Synthesis, characterization, crystal structures and electrochemical studies of organotin(IV) carboxylates

Article abstract of DOI:10.1016/j.jorganchem.2012.10.006

Four new organotin(IV) compounds [Me₃SnL] (1), {[Me ₂SnL]₂O}₂ (2), Bu₂SnL₂ (3) and [Bu₃SnL] (4), where L = O₂C(C₂H ₅)CCHC₆H₄F have been synthesized by refluxing the corresponding organotin(IV) chlorides with sodium (E)-2-(2- fluorobenzylidene) butanoate (NaL) in dry toluene except 2 for which Me ₂SnO was refluxed with (E)-2-(2-fluorobenzylidene) butanoic acid (LH) in the same solvent. NaL and all the compounds were characterized by FT-IR, ¹H NMR and ¹³C NMR spectroscopy. NaL and compounds 1 and 2 were also characterized by X-ray single crystal analysis which confirmed a linear polymeric structure for compound 1 and a stannoxane for 2. The structural analyses have shown a bridging behavior of the ligand in trimethyl- and tributyl-, chelating as well as bridging behavior in dimethyl- and a chelating bidentate mode of coordination in dibutyltin(IV) compound which is also confirmed from the crystal analysis for compounds 1 and 2. Cyclic voltammetry was used to evaluate the electrochemical, kinetic and thermodynamic parameters for the electron transfer processes of compounds 1-4. Predominantly adsorption controlled redox processes were exhibited by the slope of log i_p vs. -log v plots. The values of the diffusion coefficient (D_o) indicated the role of solvent in the mobility of the electro-active species to some extent but the overall rate of the redox process was typical of the adsorption controlled process and could not be explained on the basis of solvation effects. The values of formal potential and charge transfer coefficient were also calculated.

Full text of DOI:10.1016/j.jorganchem.2012.10.006

Journal of Organometallic Chemistry 723 (2013) 214e223
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, crystal structures and electrochemical studies of
organotin(IV) carboxylates
,
b
c
d
Muhammad Iqbal a, Saqib Ali a , Niaz Muhammad , Masood Parvez , Peter Langer ,
*
Alexander Villinger d
a Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
b Department of Chemistry, Abdul Wali Khan University, Mardan, KPK, Pakistan
c Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
d Department of Organic Chemistry, Institute of Chemistry, University of Rostock, Albert-Einstein-Strasse 3a, 18059 Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new organotin(IV) compounds [Me3SnL] (1), {[Me2SnL]2O}2 (2), Bu2SnL2 (3) and [Bu3SnL] (4), where
L ¼ O2C(C2H5)CCHC6H4F have been synthesized by refluxing the corresponding organotin(IV) chlorides
with sodium (E)-2-(2-fluorobenzylidene) butanoate (NaL) in dry toluene except 2 for which Me2SnO was
refluxed with (E)-2-(2-fluorobenzylidene) butanoic acid (LH) in the same solvent. NaL and all the
compounds were characterized by FT-IR, 1H NMR and 13C NMR spectroscopy. NaL and compounds 1 and
2 were also characterized by X-ray single crystal analysis which confirmed a linear polymeric structure
for compound 1 and a stannoxane for 2. The structural analyses have shown a bridging behavior of the
ligand in trimethyl- and tributyl-, chelating as well as bridging behavior in dimethyl- and a chelating
bidentate mode of coordination in dibutyltin(IV) compound which is also confirmed from the crystal
analysis for compounds 1 and 2. Cyclic voltammetry was used to evaluate the electrochemical, kinetic
and thermodynamic parameters for the electron transfer processes of compounds 1e4. Predominantly
adsorption controlled redox processes were exhibited by the slope of log ip vs. ꢀlog v plots. The values of
the diffusion coefficient (Do) indicated the role of solvent in the mobility of the electro-active species to
some extent but the overall rate of the redox process was typical of the adsorption controlled process and
could not be explained on the basis of solvation effects. The values of formal potential and charge transfer
coefficient were also calculated.
Received 24 July 2012
Received in revised form
24 September 2012
Accepted 2 October 2012
Keywords:
Organotin(IV) carboxylates
Sodium (E)-2-(2-fluorobenzylidene)
butanoate
Crystal structures
Stannoxane
Cyclic voltammetry
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
ladder shaped, hexameric [8] and polymeric (linear [9], zigzag [10]
and stannoxanes such as 1D, 2D, and 3D [8]) structures. Apart from
Owing to the ability of tin to form stable complexes with carbon
and hetero atoms, organotin derivatives are becoming the focal
point of many research areas such as organic synthesis, synthesis of
polyesters, polyurethanes and in the crosslinking of silicon and
catalysis such as esterification and trans-esterification reactions [1].
Organotin(IV) carboxylates constitute a class of organoitn(IV)
compounds and are extensively being studied nowadays due to
their potential biocidal activity, cytotoxicity, industrial and agri-
cultural applications. Organotin(IV) compounds containing
carboxylate ligand with additional donor atoms such as N, O or S
have been shown to exhibit new structural geometries and hence
new potential activities [1,2,7]. The most common geometries are
monomeric (tetrahedral, octahedral [3], skew trapezoidal [4],
trigonal bipyramidal [5,6]), dimeric, tetrameric [7], oligomeric,
these, some cyclic species have also been reported [8]. The mono-
alkyltin(IV) tricarboxylates exhibit relatively less structural diver-
sity and are not as widespread as the di- and triorganotin(IV)
carboxylates [11].
