be assigned in the complexes
[21,24,33,34]. The typical νasy(COO) asymmetric stretching frequency
bands for complexes 1 and 2 were observed at 1622 and 1639 cm−1
while the symmetric stretching absorption bands observed at
1351 cm−1 and 1352 cm−1 and their corresponding Δν was found to be
271 cm−1 and 287 cm−1 respectively. Hence it can be suggested, that
in complexes 1 and 2, the carboxylate ligand coordinate to Sn-atom in
mono-dentate mode of coordination [21,35]. Furthermore, complex 3
shows asymmetric IR stretching frequency band at 1627 cm−1 while
the symmetric stretching frequency band at 1440 cm−1 and the value of
Δν was found to be 187 cm−1. This suggests bi-dentate mode of co-
O(9)-H(9A)…O(4)
O(9)-H(9B)…O(10)
O(10)-H(10B)…O(2)II
C(4)-H(4)…O(5)I
ordination of carboxylate ligand to the Sn-atom in complex
3
C(24)-H(24)…O(2)II
N(1)-H(1)…O(2)
[21–24,35]. The coordination mode of carboxylate ligands in these
complexes were found to be in fully consistent with the single crystal
structures of the complexes (vide infra). Moreover, the IR absorption
bands observed in the range 675–681 cm−1 and 495–522 cm−1 in
complexes 1–3 are assigned for Sn-C and Sn-O respectively [23,36,37].
1
2
0.86(3) 1.84(4)
C(23)-H(23A)…O(3)III
N(1)-H(1)…O(2)
0.99
2.56
3.285(4) 130
2.592(7) 134
2.734(8) 163
2.842(7) ——
3.001(7) ——
2.789(8) 163
2.897(8) 170
0.88
1.90
O(1 M)-H(1 M)…O(2)
0.84
1.92
O(5 W)…O(3)IV
———
———
———
———
1.98
O(5 W)…O(3)V
3.2.2. Multinuclear (1H, 13C and 119Sn) NMR spectroscopy
O(4)-H(4O)…O(1 M)VI 0.84
1H-, 13C- NMR spectroscopic characterization data of the azo-car-
boxylate ligand was recorded in DMSO‑d6 while 1H, 13C and 119Sn NMR
spectroscopic data for the complexes 1–3 were recorded in CDCl3 and
their spectroscopic data are provided in the experimental section. Some
representative spectra for the ligand and complexes were also provided
in the Electronic Supplementary Information (ESI). The assignments of
1H-, 13C- NMR spectra of the ligand was accomplished by examining its
chemical shift values, integration values, multiplicity patterns and also
by careful comparison of the proton NMR spectroscopic data of similar
type of azo-ligands derived from either 2-amino benzoic acid
[21,23,24] or 1-naphthol reported earlier [38–40]. The assignments of
the chemical shift position of the protons and carbons for the complexes
were achieved by examining their chemical shift values, integration
values, multiplicity patterns and also by comparing ligand NMR data
with those of the complexes because of the similarity in their spectral
data. The 1H NMR spectroscopic data for aromatic protons in complexes
were obtained in the range 6.67–8.48 ppm. In case of complexes 1 and
3, the tin butyl protons showed a triplet and multiplet peaks in the
range 0.93–0.94 ppm and 1.40–1.95 ppm which can be attributed to
methyl and three methylene protons of the tin butyl protons respec-
tively [23,24]. In complex 2, a singlet peak was observed at 0.75 ppm
which is assigned to tin-methyl protons [21,24]. The 13C NMR spectra
of the azo carboxylate ligand showed δ (COO) signal at 179.9 ppm but
in case of complexes 1–3, the δ (COO) signals were appeared in the
range 179.2–179.9 ppm. The 13C NMR signals for aromatic ring carbons
of the free azo carboxylate ligand was observed in the range
115.0–169.8 ppm whereas in case of complexes 1–3, these signals were
observed in the range 114.0–176.7 ppm. The number of 1H and 13C
NMR signals observed in the ligand and the complexes were found to be
in consistent with the formulation of expected compounds. The geo-
metry and coordination number around tin atoms in solution state for
tin complexes can be determined by using 2J(119/117Sn-1H) and nJ
C(12)-H(12A)…O
(5)VII
0.95
1.95
3
N(2)-H(2A)…O(2)
