polystannanes - reaction mechanism and products
TRANSCRIPT
Research Collection
Doctoral Thesis
Polystannanesreaction mechanism and products
Author(s): Trummer, Markus
Publication Date: 2011
Permanent Link: https://doi.org/10.3929/ethz-a-006741780
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ETH Library
DISS. ETH No. 19 795
POLYSTANNANES - Reaction Mechanism and Products
A dissertation submitted to
ETH ZURICH
for the degree of
Doctor of Sciences
presented by
MARKUS TRUMMER
DI
22.03.1980
citizen of Austria
accepted on the recommendation of
Prof. Paul Smith, examiner
Prof. Walter Caseri, co-examiner
Prof. Frank Uhlig, co-examiner
PD Dr. Wolfram Uhlig, co-examiner
2011
Contents
Summary I
Zusammenfassung V
Chapter I 1
Introduction
Chapter II 17
Diorganostannide Dianions (R2Sn2−) as Reaction Intermediates Revisited:
In-situ 119Sn NMR Studies in Liquid Ammonia
Chapter III 39
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
Chapter IV 73
Poly(dialkylstannane)s and Poly(diarylstannane)s Homo- and Copolymers
Synthesized in Liquid Ammonia
Chapter V 107
From Poly(dialkylstannane)s to Poly(diarylstannane)s:
Comparison of Synthesis Methods and Resulting Polymers
Chapter VI 133
Stability of Polystannanes Towards Light
Chapter VII 151
Conclusions and Outlook
Acknowledgements 165
Curriculum Vitae 167
I
Summary
Polystannanes are specified by a polymer main chain consisting of
covalently interconnected tin atoms, which is to our knowledge unprecedented for
other metals and therefore of fundamental interest. Due to the delocalization of
the electrons in the polymer backbone (σ-delocalization), polystannanes are
potentially appealing materials concerning their chemical, optical, thermal and
electrical properties.
The first synthesis providing pure high molar mass polystannanes was based
on dehydropolymerization of dialkylstannanes (H2SnR2) with the catalyst
[RhCl(PPh3)3] (Wilkinson’s catalyst). This route allows to obtain and isolate pure
linear poly(dialkylstannane)s without cyclic oligomers; but on the other hand, has
some substantial drawbacks. In particular, this method has so far not been suited
to synthesize poly(diarylstannane)s. Hence, to create such materials a new
synthetic route is required, for instance reaction of dichlorodiorganostannanes in
liquid ammonia.
It has frequently been proposed that diorganostannide dianions, SnR22-,
form during reactions of dihalodiorganostannanes with sodium in liquid ammonia.
The formation of this intermediate has been advanced to be an important step in
the synthesis of polystannanes. However, our investigations conducted with 119Sn
NMR spectroscopy in liquid NH3 of reaction intermediates formed in-situ during
the exposure of dichlorodiphenylstannane, dichlorodibutylstannane and dichloro-
dioctylstannane to a stoichiometric amount of sodium (i.e. 4 molar equivalents
sodium per tin atom) unveiled that the proposed SnR22- dianion was not present.
Tetraorganodistannides, (R2Sn-SnR2)2-, and hydrodiorganostannides (tin hydri-
II
des), R2SnH-, were detected instead. Also, products resulting from mixtures of
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 composition of the
respective compounds was found to be essentially independent of the R2SnCl2/Na
ratio. Our experiments showed that the chemical structure of the in-situ generated
species did not permit to draw conclusions about the composition of the
corresponding reaction products with bromoethane and vice versa – a practice
commonly employed. Furthermore, we observed migration of butyl groups both
in-situ during the reaction of dichlorodibutylstannane with sodium in liquid
ammonia, as well as in the final reaction products. By contrast, in the case of
phenyl substituents, migration was not detected in liquid ammonia, unless a large
excess of sodium was present. These observations imply a different mechanism for
butyl and phenyl group migration.
At a molar ratio of R2SnCl2/Na of 1:2, polystannanes precipitated from the
reaction mixture, in some cases accompanied by cyclic oligostannanes. Therefore,
in the case of dichlorodibutylstannane, Bu2SnCl2, and dichlorodiphenylstannane,
Ph2SnCl2, two different reaction pathways could be applied: the monomers were
either directly treated with 2 molar equivalents of sodium, or the reactive
organostannides formed in-situ were further converted with the respective
R2SnCl2. The polymers obtained with the new synthesis route were compared to
the products obtained in polymerization with Wilkinson’s catalyst and tetra-
methylethylenediamin (TMEDA). The route employing Wilkinson’s catalyst was
most beneficial for preparation of poly(dibutylstannane) and TMEDA for
polystannanes containing at least one aromatic group per Sn atom, whereas
synthesis in Na/NH3 yielded best results for polystannanes comprising two
aromatic groups per Sn atom – poly(diarylstannane)s.
III
To expand the range of polystannanes, poly(diarylstannane)s and copoly-
mers of dialkylstannanes (butyl, octyl and dodecyl) and diarylstannanes were
synthesized and characterized. UV/Vis absorption spectroscopy unveiled the
presence of σ-delocalization and σ-π-delocalization in the copolymers with the
σ-π-delocalization originating from the SnPh2 moieties in the polymer. The
copolymers were mainly soluble, dichroic materials which could easily be oriented.
Depending on the length of the alkyl side chain, the orientation was parallel or
perpendicular to the direction of external stimuli.
Finally, the influence of pendant side groups on the stability towards light
of polystannanes in solution was studied; more specifically poly[bis(4-butyl-
phenyl)stannane] and poly(dibutylstannane) in solutions of tetrahydrofuran and
dichloromethane. In both solvents, the poly(diarylstannane) was found to be more
resistant towards light than the poly(dialkylstannane). Experiments with laser flash
photolysis and gel permeation chromatography (GPC) analysis of irradiated
polymer solutions resulted in the conclusion that two different decomposition
mechanisms can occur: either random scission of polymer chains or unzipping,
depending on the polymer architecture and the nature of the solvent.
V
Zusammenfassung
Polystannane sind Verbindungen mit kovalent gebundenen Zinn-Atomen
im Polymerrückgrat. Solche Substanzen sind für andere Metalle bislang unbekannt
und deswegen von fundamentalem Interesse für die Wissenschaft. Die Delokalisa-
tion der Elektronen entlang der Polymer Hauptkette macht Polystannane zu
attraktiven Materialien in Bezug auf ihre chemischen, optischen, thermischen und
elektronischen Eigenschaften.
Die erste Synthese reiner Polystannane mit hoher molarer Masse gelang
mittels der katalytischen Dehydropolymerisation von Dialkylstannanen Alk2SnH2
mit dem Katalysator [RhCl(PPh3)3] (Wilkinson Katalysator). Dieser Weg
ermöglicht es, reine Polymere ohne zyklische Nebenprodukte zu erhalten, birgt
allerdings den Nachteil, dass Diarylstannane Ar2SnH2 nicht zu Poly(diaryl-
stannane)n umgesetzt werden können. Zur Herstellung dieser erstrebenswerten
Materialien, musste ein neuer synthetischer Ansatz gefunden werden: Die
Reaktion von Dichlordiorganostannanen mit Natrium in flüssigem Ammoniak.
Generell wurde bis anhin in der Literatur davon ausgegangen, dass die
Umsetzung von Dichlordiorganostannanen mit Natrium in flüssigem Ammoniak
zur Bildung des Dianions R2Sn2- führt. Die entstehenden Produkte sind ein
wichtiger Schritt bei der Umsetzung zu Polymeren und wurde deswegen mittels
in-situ 119Sn NMR Messungen in flüssigem Ammoniak genauer untersucht. Dabei
zeigte sich, dass bei eine stöchiometrischen Umsatz (4 molare Equivalente
Natrium pro Zinn-Atom) das Dianion R2Sn2- nicht gebildet wird. In der Lösung
detektiert wurden Tetraorganodistannide (R2Sn-SnR2)2- und Diorganohydro-
stannide R2SnH-. Auch durch die Veränderung des Stannan : Natrium
VI
Verhältnisses zwischen 1:3 und 1:10 konnte das Dianion nicht erzeugt werden,
vielmehr wurde festgestellt, dass die entstehenden Zwischenprodukte größtenteils
unabhängig von der eingesetzten Menge an Natrium sind. Weiters wurden die
Reaktionslösungen mit Bromethan umgesetzt. Diese Experimente zeigen keinen
direkten Zusammenhang zwischen den in-situ gebildeten Stanniden und den
später gebildeten Reaktionsprodukten, welche früher öfters zur Identifizierung der
Zwischenprodukte herangezogen wurden. Bei der Reaktion von Dichlordibutyl-
stannan mit Natrium wurde eine Wanderung der Alkylgruppen sowohl in
flüssigem Ammoniak als auch bei den Reaktionsprodukten mit Bromethan
festgestellt. Im Gegensatz dazu wurden bei aromatischen Substituenten ähnliche
Phänomene nicht gefunden, außer es wurde ein sehr großer Überschuss an
Natrium eingesetzt. Dies lässt auf unterschiedliche Mechanismen bei der Alkyl-
und Arylgruppen Migration schließen.
Ein Mischungsverhältnis von 1:2 zwischen R2SnCl2 und Natrium führt zur
Bildung von Polystannanen, teilweise begleited von zyklischen Oligostannane.
Dadurch konnte zur Herstellung von Polystannanen zwischen zwei Wegen
gewählt werden: einerseits die direkte Route mit zwei molaren Equivalenten
Natrium oder die Umsetzung der in-situ gebildeten, reaktiven Zwischenstufen mit
weiterem R2SnCl2. Die erhaltenen Polymere wurden mit den Produkten der
Dehydropolymerisation mit dem Wilkinson Katalysator und Tetramethylethylen-
diamin (TMEDA) verglichen. Es stellte sich heraus, dass der Wilkinson Katalysa-
tor die besten Resultate für die Synthese von Poly(dialkylstannan)en liefert,
hingegen der Weg mit TMEDA für Polymere mit mindestens einer Arylgruppe
pro Zinn die beste Leistung zeigte. Die Synthese mit Natrium in flüssigem
Ammoniak lieferte die besten Resultate für Stannane mit zwei aromatischen
Gruppen pro Zinn-Atom.
VII
Durch die Herstellung von Poly(diarylstannan)en und Copolymeren
zwischen Dialkylstannanen (Butyl, Octyl, und Dodecyl) und Diarylstannanen
wurde der Bereich der zugänglichen Materialien deutlich erweitert. Mittels
UV/Vis Spektroskopie wurde die -Delokalisierung des aliphatischen- und die -
-Delokalisierung des aromatischen Anteils in den Polymeren verdeutlicht. Die
Copolymere sind größtenteils lösliche, dichroitische Materialien, die leicht
orientierbar sind. Je nach Länge der Alkyl-Seitenkette orientiert sich die Sn-Sn
Hauptkette parallel oder senkrecht zur Orientierungsrichtung.
Am Schluss wurde noch der Einfluss der Seitenketten auf die Lichtstabilität
in Lösung erforscht. Dazu wurden Lösungen von Poly[bis(4-butylphenyl)stannan]
und Poly(dibutylstannan) in THF und Dichlormethan untersucht. In beiden
Lösungsmitteln war das aromatische Polymer deutlich stabiler gegenüber Licht-
einflüssen, verglichen mit Poly(dibutylstannan). Blitzphotolyse und gelperme-
ations-chromatographische Untersuchungen an bestrahlten Proben enthüllten
zwei unterschiedliche Abbaumechanismen: zufällige Hauptkettenspaltung und
fortlaufender Kettenabbau.
Chapter I
Introduction
3
Preface
Tin is located in group 14 of the periodic table and the first metal therein. In
fact, the elements in this group - carbon, silicon, germanium, tin and lead -
represent the transition from non-metallic elements (C), to semi-metals (Si, Ge)
and metals (Sn, Pb). Notably, elemental tin exists in two allotropes that also
indicate this change. Grey tin, or the -form is a semiconductor with the same
diamond type crystal structure like the prominent crystalline semiconductors
silicon and germanium, whereas the second modification, white – or - tin is a
silvery, ductile metal with a distorted octahedral structure and a melting
temperature of 232 °C. It is a typical metallic conductor. The metallic form slowly
converts into grey tin below 13 °C (tin disease or tin pest) [1-3].
Organotin Compounds
First reports of organotin compounds, i.e. species comprising both Sn and
organic moieties, date back to 1849, when Frankland heated iodomethane in the
presence of metallic tin [4, 5]. Thereafter, formation of these so-called
organostannanes was also reported by reaction of tin/sodium with iodoalkanes [6]
and by exposure of organozinc or organomercury compounds to tin halides [7].
The yields of these reactions were not satisfying due to the occurrence of secondary
reactions. This could be improved by application of Grignard reactions as shown
by Pope and Peachey for tetraalkyltin derivatives [8], as well as by Pfeifer et al. for
alkyl- and arylstannanes [9, 10]. Synthesis of chlorotriphenylstannane by Krause in
1920 via Grignard reaction and further conversion with Na to hexaphenyldi-
stannane Ph3Sn-SnPh3 in benzene and absolute alcohol resulted in high purities
and good yields of the species [11]. The compound was identified by its elemental
composition and was the first example of an aromatic distannane. The existence of
4
Scheme 1. Preparation of organotin (IV) compounds starting from SnO2 by subsequent alkylation
of SnCl4 with organometallic reagents (RMgX or RLi) and Kocheshkov comproportionation [1].
hexaethyldistannane was already shown with vapor-density measurements by
Ladenburg in 1870 [12], and related work was continued by the synthesis of
various hexaalkyldistannanes by Grüttner [13]. The oxidation number of tin in the
distannanes amounts to +III, in contrast to the most common organic and
inorganic compounds which comprise Sn(II) or Sn(IV). In 1964, Neumann and
König [14] published an excellent overview about compounds with the proposed
structure (SnPh2) obtained by various available methods; in addition, these authors
reported the synthesis of pure dodecaphenylcyclohexastannane in excellent yields
by catalytic condensation of diphenyltin dihydride and by coupling of dichlorodi-
phenylstannane with sodium naphtalide.
Nowadays, aromatic and aliphatic organostannanes are extensively used in
organic chemistry for the formation of C-C bonds in small molecules, as well as
polymers, by palladium-catalyzed Stille cross-coupling [15-18]. Typical industrial
production of organostannanes utilizes the most common tin source, i.e. the oxide-
ore-mineral cassiterite (SnO2), as illustrated in Scheme 1. Industrial applications of
such compounds are the stabilization of poly(vinylchloride) [19-21] (PVC, 20 000
tons of tin/year [22]) and antifouling agents [22]. While the market of PVC stab-
ilization is still growing, that of antifouling is decreasing due to environmental
issues.
SnCl4SnO2
C
CO2
SnCl4
SnCl2
SnCl4 SnR4
MR
MClRxSnCl4-x
+200 °C
+ 2
+ 4
4
+
5
Liquid Ammonia as Medium for the Synthesis of Organostannanes
Reactions of sodium and ammonia were first described by Davy in 1807 [23].
Subsequently, Weyl dissolved metallic sodium and potassium together with
mercury in liquid ammonia in 1864 (“Weylsche Flüssigkeiten”) [24, 25]. In 1878,
Bleekrode [26] mentioned liquid ammonia as a good electric conductor and
observed a blue coloration by applying an electric current on pure liquid ammonia,
and Cady [27] reported in 1897 a high conductivity of sodium solutions in liquid
ammonia. Kraus explained this behavior and the intense blue color of the solutions
by advancing the concept of solvated electrons – i.e. sodium dissolved in liquid
ammonia acts like metal cations and solvated electrons in equilibrium with metal
atoms [28]. This approach gained large interest in organic chemistry after Birch
introduced reduction of aromatic compounds, e.g. benzene to 1,4-cyclohexadiene,
by the action of electrons dissolved in liquid ammonia and ethanol [29-34].
First reports of reactions of organostannanes with sodium in liquid ammonia
were published by Kraus in 1925 [35, 36] with chloro- and bromomethylstannanes
and Chambers 1926 [37] with chloro- and bromophenyltin compounds. Attempts
to produce free SnR2 moieties failed in both cases due to the formation of
polymeric species. As Kraus stated: “In no case was the molecular weight found to
correspond, even roughly, to the monomolecular formula” [35]. Reactions of mono- and
dichloroorganostannanes with sodium in liquid ammonia and subsequent exposure
to haloalkanes resulted in the formation of tetraorganostannanes and, depending
on the sodium : stannane ratio, also di-, tri- and pentastannanes were proposed
[35, 38, 39]. Therefore, interest arose on products of the reaction between
haloorganostannanes and sodium in liquid ammonia. The reaction intermediates
were analyzed by conductivity measurements in liquid ammonia [40-42], or by
synthetic experiments and characterization of the obtained products after removal
6
of the liquid ammonia [38, 39, 43-52]. It was shown that sodium salts of
stannanes behave like strong electrolytes with a high degree of dissociation in
liquid ammonia. However, neither the reaction intermediates that were formed in-
situ with sodium in liquid ammonia, nor the obtained polymeric products were
explored and characterized to satisfaction until now.
Polystannanes
Polymeric materials, i.e. species consisting of a backbone of covalently bound
group 14 elements are known for all corresponding elements, starting from the
traditional organic polymers based on a backbone of carbon atoms (e.g.
polyethylene -CH2-) [53, 54] to polysilanes [55-57], polygermanes [58],
polystannanes as well as (poly)plumbanes (diplumbanes) [59]. The characteristics
of the -bond in the polymer backbone change significantly from the C-C bond to
the Sn-Sn bond. The interaction between the Sn sp3 orbitals in the chain give rise
Scheme 2. Common synthesis methods of polystannanes starting from
dichlorodiorganostannanes, R2SnCl2, with Wurtz-type coupling or electropolymerization, and
from dihydrodiorganostannanes, R2SnH2, by catalytic dehydropolymerization with release of
hydrogen, H2.
Sn
R
R
ClCl
Sn
R
R
ClCl
Sn
R
R
HH
Sn
R
R
Sn
R
R
Sn
R
R
Wurtz-Type Coupling
Electrochemical Synthesis
Catalytic Dehydropolymerization
x
x
Na / Tol.
e-
catalyst
- NaCl
- Cl2
- H2
x
x
x
7
to delocalization of the electrons which is described as linear combination of
* Sn-Sn orbitals [22, 60]. Due to the electronic configuration of Sn, [Kr] 4d10 5s2
5p2, also overlapping with d-orbitals is possible, which could lead to σ--deloca-
lization of electrons, which is not very pronounced for silicon and not possible for
carbon (C: 1s2 2s2 2p2).
Polystannanes were first described by Löwig already in 1852 [61] by reaction
of Sn/K and Sn/Na alloys with iodoethane. The elemental composition of the
obtained materials corresponded to the formula (SnEt2). Also Cahours found
similar products by heating of iodoethane with metallic tin [62-65].
Today, Wurtz-type coupling with Na in organic solvents is still used to obtain
high molar mass poly(dialkylstannane)s (Scheme 2) [66-69], even though low
yields and (cyclic) oligomeric byproducts are commonly reported. Electrochemical
synthesis of dichlorodiorganostannanes, R2SnCl2, has also been applied to create
poly(dialkylstannane)s (Scheme 2), as well as polystannane-polysilane and
polystannane-polygermane copolymers and polystannane networks [70-73].
Formation of poly(dialkylstannane)s by catalytic dehydropolymerization of
R2SnH2 with R = alkyl was often mentioned in literature [74-78], but only
recently, Choffat et al. developed a synthesis route that resulted in linear
poly(dialkylstannane)s in high yields and without the occurrence of cyclic
oligomers, by reaction of Alk2SnH2 with Wilkinson’s catalyst [RhCl(PPh3)3]
(Scheme 2) [79-81]. This procedure allowed for systematic investigation of the
materials properties of polystannanes without the influence of those of the
oligomers. It was found, for instance, that poly(dibutylstannane) is highly
birefringent at room temperature [80], it could easily be oriented by various
methods [82] and possesses a high mobility of charge carriers along the tin atoms
in the polymer chain [83]. Unfortunately, the stability towards light of the
8
materials produced was limited [84]. One possible option to enhance that stability
could be to employ aryl moieties [85, 86] due to the enhanced delocalization of the
electrons (-delocalization), which was also observed for poly(diarylsilanes) by
West [55]. However, in the case of poly[bis(-phenylalkyl)stannane]s [87] this
also resulted in poor resistance.
Only a limited number of reports refer to the synthesis of
poly(diarylstannane)s. Unfortunately, experiments with polymerization via the
catalyst Cp2ZrMe2 by Lu and Tilley yielded only mixtures with cyclic- and low
molar mass oligomers.
9
Objective and Scope
The main objective of this thesis was to extend the available range of
polystannane compounds from poly(dialkylstannane)s to polystannanes with
aromatic side groups. This study also included a range of materials properties of
those materials, especially concerning their interaction with light. A high stability
towards light might be associated with a bathochromic shift in the absorption
maximum, i.e. a lower band-gap and therefore potentially higher conductivity,
compared to the poly(dialkylstannane)s produced so far. As catalytic dehydropoly-
merization with Wilkinson’s catalyst was not successful for the synthesis of poly-
stannanes with aromatic side groups [87], the objective was to develop an adequate
synthesis route for such polymers and thoroughly characterize them.
Thus, in Chapter II and III the reaction of sodium in liquid ammonia and di-
chlorodibutylstannane and dichlorodiphenylstannane is revisited to evaluate the
applicability of this route to produce polystannanes. Reaction intermediates were
characterized in-situ by 119Sn NMR spectroscopy and exposed to bromoethane to
analyze the reaction products.
In Chapter IV poly(dibutylstannane) and poly(diphenylstannane) prepared
with sodium in liquid ammonia are presented, and the synthesis of copolymers of
poly(diphenylstannane) and various dialkylstannanes, ranging from butyl to
dodecyl explored.
A study of the applicability of different synthetic routes to poly(dialkyl-
stannane)s and poly(diarylstannane)s, including the catalytic dehydropolymeriza-
tion, the reaction of sodium and dichlorodiorganostannanes in liquid ammonia and
of dihydrodiorganostannanes with N,N,Nʹ,Nʹ-tetramethyl-1,2-ethylendiamin,
10
TMEDA, is presented in Chapter V. These experiments were performed in
cooperation with Prof. Frank Uhlig and Dr. Marie-Luise Lechner (TU-Graz).
In Chapter VI the stability of poly(diarylstannane)s and poly(dialkylstannane)s
in solution towards light was explored by laser flash photolysis and irradiation
experiments, in collaboration with Dr. Thomas Nauser from the Laboratory of
Inorganic Chemistry at ETH Zürich.
Finally, general conclusions and an outlook on the prospering future of poly-
stannanes are presented in Chapter VII.
11
This thesis is based on manuscripts, which have been published, have been
submitted for publication or are in preparation:
Chapter II:
M. Trummer, W. Caseri, Diorganostannide Dianions (R2Sn2−) as Reaction
Intermediates Revisited: In-situ 119Sn NMR Studies in Liquid Ammonia,
Organometallics, 29 (2010) 3862-3867.
Chapter III:
M. Trummer, J. Zemp, C. Sax, P. Smith, W. Caseri, 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, J. Organomet. Chem., (2011) accepted.
Chapter IV:
M. Trummer, D. Solenthaler, P. Smith, W. Caseri, Poly(dialkylstannane)s and
Poly(diarylstannane)s Homo- and Copolymers Synthesized in Liquid Ammonia,
(2011) submitted
Chapter V:
M.-L. Lechner, M. Trummer, I. Bräunlich, P. Smith, W. Caseri, F. Uhlig, From
Poly(dialkylstannane)s to Poly(diarylstannane)s: Comparison of Synthesis
Methods and Resulting Polymers, Appl. Organomet. Chem., (2011) submitted.
Chapter VI:
M. Trummer, T. Nauser, M.-L. Lechner, F. Uhlig, W. Caseri, Stability of
Polystannanes Towards Light, Polym. Degrad. Stab., (2011) submitted.
In addition, the following publication is related to this work:
M. Trummer, F. Choffat, M. Rämi, P. Smith, W. Caseri, Polystannanes –
Synthesis and Properties, Phosphorus, Sulfur, and Silicon, 186 (2011) 1-3
12
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Chapter II
Diorganostannide Dianions (R2Sn2-) as Reaction
Intermediates Revisited:
In-Situ 119Sn NMR Studies in Liquid Ammonia
19
1. Introduction
The in-situ formation of organotin anions in liquid ammonia is a key step in
the synthesis of a wide spectrum of organic tin compounds [1-7]. In this context,
diorganostannides R2Sn2- have been advanced as intermediates for different
organometallic syntheses, including deuteration of carbonyl compounds [8],
preparation of asymmetric stannanes [9, 10] and formation of polymers [11].
