Reaction products of dichlorodiorganostannanes with sodium in liquid ammonia: In-situ investigations with 119Sn NMR spectroscopy and usage as intermediates for the synthesis of tetraorganostannanes

Article abstract of DOI:10.1016/j.jorganchem.2011.05.001

Dichlorodibutylstannane, dichlorodioctylstannane and dichlorodiphenylstannane were reacted with different amounts of sodium in liquid ammonia. At a molar ratio of R₂SnCl₂/Na of 1:2, polystannanes precipitated, in some cases accompanied by cyclic oligostannanes. The products resulting from mixtures with R₂SnCl₂/Na ratios of 1:3 to 1:10 were soluble and, hence, could be studied in-situ in liquid ammonia with ¹¹⁹Sn NMR spectroscopy. The compounds obtained, tin hydrides of the type R₂SnH^- and in certain cases distannides of the composition R₄Sn₂^2-, formed essentially independent of the R₂SnCl₂/Na ratio; this, in contrast to views expressed in the literature. Our experiments showed that the chemical structure of the in-situ generated species did not permit to draw conclusions about the composition of the reaction products with bromoethane and vice versa - a practice commonly described. Furthermore, we observed migration of the butyl groups both in-situ during the reaction of dichlorodibutylstannane with sodium in liquid ammonia, as well as in the final reaction product. By contrast in the case of the phenyl substituent, migration was detected not during the chemical event in liquid ammonia, but only in the compounds formed. These observations imply a different mechanism for butyl and phenyl group migration.

Full text of DOI:10.1016/j.jorganchem.2011.05.001

Journal of Organometallic Chemistry 696 (2011) 3041e3049
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Reaction products of dichlorodiorganostannanes with sodium in liquid ammonia:
In-situ investigations with 119Sn NMR spectroscopy and usage as intermediates for
the synthesis of tetraorganostannanes
*
Markus Trummer, Jérôme Zemp, Cédric Sax, Paul Smith, Walter Caseri
Department of Materials, Eidgenössische Technische Hochschule (ETH1) Zurich, Wolfgang Pauli Strasse 10, CH 8093 Zurich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Dichlorodibutylstannane, dichlorodioctylstannane and dichlorodiphenylstannane were reacted with
different amounts of sodium in liquid ammonia. At a molar ratio of R2SnCl2/Na of 1:2, polystannanes
precipitated, in some cases accompanied by cyclic oligostannanes. The products resulting from mixtures
with R2SnCl2/Na ratios of 1:3 to 1:10 were soluble and, hence, could be studied in-situ in liquid ammonia
with 119Sn NMR spectroscopy. The compounds obtained, tin hydrides of the type R2SnHꢀ and in certain
cases distannides of the composition R4Sn22ꢀ, formed essentially independent of the R2SnCl2/Na ratio;
this, in contrast to views expressed in the literature. Our experiments showed that the chemical structure
of the in-situ generated species did not permit to draw conclusions about the composition of the reaction
products with bromoethane and vice versa e a practice commonly described. Furthermore, we observed
migration of the butyl groups both in-situ during the reaction of dichlorodibutylstannane with sodium in
liquid ammonia, as well as in the final reaction product. By contrast in the case of the phenyl substituent,
migration was detected not during the chemical event in liquid ammonia, but only in the compounds
formed. These observations imply a different mechanism for butyl and phenyl group migration.
Ó 2011 Elsevier B.V. All rights reserved.
Received 18 March 2011
Received in revised form
21 April 2011
Accepted 3 May 2011
Keywords:
Stannides
Polystannanes
Liquid ammonia
Alkyl group migration
119Sn NMR spectroscopy
1. Introduction
Further, it has been suggested (but not spectroscopically veri-
fied) [5,26] that the composition of the reaction intermediates
The reaction of haloorganostannanes with sodium in liquid
ammonia has attracted attention since the early part of the past
century [1e11]. The resulting products have been used in-situ as
intermediates for the preparation of organotin compounds (orga-
nostannanes). In particular, conversion of such intermediates with
haloalkanes and haloarenes yielded tetraorganostannanes [12e25].
In this context, also dihalodiorganostannanes, R2SnCl2, have been
exposed to sodium, and were regarded to yield diorganotin dia-
nions (Fig. 1, diorganostannide dianions, R2Sn2ꢀ), which can
subsequently act as reaction intermediates for the synthesis of
tetraorganostannanes, with the respective sodium halides as
reaction byproducts [18,26e30]. However, surprisingly, recent
studies performed in-situ with 119Sn NMR spectroscopy experi-
ments indicated that diorganostannide dianions were not formed
when stoichiometric ratios of dichlorodiorganostannanes and
sodium are present in liquid ammonia (i.e. 4 molar equivalents of
sodium per mol stannane) [31].
changes upon variation of the diorganostannane/sodium ratio. In
addition, it has been claimed that treatment of dihalodiorgano-
stannanes with less than 4 molar equivalents of sodium (prefer-
entially two equivalents) gives rise to the formation of
oligostannanes or polystannanes (Fig. 1) [1,4,5]; however the
resulting products were not thoroughly characterized. Poly-
stannanes are a unique class of polymers as their backbone consists
of covalently bound metal atoms; they were first prepared by Löwig
[32] and then mainly during the last two decades in various labo-
ratories by different methods [33e52].
In order to resolve the various discrepancies alluded above, we
carried out in-situ investigations of the species arising from reac-
tions of dichlorodiorganostannanes and sodium at different molar
ratios in liquid ammonia and explored their applicability as reac-
tion intermediates for the synthesis of tetraorganostannanes by
conversion with bromoethane.
2. Results
* Corresponding author. Tel.: þ41 44 632 22 18; fax: þ41 44 632 11 78.
E-mail address: wcaseri@mat.ethz.ch (W. Caseri).
www.polytech.mat.ethz.ch.