Although a number of new synthetic techniques such as sol-
vothermal synthesis and other are commonly employed for the
synthesis of organotin(IV) carboxylates, the conventional synthetic
techniques such as standard schlenk technique [8,12] and reflux
methods are still in common use [10]. In this research work the
complexes are synthesized using the reflux method. The ligand is
used to react with the organotin(IV) chlorides under reflux condi-
tions. The synthesized organotin(IV) compounds consist of linear
polymeric carboxyl bridged [Me3SnL]n (1) and [Bu3SnL]n (4), tetra-
meric tetraorganodistannoxane {[Me2Sn(OOC(C2H5)CCHC6H4F)]2O}2
(2)andmonomericskewtrapezoidalBu2SnL2 (3);eachselfassembled
from the respective organotin(IV) derivative and the ligand under
reflux conditions.
* Corresponding author. Tel.: þ92 51 90642130; fax: þ92 51 90642241.
E-mail address: drsa54@yahoo.com (S. Ali).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.10.006

M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
215
Table 1
Regarding the biological activities of metal complexes, the
oxidation state and redox behavior of the central metal atom are of
prime importance among several other parameters [13]. The labili-
zation of the attached ligands, resulting in the biologically active
species as well as the inherent activity of the complex without ligand
dissociation is critically affected by the former of the two mentioned
properties. Hence the in vivo interconversion of different oxidation
states of the metal complexes has been reported to alter their
properties drastically [14,15]. The redox potential of an organome-
tallic compound is a function of several factors. Besides synthesis, the
role of the electronic properties of the attached ligands, geometry of
the complex and the nature of organic moiety attached to metal in
determining the redox potential of the synthesized organotin(IV)
carboxylates has been studied and discussed in the present work.
Crystal data and structure refinement parameters for NaL and compounds 1 and 2.
Compound
NaL
1
2
Moiety Formula
Formula mass
C
11 H18 F Na O5.50
C14 H19 F O2 Sn
356.98
C26 H32 F2 O5 Sn2
699.90
280.24
(g molꢀ1
)
Crystal system
Space group
Orthorhombic
F d d 2
Orthorhombic
P 21 21 21
Monoclinic
P 21/n
Unit cell dimensions
ꢀ
a (A)
12.4218(8)
75.112(5)
5.6337(3)
90.00
90.00
90.00
6.6797(3)
10.1047(4)
21.7370(11)
90.00
90.00
90.00
10.5396(3)
15.5407(4)
17.3810(4)
90.00
106.7558(15)
90.00
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
3
ꢀ
V (A )
5256.4(6)
1467.17(12)
2726.01(12)
q
Ranges
for data
2. Experimental
Collection (ꢂ)
2.17 ꢀ 25.00
16
3.58 ꢀ 30.02
4
2.45 ꢀ 26.04
4
2.1. Materials and methods
Z
rcalc.(g cmꢀ3
F(000)
)
1.417
1.616
1.705
2368
712
1384
Dimethyltin(IV) oxide, trimethyl-, tributyl- and dibutyltin(IV)
chlorides were purchased from Aldrich and used without further
purification. All the solvents were dried according to the reported
standard procedures [16]. The melting points were obtained in
a capillary tube using a Gallenkamp, serial number C040281, UK,
electro-thermal melting point apparatus. FT-IR spectra were
recorded on a Nicolet-6700 FT-IR spectrophotometer, thermo-
scientific, USA, in the range from 4000 to 400 cmꢀ1. 1H and 13C NMR
spectra were recorded on Bruker Advance Digital 300 MHz NMR
spectrometer, Switzerland, using CDCl3 as an internal reference
Crystal
size (mm)
T (K)
0.09 ꢃ 0.08
ꢃ 0.02
123(2)
0.16 ꢃ 0.12
ꢃ 0.12
123(2)
0.16 ꢃ 0.14
ꢃ 0.04
173(2)
Index ranges
h
k
l
Total data
Unique data
Final R indices
ꢀ14e14
ꢀ88e87
ꢀ6e6
ꢀ9e9
ꢀ13e13
ꢀ19e18
ꢀ21e21
5272
ꢀ14e14
ꢀ30e30
2447
1268
1169
2419
4807
R1 ¼ 0.0416
R1 ¼ 0.0305
R1 ¼ 0.0302
[I>/2s(I)]
wR2 ¼ 0.0781
wR2 ¼ 0.0918
wR2 ¼ 0.0761
[
d
1H (CDCl3) ¼ 7.25 and
d
13C (CDCl3) ¼ 77.0 ppm]. The spectra were
acquired at room temperature (298 K).
2.2. X-ray crystallographic studies
containing 0.1
M tertiarybutylammonium perchlorate (TBAP)
under a N2 saturated environment in a conventional three-
electrode cell with saturated calomel electrode as reference,
a Platinum wire having diameter of 0.6 mm as counter and a bare
glassy carbon electrode (GCE) with a surface area of 0.196 cm2 as
the working electrode. Prior to experiment the GCE was polished
with alumina (Al2O3) on a nylon buffing pad followed by washing
with acetone and finally with distilled water. Electrochemical
measurements were carried out at room temperature (25 ꢁ 0.5 ꢂC).
