An ylide-like phosphasilene and striking formation of a 4π-electron, resonance-stabilized 2,4-disila-1,3-diphosphacyclobutadiene

Article abstract of DOI:10.1021/ja200462y

The first N-donor-stabilized phosphasilene LSi(SiMe₃)=PSiMe ₃ (L = PhC(NtBu)₂) has been synthesized in 87% yield through 1,2-silyl migration of the (Me₃Si)₂P-substituted, N-heterocyclic silylene [LSi-

Full text of DOI:10.1021/ja200462y

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An Ylide-like Phosphasilene and Striking Formation of a 4π-Electron,
Resonance-Stabilized 2,4-Disila-1,3-diphosphacyclobutadiene
Shigeyoshi Inoue,* Wenyuan, Wang, Carsten Pr€asang, Matthew Asay, Elisabeth Irran, and Matthias Driess*
Institute of Chemistry: Metalorganics and Inorganic Materials, Technische Universit€at Berlin, Strasse des 17, Juni 135, Sekr. C2, D-10623
Berlin, Germany
S Supporting Information
b
Scheme 1. Synthesis of Phosphasilene 1 from N-Heterocyc-
lic Phosphino Silylene 2
ABSTRACT: The first N-donor-stabilized phosphasilene
LSi(SiMe3)dPSiMe3 (L = PhC(NtBu)2) has been synthe-
sized in 87% yield through 1,2-silyl migration of the
(Me3Si)2P-substituted, N-heterocyclic silylene [LSi-P-
(SiMe3)2]. Remarkably, the latter reacts with dichlorotri-
phenylphosphorane Ph3PCl2 to give the unprecedented 4π-
electron Si2P2-cycloheterobutadiene [(LSi)2P2] with two-
coordinate phosphorus atoms. The striking molecular struc-
tures as well as the 29Si and 31P NMR spectroscopic features
of both products indicate the presence of zwitterionic SidP
bonds which is also in accordance with results by DFT
calculations.
ompounds with a heteroleptic multiple bond between
C
heavier main-group elements, in particular those between
group 14 and 15 elements, have attracted considerable attention
because of their unique reactivity and electronic properties, which
can be attributed to the pronounced polarity of the E14-E15 multiple
bonds.1 Since the first isolable phosphasilene with a three-coordinate
Si and a two-coordinate P atom reported by Bickelhaupt and co-
workers in 1984,2 several derivatives could be synthesized and
structurally characterized containing localized and delocalized SidP
bonds.3 Their rich chemistry opened new doorways to functional
Si-P compounds. The availability of elusive phosphasilyne species,
that is, compounds with a silicon-phosphorus triple bond, would be
a highly desired progression in that field with respect to the synthesis
of polyfunctional silicon-phosphorus containing materials. How-
ever, the chances of success for the preparation of stable phospha-
silynes seem to be much more difficult due to the high tendency of
highly polarized Si-P multiple bonds to undergo dimerization or
even oligomerization. In fact, to the best of our knowledge, an
isolable phosphasilyne is hitherto unknown.4 Beside the spatial
protection of a Si-P multiple bond by steric congestion with suitable
substituents, the oligomerization of highly polarized P-Si multiple
bonds could be prevented additionally by taking advantage of the
donor-acceptor stabilization of a Si-P multiple bond.3d Herein, we
report the facile synthesis of the first N-heterocyclic zwitterionic
phosphasilene [LSi(SiMe3)dPSiMe3] 1with four-coordinate silicon
and of the unique 2,4-disila-1,3-diphosphacyclobuta-1,3-diene
[(LSi)2P2] 3, starting from the new N-heterocyclic bis(trimethyl-
silyl)phosphino silylene precursor 2.
Figure 1. Molecular structures of 1 (left) and 2 (right). Thermal
ellipsoids are drawn at 30% probability level. Hydrogen atoms are
omitted for clarity.
