Zwitterionic Si-C-Si-P and Si-P-Si-P four-membered rings with two-coordinate phosphorus atoms

Article abstract of DOI:10.1002/anie.201005866

(Figure Presented) It's hip to be square: The reaction of amidinato chlorosilylene (PhC-(NtBu)₂SiCl) with adamantyl phosphaalkyne and white phosphorus affords the formation of Si-C-Si-P and Si-P-Si-P (see picture) four-membered rings. Both compounds were characterized by singlecrystal X-ray studies and by solution and solid-state NMR spectroscopy. DFT calculations elucidated the bonding situation.

Full text of DOI:10.1002/anie.201005866

Communications
DOI: 10.1002/anie.201005866
Silicon Chemistry
Zwitterionic SiK-C-Si-PL and SiK-P-Si-PL Four-Membered Rings with
Two-Coordinate Phosphorus Atoms**
Sakya S. Sen, Shabana Khan, Herbert W. Roesky,* Daniel Kratzert, Kathrin Meindl, Julian Henn,
Dietmar Stalke,* Jean-Philippe Demers, and Adam Lange
Dedicated to Professor Roald Hoffmann
In contrast to the rich and ubiquitous chemistry displayed by
silicon,[1] the chemistry of unsaturated four-membered het-
erocycles containing silicon and phosphorus is not known.
This situation is perhaps due to the instability associated with
antiaromaticity for the cyclobutadiene ring systems, although
it is known that the introduction of heteroatoms will
substantially stabilize such rings. In a 1998 review, Bertrand
clarified the bonding situation of four-membered hetero-
cycles with four p electrons containing a phosphorus atom
that has no p orbital available for contributing to the
p system.[2] He showed that in these compounds either there
is interrupted cyclic p delocalization or they are stable owing
to a zwitterionic character, and he concluded that these
compounds are not antiaromatic. This observation prompted
us to design four-membered rings containing silicon and
phosphorus as heteroatoms and to elucidate their bonding
properties. Furthermore, nitrogen- or phosphorus-doped SiC
systems have commercial applications as semiconducting
materials.[3]
Scheme 1. Preparation of compound 3.
À308C in 1:3 molar ratio resulted in the formation of 3, which
was isolated in 68.2% yield as reddish-yellow air- and
moisture-sensitive crystals (Scheme 1). The constitution of 3
was unequivocally characterized by single-crystal X-ray
diffraction.[7] Figure 1 depicts the molecular view of 3, while
selected bond lengths and bond angles are provided in the
legend of Figure 1. Compound 3 crystallizes in the ortho-
rhombic space group P212121. The core structure consists of
two silicon atoms, one carbon atom, and one phosphorus
atom, which together form a SiK-C-Si-PL four-membered ring.
The predominant structural feature of 3 is the “naked”
phosphorus atom, which connects the two silicon atoms. The
two silicon atoms are each four-coordinate and display a
distorted tetrahedral geometry by coordination with two
nitrogen atoms from the amidinato ligand. The remaining two
The easy availability of heteroleptic chlorosilylene LSiCl
[5]
(1)[4b] and bis-silylene (LSi SiL) (2) (L = PhC(NtBu)2)
À
creates new dimensions in silicon chemistry. The reactions
of these systems with unsaturated organic compounds are
thermodynamically favorable owing to the oxidation of
silicon from the divalent to the tetravalent state.[4b,6] Herein
we report the synthesis of the 1,3-disilacarbaphosphide 3,
containing three heteroatoms, that resulted from the reac-
ꢀ
ꢀ
tions of 1 and 2 with RC P under cleavage of the C P bond.
The synthesis of 3 is straightforward. Addition of a
À ꢀ
toluene solution of 1-Ad C P to the toluene solution of 1 at
[*] Dr. S. S. Sen, Dr. S. Khan, Prof. Dr. H. W. Roesky, D. Kratzert,
Dr. K. Meindl, Dr. J. Henn, Prof. Dr. D. Stalke
Institut fꢁr Anorganische Chemie, Universitꢂt Gꢃttingen
Tammannstrasse 4, 37077 Gꢃttingen (Germany)
Fax: (+49)551-39-3373
E-mail: hroesky@gwdg.de
dstalke@chemie.uni-goettingen.de
J.-P. Demers, Dr. A. Lange
Solid-State NMR, Max-Planck-Institut fꢁr biophysikalische Chemie
Am Faßberg 11, 37077 Gꢃttingen (Germany)
Figure 1. Molecular structure of compound 3. Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms have been removed for clarity. Selected bond lengths [ꢀ] and
angles [8] N(1)–Si(1) 1.8646(17), N(2)–Si(1) 1.8653(18), P(1)–Si(2)
2.1894(7), P(1)–Si(1) 2.2012(7), Si(1)–C(31) 1.7808(18), Si(2)–C(31)
1.7834(18); Si(2)-P(1)-Si(1) 68.86(3), C(31)-Si(1)-P(1) 101.23(6),
N(1)-Si(1)-P(1) 117.63(6), N(2)-Si(1)-P(1) 119.32(7), C(31)-Si(2)-P(1)
101.59(6), N(3)-Si(2)-P(1) 119.44(7), N(4)-Si(2)-P(1) 120.78(7).