A colorless prismatic crystal of 1 and colorless plate crystals of 2
and NaL were used for data collection. The crystals were coated with
Paratone 8277 oil (Exxon) and mounted on a glass fiber. All
measurements were made on a Nonius Kappa CCD diffractometer
with graphite monochromated Mo-Ka radiation. Details of crystal
data and structure refinement have been provided in Table 1. The
datawere collected at a temperature of173(2)K for1 and123(2) K for
2 and NaL using
u and 4 scans, corrected for Lorentz and polarization
effects and for absorption using multi-scan method [17,18].
The structures were solved by the direct methods [19] and
expanded using Fourier techniques [20]. The non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were included at
geometrically idealized positions and were not refined. The final
cycle of full-matrix least-squares refinement using SHELXL97 [21a]
converged with unweighted and weighted agreement factors, R
and wR ¼ 0.0305 and 0.0918 (all data) for 1, 0.0302 and 0.0761 (all
data) for 2 and 0.0416 and 0.0781 for NaL, and goodness of fit,
S ¼ 1.076, 1.053 and 1.154, respectively. The absolute structure
parameter x [21b] indicated the presence of racemic twinning in NaL.
ATWIN instructionwiththedefaultmatrixR¼ (ꢀ10 0, 0ꢀ10, 0 0ꢀ1)
in the least-squares refinement cycles lead to a BASF parameter of
0.2147. The Friedel pairs of reflections (865) were merged.
2.4. Synthesis
Reactions of the sodium salt of the ligand (NaL) with trimethyl,
dibutyl and tributyltin(IV) chlorides in 1:1, 2:1 and 1:1 molar ratios,
respectively afforded air stable compounds 1, 3 and 4 whereas, to
affect the synthesis of 2, the ligand (LH) was refluxed with dime-
thyltin(IV) oxide in 2:1 molar ratio. The general synthetic proce-
dures and numbering pattern are shown in Scheme 1.
R3SnCl þ NaL/R3SnL þ NaCl
(1)
R ¼ Methylð1Þ; n ꢀ Butylð4Þ
4ðmethylÞ2SnO þ 4LH/ðmethylÞ8Sn4L4O2ð2Þ þ 2H2O
ðn ꢀ ButylÞ2SnCl2 þ 2NaL/ðn ꢀ ButylÞ2SnL2ð3Þ þ 2NaCl
(2)
(3)
The weighting schemes were based on counting statistics and
the final difference Fourier maps were essentially featureless. The
figures were plotted with the aid of ORTEP-3 [22].
2.3. Electrochemistry
2.4.1. Synthesisofsodium(E)-2-(2-fluorobenzylidene) butanoate (NaL)
The ligand viz., (E)-2-(2-Fluorobenzylidene) butanoic acid (LH)
has been synthesized and reported earlier [23]. The sodium salt of
the ligand was prepared by drop wise addition of an equimolar
amount of sodium hydrogen carbonate dissolved in distilled water
Voltammetric experiments were performed using an SP-300
potentiostate, serial number 0134, BioLogic Scientific Instruments,
France. Measurements were carried out in DMSO solution

216
M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
10 11
H2C
CH3
9
8
3
2
1
4
7
C
C
C
H
OH
6
F
O
5
Ligand (LH)
γ
α
δ
CH3
Sn
β
α
Sn
CH3
Scheme 1. General synthetic procedure and numbering pattern of LH and compounds 1e4.
to an ethanolic solution of ligand. The solution was stirred for 1 h at
60 ꢂC followed by filtration. The filtrate was allowed to evaporate to
get the colorless crystals of the salt. M.p. 88e89 ꢂC. IR (cmꢀ1): 1643
(0.864 g, 4 mmol). (Yield: 1.15 g, 80%). FT-IR (cmꢀ1): 1610
(OCO)asym,1483 (OCO)sym (SneC), 456
(SneO). 1H
y
y
,
Dy ¼ 127, 523
y
y
NMR (CDCl3, ppm): 7.75 (s, 1H, H3), 7.34e7.09 (m, ArꢀH, 4H), 2.52
(q, H10, 2H), 1.20 (t, H11, 3H), 1.78-1.77 (m, Ha, and Hb, 8H), 1.49e
1.42 (m, Hg, 4H), 0.95 (t, Hd, 6H). 13C NMR (CDCl3, ppm): 177.5
(C-1), 130.1 (C-2), 136.9 (C-3), 124.0{14.2} (C-4), 162.4{248.2} (C-5),
115.7{21.8} (C-6), 132.4{8.1} (C-7), 131.0{3.8} (C-8), 130.1{3.0} (C-9),
y
(OCO)asym, 1388
y
(OCO)sym
,
Dy ¼ 255. 1H NMR (CDCl3, ppm): 7.2 (s,
1H, H3), 7.25e6.92 (m, 4H, Ar-H), 2.22 (q, H10, 2H), 0.86 (t, H11, 3H).
