Synthesis of metastable SiIIX2 solutions (X = F, Cl). A novel binary halide for synthesis

Article abstract of DOI:10.1002/zaac.201400122

Thermodynamic data for gaseous SiF₂ and SiCl₂ known from literature show that these binary subhalides are formed nearly quantitatively at 1376°C and 1076°C, respectively, as products of the reaction of elemental silicon with SiF₄ or SiCl₄ at 1×10^-2 mbar. Applying the co-condensation technique SiF ₂ as well as SiCl₂ can be trapped at -196°C and prepared in synthetic scale. Herein, first analysis of metastable SiF ₂ and SiCl₂ solutions are presented, showing that SiF ₂ is more reactive than SiCl₂ leading to compounds of low boiling point of so far unknown composition. The metastable SiCl₂ solution can be used as a SiCl₂ source, which is shown by the trapping reaction with Idipp [Idipp = 1, 3-bis(2, 6-iPr₂-C ₆H₃)imidazol-2-ylidene] leading to Idipp-SiCl₂. The reactivity of the solution might be fine-tuned by changing the donor opening the door for further applications. Copyright

Full text of DOI:10.1002/zaac.201400122

Job/Unit: Z14122
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Date: 30-04-14 12:50:05
Pages: 8
ARTICLE
DOI: 10.1002/zaac.201400122
Synthesis of Metastable SiIIX2 Solutions (X = F, Cl). A Novel Binary Halide
for Synthesis
Fabian Uhlemann,[a] Ralf Köppe,[b] and Andreas Schnepf*[a]
Keywords: Thermochemical calculations; Silicon; Subhalides; Thermodynamics
Abstract. Thermodynamic data for gaseous SiF2 and SiCl2 known
SiF2 is more reactive than SiCl2 leading to compounds of low boiling
from literature show that these binary subhalides are formed nearly point of so far unknown composition. The metastable SiCl2 solution
quantitatively at 1376 °C and 1076 °C, respectively, as products of the
reaction of elemental silicon with SiF4 or SiCl4 at 1ϫ10–2 mbar. Ap-
plying the co-condensation technique SiF2 as well as SiCl2 can be
trapped at –196 °C and prepared in synthetic scale. Herein, first analy-
sis of metastable SiF2 and SiCl2 solutions are presented, showing that
can be used as a SiCl2 source, which is shown by the trapping reaction
with Idipp [Idipp = 1,3-bis(2,6-iPr2-C6H3)imidazol-2-ylidene] leading
to Idipp-SiCl2. The reactivity of the solution might be fine-tuned by
changing the donor opening the door for further applications.
can be used for the synthesis of Idipp-SiX2 (X = Cl, Br, I),
which might be described as a stabilized form of the silicon(II)
halide. However the stabilization strongly reduces the reactiv-
ity of the halide so that it can be e.g. dissolved in tetrahydrofu-
rane (thf) without decomposition. Nevertheless starting from
the formal silicon(II) compound Idipp-SiCl2 or Idipp-SiBr2 a
variety of further reactions were possible leading e.g. to the
trisilaallene Si3(SitBu2Me)4,[9] the silaisonitrile dimer
(SiNAr)2 (Ar = 2,6-iPr2-C6H4),[10] or the NHC-stabilized
halosilyliden complex (C5R5)(CO)2Cr=SiBr-Idipp (R = H,[11]
Me[12]) indicating the synthetic potential of the Idipp-SiX2
compounds.[13] As the reactivity of the silicon(II) chloride is
strongly reduced by the NHC we asked ourselves if it is pos-
sible to obtain silicon(II) halides with weaker donors exhibit-
ing a fine-tuned reactivity. Thereby starting from the prototype
high temperature gas phase molecule seems most appropriate
as this compound already exhibits the correct anticipated oxi-
dation state of the silicon atom.
