Consecutive insertion of a silylene into the P4 tetrahedron: Facile access to strained SiP4 and Si2P4 cage compounds

Article abstract of DOI:10.1002/anie.200701203

Silylene bites twice in the first insertion of a silylene into a P-P bond of the P₄ tetrahedron. Reaction of the silylene 1 (see scheme; R = 2,6-diisopropylphenyl) with white phosphorus at ambient temperature gives 2 with a tricyclic SiP_4<

Full text of DOI:10.1002/anie.200701203

Angewandte
Chemie
DOI: 10.1002/anie.200701203
P4 Activation
Consecutive Insertion of a Silylene into the P4 Tetrahedron: Facile
Access to Strained SiP4 and Si2P4 Cage Compounds**
Yun Xiong, Shenglai Yao, Markus Brym, and Matthias Driess*
Dedicated to Professor Michael F. Lappert
Organosilicon–phosphorus compounds are versatile building the first SiP4 and Si2P4 cage compounds which result from
blocks in contemporary organophosphorus chemistry[1] and consecutive P4 activation with the stable silylene
for the synthesis of metal phosphide semiconducting materi- (Scheme 1).
als or respective molecular clusters.[2]
1
Likewise,cyclic silicon–phosphorus
compounds can serve as chelating
electron-rich ligands in transition-
metal complexes,modeling unusual
coordination modes of metal centers
in catalysts.[3] The formation of silicon–
phosphorus compounds is commonly
achieved by salt metathesis reactions
using silicon halides and main-group-
metal phosphides (for example,lith-
ium phosphides).[1,4] However,the
latter method is hardly suitable for
the synthesis of strained silicon–phos-
phorus cycles and cages because of
facile skeletal rearrangement reactions
(for example,intermolecular ring
enlargement) under the reaction con-
ditions.[5] Hence,other synthetic meth-
ods are desirable to gain access to well-
defined,strained silicon–phosphorus
Scheme 1. Synthesis of 2 and 3 from 1 and P4. R=2,6-iPr2C6H3.
cycles and cages. A promising alterna-
tive strategy represents the utilization
of unsaturated silicon compounds and white phosphorus (P4)
Although several stable silylenes have been described
=
as shown by the gentle reaction of disilenes (R2Si SiR2) and thus far,to our knowledge,both the highly electrophilic
phosphasilenes (R2Si PR’) with P4 at 408C,leading to the
silylene (CH2)2[C(SiMe3)2]2SiD[8] and the silicon analogues of
=
unusual butterfly-shaped Si2P2 and SiP3 heterobicyclo- nucleophilic Arduengo-type carbenes[9] seemed to be unsuit-
[1.1.0]butanes,[6a,7] respectively. The mechanism of this able for P4 activation. In contrast,the zwitterionic silylene
remarkably mild P4 activation and degradation process,
initiated by low-valent silicon,is not clearly understood as
1
[10a] reacts gently with P4 in the molar ratio of 1:1 at ambient
temperature to give solely the compound 2. Remarkably,the
yet.[6b] The latter results prompted us to investigate whether germylene analogue of 1,[10b] in turn,is resistant towards P
4
stable silylenes (R2SiD) are also capable of P4 activation to give even in boiling toluene owing to the lower reduction potential
novel strained silicon–phosphorus cycles. Herein we report of GeII versus SiII (inert-pair effect). Compound 2 can be
isolated in the form of colorless crystals in 60% yield.[11]
According to EI-MS spectra and combustion analysis, 2 is
1
[*] Dr. Y. Xiong, Dr. S. Yao, Dr. M. Brym, Prof. Dr. M. Driess
Institute of Chemistry: Metalorganics and Inorganic Materials
Technische Universität Berlin
a 1:1 adduct of 1 and P4. Its H NMR spectrum proves the
integrity of the C3N2 chelate ligand at the silicon atom. As
shown by 31P NMR spectroscopy, 2 bears three chemically
different sorts of 31P nuclei (A,B,and X),giving rise to
temperature-invariant resonance signals at dX = 131.9, dA =
À342.4,and dB = À348.0 ppm. The low-field resonance signal
at dX splits into a doublet of doublets (1J(PX,PA) = 146.8,
1J(PX,PB) = 144.7 Hz) while each of the two high-field signals
Strasse des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
Fax: (+49)30-314-22168
E-mail: matthias.driess@tu-berlin.de
[**] We thank the Deutsche Forschungsgemeinschaft for financial
support and Prof. Dr. Konstantin Karaghiosoff (University of
Munich) for his help on spectra simulations.
exhibits a doublet of triplets (1J(PX,PA) = 144.7, J(PA,PB) =
188.0 Hz),indicating an ABX
1
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
splitting pattern.[11] The
2
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À
unusual low-field shift of PX is similar to values observed for
into the P3 P4 bond in 2. Thus, 1 reacts slowly with 2 in
À
M P-bonded P atoms in isovalent [LnMP4] complexes with a
toluene at 258C to give the insertion product 3 (Scheme 1),
which was isolated in 27% yield in the form of colorless
crystals.[11] The reaction progress for the second insertion step
tricyclo[3.1.0]pentane-like core ({LnM} = diamagnetic transi-