However, conclusive spectroscopic evidence for the formation of diorganostannide
dianions is not available; for instance 119Sn Mössbauer spectroscopy [12] did not
lead to identification of specific compounds. Driven by our interest in the synthesis
of macromolecular polystannanes [13-19], we revisited the purported existence of
diorganostannide dianions. Diorganostannides are commonly generated in-situ by
reaction of dihalodiorganostannanes with sodium in liquid ammonia; the
intermediate products are further converted for synthetic purposes to tetra-
organostannanes according to Scheme 1. Early reports from Kraus et al. [20, 21]
propose the formation of trimethylstannide, SnMe3-, and dimethylstannide
dianion, SnMe22-, by reaction of sodium with bromotri methylstannane, Me3SnBr,
or dibromodimethylstannane, Me2SnBr2, respectively. Depending on the applied
ratio between the dibromodimethylstannane and sodium, also oligomeric stannides
of the type (Me2Sn)x2- were postulated. Similar conclusions were drawn for the
formation of monostannides and distannides upon conversion of the corresponding
Scheme 1. Formation of the previously proposed diorganostannide dianions by conversion of
diorganostannanes with sodium in liquid ammonia and subsequent reaction with haloalkanes [8-
10, 21-25] (Y = Cl, Br, H; X = Cl, Br, I).
Sn
R
R
YY Na Sn2-
R
R
NH3Sn
R
R
R'R'+, 195 K R'X+, ℓ
20
phenyl [22] and ethyl halostannanes [23]. More recently, synthesis of substituted
diaryldimethylstannanes was proposed to proceed via the intermediate Me2Sn2-;
but the formation of the dimeric species (Me2Sn)22- was anticipated, since the
dimer (PhMe2Sn)2 emerged among the reaction products [25]. In-situ formed
tetraorganodistannides were suggested to arise on the basis of crystallization of
salts comprising the dimeric dianion (Ph2Sn)22- from lithium-treated dichlorodi-
phenylstannane in liquid ammonia in the presence of tetraammine lithium [26] or
(18-crown-6)diammine potassium [27] counterions.
The dianion SnMe22- is believed to be formed by reaction of dihydrodimethyl-
stannane (dimethyltin dihydride, dimethylstannane), Me2SnH2, with sodium in
liquid ammonia. Conductivity measurements [28-30] and conductometric titration
of the sodium with dihydrodimethylstannane [24] (and also stannane, SnH4 [31])
were interpreted to indicate the formation of ionic products. A conductivity
minimum was found at a Me2SnH2:Na ratio of ca. 0.5. Up to this ratio,
development of one molar equivalent hydrogen gas per dimethylstannane was
reported, which might at first glance point to the formation of dimethylstannide
dianions by replacement of the hydrogen atoms. At larger Me2SnH2:Na ratios,
however, less than one equivalent hydrogen per dimethylstannane evolved, and it
was assumed – but not proven – that dimethylstannide reacted with dimethyl-
stannane also to form tetramethyldistannide, (SnMe2)22-, or hydrodimethyl-
stannide, Me2SnH-, respectively.
In order to bring to light the nature of the intermediate form of stannides
resulting from the reaction of dihalodiorganostannides or dihydrodiorgano-
stannides with sodium, we employed 119Sn NMR spectroscopy in liquid ammonia
to identify the in-situ formed species – previously claimed to be R2Sn2- – and
examined their reactivity.
21
2. Results and Discussion
In-situ electric conductivity. Reactions were carried out by treatment of dichloro-
dibutylstannane, Bu2SnCl2, and dichlorodiphenylstannane, Ph2SnCl2, with four
molar equivalents of sodium in liquid ammonia. This stoichiometric ratio is just
required for the formation of the hypothetical SnR22- under release of two
equivalents of NaCl as byproduct (upon application of two molar equivalents of
sodium, yellow precipitates emerged, apparently oligostannanes or polystannanes).
In order to determine the time needed for completion of the reactions, its course
was monitored in-situ by recording the electric conductivities of the reacting
solutions (Figure 1). The conductivity reached a plateau value within 30 min
during the conversion of both stannanes with sodium – implying that the reaction
had terminated. The conductivities at the plateau value corresponded to 30 % and
20 % of the initial value for dichlorodiphenylstannane and dichlorodibutyl-
Figure 1. Electric conductivity of reaction mixtures upon addition of dichlorodiphenylstannane,
Ph2SnCl2 (□), or dichlorodibutylstannane, Bu2SnCl2 (■) to a solution of sodium in liquid
ammonia at 195 K (t = 0 indicates the moment of stannane addition; conductivity levels are
normalized for comparison to 100 % at t = 0). Insert: Schematic illustration of the experimental
setup of the in-situ conductivity measurements.
0 20 40 60 800
20
40
60
80
100
Ele
ctric
co
nd
uct
ivity
/ %
Time / min
N2
0 4020 60 80
Time / min
100
60
40
20
0
80
Ele
ctric
alco
nduc
tivity
/ %
22
stannane, respectively. Qualitatively, a pronounced decrease in conductivity is
expected indeed, as highly mobile and conductive solvated electrons, generated
upon dissolution of sodium in liquid ammonia [32], are consumed by the reaction
with the stannanes to form negatively charged stannides.
In-situ 119Sn NMR spectroscopy. In order to characterize the in-situ formed
stannides, the solutions in liquid ammonia were transferred to NMR tubes and
119Sn NMR spectra were recorded at 200 K and 220 K. The higher of these
temperatures resulted in a significantly better resolution of 119Sn-1H and 119Sn-2D
couplings, but was relatively close to the boiling temperature of ammonia;
therefore a temperature of 200 K was adopted for measurements which were
unrelated to the aforementioned couplings. In particular at very long measurement
times, often additional signals emerged, probably due to slow diffusion of water or
oxygen from the atmosphere through the cap of the NMR tube lead to subsequent
reactions.
Reaction of dichlorodiphenylstannane and dihydrodiphenylstannane with Na in
liquid ammonia. 119Sn NMR spectra of dichlorodiphenylstannane treated with
sodium in liquid ammonia featured two strong signals at -132 ppm and -197 ppm
when measured at 200 K (Figure 2a). The chemical shifts showed a pronounced
temperature dependence (Δδ = 0.2 to 0.8 ppm K-1; Table 1). Notably, the same
signals were found when dihydrodiphenylstannane, Ph2SnH2, was exposed to four
molar equivalents of sodium (Figure 2b), i.e. under conditions where the formation
of diorganostannide dianions was expected [24], indicating that in the cases of
dichlorodiphenylstannane and dihydrodiphenylstannane the same products are
formed, although possibly via different processes. The signal at -132 ppm detected
23
Figure 2. 119Sn NMR spectra recorded in-situ in liquid ammonia (at 200 K) of reaction products
of (a) Ph2SnCl2, (b) Ph2SnH2 and (c) Bu2SnCl2 formed during treatment with four molar
equivalents of sodium.
at 200 K (-116 ppm at 220 K) is accompanied by those of satellites of tin (totally
~8 % of the integrated intensity of the main signal, corresponding to the natural
abundance of 117Sn) with a coupling constant 1J(119Sn,117Sn) = 1950 Hz (Figure 3a)
This indicates the presence of a binuclear species in the solution, with the coupling
constant being in the range common to distannanes [33]. Hence, we attribute this
signal – which remained unaffected in hydrogen-coupled 119Sn NMR spectra – to
tetraphenyldistannide, (Ph2Sn)22- [1]. Remarkably, the signal at -193 ppm (measured at
220 K) split up into a well-defined doublet in the hydrogen coupled spectra indicating the
presence of a monohydrostannide (Figure 3a; the coupling typically poorly resolved at 200
K); the coupling constant 1J(Sn,H) = 145 Hz is in good agreement with the coupling
constant for hydrostannides reported earlier [34]. Therefore, we assign this signal to
hydrodiphenylstannide, Ph2SnH-. Obviously, we failed to detect any evidence for the
presence of diphenylstannide, Ph2Sn2- – in surprising variance with literature.
a
b
c
300 200 100 0 -100 -200
Chemical shif t δ / ppm
-300
24
Figure 3. (a) Proton-coupled 119Sn NMR spectrum of Ph2SnCl2/Na in liquid ammonia (at 220
K). The signal at -116 ppm shows a 119Sn-117Sn coupling with a coupling constant of 1950 Hz
and the signal at -192 ppm a 119Sn-1H coupling with a coupling constant of 145 Hz. (b)
Proton-coupled and (c) proton-decoupled 119Sn NMR spectrum showing the hydride region of
Ph2SnD2/Na in liquid ammonia after 60 min.
Reaction of dideuterodiphenylstannane with Na in liquid ammonia. In order to
elucidate if ammonia is the hydrogen source for the hydrodiphenylstannide
resulting from dihydrodiphenylstannane, or if the corresponding hydrogen atom
remained from the starting compound, dideuterodiphenylstannane, Ph2SnD2, was
-190 -192 -194 -196 -198 -200
c
b
-100 -120 -140 -160 -180 -200
a
Sn Sn Sn H
Chemical shif t δ / ppm
Chemical shif t δ / ppm
25
exposed to four equivalents of sodium in liquid ammonia. Water and other highly
active hydrogen donating impurities can be excluded as hydrogen source, since
they are expected to react rapidly with sodium in liquid ammonia, before the
addition of the stannanes.
After exposure of dideuterodiphenylstannane to four equivalents of sodium in
liquid ammonia for 60 min, the 119Sn NMR spectrum at 220 K showed, as
expected, on one hand the signal at -116 ppm of tetraphenyldistannide (see above),
and on the other hand two signals in the hydride region. A small signal, which
appeared as a singlet at -192.9 ppm in the proton-broadband-decoupled spectrum
(Figure 3c), split into a doublet in the proton-coupled spectrum (Figure 3b; a
series of additional spectra recorded at reaction times between 30 min and 10 h are
displayed in Figure 4), which is indicative of the above-mentioned hydroidphenyl-
stannide. Further, a pronounced three line feature with 1J(Sn,D) = 25 Hz at
-196.8 ppm (center peak) is indicative of the presence of deuterodiphenylstannide,
Ph2SnD-. (NB the three signals in the proton-coupled spectra are poorly resolved
due to additional couplings with the phenyl protons). The intensity of the
Ph2SnH- signal observed after one hour is by far too strong to be due to residual
hydrogen atoms in the starting compound Ph2SnD2 (as evident from analysis of 1H
and 119Sn NMR spectra of Ph2SnD2 in organic solvents). The ratio between
hydrodiphenylstannide and deuterodiphenylstannide increased steadily during a
period of more than one week, indicating that further H-D exchange occurred very
slowly (~25 % exchange after 12 h at 220 K and 45 % after one week, monitored
directly in the NMR test tube). It appears, therefore, that at least two processes
contribute to the H-D exchange: one largely advancing within one hour or less,
probably along the reaction path from Ph2SnD2 to Ph2SnD-, and another one
lasting for days or weeks. Since Ph2SnD2 itself is essentially insoluble in liquid
ammonia (it only dissolves upon treatment with sodium) it seems unlikely
26
Figure 4. 119Sn NMR spectra of in-situ formed products resulting from Ph2SnD2 exposed to four
molar equivalents of sodium in liquid ammonia, measured at 220 K at different reaction times in
order to monitor Ph2SnH-. The series shows certain fluctuations of the position of the main
signal, attributed to Ph2SnD-, most likely reflecting temperature fluctuations during the
experiment. Temperature fluctuations, which are expected to be more pronounced in the initial
phase where the temperature of the sample is adjusted, are assumed to cause the broadness of the
signal obtained after 30 min (note that 2000 scans were accumulated which took about 20 min;
the indicated times refer to the time at the middle of the accumulation process).
30 min
100 min
200 min
300 min
400 min
500 min
600 min
-180 -190 -200
Chemical shif t δ / ppm
-210
27
that the faster of the two processes is due to a H-D exchange of Ph2SnD2 with the
solvent (ammonia). Besides, it is worth to note that the percentage of tetraphenyl-
distannide in relation to the sum of the two monostannides remained constant
over time, within experimental error.
Since the hydrogen atoms of hydrodiphenylstannide extracted from dichloro-
diphenylstannane or dihydrodiphenylstannane at least partially stem from different
sources, hydrodiphenylstannide is formed by different processes. This might be
due to the higher reactivity of sodium to Sn-Cl than Sn-H groups at initial stages
of the reaction. Dichlorodiphenylstannane may initially lose both chlorine atoms
and quickly react with hydrogen atoms from the solvent (liquid NH3) to form the
hydrodiphenylstannide. Dihydrodiphenylstannane preferentially loses only one
hydrogen atom initially, while the other remains bound to the tin atom for a long
time. The exchange of the remaining hydrogen (deuterium) atom with hydrogen
atoms from the solvent is a second reaction step proceeding at a different time
scale and with complex reaction order. The initial reactions which lead to the
distannide and the hydrostannide (or deuterostannide) are completed within less
than 60 min, whereas the second step takes days.
Reaction of dichlorodibutylstannane with Na in liquid ammonia. 119Sn NMR
spectra of dichlorodibutylstannane exposed to sodium in liquid ammonia were
more complex than those of the diphenylstannanes. In the case of
dichlorodibutylstannane, four major signals emerged (Figure 2c; additional spectra
in Figure 5), i.e. two additional signals featured when compared to those resulting
from experiments with dichlorodiphenylstannane (occasionally, a minor signal at
27 ppm was also present, probably as a result of a reaction with traces of oxygen).
The two main signals (with respect to the integrated intensities) were linked to the
28
Figure 5. 119Sn NMR spectra of in-situ formed products resulting from Bu2SnCl2 converted with
four molar equivalents of sodium in liquid ammonia, measured at 200 and 220 K with and
without proton broadband decoupling as indicated (see main text). An additional peak at -139.7
in the spectra at 220 K is probably caused by a reaction product of stannides with atmospheric
water or oxygen which diffused via the cap into the NMR tube; note that the spectra at 220 K
were measured after those at 200 K in the same NMR tube.
binuclear species tetrabutyldistannide, (Bu4Sn)22- (-161 ppm at 200 K), and
hydrodibutylstannide, Bu2SnH-(-228 ppm at 200 K). Accordingly, the latter signal
splits up into a doublet in proton-coupled 119Sn NMR spectra
(1J(119Sn-1H) = 96 Hz at 220 K), but the resolution is relatively low due to the
coupling with the methylene protons of the butyl groups. The resolution decreased
even more upon very long measurement periods (>7 h), which is probably a result
of the limits in temperature control: the chemical shift strongly depends on the
temperature (Δδ = 0.5 ppm K-1; cf. Table 1); compare also to the fluctuations of
chemical shifts of phenylstannides during NMR measurements as displayed in
Figure 4. The additional signals are presumably due to butyl group migration (see
also below, in the section describing reactions with bromoethane). The signal at
-136 ppm (at 200 K) obviously represents tributylstannide, SnBu3-, since the
decoupled200 K
coupled200 K
decoupled220 K
coupled220 K
-50 -100 -150 -200 -250 -300 -350
Chemical Shif t δ / ppm
29
Table 1. Chemical shifts (), coupling constants (J) and full-widths at half-maximum (fwhm) in
proton-decoupled and proton-coupled 119Sn NMR spectra of different diorganostannanes exposed
to four molar equivalents of sodium in liquid ammonia.
reaction mixture of chlorotributylstannane and two equivalents of sodium in liquid
ammonia resulted in a single peak at the same chemical shift. The signal at
-212 ppm showed pronounced broadening in the proton-coupled 119Sn NMR
spectra, at least partially due to non-resolved couplings to protons of the butyl
groups. Yet the coupling of 119Sn nuclei to protons of Sn-H bonds is significantly
stronger and, therefore, the line broadening increases additionally in species with
non-resolved Sn-H bonds. The full-width at half-maximum (fwhm) of the peak at
-212 ppm (-208 ppm at 220 K) extended in the proton-coupled spectra by 140 %
(170 % at 220 K) compared to the decoupled spectra (cf. Table 1). This increase in
fwhm is even more pronounced than that seen for hydrostannide at -228 ppm
proton decoupled proton coupled
Measurement Temp
K
δ 119Sn
ppm
fwhm
Hz
1J
Hz
δ 119Sn
ppm
fwhm
Hz
1J
Hz
Ph2SnCl2 200 -131.7 s 100 -131.7 s 124
-197.2 s 58 -197.1 d 111 147 (Sn,H)
220 -115.9 s 38 2027 (Sn,Sn) -116.2 s 45 2018 (Sn,Sn)
-192.7 s 30 -191.8 d 35 145 (Sn,H)
Ph2SnH2 200 -132.2 s 102 -131.5 s 130
-197.2 s 75 -197.1 d 90 148 (Sn,H)
220 -116.1 s 44 2029 (Sn,Sn)
-192.8 s 28
Ph2SnD2 220 -116.4 s 18 2023 (Sn,Sn) -116.5 s 26 2023 (Sn,Sn)
-192.9 s 9 -192.9 d 22 144 (Sn,H)a)
-196.8 m 9 24 (Sn,D) -196.8 m 26 23 (Sn,D)b)
Bu2SnCl2 200 -136.3 s 104 -136.3 s 92
-161.1 s 125 -161.1 s 120
-212.0 s 57 -212.0 m 155
-228.0 s 118 -228.0 m 220
220 -136.7 s 67 -137.4 s 80
-143.0 s 75 -141.6 s 86
-207.9 s 60 -208.1 m 144
-219.5 s 120 -218.6 m 195 96 (Sn,H)b)
a) Hydrostannide formed by D-H exchange (see text); b) determinded by peak deconvolution.
30
(-218 ppm at 220 K) which was broadened by about 80 % (60 % at 220 K). For
comparison, the fwhm of the two peaks at -136 ppm and -161 ppm (-137 ppm
and -142 ppm at 220 K), which represent species without Sn-H bonds, were only
weekly influenced by the change from proton-coupled to proton-decoupled
spectra; the broadening amounted only to 15 - 20 % at 220 K and even less at
200 K. Thus, the signal at -212 ppm may represent, for instance, a mononuclear
dihydrostannide or, since the signal intensity was not sufficiently high to allow
detection of tin satellites, a binuclear hydrostannide.
Reaction of the stannide intermediates with bromoethane. As mentioned above, it
has been postulated that the intermediate diorganostannide dianion can be trapped
by reaction with organohalides (Scheme 1) [20-23]. Accordingly, we transferred
solutions with the in-situ prepared stannides in the final state into a large excess of
precooled bromoethane. In the case of the intermediates resulting from conversion
of dichlorodiphenylstannane, only diethyldiphenylstannane, Et2Ph2Sn, was found
after reaction (Figure 6a; for chemical shifts see Experimental Section). The
stannides resulting from conversion of dichlorodibutylstannane with bromoethane
yielded two additional products compared to the analogous conversion with di-
chlorodiphenylstannane. Note that in the former case also two additional reaction
intermediates were detected (see above). Besides the expected main product
dibutyldiethylstannane, Bu2Et2Sn, also tributylethylstannane, Bu3EtSn, and
butyltriethylstannane, BuEt3Sn, were found by 119Sn NMR analysis (Figure 6b;
chemical shifts see Experimental Section), in line with the alkyl group migration
implied by the reaction intermediates. Consequently, reaction experiments with
1-bromobutane yielded only one product, i.e. tetrabutylstannane. Thus, the two
different stannides (R2Sn)22- and HR2Sn- react with bromoethane to the same
product (Et2R2Sn) which would be expected from a reaction of R2Sn2- with
31
Figure 6. 119Sn NMR spectra of (a) Ph2SnCl2 and (b) Bu2SnCl2 converted with 4 molar
equivalents of sodium in liquid ammonia and subsequently reacted with an excess of
bromoethane.
bromoethane. These findings show that the reaction products of the intermediates
in liquid ammonia with haloalkanes allow only limited conclusions on the
composition of the tin species present in liquid ammonia, although starting from
Bu2SnCl2 they appear to reflect at least the in-situ migration of organic groups. As
a final remark, note that the quantity of sodium in the system corresponds to the
stoichiometry of the overall reaction according to Scheme 1 (Na:Ph2SnCl2 = 4:1);
i.e. some sodium is also involved in the reaction of the stannides with
bromoethane, since sodium is only partially consumed upon formation of
tetraorganodistannide and hydrodiorganostannide. In the case of R2SnH2, there is
sufficient sodium for a reduction under formation of NaH (it is not evident if H2
formed, in this case sodium would be present in excess).
300 200 100 0 -100 -200
a
b
Chemical shif t δ / ppm
-300
32
3. Conclusions
119Sn NMR measurements in liquid ammonia showed that, in contrast to the
generally accepted view, diorganostannide dianions are not formed significantly by
exposure of dichlorodiorganostannanes or dihydrodiorganostannanes with four
equivalents of sodium in liquid ammonia, as deduced previously from indirect
experiments. The species that are present in the reacting medium, as a matter of
fact, are tetraorganodistannide, (R2Sn-SnR2) 2- and hydrodiorganostannide,
HR2Sn-. The latter slowly exchanges hydrogen atoms with the solvent. In
addition, butyl group migration takes place in liquid ammonia, while significant
phenyl group migration does not occur under the applied reaction conditions.
All experiments showed that, in contrast to previous assumptions, reaction
products of the in-situ generated diorganostannides with haloalkanes do not
represent the chemical nature of the intermediates in liquid ammonia.
33
4. Experimental Section
Materials. Ammonia was purchased from PanGas, (Dagmarsellen, Switzerland,
99.999 %), dichlorodibutylstannane from ABCR GmbH (Karlsruhe, Germany)
and dichlorodiphenylstannane from Sigma Aldrich (Buchs, Switzerland). Both
substances were recrystallized twice by dissolving in boiling pentane and
subsequent precipitation of the product at 250 K. Deuterated dichloromethane
(99.9% D) was purchased from Cambridge Isotope Laboratories (ReseaChem
GmbH, Burgdorf, Switzerland), and organic solvents from Fluka (Buchs,
Switzerland).
Conductivity measurements. Electrical conductivity measurements in liquid
ammonia solutions were performed with a TetraCon 325/Pt electrode from WTW
(Weilheim, Germany) in combination with a WTW MultiLab 540 instrument. In
a typical reaction, 150 mL of ammonia were condensed with a cold finger
condenser in a flame-dried 200 mL three-neck flask with flat bottom under
nitrogen atmosphere. The conductivity cell was immersed into the cold solution
(195 K) and 8 mmol of sodium were introduced in a nitrogen counter flow. The
reaction mixture was stirred until the conductivity level was constant, which took
about 15 min. Subsequently 2 mmol of the respective dichlorodiorganostannane
were added at 195 K and the electrical conductivity was monitored as a function of
time.
NMR spectroscopy. 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 decoupling, 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 μs (90°) was applied. The sweep width was 700 ppm with a 16 k data
34
point acquisition range resulting in a digital resolution of 4.78 Hz. Chemical shifts
(δ) are reported in ppm referring to tetramethylstannane (δ(Me4Sn) = 0 ppm).
Syntheses of dihydrodiphenylstannane and dideuterodiphenylstannane. The com-
pounds were synthesized according to the literature [35] but with LiAlD4 instead
of LiAlH4 for the deuterated compound. NMR analysis (in CD2Cl2, room
temperature, chemical shifts in ppm, coupling constants in Hz, q: quintet, m:
multiplet): 1H: = 8.07 (m, 2 H), 7.81 (m, 3 H), 13C: = 129.3 (J(C,117Sn/119Sn)
51.7/54.2), 129.6, 129.6 (J(C,Sn) 11.8), 138.2 (J(C,117Sn/119Sn) 39.3/40.7), 119Sn:
= -233.6 (q, 1J(119Sn/D) 296.6), proton-coupled 119Sn: -233.6 (qq, 3J(119Sn,H)
53.9, 4J(119Sn,H) 11.4).
Reactions in liquid ammonia. The conversion of dichlorodiorganostannanes or
dihydrodiorganostannanes with sodium in liquid ammonia was conducted in a
flame-dried three-neck flask with flat bottom under nitrogen atmosphere in which
ca. 100 mL of ammonia were condensed with a cold finger condenser. A quantity
of 8 mmol of sodium were introduced in a nitrogen counter flow and dissolved by
stirring with a magnetic glass stirring bar, resulting in a homogeneous blue
solution (15 min). Thereafter, 2 mmol of dichlorodibutylstannane,
dichlorodiphenylstannane or dihydrodiphenylstannane were dissolved in 1 mL of
THF (dried over molecular sieve) and slowly added to the sodium/ammonia
solution with a syringe through a septum (to examine the influence of the THF in
the reaction mixture, some reactions were conducted by directly adding solid
dichlorodiorganostannane under a nitrogen counter flow, which led to the same
results). The color of the solution changed from clear blue to dark red. The
reaction mixture was stirred for 30 min in order to complete the reaction (as
previously determined with conductivity measurements). Syntheses of
phenylstannides from dideuterodiphenylstannane were performed in the same way.
35
For in-situ investigations with low-temperature NMR spectroscopy in liquid
ammonia, the reaction mixture containing the final stannides was transferred via a
bended glass tube into a flame dried and precooled NMR tube (195 K, Type 5UP
5×178 mm; ARMAR AG, Döttingen, Switzerland) equipped with a sealed
capillary with deuterated dichloromethane. The NMR tube was stored under
argon atmosphere in a 250 mL Schlenk tube and the transfer of the reaction
mixture was performed with nitrogen overpressure by carefully excluding oxygen
(argon counterflow from the Schlenk tube). The first 5-10 mL were poured into
the Schlenk tube before filling about 0.5 mL into the NMR tube. The filled NMR
tube was flushed with argon and stored at 195 K before it was inserted into the
precooled NMR spectrometer.