Introduction
Subvalent halides of silicon in oxidation state +2 with the
general formulae SiX2 (X = F, Cl, Br, I) exhibit a lone pair of
electrons at the silicon atom and possess a
1
+
∑
electronic
g
ground state. Such silylenes are key intermediates in a variety
of thermal and reduction reactions of silicon halides.[1] Hence
such silylenes have first been analyzed spectroscopically in the
gas phase during high temperature gas phase reactions.[2] First
reactions applying these high temperature molecules were re-
ported by Timms et al. in 1968, where they co-condensed SiCl2
as well as SiF2 with various reagents like acetylene, benzene,
PCl3, etc.,[3] showing that the expected high reactivity of the
silicon(II) halide can be used even to attack benzene to yield a
bicyclic product, where cyclo-butadiene is bridged by a (SiF2)n
group (n = 2,3), indicating that pure SiF2 is even able to react
with aromatic compounds.[4] Without a reagent SiX2 polymer-
izes to (SiX2)n or oligomerizes to (SiX2)n rings (n = 4–6) or it
disproportionates to Si and SiX4.[5]
In 1998 it was shown by West et al. that the cyclic perchloro-
silane Si4Cl8 can be transferred into the perchlorpolysilane
(SiCl2)x, which could be structurally characterized, showing a
Si–Si bound backbone.[6] Recently, Roesky et al.[7] and
Filippou et al.[8] showed that N-heterocyclic carbenes (NHC’s)
like Idipp [Idipp = 1,3-bis(2,6-iPr2-C6H3)imidazol-2-ylidene]
Results and Discussion
For the synthesis of metastable silicon(II) halide solutions
the preparative co-condensation technique seems most suitable
as one can start directly from the high temperature molecule
in the gas phase. This procedure has already been shown to
work for other high temperature molecules like the group 13
monohalides AlIX / GaIX (X = Cl, Br, I)[14] or the group 14
monohalides GeIX and SnIX (X = Cl, Br).[15] Thereby, the tem-
perature as well as the pressure dependence of the central high
temperature reaction SiX4(g) + Si(f) Ǟ 2SiX2(g) is calculated
using tabulated thermodynamic data (see below). The gas
phase molecules can afterwards be condensed with a variety
of different mixtures of solvents as well as donor molecules so
that a large spectrum of donor as well as solvent properties can
* Prof. Dr. A. Schnepf
E-Mail: andreas.schnepf@uni-tuebingen.de
[a] Institute of Inorganic Chemistry
University Tübingen
Auf der Morgenstelle 18
72076 Tübingen, Germany
[b] Institut für Anorganische Chemie
Karlsruher Institut für Technologie (KIT)
76131 Karlsruhe, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201400122 or from the au-
thor.
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F. Uhlemann, R. Köppe, A. Schnepf
ARTICLE
Table 1. Enthalpy and entropy changes of the independent reactions in the systems SiX4/Si (X = F, Cl). The standard values16 ΔfH°298 and fS°298
a)
taken for the calculations are given in the legend
.
X = F
X = Cl
ΔrH°298 /kJ·mol–1
ΔrS°298 /J·mol–1·K–1
ΔrH°298 /kJ·mol–1
ΔrS°298 /J·mol–1·K–1
Si(s) + 2X(g) = SiX2(g)
Si(s) + X(g) = SiX(g)
SiCl2(g) + 2X(g) = SiX4(g)
X2(g) = 2X(g)
–746.7
–99.5
–1185.8
158.8
–79.8
48.2
–291.4
114.8
–411.2
77
–736.7
242.6
–67.9
53.8
–280.8
107.3
a) ΔfH°298 (fS°298) values in kJ·mol–1 (J·mol–1·K–1) taken for the calculations: SiF: –20.1 (225.8); SiCl: 198.3 (237.8); SiF2: –578.9 (256.6);
SiCl2: –168.6 (281.3); SiF4: –1614.9 (282.8); SiCl4: –662 (330.9); F2: 0 (202.8); Cl2: 0 (223.1); Si(f): 0 (18.8); F: 79.4 (158.8); Cl: 121.3
(165.2).
be introduced into silicon(II) halide chemistry maybe gradually
lowering the reactivity of the silicon(II) halide.
Composition of the Gas Phase, Thermochemical
Calculations
By means of matrix isolation spectroscopy as well as by co-
condensation techniques we proved that metastable solutions
of subvalent group 13 monohalides are produced by high tem-
perature reaction of the condensed phase (aluminum or gal-
lium) with a gaseous component in a completely thermo-
dynamic controlled manner.[14] Therefore it is possible to de-
termine the composition of the gas phase over such a solid or
liquid by thermochemical calculations. Accurately determined
experimental standard values of enthalpy and entropy are
known for all the possible components of the present prob-
lem.[16] In the chemical equilibrium of SiX4 (X = F, Cl) over
solid or liquid silicon we assume the gaseous molecules X, X2,
SiX, SiX2, and SiX4 to appear depending on the temperature
and the pressure. The four independent reactions, their heats
of reaction ΔrH°298 and their entropy changes ΔrS°298 together
with the applied standard values ΔfH°298 and S°298 are given
f
in Table 1. The temperature dependence of ΔfH° and S° was
f
not taken into account – the error resulting from this approxi-
mation is negligible.
