Sn(I) halides: Novel binary compounds of tin and their application in synthetic chemistry

Article abstract of DOI:10.1016/j.jorganchem.2009.08.038

Metalloid cluster compounds are ideal model compounds for the area between the molecular and solid state, i.e. the nanometer regime. For the synthesis of metalloid cluster compounds, the disproportionation reaction of a metastable subhalide is a fruitful synthetic route. In the case of tin, monohalides are needed for this synthetic route as tin(II) halides are too stable to be used. Due to thermodynamic data, gaseous SnBr should be formed at 1370 °C, and by applying a co-condensation technique it can be trapped at -196 °C and prepared in synthetic scale. Herein first analyses of SnBr are presented, showing that SnBr is more reactive than the corresponding GeBr, already disproportionating quantitatively to elemental tin and SnBr₂ on heating to room temperature. By applying nitrogen-based donor molecules like NnBu₃ or pyridine, the reactivity can be moderated and the solubility is enhanced leading e.g. to an SnBr emulsion, which can be used for the synthesis of metalloid cluster compounds of tin.

Full text of DOI:10.1016/j.jorganchem.2009.08.038

Journal of Organometallic Chemistry 695 (2010) 941–944
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Sn(I) halides: Novel binary compounds of tin and their application
in synthetic chemistry
*
Andreas Pacher, Claudio Schrenk, Andreas Schnepf
Institut für Anorganische Chemie der Universität Karlsruhe, Engesserstraße 15, Geb. 30.45, 76128 Karlsruhe, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Metalloid cluster compounds are ideal model compounds for the area between the molecular and solid
state, i.e. the nanometer regime. For the synthesis of metalloid cluster compounds, the disproportionation
reaction of a metastable subhalide is a fruitful synthetic route. In the case of tin, monohalides are needed
for this synthetic route as tin(II) halides are too stable to be used. Due to thermodynamic data, gaseous
SnBr should be formed at 1370 °C, and by applying a co-condensation technique it can be trapped at
ꢀ196 °C and prepared in synthetic scale. Herein first analyses of SnBr are presented, showing that SnBr
is more reactive than the corresponding GeBr, already disproportionating quantitatively to elemental tin
and SnBr2 on heating to room temperature. By applying nitrogen-based donor molecules like NnBu3 or
pyridine, the reactivity can be moderated and the solubility is enhanced leading e.g. to an SnBr emulsion,
which can be used for the synthesis of metalloid cluster compounds of tin.
Received 4 August 2009
Accepted 26 August 2009
Available online 31 August 2009
Keywords:
Metalloid clusters
Tin
Subhalides
Thermodynamics
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
polyhedrons are even stable for the higher congeners of carbon
hinting to a possible route to ‘‘germa-fullerenes”.
Metalloid cluster compounds are ideal model compounds to get
an insight into the area between the molecular and solid state, i.e.
the nanometer regime [1]. This borderland is of particular interest
especially for metals or semi-metals as a drastic change of the
physical properties takes place from oxidized species (salts, non-
conducting behaviour) to the bulk phase (metals/semi-metals, con-
ducting/semi-conducting behaviour) [2]. For the heavier group 14
elements [3] this area is also of great technical interest as the mod-
ule size in computer industry reaches more and more the nanome-
ter range [4]. During the last years metalloid cluster compounds of
germanium of the general formulae GenRm (n > m) could be estab-
lished as a novel group of substances [5]. Thereby we have been
able to establish a fruitful synthetic route via the disproportion-
ation reaction of a metastable Ge(I) halide (GeBr or GeCl), leading
to cluster compounds with up to 14 germanium atoms in the clus-
ter core [6]. The structural characterization of the so far synthe-
sized compounds reveals that novel structural motives can be
realized on the way to the bulk phase of elemental germanium.
Thus the arrangement of the 10 germanium atoms in (THF)18Na6-
Ge10[Fe(CO)4]8 1 could be described as a centaur polyhedron [7],
a motif known from solid state chemistry for coordination number
10. However, in the case of 1, the polyhedron is empty. Addition-
ally the 14 germanium atoms in (THF)6Li3Ge14[Ge(SiMe3)3]5 2 [8]
form an empty polyhedron (Fig. 1), showing that larger empty
Due to these achievements in germanium chemistry we asked
ourselves if this synthetic concept might be extended to other
group 14 elements. Among these, tin is most interesting as here,
due to the more metallic character, a greater structural diversity
might be expected [3]. Some examples of metalloid cluster com-
pounds of tin could be synthesized by the groups of Power and
co-workers [9] and Fischer and co-workers [10] in recent years
via a reductive coupling reaction. Thereby the more metallic char-
acter of tin becomes obvious in the case of the metalloid cluster
compound Sn15Dipp6
3 (Dipp = NArSiMe3; Ar = 2,6-iPr2-C6H3),
where the central tin atom (Fig. 1) exhibits the coordination num-
ber 8 + 6 (tungsten type).
2. Results and discussion
The disproportionation reaction of a metastable tin halide leads
at the end to elemental tin and a halide in a higher oxidation state.
During this disproportionation reaction tin-rich compounds are
formed on the way to elemental tin as sketched in Scheme 1. These
intermedially formed metalloid cluster compounds might be kinet-
ically stabilized by substituting the halides by bulky ligands as the
reactive metal core is then shielded by a ligand shell. This synthetic
concept can lead to giant metalloid cluster compounds, first of all
2ꢀ
shown for group 13, as the metalloid clusters Al77[N(SiMe3)2]20
xꢀ
[11] and Ga84[N(SiMe3)2]20 (x = 3, 4) [12] are synthesized apply-
ing the disproportionation reaction of a metastable Al(I) or Ga(I)
solution, respectively.
* Corresponding author.
E-mail address: schnepf@chemie.uni-karlsruhe.de (A. Schnepf).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.08.038