13C NMR (CDCl3, ppm): 177.7 (C-1), 130.4 (C-2), 144.5 (C-3), 124.24
{15.0} (C-4), 159.8{243.8} (C-5), 115.2{21.8} (C-6), 129.4{8.2} (C-7),
128.2{3.0} (C-8), 123.91{3.0} (C-9), 21.4 (C-10), 12.5 (C-11).
21.5 (C-10), 13.8 (C-11), 25.4 [C-
a
,
1J (119Snꢀ13C) ¼ 790 Hz], 26.8
[C-b
, 2J (119Snꢀ13C) ¼ 38 Hz], 26.4 [C-
g
, 3J (119Snꢀ13C) ¼ 82 Hz], 13.6
2.4.2. Trimethylstannyl 2-(2-fluorobenzylidene) butanoate (1)
The sodium salt of the ligand (NaL) (0.500 g, 2.33 mmol) was
refluxed for 6 h with trimethyltin(IV) chloride (0.465 g, 2.33 mmol) in
dry toluene in a two necked round bottom flask. At the end the turbid
solution was allowed to cool down to room temperature. The sodium
chlorideformedwasfilteredoffandthefiltratewasrotaryevaporated.
The resultant solid was recrystallized from ethanolic solution. (Yield:
(C-d).
2.4.5. Tributylstannyl 2-(2-fluorobenzylidene)butanoate (4)
Compound 4 was prepared and crystallized in the same way as 1
using tributylstannyl chloride (0.651 g, 2 mmol) and NaL (0.432 g,
2 mmol). (Yield: 0.70 g, 75%). FT-IR (cmꢀ1): 1649
y
(OCO)asym, 1454
y(OCO)sym
,
Dy ¼ 195, 523
y(SneC), 455 y
(SneO). 1H NMR (CDCl3,
0.80 g, 83%). M.p. 104e105 ꢂC. FT-IR (cmꢀ1): 1641
y
(OCO)asym, 1453
ppm): 7.65 (s, 1H, H3), 7.65e7.06 (m, ArꢀH, 4H), 2.48 (q, H10, 2H),
1.15 (t, H11, 3H), 1.70-1.62 (m, Ha and Hb, 12H), 1.44e1.28 (m, Hg,
6H), 0.94 (t, Hd, 9H). 13C NMR (CDCl3, ppm): 172.8 (C-1), 130.1 (C-2),
138.9 (C-3), 124.7{14.3} (C-4), 160.3{247.5} (C-5), 115.5{21.8} (C-6),
129.6{8.2} (C-7), 130.0{3.8} (C-8), 123.8{3.8} (C-9), 21.6 (C-10), 13.8
y
(OCO)sym (SneC), 541
,
Dy¼ 188, 543
y
y
(SneO).1HNMR(CDCl3, ppm):
7.7 (s, 1H, H3), 7.19e7.06 (m, Ar-H, 4H), 2.47 (q, H10, 2H), 1.15 (t, H11,
3H), 0.63 [Ha, 9H, 2J (119/117Snꢀ1H) 58/56 Hz]. 13C NMR (CDCl3, ppm):
177.7 (C-1),130.4 (C-2),144.5 (C-3),124.2{14.2} (C-4),159.8{247.5} (C-
5), 115.2{21.8} (C-6),129.4{8.3} (C-7), 128.2{3.8} (C-8), 123.9{3} (C-9),
(C-11), 16.6 [C-
(
a
,
1J (119/117Snꢀ13C) ¼ 358/343 Hz], 27.9 [C-
b,
2J
).
21.4 (C-10), 12.5 (C-11), ꢀ2.2 [C-
a
, 1J (119Snꢀ13C) ¼ 397 Hz].
119Snꢀ13C) ¼ 21 Hz], 27.0 [C-
g
, 3J (119Snꢀ13C) ¼ 64 Hz], 13.7 (C-
d
2.4.3. Bis(-2-(2-fluorobenzylidene)butanoato)tetra-
methyldistannoxane (2)
3. Results and discussion
3.1. Spectroscopic studies
3.1.1. FTꢀIR spectra
Compound 2 was prepared by fluxing dimethylstannyl oxide
(0.659 g, 4 mmol) with LH (0.776 g, 4 mmol) in dry toluene for 6 h
using Dean-stark apparatus. The reaction mixture was then cooled,
filtered and subsequently rotary evaporated to get the dry product
which was then recrystallized in the same way as 1. (Yield: 0.95 g,
The frequencies of interest are those associated with COO, SneC
and SneO groups. The absorption band appearing in range 455e
476 and 523e551 cmꢀ1 in the respective FT-IR spectra of the
compounds 1e4, that were absent from the spectrum of the free
ligand (NaL), were assigned to the SneO [1,6,24,25] and SneC
stretching modes of vibration, respectively. The asymmetric and
symmetric vibrations due to COO moiety were observed at 1649e
1610 and 1375e1483 cmꢀ1, respectively. The difference between
these two modes of vibration (Dy) indicates the mode of coordi-
nation of the ligand to the tin centre [26]. The reduction in Dy
values of COO moiety of the ligand is attributed to the reduction of
electron density on its oxygen atoms consequent upon coordina-
tion to tin(IV) moiety. In the present work, 1, 2 and 4 showed
70%). M.p. 122e123 ꢂC. FT-IR (cmꢀ1): 1540
(OCO)sym (SneC), 476
Dy ¼ 142, 551
y
(OCO)asym, 1398
y
,
y
y
(SneO). 1H NMR (CDCl3,
ppm): 7.8 (s, 1H, H3), 7.35 - 7.09 (m, Ar-H, 4H), 2.51 (q, H10, 2H), 1.19
(t, H11, 3H), 0.93 and 0.99 [Ha, 6H, 2J (119/117Snꢀ1H) 75/71 Hz]. 13
C
NMR (CDCl3, ppm): 177.5 (C-1), 130.2 (C-2), 136.5 (C-3), 123.7{14.3}
(C-4), 160.4{248.2} (C-5), 115.7{21.8} (C-6), 130.3{8.2} (C-7), 130.02
{3.7} (C-8), 128.6{3.0} (C-9), 21.5 (C-10), 13.8 (C-11), 4.8 (C-a).