Conversion of the in-situ prepared stannides with bromoethane. For the reactions
of the in-situ prepared stannides with bromoethane, 5-10 mL of the ammonia
solutions containing the final stannides were transferred at 195 K to 20 mL of
precooled bromoethane (195 K) in a 100 mL two-neck round bottom flask via a
bended glass tube with nitrogen overpressure. The ammonia was evaporated by
warming the flask to room temperature in a N2 stream. The remaining products
were dried in vacuum (ca. 0.1 mbar) for 24 hours, and the solids thus obtained
were dissolved in deuterated dichloromethane for analysis with NMR
spectroscopy. 119Sn NMR analysis (CD2Cl2, Me4Sn), chemical shifts in ppm
(discussion of the products see text): Bu4Sn = -11.8, Bu3EtSn = -7.9, Bu2Et2Sn
= -4.1, BuEt3Sn = -0.5, Et2Ph2Sn = -65. Selected literature values for
comparison: Bu4Sn = -11.5[33], Et2Ph2Sn = -66 [36]; since we did not find
chemical shifts of Bu2Et2Sn, EtBu3Sn and Et3BuSn, we also quote the value of
Et4Sn ( = 1.4 [36]) which discloses that the chemical shifts of the stannanes
comprising mixed alkyl groups are located between the chemical shifts of Bu4Sn
and Et4Sn. Further, the chemical shifts reported for Et3PhSn = -34 [36] and
36
EtPh3Sn = -98 [36] reveal that these products did not appear in the spectra
obtained by conversion of the related phenylstannides with bromoethane. The
same reaction procedure with 1-bromobutane gave only one product, Bu4Sn.
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Compounds, Inorg. Chem., 2 (1963) 736-740.
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Arylstannanes via SRN1, Organometallics, 21 (2002) 1425-1429.
[4] A.B. Chopa, M.T. Lockhart, G. Silbestri, Synthesis of Arylstannanes from Arylamines,
Organometallics, 20 (2001) 3358-3360.
[5] V.B. Dorn, M.A. Badajoz, M.T. Lockhart, A.B. Chopa, A.B. Pierini, Synthesis of
Cyclohexadienylstannanes - Novel Example of Vinylic SRN1 Mechanism: A Theoretical and
Experimental Study, J. Organomet. Chem., 693 (2008) 2458-2462.
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52 (1930) 4426-4433.
[7] M. Mąkosza, K. Grela, Preparation of Allylstannanes and Distannanes using Zinc in Liquid
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[8] K. Kühlein, W.P. Neumann, H. Mohring, A Versatile Method for Preparation of
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[9] R.H. Bullard, F.R. Holden, Action of Hydrogen Chloride on Stannanes of the Type
R2SnR'2, J. Am. Chem. Soc., 53 (1931) 3150-3153.
[10] R.K. Ingham, S.D. Rosenberg, H. Gilman, Organotin Compounds, Chem. Rev., 60 (1960)
459-539.
[11] R.K. Ingham, H. Gilman, Inorganic Polymers, Academic Press, New York, 1962.
[12] T. Birchall, J. Vetrone, Organotin Anions in Solution and in the Solid State, Hyperfine
Interact., 40 (1988) 291-294.
[13] F. Choffat, S. Fornera, P. Smith, W.R. Caseri, D.W. Breiby, J.W. Andreasen, M.M.
Nielsen, Oriented Poly(dialkylstannane)s, Adv. Funct. Mater., 18 (2008) 2301-2308.
37
[14] F. Choffat, D. Schmid, W. Caseri, P. Wolfer, P. Smith, Synthesis and Characterization of
Linear Poly(dialkylstannane)s, Macromolecules, 40 (2007) 7878-7889.
[15] F. Choffat, P. Smith, W. Caseri, Facile Synthesis of Linear Poly(dibutylstannane), J. Mater.
Chem., 15 (2005) 1789-1792.
[16] F. Choffat, P. Smith, W. Caseri, Polystannanes: Polymers of a Molecular, Jacketed Metal-
Wire Structure, Adv. Mater., 20 (2008) 2225-2229.
[17] F. Choffat, P. Wolfer, P. Smith, W. Caseri, Light-Stability of Poly(dialkylstannane)s,
Macromol. Mater. Eng., 295 (2010) 210-221.
[18] M.P. de Haas, F. Choffat, W. Caseri, P. Smith, J.M. Warman, Charge Mobility in the
Room-Temperature Liquid-Crystalline Semiconductor Poly(di-n-butylstannane), Adv. Mater.,
18 (2006) 44-47.
[19] F. Choffat, Y. Buchmüller, C. Mensing, P. Smith, W. Caseri, Poly(di(ω-
alkylphenyl)stannane)s, J. Inorg. Organomet. Polym. Mater., 19 (2009) 166-175.
[20] C.A. Kraus, W.N. Greer, The Dimethyltin Group and Some of its Reactions, J. Am. Chem.
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[21] C.A. Kraus, W.V. Sessions, Chemistry of the Trimethyltin Group, J. Am. Chem. Soc., 47
(1925) 2361-2368.
[22] R.F. Chambers, P.C. Scherer, Phenyltin Compounds, J. Am. Chem. Soc., 48 (1926)
1054-1062.
[23] T. Harada, On the Metallo-Organic Compounds, Sci. Papers Inst. Phys. Chem. Res., 35
(1939) 302-313.
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[25] P.M. Uberman, S.E. Martin, R.A. Rossi, Synthesis of Functionalized
Diaryldimethylstannanes from the Me2Sn2- Dianion by SRN1 Reactions, J. Org. Chem., 70 (2005)
9063-9066.
[26] N. Scotti, U. Zachwieja, H. Jacobs, Tetraammin-Lithium-Kationen zur Stabilisierung
phenylsubstituierter Zintl-Anionen: Die Verbindung, Z. Anorg. Allg.Chem., 623 (1997)
1503-1505.
[27] K. Wiesler, C. Suchentrunk, N. Korber, Syntheses and Crystal Structures of Ammoniates
with the Phenyl-Substituted Polytin Anions Sn2Ph42-, cyclo-Sn4Ph4
4- and Ph6Ph122-, Helv. Chim.
Acta, 89 (2006) 1158-1168.
[28] C.A. Kraus, E.G. Johnson, Properties of Electrolytic Solutions. VII. Conductance of
Sodium Trimethylstannide and of the Sodium Salts of Certain Phenols and Thiols in Liquid
Ammonia, J. Am. Chem. Soc., 55 (1933) 3542-3547.
[29] C.A. Kraus, W.H. Kahler, Properties of Electrolytic Solutions. VI. Conductance of Sodium
Triphenylstannide, Sodium Triphenylgermanide and Sodium Triphenylmethide in Liquid
Ammonia, J. Am. Chem. Soc., 55 (1933) 3537-3542.
38
[30] C.A. Kraus, P.B. Bien, Properties of Electrolytic Solutions. VIII. Conductance of Some
Ternary Salts in Liquid Ammonia, J. Am. Chem. Soc., 55 (1933) 3609-3614.
[31] H.J. Emeléus, S.F.A. Kettle, Sodium Derivates of Stannane, J. Chem. Soc., (1958) 2444-
2448.
[32] C.A. Kraus, Solutions of Metals in Non-metallic Solvents. VI. The Concuctance of the
Alkali Metals in Liquid Ammonia, J. Am. Chem. Soc., 43 (1921) 749-770.
[33] B. Wrackmeyer, G.A. Webb, Annu. Rep. NMR Spectrosc., 16 (1985) 73-186.
[34] R.E. Wasylishen, N. Burford, Large Isotope effects on the 119Sn NMR parameters of the
Stannyl Ion, J. Chem. Soc.,Chem. Commun., (1987) 1414-1415.
[35] T. Imori, V. Lu, H. Cai, T.D. Tilley, Metal-Catalyzed Dehydropolymerization of
Secondary Stannanes to High Molecular Weight Polystannanes, J. Am. Chem. Soc., 117 (1995)
9931-9940.
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Chapter III
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
41
1. Introduction
The reaction of haloorganostannanes with sodium in liquid ammonia has
attracted attention since the early part of the past century [1-11]. The resulting
products have been used in-situ as intermediates for the preparation of organotin
compounds (organostannanes). In particular, conversion of such intermediates
with haloalkanes and haloarenes yielded tetraorganostannanes [12-25]. In this
context, also dihalodiorganostannanes, R2SnCl2, have been exposed to sodium, and
were regarded to yield diorganotin dianions (Scheme 1, diorganostannide dianions,
R2Sn2-), which can subsequently act as reaction intermediates for the synthesis of
tetraorganostannanes, with the respective sodium halides as reaction byproducts
[18, 26-30]. However, surprisingly, our recent studies performed in-situ with 119Sn
NMR spectroscopy experiments 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], Chapter II.
Scheme 1. Previously advanced reactions of dihalodiorganostannanes with sodium.
SnCl
R
R
ClSn
R
RNa
+Cl
SnCl
R
R
ClSn
R
RSn
R
R
Na+
Cl
SnCl
R
R
ClSn
R
R n
Na+
Cl
+ 4 Na + +4 2
+ 3 Na + +3 2
2-
1/2
+ 2 Na + +2 2 (1)
(2)
(3)2- -
-
-
42
Further, it has been suggested (but not spectroscopically verified) [5, 26] that
the composition of the reaction intermediates changes upon variation of the
diorganostannane/sodium ratio. In addition, treatment of dihalodiorganostannanes
with less than 4 molar equivalents of sodium (preferentially two equivalents) was
claimed to give rise to the formation of oligostannanes or polystannanes
(Scheme 1) [1, 4, 5]; however, the resulting products were not thoroughly
characterized. Polystannanes 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 laboratories by different
methods [33-52].
In order to resolve the various discrepancies alluded above, we carried out in-
situ investigations of the species arising from reactions of dichlorodiorgano-
stannanes and sodium at different molar ratios in liquid ammonia, and explored
their applicability as reaction intermediates for the synthesis of tetraorgano-
stannanes by conversion with bromoethane.
43
2. Results
For the experiments, dichlorodibutylstannane and dichlorodioctylstannane
were employed as representatives for alkylstannanes, 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.
Dihalodiorganostannane/Sodium 1:2. A general feature of reactions with a di-
chlorodiorganostannane/sodium ratio of 1:2 was the formation of polymers (cf.
Table 1). Below, the results obtained for the different systems are described in
more detail.
Table 1. Overview of the detected products that emerged from reaction of dichlorodiorgano-
stannanes, R2SnCl2, with different ratios of sodium in liquid ammonia.
Dichlorodibutylstannanes
Exposure of dichlorodibutylstannane to two molar equivalents of sodium in
liquid ammonia caused immediate precipitation of a yellow product, which is
R2SnCl2 : Na ratio R = butyl R = octyl R = phenyl
1:2 (SnBu2)n polymer and cyclic oligomers
(SnOct2)n polymer and cyclic oligomers
(SnPh2)n polymer
1:3 Bu2SnH-, (Bu4Sn2)2-, Bu3Sn-, one unidentified product Oct2SnH- Ph2SnH-, (Ph4Sn2)2-a)
1:4 Bu2SnH-, (Bu4Sn2)2-, Bu3Sn-, one unidentified product Oct2SnH- Ph2SnH-, (Ph4Sn2)2-
1:10 Bu2SnH-, (Bu4Sn2)2-, Bu3Sn-, one unidentified product Oct2SnH- Ph2SnH-, (Ph4Sn2)2-
a) Additional signals are attributed to degradation products formed during transfer of the reaction solutions to the NMR tubes that could not be avoided.
44
typical for poly(dibutylstannane). The gel permeation chromatography (GPC)
diagrams revealed the presence of polymer with a molar mass of 8 kg/mol. In
addition, products in a mass range of cyclic byproducts (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 was consistent with products of the composition (SnBu2)n, which is in
agreement both with the composition of linear polymers and cyclic oligomers.
Dichlorodioctylstannane
Conversion of dichlorodioctylstannane with two molar equivalents of sodium
was performed in the same manner as the above described reaction with dichloro-
dibutylstannane, resulting in precipitation of a yellow, pasty material. Analogously,
GPC analysis indicated the generation of poly(dioctylstannane) (molar mass
around 6 kg/mol), together with cyclic pentamers and hexamers. The elemental
composition was in agreement with that of (Oct2Sn)n. 119Sn NMR spectroscopy in
deuterated dichloromethane revealed a broad signal at -192 ppm which correspon-
ds to the value of poly(dioctylstannane), and signals at -203 ppm and ‐205 ppm for
the cyclic byproducts [50].
Dichlorodiphenylstannane
Also treatment of dichlorodiphenylstannane with two 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,
as well as at elevated temperatures (close to the boiling point of the solvents).
Therefore, it was not possible to determine its molar mass. The product was
45
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(diphenylstannane)s. 119Sn NMR analysis of the concentrated
extracts indicated that no significant amounts of cyclic byproducts were formed.
Elemental analysis of the material was consistent with that of (Ph2Sn)n.
Dichlorodiorganostannane/Sodium 1:3, 1:4 and 1:10. In the previous Chapter
and [31], we reported that in contrast to general views (Scheme 1),
dichlorodibutylstannane and dichlorodiphenylstannane do not react with four
equivalents of sodium to yield the respective diorganostannide dianions. Instead,
the anions HSnR2– and (R2Sn-SnR2)2– formed in quantities of a similar order of
magnitude, and in the case of dichlorodibutylstannane additionally R3Sn– and a
fourth unidentified product (e.g. H2RSn– or (HRSn-SnHR)2-) arose by alkyl group
migration. Considering that Na is in fact present in liquid ammonia as Na+ and
solvated electrons, the latter causing the typical blue color of the corresponding
solutions [53], the existence of (R2Sn-SnR2)2– may be somewhat surprising, as two
of the highly reactive solvated electrons per dianion may remain under a dichloro-
diorganostannane/sodium ratio of 1:4. In order to investigate if larger quantities of
sodium would ultimately lead to cleavage of Sn-Sn bonds in the dianions and if
lower amounts influence the ratio between the aforementioned stannides,
dichlorodiorganostannanes were exposed to 3, 4 and 10 molar equivalents of Na.
According to the literature [5, 26] substantial differences in the in-situ formed
reaction products are to be 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
46
expected to decrease during the reaction of dichlorodiorganostannanes with
sodium, as highly mobile electrons are removed from the system. Thus, we
employed in-situ measurements of the electric conductivity in liquid ammonia to
qualitatively monitor the course of the reaction.
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.
Table 2. Overview of the soluble products that emerged after treatment of dichlorodiorgano-
stannanes with sodium in liquid ammonia identified by in-situ 119Sn NMR spectroscopy.
Dichlorodibutylstannane
The electric conductivity of mixtures of dichlorodibutylstannane/sodium 1:4
versus reaction time is presented in Figure 1a. The data show that the conductivity
reached a constant value after 30 min, indicating that the reaction was terminated
within this period (Figure 1a). 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,
δ 119Sn (ppm) 200 K 220 K
Bu2SnH- -228 -219 (Bu4Sn2)2- -161 -143 Bu3Sn- -136 -137 Oct2SnH- -223 -219 Ph2SnH- -197 -192 (Ph4Sn2)2- -132 -116
47
Figure 1. (a) Electrical conductivity of a mixture of dichlorodibutylstannane (■), dibromodibutyl-
stannane (□), dichlorodioctylstannane (●) 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.
not only characteristic signals of the same intermediates emerged, but also their
relative distribution did not alter strikingly (Figure 1b). These intermediates are
attributed to dibutylhydrostannide Bu2SnH- (-228 ppm), tetrabutyldistannide
0 20 40 600
10
20
30
40
50
60
70
80
90
100
elec
tric
cond
uctiv
ity /
%
Time / min
100
20
40
0
80
60
Ele
ctric
alco
nduc
tivity
/ %
10 20 30 40 50 60
Time / min
0
0 -100 -200 -300
Chemical shif t δ / ppm
1:10
1:4
1:3
a
b
48
(Bu4Sn2)2- (-161 ppm), tributylstannide Bu3Sn- (-136.3 ppm) and a fourth
unidentified product that arose by alkyl group migration (e.g. H2RSn– or (HRSn-
SnHR)2–) (-212 ppm) [31]. Thus, dichlorodibutylstannane was found to undergo
alkyl group migration at molar stannane/sodium ratios between 1:3 and 1:10.
Dichlorodioctylstannane
Conversion of dichlorodioctylstannane with sodium in liquid ammonia was
slow compared to that of dichlorodibutylstannane, as revealed by conductivity
measurements. It took well over one hour until equilibrium was established in a
reaction mixture of dichlorodioctylstannane/sodium at a ratio of 1:4 (Figure 1a).
Notably, 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,
Δσ = 0.2 ppm/K), i.e. in the range of the above mentioned Bu2SnH- 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 119Sn-1H coupling (Figure 2). Therefore we assign the signal to
dioctylhydrostannide Oct2SnH-. The coupling constant of 1J(Sn,H) = 95 Hz is in
good agreement with values reported earlier for Bu2SnH- [31, 54]. I 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 tend 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.
49
Figure 2. (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. (Δσ = 0.2 ppm/K). (b) 1H-decoupled
and 1H-coupled 119Sn NMR spectra of the signal at -223.3 ppm (1J(Sn,H) = 95 Hz).
Dichlorodiphenylstannane
The reaction of dichlorodiphenylstannane with sodium was terminated within
30 min, as evident from conductivity measurements (Figure 1a). After this period,
two strong signals were visible in 119Sn NMR spectra of in-situ formed
intermediates at dichlorodiphenylstannane/sodium ratios of 1:4 and 1:10
(Figure 3), representing diphenylhydrostannide, Ph2SnH-, (-197.2 ppm) and tetra
-100 -200-150 -250-50Chemical shif t δ / ppm
a
1:10200 K
1:4200 K
1:3220 K
-220 -240
Chemical shif t δ / ppm
b
1:4, 200 KH-decoupled
1:4, 200 KH-coupled
-220 -240
50
Figure 3. In-situ 119Sn NMR spectra of dichlorodiphenylstannane exposed to 10, 4 and 3 molar
equivalents sodium in liquid ammonia including a mixture of 1:3 ratio that was aged for 2 h.
phenyldistannide, (Ph4Sn2)2- (-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 as compared to those with
dichlorodibutylstannane, since a higher tendency to form precipitates (polymers or
other products) was observed in the former. When such solutions (still dark red)
were investigated by 119Sn NMR spectroscopy, several signals emerged. In addition
to the peak of diphenylhydrostannide at -197.2 ppm, two other strong signals
at -65 ppm and -125 ppm arose (Figure 3). Unfortunately, the species causing the
latter signals could not be identified. The solutions lost their characteristic dark red
color after about 2 h at 200 K; a yellow precipitate was formed, but the NMR
-100 -200
1:10
1:3aged
1:3
1:4
-150 -250-50
Chemical shif t δ / ppm
51
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 particular
compound.
In-situ prepared alkylstannides as intermediates: Reactions with bromoethane.
The above described invariability of detected products arising at dichlorodialkyl-
stannane/sodium ratios between 1:3 and 1:10, as well as in dichlorodiphenyl-
stannane/sodium 1:4 and 1:10 mixtures are not in agreement with previous
suggestions in the literature [5, 26]. In fact, it has been argued that after reactions
of in-situ formed products with haloalkanes, the composition of the formed
products should change upon variation of the dihalodiorganostannane/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
Table 3. Summary of the compounds that resulted from reaction of in-situ prepared dichlorodi-
organostannane/sodium mixtures in liquid ammonia with bromoethane (30 min equilibration
time for butyl and phenyl; 90 min for octyl).
R2SnCl2 : Na ratio R = butyl R = octyl R = phenyl
1:10 Bu3SnOH Bu2SnEt2 Bu3SnEt Bu4Sn2Et2 Bu3Sn2Et3
Oct2SnEt2a)
OctxSn2Et6-x unident. prod. (-125 ppm)
Ph2SnEt2 Ph4Sn2Et2 unident. prod. (-28 ppm)
1:4 BuSnEt3
Bu2SnEt2 Bu3SnEt
OctSnEt3 Oct2SnEt2 Oct3SnEt OctxSn2Et6-x Oct6Sn2O
Ph2SnEt2
1:3 BuSnEt3
Bu2SnEt2 Bu3SnEt
Oct2SnEt2 OctxSn2Et6-x Oct6Sn2O
PhSnEt3 Ph2SnEt2 Ph3SnEt
a) Minor quantities of OctSnEt3 and Oct3SnEt might be present but were not observed due to a relatively low signal/noise ratio in the spectra.
52
indeed provide information about in-situ formed products and vice versa,
exemplarily with reactions with bromoethane. An overview of the results is
presented in Table 3, and will be detailed below.
Conversion of butylstannides
Butylstannides were exposed to an excess of bromoethane and products studied
with 119Sn NMR spectroscopy. At dichlorodibutylstannane/sodium ratios of 1:4
and 1:10, the spectra did not differ considerably (Figure 4a). A main species
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
chemical shifts of the Sn-CH2 signals in 13C NMR spectra (around 0 ppm,
Table 5). Experiments with 13C-labeled bromoethane, Br-13CH2CH3, allowed
identification of the reaction products via the signal splitting pattern caused by
119Sn-13C couplings (Figure 5 and Table 4 and 5). Thus, it was concluded that the
primary product of the mixtures with 1:4 and 1:10 ratios was dibutyldiethyl-
Figure 4. 119Sn NMR spectra of the reaction products of (a) dichlorodibutylstannane, (b)
dichlorodioctylstannane and (c) dichlorodiphenylstannane exposed first to 10, 4 and 3 molar
equivalents of sodium and then converted with bromoethane.
Chemical shift δ / ppm0100 -100
1:10
1:3
1:4
Chemical shift δ / ppm
1:10
1:3
1:4
0100 -100Chemical shift δ / ppm
1:10
1:3
1:4
0100 -100
a b c
53
Figure 5. (a) 119Sn NMR spectrum of the reaction products of dichlorodibutylstannane/sodium
1:4 exposed to Br13CH2CH3. (b) magnification of the region of the monostannanes.
SnC13 H2
BuBuBu
SnC13 H2
C13 H2
Bu C13 H2
SnC13 H2
C13 H2
BuBu
0 -2 -4 -6 -8 -10 -12246
Chemical shift δ / ppm
50 0 -50 -100100
Chemical shift δ / ppm
a
b
54
Table 4. 119Sn NMR data of the products resulting from conversion of a mixture of Bu2SnCl2/Na
1:4 with 13C labeled bromoethane, Br-13CH2CH3.
Table 5. 13C NMR data of the reaction products of a mixture Bu2SnCl2/Na, 1:4, with 13C labeled
bromoethane Br-13CH2CH3.
δ 1J(Sn,C) 2J(Sn,C) 1J(Sn,Sn) ppm Hz Hz Hz Bu3 Sn (13CH2CH3) d -7.9 318.8 Bu2 Sn (13CH2CH3)2 t -4.1 319.2 Bu Sn (13CH2CH3)3 q -0.6 319.3 Bu2 Sn (13CH2CH3) - Bu2 Sn (13CH2CH3) dd -75.3 245.3 41.7 2580 Bu2 Sn (a)(13CH2CH3) - Bu Sn (b)(13CH2CH3)2 a dt -73.8 246.0 42.1 b td -69.1 245.3 41.4 Bu2 Sn (13CH2CH3) OH d 99.9 374.0 Bu2 Sn (13CH2CH3) - O - Bu2 Sn (13CH2CH3) d 90.1 374.9 Bu Sn (a) (13CH2CH3)2 - O - Bu2 Sn(b) (13CH2CH3) a t 90.4 375.4
b d 90.6 374.6
δ 1J(119Sn,C) 1J(117Sn,C) ppm Hz Hz Bu3 Sn (13CH2CH3) 0.11 321.4 309.0 Bu2 Sn (13CH2CH3)2 0.28 319.1 305.0 Bu Sn (13CH2CH3)3 0.67 318.8 304.6 Bu2 Sn (13CH2CH3) - Bu2 Sn (13CH2CH3) 1.53 245.5 234.2 Bu Sn (13CH2CH3)2 - Bu2 Sn (13CH2CH3) 1.88 245.4 234.5 Bu2 Sn (13CH2CH3) - O - Bu2 Sn (13CH2CH3) 7.73 373.8 358.8 Bu Sn (13CH2CH3)2 - O - Bu2 Sn (13CH2CH3) 8.23 376.3 358.8
55
stannane, Bu2SnEt2, and the side products were butyltriethylstannane, BuSnEt3,
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 tetrabutyldiethyldistannane.
Tributylethylstannane and dibutyldiethylstannane were found in similar quantities
for dichlorodibutylstannane/sodium ratios of 1:3, but in addition distannanes
formed (Figure 4a). 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 (Table 4 and Table 5). The broad signal at
105 ppm was attributed to Bu3SnOH [31].
Conversion of octylstannides
In contrast to the other stannanes, the product composition resulting from the
reaction of bromoethane with dichlorodioctylstannane/sodium mixtures at molar
ratios between 1:3 and 1:10 strongly depended on the sodium ratio (Figure 4b).
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 ratio.