N-heterocyclic chlorosilylene LSiCl5 with the corresponding lithium
phosphide LiP(SiMe3)2 at room temperature (Scheme 1). The
molecular structure of 2 has been elucidated by NMR spectros-
copy and single-crystal X-ray diffraction analysis (Figure 1). The
Si1-P1 distance of 2 (2.2838(12) Å) is slightly shorter than that
of the previously reported LSiPiPr2 derivative (2.307(8) Å),6 and
the elongated P1-Si2 and P1-Si3 single bonds in 2 [2.2264(13)
and 2.2321(12) Å] reflect steric congestion within the P(SiMe3)2
subunit.
The relatively long Si-P bonds of the P(SiMe3)2 subunit in
close vicinity to the unsaturated ring Si(II) atom could enable a
silyl group shift similar to the situation reported for the isomer-
ization of silylenes into silenes via 1,2-silyl migration.7 For
instance, Kira and co-workers have reported the formation of a
cyclosilene from a cyclic dialkylsilylene via a 1,2-shift of a Me3Si
group.8 The latter result encouraged us to probe whether 1,2-silyl
migration in 2 leads to the formation of the corresponding
The starting material, bis(trimethylsilyl)phosphino silylene 2,
is facilely accessible in 83% yield by phosphanylation of
Received: January 17, 2011
Published: February 15, 2011
r
2011 American Chemical Society
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|

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Scheme 2. Resonance Structures of 1
Scheme 3. Synthesis of 3
phosphasilene 1. In fact, heating solutions of 2 in toluene at
100 °C furnishes the desired phosphasilene 1 quantitatively
(NMR spectroscopy), which could be isolated as air-sensitive
pale yellow crystals in 87% yield (Scheme 1).
The composition of 1 has been confirmed by X-ray crystal-
lography (Figure 1, left). The four-coordinate Si1 atom is bonded
to the phosphorus atom (P1), the amidinato chelate ligand L, and
the Si2 atom of the trimethylsilyl group. Compound 1 exhibits an
E-configuration with a Si2-Si1-P-Si3 torsion angle of 177.5°.
Although the Si1 atom is four-coordinate, the Si1-P1 distance of
2.095(3) Å, which is 8.4% shorter than the corresponding Si-P
single bond of 2, represents a SidP bond; similar values were
reported for phosphasilenes with three-coordinate silicon.3
However, as shown previously for related donor-stabilized SidX
compounds containing four-coordinate silicon,9 the additionally
dative NfSi bond favors an ylide-like character of the Si1dP1
subunit of 1 (see Scheme 2 and discussion below). The Si1-N1
[1.862(2) Å] and Si1-N2 [1.849(2) Å] bond lengths of 1 are
slightly shorter than those of 2 [1.877(3) and 1.882(2) Å], owing
to a higher formal oxidation state of the ring Si atom in 1.
The 29Si NMR spectrum of 1 exhibits three doublet signals at
Figure 2. Molecular structure of 3. Thermal ellipsoids are drawn at 30%
probability level. Hydrogen atoms are omitted for clarity.
which could be isolated as yellow crystals in 72% yield
(Scheme 3). Compound 3 represents a head-to-tail dimer of
the desired phosphasilyne and was fully characterized by multi-
nuclear NMR spectroscopy (see Supporting Information) and a
single-crystal X-ray analysis. The 29Si NMR spectrum of 3 shows
a triplet resonance at δ = 26.5 ppm (1JSiP = 99.8 Hz) which
δ = -17.5 (Me3Si-Si1,2JSiP = 36.3 Hz), 3.0 (Me3Si-P1, 1JSiP
=
indicates the presence of two P atoms bonded to silicon. The 31
P
70.5 Hz), and 40.5 ppm (SidP, 1JSiP = 191.4 Hz), respectively.
Theoretical calculations of the NMR chemical shifts [GIAO/
B3LYP/6-311(d) (H, C, N, P), 6-311þG(3d) (Si)// B3LYP/6-
31G(d)] of 1 (-15.3, 7.9, and 45.5) confirmed the assignment,
and the calculated values agree well with the experimental data.10
Owing to the higher coordination of the ring Si atom, its δ value
is shifted significantly upfield in comparison to those of phos-
phasilenes with three-coordinate silicon (δ > 200 ppm).3 Corre-
spondingly, the 31P NMR signal of δ = -252.9 ppm is shifted
drastically to higher field in comparison to related P-silyl
substituted phosphasilenes with three-coordinate silicon (δ >
-40 ppm).3 Clearly, the drastic changes of the NMR data can
only be rationalized by the NfSi donor coordination in 1 which
favors a stronger polarization of the SiδþdPδ- π bond and thus
leads to a significant contribution of betaine (ylide-like) struc-
tures to its electronic ground state (Scheme 2). The UV-vis
spectrum of 1 in hexane reveals an absorption maximum at 333
nm, due to the π-π* transition of the SidP bond, a value that is
similar to those observed for other polarized phosphasilenes (331-
343 nm).3g In other words, both NMR and UV-vis spectroscopic
data support the ylide-like resonance structures of 1.