942
A. Pacher et al. / Journal of Organometallic Chemistry 695 (2010) 941–944
Fig. 1. Molecular structure of selected metalloid cluster compounds of germanium (left) and tin (right) without hydrogen atoms: (THF)6Li3Ge14[Ge(SiMe3)3]5 2 and Sn15Dipp6
3 (Dipp = NArSiMe3; Ar = 2,6-iPr2-C6H3). The arrangement of the germanium as well as tin atoms is highlighted by a polyhedral presentation.
However, for a kinetic stabilization low reaction temperatures
are needed and therefore the well known Sn(II) halides do not rep-
resent suitable starting materials as e.g. SnBr2 is stable up to the
boiling point of 619 °C [13]. Consequently, to establish a synthetic
route to metalloid cluster compounds via a disproportionation
reaction, a more reactive subhalide is needed; a promising candi-
date might be a Sn(I) halide, which has been spectroscopically ana-
lyzed in the gas phase [14]. For the synthesis of this monohalide in
the gas phase drastic reaction conditions are needed, so a first esti-
mation based on quantum chemically computed thermodynamic
data shows that Sn(I)Br should be formed as the major gaseous
compound by reaction (1) at 1370 °C at a reaction pressure of
10ꢀ2 mbar [16].
For the synthesis, elemental tin is placed in the graphite reactor
(a in Fig. 2), which is heated inductively to ca. 1240 °C – measured
via a spectral pyrometer (c in Fig. 2). Afterwards HBr is passed
through the reactor and the resulting gaseous products are con-
densed together with an added solvent at ꢀ196 °C. The simulta-
neously formed hydrogen gas is pumped off by a high vacuum
pumping system (oil diffusion pump) leading to a constant pres-
sure of 5 ꢁ 10ꢀ5 mbar during the reaction. After the reaction, the
liquid nitrogen is removed and the apparatus is vented with nitro-
gen. During the slow warming, the internal matrix melts and the
resulting solution can be transferred into a cooled Schlenk vessel
by overpressure via a steel cannula (h in Fig. 2).
1
2
Snðl:Þ þ HBrðg:Þ ! SnBrðg:Þ
þ
H2ðg:Þ
ð1Þ
For the synthesis of SnBr in preparative scale the preparative
co-condensation is applied, which is useful to trap such reactive
species and which is performed in a home made apparatus, whose
principal construction is shown in Fig. 2.
Fig. 2. Schematic presentation of the co-condensation apparatus: (a) graphite
reactor; (b) stainless steel vessel; (c) optic with glass fibre to spectral pyrometer;
(d) induction coil; (e) solvent inlet; (f) copper cooling shield; (g) main lock valve;
(h) steel cannula; (i) HV = high vacuum.
Scheme 1. Schematic presentation of the synthetic concept of the synthesis of a
metalloid cluster compound of tin via the disproportionation reaction of
metastable subvalent subhalide.
a

