A P4 chain and cage from silylene-activated white phosphorus

Article abstract of DOI:10.1002/anie.201105610

An acyclic P₄ chain supported by a silicon-substituted amidinato ligand was formed from the reaction of PhC(NtBu)₂SiN(TMS) ₂ with P₄ (TMS=Me₃Si). This is the first example of an acyclic Si-P chain tha

Full text of DOI:10.1002/anie.201105610

Communications
DOI: 10.1002/anie.201105610
P4 Activation
A P4 Chain and Cage from Silylene-Activated White Phosphorus**
Shabana Khan, Reent Michel, Sakya S. Sen, Herbert W. Roesky,* and Dietmar Stalke*
Dedicated to Professor Philip P. Power
Transition-metal-mediated degradation of white phosphorus
has been extensively studied,[1] and it has attracted consid-
erable interest owing to its demand in industrial applications
for a more facile production of organophosphorus derivatives.
Recently in a seminal review,[2] Power pointed out that main-
group elements in lower oxidation states exhibit an amazing
resemblance to transition metals in many ways, for example
their ability to activate small molecules, such as dihydro-
gen.[3–6] In fact, Bertrand et al. mentioned that cyclic alkyl
amino carbene (CAAC) can activate NH3,[7] which is a much
more difficult task for transition metals.[8] The carbene
footsteps were immediately followed by its heavier conge-
ners: silylene, germylene, and stannylene, which all display
the activation of NH3 under ambient condition.[9–11] Another
remarkable breakthrough in this field is the fragmentation of
Scheme 1. Examples of P4 activation by compounds with low-valent
silicon.
P4 by a N-heterocyclic carbene (NHC) and related carbene
ligands.[12–14] Among these, the most intriguing example was
the activation of P4 by a bulky rigid CAAC and the isolation
of the resulting acyclic P4 chain.[13]
zwitterionic Si2P2 ring (D) by P4 activation with silicon(II)
chloride [PhC(NtBu)2SiCl] and bis(silylene) [(PhC-
(NtBu)2Si)2].[20] The latter, compound D, was also isolated
by Driess et al. by treating [PhC(NtBu)2SiP(SiMe3)2] with
Ph3PCl2.[21]
In a surprisingly short period of time, the activation of
phosphorus by subvalent Group 14 elements has flowered
into an engaging, intriguing, and emerging area of chemis-
try.[22,23] Nevertheless, a neutral acyclic P4 chain stabilized by
an N-heterocyclic silylene is still elusive. Herein, we present
the synthesis of a neutral acyclic P4 chain stabilized by the
recently isolated silicon(II) bis(trimethylsilyl)amide [PhC-
(NtBu)2SiN(SiMe3)2] (1).[24]
Addition of toluene to the mixture of 1 and P4 at À108C in
a 2:1 molar ratio immediately gave rise to a dark bluish-green
solution that turned purple after stirring for 30 minutes at
room temperature. The solution was concentrated and kept at
À308C to afford 2 in a 50% yield as purple-colored,
extremely air- and moisture-sensitive crystals (Scheme 2).
Compound 2 is stable in solution at À308C for a long time, but
decomposes rapidly (within a few minutes) after separation of
crystallized material from the supernatant mother liquor and
subsequent drying. The constitution of 2 was unequivocally
Owing to the similar properties between N-heterocyclic
carbene and silylene (both having a singlet ground state[15,16]
and a lone pair of electrons on the low-valent carbon and
silicon atom), it is expected that subvalent silicon compounds
can also activate P4; however, only four examples of P4
activation by compounds with low-valent silicon have been
reported to date (Scheme 1). West et al. described the
isolation of 1,3-diphospha-2,4-disilabicyclo[1.1.0]butane
(A),[17] which was later structurally characterized.[18] Driess
et al. described the activation of P4 by a N-heterocyclic
=
À
=
silylene
[{RNC( CH2) CH C(Me)NR}Si]
(R = 2,6-
iPr2C6H3) to afford bicyclotetraphosphine derivatives B and
C.[19] Very recently, we isolated a novel four-membered
[*] Dr. S. Khan, R. Michel, Dr. S. S. Sen, Prof. Dr. H. W. Roesky,
Prof. Dr. D. Stalke
Institut fꢀr Anorganische Chemie, Georg-August-Universitꢁt
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: hroesky@gwdg.de
dstalke@chemie.uni-goettingen.de
Homepage: http://www.roesky.chemie.uni-goettingen.de
http://www.stalke.chemie.uni-goettingen.de
[**] H.W.R. thanks the Deutsche Forschungsgemeinschaft (DFG RO
224/55-3) for support. S.K. is indebted to the DAAD for a research
fellowship. D.S. is grateful for funding from the DFG Priority
Programme 1178, the DNRF-funded Center for Materials Crystal-
lography (CMC) for support, and the Land Niedersachsen for
providing a fellowship in the Catalysis for Sustainable Synthesis
(CaSuS) Ph.D. program.
Supporting information for this article, including experimental
details, is available on the WWW under http://dx.doi.org/10.1002/
anie.201105610.
Scheme 2. Synthesis of 2.
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Angew. Chem. Int. Ed. 2011, 50, 11786 –11789