For the experiments, dichlorodibutylstannane and dichloro-
dioctylstannane were employed as representatives for alkyl
1
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.05.001

3042
M. Trummer et al. / Journal of Organometallic Chemistry 696 (2011) 3041e3049
R
elemental composition of the isolated product was in agreement
R
Cl
Cl
with that of (Oct2Sn)n, and 119Sn NMR spectroscopy in deuterated
dichloromethane revealed a broad signal at ꢀ192 ppm which
corresponds to the value of poly(dioctylstannane), and signals
at ꢀ203 ppm and ꢀ205 ppm for the cyclic byproducts [50].
2Na+
3Na+
4 Na+
2 Cl
2 Cl
2 Cl
-
-
-
(1)
(2)
(3)
Sn
2 Na
3 Na
4 Na
Sn
R
+
+
+
+
+
+
+
R
n
2-
R
R
Cl
R
R
R
Sn
Sn Sn
1/2
+
2.1.3. Dichlorodiphenylstannane
Cl
R
Also treatment of dichlorodiphenylstannane with 2 molar
equivalents of sodium resulted in immediate precipitation of
a yellow, shiny product. The material obtained was insoluble in all
tested organic solvents at room temperature and as well at elevated
temperatures (close to the boiling point of the solvents). Therefore,
it was not possible to determine its molar mass. The product was
washed with a water/ethanol mixture (9:1) to remove sodium
chloride and, thereafter, extracted with hot dichloromethane to
dissolve potential byproducts, in particular cyclic oligo(diphenyl-
stannane)s. 119Sn NMR analysis of the concentrated extracts indi-
cated that no significant amounts of cyclic byproducts were formed.
Elemental analysis of the material was consistent with that of
(Ph2Sn)n.
R
R
Cl
Sn
Cl
R
R
2-
Sn
+
Fig. 1. Previously advanced reactions of dihalodiorganostannanes with sodium.
stannanes and dichlorodiphenylstannane as the most convenient
arylstannane. All those compounds were exposed to 2, 3, 4 and 10
equivalents of sodium, respectively. A summary of the resulting
products is presented in Table 1.
2.1. Dihalodiorganostannane/sodium 1:2
2.2. Dichlorodiorganostannane/sodium 1:3, 1:4 and 1:10
A general feature of reactions with a dichlorodiorganostannane/
sodium ratio of 1:2 was formation of polymers (cf. Table 1). Below,
the results obtained for the individual systems are described in
more detail.
Recently, we reported [31] that in contrast to general views
(Fig. 1), dichlorodibutylstannane and dichlorodiphenylstannane do
not react with four equivalents of sodium to yield the respective
e
diorganostannide dianions. Instead, the anions HSnR2 and
(R2SneSnR2)2e formed in quantities of similar order of magnitude,
and in the case of dichlorodibutylstannane additionally R3Sne and
a fourth unidentified product (e.g. H2RSne or (HRSneSnHR)2ꢀ) arose
by alkyl group migration. Considering that Na in fact is present in
liquid ammonia as Naþ and solvated electrons, the latter providing
the typical blue color of the corresponding solutions [53], the exis-
tence of (R2SneSnR2)2e may be somewhat surprising, as two of the
highly reactive solvated electrons per dianion may remain under
a dichlorodiorganostannane/sodium ratio of 1:4. In order to inves-
tigate if larger quantities of sodiumwould ultimately lead to cleavage
of SneSn bonds in the dianions and if lower amounts influence
the ratio between the aforementioned stannides, dichloro-
diorganostannanes were exposed to 3, 4 and 10 molar equivalents of
Na. According tothe literature [5,26] substantial differences in the in-
situ formed reaction products are expected at these different ratios.
Since the solvated electrons produced upon dissolution of
metallic sodium are consumed when dichlorodiorganostannanes
react - due to the generation of stannides and chloride ions - the
electric conductivity of the reaction mixture is expected to decrease
during the reaction of dichlorodiorganostannanes with sodium
because highly mobile electrons are removed from the system.
Thus, we employed in-situ measurements of the electric conduc-
tivity in liquid ammonia to qualitatively follow the course of the
reaction.
2.1.1. Dichlorodibutylstannane
Exposure of dichlorodibutylstannane to 2 molar equivalents of
sodium in liquid ammonia caused immediate precipitation of
a yellow product, which is typical for poly(dibutylstannane). The
GPC diagrams revealed the presence of polymer with a molar mass
of 8 kg/mol. In addition, products in a mass range of cyclic byprod-
ucts (cyclopentastannane and cyclohexastannane) were detected.
119Sn NMR spectra showed a broad signal at ꢀ190 ppm characteristic
for poly(dibutylstannane) and signals at ꢀ202 ppm and ꢀ203 ppm,
which are typical for cyclic byproducts [50]. The elemental
composition of the isolated yellow product was consistent with
products of the composition (SnBu2)n, which is in agreement both
with the composition of linear polymers and cyclic oligomers.
2.1.2. Dichlorodioctylstannane
Conversion of dichlorodioctylstannane with 2 molar equivalents
of sodium was performed in the same manner as the above
described reaction of dichlorodibutylstannane, resulting in
precipitation of a yellow, pasty material. Analogously, GPC analysis
indicated the generation of poly(dioctylstannane) (Mp around
6 kg/mol), together with cyclic pentamers and hexamers. The
Table 1
Overview of the detected products that emerged from reaction of dichloro-
diorganostannanes, R2SnCl2, with different ratios of sodium in liquid ammonia.
An overview of the soluble products detected by in-situ 119Sn
NMR spectroscopy in liquid ammonia obtained with the different
compounds is displayed in Table 2; the results are described in
more detail in the following sections.