A. Pacher et al. / Journal of Organometallic Chemistry 695 (2010) 941–944
943
When toluene is used as solvent, a colourless solution is ob-
tained, in which a black solid is suspended, that looks dark red in
small layers. During the reaction normally 17–19 mmol tin are
consumed when 20 mmol HBr are used, thus SnBr is formed nearly
quantitatively during the high temperature reaction. The black res-
idue can be filtered and dried under vacuum, while it is heated to
room temperature, leading to a dark grey solid, in which a Sn:Br ra-
tio of nearly 1:1 is found. However powder diffraction studies as
well as 119Sn Mössbauer spectroscopy show that the residue is a
1:1 mixture of Sn and SnBr2 indicating that the disproportionation
reaction (2SnBr ? Sn + SnBr2) has quantitatively taken place [16].
However the dark red colour at low temperatures hints to a certain
amount of subhalides. Consequently Sn(I)Br is more reactive than
the corresponding Ge(I)Br, which is stable in the solid state up to
90 °C [15].
tacts (Sn1–Br20: 356.0(2) pm, Sn1–Br30: 353.8(2) pm) are formed
leading to a distorted octahedral arrangement around the central
Sn atom, a similar situation as found in SnBr2ꢂ(acetone)2 and
SnBr2ꢂ(thf)2 [16].
The appearance of larger amounts of SnBr2 (1.7 g of 5) shows
that beside the reaction with dichloromethane leading to an or-
ange solid, a disproportionation reaction has taken place leading
to SnBr2. Interestingly, although the primary product was redis-
solved in thf, one pyridine molecule is present in the crystal, indi-
cating that pyridine is a stronger donor for tin subhalides than thf.
As chlorinated solvents are not inert against tin monohalides, we
changed to fluorinated ones as the C–F bond is more stable than
the C–Cl bond. Thus the co-condensation reaction with a 4:1 mix-
ture of
a,a,a-trifluorotoluene yields a suspension of a black residue
and a colourless solution. The solution is filtered and stored at
Further co-condensation reactions with solvent mixtures con-
taining oxygen-based donor molecules like acetone or thf show
that the disproportionation reaction is the favourable reaction
channel of Sn(I)Br. Thus no reaction products with e.g. acetone
are observed and only SnBr2ꢂ(acetone)2 and elemental tin are ob-
tained [16]. Thus we changed to nitrogen-based donor molecules
like pyridine and NnBu3 and also used a solvent that exhibits a high
dielectric constant, which in the case of Ge(I) halides has led to iso-
lable solutions [7]. First of all we performed a co-condensation
reaction with a 4:1 mixture of dichloromethane and pyridine, lead-
ing at ꢀ78 °C to a dark red solution, in which a small amount of an
orange solid is suspended. This result looks quite promising. How-
ever, after a couple of days, the colour of the solution vanishes even
at ꢀ78 °C leading to more orange precipitate, in which due to an X-
ray fluorescence analysis, a nearly equimolar amount of tin, bro-
mine and chlorine is present. The presence of chlorine indicates
that chlorinated solvents are not useful as they seem to react with
the subhalide leading to an orange precipitate. The solvent of the
overlaying solution was evaporated leading to a pale yellow resi-
due which was redissolved in thf. On cooling this solution to
ꢀ28 °C colourless rod-like crystals of SnBr2ꢂthfꢂpyridine 5 were ob-
tained (Fig. 3). In the crystal of 5, SnBr2 molecules are present
(Sn1–Br2: 261.96(15) pm, Sn1–Br3: 263.89(14) pm), which are
coordinated by a pyridine (Sn–N: 238.5(8) pm) as well as a thf
(Sn–O 270.8(3) pm) molecule. Additionally two longer Sn–Br con-
ꢀ15 °C leading to colourless rod-like crystals of SnBr2ꢂpyridine2
6
(figures can be found in the Supporting information). The black res-
idue exhibits a Sn:Br ratio of 1:1.18 being nearly a 1:1 mixture of
Sn and SnBr2. This result indicates that the disproportionation
reaction has quantitatively taken place; this might be due to the
high temperature (ꢀ35°), which has to be used with respect to
the melting point of
a,a
,a-trifluorotoluene of ꢀ35 °C.
Thus in the following we performed a co-condensation reac-
tion with a 4:1 mixture of toluene and NnBu3 leading at ꢀ78 °C
to a dark emulsion of a black oil in a colourless solution. When
this emulsion is treated with a ꢀ78 °C cold toluene solution of Li-
Si(SiMe3)3 and subsequently heated to room temperature a black
reaction solution is isolated with no residue remaining in the co-
condensation apparatus. After work-up procedures, dark red crys-
tals of Sn10[Si(SiMe3)3]6 7 are obtained [17], representing the first
metalloid cluster compound synthesized via a disproportionation
reaction of a Sn(I) halide and showing that the oily form of the
subhalide can be used for the synthetic route emphasized in
Scheme 1.
3. Conclusions
Sn(I) halides can be synthesized in preparative scale via a
co-condensation technique. These Sn(I) halides already dispropor-
Fig. 3. Left: molecular structure of SnBr2ꢂpyridineꢂthf 5; right: first coordination sphere of the tin atom within the crystal, the pseudo octahedral arrangement is emphasized
via a polyhedral presentation: (displacement ellipsoids with 50% probability).

