Bis(stannyl)phosphanyl-substituted dichlorosilanes/germanes - Potential precursors for a novel strategy toward P-Si/Ge multiple bonds?

Article abstract of DOI:10.1002/ejic.200500646

A general synthetic route to bis(stannyl)phosphanyl-substituted dichlorosilanes/germanes of the type R-SiCl₂-P(SnMe₃) ₂ and R-GeCl₂-P(SnMe₃)2 is reported. For R = Cp* (= 1,2,3,4,5-pentamethylcyclopentadienyl) crystal structures for the corresponding silane and germane could be obtained. These compounds are the first structurally characterized bis-(stannyl)phosphanyl-substituted dichlorosilanes/germanes that due to their functionalities should be ideal precursors for the generation of unsaturated PSi and PGe units and possibly phosphasilynes and phosphagermynes. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200500646

FULL PAPER
DOI: 10.1002/ejic.200500646
Bis(stannyl)phosphanyl-Substituted Dichlorosilanes/Germanes − Potential
Precursors for a Novel Strategy Toward P–Si/Ge Multiple Bonds?
Marion Bender,[a] Edgar Niecke,*[a] Martin Nieger,[a] and Rudolf Pietschnig*[b]
Keywords: Silicon / Tin / Phosphorus / Hyperconjugation
A general synthetic route to bis(stannyl)phosphanyl-substi-
tuted dichlorosilanes/germanes of the type R–SiCl2–
P(SnMe3)2 and R–GeCl2–P(SnMe3)2 is reported. For R = Cp*
(= 1,2,3,4,5-pentamethylcyclopentadienyl) crystal structures
for the corresponding silane and germane could be obtained.
These compounds are the first structurally characterized bis-
(stannyl)phosphanyl-substituted dichlorosilanes/germanes
that due to their functionalities should be ideal precursors for
the generation of unsaturated PSi and PGe units and possibly
phosphasilynes and phosphagermynes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
GeCl3 (2), respectively. In order to introduce the bis-
(stannyl)phosphanyl group we reacted 1 and 2 with tris-
(trimethylstannyl)phosphane P[Sn(CH3)3]3 neat in the ab-
sence of any solvent. At elevated temperature the reaction
proceeds under chlorotrimethylstannane elimination, which
is continuously sublimed out of the reaction mixture. This
approach is necessary to avoid further exchange reactions
of the stannaphosphanes.[9,10] In both cases the desired
products (3a) and (4) are obtained in high yield. In addition
the related bis(stannyl)phosphanyl dichlorosilanes R-SiCl2–
P(SnMe3)2 [R = Mes (3b) (Mes = 2,4,6-trimethylphenyl),
Tip (3c) (Tip = 2,4,6-triisopropylphenyl)] have been pre-
pared which underlines the generality of this method
(Scheme 1).
Introduction
After the recent success of preparing stable disilynes,[1–3]
the synthesis of triple-bonded systems of silicon and germa-
nium towards other elements remains one of the major
challenges in main group chemistry. While examples for
transition-metal complexed silylynes and germylynes have
been reported,[4–6] triple bonds of silicon and germanium
towards other main group elements have not to date been
stabilized, despite theoretical predictions.[7,8] In this respect,
1,2-chlorosilane elimination has proven to be unsuccessful,
while the analogous chlorostannane elimination has not
been exploited for this purpose to date. This prompted us
to investigate whether chlorostannane elimination might be
a better alternative for generating unsaturated PSi and PGe
units.
In this contribution we report on the synthesis and struc-
tural characterization of Cp*–SiCl2–P(SnMe3)2 and Cp*–
GeCl2–P(SnMe3)2 (Cp* = 1,2,3,4,5-pentamethylcyclopen-
tadienyl). They are the first structurally characterized bis-
(stannyl)phosphanyl-substituted dichlorosilanes/germanes.
Owing to their functionalities, they should be ideal precur- Scheme 1. Synthesis of bis(stannyl)phosphanyl dichlorosilanes/ger-
manes 3a–c [E = Si; R = Cp* (3a), Mes (3b), Tip (3c)], 4 (E = Ge;
R = Cp*) from 1 (E = Si) and 2 (E = Ge).
sors for the generation of unsaturated PSi and PGe units
and possibly phosphasilynes and phosphagermynes.