Distannanes dominated at a ratio of 1:3, while dioctyldiethylstannane, Oct2SnEt2,
was the main product at 1:4 and 1:10. Alkyl group exchange was little pronounced
at a 1:4 ratio, while at a 1:3 ratio strong NMR signals at -69.4 ppm and -70.1 ppm
indicated exchange of alkyl groups in distannanes, resulting in derivatives of the
general composition EtxOct6 - xSn2 (Figure 4b). The compound causing the signal
at about -125 ppm observed in the case of a 1:10 ratio could not be identified. NB
56
oxidation products, Oct3Sn-OH (99 ppm) and Oct3Sn-O-SnOct3 (96 ppm), were
observed in some cases.
Evolution of the octylstannides
The low reaction rate of dichlorodioctylstannane with sodium (Figure 6a) was
beneficially employed to convert portions of dioctylstannane/sodium 1:4 mixtures
Figure 6. (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 bromoethane after the times
indicated in Figure 3a.
-40 -20 0 20 40 60 80 100 120 140
20
30
40
50
60
70
80
elec
trica
l con
duct
ivtiy
/ m
S c
m-1
time / minTime / min
Ele
ctric
alco
nduc
tivity
/ mS
cm-1
40
20
30
70
50
60
7 min
14 min21 min
100 min
-40 0 40 80 120
7 min
14 min
21 min
100 min
300 0 -300150 -150Chemical shif t / ppm
a
b
57
with bromoethane, already before the equilibrium of the intermediate products was
established in-situ. The 119Sn NMR spectra of the reaction products (Figure 6b)
indicate 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 diethyldioctylstannane (-4.4 ppm),
tetraoctyldiethyldistannane and derivatives of the composition EtxOct6 - xSn2 at
about -70 ppm were detected, and the spectra of the reaction products after 7 and
14 minutes equilibration time (Figure 6b) showed the signals of chloroethyl-
dioctylstannane (148 ppm [55]), together with numerous signals
between -150 ppm and -200 ppm. After 21 minutes a group of signals
around -210 ppm was present. These signals might represent oligostannanes
(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.
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 in 119Sn NMR spectra after conversion with
bromoethane, namely at -65.4 ppm – diethyldiphenylstannane [56] (Figure 4c)
(occasionally a small signal around -90 ppm was detected, which was not due to
either Ph3SnEt, δ 119Sn = -98 ppm or to dimeric species Ph4Et2Sn2,
δ 119Sn = -116 ppm). A stannane/sodium ratio of 1:3 might favor formation of
58
distannanes due to stoichiometric considerations (equation 2 in Scheme 1), and
indeed a adominating additional signal at -116 ppm was observed in 119Sn NMR
spectra (Figure 4c, identified by the accompanied tin satellites 1J(Sn,Sn) = 3472 Hz
in other experiments), attributed to diethyltetraphenyldistannane (Table 6). When
dichlorodiphenylstannane was exposed to ten equivalents of sodium and
subsequently converted with bromoethane, the main product was again diethyldi-
phenylstannane. But the additional sodium seemed to trigger the breakage of the
tin-carbon 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.
Table 6. 119Sn NMR data of the products resulting from conversion Oct2SnCl2/Na and
Ph2SnCl2/Na mixtures with bromoethane.
δ 119Sn (ppm)
Oct3SnEt -8.3 Ph3SnEt -34 Oct2SnEt2 -4.4 Ph2SnEt2 -65 OctSnEt3 -0.7 PhSnEt3 -98
Oct4Et2Sn2 -75.7 Ph4Et2Sn2 -116 Oct3Et3Sn2 -70.1 / -70.2 Oct2Et4Sn2 -69.7
59
3. Discussion
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(dialkylstannane)s, while no evidence for cyclic oligo(diphenylstanna-
ne)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 Figure
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 literature, the
differences in the distribution of the in-situ formed products at dichlorodiorgano-
stannane/sodium ratios of 1:3 to 1:10 were not significant. Stannides of the
composition R2SnH- were detected in all cases and dinuclear compounds of the
type R4Sn22- with R = butyl and R = phenyl. In non of the solutions was found
evidence of the presence of the frequently proposed SnR22- dianion. In fact, SnR2
2-
is expected to act as a very strong base, which may readily be able to undergo an
acid-base reaction with ammonia, thus forming NH2- and R2SnH-. Therefore, it
cannot be excluded that the detected R2SnH- developed from very short-living
SnR22-.
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
60
both formation of dimers and alkyl group migration in liquid ammonia, which
might be due to steric hindrance.
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, R2SnH- (which were
present in all reaction solutions of dichlorodiorganostannanes with sodium), as
well as the appearance of the dinuclear species R4Sn22- in the in-situ reactions of
dichlorodibutylstannane 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 composition of the products emerging after subsequent
reaction with bromoethane. For example, the presence of in-situ generated
R2SnH- does not result in formation of tin hydrides after conversion 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.
Remarkably, even an excess of sodium is not able to cleave Sn-Sn bonds in
R4Sn22- (detected with R = butyl and R = phenyl), while subsequent cleavage of
those Sn-Sn bonds after addition of bromoethane was observed - in particular
61
when four equivalents of sodium per tin atom were present. This implies that
bromoethane or reaction products of bromoethane, respectively, were involved in
Sn-Sn cleavage. Contrariwise, the occurrence of binuclear species in reaction
products with bromoethane does not allow to conclude that the in-situ generated
intermediate products also contained binuclear stannanes, as especially evident
from the solutions with dichlorodioctylstannane/sodium 1:3, where Oct2SnH-
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 generated 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 dichlorodiphenyl-
stannane/sodium; phenyl group exchange was not detected 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.
62
Finally, it appears that the halogen atoms present in the system do not exert a
pronounced influence on the product distributions. Substitution of dichlorodi-
butylstannane by dibromodibutylstannane did not considerably change the course
of the conductivity of reaction solutions with four equivalents sodium (Figure 1a),
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 solutions of dichlorodibutylstannane and four
equivalents of sodium, product formation essentially did not differ.
4. Conclusions
Reaction of dichlorodiorganostannanes with two equivalents of sodium in
liquid ammonia resulted in the formation of polystannanes, which were
accompanied by cyclic byproducts in the case of the alkylstannanes. However,
conversion of dichlorodiorganostannanes with three to ten molar equivalents of
sodium yielded soluble intermediates. Instead of the frequently proposed R2Sn2-
dianions, R2SnH- and in the case of R = butyl and R = phenyl also dinuclear
species of the type R4Sn22- 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.
63
5. Experimental
Materials. Ammonia was purchased from PanGas (Dagmarsellen, Switzerland,
99.999 %), dichlorodibutylstannane, dichlorodioctylstannane from ABCR GmbH
(Karlsruhe, Germany) and dichlorodiphenylstannane 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, deuterated
dichloromethane (99.9% D) from Cambridge Isotope Laboratories (ReseaChem
GmbH, Burgdorf, Switzerland), and organic solvents from Fluka (Buchs,
Switzerland).
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 decoupling, 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 μs (90°) were applied. 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 (δ)
are reported in ppm referred to tetramethylstannane (δ(Me4Sn) = 0 ppm).
Electrical conductivity measurements in liquid ammonia solutions 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 ~200 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
64
during the reaction of dichlorodiorganostannanes with four 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. For differential scanning
calorimetry (DSC), a DSC822e instrument (Mettler Toledo, Greifensee, Switzer-
land) equipped with an intracooler was applied, and the measurements were
carried out under nitrogen atmosphere.
Gel permeation chromatography was performed with a Viscotek VE7510
equipped with degasser, VE1121 solvent pump, VE520 autosampler and Model
301 triple detector array. A PL gel 5 μm Mixed-D column from Polymer
Laboratories Ltd. (Shropshire, United Kingdom) was used, calibrated with
poly(styrene) standards.
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). The flask was evacuated, 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 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 sec. 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 surrounded 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 conductivity measurements), the ammonia flow was stopped and a
smooth nitrogen flow restarted.
65
Preparation of reaction mixtures for in-situ NMR investigations. For reactions of
dichlorodibutylstannane and dichlorodiphenylstannane typically 8 mmol of sodium
were introduced in a nitrogen counterflow into 90 mL condensed ammonia and
dissolved by stirring for 15 min to result in a homogeneous blue solution.
Thereafter 2.67 mmol, 2 mmol and 0.6 mmol respectively of dichlorodiorgano-
stannane dissolved in 1 mL THF were added slowly. The color changed from blue
to dark red. The mixtures were stirred for 30 min in order to complete the
reactions and subsequently transferred via a bended glass tube directly into
precooled (200 K) and dried NMR tubes (Type 5UP 5x178mm; ARMAR AG,
Döttingen Switzerland) equipped with a sealed capillary with deuterated dichloro-
methane. The transfer was performed with nitrogen overpressure and argon
counterflow 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 dichlorodiorganostannanes. 2 mmol sodium were
dissolved in liquid ammonia by stirring for 15 min followed by the addition of
0.67 mmol (1:3 ratio) or 0.5 mmol (1:4 ratio) dichlorodioctylstannane. The
mixtures with the 1:10 ratio were prepared by dissolving 4 mmol sodium together
with 0.4 mmol dichlorodioctylstannane. Apart from the differences in concen-
tration, the reaction was performed as described above for the other stannanes.
Reactions with two 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
66
Table 7. Quantities applied for the conversion of dichlorodiorganostannanes with 2 molar
equivalents of sodium.
sodium solution under continuous stirring, whereupon the deep blue color
disappeared and a yellow precipitate formed. After about two minutes, the
ammonia was removed under a constant nitrogen flow by warming the flask to
room temperature, and THF was evaporated at room temperature in vacuo
(0.1 mbar, 12 h). For gel permeation chromatography (GPC) analysis of the
obtained material was dissolved in THF; this solution was directly used after
filtration with syringe filters (0.45 μm PTFE filters). The residues stemming from
the reactions of the different stannanes were further processed as follows:
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).
Dichlorodioctylstannane: The remaining product was dissolved 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).
sodium R2SnCl2 mg mmol mg mmol
Bu2SnCl2 151.5 6.59 1000.7 3.29 Ph2SnCl2 181.9 7.91 1360.5 3.96 Oct2SnCl2 227.0 9.87 2053.6 4.94
67
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 3-4 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).
Reactions of organostannide intermediates with bromoethane. In a typical
reaction, 218 mg of sodium (9.45 mmol) were dissolved 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
tetrahydrofuran (THF) were added. The deep red mixture of
dichlorodiorganostannanes and sodium was transferred after establishment of the
equilibrium (30 min for dichlorodibutylstannane and dichlorodiphenylstannane,
90 min for dichlorodioctylstannane) 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.
68
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Chapter IV
Poly(dialkylstannane)s and Poly(diarylstannane)s
Homo- and Copolymers Synthesized in Liquid
Ammonia
75
1. Introduction
Polystannanes are defined as polymers of which the main chain consists of
covalently connected tin atoms. This, to our knowledge, has not been reported for
other metallic elements and, therefore, is of fundamental interest. Due to delocal-
ization of electrons along the polymer backbone (σ-delocalization) [1-3], poly-
stannanes were regarded to be appealing materials with respect to their chemical,
optical, thermal and electrical properties [4-10]. Moreover, polystannanes with
aromatic side groups were reported to feature σ-π-delocalization of the electrons
[11]. While some research has been addressed to copolymers of stannanes and
silanes or germanes (Scheme 1) [12-15], little is known about polystannane
copolymers constituted of tin atoms with different organic side groups. Only a
copolymer comprising the repeat units dibutylstannane and bis(3-phenylpropyl)-
Scheme 1. Schematic of the reported synthesis of copolymers with diorganostannane moieties in
the main chain: poly(dibutylstannane-co-methylphenylsilane), poly(dibutylstannane-co-dibutyl-
silane), poly(dibutylstannane-co-dibutylgermane) and poly[dibutylstannane-co-di(ω-phenyl-
propyl)stannane]. Refs. indicated.
Sn
Bu
Bu
ClCl Si
Me
ClCl
Ph
Sn
Bu
Bu
ClCl E
Bu
Bu
ClCl
Sn
Bu
Bu
ClCl Si
Me
ClCl
Ph
Sn
Bu
Bu
HH
Sn
Bu
Bu
Si
Me
Ph
Sn
Bu
Bu
E
Bu
Bu
Sn
Bu
Bu
Si
Me
Ph
Sn
Bu
Bu
Sn
Ph
Ph
Sn HH
Ph
Ph
+
+
+
+
[12]
[15]
[13]
[4]
x Y
x Y
x Y
x Y
Na / 120 °C
e-
Na / tol.
[RhCl(PPh3)3]
E = Si, Ge
76
stannane (i.e. with the propyl and not the phenyl groups bound to the tin atoms)
has been described [4]. Hence, in order to further explore the spectrum of
polystannanes and their material properties, copolymers comprising
dialkylstannane and diarylstannane moieties are aimed for in this study, as
indications exist that incorporation of the latter may enhance the environmental
stability of these materials.
Efficient synthesis of polystannanes was found to be the dehydropoly-
merization of R2SnH2 with Wilkinson’s catalyst [5], resulting in pure linear poly-
(dialkylstannane)s. This route enabled to uncover material properties of
polystannanes without the influence of undesirable byproducts, in particular cyclic
oligomers. Unfortunately, however, Wilkinson’s catalyst was of limited
applicability for polymerization of diarylstannanes, (e.g. Ph2SnH2), and thus likely
to be of limited use for the synthesis of copolystannanes comprising this moieties.
Therefore, we adopted the reaction of dichlorodiorganostannanes with sodium in
liquid ammonia for the preparation of poly(dialkylstannane-co-diarylstannane)
copolymers of the general composition (SnAlk2)x(SnAr2)y, with Alk representing
an alkyl- and Ar an aryl group and, for reference purposes, the corresponding
homopolymers (Scheme 2). Conveniently, reactions of this type have found
attention in the literature for some time [16-19] with a focus to form tin-carbon
bonds as well as tin-tin bonds [20-35], and have been investigated in more detail
more recently [36].
Scheme 2. Overall reaction scheme of dichlorodialkylstannanes and dichlorodiarylstannanes with
sodium for the synthesis of homo- and copolymers in liquid ammonia.
Sn ClCl
R
R
Sn
R'
ClCl
R'
Sn Sn
R'
R'
R
R
+Na / NH3
-78°C
x Y
Homopolymerization: R = R' = alkyl, arylCopolymerization: R = alkyl, R' = phenyl
77
2. Results and Discussion
Polymerization. Reactions of dichlorodiorganostannanes, R2SnCl2, with
sodium in liquid ammonia offer two different pathways to create polymers – either
in a one-step polymerization by application of two molar equivalents of sodium
(Scheme 3), or a two-step reaction of dichlorodiorganostannane with four
equivalents of sodium to generate a mixture of soluble stannides as intermediates
[36], followed by addition of another equivalent of dichlorodiorganostannane
(two-step polymerization, Scheme 3). The latter quantities result again in an
overall dichlorodiorganostannane : Na stoichiometry of 1 : 2 (one sodium atom per
chlorine), which corresponds to the theoretical ratio to yield “free (SnR2) groups”
and the polymer (SnR2)n, respectively.
Scheme 3. Schematic of two pathways towards poly(diorganostannane)s by reaction of dichlorodi-
organostannanes with sodium in liquid ammonia.
Sn
R
R n
SnCl
R
R
Cl
+ 2 Na; NH3 (l)- 2 NaCl
+ 4 Na; NH3 (l)- 2 NaCl
intermediate stannides
+ R2SnCl2- 2 NaCl
one-steppolymerization
two-steppolymerization
78
In the present study poly(dibutylstannane) and poly(diphenylstannane) and
copolymers comprising their respective repeat units were selected as representatives
for polystannanes with aliphatic and aromatic substituents. A priori it should be
noted that chemical structures, and, hence the properties of polymers resulting
from treatment of dichlorodiorganostannanes with sodium using the one- or
two-step synthesis route may differ significantly. For instance, homopoly-
merization of dichlorodibutylstannane Bu2SnCl2 by the two-step process is expec-
ted to result in branched polymer chains, due to the exchange of alkyl groups in
the intermediate stannides [36]. However, such phenomena have not been
observed for reactions with dichlorodiphenylstannane, Ph2SnCl2, i.e. there was no
evidence for exchange of aryl groups in the liquid ammonia solutions.
Thus, copolymers with Bu2SnCl2 and Ph2SnCl2 and their reference
homopolymers were synthesized by the above mentioned one- and two-step
polymerization protocols. Since poly(diphenylstannane) is insoluble - at least in
common solvents - and since increasing the length of alkyl side groups is generally
reported to show positive influence on the solubility of polymers [37-43], also
copolymers of dichlorodiphenylstannane with dichlorodialkylstannanes comprising
longer alkyl groups were synthesized, i.e. with dichlorodioctylstannane, Oct2SnCl2,
(Oct = octyl) and dichlorodidodecylstannane, Dod2SnCl2 (Dod = dodecyl). An
overview of the homo- and copolymers explored is shown in Table 1.
Table 1. Overview of the suitability of the applied methods for the synthesis of homo- and
copolymers discussed in the present work: + good; - bad due to crosslinking/branching; x no
polymer formed/no reaction.
Phenyl Butyl Octyl Dodecyl 1step 2step 1step 2step 1step 2step 1step 2step
Phenyla) + + + + x + x + Butyla) + - + - not applied not applied a)present in intermediate stannides in the case of two-step synthesis
79
Poly(dibutylstannane)
When the one-step synthesis was applied, polymeric material precipitated
within 10 to 20 s after addition of dichlorodibutylstannane to the solution of Na in
liquid ammonia. After isolation, the molar mass of the polymer dissolved in THF
was estimated by gel permeation chromatography (GPC). This analysis revealed a
weight-average molar mass MW of 8 kg/mol (Table 2). The molar mass of the
polymer obtained by two-step polymerization was higher (MW = 15 kg/mol).
Reassuringly, elemental analysis of the two products did not differ significantly
(Table 2).
In subsequent experiments, using the two-step procedure, the stoichiometric
ratios of the two Bu2SnCl2 portions of the first and the second addition were
varied between 0.9 and 1.1. Isolation and dissolution of the products in THF
allowed the determination of the molar mass as a function of this stoichiometry.
However, no significant influence of the stoichiometric ratios on the molar masses
was observed (Mw = 15 kg/mol at a ratio of 1.1, Mw = 13 kg/mol at a ratio of 0.9).
Importantly, a pronounced decrease in molar mass is expected for a poly-
condensation type reaction of the in-situ formed stannides in the 1st step with
dichlorodiorganostannane added in the 2nd step, according to Carother’s equation
[44]. As this was not the case, a chain-growth polymerization is presumed to
prevail. Radical reactions of organostannanes in liquid ammonia are well known
[29-34] indicating that the chain-growth polymerization could possibly be
initiated by radicals. Since the intermediates formed in-situ in the first step did not
react with each other to yield polymers, the species which initiated polymerization,
therefore, must have been generated upon addition of the second portion of
Bu2SnCl2 - perhaps by reaction with Na, since the entire quantity of Na is not re-
quired for the formation of the observed stannides generated in the first step [36].
80
Table 2. Calculated and found elemental compositions (in % m/m, products analyzed after
extraction of NaCl from reaction mixtures) and molar masses MW of the polymers obtained. The
theoretical elemental compositions of the copolymers were calculated on the basis of the applied
equimolar ratio of the monomers.
Poly(diphenylstannane)
One-step polymerization of dichlorodiphenylstannane, Ph2SnCl2, with two
molar equivalents of sodium resulted in precipitation of a yellow product. The
reaction appeared to be completed - as far as could be detected by visual
observation - within a few seconds after the addition of Ph2SnCl2. Interestingly,
rapid addition triggered inhomogeneity in the solution yielding, after evaporating
the ammonia, a product with red, pasty parts which were soluble in toluene and
dichloromethane to a certain degree. Drop-wise addition of Ph2SnCl2 to the
sodium solution in ammonia, by contrast, resulted in a yellow, bright precipitate.
However, this reaction appears to represent an intermediate stage between one-
and two-step polymerization, as the first quantities of Ph2SnCl2 reacted with a
high excess of sodium to form soluble intermediates. Therefore, the resulting
polymer showed the same characteristics as the material obtained by the two-step
Polymer C (% m/m) H (% m/m) MW
found calculated found calculated kg/mol (SnBu2)n 1 step 41.09 41.25 7.61 7.79 8 2 step 40.75 7.56 15 (SnPh2)n 2 step 51.87 52.81 3.74 3.69 insoluble (SnBu2)n(SnPh2)m 1 step 46.88
47.48 5.37
5.59 10
2 step (Bu-stannides) 37.87 5.19 insoluble 2 step (Ph-stannides) 47.74 5.50 10 (SnOct2)n(SnPh2)m 53.36 54.24 7.32 7.48 10 (SnDod2)n(SnPh2)m 57.86 59.21 8.89 8.28 12
81
polymerization – i.e. a homogeneous bright yellow powder that was insoluble in all
tested organic – low and high boiling – solvents, also at elevated temperatures.
Nonetheless, elemental analysis of the purified species were in good agreement
with the composition of pure (Ph2Sn)n (Table 2).
Poly(dibutylstannane-co-diphenylstannane)
Mixtures of dichlorodibutylstannane and dichlorodiphenylstannane in various
ratios were introduced into a homogeneous mixture of sodium in liquid ammonia
(Na : total stannane = 2 : 1, one-step synthesis). This procedure resulted in
precipitation of polymeric species within 10 and 40-50 seconds. The isolated
polymers were largely soluble in toluene and dichloromethane (Mw = 10 kg/mol,
Table 2), but also contained an insoluble fraction. Since the corresponding homo-
polymers poly(dibutylstannane) and poly(diphenylstannane) differ in their solubility
(see above), it would be reasonable to assume that the insoluble part consisted of
copolymers of a high content of diphenylstannane. Application of mixtures of 75 %
mol/mol Bu2SnCl2 and 25 % mol/mol Ph2SnCl2, as well as 25 % mol/mol
Bu2SnCl2 and 75 % mol/mol Ph2SnCl2 also resulted in the formation of soluble
and insoluble copolymer.
The two-step copolymerization started with the in-situ formation of stannides.
Accordingly, there are two possibilities to produce a copolymer. If in the first step
Bu2SnCl2 was applied, followed by the addition of Ph2SnCl2 in the 2nd step, the
product obtained was largely insoluble in toluene and dichloromethane. Elemental
analyses deviated significantly from theoretical values (Table 2). Oxygen contents
of up to 5 % w/w were found in the elemental compositions; obviously the
materials formed were sensitive to ambient oxygen. On the other hand, employing
Ph2SnCl2 in the 1st step, followed by addition of Bu2SnCl2 led to the formation of
82
partly soluble reaction products. The soluble fraction possessed a molar mass
Mw = 10 kg/mol, and the elemental analysis were in good agreement with the
theoretical composition (Table 2).
Poly(dioctylstannane-co-diphenylstannane)
One-step synthesis of Oct2SnCl2 and Ph2SnCl2 was not successful in liquid
ammonia since Oct2SnCl2 precipitated on the bottom of the flask and was inert to
further reactions. Hence, two-step polymerization was conducted by reaction of
Ph2SnCl2 in the 1st step followed by slow addition of Oct2SnCl2 to circumvent the
monomer to precipitate prior to reaction. About 60 s after the addition the liquid
reaction mixture turned yellow, and a yellow compound precipitated and could be
isolated by removing NH3. The polymeric material was partly soluble in THF and
dichloromethane. The soluble fraction was of a molar mass Mw = 10 kg/mol with
an elemental composition close to the theoretical values of the copolymer (cf.
Table 2).
Poly(didodecylstannane-co-diphenylstannane)
This copolymer could be synthesized only by the two-step procedure, since
Dod2SnCl2 accumulated at the bottom of the flask without reaction in the
one-step reaction. Therefore, in-situ created phenylstannide intermediates were
exposed to Dod2SnCl2. The resulting polymer precipitated immediately during
addition of Dod2SnCl2. The material thus produced was partly soluble in toluene
and dichloromethane, and its molar mass amounted to Mw = 12 kg/mol (Table 2).
83
Characterization
NMR Spectroscopy
All polymers were analyzed by NMR spectroscopy. The results are
summarized in Table 3 and will be discussed below.
Table 3. 1H and 119Sn NMR data for the polymers produced; 119Sn NMR chemical shifts δ refer
to tetramethylstannane (δ(Me4Sn) = 0 ppm); t = triplet, m = multiplet.
Poly(dibutylstannane)
119Sn NMR spectra of poly(dibutylstannane) recorded for
poly(dibutylstannane) from the one-step synthesis dissolved in deuterated
dichloromethane, CD2Cl2, featured a relatively broad signal at -190 ppm and two
sharp signals at -202 ppm and -203 ppm. The former corresponds to that of linear
poly(dibutylstannane), while the others correspond to decabutylpentastannane,
(Bu2Sn)5, and dodecabutylhexastannane, (Bu2Sn)6 respectively [5, 6]. The product
obtained from two-step synthesis showed a somewhat broader signal at -190 ppm,
attributed to linear poly(dibutylstannane) and signals of the cyclic oligomers at
Polymer δ 1H (ppm) Ratio Ar/Alkyl δ 119Sn (ppm)
-CH3 -CH2- Ar-H Sn
(SnBu2)n 0.9-1.0 [m, 3H] 1.1-1.8 [m, 6H] - - -190, -202, -203 -410 to -430a)
(SnPh2)n - - - - -197b)
(SnBu2)n(SnPh2)m 1.1 [m, 3H] 1.3-2.3 [m, 6H ] 6.7-8.1 [m, 3.7H] 0.7-0.9 -160 to -190 -200 to -220
(SnOct2)n(SnPh2)m 0.9 [t, 3H] 1.0-1.55 [m, 14] 6.7-7.6 [m, 5H] 1 -157 to -195 -200 to -220
(SnDod2)n(SnPh2)m 0.9 [t, 3H] 1.0-1.65 [m, 22 H] 6.7-7.6 [m, 5H] 1 -155 to -180 -187 to -211
a) the signal at -410 to -430 ppm was only observed in the two-step synthesis; b) determined by solid-state MAS NMR spectroscopy.