2.4.4. Dibutylstannyl bis(-2-(2-fluorobenzylidene)butanoate (3)
Compound 3 was prepared and crystallized in the same way as 1
employing dibutylstannyl chloride (0.608 g, 2 mmol) and NaL

M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
217
Table 2
density transfer from the ligand to the acceptor tin atom. The
positions of carbon atoms in fluorophenyl group are easily assigned
with the help of nJ(19Fꢀ13C) coupling, where n ¼ 1,2,3,4. The
nJ(19Fꢀ13C) coupling constants calculated, are comparable to those
reported in the literature. The reported ranges of the nJ(19Fꢀ13C)
coupling, where n ¼ 1,2,3 and 4 are 230e265,15e25, 7e12 and 3 Hz,
respectively [28e30]. The values observed for nJ(19Fꢀ13C) coupling
are 244e248, 15ꢀ21, 8 and 3ꢀ4 Hz for n ¼ 1,2,3 and 4, respectively.
The satellites due to nJ [119/117Snꢀ13C] coupling, shown by the
compounds 1, 3 and 4, have been calculated which are important
indicators for the structural elucidation of organotin(IV) carboxyl-
ates. Among the synthesized compounds, 1 exhibited 1J [119Snꢀ13C]
being of the order of 397 Hz, confirming a monomeric tetrahedral
geometry in solution [31]. In case of compounds 3 and 4, nJ
ꢂ
ꢀ
Selected bond lengths (A) and bond angles ( ) of NaL.
Bond angles (ꢂ)
ꢀ
Bond lengths (A)
O1eC1
O1eH10
O2eC1
1.282(4)
0.8400
C1eO1eH10
O2eC1eO1
O2eC1eC2
O1eC1eC2
O3WeNa1eO4W
O4WeNa1eO5W
O5WeNa1eO6W
O3WeNa1eO6W
109.5
123.2(4)
118.8(3)
118.0(4)
90.37(11)
108.26(12)
76.13(10)
85.65(10)
1.246(5)
1.515(5)
2.379(3)
2.388(3)
2.401(3)
2.469(4)
C1eC2
Na1eO3W
Na1eO4W
Na1eO5W
Na1eO6W
a bridging structure while 3 showed a chelating mode of coordi-
nation for the ligand. These observations were further supported by
the crystal structures of the compounds. The stannoxane structure
for 2 was supported from its characteristic absorption band of
medium intensity observed at 693 cmꢀ1 assignable to SneOeSn
functionality [24,27].
[
119Snꢀ13C] values are 790, 38, 82 and 358, 21, 64 Hz respectively.
These values suggest a skew trapezoidal geometry around tin for 3
and a monomeric tetrahedral geometry for 4 in solution form. This
shows that a reduction of coordination number of tin atom in
solution occurs in triorganotin(IV) carboxylates (1 and 4). On the
other hand, the diorganotin(IV) compounds (2 and 3) retain their
geometries in solution as well.
3.1.2. 1H NMR spectra
The methyl protons attached to the tin atom in compounds 1
and 2 exhibited a well defined singlet with 2J (119/117Snꢀ1H) values
58/56 and 75/71 Hz, respectively. The terminal methyl protons of
butyl groups attached to tin atom in compounds 3 and 4 gave
a clear triplet each while the rest of the protons exhibited a multi-
faced pattern. The sharp singlet observed for the single proton
attached to C-3 was shifted from 7.8 ppm, to 7.6e7.7 ppm, observed
for free ligand and compounds 1e4, respectively. The methyl and
methylene protons of the coordinated ligand gave rise to triplet and
quartet at 0.86 and 2.22 ppm, respectively in the spectra of all the
compounds. 2J [119/117Snꢀ1H] in Hz are also calculated and helped
to confirm four coordinated geometry around tin atom in
compound 1 and the retention of six coordinated geometry around
tin centre in 2, in solution form.
3.2. Crystal structure
3.2.1. Structure of NaL
The structure refinement parameters and selected bond lengths
and bond angles of the sodium salt of the ligand (NaL) are given in
Tables 1 and 2 while its ORTEP is shown in Fig. 1 which reveals that
each of the sodium ions is coordinated by six water molecules.