Compounds 3a–c and 4 show an NMR signal in the high
field region of the 31P NMR spectra (Table 1), which is typi-
cal for stannylated phosphanes.[11] Due to the presence of
Results and Discussion
1
the isotopes 117/119Sn in the molecules, the JP,Sn coupling
To synthesize Cp*–SiCl2–P(SnMe3)2 (3a) and Cp*–
GeCl2–P(SnMe3)2 (4), we started from the corresponding
trichlorosilane Cp*–SiCl3 (1) and trichlorogermane Cp*–
constants can be determined directly from the satellite tran-
sitions in these spectra. These coupling constants provide
valuable information about the P–Sn bond situation which
is significantly affected by the electronic behavior of the
other substituents at the phosphorous atom. In principle,
changing to a more electron-withdrawing substituent (R)
should decrease the s-character of the P–Sn σ-bond in com-
pounds of this constitution and therefore the 1JP,Sn coupling
[a] Institut für Anorganische Chemie, Rheinische Friedrich-
Wilhelms-Universität Bonn,
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
[b] Institut für Chemie, Karl-Franzens-Universität Graz,
Schubertstraße 1, 8010 Graz, Austria
E-mail: rudolf.pietschnig@uni-graz.at
380
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Bis(stannyl)phosphanyl-Substituted Dichlorosilanes/Germanes
FULL PAPER
constants. Surprisingly at first sight, we observe just the op- displaced by a bis(stannyl)phosphanyl group (Figure 1).
posite trend in our series of compounds (Table 1). In the The phosphorus atom shows a pyramidal coordination en-
same direction the 31P NMR shift values were found to be vironment with an angular sum of 309.83(7)°. The silicon
shifted downfield. Similar observations have been made for atom shows a slightly distorted tetrahedral environment
other stannylphosphanes.[9–11] A notable feature is the rela- with a Cl–Si–Cl angle of 100.6(1)° and a Cl(2)–Si–P angle
1
tively high JP,Si coupling constant observed in 3a–c, which of 114.31(9)°. The central Si–P bond is at 2.209(2) Å short
can be interpreted as an indication of strong electronic in- compared to standard bond lengths (2.24–2.28 Å) and the
teraction within the silylphosphane unit. This is also sup- sum of the covalence radii (2.28 Å).[15,16] Again this con-
ported by the short Si–P bond length found in the crystal traction can be understood assuming hyperconjugative in-
1
structure of 3a. Usually typical JP,Si values for silylphos- teraction of occupied σ-states assigned to the P–Sn bond
phanes range between 16 and 53 Hz [exception LiP- with vacant σ*-states derived from the Si–Cl bonds.[13,14]
(SiH3)2, 256 Hz].[12] This strong electronic interaction This interaction should be concomitant with an elongation
within the silylphosphane unit can be understood assuming of the Si–Cl bonds, which in fact is observed. The Si–Cl
hyperconjugative interaction of occupied states at the phos- distances are at 2.078(2) and 2.080(2) Å longer than the
phorus atom with vacant σ*-states derived from the Si–Cl standard bond length of 2.02 Å.[15] As a consequence of
1
bonds.[13,14] The decrease of JP,Si on going from 3a to 3b,c such hyperconjugative interaction also the Sn–P bond
might be explained by the reduced phosphorus s-character length should be affected. However, the values are at
of the P–Si bond due to an increase of the electron demand 2.505(1) and 2.510(1) Å for 3a in the typical range for P–
at the silicon atom. In line with these interpretations, the Sn single bonds.[23] The significant elongation of the Si–Cl
31P chemical shift of 4 appears at a lower field than that of bonds with no structural change of the P–Sn bonds might
3a, since germanium is more electronegative than silicon. be an indication that the hyperconjugative interaction of the
Similarly, the formal sp2 centers attached to silicon in 3b,c phosphorus lone pair with σ*-states derived from the Si–Cl
should be more electron withdrawing than the formal sp3 bonds might in fact be more significant in this case. From
carbon atoms in 3a resulting in a deshielding of the phos- a Newman projection along the Si–P bond, the eclipsed
phorus center. Somewhat more subtle is the situation for conformation becomes obvious (Figure 2). The lone pair of
1
the JP,Sn coupling constants for which an increase is ob- the phosphorus atom points towards the Cp* moiety. The
served with increasing electron demand at the E(IV) center. almost perfect synperiplanar arrangement of the stannyl
This trend is most likely a result of two counteracting tend- groups relative to the Cl atoms deviates only slightly, with
encies. Again a higher electron demand on E(IV) should dihedral angles Sn–P–Si–Cl of 0.2° and 7.4°. Within these
result in reduced phosphorus s-character of the P–Sn bond planes the Sn···Cl distances are at 3.639 and 3.706 Å shorter
and therefore lower 1JP,Sn coupling constants. This however, than the sum of the van der Waals radii (Figure 3).[17]
is not observed, and therefore this effect is obviously not
the dominant mechanism. The dominant effect leading to
1
the observed increase of the JP,Sn coupling constants
might, however, be attributed to the above-mentioned hy-
perconjugative interaction of occupied states at the phos-
phorus atom with vacant σ*-states derived from the Si–Cl
bonds. Such occupied states could involve the σ-P–Sn
bonds or alternatively the phosphorus lone pair. Increasing
the electron demand on the E(IV) center should also in-
crease this hyperconjugative interaction. Therefore, the si-
multaneous interaction of the phosphorus lone pair (pre-
dominant s-character) and the σ-P–Sn bonds with the σ*-
states at the E(IV) atom might be responsible for an in-
1
crease of the JP,Sn coupling that even overcompensates the
decrease expected for more electron-withdrawing groups.