R2SnCl2:
Na ratio
R ¼ butyl
R ¼ octyl
R ¼ phenyl
1:2
(SnBu2)n polymer
and cyclic oligomers
(SnOct2)n
polymer
(SnPh2)n
polymer
and cyclic
oligomers
Oct2SnHꢀ
2.2.1. Dichlorodibutylstannane
1:3
Bu2SnHꢀ, (Bu4Sn2)2ꢀ, Bu3Snꢀ,
one unidentified product
Bu2SnHꢀ, (Bu4Sn2)2ꢀ, Bu3Snꢀ,
one unidentified product
Bu2SnHꢀ, (Bu4Sn2)2ꢀ, Bu3Snꢀ,
one unidentified product
Ph2SnHꢀ, (Ph4Sn2)2ꢀ
The electric conductivity of mixtures of dichlorodibutyl-
stannane/sodium 1:4 versus reaction time is presented in Fig. 2a.
The data show that the conductivity reached a constant value after
30 min, indicating that the reaction was terminated within this
period (Fig. 2a). Accordingly, in-situ NMR measurements in liquid
ammonia were performed after a reaction time of 30 min.
Remarkably, regardless of the dichlorodibutylstannane/sodium
molar ratio in the range of 1:3 up to 1:10, not only the same
a
1:4
Oct2SnHꢀ
Oct2SnHꢀ
Ph2SnHꢀ, (Ph4Sn2)2ꢀ
Ph2SnHꢀ, (Ph4Sn2)2ꢀ
1:10
a
Additional signals are attributed to degradation products formed during transfer
of the reaction solutions to the NMR tubes that could not be avoided. Thus, the signal
of the distannide was not always present in the spectra.

M. Trummer et al. / Journal of Organometallic Chemistry 696 (2011) 3041e3049
3043
Table 2
equilibrium was established in a reaction mixture of dichloro-
dioctylstannane/sodium at a ratio of 1:4 (Fig. 2a).
Overview of the soluble products which emerged after treatment of dichlor-
odiorganostannanes with sodium in liquid ammonia identified by in-situ 119Sn NMR
spectroscopy.
Remarkably, the reaction mixture of dichlorodioctylstannane
with three, four and ten equivalents of sodium gave rise to only one
signal in in-situ 119Sn NMR spectra within the detection limits,
at ꢀ223 ppm at 200 K (ꢀ219 ppm at 220 K, Ds ¼ 0.2 ppm/K), i.e. in
the range of the above mentioned Bu2SnHꢀ anion. This signal is
present as a singulet in broadband proton-decoupled 119Sn NMR
spectra, in contrast to the corresponding signal in proton-coupled
spectra, where a doublet is visible, due to a 119Snꢀ1H coupling
(Fig. 3). Therefore, we assign the signal to dioctylhydrostannide
Oct2SnHꢀ. The coupling constant of 1J(Sn,H) ¼ 95 Hz is in good
agreement with values reported earlier for Bu2SnHꢀ [31,54]. We
would like to note that the transfer of the reaction products in
liquid ammonia into NMR tubes was more challenging for the octyl
than for the butyl compounds. Due to the lower solubility of the
octyl compounds in liquid ammonia, they tended to precipitate
during the transfer to the NMR test tube (yellow polymeric
residue). To avoid this problem lower stannane concentrations
were used - leading to a decreased sensitivity of the in-situ 119Sn
NMR spectra and therefore increased acquisition times.
d
119Sn (ppm)
200 K
220 K
Bu2SnHꢀ
(Bu4Sn2)2ꢀ
Bu3Snꢀ
ꢀ228
ꢀ161
ꢀ136
ꢀ223
ꢀ197
ꢀ132
ꢀ219
ꢀ143
ꢀ137
ꢀ219
ꢀ192
ꢀ116
Oct2SnHꢀ
Ph2SnHꢀ
(Ph4Sn2)2ꢀ
intermediates emerged, but also their relative distribution did not
alter strikingly (Fig. 2b). These intermediates are attributed to
dibutylhydrostannide Bu2SnHꢀ (ꢀ228 ppm), tetrabutyldistannide
(Bu4Sn2)2ꢀ (ꢀ161 ppm), tributylstannide Bu3Snꢀ (ꢀ136 ppm) and
a fourth unidentified product that arose by alkyl group migration
(e.g. H2RSne or (HRSneSnHR)2e
)
(ꢀ212 ppm) [31]. Thus,
dichlorodibutylstannane was found to undergo alkyl group
migration at molar stannane/sodium ratios between 1:3 and 1:10.
2.2.3. Dichlorodiphenylstannane
2.2.2. Dichlorodioctylstannane
The reaction of dichlorodiphenylstannane with sodium was
terminated within 30 min, as evident from conductivity measure-
ments (Fig. 2a). After this period two strong signals were visible in
119Sn NMR spectra of in-situ formed intermediates at dichloro-
diphenylstannane/sodium ratios of 1:4 and 1:10 (Fig. 4), repre-
senting diphenylhydrostannide, Ph2SnHꢀ, (ꢀ197.2 ppm) and
Conversion of dichlorodioctylstannane with sodium in liquid
ammonia was slower compared to dichlorodibutylstannane, as
revealed by conductivity measurements. It took well over 1 h until
Fig. 2. a) Electrical conductivity of a mixture of dichlorodibutylstannane (-), dibro-
modibutylstannane (,), dichlorodioctylstannane (C) and dichlorodiphenylstannane
(:) and 4 equivalents of sodium measured in situ in liquid ammonia as a function of
reaction time. b) 119Sn NMR spectra recorded in-situ of dichlorodibutylstannane
exposed to 10, 4 and 3 molar equivalents of sodium in liquid ammonia.
Fig. 3. a) In-situ 119Sn NMR spectra of dichlorodioctylstannane exposed to 10, 4 and
3 molar equivalents of sodium in liquid ammonia. The spectrum of the 1:3 ratio was
recorded at 220
K whereas the other measurements were performed at 200 K
(
Ds ¼ 0.2 ppm/K). b) 1H decoupled and 1H coupled 119Sn NMR spectra of the signal at
ꢀ223.3 ppm (1J(Sn,H) ¼ 95 Hz).