84
-202 ppm and -203 ppm. In addition, weak signals around -420 ppm were found,
which most likely originate from tertiary tin atoms (i.e. Sn atoms bound to three
other Sn atoms) in the polymer chain [45]. This also explained the broadening of
the polymer signal at -190 ppm, as a result of variances in the neighboring tin
atoms of the main chain.
Poly(diphenylstannane)
The soluble fractions extracted from the inhomogeneous product resulting
from the one-step synthesis did not feature any signal in 119Sn NMR experiments.
As the product from the two-step polymerization was insoluble, solid-state magic-
angle-spinning (MAS) 119Sn NMR spectra were acquired, which revealed one
(broad) signal at about -197 ppm, i.e. in the range typical for polystannanes [5, 6].
Poly(dibutylstannane-co-diphenylstannane)
1H NMR spectra of the soluble products (one- and two-step synthesis)
showed the typical signals of phenyl, as well as alkyl groups (Table 3). Integration
of the aliphatic and the aromatic signals indicated that the butyl content was
higher than expected – typical ratios between 0.9 and 0.7 were found (calculated
on the basis of the total number of hydrogen atoms). 119Sn NMR spectra featured
two very broad signals, one ranging from -160 ppm to -190 ppm and another from
-200 ppm and -220 ppm – the first is attributed to SnBu2 and the latter to SnPh2
moieties.
Poly(dioctylstannane-co-diphenylstannane)
The soluble fraction of this material featured signals in 1H NMR spectra of
the hydrogen atoms of the aromatic and aliphatic units; integration of these signals
85
were consistent with a 1 : 1 ratio between octyl and phenyl groups (Table 3). 119Sn
NMR spectra displayed broad signals ranging from -157 ppm to -195 ppm
resulting from SnOct2 and from -200 ppm to -220 ppm resulting from SnPh2
moieties.
Poly(didodecylstannane-co-diphenylstannane)
1H NMR spectra of this compound featured evidence of a 1 : 1 ratio of
dodecyl- and phenyl groups in the copolymer (Table 3). 119Sn NMR spectra of the
soluble polymer showed two broad peaks ranging from -155 ppm to -180 ppm,
assigned to SnDod2 moieties, and from -187 ppm to -211 ppm representing SnPh2
units.
UV/Vis spectroscopy
Delocalization of σ−electrons in the backbone of polystannanes was found to
be responsible for the characteristic yellow color and the absorption maxima
around 390 nm for poly(dialkylstannane)s (σ-delocalization) [5, 10]. In the case of
aromatic side groups, additional delocalization is expected to result in a
bathochromic shift of the absorption maximum, due to σ-π-delocalization [1, 11].
A main goal of the UV/Vis investigation was to investigate, if σ-π-delocalization
also occurs in copolymers with phenyl and alkyl side groups.
Homopolymers
UV/Vis absorption spectra of poly(dibutylstannane) featured an absorption
maximum at 390 nm independent of the synthesis method (one- or two-step),
consistent with values reported in the literature [5, 6, 10]. Absorption spectra of
poly(diphenylstannane), however, displayed an absorption edge around 480 nm,
86
which was attributed to enhanced delocalization of the electrons along the
backbone and the aromatic side chains (σ-π-delocalization) [1, 11].
Copolymers
UV/Vis spectra of poly(dibutylstannane-co-diphenylstannane) synthesized by
one-step reactions featured two peaks - one around 400 nm originating in the
σ-delocalization of the electrons of dibutylstannane units, and one around 470 nm
related to the σ-π-delocalization in diphenylstannane units. Comparing the
UV/Vis spectra of the insoluble and the soluble part of poly(dibutylstannane-co-
diphenylstannane), obtained from a 1 : 1 mixture of dichlorodibutylstannane and
dichlorodiphenylstannane (Figure 1a) indicated that the fraction of Ph2Sn moieties
was larger in the insoluble part, as indicated by the obervation that the maximum
at 470 nm was more pronounced in the absorption spectra of this part (Figure 1).
The normalized spectra of the insoluble part showed an intense shoulder at a level
of about 75 % compared to the signal at 400 nm, whereas in the soluble part this
signal was of only ~50 % of the corresponding intensity. Taking into account that
the same initial molar amount of Bu2SnCl2 and Ph2SnCl2 was employed, it is
obvious from the spectra that the extinction coefficient of the dibutyl stannane
units was higher than that of the diphenylstannane units, as absorbance at 400 nm
was higher in the soluble - as well as the insoluble fractions.
UV/Vis absorption spectra of the soluble polymer produced with mixtures
containing 25 to 75 % mol/mol Bu2SnCl2 also featured the two absorption
maxima at 400 nm and 470 nm. The signal at 400 nm, attributed to dibutyl-
stannane units, increased with increasing amount of dichlorodibutylstannane in the
starting mixtures concomitant with a decrease of the intensity of the signal at
470 nm, representing a decreasing fraction of diphenyltin moieties (Figure 2).
87
Figure 1. UV/Vis absorption spectra of the insoluble (dashed) and soluble (solid) fraction of poly-
(dibutylstannane-co-diphenylstannane) obtained by one-step polymerization (a) and two-step
polymerization (b).
Surprisingly, analysis of UV/Vis spectra indicated that the soluble polymer
resulting from the mixture with 75 % dichlorodiphenylstannane comprised a higher
aromatic content than the insoluble part of the 50 % mixture. Thus, it appears that
not only the amount of phenyl groups, but also their arrangement had an influence
on the solubility – larger blocks of poly(diphenylstannane) could lead to a decreased
solubility, whereas randomly distributed SnPh2 moieties, even at a higher overall
diphenyltin content, can still result in solubility of the respective copolymer.
A shift of the absorption maxima towards higher wavelengths was observed in
the soluble part from 459 nm (25 % Ph2SnCl2) to 465 nm (75 % Ph2SnCl2),
0.0
a
Rel
. Abs
orba
nce
0.5
1.0
b
Wavelength / nm400 600500
Rel
. Abs
orba
nce
0.5
0.0
1.0
88
Figure 2. UV/Vis absorption spectra of poly(dibutylstannane-co-diphenylstannane) produced by
one-step polymerization with a molar dichlorodibutylstannane content of 75 % (solid), 50 %
(dashed) and 25 % (dotted) in the starting mixtures.
indicating an increasing σ-π-delocalization of the electrons in the phenyl fraction in
the latter. At the same time the absorption maxima assigned to dibutylstannane
units shifted from 399 nm (75 % Bu2SnCl2) to 383 nm (25 % Bu2SnCl2) indicative
of decreasing delocalization and the influence of the (Bu2Sn) moieties (Table 4). In
the case of pure poly(dibutylstannane) only σ - delocalization of the electrons
occurs. The results of the above analysis represent conclusive evidence of the
formation of copolymers with both diarylstannane (SnAryl2) and dialkylstannane
(SnAlkyl2) moieties in the polymer main chain, as the absorption maxima shifted
depending on the composition of the copolymer. This, would of course not be the
case for blends of poly(dialkylstannane) and poly(diarylstannane) homopolymers.
UV/Vis absorption spectra of the materials obtained in two-step synthesis
(Figure 1b) showed a high phenyl content (assessment see above) in the insoluble
part, whereas the shoulder at 470 nm in the soluble polymer, indicating the
presence of diphenylstannane units, was detected, but not very pronounced.
Rel
. Abs
orba
nce
Wavelength / nm
400 600500
89
Figure 3. UV/Vis absorption spectra of the insoluble (dashed) and soluble (solid) fraction of poly-
(didodecylstannane-co-diphenylstannane).
Also the UV/Vis absorption spectra of poly(dioctylstannane-co-diphenylstannane)
featured an absorption maximum at 400 nm and a shoulder at 460 nm,
representative of dioctylstannane and diphenylstannane moieties, respectively.
Again, the intensity of the latter was more pronounced in the spectrum of the
insoluble part. UV/Vis absorption spectra of the soluble and insoluble parts of
poly(didodecylstannane-co-diphenylstannane) are shown in Figure 3. Both spectra
show an absorption maximum at 400 nm, together with an intense shoulder at
473 nm, which represents a bathochromic shift of the diphenylstannane related
absorption maxima compared to the corresponding copolymers comprising butyl
Table 4. Absorbance E and absorption maxima λmax of the soluble poly(dibutylstannane-co-di-
phenylstannane) synthesized by copolymerization with different molar fractions X of dichlorodi-
butylstannane.
Wavelength / nm400 600500
Rel
. Abs
orba
nce
0.5
0.0
1.0
dibutylstannane
segments diphenylstannane
segments XButyl EButyl λmax EPhenyl λmax
0.75 0.434 399 0.080 459 0.50 1.835 398 0.934 461 0.25 0.226 383 0.2422 465
90
and octyl groups, respectively. The shoulder was more pronounced in the insoluble
part. Furthermore, also the second absorption maximum showed a shift towards
higher wavelengths when compared to the poly(dibutylstannane-co-
diphenylstannane) polymers (400 nm to 406 nm).
Overall, the UV/Vis spectra are consistent with the incorporation of aromatic
moieties in the copolymer. However, it was not evident if the σ−π-delocalization is
caused by the existence of randomly distributed aromatic groups in the copolymer,
or by the occurrence of aromatic blocks of several (SnPh2) units in sequence.
Thermal Properties
Previously, distinctly different thermal characteristics have been reported for
poly(dialkylstannane)s [5, 6] and poly[di(ω-phenylalkyl)stannane]s [4]. For
instance, poly(dibutylstannane) synthesized with Wilkinson’s catalyst was found to
exhibit a phase transition at ca. 0 °C on heating and -25 °C on cooling, whereas
poly[bis(4-phenylbutyl)stannane] revealed a glass transition at -52 °C [4]. Also the
thermal stability differed significantly between polystannanes with aliphatic and
aromatic side groups. Therefore, thermal analysis should yield indications whether
a mixture of two homopolymers or a copolymer was formed in the present
synthetic efforts. Accordingly, thermal properties were investigated by differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA).
Poly(dibutylstannane) synthesized with sodium in liquid ammonia did not
feature the characteristic phase transition at 0 °C from a low temperature
crystalline solid phase to a liquid-crystalline mesophase [5, 7] (Table 5). This was
attributed to the presence of cyclic oligomers and the occurrence of branching
points in the products obtained from liquid ammonia, due to exchange of alkyl
91
Table 5. Peak transition temperatures and enthalpies of poly(dialkylstannane) and poly(diaryl-
stannane) homo- and copolymers, determined by differential scanning calorimetry (DSC).
groups [36], which would supress crystallization of the polymer. The
decomposition temperature (270 °C), however, was found to be virtually
independent of the synthesis method employed; i.e. no significant differences in
degradation temperatures were detected between polymers synthesized with
Wilkinson’s catalyst or sodium in liquid ammonia.
Poly(diphenylstannane), displayed no phase transition between -50 °C and
200 °C in DSC thermograms. This observation is consistent with polarization
microscopy studies which revealed no change in birefringence upon heating
polymer films up to 200 °C. Thermogravimetric analysis (TGA) indicated a
decomposition temperature of 350 °C with an onset at about 270 °C, leading to
the conclusion that poly(diphenylstannane) decomposes prior to melting.
Polymer Peak Transition Temperature °C
Transition Enthalpy J/g
1st trans. 2nd trans. 1st trans. 2nd trans. (SnBu2)n heating -1 - 10.1 - Wilkinson’s Cat.[2] cooling -26 - -9.3 - (SnBu2)n heating - - - - cooling - - - - (SnPh2)n heating - - - - cooling - - - - (SnBu2)n(SnPh2)m heating - - - - cooling - - - - (SnOct2)n(SnPh2)m heating -5 58 1.25 0.65 cooling -16 52 -1.28 -0.64 (SnOct2)n heating 29 74 14.3 5.2 Wilkinson’s Cat. [2] cooling 13 58 -13.5 -5.8 (SnDod2)n(SnPh2)m heating 94 - 43.0 - cooling 83 - -43.7 - (SnDod2)n heating 55 91 12.7 2.1 Wilkinson’s Cat. [2] cooling 39 80 -24.9 -3.8
92
Figure 4. Differential scanning calorimetry (DSC) thermograms of poly(dioctylstannane-co-
diphenylstannane) recorded at heating- and cooling rates of 10 °C/min. Two phase transitions are
observed at -5 °C and 58 °C upon heating and at 52 °C and -16 °C upon cooling in the second
respective thermograms.
Poly(dibutylstannane-co-diphenylstannane) copolymers did not show any phase
transition (Table 5), independent of the synthesis route (one- or two-step
procedure) and the butyl/phenyl ratio (1 : 3 to 3 : 1). Also these polymers reminded
birefringent in the entire temperature range. Similar to poly(dibutylstannane),
thermal decomposition occurred at 270 °C.
By contrast, poly(dioctylstannane-co-diphenylstannane) featured two weak
transitions in DSC thermograms at -5 °C and at 58 °C on heating (73 °C in the
first heating) and at -16 °C and 52 °C upon cooling (Figure 4). The crystallization
enthalpy of the first transition was about 1.25 J/g and the second 0.65 J/g
(Table 5). In the corresponding homopolymer poly(dioctylstannane), two phase
transitions were reported at 29 °C and 74 °C upon heating with melting enthalpies
of 14.3 and 5.2 J/g, respectively [5]. The decrease in the phase transition
temperatures and enthalpies of the copolymers was ascribed to the lower content of
octyl groups in the copolymer compared to that in the homopolymer, and to the
presence of rather amorphous segments of randomly dispersed diorganostannane
Hea
t flo
w W
/g
100500
endo
1st heating
2nd heating
1st cooling
2nd cooling
0.01 W/g
93
Figure 5. DSC thermograms of poly(didodecylstannane-co-diphenylstannane). In contrast to poly-
(didodecylstannane), only one phase transition upon heating and cooling were detected.
units. Thermogravimetric analysis unveiled a degradation temperature of 300 °C,
i.e. slightly higher than that of the dibutylstannane-diphenylstannane copolymer.
Poly(didodecylstannane) homopolymer synthesized with Wilkinson’s catalyst
from didodecylstannane (Dod2SnH2) showed two phase transitions in DSC
diagrams – at 55 °C upon heating and at 91 °C and upon cooling at 39 °C and
80 °C, with melting enthalpies of 13 J/g for the first and 2.1 J/g for the second
transition. For the copolymer poly(didodecylstannane-co-polydiphenylstannane)
only one, intense transition was observed at 94 °C on heating and 83 °C on cooling
with a melting enthalpy of 43 J/g (Table 5 and Figure 5). The temperature of this
transition corresponds well with the second transition of the homopolymer. It
seems, therefore, that the aromatic moieties suppressed the first phase transition at
55 °C, while melting occured at the same temperature as for the poly(didodecyl-
stannane) homopolymer – most likely due to the long alkyl chains. The
decomposition temperatures (330 °C) were found to increase compared with the
corresponding homopolymers.
Hea
t flo
w W
/g
100500-50
0.5 W/g
endo
1st heating
2nd heating
1st cooling
2nd cooling
94
Orientation
Previously, molecular orientation of poly(dialkylstannane)s was studied [10],
since orientation of polymeric materials is of great relevance, e.g. with regard to
mechanical, electrical and optical properties. For instance, poly(dibutylstannane)
was oriented by shearing, drawing of blends with ultra-high molecular weight
polyethylene (UHMWPE) and by crystallization onto friction-deposited layers of
uniaxially oriented poly(tetrafluoroethylene) (PTFE) molecules. The same
methods were also used for poly(dioctylstannane) and poly(didodecylstannane)
[10]. Remarkably, it has been described earlier [10] that the polymer main chain
can orient either preferentially parallel or perpendicular to the orientation direction
of the external stimulus, depending on the orientation method employed and the
side group of the polystannanes. Therefore, also in this study we investigated the
influence of the side group on the orientation direction of copolymers.
Poly(diphenylstannane), poly(dibutylstannane-co-diphenylstannane), poly(di-
octylstannane-co-diphenylstannane) and poly(didodecylstannane-co-diphenyl-
stannane) were examined. Orientation was induced by shearing on a glass slide
with a spatula, by drawing of blends with UHMWPE on a hot-stage at 110 °C
and, in the case of the soluble polymers, by crystallization onto PTFE orientation
layers. Alignment of the polymer was analyzed by means of polarized optical
microscopy, UV/Vis spectroscopy with polarized light and wide-angle X-ray
diffraction (WAXD).
Shearing the polymers at room temperature induced orientation of the main
chain for all materials parallel to the direction of shear, as clearly evident from
Figures 6 and 7 - shown by the example of poly(diphenylstannane) and
poly(dibutylstannane-co-diphenylstannane) (synthesized by the two-step synthesis).
95
Figure 6. Polarized optical microscopy images taken between crossed polarizers of poly(diphenyl-
stannane) prepared by two-step polymerization and oriented by shearing on a glass slide (top row)
and by drawing of a blend with UHMWPE (second row), at 0° and 45 ° angles between the
orientation axis of the sample and polarization direction of the light. At the bottom wide angle
X-ray diffraction patterns of the oriented materials are shown; left, oriented by shearing; right, by
drawing of blends with UHMWPE. Arrows indicate signals due attributed to the polystannane,
whereas the other signals originate from UHMWPE; orientation direction vertical.
Tensile deformation of blends with UHMWPE provided the same results for
poly(diphenylstannane), poly(dibutylstannane-co-diphenylstannane) and poly(di-
octylstannane-co-diphenylstannane). The orientation of these polymers was parallel
to the drawing direction. Drawing of blends with poly(didodecylstannane-co-
diphenylstannane), on the other hand, showed unexpected results. UV/Vis
spectroscopy with polarized light yielded different results compared to those
recorded in the shearing experiments. The development of the shoulder at 470 nm,
1 mm
45° 0°
96
Figure 7. Polarized UV/Vis absorption spectra (angles relative to the orientation direction
indicated) and wide angle X-ray diffraction pattern of poly(dibutylstannane-co-diphenylstannane)
oriented by shearing (orientation direction vertical).
attributed to the aromatic content of the copolymer, with the angle between
polarization direction of incident light and drawing direction was essentially the
same for both methods (Figure 8). By contrast, that of the signal around 400 nm
differed strikingly (cf. Figure 8b). Dichroism at 400 nm was more pronounced and
the absorbance perpendicular to the drawing direction and polarization plane of
incident light was higher, i.e. the dichroic ratio changed from 1.2 to -1.7 (the
negative sign indicates that absorption perpendicular to the polarization direction
is higher than parallel). This result implies that the copolymerization of dichloro-
diphenylstannane and dichlorodidodecylstannane did not yield an alternating
copolymer, but contains didodecylstannane blocks which cause orientation of side
groups parallel to the drawing axis thus forcing orientation of the corresponding
main chain segment to be perpendicular to the drawing axis, as was observed with
poly(didodecylstannane) homopolymer [5]. Inducing orientation by crystallization
from solution onto pre-oriented PTFE layer was not possible for poly(diphenyl-
stannane) due to its insolubility and was not successful for the copolymers.
Wavelength / nm
400 500
Abs
orba
nce
0.4
0.0
0.2
0.6
0
9045
97
Figure 8. Polarized optical absorption spectra of poly(didodecylstannane-co-diphenylstannane)
oriented by shearing on a glass slide at room temperature (a) and by drawing of a blend with
UHMWPE (b). Note that for orientation by shearing the absorption at 0° is higher for both
signals, whereas in the drawing experiments the absorption around 400 nm is higher at 90°.
Rel
. Abs
orba
nce
Wavelength / nm350 400 450 500
Rel
. Abs
orba
nce
a
b
0
9045
0
9045
98
3. Conclusions
We demonstrated that formation of polystannane homopolymers and copoly-
mers with sodium in liquid ammonia can be achieved via two different reaction
paths. That is, starting from dichlorodiorganostannanes, R2SnCl2, one-step
synthesis by reaction with two molar equivalents sodium and two-step synthesis,
where in the first step reactive intermediates are generated by the reaction with
four molar equivalents sodium. The two-step polymerization does not follow a
step-growth mechanism, but rather chain growth polymerization, probably
initiated by radicals generated during the second synthetic step. Migration of alkyl
groups- but not of phenyl groups - during the reaction with sodium in liquid
ammonia was confirmed, which can lead to branched polymers if alkylstannide
intermediates are formed.
By means of 119Sn NMR spectroscopy, UV/Vis spectroscopy and thermal
analysis it is demonstrated that copolymers were created as opposed to blends of
the respective homopolymers. UV/Vis spectra indicated the presence of both σ-π-
delocalization and pure σ-delocalization in poly(dialkylstannane-co-diphenyl-
stannane). From the spectra it could not be unequivocally concluded if the
corresponding signals were just due to occurrence of aromatic moieties in the
polymer or due to formation of block-copolymers.
Phase transitions observed in homopolymers were not, or only partially,
reflected in the related copolymers. The decomposition temperatures of poly(di-
alkylstannane-co-diphenylstannane)s increased with increasing length of the alkyl
group.
Like the homopolymers, the copolymers could readily be oriented by shearing
or by drawing of blends with ultra-high molecular weight polyethylene
(UHMWPE). In most cases, the polystannane main chains oriented preferentially
99
parallel to the orientation direction of the external stimulus. However,
interestingly, the orientation of poly(didodecylstannane-co-diphenylstannane)
depended on the orientation method. Whilst in simple shearing experiment the
copolymer chain axis oriented into the direction of shear, this polymer revealed
preferential orientation of segments with dodecylstannane perpendicular and
segments with diphenylstannane parallel to the drawing direction of blends with
UHMWPE. This observation appears to indicate the presence of blocks of
dodecylstannane and phenylstannane units, respectively, where the dodecyl groups
oriented parallel to the direction of the external stimulus, therewith forcing the
main chain to a perpendicular orientation.
100
4. Experimental
Materials. Ammonia was purchased from PanGas (Dagmarsellen, Switzerland,
99.999 %). Dichlorodibutylstannane and dichlorodioctylstannane was acquired
from ABCR GmbH (Karlsruhe, Germany) and dichlorodiphenylstannane from
Sigma Aldrich (Buchs, Switzerland). The substances were recrystallized twice by
dissolving in boiling pentane and subsequent precipitation at -78 °C. Deuterated
dichloromethane CD2Cl2 (99.9% D) was purchased from Cambridge Isotope
Laboratories (ReseaChem GmbH, Burgdorf, Switzerland), and organic solvents
from Fluka (Buchs, Switzerland). Dichlorodidodecylstannane was synthesized
according to literature [5].
Characterization. NMR spectra were recorded on a Bruker UltraShield 300
MHz/54 mm Fourier transform spectrometer with standard 5 mm broad band
probe. All soluble samples were dissolved in CD2Cl2. Solid-state magic-angle
spinning experiments were executed on a Brucker UltraShield 500 MHz/54 mm
Fourier transform spectrometer, at a spinning rate of 14’000 rpm. Samples were
prepared in Bruker Ph MAS ZrO2 Rotors with Kel-F caps.
Elemental analyses were performed by the Microelemental Analysis
Laboratory of the Department of Chemistry at ETH Zürich. Gel permeation
chromatography was conducted with a GPC instrument from Viscotek (VE7510)
equipped with degasser, VE1121 solvent pump, VE520 autosampler and Model
301 triple detector array. A PL gel 5 μm Mixed-D column from Polymer
Laboratories Ltd. (Shropshire, United Kingdom) calibrated with atactic
poly(styrene) standards from Fluka (Buchs, Switzerland) were used. Samples were
dissolved in THF containing 2.5 % v/v toluene, which served as a marker. THF
eluent flow amounted to 1 mL/min. For optical microscopy, a Leica DMRX
101
microscope equipped with two polarizers and a Mettler Toledo FP82 HT hot
stage was used. UV/Vis spectra were recorded in transmission with a Perkin Elmer
Lambda 900 spectrophotometer equipped with rotating polarizers. Thermal
analysis were carried out with a differential scanning calorimeter (DSC) DSC822e
instrument (Mettler Toledo, Greifensee, Switzerland) equipped with an
intracooler, and thermal gravimetric analysis (TGA) with a TGA/SDTA851e
from Mettler Toledo under nitrogen atmosphere. The heating and cooling rates
were 10 °C/min. Maximum decomposition was determined by the maximum of
the 1st derivative of the TGA thermogram. Wide-angle X-ray diffraction pattern
were taken with a Diffraction Xcalibur™ PX (Oxford Instruments, Scotts Valley,
USA), using MoKα radiation.