Table 1. Characteristic NMR spectroscopic data of 3a–c and 4.
δ(31P) [ppm] 1J(P,117Sn) [Hz] 1J(P,119Sn) [Hz] 1J(P,Si) [Hz]
Figure 1. ORTEP plot of 3a (thermal ellipsoids at 30% probability
3a
3b
3c
4
–272.1
–242.5
–235.0
–242.4
669.2
681.2
692.9
718.4
721.0
713.4
724.7
751.5
116.1
99.0
101.4
–
level).
A similar situation is found in the crystal structure of
dichlorogermane 4 (Figure 4). The P–Ge distance is at
2.274(2) Å in the short range of single bond lengths ob-
We were able to obtain single crystals of 3a and 4 that served for this element combination (2.30–2.36 Å).[18,19]
were suitable for X-ray diffraction. The crystal structure Again, the substituents flanking this central bond show an
analysis of 3a confirms that one chlorine atom in 1 has been eclipsed conformation concomitant with attractive Sn···Cl
Eur. J. Inorg. Chem. 2006, 380–384
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
381

M. Bender, E. Niecke, M. Nieger, R. Pietschnig
FULL PAPER
Figure 2. Newman projection and selected bond lengths of 3a and
4 (in brackets).
Figure 4. ORTEP plot of 4 (thermal ellipsoids at 30% probability
level).
tion processes via chlorostannane elimination are favored
over intramolecular elimination under these conditions.[20]
As outlined earlier, the structural and spectroscopic pa-
rameters of 3a–c and 4 suggest significant hyperconjugative
interaction in this type of molecules. To get further insight
into the bonding situation we performed quantum chemical
calculations on the germanium compound 4 which shows
the largest hyperconjugative effect. As a result we find good
agreement of the theoretical with the experimental struc-
tural data. Moreover, a population analysis provides evi-
dence that bonding states at the phosphorus atom as well
as the lone pair interact with σ*-states derived from the
Ge–Cl bonds. The HOMO-1 of 4 is mainly located at the
phosphorus atom but shows some mixing with the Ge–Cl
bonds (Figure 5). Quite similar is the situation for the Cp*–
centered HOMO-2 that also partially involves the phospho-
rus atom. In the HOMO-3 the antibonding Ge–Cl σ*-states
mix with bonding Sn–P σ-states (Figure 6).
Figure 3. Relevant intramolecular distances of 3a and 4 (in brack-
ets).
interaction. The Sn···Cl distances are at 3.701 and 3.784 Å
close to those found in 3a. The Ge–Cl distances show values
of 2.189(2) and 2.192(2) Å. As in 3a, the P–Sn distances in
4 show with values of 2.511(2) and 2.519(2) Å no peculiari-
ties. The dihedral angles, by which the Sn–P–Ge–Cl planes
differ from a perfect synperiplanar arrangement, are 1.1°
and 6.2°.
In both structures the short Si–P or Ge–P bond lengths
in addition to the attractive Sn···Cl interaction seem to
underline the tendency of both molecules to eliminate chlo-
rostannane. Interestingly in the mass spectra of 3a and 4,
the molecular ions both lose one equivalent of chlorotri-
methylstannane. The elimination of a second equivalent of
chlorotrimethylstannane from the resulting phosphasilene
and phosphagermene cations is not observed, however.
While solutions of these compounds in aprotic solvents
are stable at room temperature, heating of 3a or 4 as neat
substances up to 200 °C leads to elimination of trimeth-
ylchlorostannane. During this process polymeric materials
Figure 5. Surface plot of the HOMO-1 (left) and the HOMO-2
(right) in 4.