2 shows an AA’XX’ system with two multiplets at d =
À77.19 ppm and + 381.72 ppm (JA,A’ = 346.14, JA,X = 295.77,
J
X,X’ = 184.18, and JA,X’ = 50.36 Hz), which can be assigned to
=
=
P of Si P and P P, respectively. In the EI-MS spectrum, the
most abundant peak with the highest relative intensity at m/
z 900.5 was observed for the fragment after the probable
elimination of a P2 unit.
The N(SiMe3)2-substituted disilene [(Me3Si)2N(h1-
1
=
Me5C5)Si Si(h -Me5C5)N(SiMe3)2] (3) was first reported by
Jutzi et al. in 2004,[36] which was later shown to exist in
equilibrium with the corresponding silylene in solution.[37]
Very recently, we have successfully synthesized 3 in 68%
yield by treating [(h1-Me5C5)SiHCl2] with KN(SiMe3)2.[38] This
high-yield access of 3 and its ability to exist in equilibrium
with the corresponding silylene prompted us to investigate its
reactivity with P4. It can be used as a source for two-
coordinate silylene [(Me5C5)SiN(SiMe3)2] and can be com-
pared with the reactivity of three-coordinate silylene 1.
Unlike 1, the reaction of 3 with P4 in a 1:1 molar ratio in
toluene at ambient temperature under stirring for 12 h yields
Figure 1. Molecular structure of 2·2C7H8. Ellipsoids are set at 50%
probability; hydrogen atoms and the two toluene molecules are
omitted for clarity. Atom P1 is part of a disordered moiety with the
highest site occupation factor. Atom P2#1 (and those shown to the
left of P2#1; #1=x, Ày+0.5, z) are symmetry-generated by a mirror
À
plane lying between and perpendicular to P2 P2#1. Selected bond
an extraordinary Si P cage (4; Scheme 3). The 1H NMR
distances [ꢂ] and angles [8]: P2–P2#1 2.0559(7), P1–P2 2.1317(14),
Si1–P1 2.1602(12), N1–Si1 1.8316(12), N2–Si1 1.8339(13), Si1–N3
1.7264(12); N1-Si1-P1 116.57(9), N2-Si1-P1 117.82(10), N3-Si1-P1
116.56(6), P2-P1-Si1 94.22(5), P2#1-P2-P1 106.93(3).
À
characterized by cryogenic single-crystal X-ray diffraction
studies.[25–27] Figure 1 depicts the molecular view of 2, which
crystallizes in the orthorhombic space group Pnma. The
molecular structure shows the presence of the Z-diphosphene
isomer (with a dihedral angle of 08) and benzamidinato-
stabilized phosphasilene substituents, whereas the reported
CAAC-supported acyclic P4 is an E isomer (major prod-
Scheme 3. Synthesis of 4.
spectrum exhibits a broad resonance for thirty protons of the
methyl groups attached to the two five-membered rings (d =
1.73–1.86 ppm). Two resonances at d = 0.41 and 0.49 ppm
correspond to protons of two different SiMe3 groups. The 29Si
NMR spectrum of 4 in C6D6 shows four resonances (d =
À78.9, 0.2, 5.8, and 8.1 ppm) for four different silicon atoms.
A quartet at d = À78.9 ppm (1J(29Si-31P) = 48.20 Hz) appears
for silicon attached to three phosphorus atoms, while the
silicon attached to two phosphorus atoms has a broad triplet
at d = 0.2 ppm (1J(29Si-31P) = 42.86 Hz).[19] The remaining two
resonances result from the silicon atoms of the SiMe3 groups.
As shown by 31P NMR spectroscopy, 4 exhibits four chemi-
cally different phosphorus atoms (A, B, X, Y) giving rise of
the resonance signals as a doublet of doublets of doublets at
uct).[13] AC2 axis passes through the middle of the central P P
=
double bond of 2. The main structure consists of two Si atoms
and four P atoms, which together form a neutral acyclic Si2P4
= À = À =
(Si P P P P Si) chain with 6p electrons contained in a
diphosphene and two phosphasilene units. The two Si atoms
are four-coordinate each and display a distorted tetrahedral
geometry by coordination with two N atoms from the