N(3)-H(3A)…O(4)
N(5)-(H5A)…O(8)
N(8)-H(8A)…O(10)
0.86
0.86
0.86
0.86
1.95
1.95
1.98
1.98
2.612(6) 133
2.613(6) 133
2.638(7) 133
2.638(7) 132
(I)1-x,1-y,2-z;
1-x,1-y,1-z
1-x,1-y,1-z;
1/3-x + y,5/3-x,2/3 + z; (IV)2-x,1-y,1-z;
(II)
(VI)
(III)
(VII)
(V)
x,1 + y,1 + z;
x,-1 + y,-1 + z;
3. Results and discussion
3.1. Synthesis
Tributyltin(IV) complex (1) and dibutyltin(IV) complex (3) were
synthesized by reacting 2-(4-hydroxynaphthylazo) benzoic acid with
bis-tributyltin(IV) oxide and dibutyltin(IV) oxide in anhydrous toluene
using dean and stark apparatus in molar reaction ratio (1:1) and (1:2)
M:L respectively. On contrary, trimethyltin(IV) complex (2) was syn-
thesized by reacting the ligand with trimethyltin(IV) chloride in an-
hydrous methanol using triethylamine base in 2:1 (M: L) molar reaction
ratio. Complex 1 was also prepared adopting different reaction route by
reacting tributyltin(IV) chloride with the ligand in presence of Et3N
base with lesser yield compared to the previous method. So, we adopted
first method for the synthesis of 1. All the complexes were obtained in
good yield and found to have good solubility in most of the common
organic solvents. The reaction scheme for the synthesis of complexes
1–3 is shown in Scheme 2.
3.2. Spectroscopic characterization
3.2.1. IR spectroscopy
The IR spectroscopic data for azo-carboxylate ligand and com-
pounds 1–3 are provided in the experimental section. IR spectroscopy is
an important useful tool for determining the coordination behavior of
(
119Sn-13C) coupling constants [16,21,41–44]. It is commonly observed
that four coordinate tributyltin compounds exhibit 1J(119/117Sn-13C)
5

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
8
5'
7
6
4'
9
6'
10
H3C
3'
H3C
N
H
O
CH3
1'
N
H
2'
5
1
Sn
CH3
Sn
OH2
2
CH3
4
O
O
O
CH3
3
[(CH3)3Sn]2(OH)(H2O)HL (2)
Bu
Sn
Bu
6
7
M:L=2:1, Methanol,
O
Me3SnCl + Et3N
Reflux for 6-7 hrs
5
8
4
Bu
3
8
9
5'
10
7
9
4'
3'
6'
1
2
(Bu3Sn)2O
10
N
N
6
M:L=1:1, Toluene,
Reflux for 6-7 hrs
1'
N
2'
H
N
5
1
1'
6'
2
O
O
4
HO
HO
O
2'
3
Bu
Bu
5'
2-(4-hydroxynaphthylazo) benzoic acid (H2L)
Sn
Bu
4'
3'
Bu3SnHL (1)
M:L=1:2, Toluene,
Reflux for 6-7 hrs
Bu2SnO
O
6
4
5
7
N
N
3
8
2
10
H
1
9
O
O
O
O
O
H
(HL)
N
O
N
O
Sn
Bu
Bu
Sn
O
1'
2'
Bu
6'
(LH)
Bu
O
[Bu2Sn(HL)2]2 (3)
3'
5'
4'
Scheme 2. Reaction scheme for the synthesis of compounds 1, 2 and 3.
coupling in the range of 325–390 Hz and five coordinated ones in the
range of 430–540 Hz while four coordinate trimethyltin compounds
[16,21,42–46]. In case of complex 1, the 1J (119Sn-13C) and 3J
(
119Sn-13C) coupling satellites were observed at 344.1 Hz and 63.3 Hz
display 2J
(
119/117Sn-1H) coupling in the range 40–59 Hz
respectively which is in consistent with the four coordinate tetrahedral
6

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
geometry of the tin atom in solution state for the given complex
[21–24,42–44]. In addition, complex 2 displays 2J(119Sn-1H) coupling
(57.2 Hz) suggesting the 4-coordinate tetrahedral geometry of Sn-atom
in solution state [22,45–46]. However, 2J(119Sn-1H) values for com-
plexes 1 and 3, nJ(119Sn-13C) couplings for the complexes 2 and 3 could
not be assigned because of the poor signal-to-noise ratio and also low
intensities of the Sn-Bu resonances [18,23]. Furthermore, the geometry
around tin atom in organotin compounds in solution state can also be
determined by calculating angle between C-Sn-C using Lockhart and
Mander’s equation [47] {θ(C-Sn-C) = 0.0161 | 2J(Sn-H) |2 – 1.32 | 2J
(Sn-H) | + 133.4}. In case of complex 2, the calculated angle was found
to be 110.57° indicating the four coordinated tetrahedral geometry
[21,22,42,47]. The geometry of the complexes was also further con-
firmed by 119Sn NMR study from δ (119Sn) values of the complexes.