[**] Support from the Deutsche Forschungsgemeinschaft (Emmy
Noether Fellowship to A.L.), Deutscher Akademischer Austausch
Dienst (Research fellowship to S.K.), and the NSERC of Canada
(scholarship to J.-P.D.) is gratefully acknowledged.
Supporting information for this article, including experimental
details, is available on the WWW under http://dx.doi.org/10.1002/
anie.201005866.
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Angew. Chem. Int. Ed. 2011, 50, 2322 –2325

sites of the tetrahedron are occupied by the carbon and
phosphorus atoms. The bond lengths of the phosphorus atom
to the atoms Si(1) and Si(2) are 2.2012(7) and 2.1894(7) ꢀ,
the phosphorus atoms are not protonated, since maximal
intensity is reached after a long contact time of 5.6 ms (31P) or
5.1 ms (29Si). This finding confirms the presence of a lone pair
of electrons at P(1). In the EI-MS spectrum, the molecular ion
is detected as the most abundant species with highest relative
intensity at m/z 696. The absorption maximum of the UV/Vis
spectrum in THF was detected at 259 nm (e =
18300 cmÀ1mÀ1).
The successful isolation of 3 prompted us to verify
whether it is possible to substitute the remaining carbon
atom in 3 by another phosphorus atom. Theoretically, the
replacement of the RC fragment in 3 with another phospho-
rus atom is possible, because these two fragments are
isolobal,[13] and Scherer and co-workers reported a myriad
of polycyclic phosphorus compounds using this relation-
ship.[14] Herein we report the synthesis and structure of the
first complex with a SiK-P-Si-PL core through activation of the P4
molecule[15] by silylene (Scheme 2).
respectively, with a Si-P-Si angle of 68.868. The bond lengths
[8a]
À
are between the average Si P single bond lengths of 2.25 ꢀ
[8b]
À
and Si P double bond length of 2.09 ꢀ.
Moreover, the
À
À
Si(1) C(31) and Si(2) C(31) bonds (1.7808(18) and
À
1.7834(18) ꢀ) are significantly shorter than the Si C single
bond length reported in literature (1.86–1.93 ꢀ)[6b,9a] and
À
=
slightly longer than the Si C double bond found in (TMS)2Si
C(OTMS)-2-Ad (1.764 ꢀ; TMS = trimethylsilyl).[9b] This find-
ing can be interpreted in terms of a zwitterionic electronic
structure in which the positive charge is located on the Si-C-Si
fragment and the negative charge at the two-coordinate
phosphorus atom. A similar explanation was given by Frank
et al. for a mixed-valent tetraphosphete, and they confirmed
the predominance of the bis-ylidic formulation over the
delocalized-double-bond description by charge density cal-
culation of the P4 ring.[10] It is noteworthy that the Si C bond
Treatment of 1 and P4 in toluene overnight resulted in the
formation of the SiK-P-Si-PL unit stabilized by the amidinato
ligand (L2Si2P2; L = PhC(NtBu)2; 4). After recrystallization
from THF, 4 was isolated as air- and moisture-sensitive yellow
crystals in 60% yield. The 1H NMR spectrum shows a singlet
at d = 1.36 ppm, which is shifted downfield in comparison to
À
lengths in 3 match excellently with those of the 1,2-disila-
benzene derivative (1.804(4) and 1.799(5) ꢀ) reported by
Sekiguchi and co-workers[11] and the recently published 1,4-
disilabenzene (1.800(3) ꢀ) in which the electron density
within the Si C bonds is delocalized.[6d] The geometry of the
À
three-coordinate C(31) atom can be best described as
distorted trigonal planar, with the sum of bond angles
around C(31) of 359.968. Atom P(1) also adopts a distorted
trigonal-planar geometry when the lone pair is also consid-
ered. A similar type of naked bridging phosphorus atom is
very unusual in the literature.[10,12] Another interesting feature
is the interatomic Si···Si distance (2.48 ꢀ), which conclusively
À
demonstrated that there is no Si Si bond in 3. Compound 3
Scheme 2. Preparation of compound 4.
contains a chain of three four-membered rings, and in the
spirocyclic structure each silicon atom is part of two four-
membered rings.
that of 1 (d = 1.08 ppm). The 31P NMR spectrum of 4 displays
a sharp singlet at d = À166 ppm. This downfield shift is
presumably due to the presence of two naked phosphorus
atoms. A similar trend is observed in the 29Si NMR spectrum,
which shows a resonance at d = 25.6 ppm that is shifted about
30 ppm downfield compared with that of 3, although the
magnitude of the coupling constant (1JSi-P = 109.02 Hz)
remains comparable. The solid-state 31P NMR spectrum of 4
shows a sharp resonance with an isotropic chemical shift of
d = À166.4 ppm. In the solid-state 29Si NMR spectrum, a
triplet is observed at d = 24.0 ppm, with a coupling constant of
1JSi-P = 97.7 Hz (see the Supporting Information, Figure S4.2).