Oxygen atom of each water molecule acts as a bridge between the
two sodium ions. This constitutes an infinite 2D sheet of the hex-
acoordinated sodium ions where the hydrogen atoms of the water
molecules are oriented outside. These hydrogen atoms are
responsible for hydrogen bonding with the oxygen atoms of the
carboxylate moiety of the ligand molecules resulting in their
attachment to the ionic layer in a perpendicular manner. Addi-
tionally, every sodium ion is in a position suitable enough to
establish weaker interaction with a lone pair of an oxygen atom of
each the two ligand molecules lying on its two opposite sides.
3.1.3. 13C NMR spectra
The 13C NMR spectra of all the complexes showed a more or less
downfield shift of all the carbon resonances compared with those of
the free ligand (NaL). The shift is a consequence of the electron
Fig. 1. ORTEP diagram of the sodium salt of the ligand (NaL).

218
M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
Fig. 2. Packing of NaL showing double layers of ligand molecules sandwitched between those of hydrated sodium ions in aoc plane.
Looking at the packing diagram of the ligand, shown in Fig. 2, it is
seen that both sides of the ionic layer are occupied by ligand
molecules constituting a double layer of the latter, sandwiched
between two consecutive layers of the former.
As is evident from the structure, the ligand molecule is lying as
such in the crystal lattice and there is no direct OꢀeNaþ bond,
however, there are some noteworthy variations in the properties of
the NaL compared to those of HL reported earlier by our research
group [23]. In this regard, apart from the change in physical prop-
erties, there is a considerable shortening of the acidic OeH bond
length in NaL compared to that in HL. The O1eH10 bond length in
The compound exhibits a carboxylate bridged motif in which the
Sn center shows trigonal-bipyramidal coordination, the equatorial
plane being defined by the three methyl groups while the axial
positions are occupied by two oxygen atoms. According to the
literature[32], the geometryaround Sn atom can be characterized by
the value of
around the Sn atom. The angle values
a square-pyramidal geometry, and the value of
s
¼ (
bea)/60, where b is the largest of the basal angles
a
¼
b
¼ 180ꢂ correspond to
a
¼ 120ꢂ corresponds
to a perfect trigonal-bipyramidal geometry. Thus the s value is equal
to zero for a perfect square-pyramidal and unity for perfect trigonal-
bipyramidal geometry. The calculated value for 1 is 0.82 which
s
ꢀ
ꢀ
NaL is 0.84 (A) as opposed to 0.97(2) (A) in HL. This may be attributed
to the reduction of electron density on O1 as a consequence of its
weak interaction with sodium ions resulting in strengthening and
shortening of OeH bond in crystal lattice. This observation is in line
indicates distorted trigonal-bipyramidal geometry around Sn atom.
The angle OeSneO is 171.17ꢂ while the sum of the equatorial CeSne
C angles is 358.9ꢂ. The overall structure is an infinite zigzag chain of
carboxylate bridged trimethyltin(IV) species (Fig. 4).
ꢀ
with the shortening of C1eO1 bond (1.282(4) (A)) of NaL compared
to that of HL (1.3006(18) (A)), again attributable to the same reason.
ꢀ
3.2.3. Crystal structure of compound 2
The molecular structure of compound 2 is shown in Fig. 5
whereas the crystal data, selected bond lengths and bond angles
are listed in Tables 1 and 4.
The geometries of all the tin atoms in 2 can be classified into two
types: pentacoordinated tin (Sn2), and hexa-coordinated tin (Sn1).
The pentacoordinated tin atoms (Sn2) form three short SneO bonds
with three oxygen atoms: two from the Sn2O2 moiety and the other
one from the carboxylate group, together with the two bonds to
methyltin groups. The Sn2 atom may be viewed as highly distorted
trigonal bipyramid with the axial site occupied by the O(5) and
3.2.2. Crystal structure of compound 1
The molecular structure of compound 1 is shown in Fig. 3
whereas the crystal data, selected bond lengths and bond angles
are listed in Tables 1 and 3.
O(3) atoms (:(O5)eSn(2)eO(3) ¼ 166.69(9)ꢂ) having
s
¼ 0.418.
The five-coordinate geometry about each of the Sn2 is due to the
monodentate coordination mode of the carboxylate ligand. Each of
the hexacoordinated tin atoms, Sn1, forms four short SneO bonds
with four oxygen atoms: one from the Sn2O2 moiety and the other
three from two different carboxylate groups. Together with the two
bonds to methyltin groups, the Sn1 atom may be viewed as distorted
Table 3
ꢂ
ꢀ
Selected bond lengths (A) and bond angles ( ) of compound 1.
ꢀ
Bond lengths (A)
Sn1eC1
Sn1eC2
Sn1eC3
Sn1eO2
Sn1eO1
2.123(4)
2.129(4)
2.131(5)
2.173(3)
2.394(3)
O1eC4
O2eC4
O2eSn1
C4eC5
1.251(5)
1.285(5)
2.173(3)
1.508(5)
Bond angles (ꢂ)
C1eSn1eC2
C1eSn1eC3
C2eSn1eC3
C1eSn1eO2
C2eSn1eO2
116.5(2)
121.2(2)
121.70(19)
87.22(15)
92.53(15)
C3eSn1eO2
C1eSn1eO1
C2eSn1eO1
C3eSn1eO1
O2eSn1eO1
97.56(17)
84.09(14)
92.75(14)
85.65(16)
171.17(11)
Fig. 3. ORTEP diagram of compound 1.

