From disilene (Si=Si) to phosphasilene (Si=P) and phosphacumulene (P=C=N)

Article abstract of DOI:10.1002/anie.201308525

The generation of heavier double-bond systems without by- or side-product formation is of considerable importance for their application in synthesis. Peripheral functional groups in such alkene homologues are promising in this regard owing to their inherent mobility. Depending on the steric demand of the N-alkyl substituent R, the reaction of disilenide Ar₂Si=Si(Ar)Li (Ar=2,4,6-iPr₃C₆H₂) with ClP(NR ₂)₂ either affords the phosphinodisilene Ar ₂Si=Si(Ar)P(NR₂)₂ (for R=iPr) or P-amino functionalized phosphasilenes Ar₂(R₂N)Si-Si(Ar)=P(NR ₂) (for R=Et, Me) by 1,3-migration of one of the amino groups. In case of R=Me, upon addition of one equivalent of tert-butylisonitrile a second amino group shift occurs to yield the 1-aza-3-phosphaallene Ar ₂(R₂N)Si-Si(NR₂)(Ar)-P=C=NtBu with pronounced ylidic character. All new compounds were fully characterized by multinuclear NMR spectroscopy as well as single-crystal X-ray diffraction and DFT calculations in selected cases. Copyright

Full text of DOI:10.1002/anie.201308525

.
Angewandte
Communications
DOI: 10.1002/anie.201308525
Heavier Double Bonds
=
=
From Disilene (Si Si) to Phosphasilene (Si P) and Phosphacumulene
(P C N)**
= =
Philipp Willmes, Michael J. Cowley, Marco Hartmann, Michael Zimmer, Volker Huch, and
David Scheschkewitz*
=
Abstract: The generation of heavier double-bond systems
without by- or side-product formation is of considerable
importance for their application in synthesis. Peripheral func-
tional groups in such alkene homologues are promising in this
regard owing to their inherent mobility. Depending on the
steric demand of the N-alkyl substituent R, the reaction of
a lead(II) moiety[11] to the Si P unit, thus yielding P-
metalated phosphasilenes.
For the prospect of the synthesis of novel polymeric
materials, the interconversion of low-valent functionalities is
crucial, preferably in reversible manner and free of by-
products.[12] While the transformation of an Si Si moiety of
=
=
=
disilenide Ar2Si Si(Ar)Li (Ar= 2,4,6-iPr3C6H2) with ClP-
anionic disilenides into heteronuclear silenes (Si C) with
concomitant LiCl elimination has been documented,[13] the
addition of a disilenide to a bulky ketone followed by
Peterson-type rearrangement to a silene remained the sole
example of such reactivity without systematic by-product
formation until very recently.[14] Earlier this year, we reported
=
(NR2)2 either affords the phosphinodisilene Ar2Si Si(Ar)P-
(NR2)2 (for R = iPr) or P-amino functionalized phosphasilenes
Ar2(R2N)Si Si(Ar) P(NR2) (for R = Et, Me) by 1,3-migration
of one of the amino groups. In case of R = Me, upon addition
of one equivalent of tert-butylisonitrile a second amino group
À
=
À
=
shift occurs to yield the 1-aza-3-phosphaallene Ar2(R2N)Si
the complete cleavage of an Si Si bond of unsymmetrically
À = =
=
Si(NR2)(Ar) P C NtBu with pronounced ylidic character.
substituted disilenes by isonitriles to afford silenes with Si C
All new compounds were fully characterized by multinuclear
NMR spectroscopy as well as single-crystal X-ray diffraction
and DFT calculations in selected cases.