The partial pressures of the five gaseous components are
then determined iteratively according to the four expressions
of the equilibrium constants and the conditional equation (i.e.,
the given constant total pressure).[17] In Figure 1 the partial
pressure diagrams of the fluorine and chlorine systems are pre-
sented for a total pressure of 10–2 mbar, which is the typical
Figure 1. Diagram of the partial pressures of the components of the
SiX4/silicon system at a total pressure of 10–2 mbar; top: X = F; bottom:
X = Cl.
pressure for the high temperature reaction performed in a reac-
tor used for the preparative co-condensation technique.
Under these pressure conditions at a temperature between
In the case of the fluorine system the calculations thus show
1600 und 1700 K (X = F) as well as 1150 and 1700 K (X = that a temperature of ca. 1700 K is needed to obtain a nearly
Cl) the vapor over silicon consists of more than 95% of the quantitative amount of SiF2 in the gas phase. This temperature
divalent species SiX2. At 2000 K the equilibrium mixture of is close to the melting point of silicon (1687 K). Hence for
the system SiF4/Si contains 52% SiF2 and 48% SiF, whereas the synthesis an oven material is needed that can be used in
in the case of SiCl4/Si 62% SiCl2, 30% SiCl, and 8% Cl atoms combination with liquid silicon. Consequently the normally
are formed. In Figure 2 the relative amount p(SiX2)/ptotal with used pyrolytic graphite, which works well in the case of ger-
respect to the temperature and the total pressure of the system manium and tin[15] cannot be used as it reacts with liquid sili-
is presented. We found that – going from a total pressure of con at this temperature to give SiC.[18] Additionally liquid sili-
10–5, 10–2, to 10 mbar – the temperature of the highest amount con penetrates the normally used graphite.[19] Nevertheless
of SiX2 increases from 1350, 1650, to 2050 K for the fluorine glassy graphite can be used as crucible at this temperature as
and from 1100, 1350, to 1700 K for the chlorine system.
liquid silicon is not penetrating the glassy graphite and only
2
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Metastable SiIIX2 Solutions (X = F, Cl)
Figure 2. Diagram of the relative amounts p(SiX2)/ptotal with respect
to temperature and total pressure (top: X = F, bottom: X = Cl).
Scheme 1. Schematic presentation of the oven construction for the
synthesis of silicon(II) halides by the high temperature reaction Si +
SiX4 Ǟ 2SiX2 (X = F, Cl). A photo of the new inlets is given in the
Supporting Information.
reacting slowly with the surface to give SiC.[20] Thus we used
a modified oven construction, where silicon is placed within
glassy carbon inlets (Scheme 1).
Metastable SiF2 Solutions
In a first experiment SiF4 was reacted with elemental silicon
slightly below the melting point at 1365 °C.[21] Thereby
20 mmol SiF4 are applied and 7.5 mmol Si are consumed dur-
ing the reaction, leading to an amount of SiF2 in the gas phase
of only 38%. The gas phase molecules are condensed together
with a solvent mixture of toluene and P(nBu)3 as a donor com-
ponent at –196 °C. After the co-condensation reaction is fin-
ished the solid matrix is heated to –78 °C with dry ice and
after melting a pale yellow solution is obtained. On heating
this solution to room temperature the color intensifies leading
to a yellow-orange precipitate at the end that cannot be dis-
solved again. To get a first idea about the halide species pres-
ent in solution we took 15% of the –78 °C cold solution (ca.
4 mmol of silicon) and added the same amount of the carbene
C(NiPrCH)2 as a trapping reagent. From this reaction solution
we were able to obtain colorless crystals of the carbene adduct
SiF4[C(NiPrCH)2]2 (1) (1 mmol; ca. 25%), whose molecular
Figure 3. Molecular structure of SiF4[C(NiPrCH)2]2 (1), thermal ellip-
soids with 25% probability. Selected bond lengths /pm and angles /
°: Si1–F1:168.38(6); Si1–F2: 168.90(6); Si1–C2: 199.96(9); N1-C2:
135.8(1); C2–N3: 135.7(1); N3–C4: 138.4(1); C4–C5: 134.1(1); C5–
structure is shown in Figure 3. Thus with the aid of the carbene N1: 138.6(1); N1–C11B: 147.9(1); C11B–C11A: 152.1(2); C2–Si1–
C2a: 180.00(3); F1–Si1–F2: 90.48(3); N1–C2–N3: 104.71(8).
we were able to trap the unreacted SiF4 to give the hypercoor-
dinated silicon compound SiF4[C(NiPrCH)2]2 (1), which crys-
¯
tallizes in the hexagonal crystal system with space group R3, leading to a stretched octahedral arrangement of the four fluor-
where the silicon atom is positioned on the inversion center ine and the two carbene ligands around the silicon atom.