The CP build-up curves reach maximal intensity after 11.6 ms
(31P) or 7.8 ms (29Si), thus confirming that silicon and
phosphorus atoms are not protonated (see the Supporting
Information, Figure S4.1). The absorption maximum of the
UV/Vis spectrum in THF was detected at 234 nm (e =
15600 cmÀ1mÀ1). In the EI-MS spectrum, the molecular ion
is detected as the most abundant species, with highest relative
intensity at m/z 580.
Additionally, 3 was characterized by solution and solid-
state NMR spectroscopy, EI-MS, and elemental analysis. In
the 1H NMR spectrum, four sets of resonances are observed.
The tBu protons appear at d = 1.26 ppm and are thus shifted
downfield compared to 1 (d = 1.08 ppm). An additional set of
resonances is also found in the 1H NMR spectrum (d =
1.17 ppm). This signal indicates the formation of LSiCl3 as
the side product. The 1H-coupled and 1H-decoupled 31P NMR
spectra of 3 show a sharp singlet at d = À243 ppm, which can
be assigned to the naked phosphorus atom. The value of the
chemical shift is consistent with the assumption of a zwitter-
ionic structure for 3. In the 29Si NMR spectrum, a sharp
doublet is detected at d = À5.1 ppm with a coupling constant
of 1JSi-P = 75.45 Hz, which is quite small compared to the Si P
À
coupling constants found in literature.[8] The formation of
LSiCl3 was confirmed by 29Si NMR spectroscopy by a sharp
resonance detected at d = À98.4 ppm, which corresponds to
the five-coordinate silicon atom of LSiCl3.[4a] In the 31P solid-
state NMR spectrum of 3, the most intense resonance appears
at an isotropic chemical shift of d = À326.1 ppm (see the
Supporting Information, Figure S4.2). The 29Si NMR spectral
resonance with the highest intensity appears at d = À8.4 ppm.
The cross-polarization (CP) build-up curves (see the Support-
ing Information, Figure S4.1) show that both the silicon and
The proposed constitution of 4 was confirmed by single-
crystal X-ray diffraction (Figure 2). Crystals of 4 were grown
by cooling a concentrated THF solution to À328C. Com-
pound 4 crystallizes in the monoclinic space group C2/c with a
mirror plane bisecting the molecule.[7] The most apparent
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Communications
of antiaromatic character with two delocalized double bonds
or if there is only one delocalized double bond in the Si-C-Si
moiety and two lone pairs of electrons at the phosphorus
atoms, we performed DFT gas-phase structural optimizations
and frequency calculations at the B3LYP/6-311G* level of
theory.[16] The Si P bond lengths (2.21372/2.21238 ꢀ) in 3 are
À
slightly elongated compared to the experimental results and
À
are clearly more in the range of a Si P single bond than of a
À
À
Si P double bond, while the Si C bond lengths are in the
range between a single and a double bond (1.79472/
1.79371 ꢀ). Isotropic and anisotropic 31P chemical shieldings
as determined by theory are in agreement with values
extracted from the spinning sideband pattern of solid-state
NMR spectra (see the Supporting Information, Table S4.1
and Figure S4.2).
Figure 2. Molecular structure of compound 4. Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms have been removed for clarity. Selected bond lengths [ꢀ] and
angles [8] N(1)–Si(1) 1.8585(14), N(2)–Si(2) 1.8524(15), P(3)–Si(2)
2.1737(6), P(3)–Si(1) 2.1742(6), Si(1)–N(1A) 1.8586(14), Si(1)-P(3A)
2.1742(6), Si(2)–P(3A) 2.1737(6); Si(2)-P(3)-Si(1) 72.45(2), N(1)-Si(1)-
N(1A) 170.53(9), N(1)-Si(1)-P(3) 118.61(4), N(1A)-Si(1)-P(3)
119.11(4), N(1)-Si(1)-P(3A) 119.11(4), N(1A)-Si(1)-P(3A) 118.61(4),
P(3)-Si(1)-P(3A) 107.54(3), P(3)-Si(2)-P(3A) 107.57(3). Symmetry gen-
erated A: Àx, y, Àz+1/2.