bond, a reaction that can be fully reversed by BEt3 as
isonitrile scavenger.[15] Indications of reversibility were also
found in case of the reaction of isonitriles with cyclotrisilenes
yielding imino-functionalized cyclic disilenes.[16] The reversi-
fter the isolation of the first disilene,[1] heteronuclear
ble formation of a base-stabilized arsasilene (Si As) as
=
A
double bonds with two heavier main-group elements soon
moved into focus.[2] In particular, phosphasilenes allowed
early on for the introduction of peripheral functionality,[3]
while the emphasis in disilene chemistry was initially directed
reported by Driess et al. constitutes a remarkable milestone
with heteronuclear double bond.[17] A related phosphasilene
is capable of transferring the parent phosphinidene fragment
to N-heterocyclic carbenes.[18]
We recently reported stable dialkylphosphino disilenes
1 (Scheme 1) and their use as ligands in palladium com-
to reactivity studies of unfunctionalized Si Si moieties.[4] For
=
several years, functional disilenes have enjoyed increased
attention,[5] which generates opportunities regarding the
exploitation of the physical and chemical properties of the
=
Si Si bond. For example, the synthesis of p-conjugated
systems lithium disilenides[6] results in better yields than the
classical reductive coupling techniques, although the latter
approach is still remarkably successful. Notably, it allowed
Tamao et al. to obtain air-stable and luminescent disilenes[7] as
Scheme 1. P-Diorganophosphino disilenes 1 (Tip=2,4,6-iPr3C6H2;
R=alkyl, aryl)[19] and degenerate rearrangement of P-fluorosilyl phos-
phasilene 2.[20]
[8]
=
well as p-conjugated systems with Si P bonds. Very
recently, a proof-of-concept for organic light-emitting devices
based on Tamaoꢀs disilenes was provided.[9] “Half-parent”
phosphasilene HP SiTip Si(tBu)3 (Tip = 2,4,6-iPr3C6H2) has
plexes.[19] In consideration of the degenerate rearrangement
of phosphasilene 2, as described by Driess and co-workers,[20]
we anticipated that the use of donor-substituted phosphino
=
À
been employed for the grafting of a ZnMe[10] as well as
=
groups as substituent at the Si Si moiety might result in the
[*] P. Willmes, Dr. M. J. Cowley, M. Hartmann, Dr. M. Zimmer,
Dr. V. Huch, Prof. Dr. D. Scheschkewitz
1,3-migration of the functional donor-group to the b-silicon
atom of the double bond and thus initiate the conversion of
Krupp-Chair of General and Inorganic Chemistry
Saarland University, 66125 Saarbrꢀcken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
Homepage: http://www.uni-saarland.de/fak8/scheschkewitz/
index.html
=
=
Si Si into Si P double bonds.
Indeed, the reactions of disilenide 3 with diamino-
(chloro)phosphines depending on the bulkiness of the
amino substituents either afford a P-diaminophosphinodisi-
lene 4c (R = iPr) or result in the isomeric phosphasilenes 5a,b
as E/Z-mixtures. Phosphasilenes 5a,b are the formal products
of 1,3-migration of one of the amino groups of the plausible
but unobserved intermediates [4a,b]. The subsequent addi-
[**] Funding by the EPSRC (EP/H048804/1) and the Alfried Krupp
Foundation is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308525.
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Scheme 2. Synthesis of P-diaminophosphino disilene 4c, phosphasi-
lenes E/Z-5a,b and conversion of 5a into 1-aza-3-phosphaallene 6a
(Tip=2,4,6-iPr3C6H2; a: R=Me; b: R=Et; c: R=iPr).
tion of tert-butyl isocyanide to 5a yields the 1-aza-3-phos-
=
=
phaallene 6a, thus converting the Si P to an Si C bond
(Scheme 2).
Figure 1. Molecular structure of 4c in the solid state (ellipsoids set at
50% probability, H atoms and disordered iPr groups omitted).
Selected bond lengths [ꢁ]: Si1–Si2 2.1968(9), Si2–P1 2.3126(10), P1–
N1 1.699(2), P1–N2 1.714(2).
The diaminophosphino disilene 4c is isolated as orange
crystals from pentane in 76% yield. The doublet resonances
in the 29Si NMR spectrum at d = 90.7 and 71.1 ppm appear at
[5,6,19]
=
characteristic chemical shifts for Tip2Si SiTip moieties.