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F. Uhlemann, R. Köppe, A. Schnepf
ARTICLE
The Si–F (168 pm) as well as the Si–C (200 pm) distances
Nevertheless beside the SiF4 adduct we were not able to
in 1 are thereby similar to a lately described isostructural ad- isolate any further compound from the reaction mixture. Hence
duct SiF4[Idipp]2 (2) [Idipp = 1,3-bis(2,6-iPr2-C6H3)imidazol- the excess of unreacted SiF4 completely dominates the chemis-
2-ylidene], which was synthesized by Roesky et al. by the reac- try of the solution. To overcome this problem we increased the
tion of SiF4 with the bulkier carbene Idipp.[22] The main differ- oven temperature during the co-condensation reaction in small
ence between both compounds in the solid state is as follows: steps from 1365 °C to 1510 °C (Table 2). Thereby the amount
in the case of 1 both carbenes are oriented parallel, whereas in of silicon consumed during the reaction increases from
the case of 2 the carbenes are oriented perpendicular to each 7.48 mmol to 19.97 mmol.
other. However in the case of 1 this is only a packing effect
Table 2. Maximum oven temperature /°C applied and amount of sili-
con /mg (mmol) consumed during the high temperature reaction. In
every reaction 20 mmol of SiF4 are used.
as we were able to isolate a second sort (1b) of crystals of 1,
where toluene is incorporated in the crystal[23] and where the
carbenes are now oriented perpendicular to each other (Fig-
ure 4).[24]
T
1365
1430
1470
1490
1510
Si con-
sumed
210
(7.48)
305
(10.86)
464
(16.52)
515
(18.33)
561
(19.97)
Consequently in the case of the oven temperature of
1510 °C only SiF2 is present as a gas phase species. SiF2 is
condensed together with a solvent mixture of toluene and
P(nBu)3 or NEt3 as a donor component at –196 °C. After the
co-condensation reaction is finished the solid matrix is heated
to –78 °C with dry ice and after melting a nearly colorless
solution is obtained, which is quite reactive against moisture
and air leading to smoke generation. However at –78 °C it can
be stored without any obvious change. Heating this solution to
room temperature the formation of a polymeric membrane on
the surface is observed, indicating that a reaction takes place.
However attempts to collect any solid product out of this sub-
sequent reaction failed so far. Also the addition of the carbene
C(NiPrCH)2 as a trapping reagent did not give any conclusive
result and in the proton NMR spectrum only the free carbene
is observed and re-isolated in high yield indicating that no re-
action with a silicon species takes place. As it has already been
shown by Timms et al. SiF2 reacts with aromatic compounds
to give solid products, where cyclohexadiene is bridged by
Si2F4 or Si3F6 groups.[4] However when we evaporate the sol-
vent in vacuo only a small amount of a pale yellow residue
(ca. 60 mg) is obtained. Thus the subhalide must have reacted
to volatile products, which we were not able to isolate and
identify so far. As the reactivity of SiCl2 should be lower than
that of SiF2 we tried to synthesize a metastable SiCl2 solution,
where the subhalide is available for further reactions.
Figure 4. Molecular structure of SiF4[C(NiPrCH)2]2 (1b), thermal el-
lipsoids with 25% probability. Selected bond lengths /pm and angles /
°: Si2–F1:167.8(2); Si2–F2: 168.4(2); Si2–F3:168.4(2); Si2–F4:
167.7(2); Si2–C2: 203.1(3); Si2–C7: 202.3(3); C2–N1: 135.5(3); N1–
C5: 138.0(4); C7–N6: 136.2(3); N6–C10: 138.2(4); C4–C5: 134.0(4);
C10–C9: 133.0(4); C2–Si2–C7: 176.8(1); F1–Si2–F3: 178.68(10); F2–
Si2–F4: 179.65(9); N1–C2–N3: 104.3(2); N6–C7–N8: 104.3(2).
In the case of 1b the adduct 1 crystallizes in the monoclinic
crystal system in space group P21/c with two independent mol-
ecules in the asymmetric unit together with one toluene mole-
cule. As the silicon atom is not placed on a symmetry center
the fluorine atoms and the carbene carbon atoms are indepen-
dent from each other, however again a nearly perfect stretched
octahedral arrangement of the six substituents is realized as it
was the case for 1.