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M. Trummer et al. / Journal of Organometallic Chemistry 696 (2011) 3041e3049
Table 3
Summary of the compounds that resulted from reaction of in-situ prepared
dichlorodiorganostannane-sodium mixtures in liquid ammonia with bromoethane
(30 min equilibration time for butyl and phenyl; 90 min for octyl).
R2SnCl2:Na ratio
1:10
R ¼ butyl
BuSnEt3
Bu2SnEt2
Bu3SnEt
R ¼ octyl
Oct2SnEt2
OctxSn2Et6ꢀx
unident. prod.
(ꢀ125 ppm)
OctSnEt3
Oct2SnEt2
Oct3SnEt
OctxSn2Et6ꢀx
Oct6Sn2O
Oct2SnEt2
R ¼ phenyl
Ph2SnEt2
Ph4Sn2Et2
unident. prod.
(ꢀ28 ppm)
Ph2SnEt2
a
1:4
1:3
BuSnEt3
Bu2SnEt2
Bu3SnEt
Bu3SnOH
Bu2SnEt2
Bu3SnEt
PhSnEt3
Ph2SnEt2
Ph3SnEt
OctxSn2Et6ꢀx
Oct6Sn2O
Bu4Sn2Et2
Bu3Sn2Et3
a
Minor quantities of Oct3SnEt and OctSnEt3 might be present but were not
observed due to a relatively low signal/noise ratio in the spectra.
2.3. In-situ prepared alkylstannides as intermediates: reactions
with bromoethane
The above described invariability of detected products arising at
dichlorodialkylstannane/sodium ratios between 1:3 and 1:10, as
well as in dichlorodiphenylstannane/sodium 1:4 and 1:10 mixtures
are not in agreement with previous suggestions in the literature
[5,26]. In fact, it has been concluded after reactions of in-situ
formed products with haloalkanes that the composition of the
formed products should change upon variation of the dihalo-
diorganostannane/sodium ratio (note that these conclusions were
not based on direct spectroscopic data). Hence, in the following we
investigated if related products with haloalkanes can indeed
provide information on in-situ formed products and vice versa, as an
example of reactions with bromoethane. An overview on the
results, which will be detailed below, is presented in Table 3.
Fig. 4. In-situ 119Sn NMR spectra of dichlorodiphenylstannane exposed to 10, 4 and
3 molar equivalents sodium in liquid ammonia including a sample of 1:3 ratio that was
aged for 2 h.
tetraphenyldistannide, (Ph4Sn2)2ꢀ (ꢀ131.7 ppm), respectively [31].
Reliable in-situ measurements of 1:3 mixtures were somewhat
more difficult to perform with reaction mixtures based on
dichlorodiphenylstannane than on dichlorodibutylstannane,
because in the former a higher tendency to form precipitates
(polymers or other products) was observed. When such solutions
(still dark red) were investigated by 119Sn NMR spectroscopy,
several signals emerged. In addition to the peak of diphenylhy-
drostannide at ꢀ197.2 ppm, two other strong signals at ꢀ65 ppm
and ꢀ125 ppm arose (Fig. 4). Unfortunately, the species causing the
latter signals could not be identified. The solutions lost their char-
acteristic dark red color after about 2 h at 200 K; a yellow precip-
itate was formed, but the NMR spectra still featured the
diphenylhydrostannide signal at ꢀ197.2 ppm together with the
strong signal at ꢀ75 ppm, which could not be attributed to
a specific compound.
2.3.1. Conversion of butylstannides
Butylstannides were exposed to an excess of bromoethane and
products studied with 119Sn NMR spectroscopy. At dichloro-
dibutylstannanes/sodium ratios of 1:4 and 1:10, the spectra did not
differ considerably (Fig. 5a). A main product accompanied by two
side products emerged in a region of 0 to ꢀ10 ppm (Table 4), i.e. in the
typical range of tetraalkylstannanes, which is consistent with the
Fig. 5. 119Sn NMR spectra of the reaction products of a) dichlorodibutylstannane, b) dichlorodioctylstannane and c) dichlorodiphenylstannane exposed first to 10, 4 or 3 molar
equivalents of sodium and then converted with bromoethane.

M. Trummer et al. / Journal of Organometallic Chemistry 696 (2011) 3041e3049
3045
chemical shifts of the SneCH2 signals in 13C NMR spectra (around
0 ppm, Table 5). Experiments with 13C labeled bromoethane,
Br13CH2CH3, allowed identification of the reaction products via the
signal splitting pattern caused by 119Sne13C couplings (Fig. 6 and
Tables 4 and 5). Thus, it was concluded that the primary product of
the mixtures with 1:4 and 1:10 ratios was dibutyldiethylstannane,
Bu2SnEt2, and the side products were butyltriethylstannane,
Bu-SnEt3, and tributylethylstannane, Bu3SnEt, which had been
established by alkyl group exchange reactions [31]. The very weak
signal at ꢀ75 ppm in the spectra representing the 1:4 ratio is
probably due to tetrabutyldiethyldistananne. Tributylethyl-
stannane and dibutyldiethylstannane were found in similar quan-
tities at a dichlorodibutylstannane/sodium ratio of 1:3, but in
addition distannanes formed (Fig. 5a). The latter were identified not
only by the chemical shifts (about ꢀ70 ppm, typical for distannanes
[55]), but also by the signal splitting patterns in 119Sn NMR spectra
(Tables 4 and 5). The broad signal at 105 ppm was attributed to
Bu3SnOH [31].