944
A. Pacher et al. / Journal of Organometallic Chemistry 695 (2010) 941–944
tionate quantitatively to tin and Sn(II) halides when heated to room
temperature, being more reactive than the corresponding Ge(I)
halides. Nitrogen-based donor molecules are capable to stabilize
the monohalides, leading in the case of NnBu3 to an oily form of
the subhalide, which can be used for the synthesis of metalloid clus-
ter compounds of tin, as the reaction with LiSi(SiMe3)3 leads to the
metalloid cluster compound Sn10[Si(SiMe3)3]6 7, in which beside 10
tin atoms only 6 ligands are present and where the 10 tin atoms are
arranged in the form of a centaur polyhedron, being a novel struc-
tural motif in tin chemistry. This synthetic route might now be
extended to other substituents, leading to larger metalloid cluster
compounds, shedding more light onto the area between the molec-
ular and solid state. Furthermore changing the donor component
from hard nitrogen bases to softer phosphorous bases might further
stabilize the monohalide, leading to, at least at low temperature,
stable solutions.
less steel vessel is heated with dry ice to ꢀ78 °C. After the melting
of the matrix the resulting solution is decanted to a cooled
schlenk vessel (dry ice) via a steel cannula with overpressure.
4.1. SnBr2ꢂthfꢂpyridine 5
During the co-condensation reaction 2.13 g Sn (17.94 mmol)
and 20 mmol HBr are consumed. The gaseous products were con-
densed with
a solvent mixture of dichloromethane/pyridine
(4:1), leading to a dark red solution with a small amount of an or-
ange solid at ꢀ78 °C. At ꢀ78 °C the solution bleaches, while more
orange precipitate is obtained. The colourless solution is filtered
and concentrated to dryness. The pale yellow residue is dissolved
in thf leading to a pale yellow solution. From this solution colour-
less crystals (Table 1) of SnBr2ꢂthfꢂpyridine 5 are obtained after
storage at ꢀ28 °C (1.74 g, 22.5%).
4.2. SnBr2ꢂ(pyridine)2
6
4. Experimental
During the co-condensation reaction 2.42 g Sn (20.4 mmol) and
20 mmol HBr are consumed. The gaseous products were con-
The co-condensation reaction is performed in the apparatus
shown in Fig. 2 which is evacuated to a pressure of 5 ꢁ 10ꢀ6 mbar.
In the graphite reactor elemental tin is placed and the reactor is
heated inductively to 1240 °C. Afterwards, the liquid tin is treated
with a constant flow rate of HBr (0.2 mmol/min), leading to a con-
stant working pressure of ca. 5 ꢁ 10ꢀ5 mbar. The resulting gas-
eous products are condensed together with a solvent mixture
(ca. 150 ml) at the surface of the stainless steel vessel, which is
cooled to ꢀ196 °C with liquid nitrogen. After the reaction, the
inductive heating is switched off and the graphite reactor is al-
lowed to cool for ca. 20 min. Then the main lock valve is closed
and the apparatus is vented with nitrogen. Afterwards the stain-
densed with a solvent mixture of
a,a,a-tri-fluorotoluene/pyridine
(4:1), leading to a black suspension of a colourless solution and a
black residue at ꢀ35 °C. The colourless solution is filtered and
stored at ꢀ15 °C leading to colourless rod-like crystals (Table 1)
of SnBr2ꢂthf2 6 (510 mg, 5.72%).
Appendix A. Supplementary material
CCDC 742644 and 742645 contain the supplementary crystallo-
graphic data for compounds 5 and 6. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.08.038.
Table 1
Crystallographic data and structural refinement details of SnBr2ꢂthfꢂpyridine
5
SnBr2ꢂ(pyridine)2 6.
References
Compound formula
SnBr2ONC9H13
5
SnBr2N2C10H10
6
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Space group
Cell parameters [Å respectively °]
a
Monoclinic
P21/n
Orthorhombic
Pmn21
8.4698(17)
9.3422(19)
16.424(3)
100.82(3)
1276.4(4)
4
2.236
8.234
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15.110(3)
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645.9(2)
2
2.246
8.135
408
Z
Density [g cmꢀ3
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0.5 ꢁ 0.1 ꢁ 0.05 0.6 ꢁ 0.1 ꢁ 0.1
STOE IPDS
150
STOE IPDS
200
0.71073
2.95–26.68
4487
0.71073
2.52–26.76
6100
2669
2009
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H
Range [°]
Measured reflections
Independent reflections
1269
1253
Observed reflections with F2 > 2
r
Programs used
SHELXL and SHELXS
SHELXL and
SHELXS
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Number of parameters
127
73
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R-value [I > 2
R-value (all data)
Goodness of fit
r
(I)]
0.0630
0.1952
0.0336
0.0913
Remaining electron density minimum/
1.702 and
ꢀ2.292
1.093 and
ꢀ0.830
maximum [e Åꢀ3
]