Metastable SiCl2 Solution
The orientation of the carbene ligands to each other thus
seems to have no significant influence on the bonding to the
When elemental silicon is reacted with 20 mmol of SiCl4 at
central silicon atom as the Si–F (168 pm) as well as the Si–C an oven temperature of ca. 1440 °C 557 mg (19.8 mmol) sili-
(203 pm) bond lengths in 1b are similar to the one in 1 within con are consumed leading thus to a quantitative yield of SiCl2
standard deviation.
in the gas phase. The gaseous SiCl2 is condensed together with
Additionally in solution both arrangements gave the same a solvent mixture of toluene and P(nBu)3 at –196 °C. After the
NMR spectroscopic signals, indicating that the different ar- co-condensation reaction is finished the solid matrix is heated
rangement of the carbene ligands in 1 and 1b is only a result to –78 °C with dry ice and after melting a pale yellow solution
of packing effects. The 29Si NMR signal appears thereby at is obtained, which is less reactive than the corresponding SiF2
–182 ppm, consistent with five or six coordinated silicon com- solution as the exposition to normal atmosphere did not lead
pounds[25] and the 19F NMR signal appears at –102.5 ppm. to smoke generation. However when this solution is heated to
29Si and 13C satellites are detected with 1J(19F–29Si) = room temperature the yellow color intensifies leading after a
2
187.4 Hz and J(19F–13C) = 61.5 Hz, showing that 1 stays in- couple of days to an orange precipitate of unknown composi-
tact in solution.
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Metastable SiIIX2 Solutions (X = F, Cl)
30 min to ca. 1300 °C. Afterwards the reactor was further heated to
the working temperature e.g. 1440 °C in the case of SiCl2. Afterwards,
SiX4 gas was passed through the reactor and the resulting gaseous
compounds were condensed together with a solvent mixture at a
–196 °C cold surface. The amount of SiX4 was measured by a flow
controller; thereby 20 mmol of SiX4 were used over the period of ca.
2 h, where additionally ca. 150 mL of solvent were condensed. Sub-
sequently heating was switched off and after the removal of the liquid
nitrogen cooling the whole apparatus was flushed with nitrogen gas
4.8. The reaction chamber was heated with dry ice to –78 °C. After
When a cold sample of the solution (ca. 4 mmol of silicon)
is reacted with the bulky carbene Idipp [Idipp = 1,3-bis(2,6-
iPr2-C6H3)imidazol-2-ylidene] we observe the formation of
SiCl2-Idipp (3), which can be identified by its proton NMR
signals. Hence the proton NMR spectrum of the reaction mix-
ture is in line with a 1:1 mixture of free carbene and 3. Thus
only 50% of the SiCl2 seems to be available for adduct forma-
tion. We were also able to crystallize Idipp-SiCl2 (3), which
is obtained as a second polymorph, i.e. it crystallizes in the
monoclinic crystal system in space group P21/n with one mole- the matrix melted the resulting solution was transferred by a steel can-
nula with nitrogen overpressure into a dry ice cooled Schlenk vessel.
cule in the asymmetric unit. However no differences concern-
ing Si–Cl and Si–C bond lengths are present with respect to
the previously published structure.[26]
Synthesis of SiF4[C(NiPrCH)2]2 (1, 1a): 1,3-Diisopropylimidazo-2-
ylidene (600 mg, 1 equiv., 4 mmol) was dissolved in toluene (20 mL)
at –78 °C and stirred for 10 min. To this solution the –78 °C cold SiF2
solution [20 mL, 40% SiF2, toluene/P(nBu)3, 4 mmol Si] were added
rapidly. The reaction mixture was warmed within 2 h to room tempera-
ture. After removal of all volatiles the remaining solid was washed
3 times with pentane (20 mL) and dried in vacuo to obtain 350 mg
(0.857 mmol, 21.4%) of pale beige solid. Crystallization of the solid
from thf gave colorless hexagonal plates of 1, while crystallization
from toluene gave rhombic plates of 1a. When the hexagonal crystals
were filtered off and dried in vacuo they got turbid, leading to a pale
yellow powder indicating that the crystal thf was evaporated. NMR
spectra of the powder indicated that only half a thf molecule per 1 was
left in the solid (NMR spectra are shown in the Supporting Infor-
mation) which is in line with results of the elemental analysis of this
Summary and Outlook
With respect to thermodynamic calculations SiF2 and SiCl2
should be obtained in maximum yield by the reaction of SiX4
(X = F, Cl) with elemental silicon at a reaction temperature of
1377 °C and 1077 °C, respectively, by a reaction pressure of
0.01 mbar. The synthesis of SiF2 and SiCl2 is done via a pre-
parative co-condensation reaction, where the high temperature
reaction is performed at 1510 °C for SiF2 and 1440 °C for
SiCl2 leading to quantitative yield of the silicon(II) halide in
the gas phase. The gaseous compounds are then condensed
together with a mixture of an inert solvent and a donor compo-
nent, e.g. toluene/P(nBu)3 at –196 °C to give a solid matrix.