M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
219
Fig. 4. Linear polymeric structure of compound 1.
octahedral in geometry. The largest angle around tin atom is 170.44ꢂ
(¼:(O(1)eSn(1)eO(4)). In contrast to the common polymeric
stannoxanes, this compound is monomeric tetra-nuclear tetraor-
ganodistannoxane. Here two of the carboxylate ligands exhibit
a bridging bidentate mode of coordination while the other two
ligands show monodentate mode of coordination (Fig. 6).
irreversible nature of the redox processes is attributed to the
instability of the resulting species in DMSO [33].
The ligand centered relatively weak anodic peaks of the
compounds 1e3 occur in the range 0.98e1.13 V. No peak was
observed in the oxidation region of the cyclic voltamogram of
compound 4 showing its electrochemical stability under the given
conditions. The anomalous behavior of butyl derivatives of orga-
notin compounds has been observed by other researchers too [34].
The reduction signals of all the compounds were observed in
between ꢀ0.95 and ꢀ1.06 V as shown in Fig. 7. The cathodic signal
of the compounds is attributed to the reduction of tin(IV) to tin(II)
species [35,36]. Owing to the similarity of the chemical environ-
ment around central metal in these compounds, the individual
reduction potential values are lying closer to each other. The more
electron donating butyl groups in 3 and 4, compared to 1 and 2,
render the former pair of compounds less prone to reduction, thus
slightly shifting their cathodic potential to more negative values.
3.3. Electrochemical studies
Cyclic voltammetry was used to explore the electrochemical
redox behavior of compounds 1ꢀ4 and the resulting voltammo-
grams at 100 mV sꢀ1 are given in Fig. 7.
As is evident from the respective voltammograms, one oxidation
and one reduction peak was observed for each of the four
compounds. The large potential difference between the oxidation
and reduction peaks (w2000 mV) provides ample evidence for the
independent nature of the irreversible redox processes. The
Fig. 5. ORTEP diagram of compound 2.

220
M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
Table 4
0.03
ꢂ
ꢀ
1
2
3
4
Selected bond lengths (A) and bond angles ( ) of compound 2.
ꢀ
Bond lengths (A)
0.02
0.01
Sn1eO5
Sn1eC2
Sn1eC1
Sn1eO1
Sn1eO4
2.023(2)
2.103(4)
2.120(4)
2.163(2)
2.265(2)
Sn2eO5
Sn2eC14
Sn2eC15
Sn2eO5
Sn2eO3
2.032(2)
2.112(4)
2.113(4)
2.174(2)
2.262(2)
Bond angles (ꢂ)
0.00
O5eSn1eC2
O5eSn1eC1
C2eSn1eC1
O5eSn1eO1
C2eSn1eO1
C1eSn1eO1
O5eSn1eO4
C2eSn1eO4
C1eSn1eO4
O1eSn1eO4
107.26(13)
111.36(13)
140.37(16)
80.88(9)
99.98(14)
94.57(13)
91.12(9)
87.34(14)
83.43(13)
170.44(9)
O5eSn2eC14
O5eSn2eC15
C14eSn2eC15
O5eSn2eO5
C14eSn2eO5
C15eSn2eO5
O5eSn2eO3
C14eSn2eO3
C15eSn2eO3
O5eSn2eO3
105.82(13)
112.14(13)
141.61(15)
75.96(9)
98.20(12)
96.30(12)
91.35(9)
89.00(12)
84.67(12)
166.69(9)
-0.01
-0.02
-0.03
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
E / V
Fig. 7. Cyclic voltammograms of 1.40 mM compounds 1ꢀ4 at 100 mV sꢀ1
.
The combined plots obtained at different scan rates for each of the
compounds are shown in Fig. 8 that clearly indicate that on
increasing scan rate, the cathodic peak potential is displaced to
more negative potential suggesting irreversibility of the reduction
process of the compounds.
This shows that solvation does play a role in determining the rate of
mass transport of these compounds to the electrode surface.
However, being adsorption controlled the values of Do and their
mutual order is not typical of a diffusion controlled process.
Fig. 9(b) shows the linear relation between log ipc vs. ꢀlog v with
slope values of 0.19, 0.22, 0.12 and 0.17 for compounds 1ꢀ4,
respectively. It reveals predominantly adsorption controlled redox
processes [38]. The general linear regression equation for this
relation is given as follows:
For irreversible processes the peak current is given by the
RandleseSevcik model (Eq. (4)) [37]:
ꢀ
ꢁ
ip
¼
2:99 ꢃ 105 nð
a
nÞ1=2AC*D1o=2v1=2
(4)
where ip is the peak current in amperes,
a
is the charge transfer
coefficient, n is the number of electrons involved in the redox
process, Do is diffusion coefficient in cm2 sꢀ1, C* is the bulk
concentration of the compound in mol cmꢀ3, A is the surface area of
the working electrode in cm2 and v is the potential scan rate in
V sꢀ1. The values of the diffusion coefficient for the redox processes
are calculated employing Eq. (4) by putting various cathodic
h
i
ꢀlog ipc½mAꢄ ¼ þslope log v Vsꢀ1 þ intercept
For getting further insight into the nature of the electrochemical
redox process, the charge transfer coefficients
compounds 1e4 were determined by the application of Bard and
(a) of the
parameters in it using the slope of ipc (m
A) vs. v1/2 (Vsꢀ1) plot
Faulkner relation [39]. According to them the value of
calculated as
a
can be
(shown in Fig. 9(a)). The values of Do thus calculated for compounds
1ꢀ4 are listed in Table 4, which show that the mobility of lower
coordinated Sn4þ species is hampered by the facile attachment of
relatively more solvent molecules. The slow diffusion of 4 is
attributed to its tetracoordinated geometry around tin(IV) center
leading to high solvation by solvent molecules and the presence of
relatively longer butyl groups. Looking at the coordination number
and steric effects of the attached groups to the tin centre, the ease of
solvation of the molecules would vary in the order: 1 > 4 > 2 > 3.