Table 5
13C NMR data of the reaction products of a mixture Bu2SnCl2: Na, 1:4, with 13C
labeled bromoethane Br13CH2CH3.
d
1J(119Sn,C)
1J(117Sn,C)
Hz
ppm
Hz
Bu3Sn (13CH2CH3)
0.11
0.28
0.67
1.53
1.88
7.73
8.23
321.4
319.1
318.8
245.5
245.4
373.8
376.3
309.0
305.0
304.6
234.2
234.5
358.8
358.8
Bu2 Sn (13CH2CH3)2
Bu Sn (13CH2CH3)3
Bu2 Sn (13CH2CH3)eBu2 Sn (13CH2CH3)
Bu Sn (13CH2CH3)2eBu2 Sn (13CH2CH3)
Bu2 Sn (13CH2CH3)eOeBu2 Sn (13CH2CH3)
Bu Sn (13CH2CH3)2eOeBu2 Sn (13CH2CH3)
derivatives of the composition EtxOct6ꢀxSn2 at about ꢀ70 ppm were
detected, and the spectra of the reaction products after 7 and 14 min
equilibration time (Fig. 7b) showed the signals of chlor-
oethyldioctylstannane (148 ppm [55]), together with numerous
signals between ꢀ150 ppm and ꢀ250 ppm. After 21 min a group of
signals around ꢀ210 ppm was present. These signals might represent
oligostannanes (For comparison: dimers ꢀ70 ppm, polystannanes
ꢀ192 ppm and cyclic oligostannanes ꢀ203 ppm [55]). Finally, after
100 min equilibration time the number of significant species was
reduced.
2.3.2. Conversion of octylstannides
The product composition resulting from the reaction of bro-
moethane with dichlorodioctylstannane/sodium mixtures at molar
ratios between 1:3 and 1:10 strongly depended on that ratio
(Fig. 5b). The corresponding products were designated by 119Sn
NMR spectroscopy on the basis of the above discussed values of
the butylstannanes (Table 6). Mono- and distannanes were found
in all cases, yet the ratio between these species shifted to the
distannanes with decreasing dichlorodioctylstannane/sodium
2.3.4. Conversion of diphenylstannides
In the case of conversion of in-situ prepared diphenylstannides,
the ratio of dichlorodiphenylstannane/sodium in the initial solution
had a significant influence on the products formed after subsequent
reaction with bromoethane. The simplest situation occurred at
a dichlorodiphenylstannane/sodium ratio of 1:4 where only one
product was observed after conversion with bromoethane in 119Sn
NMR spectra, namely at ꢀ65.4 ppm e diethyldiphenylstannane [56]
(Fig. 5c) (sometimes a small signal around ꢀ90 ppm was detected,
ratio. Distannanes dominated at
a ratio of 1:3, while dio-
ctyldiethylstannane, Oct2SnEt2, was the main product at 1:4 and
1:10. Alkyl group exchange was little pronounced at 1:4 ratio, while
at 1:3 ratio strong NMR signals at ꢀ69.7 ppm and ꢀ70.1 ppm indi-
cated alkyl group exchange in distannanes, resulting in derivatives
of the general composition EtxOct6ꢀxSn2 (Fig. 5b). The compound
causing the signal at about ꢀ125 ppm observed in the case of a 1:10
ratio could not be identified. NB oxidation products, Oct3SneOH
(99 ppm) and Oct3SneOeSnOct3 (96 ppm), were observed in some
cases.
which was not due to either Ph3SnEt,
dimeric species Ph4Et2Sn2,
d
119Sn ¼ ꢀ98 ppm or to
d
119Sn ¼ ꢀ116 ppm). A stannane/sodium
ratio of 1:3 might result in favored formation of distannanes due
to stoichiometric considerations (equation (2) in Fig. 1), and
indeed a dominating additional signal at ꢀ116 ppm was observed
in 119Sn NMR spectra (Fig. 5c, identified by the accompanied tin
satellites 1J(Sn,Sn) ¼ 3472 Hz in other experiments), attributed to
diethyltetraphenyldistannane (Table 6).
When dichlorodiphenylstannane was exposed to ten equiva-
lents of sodium and subsequently converted with bromoethane,
the main product was again diethyldiphenylstannane. But the
additional sodium seemed to trigger the breakage of the
tinecarbon bond, as the signals at ꢀ34 ppm and ꢀ98 ppm in the
119Sn NMR spectra can be attributed to triethylphenylstannane
(ꢀ34 ppm) and ethyltriphenylstannane (ꢀ98 ppm) [56,57]. A signal
at ꢀ28 ppm could not be identified.
2.3.3. Evolution of the octylstannides
The low reaction rate of dichlorodioctylstannane with sodium
(Fig. 7a) was beneficially employed to convert portions of dioctyl-
stannane/sodium 1:4 mixtures with bromoethane already before the
equilibrium of the intermediate products was established in situ. The
119Sn NMR spectra of the reactionproducts (Fig. 7b) imply that during
the decrease of the conductivity in the reaction mixtures various
intermediates formed, which, however, could not be detected in-situ
by 119Sn NMR during the formation and after the equilibrium was
reached (see above). For example, besides the signals of dieth-
yldioctylstannane (ꢀ4.4 ppm), tetraoctyldiethyldistannane and
Table 4
119Sn NMR data of the products resulting from conversion of a mixture of Bu2SnCl2/Na 1:4 with 13C labeled bromoethane, Br13CH2CH3.
d
1J(Sn,C)
2J(Sn,C)
1J(Sn,Sn)
ppm
Hz
Hz
Hz
Bu3 Sn (13CH2CH3)
d
t
q
dd
dt
td
d
d
t
ꢀ7.9
318.8
319.2
319.3
245.3
246.0
245.3
374.0
374.9
375.4
374.6
Bu2 Sn (13CH2CH3)2
ꢀ4.1
ꢀ0.6
ꢀ75.3
ꢀ73.8
ꢀ69.1
99.9
Bu Sn (13CH2CH3)3
Bu2 Sn (13CH2CH3)eBu2 Sn (13CH2CH3)
Bu2 Sn (a) (13CH2CH3)eBu Sn (b) (13CH2CH3)2
41.7
42.1
41.4
2580
a
b
Bu2 Sn (13CH2CH3) OH
Bu2 Sn (13CH2CH3)eOeBu2 Sn (13CH2CH3)
Bu Sn (a) (13CH2CH3)2eOeBu2 Sn(b) (13CH2CH3)
90.1
90.4
90.6
a
b
d

3046
M. Trummer et al. / Journal of Organometallic Chemistry 696 (2011) 3041e3049
Fig. 6. a) 119Sn NMR spectrum of the reaction products of dichlorodibutylstannane
exposed first to 4 equivalents of sodium and then converted with Br13CH2CH3, b)
magnification of the region of the monostannanes.