Generally, if δ(119Sn) values are found in the range −210 to −
400 ppm, the geometry corresponds for six coordinate, −90 to −190
for five coordinate and − 60 to + 200 ppm were assigned for four
coordinate geometry of tin in organotin complexes [18,23,24,48]. The
119Sn chemical shift values for the complexes 1, 2, and 3 were observed
at + 127.2, +139.2 and −133.4 ppm respectively. Hence, the geo-
metry around tin atom for complexes 1 and 2 are suggested to adopt
four coordinate structure in solution state [21,24,43–45] while in case
of 3, the geometry is five coordinate and the observed tin chemical shift
values are also in consistent with those for other reported R2Sn[O2CR′]2
type complexes [18,49–52].
asymmetric unit. Each 2-(4-hydroxynaphthylazo)-benzoic acid depro-
tonated at the carboxylic acid site with triethylammonium counter ions
is solvated by water molecule. Despite the fact that the geometry (see
Table 2) and conformation of both anions are similar, the search for
higher symmetry with the PLATON program [31] did not give a positive
result. The molecules are approximately planar with just small twists
about the central CeN bonds linking the aromatic rings, as indicated by
the torsion angles N(2)-N(1)-C(7)-C(6) = -2.9(4)° and N(1)-N(2)-C(8)-
C(9) = 0.2(4)° for molecule A and N(4)-N(3)-C(27)-C(26) = -1.5(4)°
and N(3)-N(4)-C(28)-C(29) = -1.6(4)° for molecule B, respectively. The
deprotonated carboxylic acid groups are also coplanar with its parent
phenyl ring [O(2)-C(1)-C(2)-C(3) = -173.1(3)° for A and O(5)-C(21)-
C(22)-C(23) = -170.5(3)° for B]. In the solid state the hydrazone
tautomeric form is observed for 2-(4-hydroxynaphthylazo)benzoic acid
anion. This enables the formation of intramolecular hydrogen bonds of
the NeH….O type (Table 3) between the oxygen atom from carboxylate
group closest to the azo N-atom, which additionally stiffen the entire
molecules (Fig. 1). The presence of water molecules and triethylamine
cations in the structure leads to the formation of a series of NeH…O
and OeH…O hydrogen bonds (Table 3, Fig. S1). As a result, a 2-D
supramolecular structure of double-layer sheets of anions, with water
and cations sandwiched in the middle, lie parallel to (0 1 0) is created.
Finally, two intermolecular CeH…O interactions [C(4)-H(4)…O(5)#1
;
#1 = 1-x,1-y,2-z and C(24)-H(24)…O(2)#2; #2 = 1-x,1-y,1-z] serve to
complete the supramolecular structure.
3.3. X-ray crystal structure description
3.3.2. Crystal structure of Bu3SnHL (1)
The complex 1 crystallizes in the trigonal system with non-cen-
trosymmetric space group R3c [Flack parameter = -0.028(3)]. The
molecular structure of complex 1 is shown in Fig. 2a. Selected bond
lengths and bond angles are presented in Table 2. The compound 1
forms in solid state a highly zig-zag 1-D polymeric chain (Fig. 2b) in
which the tin atom is five coordinated with the three butyl groups oc-
cupying the equatorial positions and the axial positions of trigonal bi-
pyramid being occupied by a carboxylate oxygen, O(1) of azo-carbox-
ylate ligand, and the phenolic oxygen, O(3)#3 [where #3 = 1/3-
x + y,5/3-x,2/3 + z], of an adjacent molecule. The deformation of the
3.3.1. Crystal structure of H2L salt with triethylamine
The single crystals of the ligand H2L salt with triethylamine were
obtained from the crystallization of the product obtained from an at-
tempted synthesis of tin complex using this ligand in presence of trie-
thyl amine base in methanol. Single crystals obtained from the re-
crystallization were analyzed and turned out to be the crystals of the
azo-ligand salt with triethylamine base. The asymmetric unit of H2L salt
is outlined in Fig. 1 and the crystal data are presented in Table 1. There
are two symmetrically independent molecules
A and B in the
Fig. 1. The molecular structure and atom numbering scheme for H2L salt with displacement ellipsoids drawn at 50% probability level.