In summary, we presented a general synthetic route to
are formed which we have not been able to characterize yet. bis(stannyl)phosphanyl-substituted dichlorosilanes and ger-
Preliminary results suggest that intermolecular condensa- manes that should allow a novel approach towards the first
382
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Bis(stannyl)phosphanyl-Substituted Dichlorosilanes/Germanes
FULL PAPER
3JH,H = 6.9 Hz), 2.73 (1 H, sept, JH,H = 10.3 Hz), 1.23 (d, 12 H,
3JHH = 6.9 Hz), 1.17 (d, 6 H, 3JH,H = 10.3 Hz), 0.29 (d, 18 H, 3JH,P
= 2.3 Hz) ppm. 13C NMR (C6D6): δ = 155.9, 152.2, 129.7, 122.7,
3
2
34.6, 33.3, 25.6, 24.0, –4.1 (d, JC,P = 7.0 Hz) ppm. MS (EI): m/z
= 662.0136 (δ = 2.6 ppm dev.) [M]+, 50%, 447 [M – ClSn(CH3)3 –
CH3]+, 70%, 278 [TipSiPCH4]+, 100%, 262 [TipSiP]+, 5%.
C21H41Cl2PSiSn2 (660.93): calcd. C 38.16, H 6.25; found C 38.34,
H 6.36.
1
4: 31P NMR (C6D6): δ = –242.4 (1JP(119),Sn = 751.5, JP(117),Sn
=
1
718.4 Hz) ppm. H NMR (C6D6): δ = 1.94 (15 H), 0.50 (d, 18 H,
3JH,P = 2.5 Hz) ppm. 13C NMR (C6D6): δ = 12.1 (m), –3.01 (d,
2JC,P = 7.5 Hz; JC(119),Sn = 160.9, JC(117),Sn = 153.6 Hz) ppm. MS
(EI): m/z = 635.8953 (δ = 0.57 ppm dev.) [M]+, 12%, 438 [M –
ClSn(CH3)3]+, 35%, 423 [M – ClSn(CH3)3 – CH3]+, 58%, 165
[Sn(CH3)3]+, 100%. C16H33Cl2GePSn2 (637.34): calcd. C 30.15, H
5.22; found C 30.26, H 5.28.
1
1
Figure 6. Surface plot of the HOMO-3 in 4.
generation of hypothetical phosphasilylynes and phosphag-
ermylynes. The molecular structures of two representative
examples show substantial hyperconjugative interaction be-
tween the stannylphosphanyl and the chlorosilyl/germyl
units, which are ideally preorganized for chlorostannane eli-
mination.
Crystallographic Details for 3a: A colorless crystal of 3a with di-
mensions 0.40×0.35×0.35 mm was sealed in a glass capillary. The
measurements were performed with a Nicolet R3M diffractometer
using graphite-monochromatized Mo-Kα radiation at 273 K. A to-
tal of 4578 reflections were collected (Θmax = 25.00°), from which
4383 were unique (Rint = 0.0361), with 4380 having I Ͼ 2σ(I). The
structure was solved by direct methods and refined by full-matrix
least-squares techniques against F2 with SHELXTL-Plus[28]. The
non-hydrogen atoms were refined with anisotropic displacement
parameters without any constraints. For 210 parameters final R
indices of R = 0.0573 and wR2 = 0.1048 (GOF = 0.989) were ob-
tained.
Experimental Section
All chemical experiments were performed under argon using stan-
dard Schlenk techniques or a glovebox. Solvents were dried with
sodium/potassium alloy and stored under argon. 1H, 13C, 31P, 29Si,
and 119Sn NMR spectra were recorded with a Bruker AMX 300 at
room temperature. Chemical shift values are given in ppm and were
referenced to external standards. Mass spectra were measured on
Kratos MS-50 and Masslab VG 12–250 spectrometers using the EI
ionization technique. Compounds 1, 2, and P(SnMe3)3 were pre-
pared according to published procedures.[21,22]
Crystallographic Details for 4: A yellow crystal of 4 with dimen-
sions 0.55×0.55×0.25 mm was sealed in a glass capillary. The mea-
surements were performed with a Nicolet R3M diffractometer
using graphite-monochromatized Mo-Kα radiation at 293 K. A to-
tal of 4549 reflections were collected (Θmax = 25.00°), from which
4399 were unique (Rint = 0.0780), with 4396 having I Ͼ 2σ(I). The
structure was solved by direct methods and refined by full-matrix
least-squares techniques against F2 with SHELX-97.[28] The non-
hydrogen atoms were refined with anisotropic displacement param-
eters without any constraints. For 210 parameters final R indices
of R = 0.0744 and wR2 = 0.1174 (GOF = 0.946) were obtained
(Table 2).