3. Discussion
Fig. 7. a) Time-dependent conductivity of a mixture of dichlorodioctylstannane and
four equivalents of sodium in liquid ammonia including marks at the times selected for
addition of bromoethane. b) 119Sn NMR spectra of the reaction products with bro-
moethane after the times indicated in Fig. 3a.
In the range of dichlorodiorganostannane/sodium ratios of 1:2
to 1:10 in liquid ammonia, polymers formed only at a ratio of 1:2,
with all three stannanes investigated. The polystannanes with
aliphatic side groups were accompanied by cyclic oligo(dialkyl-
stannane)s, while no evidence for cyclic oligo(diphenylstannane)s
was found. Indeed, polymer as well as cyclic oligomer formation
agrees with a 1:2 ratio of dichlorodiorganostannane and sodium
(cf. reaction scheme in Fig. 1). Already at a ratio of 1:3 formation of
polymeric material was completely suppressed. Yet this does not
necessarily imply that polymers never formed at this ratio, as
macromolecules might have been generated in an early stage of the
reaction and decomposed later.
Surprisingly, and in contrast to concepts advanced in the liter-
ature (see Introduction), the differences in the distribution of the
in-situ formed products at dichlorodiorganostannane/sodium
ratios of 1:3 to 1:10 were not significant. Stannides of the compo-
sition R2SnHꢀ were detected in all cases and dinuclear compounds
of the type R4Sn22ꢀ with R ¼ butyl and R ¼ phenyl. In any solution,
there was no evidence for the presence of the frequently proposed
2ꢀ
2ꢀ
SnR2 dianion. In fact, SnR2 is expected to act as a very strong
base, which may readily be able to uꢀndergo an acidebase reaction
with ammonia, thus forming NH2 and R2SnHꢀ. Therefore, it
cannot be excluded that the detected R2SnHꢀ developed from very
short-living SnR2
2ꢀ
.
Migration of alkyl groups in the in-situ formed species was
found to occur with butyl but not with octyl moieties. Also,
formation of tetraalkyldistannides was observed with butyl but not
with octyl groups. Obviously, octyl moieties impede both formation
of dimers and alkyl group migration in liquid ammonia, which
might be due to steric reasons.
Further, it is evident that, again in contrast to previous concepts
proposed in the literature, the use of the reaction products of
dichlorodiorganostannane with sodium as soluble intermediates
for subsequent reactions with haloalkanes only allows limited
conclusions about the nature of the intermediates. In particular the
existence of the intermediate hydrodiorganostannides, R2SnHꢀ
(which were present in all reaction solutions of dichloro-
Table 6
119Sn NMR data of the products resulting from conversion of Oct2SnCl2/Na and
Ph2SnCl2/Na mixtures with bromoethane.
d
119Sn (ppm)
diorganostannanes with sodium) as well as the appearance of the
Oct3SnEt
Oct2SnEt2
OctSnEt3
Oct4Et2Sn2
Oct3Et3Sn2
Oct2Et4Sn2
ꢀ8.3
Ph3SnEt
Ph2SnEt2
PhSnEt3
Ph4Et2Sn2
ꢀ34
ꢀ65
2ꢀ
dinuclear species R4Sn2
in the in-situ reactions of dichloro-
ꢀ4.4
ꢀ0.7
ꢀ98
dibutylstannane and dichlorodiphenylstannane with sodium were
not reflected in the products arising after subsequent reaction
with bromoethane. On the other hand, the chemical structure
of the intermediates does not allow definite conclusions for the
ꢀ75.7
ꢀ70.1/70.2
ꢀ69.7
ꢀ116

M. Trummer et al. / Journal of Organometallic Chemistry 696 (2011) 3041e3049
3047
Table 7
solutions of dichlorodibutylstannane and four equivalents of
sodium, product formation essentially did not differ.
Quantities applied for the conversion of dichlorodiorganostannanes with 2 molar
equivalents of sodium.
sodium
mg
R2SnCl2
mg
4. Conclusions
mmol
mmol
Reaction of dichlorodiorganostannanes with two equivalents
of sodium in liquid ammonia resulted in the formation of poly-
stannanes, which were accompanied by cyclic byproducts in the
case of the alkylstannanes. However, conversion of dichloro-
diorganostannanes with three to 10 molar equivalents of sodium
yielded soluble intermediates. Instead of the frequently proposed
R2Sn2ꢀ dianions [4,5,12,17,26,29,30], R2SnHꢀ and in the case of
Bu2SnCl2
Ph2SnCl2
Oct2SnCl2
151.5
181.9
227.0
6.59
7.91
9.87
1000.7
1360.5
2053.6
3.29
3.96
4.94
composition of the products emerging after subsequent reaction
with bromoethane. For example, the presence of in-situ generated
R2SnHꢀ does not result in formation of tin hydrides after conver-
sion with bromoethane. Also, solutions with virtually the same
in-situ prepared products can lead to different reaction products
after conversion with bromoethane if different ratios between
dichlorodiorganostannane and sodium were initially present. Note
that the formation of the detected in-situ formed products requires
less than four equivalents sodium per dihalodiorganostannane, i.e.
unreacted sodium was present at least in the case of four or ten
equivalents. This leads to the conclusion that unreacted sodium is
involved in the conversion of intermediate stannides with
bromoethane.
2ꢀ
R ¼ butyl and R ¼ phenyl also dinuclear species of the type R4Sn2
emerged. These species did not allow reliable conclusions about the
reaction products resulting from exposure to bromoethane and vice
versa. Migration of organic groups in in-situ formed reaction
products was reflected also in products resulting after conversion
with bromoethane, while the reverse conclusion was not true.