Polymerization
One-step polymerization
Typically, around 350 mg (15 mmol) of sodium were added under a nitrogen
counterflow to 70 mL of liquid ammonia at -78°C and stirred for 15 min to obtain
a homogeneous solution (quantities for each synthesis in Table 6). Subsequently,
the flask was wrapped with soft tissue and aluminum foil to exclude light, and
dichlorodiorganostannane (half of the molar amount of sodium, or mixtures of two
dichlorodiorganostannanes which together contained half of the molar amount of
sodium) dissolved in about 5 mL THF was added with a syringe through a
septum. The reaction mixture was stirred for a few minutes to complete the
reaction before the flask was allowed to warm up to room temperature under a
gentle nitrogen stream. Thereafter, the nitrogen outlet was closed and the products
dried in vacuum (0.1 mbar, 12 h). Subsequently, the resulting solids were washed
102
Table 6. Synthesis parameters for the one- and two-step preparation of homo- and copolymers.
with 50 mL of a water/ethanol (9:1) mixture until no chloride was detected in the
washing solution (usually 3-4 times, until the addition of 5 mL saturated AgNO3
solution did not lead to the formation of visible AgCl precipitates) and again dried
in vacuum (0.1 mbar, 24 h).
Two-step polymerization
Approximately 350 mg (15 mmol) of sodium were dissolved in 70 mL liquid
ammonia (N2 atmosphere) by stirring for 15 min, before a quarter of the amount
of substance (3.75 mmol) dichlorodiorganostannane dissolved in 5 mL THF was
slowly added with a syringe through a septum (exact quantities see Table 6). The
# steps Na
(mmol) 1st step(mmol)
2nd step(mmol)
Homopolymers
Poly(dibutylstannane) 1 15.21 7.610 Bu
2 3.72 1.860 Bu 1.865 Bu
Poly(diphenylstannane) 1 13.24 6.625 Ph
2 19.42 4.850 Ph 4.852 Ph
Copolymers
Poly(dibutylstannane -co- diphenylstannane)
1 14.58 3.645 Bu+ 3.64 Ph
2 15.11 3.780 Bu 3.775 Ph
2 16.94 4.242 Ph 4.239 Bu
Poly(dioctylstannane -co- diphenylstannane)
2 14.84 3.710 Ph 3.710 Oct
Poly(didodecylstannane -co- diphenylstannane)
2 19.88 4.970 Ph 4.968 Dod
103
reaction mixture was vigorously stirred for 30 min to ensure complete and
homogeneous mixing. Thereafter, another portion of dichlorodiorganostannane
(3.75 mmol) dissolved in 5 mL THF was added with a syringe through a septum,
whereupon the polymer precipitated. The ammonia was evaporated by warming
the flask to room temperature under a gentle nitrogen stream. Afterwards the flask
was closed and the material dried in vacuum (0.1 mbar, 12 h). Results from
elemental analysis of the products obtained are listed in Table 2, and 1H and 119Sn
NMR data in Table 3. The resulting solids were washed with 50 mL of a
water/ethanol (9:1) mixture until no chloride was detected in the washing solution
(as before, usually 3-4 times, until the addition of 5 mL saturated AgNO3 solution
did not lead to the formation of AgCl precipitates) and again dried in vacuum
(0.1 mbar, 24 h).
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Chapter V
From Poly(dialkylstannane)s to
Poly(diarylstannane)s: Comparison of Synthesis
Methods and Resulting Polymers
109
1. Introduction
Polystannanes are hitherto the only class of characterized, organometallic poly-
mers which comprise a linear polymer backbone of covalently interconnected metal
atoms. A number of methods have been advanced to synthesize poly(diorgano-
stannane)s, in particular Wurtz reactions [1-4], electrochemical reactions [5-7]
and hydrostannylation reactions [8, 9]. However, most of these reactions suffer
from drawbacks such as pronounced formation of cyclic stannanes as byproducts,
low yields, low molecular weights or poor reproducibility.
Recently, a facile synthesis route was developed for poly(dialkylstannane)s [10]
and poly[bis(ω-phenylalkyl)stannane]s [11]. Thereby, diorganostannanes R2SnH2
are polymerized in the presence of the catalyst precursor chlorotris(triphenylphos-
phine)rhodium(I) [RhCl(PPh3)3] by dehydrogenation to result in linear polymers
which can be isolated in high yields. Depending on the side groups, uncommon
thermal behavior (e.g. liquid crystallinity below room temperature) was observed.
Further, due to σ-delocalization of the electrons along the polymer main chain,
electric semiconductivity was anticipated and indeed demonstrated for
poly(dibutylstannane) so far [12].
Interesting properties are expected for polymers with extended σ-π-
delocalization of the electrons along the main chain and the side groups, as
reported earlier for poly(diarylsilane)s [13] and poly(diarylstannane)s [14]. These
aspects, together with the fact that polystannanes can be oriented by various
techniques to yield materials with anisotropic behavior, such as dichroism, attract
significant attention to such materials. Remarably, while poly(dialkylstannane)s
have been extensively studied [1-3, 5-7, 10, 12, 15-18], the synthesis of poly(di-
arylstannane)s was little considered due to the insolubility of typical representatives
110
Scheme 1. Overview of the reaction types for the preparation of polystannanes investigated in this
work.
like poly(diphenylstannane) [19], and therefore materials properties of well defined
poly(diarylstannane)s have still only been modestly explored.
Accordingly, in this study we compare the efficiency and applicability, respect-
ively, of polystannanes prepared by catalytic dehydropolymerization with two new
methods, i.e. polymerization of diorganostannanes, R2SnH2, under the action of
tetramethylethylenediamine (TMEDA) and polymerization of dichlorodiorgano-
stannes, R2SnCl2, with sodium in liquid ammonia (Scheme 1). All methods were
applied to monomers of the type R2SnX2, with X = H or Cl (as appropriate for the
particular route) and R = butyl, phenyl, 4-butylphenyl, as well as to Bu(Ph)SnX2.
This permits systematic investigation of the influence of alkyl and aryl groups on
the polymerization method and the properties of the resulting polymers, respecti-
vely. For instance, the presence of flexible chains bound directly or via aryl groups
to the polymer backbone may increase the solubility [20-26], while the presence of
aryl groups might increase the stability towards light, which has been reported to
be low for dissolved- and moderate for solid poly(dibutylstannane) [18].
R2SnH2[RhCl(PPh3)3]
- H2(R2Sn)n
R2SnH2TMEDA
- H2(R2Sn)n
R2SnCl2 - NaCl(R2Sn)n
Na / NH3 (l)
111
2. Polymerization
Diorganostannanes and dichlorodiorganostannanes were used as starting
materials for the synthesis of poly(diphenylstannane), poly[bis(4-butylphenyl)-
stannane], poly[butyl(phenyl)stannane] and poly(dibutylstannane). The
monomers, when not commercially available, were synthesized by Grignard
reaction of tetrachlorostannane with the corresponding organomagnesium halide
to obtain the tetraorganostannane, which was subsequently converted in a
Kozeschkow type reaction with additional tetrachlorostannane to result dichlorodi-
organostannane [27, 28]. To yield diorganotin dihydrides, the latter was treated
with an excess of LiAlH4 [29, 30].
Three different polymerization methods were employed (Scheme 1). The
reaction with Wilkinson’s catalyst, [RhCl(PPh3)3], probably proceeds by oxidative
addition of Sn-H bonds to Rh(I) centers [17]; the one with TMEDA most likely
via radicals as described for the reaction of R2SnXH with pyridine [31]; and that
with Na in liquid ammonia NH3 (l) via stannide ions [32, 33], and including a
radical process, as found in Chapter IV. The polystannanes indicated above could
indeed be synthesized; yet the schematic overview in Scheme 2 shows that, in fact,
each polymerization method is favorable for polymers with particular substitutents.
While polymerization with Na/NH3 (l) was especially appropriate for poly(diaryl-
stannane)s, the competence of Wilkinson’s catalyst was quite complementary to
that of TMEDA for the compounds explored. Obviously, phenyl and 4-butyl-
phenyl groups restrict the efficiency of the former and promote the performance of
the latter method. In the following, we will refer to characteristics of the individual
reactions. Note that molar masses of the polystannanes were estimated by GPC
analyses (see Experimental section and Table 2), as discussed elsewhere [17].
112
Scheme 2. Polymerization methods for the preparation of polystannanes. Symbols + : Mw above
8 kg/mol, no cyclic byproducts; the presence of polydiphenylstannane (not accessible to GPC
analysis due to insolubility) was deduced from other methods (elemental analysis, UV/Vis
spectra); - : molar masses below 8 kg/mol and/or cyclic byproducts; X : very slow or no reaction
and/or absence of polymeric products.
Polymerization with Wilkinson’s catalyst
Wilkinson’s catalyst is suited mainly for the polymerization of dibutylstannane.
The reaction proceeded rapidly in toluene at room temperature, and poly(dibutyl-
stannane) was isolated by precipitation from methanol at -78°C. 119Sn NMR
spectra showed one signal at -190 ppm, in agreement with literature values [18],
while signals of cyclic byproducts were absent in the spectra.
Poly[butyl(phenyl)stannane] also formed under the action of Wilkinson’s
catalyst; however, precipitation from methanol or other solvents was not successful.
Sodium in liquid ammonia + + ‐
Snn
Sn
n
Sn
n
Sn
n
Wilkinson’s catalyst X +X ‐TMEDA + + X
‐+
113
Thus, the solvent was evaporated to leave the reaction products. The resulting
compound featured a wide molar mass distribution (cf. Table 2). 119Sn NMR
spectra showed a broad signal at -197 ppm, in the common range of
polystannanes.
Polymers containing two aromatic substituents per tin atom essentially could
not be obtained with Wilkinson’s catalyst, apart from a minor fraction of
poly(diphenylstannane) when the reaction was performed at 70 °C. The reaction
mixtures showed the complete disappearance of Sn-H vibrations at 1854 cm-1 in
IR spectra, and of the signals associated to Ph2SnH2 in 1H and 119Sn NMR
spectra, but red or brown solids or viscous oils arose as main products. These
products featured no signal in 119Sn NMR spectra and we failed to consistently
interpret the results of other analyses.
Polymerization with TMEDA
Reactions in the presence of TMEDA were performed in diethyl ether at
room temperature. While the conversion of dibutylstannane was not efficient (see
Experimental section), the reaction of diphenylstannane with TMEDA yields a
yellow precipitate. Elemental analysis revealed only few impurities in the
poly(diphenylstannane). As this polymer was insoluble in all solvents tested, the
molar mass could not be determined. Extraction of the products with
dichloromethane CH2Cl2 did not give rise to any signals in 119Sn NMR spectra,
indicating that cyclic oligo(diphenylstannane)s did not form in significant
quantities, since those species dissolve in the solvent. Butyl(phenyl)stannane and
bis(4-butylphenyl)stannane polymerized to highly viscous oils which were soluble
in common organic solvents. The monomer concentration was varied between
10 g/L and 40 g/L and the reaction time between 10 min and 45 h. Suitable
114
conditions were found at a concentration of 10 g/L (26 mmol/L) and a reaction
time of 30 min for bis(4-butylphenylstannane). The vanishing of the Sn-H
vibration at 1850 cm-1 in IR spectra of reaction mixtures indicated complete
conversion of this monomer. For the polymerization of butyl(phenyl)stannane also
monomer concentrations of 10 g/L (39 mmol/L) was applied but the reaction
time was elongated to 40 min to yield the highest molar masses. Under these
conditions, weight-average molar masses (Mw) of 46 kg/mol and 13 kg/mol were
found for the two polymers (Table 2). The polydispersity index (PDI) amounted
for poly[butyl(phenyl)stannane] to 1.8 – 2.1, i.e. to a value frequently obtained for
radical polymerizations and polycondensations, while the PDI of poly[bis(4-butyl-
phenyl)stannane] had a value of 3.2 – 3.3. It appears that the molar mass, at least
of poly[butyl(phenyl)stannane], decreased upon increase of the reaction time
beyond 30 min. Higher monomer concentrations resulted in lower molar masses.
Poly[bis(4-butylphenyl)stannane] and poly[butyl(phenyl)stannane] featured a
signal at -197 ppm in 119Sn NMR spectra, i.e. in the typical region of
polystannanes. Again, no evidence for formation of cyclic oligomers was observed.
Polymerization with sodium in liquid ammonia
The monomers with two aromatic groups connected to the tin atom were
better suited for polymerization in liquid ammonia than the alkyl substituted
monomers. Conversion of dichlorodiphenylstannane with two molar equivalents of
sodium resulted in immediate precipitation of a shiny yellow product. The material
obtained was insoluble even at elevated temperature. The product was extracted
with hot CH2Cl2 to detect soluble reaction byproducts such as cyclic
oligostannanes by 119Sn NMR analysis, but no indication on their formation was
115
found. Elemental analysis of the material was in agreement with the composition
of poly(diphenylstannane).
Polymerization of dichlorobis(4-butylphenyl)stannane resulted in a product
with a bimodal molar mass distribution but an unsatisfactory elemental analysis.
119Sn NMR experiments displayed a signal at -197 ppm indicating the presence of
polystannane. Also in the case of poly[butyl(phenyl)stannane), 119Sn NMR
spectroscopy revealed a broad signal at -197 ppm and did not show the presence of
cyclic byproducts.
Treatment of dichlorodibutylstannane with two molar equivalents of sodium
in liquid ammonia caused immediate precipitation of a yellow product. Yet 119Sn
NMR spectra disclosed not only a broad signal at -190 ppm representing linear
polystannane but also signals of cyclic oligostannanes at -202 ppm and -203 ppm.
3. Materials Properties
The molar masses determined for the soluble polymers by GPC analysis are
summarized in Table 2.
Table 2. Weight-average molar mass (Mw kg/mol) and polydispersity indices (PDI) of polystan-
nanes prepared by different synthetic methods.
Wilkinson’s cat. TMEDA NH3/Na
Mw PDI Mw PDI Mw PDI
(SnBu2)n 57 2.2
no polymer obtained
5 b) 2.3
(SnBuPh)n a) a)
46 2
<5 ~2.5
[Sn(4-BuPh)]n no polymer obtained 13 3.2 8c) 1.5c)
a) Broad molar mass distribution with the highest value detected at about 30 kg/mol and a high fraction of low molar mass products as low as 1.5 kg/mol. b) Contains also cyclic oligomers. c) Bimodal molar mass distribution; the value only refers to that of the high molar mass fraction.
116
Figure 1. GPC traces of the products of the reactions intended to generate (a) poly(dibutyl-
stannane), (b) poly[butyl(phenyl)stannane] and (c) poly[bis(4-butylphenyl)stannane], synthesized
with Wilkinson’s catalyst (solid), TMEDA (dashed) and sodium in liquid ammonia (dotted).
The respective values (Mw between 5 kg/mol – 60 kg/mol) and polydispersity
indices (around 2 – 3), of the polystannanes obtained in this work are in the range
of those reported previously [11]. Clearly, the polymerization method strongly
influenced the molar mass, as reflected by the GPC traces shown in Figure 1. The
a
103 104 105 106 107
Inte
nsity
RI
Molar Mass / g mol-1
103 104 105 106 107
Inte
nsity
RI
Molar Mass / g mol-1
103 104 105 106 107
Inte
nsity
RI
Molar Mass / g mol-1
1 10 100 1000
1 10 100 1000
1 10 100 1000molar mass / kg mol-1
molar mass / kg mol-1
molar mass / kg mol-1
Inte
nsity
RI
Inte
nsity
RI
b
c
Inte
nsity
RI
Snn
Snn
Snn
117
highest molar masses were obtained for poly(dibutylstannane) synthesized with
Wilkinson’s catalyst (57 kg/mol) and poly[bis(4-butylphenyl)stannane] prepared in
the presence of TMEDA (46 kg/mol). Rather low molar masses (with respect to
the soluble polymers) were obtained with the Na/NH3 synthetic route.
In the case of poly(diphenylstannane), material properties were investigated for
the polymer that featured the best values in elemental analysis, i.e. the polymer
resulting from the Na/NH3 synthesis method. For studies of the other polymers,
those obtained with the method which yielded the highest molar masses were
used. Degradation temperatures, relative stability, wavelength at maximum
absorption in UV/Vis spectra (λmax) and dichroic ratio of polymer films produced
by shearing, solubility and phase transitions are summarized in Table 3, and
discussed in the following.
Table 3. Selected material properties of polystannanes synthesized with the suitable method.
compound: (SnBu2)n [SnBu(Ph)]n [Sn(4-BuPh)2]n (SnPh2)n
synthesized with: Wilk. Cat. TMEDA TMEDA Na/NH3
molar mass [kg/mol] 57 13 46 -
degradation temperature [°C] 250 300 320 350
thermal phase transitions [°C] 0 no phase transition observed
absorption max. UV/Vis [nm] 390 410 420 470
dichroic ratio after shearing 2 2.1 1 1.7
solubility common organic solventsa) not solublea)
a) e.g. dichloromethane, toluene, hexane, diethyl ether, THF
118
Thermal properties
Thermogravimetric analysis (TGA) of the polystannanes (Figure 2) clearly
showed increasing thermal stability with increasing aromatic content in the
polymers. The decomposition temperature increased markedly from 250 °C for
poly(dibutylstannane) to 350 °C for poly(diphenylstannane). At 400 °C,
decomposition appears to be complete for all polystannanes. However, it is evident
that the residual mass is clearly below the mass fraction of tin in the polymers; this
can be explained by formation of volatile organotin compounds at elevated
temperatures.
Differential scanning calorimetry revealed the previously described phase
transition of poly(dibutylstannane) synthesized with Wilkinson’s catalyst [10],
while no phase transition was observed between -50 °C and 200 °C for the other
polymers.
Figure 2. Thermogravimetric analysis (TGA) of poly(dibutylstannane) (solid), poly[butyl-
(phenyl)stannane] (dotted), poly[bis(4-butylphenyl)stannane] (dash-dot) and poly(diphenyl-
stannane) (dashed).
Temperature / °C
wei
ght /
%
100 200 300 400 500
20
40
60
100
80
0
119
Relative stability at ambient temperature
Visual examination of solutions and solid polymers showed again, as in the
case of thermal stability, that polystannanes with two aryl groups bound to each tin
atom were more stable at ambient than polymers which contained Sn-alkyl bonds.
Degradation, as judged by the loss of the characteristic yellow color, was far slower
for the poly(diarylstannane)s than for the poly(dialkylstannane)s and
poly[alkyl(aryl)stannane]s. Obviously, not only one but two aryl groups are
required to provide enhanced stability at ambient.
UV/Vis spectra and dichroic ratio of sheared poly(stannane)s
Poly(dibutylstannane) with pure σ-delocalization showed an absorption
maximum (λmax) at 390 nm, i.e. in the range common for polystannanes [16, 34].
UV/Vis absorption spectra reveal that phenyl groups induce a bathochromic shift
of the wavelength at maximum absorbance (Table 3 and Figure 3), which might be
associated with increasing delocalization of electrons with increasing number of
aromatic groups in the polymer. For instance a bathochromic shift of 20 nm was
found for poly[butyl(phenyl)stannane] in comparison to poly(dibutylstannane).
However, apart from the nature of the side chains, also the conformation of the
polystannane main chain may influence λmax. This phenomenon is well known for
polysilanes [35] and was also described for polystannanes, where delocalization was
found to be at a maximum for planar zig-zag structures [14, 36]. Thus, part of the
poly(diphenylstannane) might be present in the planar zig-zag conformation,
leading to a low band gap polymer with a pronounced bathochromic shift of the
absorption maximum. In contrast to poly(diphenylstannane) no pronounced
bathochromic shift was observed for poly[bis(4-butylphenyl)stannane]. Apparently
120
Figure 3. Optical absorption spectra of poly(dibutylstannane) (solid), poly[butyl(phenyl)stannane] (dotted),
poly[bis(4-butylphenyl)stannane] (dash-dot), and poly(diphenylstannane) (dashed) recorded for thin films
on glass slides and arbitrarily adjusted in intensity for facile comparison.
the 4-butylphenyl side group inhibits the planar zig-zag conformation and there-
fore a further bathochromic shift.
Optical microscopy investigations of samples placed between crossed polarizers
revealed that all polymers, except poly[bis(4-butylphenyl)stannane], could readily
be oriented (Figure 4); preferred orientation in the direction of shear was also
evident from UV/Vis spectroscopy using polarized light. Light was preferentially
absorbed for a parallel position of the polarization plane to the orientation
direction of the polymer, leading to dichroic ratios around 2 at the absorption
maximum (Table 3).
When subjected to shear, poly[bis(4-butylphenyl)stannane] featured hardly
any alignment at all (Figure 4b; Table 3). Notably, in contrast to the other
polymers which exhibited the consistence of soft powders, poly[bis(4-butyl-
phenyl)stannane] was a highly viscous oil. Apparently the quasi-liquid state of
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
[nor
mal
ised
]
Wavelength / nm
0.0
0.6
0.4
0.2
1.0
0.8R
el. A
bsor
banc
e
400 500 600 700 800Wavelength / nm
300
121
Figure 4. Polarized optical absorption spectra of sheared films of poly[butyl(phenyl)stannane] (a)
and poly[bis(4-butylphenyl)stannane] (b) at different angles φ between the polarization plane of
the light and the shearing direction (parallel: φ=0 °, perpendicular: φ=90 °). (c) Optical microscopy
images of sheared films taken between crossed polarizers at angles of 45 ° to the polarizers and 0 °
to one of the polarizers (i.e. 90 ° to the other polarizer). The films were deposited on glass slides.
this polymer allowed rapid rearrangement of the polymer chains back into the
isotropic state during shearing.
Solubility
As already indicated above, poly(dibutylstannane) , poly[butyl(phenyl)-
stannane] and poly[bis(4-butylphenyl)stannane] were soluble in common organic
solvents (e.g. CH2Cl2, toluene, hexane, THF). In fact, it is not uncommon that
alkyl groups provide solubility to polymers, mainly as a result of entropy gain upon
dissolution - due to an increase in mobility of alkyl groups upon transition from
the solid to the dissolved state. By contrast, however, no solvent for poly(diphenyl-
stannane), the only polymer without alkyl groups, could be found.
0°45°Wavelength /nm
Abs
.
300 500400 600
Wavelength /nm
Abs
.
300 500400 600
a b c
0°
90°
45°
0°
90°45°
122
4. Conclusions
Each of the three polymerization methods here explored is particularly suited
for the synthesis of specific polymers. The route employing Wilkinson’s catalyst is
most beneficial for the preparation of poly(dibutylstannane), TMEDA for
polystannanes containing at least one aromatic group per Sn atom and Na/NH3 for
polystannanes with two aromatic groups per Sn atom. With the most suited
method, polymers of weight-average molar masses in the range of roughly 10
kg/mol to 60 kg/mol were obtained, depending on the particular structure of the
macromolecules.
Not surprisingly, the material properties are strongly influenced by the
substituents along the polymeric chains. Poly(diarylstannane)s exhibited higher
thermal stability and were more resistant at ambient than the other two
polystannanes, while butyl groups, also in 4-butylphenyl, improved the solubility.
Finally, phenyl groups resulted in a bathochromic shift, which might be due to
delocalization of electrons as well as the particular conformations of the polymer
chains. Except in the case of poly[bis(4-butylphenyl)stannane], which was present
in a liquid-like state, the polystannanes could readily be oriented.
123
5. Experimental
Materials. Ammonia was purchased from PanGas (Dagmarsellen, Switzerland,
99.999 %), dichlorodibutylstannane from ABCR GmbH (Karlsruhe, Germany)
and dichlorodiphenylstannane from Sigma Aldrich (Buchs, Switzerland). Both
substances were recrystallized twice by dissolving in boiling pentane and
subsequent precipitation of the product at -78°C. CD2Cl2 (99.9% D) was
purchased from Cambridge Isotope Laboratories (ReseaChem GmbH, Burgdorf,
Switzerland), and organic solvents from Fluka (Buchs, Switzerland). TMEDA was
dried with molar sieve; all other chemicals were used as received from the
respective chemical suppliers.
Methods. NMR spectra were recorded on a Bruker UltraShield 300 MHz/54 mm
Fourier transform spectrometer employing standard 5 mm broad band probes. The
samples were dissolved in CD2Cl2. For the investigation of reaction solutions,
D2O-capillaries was inserted into the NMR tube. In order to inhibit
decomposition of the samples by ambient light, the NMR tubes were wrapped in
white tissue and subsequently covered with aluminum foil which was only removed
immediately before inserting the samples in the spectrometer. Elemental analysis
were performed by the Microelemental Analysis Laboratory of the Department of
Chemistry at ETH Zürich. Gel permeation chromatography was performed with a
GPC instrument from Viscotek (VE7510) equipped with degasser, VE1121
solvent pump, VE520 autosampler and Model 301 triple detector array. A PL gel
5 μm Mixed-D column from Polymer Laboratories Ltd. (Shropshire, United
Kingdom) was used. Preliminary test showed that the refractive index detector
revealed the most reproducible values. Therefore, the reported data refer to molar
masses obtained with this detector. For calibration, atactic poly(styrene) standards
from Fluka were employed. Samples were dissolved in THF with 2.5 % v/v
124
toluene which served as marker. The THF eluent flow amounted to 1 mL/min.
For optical microscopy a Leica DMRX polarizing microscope was used at 20 fold
magnification. UV/Vis measurements were performed in transmission with a
Perkin Elmer Lambda 900 spectrophotometer equipped with rotating polarizers.