The successful access to phosphino silylene 2 with its poten-
tially reactive Me3Si (leaving) groups at phosphorus also led us to
investigate its suitability as a precursor for a phosphasilyne with a
Si-P triple bond.11 Because of the labile nature of the P-SiMe3
bonds in 2, the compound was exposed to dichlorotriphenylpho-
sphorane, Cl2PPh3, as a promising smooth oxidation reagent for
desilylation with the intention to produce the desired phospha-
silyne along with Me3SiCl and PPh3. To our surprise, the
reaction of 2 with 1 molar equiv of Cl2PPh3 in toluene at room
temperature leads to the formation of the unique compound 3
NMR spectrum exhibits a singlet at δ = -164.9 ppm which is
more deshielded (ca. 90 ppm) than that in 2 and reflects the four-
membered ring structure of 3 with two polarized Si-P π bonds.
An X-ray crystal structure analysis confirmed that 3 is a 2,4-
disila-1,3-diphosphacyclobuta-1,3-diene (Figure 2). The Si2P2
core in 3 is planar and diamond-shaped and possesses two-
coordinate P atoms and four-coordinate Si atoms each with a
dative NfSi bond. The Si-P bond lengths in 3 are with
2.1701(12) and 2.1717(11) Å practically equal and represent
intermediate values between the SidP bond length of 1
(2.095(3) Å) and the ring Si-P single bond length of 2
(2.2838(12) Å). The equal Si-P distances in 3 already indicate
σ- and π-electron resonance stabilization within the Si2P2 cycle
which has been confirmed by theoretical calculations (see
below). Of particular interest seem the trans-annular Si Si
3 3 3
(2.5626(16) Å), and P P (3.505 Å) separations in 3: Inter-
3 3 3
estingly, the Si Si separation is shorter than the Si-Si distance
3 3 3
of disilacyclopropene [(Ad)(tBu3Si)2]Si2C [2.5797(8) and
2.5991(9) Å]12 (Ad = adamantyl) and hexa-tert-butyldisilane,
tBu3Si-SitBu3 (2.697(9) Å).13 The P P distance in 3 is much
3 3 3
larger than that for P-P bonds observed in tetrahedral P4 (2.21
Å).14 Evidently, the peculiarly short Si Si distance is caused by
3 3 3
the geometric constrains of the Si1-P-Si1* and endocyclic P1-
Si1-P1* angles of 72.34(4)° and 107.66(4)°, respectively, and
not driven by attractive Si-Si interaction.
Although the mechanism is still unknown, we propose that 3 is
formed in a multistep process as portrayed in Scheme 4. At first,
we assume that one of the Si-P bonds of the P(SiMe3)2 subunit
is cleaved by Ph3PCl2 to form the corresponding chloro-
(silyl)phosphino silylene intermediate 4, LSiP(Cl)(SiMe3). Sub-
sequent liberation of ClSiMe3 from 4 affords a reactive
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Journal of the American Chemical Society
COMMUNICATION
Scheme 4. Proposed Mechanism for the Formation of 3
filled Si-P π bond orbitals, with 1.770e and 1.770e for the Si1-
P1 and Si1*-P1* π bonding interaction, respectively.
The latter canonical π bond orbitals are strongly polarized
toward the P atoms (87.53% at the P and 12.47% at the Si).