Finally, when in-situ formed reaction products are employed as
intermediates for the preparation of tetraorganostannanes by
conversion with haloalkanes, a large number of products can arise
when the haloalkanes are added before that equilibrium is reached.
Remarkably, even an excess of sodium is not able to cleave
SneSn bonds in R4Sn22ꢀ (detected with R ¼ butyl and R ¼ phenyl),
while subsequent cleavage of those SneSn bonds after addition of
bromoethane was observed - in particular when four equivalents
of sodium per tin atom were present. This implies that bromo-
ethane or reaction products of bromoethane, respectively, were
involved in SneSn cleavage. On the other hand, the occurrence of
binuclear species in reaction products with bromoethane does not
allow to conclude that the in-situ generated intermediate prod-
ucts also contained binuclear stannanes, as especially evident
from the solutions with dichlorodioctylstannane/sodium 1:3,
where Oct2SnHꢀ dominated in the in-situ generated reaction
solution, while binuclear species dominated after conversion with
bromoethane.
However, an exchange of organic side groups in in-situ gener-
ated species was also reflected in the reaction products with
bromoethane. Yet the reverse conclusion about side group
exchange in in-situ generated species derived from the presence of
side group exchange after exposure to bromoethane, is not valid.
While considerable breakup of tin-carbon bonds happened during
the reaction of dichlorodibutylstannane with sodium, exchange of
phenyl groups was observed only after the reaction of bromoethane
with the 1:10 mixture of dichlorodiphenylstannane/sodium;
phenyl group exchange was not visible in the in-situ 119Sn NMR
measurements. This implies that exchange of phenyl groups took
place simultaneously with the reaction of the in-situ generated
intermediates with bromoethane.
As a consequence of the above considerations, conclusions
regarding the composition of solutions of dichlorodioctylstannane
and sodium at different reaction times by analysis of the reaction
products observed after addition of bromoethane should be taken
with care. These experiments may only indicate that numerous
intermediates are formed initially which ultimately converge to
few products.
Finally, it appears that the halogen atoms present in the system
do not exert a pronounced influence on the product distributions.
Substitution of dichlorodibutylstannane by dibromodibutyl-
stannane did not change, considerably, the course of the conduc-
tivity of reaction solutions with four equivalents sodium (Fig. 2a),
and there was no fundamental alteration in the reaction products
after subsequent conversion with bromoethane. Moreover, when
bromoethane was substituted by iodoethane added to reaction
5. Experimental
5.1. Materials
Ammonia was purchased from PanGas (Dagmarsellen,
Switzerland, 99.999%), dichlorodibutylstannane, dichlorodioctyl-
stannane from ABCR GmbH (Karlsruhe, Germany) and dichlor-
odiphenylstannane from Sigma Aldrich (Buchs, Switzerland). All
substances were recrystallized twice by dissolving in boiling
pentane and subsequent precipitation of the products at 200
K. Bromoethane was purchased from Acros Organics (Basel,
Switzerland), Br13CH2CH3 and deuterated dichloromethane (99.9%
D) from Cambridge Isotope Laboratories (ReseaChem GmbH,
Burgdorf, Switzerland), and organic solvents from Fluka (Buchs,
Switzerland).
5.2. Methods
119Sn NMR spectra were recorded with a Bruker UltraShield
300 MHz/54 mm Fourier-transform spectrometer at a frequency of
112 MHz with either inverse-gated decoupling or without decou-
pling, as indicated in the text. In both cases a delay time of 0.5 s, an
acquisition time of 0.1 s and a pulse angle of 3
The sweep width was 700 ppm with a 16 k data point acquisition
range resulting in a digital resolution of 4.78 Hz. Chemical shifts (
are reported in ppm referred to tetramethylstannane
m
s (90ꢁ) were applied.
d
)
(d
(Me4Sn) ¼ 0 ppm).
Electrical conductivity measurements in liquid ammonia solu-
tions were performed with a TetraCon 325/Pt electrode from WTW
(Weilheim, Germany) in combination with a WTW MultiLab 540
instrument. A 100 mL three-neck flask was filled to the top with
ammonia (about 150 mL) to ensure complete coverage of the
electrodes. After w200 mg of sodium were dissolved in the
ammonia the conductivity cell was immersed under nitrogen
counter flow. The dichlorodiorganostannanes were introduced
through the side necks of the flask and the conductivity monitored
as a function of time. Conductivity was recorded during the reac-
tion of dichlorodiorganostannanes with 4 molar equivalents of
sodium until a plateau value was reached. At this stage of the
reaction only soluble products were present.
Elemental analysis was performed by the Microelemental
Analysis Laboratory of the Department of Chemistry at ETH Zürich.

3048
M. Trummer et al. / Journal of Organometallic Chemistry 696 (2011) 3041e3049
Gel permeation chromatography was performed with a Visco-
tek VE7510 equipped with degasser, VE1121 solvent pump, VE520
autosampler and Model 301 triple detector array. A PL gel 5
Mixed-D column from Polymer Laboratories Ltd. (Shropshire,
United Kingdom) was used, calibrated with poly(styrene)
standards.
a constant nitrogen flow by warming the flask to room tempera-
ture, and THF was evaporated at room temperature in vacuo
(0.1 mbar, 12 h). For gel permeation chromatography (GPC) analysis
the obtained material was dissolved in tetrahydrofuran (THF); this
solution was directly used after filtration with syringe filters
mm
(0.45 mm PTFE filters). The residues stemming from the reactions
Liquid ammonia was condensed in a three-neck flask with flat
bottom equipped with a magnetic glass-coated stirring bar (Teflon-
coated stirring bars were attacked by the sodium) that was evac-
uated, flame-dried and flushed with nitrogen three times. A cold
finger condenser equipped with a calcium chloride drying tube was
mounted under nitrogen counter flow and was flushed with
nitrogen for another 10 min. Subsequently the ammonia gas bottle
was connected via a plastic tube, which was flushed with ammonia
for 30 s. The nitrogen flow was arrested and the equipment was
flushed with ammonia for 5 min. The drying tube outlet was closed
with a balloon before the cold finger condenser was filled with
a dry ice/isopropanol mixture. Also, the reaction vessel was sur-
rounded by a dry ice/isopropanol bath to cool the condensed
ammonia to 200 K. When the desired amount of ammonia was
condensed (90 mL for NMR experiments and 150 mL for conduc-
tivity measurements), the ammonia flow was stopped, a smooth
nitrogen flow restarted and the apparatus was opened by removing
the balloon.
