2-hydro-2-aminophosphasilene with N-Si-P π conjugation

Article abstract of DOI:10.1021/om3008184

The first 2-aminophosphasilene, [Ar(Me₃Si)N]HSi=PAr′ (4, Ar = 2,6-iPr₂C₆H₃, Ar′ = 2,6-Mes ₂C₆H₃), bearing a hydride ligand on the three-coordinate silicon atom has been synthesized and structurally characterized. Both X-ray data of 4 and DFT calculations on the model compound (H₂N)HSi=PH (4′) disclosed that the amino group on the silicon atom results in significant N-Si-P π conjugation.

Full text of DOI:10.1021/om3008184

Communication
pubs.acs.org/Organometallics
2‑Hydro-2-aminophosphasilene with N−Si−P π Conjugation
Haiyan Cui, Jianying Zhang, and Chunming Cui*
State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: The first 2-aminophosphasilene, [Ar(Me3Si)-
N]HSiPAr′ (4, Ar = 2,6-iPr2C6H3, Ar′ = 2,6-Mes2C6H3),
bearing a hydride ligand on the three-coordinate silicon atom
has been synthesized and structurally characterized. Both X-ray
data of 4 and DFT calculations on the model compound
(H2N)HSiPH (4′) disclosed that the amino group on the
silicon atom results in significant N−Si−P π conjugation.
here has been great interest in isolable multiply bonded
precursors for the synthesis of hydrophosphasilenes RHSi
Tsilicon compounds because of their unique chemistry and =PR via a 1,2-hydrogen shift. However, this type of
physical properties.1 These species are either kinetically
phosphasilylene has not been reported, as far as we know.
We have recently reported the synthesis of the NHC-
stabilized aminochlorosilylene Ar(SiMe3)NSiCl(NHC) (1; Ar
= 2,6-iPr2C6H3, NHC = 1,3-diisopropyl-4,5-dimethyl-imidazol-
2-ylidene) (Scheme 1),9 which appears to be a suitable
stabilized by bulky groups on both sides of the multiple
bonds or stabilized by a Lewis base via coordination to the
silicon atom. In contrast, doubly bonded silicon compounds in
which the three-coordinate silicon atom bears at least one
hydrogen atom are usually extremely reactive due to lack of
efficient protection on the silicon atoms and intrinsic high
reactivity of the Si−H bond. Nevertheless, they have been
extensively studied theoretically as simple models for the
understanding bonding natures in these compounds and also
investigated experimentally in noble-gas matrixes. Only very
recently a couple of hydrodisilenes (RHSiSiR2)2 and a 1-
hydrosilene (RHSiCR2)3 with a low-coordinate silicon atom
have been isolated by the Sekiguchi and Tokitoh groups, in
which the silicon atoms are protected by extremely bulky
groups. However, silicon−-group 15 double-bonded species of
the formula RHSiER (E = N, P) with a three-coordinate
silicon atom are still elusive, most likely, because of partial
cationic character of the silicon atom arising from the polarized
Siδ+−Eδ− (E = N, P) double bond. We report here the first
isolable phosphasilene with the sp2 silicon atom attached to a
hydride and an amino group. Interestingly, the compound
features a pronounced N−Si−P π conjugation, which efficiently
stabilizes the molecule.
Scheme 1. Synthesis of 2−4
precursor for the preparation of a Lewis base supported 1-
hydrophosphasilylene for the investigation of its 1,2-hydrogen
shift in the solution phase. As expected, reaction of 1 with
Ar′PHLi (Ar′ = 2,6-Mes2C6H3, Mes =2,4,6-Me3C6H2)10 in
THF at −78 °C yielded the NHC-supported 1-hydro-
phosphasilylene 2, along with another new product as indicated
by the NMR analysis of the crude product. Attempts to
crystallize the crude product from n-hexane led to a complete
conversion of 2 to the new compound. Obviously, 2 is not
stable in solution and is easily subject to an isomerization
process to give the new compound. In fact, the isomerization
even occurs in solution at low temperatures. By reduction of
the reaction time at 0 °C, 2 can be obtained as a major product
after the removal of the volatiles under vacuum. The
isomerization reaction has been studied in C6D6 and d8-THF
in NMR tubes at different temperatures with the crude product
by recording 31P NMR spectra. It was found that ca. 75% and
Several types of phosphasilenes (R2SiPR) with alkyl, aryl,
silyl, and phosphorus substituents on the silicon atoms have
been structurally characterized since their first synthesis in
1984.4−6 The half parent phosphasilene HPSi(SiBut3)(Trip)
(Trip = 2,4,6-iPr3C6H3) and its P-metalated products have been
reported by Driess and co-workers.5g,h However, phosphasi-
lenes with an amino substituent on the silicon atom have not
been reported so far. Theoretical calculations on the parent
phosphasilene H2SiPH and its silylene isomer HSiPH2
indicate that the silylene is only 14.3 kcal higher in energy
than H2SiPH.7 A matrix-spectroscopic study showed that
both isomers can be generated and easily isomerize to each
other under photochemical conditions.8 It appears that
phosphasilylenes with the formula RSi−PHR would be suitable
Received: August 24, 2012
Published: October 2, 2012
© 2012 American Chemical Society
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88% of 3 have been formed in d8-THF and C6D6 at 25 °C in 20
h, respectively. A complete isomerization to 3 was observed at
60 °C in 30 min in d8-THF. Heating the solid sample at 100 °C
for 2 h under vacuum also led to the complete isomerization
reaction. Since 2 cannot be isolated in pure form, only the
spectroscopic analyses of 2 have been carried out. The 29Si
NMR spectrum of 2 shows a resonance at δ −8.78 ppm (d,
1JSi−P = 145.4 Hz) for the divalent silicon atom, which is shifted
upfield in comparison with that observed for 1 (δ 3.14 ppm)
because of the electron-donating properties of the Ar′PH
adopts a distorted-tetrahedral geometry. The Si1−P1 bond
length of 2.1652(8) Å is much longer than those found in the
structurally characterized phosphasilenes (ca. 2.053−2.111 Å)
but shorter than a typical Si−P single bond (average 2.25 Å).