Likewise, the WBI (Wiberg Bond Index) values of the Si-P
bonds (1.142 and 1.140) are higher than that of the phosphino
silylene 2 (0.920) and lower than that of phosphasilene 1
(1.552). Owing to the aforementioned betaine (ylide)-like
resonance stabilization, 3 is distinctly different from cyclobuta-
1,3-diene derivatives. This is supported by calculations of the
nucleus independent chemical shift (NICS)16 of 3 which re-
vealed negative values [NICS(1) = -2.57 and NICS(0) = -6.01
ppm], indicating that 3 has a somewhat aromatic character, in
contrast to cyclobuta-1,3-dienes which are antiaromatic.17 Alto-
gether, the theoretical analysis is consistent with the picture that
3 is best described by the ylide-like resonance structure 3a with
some contribution of the resonance forms 3b and 3c (Scheme 5).
As a result, the endocyclic angle at phosphorus is smaller than
that at silicon due to repulsion between the negatively charged
P atoms.
Scheme 5. Resonance Structures of 3
In summary, we have reported the synthesis and isomerization
of bis(trimetylsilyl)phosphino silylene 2 into the unprecedented
zwitterionic phosphasilene 1 via 1,2-silyl migration. Unexpect-
edly, precursor silylene 2 reacts with Ph3PCl2 to form the first
2,4-disila-1,3-diphosphacyclobuta-1,3-diene 3 via dimerization of
the corresponding phosphasilyne as a reactive intermediate.
Skillful modification of the latter synthetic protocol could be
useful for the synthesis of elusive phosphasilyne derivatives with a
SitP bond. This and the investigation of the reactivity of 3
toward metals are currently in progress.
Figure 3. Molecular orbitals of 3 representing the presence of σ- and π-
electron delocalization within the Si2P2 cycle: HOMO-2 (left) and
HOMO-5 (right).
’ ASSOCIATED CONTENT
intermediate, which could represent either a silylene-phosphini-
dene 5 [L(Si:)—P:] or a phosphasilyne 50, LSitP. Dimerization
of the latter intermediate produces the observed product 3. A
similar dimerization process has been proposed by Wiberg et al.
for disilynes (Si-Si triple bonds) as reactive intermediates to
produce tetrasilatetrahedranes.15 This raised the question why is
3 preferred over its hypothetical Si2P2-tetrahedrane isomer 6
(Scheme 5)? To answer this question we performed DFT
calculations of 3 and of disiladiphosphatetrahedrane 6. It turned
out 6 is far less stable than compound 3 by 46.2 kcal mol-1, which
agrees with the experimental result. The planar structure of 3 is
most likely stabilized from the dative NfSi coordination of the
bulky amidinato ligands. The NfSi donor stabilization of 3
further suggests that the compound has an ylide-like electronic
structure (Scheme 5) akin to the situation in 1.
In order to clarify the electronic nature of 3, we carried out
DFT calculations at the B3LYP/6-31G(d) level using the para-
meters obtained by X-ray crystallographic analysis as the initial
structure.10 The optimized structure closely matched the experi-
mental data (for complete details see Supporting Information).
According to the NPA charges, each Si atom in the Si2P2 ring
bears a large positive net charge (þ1.204), while the P atoms in
the ring have somewhat smaller negative charges (-0.765) as
expected. Furthermore, the HOMO-2 of 3 represents mainly
the delocalized two π bonding orbitals (Figure 3, left), whereas
HOMO-5 depicts a set of Si-P σ bonding orbitals (Figure 3,
right). Accordingly, an NBO analysis of 3 leads to a Lewis
structure of the Si2P2 core with four occupied σ bond orbitals
for the Si-P bonds (1.917e for Si1-P1, 1.916e for Si1-P1*,
1.916e for Si1*-P1, and 1.918e for Si1*-P1*) as well as two
S
Supporting Information. Experimental details including
b
spectroscopic data, CIF files, and DFT computational details
for compounds 1-3 and 6; complete ref 10. This material is
available free of charge via Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
matthias.driess@tu-berlin.de;
shigeyoshi.inoue@mailbox.tu-berlin.de
’ ACKNOWLEDGMENT
This work was financially supported by the Deutsche For-
schungsgemeinschaft (DR 226/17-1). We thank the Alexander
von Humboldt Foundation (S.I. and M.A.) for financial support.
This work is dedicated to Prof. Siegfried Blechert on the occassion of
his 65th bithday.
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