The polymers were applied as films on glass slides. Infrared spectra were recorded
with a Bruker Vertex 70 FTIR spectrometer with the attenuated total reflection
(ATR) technique by using a Si-crystal. The samples were directly deposited on the
crystal with a syringe or a spatula. Differential scanning calorimetry (DSC)
analysis were performed with a DSC822e instrument (Mettler Toledo, Greifensee,
Switzerland) equipped with an intracooler, and thermal gravimetric analysis
(TGA) with a TGA/SDTA851e from Mettler Toledo under nitrogen
atmosphere. The heating and cooling rates were 5 °C/min.
Synthesis of the starting materials
Tetrakis(4-butylphenyl)stannane
42.6 g (0.2 mol) 4-butylphenylbromide were treated with 5.8 g (0.24 mol)
magnesium in 300 mL THF and heated under reflux for 1 h to obtain 4-butyl-
phenylmagnesium bromide. Subsequently, the mixture was cooled to 0 °C and
4.7 mL (0.4 mol) SnCl4 suspended in 200 mL THF were added before heating for
another hour under reflux. Thereafter, the THF was removed in vacuo and the
product extracted with hexane in a soxhlet extractor. The solvent was removed in a
rotary evaporator and the resulting product dried in vacuum (0.1 mbar, 12 h).
Yield: 80 %; 1H NMR (299.948 MHz, CDCl3, in ppm): δ = 0.4-1.0 [t, 12 H],
1.3-1.5 [m, 8 H], 1.6-1.7 [m, 8 H], 2.55-2.6 [t, 8 H], 7.2-7.3 [d, 8 H], 7.8-7.9 [d,
8 H]; 13C NMR (75.50 MHz, CDCl3, in ppm): δ= 13.93, 22.45, 33.73, 35.82,
135.2, 129.0, 137.5, 143.7; 119Sn NMR (111.96 MHz, CDCl3, in ppm): δ= -125.
125
Dichlorobis(4-butylphenyl)stannane
25.8 g (0.04 mol) tetrakis(4-butylphenyl)stannane and 4.6 mL (0.04 mol)
SnCl4 in 200 mL heptane were heated under reflux for 4 h. The hot solution was
filtered to remove inorganic side products and the solvent was removed in a rotary
evaporator. Thereafter, the product was dried in vacuum (0.1 bar, 12 h). Yield:
83 %; 1H NMR (299.948 MHz, CDCl3, in ppm): δ = 0.94-1.01 [t, 6 H], 1.2-1.4
[m, 4 H], 1.5-1.6 [m, 4 H]; 2.5 [t, 4 H]; 7.1-7.2 [d, 4 H]; 7.6 [d, 4 H]; 119Sn
NMR (111.96 MHz, CDCl3, in ppm): δ = -19.6.
Bis(4-butylphenyl)stannane
0.5 g (0.013 mol) LiAlH4 were suspended under nitrogen atmosphere in
150 mL degassed diethyl ether. 6 g (0.013 mol) dichlorobis(4-butylphenyl)-
stannane were placed in a dropping funnel and dissolved in 100 mL degassed
diethyl ether. This solution was added dropwise while cooling the reaction mixture
to 0 °C. Subsequently, the reaction mixture was stirred for one hour. Unreacted
LiAlH4 was neutralized by drop wise addition of 100 mL degassed water. The
organic phase was separated with a cannula and washed with 200 mL degassed
saturated aqueous disodium tatrate solution. Afterwards the organic phase was
dried with CaCl2 for 30 minutes. The solution was filtered and the diethyl ether
was removed. The product was purified by drying in vacuo (about 0.1 mbar) for 1h.
The product was a colourless liquid. No melting point could be measured. Due to
instability the compound was finally stored in brown-colored septum vials at 4 °C.
Yield: 51%; 1H NMR (299.948 MHz, D2O, in ppm): δ = 0.94-0.99 [t, 6 H],
1.3-1.4 [m, 4 H], 1.5-1.6 [m, 4 H]; 2.5-2.6 [t, 4 H]; 7.2 [d, 4 H]; 7.6 [d, 4 H],
6.3 [s, 2 H, 1J(H-119/117Sn) = 1907/1821 Hz]; 119Sn NMR (111.96 MHz, D2O, in
ppm): δ = -234.3.
126
Butyltriphenylstannane, dichlorobutyl(phenyl)stannane and butyl(phenyl)-
stannane were synthesized according to literature [37-39].
Reactions of diorganostannanes with Wilkinson’s catalyst
In a typical reaction 0.04 mmol (3 mol% with respect to diorganostannane)
Wilkinson’s catalyst were placed under nitrogen atmosphere in a Schlenk tube and
dissolved in 10 mL of toluene. The flask was wrapped with white tissue which was
subsequently surrounded by aluminum foil before 1.3 mmol of the respective
diorganostannane were added dropwise with a syringe through a septum.
Reaction mixtures with dibutylstannane were stirred for one hour before
cooling to -78 °C in an isopropanol/dry-ice bath and then poured into 50 mL of
precooled methanol at -78 °C. Poly(dibutylstannane) precipitated and was filtered
off under nitrogen atmosphere and dried in vacuo (24 h, 0.1 mbar). Elemental
analysis (in % w/w, calculated values in brackets): C 40.82(41.25), H 7.53 (7.79).
Obtained molar masses: 57 kg/mol (corresponds to approximately 250 n-Bu2Sn
units);
Applying the above conditions to diphenylstannane yielded a red solid with an
elemental composition well apart from that of poly(diphenylstannane). Elemental
analysis (in % w/w, calculated values in brackets): C 43.86 (52.81), H 3.42 (3.69).
If the reaction solution was heated to 70 °C for one hour after adding
diphenylstannane and stirring of about 5 min at room temperature, a minor
quantity of yellow precipitate formed which was filtered and dried under reduced
pressure (0.1 mbar). The composition of this product was in the range of the
corresponding polymer. Elemental analysis (in % w/w, calculated values in
brackets): C 51.89 (52.81), H 3.69 (3.69). When the filtrate was cooled to -78 °C
and poured into 100 mL methanol of the same temperature, a light yellow solid
127
precipitated which turned immediately red upon filtration. Elemental analysis (in
% w/w, calculated values in brackets): C 43.82 (52.81), H 3.42 (3.69).
The reaction mixture of bis(4-butylphenyl)stannane was also warmed to 70 °C
after stirring for one hour at room temperature. Subsequently the solution was
cooled to -78 °C and poured into 100 mL methanol; As no precipitate formed, the
solvents were removed completely in vacuo (0.1 mbar) resulting in a product with
an elemental analysis different from that of poly[bis(4-butylphenyl)stannane].
Elemental analysis (in % w/w, calculated values in brackets): C 52.69 (62.38),
H 5.53 (6.80).
Catalytic dehydrogenation of butyl(phenyl)stannane was performed at room
temperature. The reaction solution was stirred for two hours and subsequently
cooled to -78 °C and poured into 50 mL methanol at the same temperature. The
solvent was removed and the product dried under reduced pressure (0.1 mbar,
24 h). Elemental analysis (in % w/w, calculated values in brackets): C 46.05
(47.49), H 5.33 (5.58).
Reactions of diorganostannanes with N,N,N’,N’- tetramethylethylenediamine
(TMEDA)
In a typical experiment 0.5 mmol monomer were placed under nitrogen
atmosphere in a schlenk tube and dissolved in 10 mL diethyl ether. The flask was
wrapped with white tissue which was subsequently surrounded by aluminum foil
before 0.5 mmol of TMEDA were added with a syringe. After a given time
(between 10 min and 45 h) the solvent was evaporated. Polymers were dried in
vacuo (ca. 0.1 mbar, 24 h).
128
Poly(dibutylstannane): After stirring the reaction mixture over night the
polymer was obtained in 10 % yield according to 119Sn NMR spectroscopy. The
rest of the starting material remained unreacted.
Poly[butyl(phenyl)stannane]: Monomer concentrations between 39 mmol/L
(10 g/L) and 157 mmol/L (40 g/L) were used. Elemental analysis (in % w/w,
calculated values in brackets): C 46.85 (47.49); H 5.62 (5.58). Obtained molar
masses: 13 kg/mol (corresponds to approximately 51 BuPhSn units).
Poly(diphenylstannane): Elemental analysis (in % w/w, calculated values in
brackets): C 50.55 (52.81); H 3.90 (3.69).
Poly[bis(4-butylphenyl)stannane]: Monomer concentrations between
26 mmol/L (10 g/L) and 103 mmol/L (40 g/L) were used. Elemental analysis (in
% w/w, calculated values in brackets): C 63.05 (62.37); H 6.92 (6.80). Obtained
molar masses: 46 kg/mol (corresponds to approximately 120 (4-BuPh)2Sn units).
Reactions of dichlorodiorganostannanes with sodium in liquid ammonia
Sodium (8 mmol) was dissolved under nitrogen atmosphere in 90 mL of liquid
ammonia at -78 °C by stirring for 15 min. After the flask was wrapped with white
soft tissue and surrounded by aluminum foil, a quantity of
dichlorodiorganostannane (4 mmol) dissolved in 10 mL THF was slowly added
through a septum under continuous stirring. The polymer precipitated after about
10 to 15 seconds and the solution was stirred for another five minutes before the
ammonia was evaporated by warming the reaction solution to room temperature in
a nitrogen stream. Thereafter the THF was removed in vacuo (about 0.1 mbar).
The resulting solids were washed with 50 mL of a water/ethanol (9:1) mixture
until no chloride could be detected in the washing solution (usually 3-4 times,
129
until the addition of 5 mL saturated AgNO3 solution did not lead to the visible
formation of AgCl precipitates) and thereafter three times with 50 mL CH2Cl2.
Finally the product was dried in vacuo (about 0.1 mbar, 24 h).
Poly(dibutylstannane): Elemental analysis (in % w/w, calculated values in
brackets): C 40.75 (41.25), H 7.56 (7.79). Obtained molar masses: 5 kg/mol
(corresponds to approximately 22 Bu2Sn units).
Poly[butyl(phenyl)stannane]: Elemental analysis (in % w/w, calculated values
in brackets): C 44.90 (47.49); H 5.66 (5.58) Obtained molar masses: <5 kg/mol
(corresponds to approximately <13 BuPhSn units).
Poly(diphenylstannane): Elemental analysis (in % w/w, calculated values in
brackets): C 51.98 (52.81); H 3,66 (3.69).
Poly[bis(4-butylphenyl)stannane] : Elemental analysis (in % w/w, calculated
values in brackets): C 59.22 (62.37); H 6.43(6.80). Obtained molar masses:
8 kg/mol (corresponds to approximately 22 (4-BuPh)2Sn units).
130
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Chapter VI
Stability of Polystannanes Towards Light
135
1. Introduction
Polystannanes are a unique class of polymers as their backbone consists of
covalently bound metal atoms, which, to our knowledge, has not been reported so
far with any other metal element. These species were first described by Löwig [1]
but gained wide interest in the field of organometallic polymers only in recent
years [2-11], often with a view to their thermal, optical and electronic properties
[2, 5, 6]. However, a considerable drawback of these materials is their limited
stability towards light, in particular when in solution [4]. The light stability and
degradation of linear polystannanes was studied comprehensively only recently [4],
exemplary with poly(dialkylstannane)s, after a facile synthesis method for such
polymers was developed (Scheme 1). Results from this study indicated that,
depending on the solvent, cyclic oligo(dialkylstannane)s or reaction products with
the solvent formed as degradation products.
The stability of poly(diarylstannane)s against light is likely to considerably
deviate from that of poly(dialkylstannane)s, as it was reported that the
photochemical behavior of silanes and polysilanes with aryl groups differs
Scheme 1. Schematic representation of the synthesis and structure of (a) poly(dibutylstannane)
and (b) poly[bis(4-(butylphenyl)stannane].
Sn
H
H
Snn
[RhCl(PPh3)3]
-H2
Sn
Cl
Cl
Snn
Na, NH3
-NaCl
a b
136
significantly from that of related alkyl-substituted compounds [12-15].
Furthermore, replacement of methyl by phenyl groups in polysiloxanes is known to
result in enhanced stability [16-21]. Thus, we devoted this study to a comparison
of stability against light of polystannanes with aromatic and aliphatic groups,
exemplary for poly[bis(4-butylphenyl)stannane] and poly(dibutylstannane), and
attempted to unveil the mechanism of degradation.
Poly[bis(4-butylphenyl)stannane] was selected since unsubstituted poly(di-
phenylstannane) – contrary to poly(dibutylstannane) which dissolves in common
organic solvents like dichloromethane, toluene, tetrahydrofuran (THF) and
hexane - is a rather intractable species that does not dissolve in common solvents.
The derivatized form allows to compare the stability of poly(dialkylstannane)s and
poly(diarylstannane)s under equivalent conditions.
In addition to irradiation experiments of the polystannanes in different
solvents with light of defined wavelengths in a UV/Vis spectrophotometer and
under a so-called daylight lamp, also laser flash photolysis was employed. This
method was previously used extensively for the characterization of compounds
based on elements of the group 14 in the periodic table. In these studies photo-
degradation intermediates were investigated ranging from low molar mass alkyl-
and aryl-substituted carbon-, silicon-, germanium- and tin-centered compounds
[12, 13, 22] to cyclic and linear oligomeric and polymeric silanes with aryl and
alkyl side groups [14, 15, 23-30].
137
2. Results
UV/Vis Light Irradiation. Prior to the light exposure experiments, it was
established that poly(dibutylstannane) dissolved in tetrahydrofuran (THF,
stabilized and unstabilized) and in dichloromethane (CH2Cl2) exhibited a
pronounced absorption maximum at 390 nm [2-4], and poly[bis(4-butylphenyl)-
stannane] dissolved in the same solvents at 422 nm (Figure 1a), which were, hence
subsequently employed to monitor the stability of both polymers.
The stability against light of both polystannanes was studied by exposing
solutions of both polymers directly to the analyzing light of the UV/Vis spectro-
photometer by repeated scanning between 500 nm and 300 nm. The decrease in
absorbance of the samples after a series of scans indicated that the intensity of the
incident light beam was sufficient to cause degradation of the dissolved polymers.
The extent of decomposition of the polymers depended both on the structure of
the polystannanes, as well as the chemical nature of the solvent. In all cases,
however, stannane containing the aromatic substituent poly[bis(4-butylphenyl)-
stannane] was more stable than poly(dibutylstannane).
In a first set of experiments the absorption of poly(dibutylstannane) was
reduced by more than 50 % as compared to the initial absorbance already after less
than three scans, whereas poly[bis(4-butylphenyl)stannane] approached that
degree of reduction after 24 scans (Figure 1b), which represents roughly a 10-fold
increase in stability when comparing poly(diarylstannane) and poly(dialkyl-
stannane) under these conditions. After 10 subsequent scans, the absorbance at the
absorption maximum of poly(dibutylstannane) amounted to ca. 2 % of the starting
value only, while the absorbance of poly[bis(4-butylphenyl)stannane] remained at
138
Figure 1. UV/Vis spectra of poly(dibutylstannane) (grey) and poly[bis(4-butylphenyl)stannane]
(black) recorded after subsequent scans in unstabilized THF (a), where (b) shows the relative
absorbance at the absorption maximum wavelength versus the number of scans. Corresponding
spectra in stabilized THF (c) and in dichloromethane (d). Note that for the solutions in
unstabilized THF the first 10 scans are displayed (a) whereas in spectra (c) and (d) the first 100
scans are shown.
77 % (Figure 1b). Evidently, though, THF turned out to be a remarkably poor
environment to protect polystannanes against light. Interestingly, commercial
THF is typically stabilized with 250 ppm of 2,6-di-tert-butyl-4-methylphenol
(butylhydroxytoluene, BHT). Remarkably, solutions of polystannanes in such
stabilized THF featured a drastic increase in their stability towards light. After 10
scans, over 99 % of the initial absorbance was recorded both for poly(dibutyl-
stannane) and poly[bis(4-butylphenyl)stannane] (Figure 1c); after 100 scans
0.2
0.0
0.8
0.6
1.0
0.4
40 60 80 1001 20Number of scans
Rel
. Abs
orba
nce
max
imum
300 400 500350 450Wavelength / nm
0.2
0.0
0.8
0.6
1.0
0.4A
bsor
banc
e
300 400 500350 450Wavelength / nm
0.2
0.0
0.8
0.6
1.0
0.4
Nor
mal
ized
Abs
orba
nce
300 400 500350 450Wavelength / nm
0.10.20.3
0.0
0.50.6
0.7
0.4
Abs
orba
nce
a b
c d
1
10
1
100
1
100
THFInhib. f ree
THF + BHT
CH2Cl2
THFInhib. f ree
139
Figure 2. Gel permeation chromatograms displayed the decrease in molar mass of (a) poly[bis-
(4-butylphenyl)stannane] (8.5 kg/mol to about 4 kg/mol) and (b) poly(dibutylstannane) (from
15 kg/mol to about 8 kg/mol) dissolved in THF (stabilized) after the different light exposure
times indicated.
96 and 98 %, respectively, remained. The stability of the polymers dissolved in
dichloromethane was in between that of the polymers dissolved in stabilized and
non-stabilized THF. For instance, 87 % of the initial absorption was observed
after 100 scans for poly(dibutylstannane) and 98 % for poly[bis(4-butylphenyl)-
stannane (Figure 1d).
The stability of both dissolved polystannanes against irradiation with a so-
called “daylight-lamp” was monitored by gel permeation chromatography
(Figure 2). Dichloromethane and stabilized THF were employed as solvents (in
non-stabilized THF, degradation was too fast to be followed by GPC). As in the
case of irradiation in the UV/Vis spectrophotometer, results obtained with the
“daylight lamp” showed that the exposure times to reach the same level of
degradation were higher for poly[bis(4-butylphenyl)stannane] than for poly(di-
butylstannane) and lower in dichloromethane than in stabilized THF.
Retention Time / min
Inte
nsity
RI
8 6 4
20 min
0 min
Retention Time / min
Inte
nsity
RI
8 6 4
5 min
0 min
a b
140
GPC analysis revealed that poly(dibutylstannane) and poly[bis(4-butylphenyl)-
stannane] dissolved in stabilized THF decomposed via a continuous reduction of
their molar mass at increasing light exposure time (Figure 2a and b). Solutions of
poly[bis(4-butylphenyl)stannane] showed the same behavior in CH2Cl2, whereas
in the case of poly(dibutylstannane) GPC indicated that the molar mass did not
decrease – only its intensity, consistent with earlier reports [4].
Laser flash photolysis. Laser flash photolysis experiments were conducted with the
setup shown in Figure 3 with solutions of both polymers dissolved in
dichloromethane and stabilized THF. These experiments unveiled the time-
resolved change in UV/Vis absorbance after a laser pulse was applied on the
dissolved polymers. As is evident from the data presented in Figure 4, the
polymers degraded rapidly by irradiation with a laser flash of 355 nm light (third
harmonic of the Nd:YAG laser): the polystannanes are bleached instantaneously
(<1 μs) upon irradiation with the laser pulse. The photochemical degradation is
extremely fast, as shown in Figure 4a and b. Absorption changes in the first milli-
seconds after the initial bleaching process are only minor. Importantly, samples
used for photostability determinations were prepared such, that their initial
Figure 3. Setup for flash photolysis experiments with a laser pulse energy of 60 mJ/pulse at
355 nm. The intensity of the analysis light beam was reduced with a cut off filter at 370 nm and
grey filter with 5 % transmission to avoid degradation of the sample upon irradiation of the
analysis light.
Laserbeam
Cuvette
Analysis light
Filter
Shutter
Detector
141
Figure 4. Time-dependent absorbance difference after a laser pulse applied to a solution of
poly(dibutylstannane) dissolved in (a) dichloromethane and (b) THF, compared to poly[bis-
(4-butylphenyl)stannane] in (c) dichloromethane and (d) in THF.
absorbance at 355 nm was similar by humble variations in the concentration.
Therefore our experiments allow for the comparison of the photosensitivity of the
compounds. This boundary condition was not strictly applied to experiments with
focus on the time resolved absorption change. The bleaching at 370 nm and
420 nm can be used to quantify the loss of poly(dibutylstannane) (50 % with 1
laser pulse) and of poly[bis(4-butylphenyl)stannane] (25 % bleaching with 5 laser
pulses) respectively. Clearly, the photo-stability of the latter is much better despite
the fact that the initial damage by the laser pulse seems to be comparable.
0 2 4
a
c d
b
ΔA
bs.
ΔA
bs.
Time / ms
Time / ms
Time / ms
0
0
-0.2
-0.2
BuCH2Cl2
4-BuPhTHF
ΔA
bs.
0
-0.1
4-BuPhCH2Cl2
0 2 4 6 0 2 4 6
Δ
Abs
.
Time / ms
0
-0.1
BuTHF
0 2 4 66
142
Figure 5. Absorbance difference at the respective absorption maximum wavelengths after laser
flash photolysis of poly[bis(4-butylphenyl)stannane] in dichloromethane (solid line) and THF
(dashed line).
Even though all traces look somewhat similar in the millisecond time range,
fundamental differences are visible if the acquisition time is prolonged three orders
of magnitude, which explains the very different overall stability: while the
bleaching of poly(dibutylstannane) by the laser pulse was largely irreversible,
bleaching of poly[bis(4-butylphenyl)stannane] was reversible to a considerable
extent: after each laser pulse, the measured absorbance of the samples recovered to
90 % of the original value, both with THF and with dichloromethane as solvents
(see Figure 5; data not shown for poly(dibutylstannane) as no significant recovery
was observed).
0 2 4 6Time / s
ΔA
bsor
banc
e
0
-0.2
143
3. Discussion
As demonstrated in this study, the stability of polystannanes against exposure
to light strongly depends on the nature of the organic side groups. Poly[bis-
(4-butylphenyl)stannane] was found to be more stable towards light than poly(di-
butylstannane), in THF as well as in dichloromethane. Although investigations by
laser flash photolysis reveal that the initial photochemical damage is comparable
for both polymers, poly[bis(4-butylphenyl)stannane] “recovered” to a large extent
(about 90 % under the applied experimenttal conditions) within a period of a few
seconds after irradiation. While poly[bis(4-butylphenyl)stannane] was efficiently
degraded by photolysis, its degradation products seem to be able to re-form
polymer chains to a remarkable extent - which results in an apparent stabilization
of this polymer. However, no comparable recombination process was observed for
poly(dibutylstannane). Radical mechanisms may explain our findings. Homolytic
cleavage of a Sn-Sn bond at a random position in a polystannane chain may lead to
two chain ends bearing a radical each. In fact, a chain end with two aromatic
groups bound to a tin atom could readily be more stable and therefore exhibit a
longer lifetime than a chain end with two aliphatic groups due to delocalization of
the radical throughout the aromatic groups. Thus, chain ends with Sn-aryl groups
may be long-lived enough to recombine to a polymer chain, in contrast to chain
ends with Sn-alkyl groups (Scheme 2). The latter more rapidly degrades to cyclic
oligostannanes or by reaction with the solvent [4]. A radical mechanism is further
supported by the fact that degradation in THF is inhibited by the radical scavenger
2,6-di-tert-butyl-4-ethylphenol (BHT), which is used in commercial THF for the
prevention of peroxide formation. Probably, the inhibitor stops the deploymeriza-
tion by radical trapping. Remarkably, different degradation mechanisms of the
polymer molecules themselves were observed. GPC measurements revealed that
144
Scheme 2. Schematic of the principal reactions involved in the decomposition of polystannanes:
Scission of polymer chains by incident light under formation of two radical end groups;
recombination of radicals is the case of poly(diarylstannane)s and the depolymerization in the case
of poly(dialkylstannane)s.
upon irradiation the molar mass of poly[bis(4-butylphenyl)stannane] in CH2Cl2
and THF is subsequently reduced. The same result was found for poly(dibutyl-
stannane) in THF. However, the decomposition of poly(dibutylstannane) in
dichloromethane lead to a decrease in the number of polymer molecules but no
substantial reduction in molar mass of the remaining polymer molecules. This
indicates two different degradation mechanisms – depolymerization by an unzip-
ping mechanism for poly(dibutylstannane) in CH2Cl2 as proposed earlier [4] and
random scission of polymer chains in the other cases. The unzipping of poly(di-
butylstannane) in dichloromethane could be favored by reaction of the tin radicals
with the solvent, as in this system related reaction products have been found [4].
Sn
R
R Sn
R
R Sn
R
R +n n - xx
Sn
R
R Sn
R
R Sn
R
R +nn - xx
Sn R
R Sn
R
R +
Chain scission
Recombination for R = Aryl
Unzipping for R = Alkyl
x x - ncyclo - (SnR2)n
hν
hν
Sn R
R Sn
R
R Sn
R
R
ClCl
+nhν
CH2Cl2 x - n
or
●
x
●
145
4. Conclusions
Poly[bis(4-butylphenyl)stannane], a representative of poly(diarylstannane)s,
was demonstrated to be more stable towards light than poly(dibutylstannane) - a
typical poly(dialkylstannane) – when dissolved in dichloromethane as well as in
THF. While degradation was found to proceed rapidly in unstabilized THF, the
radical scavenger 2,6-di-tert-butyl-4-methylphenol (BHT) strongly reduced the
rate of degradation, indicating that degradation of polystannanes proceeds via a
radical process. Results obtained with laser flash photolysis indicate that the
observed enhanced stability of the polymer with aromatic substituents is, in fact,
not due to a higher stability of Sn-Sn bonds but due to recombination of radicals.