Synthesis of 3a–c and 4: Tris(trimethylstannyl)phosphane (1.05 g,
2 mmol) was placed in a sublimation apparatus together with an
equimolar amount of the corresponding trichlorosilanes 1a–c or
trichlorogermane 2. The mixture was heated to 150 °C until all the
chlorotrimethylstannane sublimed off (approximately 1 h). The re-
maining residue was dissolved in pentane (10 mL) and stored at
–20 °C. After 12 h, 3a–c precipitated as colorless (3a) to slightly
yellow (3b,c; 4) crystalline solids in 95% (3a), 92% (3b), 94% (3c)
and 95% (4) yield.
Table 2. Crystal data and structure refinement for 3a and 4.
3a: M.p. 181 °C. 31P NMR (C6D6): δ = –272.1 (1JP(119),Sn = 721.0,
1
1JP(117),Sn = 669.2 Hz) ppm. 29Si NMR (C6D6): δ = 33.1 (d, JSi,P
Reference
3a
4
1
= 116.1 Hz) ppm. 119Sn NMR (C6D6): δ = 36.6 (d, JSn,P = 721.0,
Formula
Formula weight
Temp. [K]
Wavelength
Crystal system
Space group
Unit cell dimensions
a [Å]
C16H33Cl2PSiSn2 C16H33Cl2GePSn2
2
2
1
1JSn,C = 320.4, JSn,Si = 36.7, J(119)Sn,(117)Sn = 219.3 Hz) ppm. H
592.8
293
637.3
293
3
NMR (C6D6): δ = 1.90 (12 H), 1.85 (3 H), 0.44 (d, 18 H, JH,P
=
2.39, JH(119),Sn = 54.9, JH(117),Sn = 52.7 Hz) ppm. 13C NMR
2
2
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
2
(C6D6): δ = 140.0, 134.5, 33.0 (d, JC,P = 12.0 Hz), 12.7, 12.2, –3.1
2
1
1
(d, JC,P = 7.6 Hz; JC(119),Sn = 320, JC(117),Sn = 305 Hz) ppm. MS
(EI): m/z = 589.9510 (δ = 0.67 ppm dev.) [M]+, 12%, 394 [M –
ClSn(CH3)3]+, 40%, 379 [M – ClSn(CH3)3 – CH3]+, 60%, 165
[Sn(CH3)3]+, 100%, 135 [Cp*]+, 50%. C16H33Cl2PSiSn2 (592.82):
calcd. C 32.42, H 5.61; found C 32.60, H 5.72.
10.004(3)
14.326(2)
17.908(3)
90
10.027(2)
14.325(2)
17.916(3)
90
b [Å]
c [Å]
α [°]
1
3b: 31P NMR (C6D6): δ = –242.5 (1JP(119),Sn = 713.4, JP(117),Sn
=
=
β [°]
γ [°]
103.56(2)
90
2495.0(9)
4
102.95(1)
90
2508.0(1)
4
1.688
3.440
0.946
0.0744
0.1174
1
681.2 Hz) ppm. 29Si NMR (C6D6):
δ
=
25.3 (d, JSi,P
Volume [Å3]
Z
99.0 Hz) ppm. 13C NMR (C6D6): δ = 156.0, 152.1, 147.6, 122.8,
2
25.6, 24.0, 7.9 (d, JC,P = 7.2 Hz) ppm. C15H29Cl2PSiSn2 (576.78):
Density (calcd.) [Mg/m3] 1.578
μ [mm–1]
0.710
0.989
0.0573
0.1048
calcd. C 31.24, H 5.07; found C 31.38, H 5.18.
Goodness-of-fit on F2
R1 (obsd. data)
wR2 (all data)
1
3c: 31P NMR (C6D6): δ = –235.0 (1JP(119),Sn = 724.7, JP(117),Sn
=
=
1
692.9 Hz) ppm. 29Si NMR (C6D6):
δ
=
24.4 (d, JSi,P
101.4 Hz) ppm. 1H NMR (C6D6): δ = 6.93 (s, 2 H), 4.13 (2 H, sept,
Eur. J. Inorg. Chem. 2006, 380–384
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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383

M. Bender, E. Niecke, M. Nieger, R. Pietschnig
FULL PAPER
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CCDC-183386 (for 3a) and -183388 (for 4) contain the supplemen-
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Huzinaga full double zeta basis set on first row elements,[24] and
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the molecular orbitals are shown at a contour value of 0.04.
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Acknowledgments
R. P. is grateful for financial support from the Austrian Science
Fund (FWF, project P17882-N11).
[20] Experiments to disfavor intermolecular elimination by dilution
with high boiling solvents or by complexation prior to elimi-
nation are currently in progress.
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