The long Si1−P1 bond distance and the high-field chemical
shift observed in the 31P NMR spectrum suggest that the
zwitterionic resonance form (Scheme 2) makes a significant
Scheme 2. Resonance Forms for 3
group.9 The 31P resonance appears at δ −88.1 ppm. The H
1
NMR spectrum of 2 exhibits a doublet at δ 4.22 ppm (1JP−H
=
197.5 Hz) for the P−H proton, and the stretching absorption
for the P−H in the infrared spectrum is centered at 2308 cm−1.
These spectroscopic data are consistent with the structure of 2.
Pure 3 can be easily obtained as red crystals by simple
crystallization of the crude product of 2 from n-hexane at room
temperature. The facile isomerization of 2 to 3 indicates that
the barrier for the 1,2-hydrogen shift from the phosphorus to
silicon is very small and 3 is more stable than 2 under these
conditions, even though there exists the Lewis base NHC
coordinated to the silicon atom. Comparison of the
experimental results with the theoretical calculations on
HSiPH2 and H2SiPH reveals that the coordination of the
Lewis base to the silicon atom does not alter their relative
energy levels.7 This is the first experimental observation of a
1,2-H hydrogen shift of phosphasilylenes from phosphorus to
silicon atom in solution and might be useful for the synthesis of
other substituted phosphasilenes with a Si−H bond. 3 has been
fully characterized by multiple NMR spectroscopy, elemental
contribution to the structure of 3. The Si1−P1−C27 bond
angle of 106.21° is comparable to that (104.2°) observed in the
first structurally characterized P-aryl-substituted phosphasilene
reported by Niecke and co-workers.4 It is noted that the only
e
known structurally characterized Lewis base supported
phosphasilene LSi(SiMe3)PSiMe3 was stabilized by a
bidentate amidinate ligand.5c Compound 3 is unique in that
it is supported by a monodentate Lewis base. Therefore, it is
possible to generate the corresponding base-free phosphasilene
from 3 by the removal of the coordinated base with a suitable
acid.12
In order to prepare the corresponding NHC-free phospha-
silene from 3, several types of Lewis acids and different
conditions were screened. The phosphasilene 4 with a three-
coordinate silicon atom was finally obtained in good yield by
the reaction of 3 with BPh3 in benzene at 80 °C for 5 h
(Scheme 1). The reaction proved to be very clean on an NMR
scale to give 4 and the adduct NHC·BPh3, on the basis of NMR
analysis. Yellow crystals of 4 were obtained in 45% yield by
analysis, and X-ray single crystal analysis. The 29Si NMR
1
spectrum shows a doublet at δ −21.1 ppm (PSi, JP−Si
=
144.4 Hz) and a singlet at δ 6.63 ppm (SiMe3). The resonance
due to the central silicon atom is shifted significantly upfield
relative to that observed in the N-donor-stabilized phosphasi-
lene LSi(SiMe3)PSiMe3 (L = PhC(NBut)2, δ 40.5 ppm) with
a four-coordinate silicon atom, owing to the strong electron
donating property of the coordinated N-heterocyclic carbene
(NHC).6c The unusually large shielding of the phosphorus
nuclei at δ −128.4 ppm observed in the 31P NMR spectrum of
3 relative to those observed in the P-aryl-substituted
phosphasilenes (δ > 65.8 ppm) is indicative of the strong
anionic character of the phosphorus atom.11
1
crystallization from n-hexane. The H NMR spectrum clearly
indicates the absence of the NHC in 4. The 29Si NMR
spectrum displays one singlet at δ 11.1 ppm for SiMe3 and one
1
doublet at δ 133.4 ppm (SiP, JP−Si = 157.0 Hz). The latter
1
greatly deshielded Si resonance and the large JP−Si coupling
constant are characteristic of an sp2 silicon atom and the
existence of a SiP π bond.4 However, this chemical shift is
shifted greatly upfield relative to those in the known
phosphasilenes (δ >170 ppm) with a three-coordinate silicon
atom, probably due to the electron-donating nature of the
amino group on the silicon atom.5,6 The 31P resonance at δ
5.42 ppm in 4 is shifted significantly downfield in comparison
to that observed for 3 but still strongly shielded in comparison
to those in the P-aryl-substituted phosphasilenes that have been
reported. These data suggest that the existence of the amino
group on the silicon atom has a significant effect on the SiP
double bond. To the best of our knowledge, phosphasilenes
bearing an amino group on the unsaturated silicon atom have
not been reported.