Additionally, the nucleus-independent chemical shift
(NICS) at 1 ꢀ above the ring centers was calculated.[17] The
NICS(1) of the SiK-C-Si-PL ring is À4.17 (NICS(0) = À12.42,
NICS(2) = + 0.14), which clearly shows that the ring has no
antiaromatic character.
To investigate the electronic situation in the vicinity of the
phosphorus atom more closely, the electron localizability
index (ELI-d) was calculated (Figure 3).[18] There are two
monosynaptic basins at the phosphorus atom which are
feature of 4 is the planar SiK-P-Si-PL four-membered ring. The
À
planar ring consists of four equivalent Si P bonds of 2.174 ꢀ.
À
The value is roughly the intermediate between the normal Si
P single bond (2.25 ꢀ) and normal Si P bond (2.09 ꢀ)[8] and
=
matches well with that of 3. The Si····Si interatomic separation
À
is 2.57 ꢀ, which shows there is no Si Si bond in 4. Like 3, the
silicon atoms in 4 exhibit a distorted tetrahedral geometry,
whereas both the phosphorus atoms adopt a trigonal-planar
geometry when the lone pairs are considered. Another
important feature are the two equivalent Si-P-Si bond
angles of 72.45(2)8. The value is increased slightly compared
to that of 3. The two P-Si-P bond angles are exactly the same
(107.5(3)8). These data indicate the formation of a bis-ylide
structure in which the two silicon atoms are formally
positively charged, whereas the two phosphorus atoms carry
partial negative charges.
Figure 3. Representation of the ELI-d of compound 4 (isosurface value,
h=1.8).
Moreover, we probed the reactions of 2 with phosphaal-
kyne and with P4 (Scheme 3). The advantage of 2 over 1 is that
À
the former has two lone pairs of electrons and a labile Si Si
bond, so it is expected that 2 would be more reactive than 1.
populated with 1.95 and 1.97 electrons. This finding indicates
two lone pairs of electrons at the phosphorus atom. These
conclusions are supported by topology investigations,[19]
because two non-bonding valence shell charge concentrations
(VSCCs) are found at the phosphorus atom. Additionally, the
two disynaptic basins between the silicon and phosphorus
atoms are occupied by 1.96 electrons each, which indicates
two single bonds. On the other hand, the disynaptic basins
between the silicon and carbon atoms are occupied by 2.84
and 2.97 electrons, thus indicating a delocalized double bond.
This result is confirmed by the leading natural lewis structure
as given by a natural bond orbital (NBO) analysis,[20] which
shows that there are 1.91 electrons distributed between each
silicon atom and the phosphorus atom and 1.94 electrons
between each silicon atom and the carbon atom, and addi-
tional 1.74 electrons between one silicon atom and the carbon
atom, thus indicating electron delocalization over the Si-C-Si
moiety but not over the Si-P-Si skeleton. The natural charges
from the NBO analysis are À0.75 for the phosphorus atom,
+ 1.56 for each silicon atom, and À1.44 for the carbon atom,
thus confirming the zwitterionic character of the ring system.
À
Accordingly, we treated 2 with equimolar amounts of 1-Ad
C P in toluene and one-half equivalent of P4 at room
ꢀ
temperature in THF, respectively. In the former case a color
change of the solution from dark to pale red was observed,
whereas the latter reaction resulted in the formation of a
bright yellow solution within minutes. Multinuclear NMR
spectroscopic and EI-MS data showed that the reactions led
quantitatively to products 3 and 4, respectively. It must be
noted that unlike in previous cases, no formation of any side
product was detected by NMR spectroscopy.
To elucidate the nature of the bonding and to investigate
whether the SiK-C-Si-PL ring of 3 and the SiK-P-Si-PL ring of 4 are
Scheme 3. Alternate synthesis of 3 and 4.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2322 –2325

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In 4 the Si P bond lengths are slightly shortened
compared to
3
(2.19835/2.19838/2.19835/2.19838 ꢀ), but
À
they are again still in the range of a Si P single bond. The
NICS(1) value is À2.56 (NICS(0) = À5.92, NICS(2) = + 0.44),
which shows that the ring has no antiaromatic character. The
ELI-d investigations reveal two monosynaptic basins at each
of the phosphorus atoms, which are occupied by 1.86 and 1.88
electrons each, again indicating two lone pairs of electrons at
each of the phosphorus atoms. The disynaptic basins between
the silicon and phosphorus atoms are occupied by 2.02, 2.03,
2.03, and 2.03 electrons, thus indicating four single bonds in
the ring. In accordance with the ELI-d findings, topological
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ˇ
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À
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Keywords: isolobal relationship · phosphaalkynes · silicon ·
small ring systems · solid-state structures
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