Thermal analyses of diorganotin dl-mandelates

Article abstract of DOI:10.1023/A:1013988930942

The compounds [(LR₂Sn)₂O] {R=CH₃ (Me), n-C₄H₉ (Bⁿ); L=C₆H₅CH(OH)COO^-} were studied by thermogravimetric and differential scanning calorimetry in a dynamic atmosphere of helium. The thermal decomposition mechanisms are similar for both compounds and occur in two consecutive steps. The TG curves of the complexes suggest the liberation of the ligand L in the first step, with probable formation of a tin oxide R₂SnO intermediate. At the end of the second step free tin is obtained in accordance with the stoichiometry of the related compounds. An inverse relation between ΔH_fusion and solubility of the compounds suggests a polymeric structure for the compound with Bⁿ ligand in relation to the compound with M ligand.

Full text of DOI:10.1023/A:1013988930942

Journal of Thermal Analysis and Calorimetry, Vol. 67 (2002) 453–458
THERMAL ANALYSES OF DIORGANOTIN
dl-MANDELATES
V. R. Terra1, R. S. Barbiéri2*, A. K. C. Dias3 and M. G. Cardoso4
1Instituto de Ciências Biológicas e da Saúde, Universidade de Uberaba, 38020-100, Uberaba-MG,
Brazil
2Faculdade Ubaense Ozanam Coelho, 36500-000, Ubá-MG, Brazil
3Instituto Superior de Educação, Universidade Vale do Rio Verde, 37410-000, Três Corações-MG,
Brazil
4Departamento de Química, Universidade Federal de Lavras, 37200-000, Lavras-MG, Brazil
Abstract
The compounds [(LR2Sn)2O] {R=CH3 (Me), n-C4H9 (Bn); L=C6H5CH(OH)COO–} were studied by
thermogravimetric and differential scanning calorimetry in a dynamic atmosphere of helium. The
thermal decomposition mechanisms are similar for both compounds and occur in two consecutive
steps. The TG curves of the complexes suggest the liberation of the ligand L in the first step, with
probable formation of a tin oxide R2SnO intermediate. At the end of the second step free tin is ob-
tained in accordance with the stoichiometry of the related compounds. An inverse relation between
∆Hfusion and solubility of the compounds suggests a polymeric structure for the compound with Bn
ligand in relation to the compound with M ligand.
Keywords: dl-mandelate compounds, organotin compounds, thermal decomposition
Introduction
The organotin carboxylate compounds consist of a vast class of compounds which
have been subject of detailed studies. A great number of these compounds, the struc-
tures of which are known, are obtained by X-ray diffratometry [1–5]. The pertinent
literature also includes extensive study of the organotin carboxylates characteristics,
with the involvement of practically all instrumental techniques of analysis [6–10]. Its
functional applications are various, and they can be used, for example, as catalysts in
industry [11], as biocides in agriculture [9, 12, 13] and as anti-tumorals in medicine
[14]. As for diorganotin compounds with α-hydroxycarboxylic acids, besides the
compounds described here, the literature is limited to the synthesis and infrared stud-
ies of tri- and diorganotin derivatives of the benzilic acid [15]. The present investiga-
tions report the use of thermogravimetric analysis (TG) and differential scanning cal-
*
Author for correspondence: E-mail: barbieri@uai.com.br
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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TERRA et al.: DIORGANOTIN dl-MANDELATES
orimetry (DSC) in helium atmosphere for study of the complexes [(LR2Sn)2O]
{R=CH3 (Me), n-C4H9 (Bn); L=C6H5CH(OH)COO–}.
Experimental
The TG curve in the range 25–700°C was obtained with the TGA-50 equipment from
Shimadzu. 8–10 mg samples were used, with a heating rate of 20°C min–1 under a dy-
namic atmosphere of helium.
The DSC curves, also in the range 25–700°C, from 5–10 mg samples, with a heat-
ing rate of 10°C min–1, also under a dynamic atmosphere of helium, were plotted with the
DSC-50 calorimetry from Shimadzu.
The infrared spectrum of the complexes in the region 5000–275 cm–1 were obtained
with a Perkin Elmer Paragon 1000 spectrophotometer, using CsI pellets.
The 119mSn Mössbauer spectra were provided by a constant acceleration spec-
trometer equipped a BaSnO3 source, at 85 K.
The complexes [(LR2Sn)2O] {R=CH3 (Me), n-C4H9 (Bn); L=C6H5CH(OH)COO–}
were prepared from dl-mandelic acid and R2SnO by methods previously described in
literature [16], and characterized through their IR spectra, by elementary carbon, hy-
drogen and tin analyses and by Mössbauer and 119mSn-NMR spectroscopies.
Results and discussion
From the reactions under reflux between acid dl-mandelic, C6H5CH(OH)COOH, and
dimethyltin oxide, Me2SnO, in ethanol medium or dibutyltin oxide, Bn2SnO, in
acetonitrile medium, the compounds [(LMe2Sn)2O] (compound I) and [(LBn2Sn)2O]
(compound II, L=C6H5CH(OH)COO–), respectively, were prepared. The compounds,