The molecular structure of 3 is shown in Figure 1 with
selected bond parameters. The Si1 atom is four-coordinate and
An X-ray single-crystal analysis of 4 reveals the essentially
planar Si atom (sum of the angles 359.53°) and the short Si1−
P1 bond length of 2.0718(6) Å (Figure 2). This distance is in
good agreement with the calculated value (2.0748 Å) for
H2SiPH at the MP2(full)/6-31G(d) level and falls within the
reported range (ca. 2.053−2.11 Å) for the structurally
characterized phosphasilenes. The other notable structural
features for 4 include a short Si1−N1 distance (1.7000(13) Å)
and almost coplanarity of the three-coordinate N1 plane with
Figure 1. Ortep drawing of 3 with 30% probability ellipsoids. Selected
bond lengths (Å) and angles (deg): Si1−P1 = 2.1652(8), Si1−N1 =
1.7561(11), Si2−N1 = 1.7633(11), P1−C27 = 1.8642(14), Si1−C16
= 1.9707(13); N1−Si1−P1 = 116.45(4), Si1−P1−C27 = 106.21(4).
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Communication
some contribution from the nitrogen lone pair. The natural
bond orbital analysis (NBO) indicates a high occupancy (1.997
27 e) of the Si−P π bond formed by the p orbitals of the Si and
P atoms and the Si−P σ bond formed by the phosphorus sp5.77
hybrid orbital with the silicon sp1.41 hybrid orbital. NBO
analysis shows a strong donor−acceptor interaction (25.18
kcal/mol) between the nitrogen lone pair and the Si−P π*
orbital, indicating significant N−Si−P π conjugation. Compar-
ison of the calculated results for 4′ with those reported for
H2SiPH indicates the remarkable effects of the amino group
attached to the silicon atom on the electronic structures of
these types of phosphasilenes.
In summary, the first 2-aminophosphasilene 4 bearing a
hydride ligand on the three-coordinate silicon atom has been
synthesized and structurally characterized. Both X-ray data of 4
and DFT calculations on 4′ show that the amino group on the
silicon atom results in significant N−Si−P π conjugation.
Studies on the reactivity of 3 and 4, especially the relative
reactivities of the Si−H bond and SiP double bond of 4
toward unsaturated molecules, are currently in progress.
Figure 2. Ortep drawing of 4 with 30% probability ellipsoids. Selected
bond lengths (Å) and angles (deg): Si1−P1 = 2.0718(6), Si1−N1 =
1.7000(13), Si2−N1 = 1.7755(13), P1−C16 = 1.8588(15); N1−Si1−
P1 = 124.03(5), Si1−P1−C16 = 96.86(5), N1−Si1−H1 = 108.8(7),
H1−Si1−P1 = 126.7(7).
the Si−P1−C16 plane. The Si1−N1 distance is much shorter
than the neighboring Si2−N1 single-bond length (1.7755(13)
Å). These features are indicative of the sp2 hybrid Si1 atom and
the significant nitrogen lone pair interaction with the Si1−P π
bond, leading to the shortened Si1−N1 bond length.
Consequently, the pronounced N1−Si1−P1 π conjugation
could result in increased electron density on the P1 atom and
thus the small Si1−P1−C16 angle (96.86°). The upfield-shifted
31P resonance of 4 relative to those observed in the other P-
aryl-substituted phophasilenes also strongly supports the π
conjugation.5c,6b 4 only exists as the E isomer in solution and in
the solid state, since there exists a small hydride ligand on the
low-coordinate silicon atom and efficient π conjugation. On the
basis of the structural and spectroscopic analysis, 4 can be best
represented by the two resonance forms depicted in Scheme 3.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF files giving details of the synthesis
and characterization data for the compounds reported in this
paper and crystallographic data for compounds 3 and 4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
■
Notes
The authors declare no competing financial interest.
Scheme 3. Resonance Forms for 4
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China and the National Basic Program (973 program, Grant
No. 2012CB821600) for support of this work.
REFERENCES
■
4 is the first example of a 2-aminophosphasilene bearing a
hydride ligand on the three-coordinate silicon atom. The
successful isolation of 4 may be largely attributed to the
significant π conjugation of the amino nitrogen lone pair with
the Si−P π bond, leading to increased electron density on the
silicon atom and reduced polarity of the Si−P bond. The
isolation and structural characterization of both 3 and 4 provide
an unprecedented opportunity for the comparison of the
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