ꢂ
ꢃ
a
n ¼ 47:7= Epa ꢀ Epa=2 mV
(5)
where Epa/2 is the potential where the current is at half the peak
value.
The values of charge transfer coefficient (a) for the compounds
were calculated using Eq. (5) and are listed in Table 5.
Fig. 6. Packing diagram of compound 2.

M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
221
0.05
A 0.04
B
-1
160 mVs
0.04
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
-0.04
-1
160 mVs
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
-1
40 mVs
-1
40 mVs
-1.5 -1.0 -0.5
0.0
E / V
0.5
1.0
1.5
-1.5
-1.0
-0.5
0.0
E / V
0.5
1.0
1.5
C
D
0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
0.04
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
-1
160 mVs
-1
160 mVs
-1
40 mVs
-1
40 mVs
-1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
-1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
E / V
E / V
Fig. 8. Cyclic voltammogramms of compounds 1(A), 2 (B), 3(C) and 4(D) each at scan rates of 40, 60, 80, 100, 120, 140 and 160 mV sꢀ1
.
For irreversible electrode processes the value of E is given by
Eq. (6) [40e42]:
Table 5
CV parameters for the reduction of the compounds.
E ¼ Eo þ 2:303 RTko=
a a
nF ꢀ 2:303 RT= nF log v
(6)
Compound
E (V)
an
D ꢃ 107
kꢂ (sꢀ1
)
(cm2 sꢀ1
)
where Eo is the formal redox potential, ko is the standard hetero-
geneous rate constant and F is the Faraday constant. The values of
heterogeneous rate constant (ko) were determined from the inter-
cept of potential vs. negative logarithm of scan rate plot (shown in
Fig. 10(a)) by putting various parameters in Eq. (6). The values of
1
2
3
4
ꢀ0.89
ꢀ0.92
ꢀ0.96
ꢀ0.99
0.25
0.28
0.27
0.30
4.80
4.02
4.27
3.13
1.36
0.70
1.66
2.11
4.86
b
1
4
a
2
19
18
17
16
15
14
3
2
1
4.83
4.80
4.77
4.74
4.71
4
3
0.20
0.25
0.30
0.35
0.40
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
1/2
v
-log v
Fig. 9. Plots of cathodic peak current vs. square root of the scan rate (a) and log of cathodic peak current vs. ꢀlog scan rate (b) for compounds 1ꢀ4.

222
M. Iqbal et al. / Journal of Organometallic Chemistry 723 (2013) 214e223
a
b1.06
4
1.06
1.04
1.02
1.00
0.98
0.96
0.94
0.92
2
3
1.04
1.02
4
1.00
1
0.98
3
0.96
2
0.94
0.92
1
0.04 0.06 0.08 0.10 0.12 0.14 0.16
-1
0.72 0.84 0.96 1.08 1.20 1.32 1.44
v / Vs
-log v
Fig. 10. Plots of cathodic peak potential vs. ꢀlog of scan rate (a) and scan rate (b) for compounds 1e4.
formal potential (E) were determined from the intercept of reduc-
tion peak potential vs. scan rate curve (Fig. 10(b)) by extrapolating
the curve to the potential axis at v ¼ 0. The values of Eo are listed in
Table 5. According to Eq. (6), there is a linear relation between
potential (E) and logarithm of scan rate (log v) and the generalized
linear regression equation thus obtained is given below:
Appendix A. Supplementary material
Crystallographic data for the structure reported in this paper has
been deposited with the Cambridge Crystallographic Data Centre,
CCDC No. 900473, 891369 and 891370 for NaL and compounds 1
and 2, respectively. Copies of this information may be obtained free
of charge from The Director, CCDC, 12, Union Road, Cambridge CB2
1EZ [Fax þ44(1223)336-033], e-mail deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk.
h
i
EpcðVÞ ¼ ꢀslope ꢃ log v Vsꢀ1 þ intercept
The values of ko evaluated using Eq. (6) are given in Table 5
which show that the small sized 4 undergoes more facile elec-
tron transfer process. Being mononuclear, 3 has higher rate
compared to the heavier tetranuclear 2. Being heaviest, 2 has the
smallest rate of electron transfer process among all the four
compounds. The order of ko supports adsorption controlled
process. That is, in case of diffusion controlled redox process, 1 and
4 would have slowest rate, being more prone to coordination by
solvent molecules.
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