7

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
2, no polymeric chain was formed, as found in the trimethyltin hy-
droxide [55]. To our knowledge, this kind of di-nuclear asymmetrical
complex with single μ-O(H) bridge, has not been described in literature
for organotin complexes yet. Each tin is pentacoordinate and the co-
ordination environment is comprised of three methyl substituents,
bridging hydroxy oxygen, carboxylate moiety for Sn(1) and oxygen
atom of a water molecule for Sn(2), respectively. The coordination
geometry around each tin is slightly distorted trigonal bipyramidal [the
deformation parameters τ are 0.88 and 0.98 for Sn(1) and Sn(2) re-
spectively], with the axial positions being occupied by one bridging
hydroxy oxygen O(4) from μ-O(H) group and the carboxylate oxygen O
(2) or water oxygen O(5) while the equatorial positions are taken up by
the three carbons of the methyl substituents (Fig. 3). The two Sn atoms
linked by the hydroxy group have similar co-ordination geometries,
with short Sn-Ohydroxy [2.188(5)Å and 2.136(5)Å] and longer Sn-
Ocarboxyl/water [2.275(5)Å and 2.402(5)Å] bond distances, respectively.
In each case the three methyl carbon atoms are slightly bent away from
the shorter Sn-O(4) bond [mean bond angles C-Sn-Ohydroxy = 93.2(2)°
and C-Sn-Ocarboxyl/water = 86.7(2)°].
Fig. 2a. The molecular structure and atom numbering scheme for compound 1
Just, like in the structure 1 the intramolecular hydrogen bond of the
type NeH…O play an important role in stabilizing the structure of the
complex. Besides, a 2-D H-bonding network in the direction (01–1) is
created. The presence of methanol molecule, water molecule and hy-
droxy group coordinated to the Sn atoms enable the formation a
number of intermolecular OeH…O hydrogen bonds (Table 3, Fig. S3).
with displacement ellipsoids drawn at 50% probability level.
Sn coordination sphere can be characterized quantitatively by para-
meter τ defined by Addison et al. [53]. The value of τ = (b-a)/60
[where b is the largest and a is the second largest basal angle around the
Sn atom] for complex 1 is 0.79. This indicates a distorted trigonal bi-
pyramidal geometry around Sn center. The sum of the C-Sn-C angles in
the trigonal plane of the complex 1 is 355.89(11)°. The shorter Sn-
3.3.4. Crystal structure of [Bu2Sn(HL)2]2 (3)
O
carboxylate distance Sn(1)-O(1) is 2.148(2)Å, while the Sn-Ophenolic bond
The molecular structure of compound 3 is depicted in Fig. 4 while
selected geometric parameters are given in Table 2. The complex 3 has
two tin centers that are surrounded by two bidentate carboxylate
groups and two butyl substituents. In addition, the Sn(1) atom forms a
bond [Sn(1)-O(9) = 2.690(5)Å] with a hydroxy O atom of one azo-
carboxylate ligand. Therefore, though uncommon, the coordination
number is seven and six around Sn(1) and Sn(2), respectively. The
donor atoms of the ligands are arranged in a distorted pentagonal bi-
pyramidal conformation around Sn(1) with five oxygen atoms spanning
the equatorial positions and two butyl carbon atoms at the apices of the
bipyramid. The number of alkyl substituents involved in coordination
to Sn(1) is probably the most important factor in fixing the geometry of
the coordination polyhedron. Generally, when three alkyl groups are
present in the organotin complex, they prefer an equatorial arrange-
ment around Sn center in a trigonal bipyramid, while the presence of
two alkyl groups, with the apparent necessity of maintaining two short
length Sn(1)-O(3)#1 is 2.537(3)Å. These values are within the range of
Sn-O bond distances usually observed for organotin azo-carboxylate
complexes [24,25,54]. The long Sn(1)….O(2) distance of 3.083(3)Å
indicates that this oxygen atom is not involved in coordination with tin
atom.
Apart from the occurring NeH…O intramolecular hydrogen bond
(Table 3), there are no significant supramolecular interactions probably
because of the azo-carboxylate ligands are not properly oriented for
efficient π-π stacking (Fig. S2).