After melting of the matrix a metastable solution of the subhal-
ide is obtained. In the case of the reaction system SiF4 / Si we
could show that at lower reaction temperatures a high amount
of SiF4 remains, which can be isolated from the solution after
reaction with the N-heterocyclic carbene C(NiPrCH)2 as the
bis-carbene adduct SiF4[C(NiPrCH)2]2, which crystallizes in
two different polymorphs (1 and 1b), where the carbene li-
gands are oriented planar or perpendicular to each other. SiF2
seems to be too reactive to be trapped by stabilizing ligands
and further experiments applying different solvents and donor
molecules during the co-condensation reaction are necessary to
transfer this subhalide from the gas phase to a useable reagent.
However in the case of SiCl2 the solvent mixture toluene/
P(nBu)3 seems useful to obtain a metastable solution of the
subhalide SiCl2. Thereby the reaction with the bulky carbene
Idipp gives Idipp-SiCl2 (3), which is also available by a dif-
ferent route using standard synthetic procedures as it has been
lately shown by Roesky et al.[7] However before trapping with
the carbene the subhalide solution shows a high intrinsic reac-
tivity,[27] which might be used in further applications, which
has to be shown in future experiments.
1
3
solid. H NMR (C6D6, 250 MHz, TMS ext.): δ = 1.25 (d, 24 H, J =
6.8 Hz, CHMe2), 6.37 [s, 4 H, J(1H29Si) = 190 Hz, CH], 6.40 (sept,
4
4 H, CHMe2) ppm. 13C NMR (C6D6, 62.9 MHz, TMS ext.): δ = 24.1
(CHMe2), 49.9 [4J(13C19F) = 4.3 Hz, CHMe2], 115.7 (CH), 171.1
[2J(13C19F) = 61.6 Hz, NCN] ppm. 19F NMR (C6D6, 376.48 MHz,
CFCl3 ext.): δ = –102.5 [1J(19F29Si) = 187.4, 2J(19F13C) = 61.5 Hz]
ppm. 29Si NMR (C6D6, 99.36 MHz, TMS ext.): δ = –182.3 [1J(29Si19F)
= 186.3 Hz] ppm. C18H32F4N2Si·0.5THF: calcd.: C 54.03, H 8.16, N
12.60%; found: C 53.93, H 8.03, N 12.28%.
Synthesis of SiCl2-Idipp (3): Bis(2,6-Diisopropylphenyl)imidazo-2-
ylidene (1.55 g, 1 equiv., 4 mmol) was dissolved in toluene (40 mL) at
–78 °C and stirred for 10 min. To this solution the –78 °C cold SiCl2
solution (20 mL, 100% SiCl2 in toluene/P(nBu)3, 4 mmol) was added
rapidly. The reaction mixture was warmed within 2 h to room tempera-
ture. After removal of all volatiles the yellowish solid was washed 3
times with pentane (20 mL) and dried in vacuo (940 mg). Slightly yel-
low crystals of 3 were formed in a concentrated toluene solution at
1
3
–30 °C. H NMR (C6D6, 400 MHz, TMS ext.): δ = 1.01 (d, 12 H, J
3
= 7.0 Hz, CHMe2), 1.43 (d, 12 H, J = 6.7 Hz, CHMe2), 2.79 (sept, 4
H, J = 6.8 Hz, CHMe2), 6.36 (s, 2 H, CH), 7.07 (d, 4 H, 3J = 7.9 Hz,
m-HPh), 7.22 (t, 2 H, J = 7.7 Hz, p-HPh) ppm.