both white, observed through microscope, presented microcrystalline constitution.
However, neither of them provided conditions to obtain single crystals with suitable
features to determine the structure by X-ray crystallography. In many attempts to
recrystalize, the complexes experienced decomposition, even under inert atmosphere,
producing crystals of the respective diorganotin oxides (R2SnO)n (R=Me, Bn).
The melting point of I appeared to be significantly higher than II, which, in ad-
dition with the results of elementary analysis of carbon, hydrogen and tin, are sum-
marized in Table 1.
Table 1 Analytical results of diorganotin dl-mandelate complexes
Mass
formula
C/%
found (calc.)
H/%
found (calc.)
Sn/%
found (calc.)
Compound
M. p./°C
II
II
614.79
783.11
323–326
177–179
40.19 (39.07)
49.14 (49.08)
3.61 (4.10)
6.19 (6.31)
39.07 (38.61)
30.41 (30.31)
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TERRA et al.: DIORGANOTIN dl-MANDELATES
455
In infrared spectrum of the compounds, it was possible to attribute the Sn–C
(555–545 cm–1), Sn–O (360 cm–1), Sn–O–Sn (680 cm–1) and alcoholic O–H
(3400 cm–1) stretching modes, which, together with the data from elementary analy-
sis, permitted the establishment of the proposed formulations and the structures
shown in Fig. 1 [17–19].
Fig. 1 Proposed diorganotin dl-mandelate structures I – [(LMe2Sn)2O], II – [(LBn2Sn)2O]
The compounds were studied through 119Sn Mössbauer spectroscopy, whose spectra
of which, presented in Fig. 2, indicate single quadrupole splitting doublets, ∆, making ev-
ident that I and II present only one site around the tin atoms, where the values of ∆ and of
isomer shift, δ, in mm s–1 (I: ∆=3.65, δ=1.27; II: ∆=3.65, δ=1.37), are consistent with
pentacoordinated tin species [20, 21], in accordance to the structures proposed.
Fig. 2 Mössbauer spectra for dl-mandelates compounds, obtained at T=85 K, using a
CaSnO3 source at room temperature, I – [(LMe2Sn)2O], II – [(LBn2Sn)2O]
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TERRA et al.: DIORGANOTIN dl-MANDELATES
Fig. 3 RMN-119mSn spectrum (400 MHz, CDCl3) for the dl-mandelate compound
[(LBn2Sn)2O]
For compound II it was possible to obtain the 1H, 13C and 119mSn NMR spectra, in
deuterated chloroform medium. The 119mSn-NMR spectrum is shown in Fig. 3, and
only one sign of absorption δ(119mSn) at –146.90 ppm was observed, which also shows
clearly its pentacoordinated geometry, in accordance with the interpretations ob-
tained from the 1H, 13C NMR spectra [22, 23].
For the compound I, due to its insolubility, it was not possible to obtain the
NMR spectra. This relative insolubility related to II, may be a sign of a certain poly-
merization degree.
The TG curves of the compounds, obtained in dynamic atmosphere of helium,
are presented in Fig. 4. One can notice that the relative thermal stability is
significantly different for each compound; while the methyl derivative starts its ther-
mal degradation at 271.5°C, that of the butyl derivated starts in 231.7°C. The lower
thermal stability of this compound may be explained in terms of a greater hindrance
effect of the butyl groups, in relation to the methyl groups of the first complex. The
Fig. 4 TG curves of dl-mandelates compound. Heating rate: 20°C min–1,
I – [(LMe2Sn)2O], II – [(LBn2Sn)2O]
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TERRA et al.: DIORGANOTIN dl-MANDELATES
457
Fig. 5 DSC curves of dl-mandelate compounds. Heating rate: 10°C min–1,
I – [(LMe2Sn)2O], II – [(LBn2Sn)2O]
thermal decomposition processes, in both cases, occur in two consecutive steps, be-
fore 500°C, producing tin as final residue. The analysis of the TG curves of the com-
plexes, according to its respective stoichiometric compositions, suggests that, the lib-
eration of the organic ligands occurs up to the end of the first step of thermal decom-
position, whereas by the end of the second step, tin is obtained. Therefore, it is possi-
ble to propose the following mechanism for the thermal decomposition process:
[(LR2Sn)2O] → R2SnO → Sn
As from the DSC curves, presented in Fig. 5, obtained in a dynamic helium at-
mosphere, the respective values of ∆Hmelting, in kJ mol–1 (I= –85.53; II= –19.67) have
been calculated. The value for the compound I was calculated taking into consider-
ation the monomeric unit.
The higher melting point value and ∆Hmelting to I in relation to II, in an inverse re-
lation with the mass formulas calculated for the monomeric units, together with the
differences of solubility observed, confirmed the possibility that I is a polymeric
compound.
* * *
The authors are grateful to Maria Jose Silva for experimental support of this research and to Prof.
Cláudia de M. S. Condé for our valuable discussions.
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