By contrast less stable radicals generated in the polystannanes with aliphatic side
groups lead to rapid degradation of the macromolecular chains (Scheme 2).
Analyses of reaction solutions by GPC unveiled that two different decomposition
mechanisms may occur - random scission of polymer chains or unzipping; the
latter might be supported by reaction of polystannane radicals with solvent
molecules.
146
5. Experimental
Materials. Ammonia was purchased from PanGas (Dagmarsellen, Switzerland,
99.999 %) and dichlorodibutylstannane from ABCR GmbH (Karlsruhe,
Germany). The latter compound was recrystallized twice by dissolution in boiling
pentane and subsequent precipitation at -20 °C. Organic solvents were acquired
from Fluka (Buchs, Switzerland), except the inhibitor-free tetrahydrofuran (THF),
which was ordered from Sigma-Aldrich (Buchs, Switzerland; typical commercial
THF is stabilized with 250 ppm of the radical scavenger 2,6-di-tert-butyl-4-
methylphenol).
Methods
Synthesis of poly[bis(4-butylphenyl)stannane]
Sodium (8 mmol) was dissolved in 90 mL of liquid ammonia at -78 °C by
stirring for 15 min. To this solution, a quantity of dichlorobis(4-butylphenyl)-
stannane (2 mmol) dissolved in 10 mL THF was added through a septum and
stirred for 30 min. Subsequently, the flask was completely wrapped with white soft
tissue and surrounded by aluminum foil; another portion of dichlorobis(4-butyl-
phenyl)stannane (2 mmol) dissolved in 10 mL THF was added through a septum.
The polymer precipitated after about 10 s to 15 s. After two minutes, the ammonia
was evaporated by warming the reaction solution to room temperature under a
nitrogen stream, and the THF was removed at room temperature in vacuo (about
0.1 mbar, 12 h). The resulting solid was dissolved in dichloromethane, insoluble
residues were filtered off, the solvent was removed and the residue dried again
(0.1 mbar, 24 h). The polymer (dissolved in THF) possessed a molar mass at the
peak maximum of gel permeation chromatography (GPC) diagrams (Mp) of
147
8.5 kg/mol, employing a PL gel 5 μm Mixed-D column from Polymer
Laboratories Ltd. (Shropshire, United Kingdom) with THF as eluent. The
calibration was performed with atactic poly(styrene) standards.
Synthesis of poly(dibutylstannane)
Poly(dibutylstannane) was synthesized according to previously reported
procedures [3, 5] by dehydropolymerization starting from dibutylstannane with
Wilkinson’s catalyst. It possessed a molar mass Mp of about 15 kg/mol according
to GPC analysis in THF.
UV/Vis Exposure
Solutions of ca. 0.01 mg/mL poly[bis(4-butylphenyl)stannane] or poly(di-
butylstannane) in THF (stabilized and unstabilized) or dichloromethane,
respectively, were prepared at room temperature and protected from light prior to
the exposure experiments. UV/Vis absorption measurements were performed with
a Perkin Elmer Lambda 900 (Schwerzenbach, Switzerland) spectrophotometer.
The experiments were conducted by means of scans between 500 nm and 300 nm
at a constant scan speed of 214.29 nm/min, a 5 nm slit, an integration time of
0.24 s and a data interval of 1 nm. All spectra shown in this report stem from a
series which was analyzed within the same week to ensure constant conditions
since the intensity of the lamp is also depending on the lamp’s life time.
148
Laser Flash Photolysis
Polymer solutions of a concentration of ca. 0.01 mg/mL in THF (stabilized)
and dichloromethane were used and thoroughly protected from light. 2 mL of the
solutions were transferred into quartz glass cuvettes and UV/Vis absorption spectra
were recorded. Subsequently, laser flash photolysis was carried out with the third
harmonic (355 nm) of a Brilliant B YAG laser (Quantel, Les Ulis, France) coupled
to an Applied Photophysics LKS 50 (Leatherhead UK) instrument. Briefly, in this
technique a sample is irradiated by a laser pulse while a single beam UV/Vis
spectrometer records time resolved spectral information, in our case with sampling
rates up to 100 MHz (setup see Figure 3). The 5 ns laser pulses had an energy of
60 mJ. Kinetic traces were recorded at 370 nm for poly(dibutylstannane) and
420 nm for poly[bis(4-butylphenyl)stannane]. Time resolved spectroscopy usually
utilizes strong light sources (here, a 150 W Xe-arc lamp) for the analyzing light
beam. Since this caused considerable photolysis, an electronic shutter, a 370 nm
cut-off filter and a grey filter with only 5 % transmission were set between light
source and sample to minimize polymer degradation by analysis light. After
irradiation of the samples by the laser pulse, UV/Vis spectra were recorded on a
dual-beam spectrophotometer to analyze the damage induced by the laser light.
“Daylight Lamp” Irradiation; GPC Analysis
Solutions of 2 mg/mL of poly[bis(4-butylphenyl)stannane] and poly(dibutyl-
stannane), respectively, were dissolved in the dark in stabilized THF and directly
measured with gel permeation chromatography. Subsequently the GPC vial with
the polymer solution was irradiated with an Osram Dulux S Luminux 7 W/860
(Daylight) lamp (Jeker Leuchten AT, Zurich, Switzerland) in a closed irradiation
149
box with a distance of 13 cm between sample and lamp, as also described
previously [4]. Irradiation times are indicated in the corresponding figures.
Measurements in CH2Cl2 were performed by irradiation of polystannanes
solutions (~20 mg/mL) in a schlenk tube and subsequent dilution of 0.2 mL in
2 mL THF in the GPC vial.
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Chapter VII
Conclusions & Outlook
153
Conclusions
A versatile method for the synthesis of polystannanes with aromatic groups
directly bound to the tin atoms was realized by coupling of dichlorodiphenyl-
stannane with sodium in liquid ammonia. The nature of the intermediates arising
in this reaction was explored in-situ by 119Sn NMR spectroscopy and exposure to
bromoethane. In contrast to the generally accepted view, diphenylstannide
dianions, Ph2Sn2-, were not formed in liquid ammonia. The species that were
present in the reacting medium were, as a matter of fact, tetraphenyldistannide,
(Ph2Sn-SnPh2)2- and hydrodiphenylstannide, HPh2Sn- (Scheme 1).
Scheme 1. Reactions of dichlorodiphenylstannane with sodium in liquid ammonia and subsequent
reaction with various compounds.
SnCl
ClPh
Ph
2 Na / NH3, (l)
4 Na NH3, (l)
Ph2SnCl2
R2SnCl2
EtBrSn
Et
EtPh
Ph
Sn Sn
Ph
Ph
Ph
Ph
Sn H
Ph
Ph
Sn
n
Ph
Ph
Sn
n
Ph
Ph
Sn
n
Ph
Ph
Sn
R
R m
154
Similar experiments with dichlorodibutylstannane yielded a mixture of four
compounds dissolved in the liquid ammonia, which were identified as HBu2Sn-
and (Bu2Sn-SnBu2)2- together with Bu3Sn- and another product - H2BuSn- or
(BuHSn-SnHBu)2- - that probably formed by exchange of alkyl groups (Scheme
2). Differences in the distribution of the in-situ formed products at dichlorodi-
organostannane : sodium ratios ranging from 1:3 to 1:10 were either not
significant (in the cases of the alkylstannanes) or moderate (in the case of the
investigated arylstannane). Migration of organic groups in in-situ formed
Scheme 2. Reactions of dichlorodibutylstannane with sodium in liquid ammonia and subsequent
reaction with various compounds. Note that the obtained polymers are not purely linear, but
branching occurs due to exchange of alkyl groups.
2 Na / NH3, (l)
4 Na NH3, (l)
Bu2SnCl2
R2SnCl2
EtBrSn
Et
EtBu
Bu
SnEt
BuBu
BuSn
Et
EtBu
Et
Sn
Bu
Bu n
Sn
Bu
Bu n
cyclo(SnBu2)
cyclo(SnBu2)
+
+
Sn Sn
Bu
BuBu
Bu
Sn
Bu
Bu
H
Sn
Bu
Bu
Bu
Sn SnBu
HH
BuSn H
H
Bu
Sn
n
Bu
Bu
Sn
R
R m
or
SnCl
ClBu
Bu
155
alkylstannides was observed in products resulting after exposure to bromoethane;
however, the reaction of the intermediates with bromoethane did not allow to
draw conclusions on the chemical structure of the intermediates, in contrast to
common practice.
Reaction mixtures comprising a dichlorodiorganostannane : sodium ratio of
1 : 2 (one-step synthesis), as well as conversion of the soluble intermediates with
dichlorodiorganostannanes (two-step synthesis) led to precipitation of
polystannanes. Characterization of the reaction products provided mechanistic
insight regarding the polymerization in liquid ammonia. The two-step
polymerization did not follow a step-growth mechanism but, rather, a radical
chain-growth process. Furthermore, the formation of cross-linked and branched
products, in the case of butyl intermediates, confirmed the migration of alkyl
groups during the reaction with sodium in liquid ammonia.
These polymerization routes allowed not only synthesis of poly(diaryl-
stannane)s, but also the formation of copolymers of the general structure poly-
(dialkylstannane-co-diarylstannane) (SnAlkyl2)n(SnAryl2)m. To create linear
polymers, it was essential to conduct the first reaction step, i.e. the formation of
stannides, with dichlorodiarylstannane to circumvent cross-linking and branching,
resulting from the migration of alkyl groups. Poly(diphenylstannane) thus
produced featured a UV/Vis absorption maximum at 480 nm assigned to σ-π-de-
localization of electrons and was a dichroic material which could easily be oriented.
The UV/Vis spectra of poly(dialkylstannane-co-diarylstannane)s displayed two
absorption maxima, attributed to the aromatic (~480 nm) and aliphatic (~400 nm)
moieties in the polymer chain.
The polymers obtained with the new synthesis route were compared to the
products produced with Wilkinson’s catalyst and TMEDA, in collaboration with
156
Scheme 3. Schematic of the principal reactions involved in the decomposition of polystannanes:
Scission of polymer chains by incident light under formation of two radical end groups;
recombination of radicals in the case of poly(diarylstannane)s and the depolymerization in the
case of poly(dialkylstannane)s.
Prof. Frank Uhlig and Marie-Luise Lechner from the TU-Graz. For this purpose,
different polymers were synthesized: poly(dibutylstannane), poly[butyl(phenyl)-
stannane], poly(diphenylstannane) and poly[bis(4-butylphenyl)stannane]. The
route employing Wilkinson’s catalyst was most beneficial for the preparation of
poly(dibutylstannane) and TMEDA for polystannanes containing at least one
aromatic group per Sn atom, whereas synthesis in Na/NH3 yield best results for
polystannanes with two aromatic groups per Sn atom – poly(diarylstannane)s.
Investigations on the stability of polystannanes in solution towards light were
performed with poly[bis(4-butylphenyl)stannane], which was demonstrated to be
Sn
R
R Sn
R
R Sn
R
R +n n - xx
Sn
R
R Sn
R
R Sn
R
R +nn - xx
Sn R
R Sn
R
R +
Chain scission
Recombination for R = Aryl
Unzipping for R = Alkyl
x x - ncyclo - (SnR2)n
hν
hν
Sn R
R Sn
R
R Sn
R
R
ClCl
+nhν
CH2Cl2 x - n
or
●
x
●
157
more stable than poly(dibutylstannane) - a typical poly(dialkylstannane) – when
dissolved in dichloromethane as well as in THF.
Results obtained with laser flash photolysis indicated that the observed
enhanced stability of the polymer with aromatic substituents was not due to a
higher stability of Sn-Sn bonds, but to recombination of the generated radicals. By
contrast, less stable radicals generated in the polystannanes with aliphatic side
groups led to rapid degradation of the macromolecular chains (Scheme 3).
Analyses of reaction solutions by GPC unveiled that two different decomposition
mechanisms may occur - random scission of polymer chains or unzipping; the
latter might be supported by reaction of the polystannane radicals with the solvent
molecules present.
158
Outlook
Enhanced stability and improved electronic properties could increase the range
of possible applications of polystannanes. To create such properties, a series of
structures are proposed in the following.
Promoted recombination of radicals in polystannanes
In Chapter VI, it was described that enhanced stability of poly(diaryl-
stannane)s in solution – when compared with poly(dialkylstannane)s - is due to the
recombination of stabilized aryl-tin radicals, that were formed by the action of
light. Therefore, to further increase the stability, either the life-time of the radicals
should be prolongued or their recombination promoted.
Recombination could be favored by external forces that keep the Sn radicals
close together. This could be realized by ligands that connect at least two tin
atoms. The geometry of the ligands should fit the Sn-Sn distance of 2.8 Å to avoid
strain in the molecule, which could actually decrease the chain stability. Recently,
the first syntheses of a six-membered ring with neighboring tin atoms was reported
(Scheme 4a) [1], as well as calculations of ring strain energies of three-, four- and
five-member rings were presented [2]. It was shown that the homodesmotic strain
energy decreases from 1,2-distannacyclopropane to 1,2-distannacyclobutane and
1,2-distannacyclopentane and that non of the rings is excessively strained.
Polymerization of these cyclic distannanes could be a first step to impart enhanced
stability to such polymers. Ideally, every tin-tin bond would be connected by a
ligand (Scheme 4b).
159
Scheme 4. (a) 1,2-distannnacycloalkanes reported in literature so far [1, 2]; (b) idealistic
schematic of a polystannane with each Sn-Sn bond additionally stabilized by an ligand.
Another concept would be incorporation of polymerizable side groups. This
could be achieved by either generating a polystannane with reactive side chains
which are polymerized (Scheme 5a), or reversely by producing a soluble polymer
with an active stannane group on each monomer unit which could be converted to
polystannanes subsequently (Scheme 5b). The latter is probably favorable due to
the higher stability of the C-C chains. With these attempts, also the mechanical
properties of the obtained products could be varied in a wide range, as they are also
influenced by the nature and molar mass of the supporting polymer.
Ladder polystannanes
Ladder components of the group 14 elements have been known already since
1927, when Zelinsky and Kozechkow first synthesized [2]ladderane (bicycle-
[2.2.0]hexane) by reduction of cis-1,4-dibromocyclohexane with sodium [3]
R R R R R R
R R R R R R
[ ]
a
b
160
Scheme 5. Schematic of polystannane stabilization concepts: (a) incorporation of reactive groups
in the polystannanes which are subsequently polymerized and (b) preliminary polymerization of
the supporting polymer with attached active stannane groups or stannane groups that can be
activated for subsequent polymerization.
Different derivates of [3]- and [4]ladderanes have been reported in 1964 [4], and
more recently longer ladderanes were synthesized by the repeated cycloaddition of
cyclobutadiene ([3], [5] and [7]ladderanes) or alternate cycloaddition of
cyclobutadiene and dimethylacetylenedicarboxylate ([n]ladderanes n=3,4,5,6,7,9).
The first silicon analogue, bicyclo[2.2.0]hexasilane ([2]ladder polysilane), was
reported in 1987 by Matsumoto et al.. In contrast to the carbon based systems the
Scheme 6. Structure of (a) [2]ladder stannane, (b) [3]ladder silane and (c) polyladderane.
a
b
Si
Si
Si
Si
Si
Si Si
Si
R R RR
RRRRR
RR
RSn
Sn
Sn
Sn
Sn
SnR R
RRR
RR
R R
R1 2 21 3
a b c
n
161
polycyclic silanes feature extended σ-electron delocalization along the backbone
and are, therefore, expected to possess intriguing electronic, optical and chemical
properties. Lowest energy absorption of ladder polysilanes are reported to arise at
464 and 483 nm for the [7]ladder polysilane and the [8]ladder polysilanes,
respectively. These molecules represent the most extended ladder polysilanes that
have been synthesized so far [5]. They show also a strong red shift in
photoluminescence compared to the corresponding linear silanes [6]. Besides their
electronic properties longer ladder polysilanes have the capability to generate
highly stable radical anions after reduction with alkali metals [7]. Only a few
ladder polygermanes are known so far. Surprisingly the UV/Vis absorption band
shows a blue shift from [3]ladder polysilane to [3]ladder polygermane [8, 9], but
the oxidation potentials of the germanium compounds are significantly lower than
those of the silicon analogous [9]. The first ladder oligostannane reported is
bicyclo[2.2.0]hexastannane ([2]ladder polystannane) [10]. It is air-stable in the
crystalline form and shows dramatic reversible thermochromic behavior, being pale
yellow at -196°C and orange-red at room temperature with an absorption
maximum at 360 nm.
Conjugated poly(carbo)stannanes
Polymers with covalent carbon-tin bonds in the backbone were first described
by Noltes and Van der Kerk 1961 [11], continued by a series of reports concerning
polycarbostannanes obtained by step growth polymerization of diorganotin
dihydrides and diolefins or diacetylenes [12-14]. With this structural concept, it
should be possible to create polycarbostannanes with σ-π-conjugation throughout
the polymer backbone. Probably, it would be necessary to insert some alkyl
sidechains to increase the solubility, as Noltes and Van der Kerk found insolubility
162
for some of the polymers produced [13]. Finally, conjugated polymer networks
could be synthesized by application of 1.3.5 – triethynylbenzene, as suggested in
Scheme 7.
Scheme 7. Synthesis routes towards (a) saturated polycarbostannanes, (b) conjugated polycarbostannanes
and (c) conjugated polycarbostannane networks.
Sn
H
R
R
H
SnR
Rn
SnR
R n
SnR
R
n
Sn
Sn
R
RR
R
+
+
+
163
References
[1] E. Zarl, J.H. Albering, R.C. Fischer, D. Flock, B. Genser, B. Seibt, F. Uhlig, Tin-containing
Indane and Tetralin Derivatives, Z. Naturforsch., 64b (2009) 1591-1597.
[2] N. Sandström, H. Ottosson, Heavy Group 14 1,(n+2)-Dimetallabicyclo[n.n.n]alkanes and
1,(n+2)-Dimetalla[n.n.n]propellanes: Are They All Realistic Synthetic Targets?, Chemistry – A
European Journal, 11 (2005) 5067-5079.
[3] N. D. Zelinsky, K.A. Kozeschkow, Synthese des Bicyclo-[0.2.2]-Hexans, Ber. Dtsch. Chem.
Ges. B, 60 (1927) 1102-1108.
[4] M. Avram, I.G. Dinulescu, E. Marica, G. Mateescu, E. Sliam, C.D. Nenitzescu,
Untersuchungen in der Cyclobutanreihe, XII. Zwei stereoisomere Dimere des Cyclobutadiens,
Chem. Ber., 97 (1964) 382-389.
[5] S. Kyushin, Y. Ueta, R. Tanaka, H. Matsumoto, Hexa-, Hepta-, and Octacyclic Ladder
Polysilanes, Chem. Lett., 35 (2006) 182-183.
[6] Y. Kanemitsu, K. Suzuki, S. Kyushin, H. Matsumoto, Optical-Properties of Small Silicon
Clusters - Chain, Ladder and Cubic Structures, Jpn. J. Appl. Phys., Par 2, 34 (1994) 101-103.
[7] S. Kyushin, Y. Miyajima, H. Matsumoto, Observation of Highly Stable Radical Anions of
Ladder Oligosilanes, Chem. Lett., (2000) 1420-1421.
[8] A. Sekiguchi, H. Naito, C. Kabuto, H. Sakurai, Synthesis and Characterization of
Cyclotetragemene and Ladder Polygermane with Functional-Groups, Nippon Kagaku Kaishi,
(1994) 248-252.
[9] H. Matsumoto, S. Kyushin, M. Unno, R. Tanaka, Syntheses, Structures, and Properties of
Ladder Oligosilanes and Ladder Oligogermanes, J. Organomet. Chem., 611 (2000) 52-63.
[10] L.R. Sita, R.D. Bickerstaff, Isolation and Molecular-Structure of the 1st
Bicyclo[2.2.0]Hexastannane, J. Am. Chem. Soc., 111 (1989) 3769-3770.
[11] J.G. Noltes, G.J.M. Van der Kerk, Studies in IVth Group Organometallic Chemistry VII -
Synthesis of Some Hetero-Polymers Containing Germanium, Tin and Lead in the Main Polymer
Chain, Rec. Trav. Chim, 80 (1961) 623-631.
[12] A.J. Leusink, J.G. Noltes, H.A. Budding, G.J.M. Van der Kerk, Studies in IVth Group
Organometallic Chemistry XVI - Synthsis of Organogermanium Compounds Containing the p-
Phenylene Group. Some Infrared Characteristics of p-Phenylene Derivatives of Silicon,
Germanium, Tin and Lead, Rec. Trav. Chim, 83 (1964) 844-856.
[13] A.J. Leusink, J.G. Noltes, G.J.M. Van der Kerk, Studies in IVth Group Organometallic
Chemistry XII - Linear Polyaddition Polymers Derived from p-Phenylene-bis-(dimethyltin
Hydride) and Diphenyltin Dihydride, Rec. Trav. Chim, 83 (1964) 609-620.
164
[14] J.G. Noltes, G.J.M. Van der Kerk, Studies in IVth Group Organometallic Chemistry VIII -
Synthesis of Linear Organotin Polymers from Organotin Dihydrides and Acetylenic Compounds,
Rec. Trav. Chim, 81 (1962) 41-48.
165
Acknowledgements
The present work would not have been possible without contributions of
numerous people and institutions which helped to conduct the research described
in this thesis the way it appears now.
First I want to thank Walter Caseri for his assistance and support throughout
the whole work, as well as his patience and endurance during the last months. No
less sincere gratitude belongs to Paul Smith for giving me the opportunity to be
part of this special group in a fantastic environment. I deeply appreciated to work,
learn and live with the given liberties and provided confidence.
Frank Uhlig (Technische Universität-Graz, Austria) is acknowledged for his
help concerning the chemistry of tin, providing stimulating discussions and for
asking the right questions at the right time, that provoked us to dive deeper into
the world of tin molecules and intermediates. The cooperation with the TU-Graz
offered the great opportunity for me to keep in contact with my home university.
I thank Wolfram Uhlig for accepting to be a co-examiner for this thesis and
his interest in my work.
Particularly acknowledged are Thomas Nauser (Department of Chemistry,
ETH Zürich) for the work performed with laser flash photolysis and his
contribution to the understanding of some important properties and Marie-Luise
Lechner for the fruitful collaboration, as well as Aitor Moreno and Heinz Rüegger
for their help with NMR experiments and the confidence given, especially
concerning the measurements in liquid ammonia in their laboratories. Also
Thomas Schweizer, Werner Schmidheiny, Marc Simonet and Martin Colussi,
contributed to the outcome of this thesis and I am grateful for their efforts.
166
I would like to thank my students Jérôme Zemp, Cédric Sax, Debora
Solenthaler, as well as Antoine Dorcier, Georgius Sotiriou and students from the
various practica who helped me to discover the diversity of polystannanes.
And of course I would like to express my thanks to all the people that
contributed to good atmosphere inside and outside our lab, and for the good times
we spent together, especially Karin Bernland and Pascal Wolfer, my lab mates
Irene Bräunlich, Joanna Wong and Stefan Busato, Felix Koch, lab chief Kirill
Feldman, Mr. Polystannane Fabien Choffat, Christian Müller, Kurt Pernstich,
Jérôme Lefèvre, Andreas Brunner, Susi Köppl, Theo Tervoort, Sara Fornera, Jan
Giesbrecht, Harald Lehman, Ueli Suter, Wolfgang Kaiser and Vappu Hämmerli.
I thank Debora Solenthaler and Rahel Bohlen for providing their picture of
poly(dibutylstannane-co-diphenylstannane) as cover for this thesis.
The Swiss National Science Foundation (Nr.: 200021_126450/1) is
acknowledged for financial support.
Am Ende möchte ich mich noch ganz besonders bei meinen Eltern für Ihr
Vertrauen und Ihre Unterstützung bedanken, sowie bei meiner Schwester und
meinem Götibueb Aron, der mir sehr viel Freude bereitet.
167
Curriculum Vitae
Markus Trummer was born on March 22, 1980 in Feldkirch, Austria. He grew up
in Schruns, attended the Gymnasium in Bludenz and graduated from the Höhere
Technische Lehranstalt (HTL) für chemische Betriebstechnik in Wels 1999. After
one year of civilian service he started his studies in Technical Chemistry at the
Technical University of Graz (TU-Graz) in 2000. During his study he completed
industrial internships at Getzner Werkstoffe GmbH (2002, 2003 and 2004) and
Chemson Polymer - Additive AG (2005). In 2006 he received the degree Diplom
Ingenieur (DI), after completing his Diploma thesis “Polyelectrolyte-clay
nanocomposites as dielectric materials” at the Institute of Chemical Process
Development and Control (Joanneum Research), in cooperation with the Institute
of Chemistry, University of Graz and the Institute for Chemistry and Technology
of Materials (TU-Graz). In 2007 he joined the Polymer Technology Group of
Prof. Paul Smith at the Department of Materials of the Eidgenössische
Technische Hochschule (ETH) Zürich where he conducted his doctoral studies
under the supervision of Prof. Walter Caseri.
Death and/or old age is coming.....we must live sweet.
After all, it is not only life, but the quality of this life.
Mike J. Libecki