3.3.3. Crystal structure of [(CH3)3Sn]2(OH)(H2O)HL (2)
The complex 2 crystallizes in the triclinic system with centrosym-
metric space group P-1. As can be seen from Fig. 3, the asymmetric unit
consists of complex [(CH3)3Sn]2(OH)(H2O)HL and methanol molecules.
Selected bond lengths and angles are listed in Table 2. In the crystal of
Fig. 2b. The 1-D infinite zig-zag polymeric chain of complex 1.
8

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
Fig. 3. The molecular structure and atom numbering scheme for compound 2, with displacement ellipsoids drawn at the 50% probability level.
Sn-C bonds in apical positions, favors a pentagonal arrangement of the
other ligands, allowing e.g. bidentate behavior of the carboxylate ions
[56,57]. The five oxygen atoms O(1), O(2), O(4), O(5) and O(9) are
coplanar with Sn(1) within the experimental errors. The Sn(2) atom
adopts the skew-trapezoidal bipyramidal configuration. The carbox-
ylate groups coordinate in an asymmetric mode around Sn(1) and Sn(2)
centers forming four short Sn-O bonds [mean Sn-O = 2.135(4)Å] and
four long Sn-O bonds [mean Sn-O = 2.520(5)Å]. The alkyl ligands lie in
axial positions, thereby completing six-coordination around the Sn(2)
atom. The bond angles of C-Sn-C [C(35)-Sn(1)-C(39) = 156.5(3)° and
C(77)-Sn(2)-C(81) = 135.5(3)°] are close to the average found for di-
organotin complexes in which the alkyl or aryl substituents do not
adopt cis- or trans-geometries around the tin atom [58]. As in the case
of compounds 1 and 2, the structure 3 is stabilized by intramolecular
hydrogen bonds of the NeH…O type (Table3, Figs. 4 and S4). No other
significant supramolecular interactions were found.
3.4. Antidiabetic activity
Complexes 1 and 3 along with the ligand were screened for their
antidiabetic activity against alpha glucosidase enzyme and the enzyme
inhibition activities of these complexes were compared with standard
compound acarbose. The results of the screening test of the anti-dia-
betic activity study against α-glucosidase enzyme revealed significant
activity for the ligand H2L and compound 1 in DMSO while compound
3 showed significant inhibition activity in ethanol. Therefore, inhibition
concentration (IC50) for the ligand H2L in DMSO (LG_D) and com-
pounds 1 (1_D) in DMSO and 3(3_E) in ethanol were determined and
compared with the standard drug, acarbose (Fig. 5). The results of the
dose effect analysis (IC50) of the ligand and compounds with acarbose
are also summarized in Table 4. From the results of the dose analysis
effect of the ligand and compounds, it was observed that compound 3
(C3_E) had higher enzyme inhibition activity (45.24 μg/mL) as
Fig. 4. The molecular structure and atom numbering scheme for compound 3, with displacement ellipsoids drawn at the 30% probability level. Hydrogen atoms of
the butyl groups have been omitted for clarity.
9

P. Debnath, et al.
InorganicaChimicaActa510(2020)119736
CRediT authorship contribution statement
Paresh Debnath: Methodology, Investigation, Writing - original
draft. Keisham Surjit Singh: Supervision, Conceptualization, Writing -
review
&
editing. Thokchom Sonia Devi: Investigation.
S.Sureshkumar Singh: Formal analysis. Ray J. Butcher: Investigation.
Lesław
Sieroń:
Investigation,
Visualization.
Waldemar
Maniukiewicz: Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Fig. 5. IC50 values for ligand and compounds 1 or 3 along with standard
compound acarbose against α-glucosidase.
Acknowledgements
Authors would like to thank SAIF, IIT Madras for providing crystal
structure data for compound 3, IISc, Bangalore for NMR spectra and
DBT, Govt. of India (grant no. BT/32/NE/2011) for Biotech hub.
Table 4
. Results of the enzyme inhibition activity (IC50).
Compounds
IC50 (µg/mL)
SD (
1.27
0.25
1.56
0.01
)
Hill Co-efficient
Appendix A. Supplementary data
1 (C1_D)
66.02
1.24
2.15
2.45
1.92
0.54
0.35
0.53
0.78
3 (C3_E)
45.24
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119736.
H2L (LG_D)
Acarbose (ACB)
74.84
13.51
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