of the different stannanes were further processed as follows:
5.3.2.1. Dichlorodibutylstannane. The residue was dissolved in
50 mL dichloromethane and the insoluble parts (sodium chloride)
were filtered off. Subsequently the solvent was removed in a rotary
evaporator and the product dried in vacuo (0.1 mbar, 24 h).
Elemental analysis (in % w/w, in brackets values calculated for
(SnBu2)n): C 40.75 (41.25), H 7.56 (7.79).
5.3.2.2. Dichlorodioctylstannane. The remaining product was dis-
solved in 50 mL dichloromethane, filtered off to remove insoluble
NaCl, the solvent was thereafter removed in a rotary evaporator and
the product dried in vacuo (0.1 mbar, 24 h). Elemental analysis (in %
w/w, in brackets values calculated for (SnOct2)n): C 55.15 (55.68), H
9.17 (9.93).
5.3.2.3. Dichlorodiphenylstannane. The resulting product was
washed with 50 mL of a water/ethanol (9:1) mixture until no
chloride could be detected in the washing solution (usually 3e4
times, until the addition of 5 mL saturated AgNO3 solution did not
lead to the visible formation of AgCl precipitates). Then, the
material was washed three times with 50 mL dichloromethane, and
finally the product was dried in vacuo (0.1 mbar, 24 h). Elemental
analysis (in % w/w, in brackets values calculated for (SnPh2)n): C
51.79 (52.81), H 3.76 (3.69).
5.3. Reactions
5.3.1. Preparation of reaction mixtures for in-situ NMR
investigations
For reactions of dichlorodibutylstannane and dichloro-
diphenylstannane typically 8 mmol of sodium were introduced in
a nitrogen counter flow into 90 mL condensed ammonia and dis-
solved by stirring for 15 min to result in a homogeneous blue
solution. Thereafter 2.67 mmol, 2 mmol and 0.8 mmol respectively
of dichlorodiorganostannane dissolved in 1 mL THF were slowly
added. The color changed from blue to dark red. The mixtures were
stirred for 30 min in order to complete the reactions and subse-
quently transferred via a bended glass tube directly into precooled
(200 K) and dried NMR tubes (Type 5UP 5 ꢂ 178 mm; ARMAR AG,
Döttingen Switzerland) equipped with a sealed capillary with
deuterated dichloromethane. The transfer was performed with
nitrogen overpressure and argon counter flow by carefully
excluding oxygen. The filled NMR tubes were flushed with argon
and stored at 200 K before they were inserted into the precooled
NMR spectrometer.
To avoid precipitation during the transfer to the NMR tube,
the applied concentration of dichlorodioctylstannane was
significantly lower than in the cases of the other two dichloro-
diorganostannanes. 2 mmol sodium were dissolved in liquid
ammonia by stirring for 15 min and subsequent addition
of 0.67 mmol (1:3 ratio) or 0.5 mmol (1:4 ratio) dichlorodioctyl-
stannane. The mixtures with the 1:10 ratio were prepared by
dissolving 4 mmol sodium together with 0.4 mmol dichloro-
dioctylstannane. Besides the differences in concentration, the
reaction was performed as described above for the other stannanes.
5.3.3. Reactions of organostannide intermediates with
bromoethane
In a typical reaction, 218 mg of sodium (9.45 mmol) were dis-
solved in 90 mL liquid ammonia and stirred for 15 min. Thereafter
720.9 mg of dichlorodibutylstannane (2.37 mmol, or other
dichlorodiorganostannanes, respectively) dissolved in 5 mL tetra-
hydrofuran (THF) were added. The deep red mixture of dichloro-
diorganostannanes
and
sodium was
transferred
after
establishment of the equilibrium (30 min for dichlorodibutyl-
stannane and dichlorodiphenylstannane, 90 min for dichlor-
odioctylstannane) via bended glass tubes by nitrogen overpressure
directly into a 100 mL 2-neck round bottom flask containing
a stirred solution of 5 mL bromoethane diluted with 20 mL THF kept
at 200 K. The solution instantaneously turned colorless and a white
precipitate formed. The ammonia was evaporated by warming the
reaction mixture to room temperature in a nitrogen flow, and THF
was removed at room temperature in vacuo (about 0.1 mbar). The
reaction products were dissolved in deuterated dichloromethane
and analyzed by means of 119Sn NMR spectroscopy.
Acknowledgments
We gratefully acknowledge financial support from the Swiss
National Foundation (Nr.: 200021_126450/1). We also thank Aitor
Moreno and Heinz Rüegger (ETH Zürich) for assistance with NMR
spectroscopy, and Frank Uhlig (TU Graz) for fruitful discussions and
input regarding the intriguing chemistry of tin.
5.3.2. Reactions with 2 molar equivalents of sodium
About 8 mmol sodium (detailed quantities see Table 7) were
stirred in 90 mL liquid ammonia at 200 K for 15 min to obtain
a homogeneous solution. The flask was wrapped with white soft
tissue and aluminum foil to exclude light and 4 mmol of the
respective dichlorodiorganostannane dissolved in 10 mL THF were
added dropwise to the sodium solution under continuous stirring,
whereupon the deep blue color disappeared and a yellow precipi-
tate formed. After about 2 min, the ammonia was removed under
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