tion-metal complex fragment[12a–c] and PI2+ [12d]). Likewise,the
high-field shifts of PA and PB in 2 signalize similar electronic
features as observed for bridgehead P atoms in [LnMP4]
complexes[12] and related butterfly-shaped Si,P compounds
(Scheme 1).[6,7] The 29Si NMR spectrum of 2 displays a
broadened triplet resonance at d = À40.4 ppm (1J(29Si,31P) =
32.0 Hz),which is in the typical range of related cyclo-
silaphosphanes with silicon–phosphorus single bonds.[13] The
molecular structure of 2 deduced from 1H, 31P,and 29Si NMR
spectroscopy was confirmed by a single-crystal X-ray diffrac-
tion analysis.[11] Compound 2 consists of a tricyclic SiP4 core
with pyramidally coordinated phosphorus atoms and silicon
atoms in a tetrahedral environment (Figure 1). The six-
membered C3N2Si skeleton is almost planar and possesses
À
of 1 into a P P bond of the P4 core is kinetically hampered
because of steric congestion.
This reaction sequence is in contrast to the spontaneous
multiple insertions of subvalent main-group-metal centers
into the P4 cage. For example,a related b-diketiminato AlI
complex reacts with P4 to give a double-insertion product with
a Al2P4 core.[15a] Likewise,the tetrameric Al compound
I
[(Cp*Al)4] reacts with P4 in
a 1.5:1 ratio to give
[(Cp*Al)6P4],[15b] whereas the isovalent GaI atom in
[(Me3Si)3CGaD] affords an insertion product with a Ga3P4
core.[15c]
The composition of 3 is proven by EI-MS (molecular ion
m/z 1013) and a correct combustion analysis while its
constitution is evident by NMR spectroscopy. As expected,
the 31P{1H} NMR spectrum displays two multiplets at dA = 153
and dB = 158 ppm,representing a AA ’BB’ splitting pattern of
higher order with unusually small magnitudes of the respec-
À
À
endocyclic C N and C C bond lengths typical for its p-
conjugated C3N2 backbone.[10] The exocyclic C1 C2 and C4
C5 bond lengths are identical owing to orientation disorder.
À
À
À
À
As expected,both the Si P (av. 224.7 pm) and P P bond
tive J(31P,31P) coupling constants.[11] The 29Si NMR spectrum
1
displays a multiplet at d = À15.9 ppm,about 24 ppm to lower
field in comparison with 2. The molecular structure of 3 was
confirmed by an X-ray diffraction analysis (Figure 2).[11] The
À
P P bond lengths in the asterane-shaped P4 core in 3 are
similar (av. 228.5(1) pm) but somewhat longer than those in 2,
presumably because of the steric congestion from the
substituents at the silane moieties. The same is true for the
À
Si P bond lengths (av. 228.5(1) pm),which are slightly longer
than those in 2. Both C3N2Si moieties are practically planar
Figure 1. Molecular structure of 2. Thermal ellipsoids (C1 through C5,
P atoms, N1, N2, Si1) are drawn at the 50% probability level. Hydro-
gen atoms (except for those at C1) are omitted for clarity. Selected
bond lengths [pm] and angles [8]: Si1-N1 172.8(3), Si1-N2 172.4(3),
Si1-P2 225.0(1), Si1-P1 224.6(1), P3-P2 223.2(1), P1-P3 223.5(2), P3-P4
215.9(2), P2-P4 222.7(1), P1-P4 222.1(2), N1-C2 141.2(4), N2-C4
142.2(4), C1-C2 141.8(4), C2-C3 139.7(5), C3-C4 139.2(4), C4-C5
141.6(5); N1-Si1-N2 104.1(1), P1-Si1-P2 87.32(5), P4-P3-P2 60.94(5),
P4-P3-P1 60.70(5), P2-P3-P1 88.03(5), P4-P2-P3 57.91(5), P4-P2-Si1
84.82(5), P3-P2-Si1 83.85(5).
Figure 2. Molecular structure of 3. Thermal ellipsoids (C1 through C5,
C30 through C34, P atoms, N1 through N4, Si1, Si2) are drawn at the
50% probability level. Hydrogen atoms (except for those at C1 and
C30) are omitted for clarity. Selected bond lengths [pm] and angles [8]:
Si1-N1 172.6(3), Si1-N2 172.9(3), Si2-N3 172.5(3), Si2-N4 173.4(3),
Si1-P1 229.1(1), Si1-P2 227.5(1), Si2-P3 229.5(1), Si2-P4 227.5(1), P1-
P3 228.0(1), P1-P4 229.7(1), P2-P3 229.6(1), P2-P4 228.0(1), C1-C2
138.9(5), C2-C3 142.0(5), C3-C4 136.6(5), C4-C5 145.5(5); N1-Si1-N2
102.9(1), N3-Si2-N4 102.8(1), P2-Si1-P1 82.18(4), P4-Si2-P3 82.30(5),
P3-P1-Si1 82.42(5), P3-P1-P4 82.16(5), Si1-P1-P4 80.00(5), P1-P3-Si2
82.31(5), P1-P3-P2 81.90(5), Si2-P3-P2 79.75(5).
lengths (av. 222.9 pm) are similar to those observed in related
strained silaphosphanes[13] and isostructural h2-P4 transition-
metal complexes,[12a–c] respectively,except for the P3 P4
À
bridgehead bond length of 215.9(2) pm,which is even shorter
than that in P4 (220 pm). This result is reminiscent of the
situation in related bicyclo[1.1.0]tetraphosphanes,R P4 (R =
2
À
organyl),which have relatively short P P bridgehead bonds
[14]
À
(212 pm) owing to partial P P p-bond character. Appa-
rently,this bonding situation favors the insertion of a Si II atom
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Chemie
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À
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Received: March 19,2007
Published online: May 4,2007
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