3
3
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-991359 (1), CCDC-991360 (1b), and CCDC-991358
(3) (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
Experimental Section
Procedure of the Co-Condensation: The co-condensation reaction
was performed within a homemade co-condensation apparatus which
has been described elsewhere.[28] The apparatus was evacuated with an
oil diffusion pump (Balzers Diff 900) to a pressure of 5ϫ10–6 mbar. In
Crystallographic data and structure refinement results are summarized
in Table 3.
a modified graphite reactor (Scheme 1) elemental silicon was placed. Supporting Information (see footnote on the first page of this article):
For bake-out the graphite reactor was first heated inductively for Pictures of the thermal behavior of different SiCl2 solutions and of the
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F. Uhlemann, R. Köppe, A. Schnepf
ARTICLE
Table 3. Crystallographic data and details for structure solution of SiF4[C(NiPrCH)2]2 (1), SiF4[C(NiPrCH)2]2 (1b), and SiCl2-Idipp (3)
[Idipp = 1,3-bis(2,6-iPr2-C6H3)imidazol-2-ylidene].
Formula
SiF4[C(NiPrCH)2]2 (1)
SiF4[C(NiPrCH)2]2·C7H8 (1b)
Idipp-SiCl2 (3)
Crystal system
Space group
hexagonal
R3
monoclinic
P21/c
monoclinic
P21/n
¯
Unit cell dimensions
a /Å
b /Å
c /Å
24.5788(6)
25.5788(6)
11.1308(7)
26.995(3)
10.643(2)
17.784(2)
104.057(4)
4934.2(13)
4
1.224
0.14
1944
9.4284(7)
14.4275(10)
19.9208(12)
100.804(2)
2661.8(3)
4
1.217
0.306
1040
β /°
Volume /Å3
5823.4(4)
9
1.049
0.127
1962
Z
Density (calculated) /g·cm–3
Absorption coefficient μ /mm–1
F(000)
Crystal size /mm
Diffractometer
Temperature /K
Wavelength /Å
Theta range for data collection /°
Index ranges
0.08ϫ0.08ϫ0.05
BRUKER APEX II
150
0.1ϫ0.1ϫ0.02
BRUKER APEX II
200
0.4ϫ0.3ϫ0.3
BRUKER APEX II
150
0.71073
0.71073
0.71073
1.66-31.66
–36 Յ h Յ 36
–36 Յ k Յ 36,
–16 Յ l Յ 15
27663
1.56–25.00
–32 Յ h Յ 30
–8 Յ k Յ 12,
–21 Յ l Յ 21
27535
1.75–27.33
–12 Յ h Յ 11,
–13 Յ k Յ 18,
–23 Յ l Յ 25
14758
Reflections collected
Independent reflections
Reflections F2Ͼ2σ
Used programs
4372
3379
8651
5890
5946
4456
SHELXL / SHELXS, PLATON SHELXL / SHELXS
124
SHELXL / SHELXS
289
Used parameters
550
Final R indices [I Ͼ 2σ(I)]
R indices (all data)
0.0395
0.1085
0.0485
0.1213
0.0456
0.1094
Largest diff. peak and hole
0.353 and –0.284
0.288 and –0.388
0.704 and –0.678
[10] R. S. Ghadwal, H. W. Roesky, K. Pröpper, B. Dittrich, S. Klein,
G. Frenking, Angew. Chem. 2011, 123, 5486–5490; Angew. Chem.
Int. Ed. 2011, 50, 5374–5378.
[11] A. C. Filippou, O. Chernov, G. Schnakenburg, Chem. Eur. J.
2011, 17, 13574–13583.
oven design as well as the molecular structure and solid-state arrange-
ment of 3 are given together with NMR spectra of 1 and 3 and of the
reaction mixture: SiCl2[P(nBu)3] solution with Idipp.
[12] A. C. Filippou, B. Baars, O. Chernov, Y. N. Lebedev, G. Schnak-
enburg, Angew. Chem. 2014, 126, 576–581; Angew. Chem. Int.
Ed. 2014, 53, 565–570.
References
[1] K. Nakayama, C. M. Aldo, J. H. Weaver, Phys. Rev. B Condens.
Matter 1999, 15893–15901.
[2] J. W. C. Johns, G. W. Chantry, R. F. Barrow, Trans. Faraday Soc.
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[3] P. L. Timms in Cryochemistry (Eds.: M. Moskovitz, G. A. Ozin)
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1441–1442.
[13] Idipp-SiCl2 can also be used as a two electron donor in coordina-
tion chemistry. For a recent review see: R. S. Ghadwal, R.
Azhakar, H. W. Roesky, Acc. Chem. Res. 2013, 46, 444–456.
[14] C. Dohmeier, D. Loos, H. Schnöckel, Angew. Chem. 1996, 108,
141–161; Angew. Chem. Int. Ed. Engl. 1996, 35, 129–149.