amidinato ligand. The remaining two sites of the tetrahedron
are occupied by the N(SiMe3)2 moiety and P atom each. The
À
central P2 P2#1 bond length is 2.0559(7) ꢀ, which falls in the
À
range of a typical double bond, while the P1 P2 distance
[28–30]
À
(2.1317(14) ꢀ) is halfway between a P P single (2.21 ꢀ)
and double bond[31,32] (1.954 to 2.044 ꢀ) and corresponds well
with the reported value (2.083(4) ꢀ) for a CAAC-stabilized
d = 257.6, 128.4, 114.2, and À141.8 ppm (JA,B = 30.9, JA,X
=
acyclic P4 unit.[13] Moreover, the Si1 P1 bond in
2
10.2, A,Y = 78.6, B,X = 126.6, B,Y = 110.2, and JX,Y
J
J
J
=
À
=
(2.1602(12) ꢀ) is longer than the Si P double bond
156.5 Hz), respectively. The composition of 4 is further
supported by EI mass spectrometry (molecular ion m/z
770), which clearly indicates the formation of a 1:1 complex of
3 with P4.
Figure 2 shows a molecule of 4. Compound 4 crystallizes
in the triclinic space group P1.
structure confirms the triply opened P4 tetrahedron, in
which one P3 face is capped, and one opened edge is bridged
by a silicon atom each, and a Cp* group has migrated from Si4
to one of the adjacent phosphorus atoms (P4) under simulta-
neous formation of a new Si P bond. Both silicon atoms (Si4
and Si7) are four-coordinate and have a distorted tetrahedral
(2.09 ꢀ),[33,34] but significantly shorter than the reported Si
À
P single bond (2.25 ꢀ)[35] and is in good accordance with the
bond lengths of the recently reported CPSi2 and P2Si2
cycles.[20] From the X-ray structural data it is clear that each
[25–27]
¯
= À
SiP2 (Si P P) moiety shows a delocalized electron density.
The X-ray crystal
Compound 2 was also characterized by multinuclear
NMR spectroscopy, EI-MS spectrometry, and elemental
1
analysis. In the H NMR spectrum, tBu protons give rise to
a singlet at d = 1.29 ppm, which confirms the presence of only
one isomer. Two resonances appear at d = 0.46 and 0.77 ppm
that are assigned to SiMe3 protons. The 31P NMR spectrum of
À
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Communications
reaction of the abovementioned silylenes with P4 confirms the
significant role of the substituents to yield the final products.
À
Such Si P compounds can be used as electron-rich chelating
ligands for transition metals and may find application in
metal-mediated catalytic process.[39–41] Furthermore, 2 and 4
may have commercial application as precursors for semi-
conducting materials.[42,43]
Received: August 8, 2011
Revised: August 24, 2011
Published online: October 11, 2011
Keywords: P4 activation · phosphorus · silicon · silylenes ·
.
solid-state structures
Figure 2. Molecular structure of 4. Ellipsoids are set at 50% proba-
bility; hydrogen atoms are omitted for clarity. Selected bond distances
[ꢂ] and angles [8]: P1–P3 2.2747(7), P2–P3 2.2831(8), P3–P4 2.2457(7),
P1–Si4 2.2829(8), P1–Si7 2.2979(6), P2–Si4 2.2868(6), P2–Si7
2.2922(8), P4–Si4 2.2600(7), P4–C1 1.9133(15), Si7–N2 1.7450(13),
Si4–N1 1.7300(13); P3-P1-Si4 69.98(2), P3-P1-Si7 77.09(3), Si4-P1-Si7
85.78(3), P3-P2-Si4 69.76(2), P3-P2-Si7 77.09(3), Si4-P2-Si7 85.82(3),
P4-P3-P1 86.44(3), P4-P3-P2 97.19(3), P1-P3-P2 86.98(3), P3-P4-Si4
70.91(2), C1-P4-P3 110.69(5), C1-P4-Si4 113.30(5), P4-Si4-P1 85.89(3),
P4-Si4-P2 96.65(2), P1-Si4-P2 86.69(3), P1-Si7-P2 86.22(3).
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À
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À
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=
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À
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1
1
=
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