[15] R. Köppe, A. Schnepf, Z. Anorg. All. Chem. 2002, 628, 2914–
2918; C. Schrenk, R. Köppe, I. Schellenberg, R. Pöttgen, A.
Schnepf, Z. Anorg. Allg. Chem. 2009, 635, 1541–1548.
[16] M. Binnewies, E. Milke, Thermochemical Data of Elements and
Compounds, Wiley, VCH, Weinheim, 2nd edn 2002.
[17] M. Binnewies, Chemische Gleichgewichte, Verlag Chemie,
Weinheim 1996.
[18] a) R. Wolf, J. Nucl. Mater. 1994, 212–215, 1174–1177; b) B. R.
Bathey, G. F. Hurley, H. E. Bates, J. Mater. Sci. Lett. 1980, 15,
2192–2196.
[19] A picture of a destroyed graphite reactor is shown in the Support-
ing Information.
[7] R. S. Ghadwal, H. W. Roesky, S. Merkel, J. Henn, D. Stalke, An-
gew. Chem. 2009, 121, 5793–5796; Angew. Chem. Int. Ed. 2009,
48, 5683–5686.
[20] I. C. Lewis, D. J. Miller, T. A. Pirro, Eur. Pat. Appl. 1993,
EP0529594A1.
[21] Due to the oven assembly a temperature gradient is present lead-
ing to a temperature range of 250 K between the hot spot, whose
temperature is given and the end of the oven, where the high
temperature gas phase molecules leave and where the temperature
is measured with a pyrometer.
[8] A. C. Filippou, O. Chernov, G. Schnakenburg, Angew. Chem.
2009, 121, 5797–5800; Angew. Chem. Int. Ed. 2009, 48, 5687–
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Chem. Int. Ed. 2013, 52, 6974–6978.
[22] R. S. Ghadwal, S. S. Sen, H. W. Roesky, G. Tavcar, S. Merkel,
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6
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Metastable SiIIX2 Solutions (X = F, Cl)
[23] In the case of 1 thf is incorporated within the crystal lattice.
The thf molecules are thereby strongly disordered so that
they could not be refined during crystal structure solution.
However SQUEEZE indicated that three thf molecules per
SiF4[C(NiPrCH)2]2 1 are present within the crystal lattice;
SQUEEZE: P. v. d. Sluis, A. L. Spek, Acta Crystallogr., Sect. A
1990, 46, 194–201.
[24] Also in the case of the sterically more demanding carbene Idipp
a less tilted orientation is possible, e.g. in the case of the cation
[Idipp-SiH3-Idipp]+ the tilt angle is only 37.7°: Y. Xiong, S. Yao,
M. Driess, Z. Naturforsch. B 2013, 68, 445–452.
[25] R. S. Ghadwal, R. Azhakar, H. W. Roesky, Acc. Chem. Res. 2013,
46, 444–456.
[26] A picture of the molecule and the packing in the solid state is
given in the Supporting Information.
[27] The reactivity of the metastable SiCl2 solution might be fine-
tuned by the stabilizing donor. Hence in a first orientating reaction
we compared the thermal behavior of a SiCl2 solution with tolu-
ene as the solvent and NEt3 or PnBu3 as the donor molecule.
Thereby obvious differences are seen (in the Supporting Infor-
mation a series of pictures are shown during the heating process
taken every 10 K): At the beginning there is no noticeable visual
change of both solutions. However at 0 °C the solution containing
NEt3 as the donor molecule becomes dull while the PnBu3 stabi-
lized SiCl2 solution becomes only slightly yellow, a color that
intensifies on heating to room temperature. In difference to that
in the case of the NEt3 stabilized SiCl2 solution a red-brown pre-
cipitate and a colorless solution is obtained on heating to room
temperature. In the case of the PnBu3 stabilized SiCl2 solution an
orange precipitate is formed only after several days. This result
hints to a higher reactivity of the amine stabilized solution with
respect to the phosphane stabilized one, which has to be eluci-
dated in future experiments.
[28] A. Schnepf, New J. Chem. 2010, 34, 2079–2092.
Received: March 18, 2014
Published Online: ᭿
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Pages: 8
F. Uhlemann, R. Köppe, A. Schnepf
ARTICLE
F. Uhlemann, R. Köppe, A. Schnepf* ............................... 1–8
Synthesis of Metastable SiIIX2 Solutions (X = F, Cl). A Novel
Binary Halide for Synthesis
8
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