À
Curiously, the two P Si coupling constants are almost
electronegative substituents induce a higher s-character of the
lone pair at the phosphorus atom.
identical (1JPSi = 115 Hz; 2JPSi = 109 Hz), which suggests a cer-
tain through-space interaction between the phosphorus lone
pair and the b-silicon atom. Indeed, the shape of the
molecular orbitals as obtained by DFT calculations on
a truncated model 4cPh (Ph groups instead of Tip; see
Supporting Info for MOs) supports the presence of such an
interaction. The HOMOÀ1, although in principle related to
the lone pair at phosphorus, is extensively delocalized and
The phosphino disilene 4c melts at 1638C with almost
complete decomposition to an intractable mixture of products
(at least eight major 31P NMR signals between d =+ 100 and
À300 ppm alongside more than 20 signals of minor intensity)
with no indication of conversion into a hypothetical phos-
phasilene 5c. Conversely, in case of P-amino substituents with
reduced steric requirements (R = Me, Et), the E/Z-isomeric
mixtures of the phosphasilenes 5a,b are directly obtained
from disilenide 3 and the appropriate chlorophosphine.
Apparently, the formation of 5a,b results from the 1,3-
migration of one of the amino groups from phosphorus to
silicon in the plausible but undetected intermediates [4a,b]
(Scheme 2). DFT calculations on model systems 4a–cPh and
5a–cPh confirm, despite truncation (Ph instead of Tip), that
the phosphasilene isomer is thermodynamically favored in the
less-congested systems (E-5aPh vs. 4aPh: DG298 = À28.8 kcal
molÀ1; E-5cPh vs. 4cPh: DG298 = À18.1 kcalmolÀ1; for details
see Supporting Information). Both crude products are
reasonably pure, but only 5a (R = Me) could be isolated as
bright yellow crystals in an excellent yield (97%). The
31P NMR spectra reveal chemical shifts at the downfield end
of the spectrum for phosphasilenes at d31P = 344.8 and
336.2 ppm for the two isomers of 5a and at d31P = 344.8 and
336.7 ppm for those of 5b (E/Z-ratio 5a 95:5, 5b 70:30). More
deshielded 31P signals were only observed by Sekiguchi et al.
in case of their “push–pull”-substituted phosphasilenes, for
À
shows a significant Si Si s-contribution. Furthermore, admix-
ture of the non-bonding orbital at the phosphorus center is
readily discerned in the predominantly Si Si p-bonding
À
HOMO (less so in the mostly phosphorus-centered
HOMOÀ3). The fairly non-diagnostic 31P NMR signal of 4c
at d = 58.4 ppm is observed in the typical region for bis(dii-
sopropylamino)-substituted phosphorus compounds between
30 and 100 ppm.[21] The longest wavelength absorption in the
UV/Vis spectrum of 4c at lmax = 441 nm is red-shifted by
about 20 nm compared to dialkylphosphino disilenes.[19]
Based on its intensity (e = 23600 LmolÀ1 cmÀ1), it is assigned
to the p–p* transition.
An X-ray diffraction study on single crystals of 4c
confirmed the constitution of
a
phosphino disilene
(Figure 1)[22] even though the Si Si and the Si P bonds are
=
À
À
À
significantly longer (Si1 Si2 2.1968(9), Si2 P1 2.3126(10) ꢁ)
than the corresponding bonds of the previously reported
dicyclohexylphosphino disilene 1a (Scheme 1, R = c-hexyl).
This observation is, however, in line with the considerably
more pronounced trans-bending of the Si Si bond in 4c
which a certain degree of inverse polarization[25] of the Si P
=
=
compared to the moderately trans-bent 1a (4c: qSiTip = 30.88;
qSiTip2 = 19.28; 1a: qSiTip = 7.78; qSiTip2 = 8.98). The Carter–
Goddard–Malrieu–Trinquier (CGMT) model predicts stron-
ger trans-bending with more electronegative substituents
(dialkylphosphino in 1a vs. diaminophosphino in 4c).[23] The
phosphorus atom in 4c is significantly more pyramidal than
that of the P-cyclohexyl-substituted counterpart 1a (sum of
angles at P1: 320.918). According to Bentꢀs rule,[24] more
bond had been implied.[26] DFT calculation of the Mulliken
charges of the model compound E-5aPh shows a truly inverse
polarization in this case (Si2: À0.095, P1: + 0.169), which
without doubt can be attributed to the electronegative, p-
donating amino group at the phosphorus end of the double
bond.[27] In contrast to the phosphino disilene 4c, the
magnitude of coupling within the typical range for phospha-
silenes readily distinguishes the two different silicon nuclei in
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Angewandte
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5a (E-5a 1JP-Si = 186.5 Hz, 2JP-Si = 40.8 Hz). For the NEt2
derivative 5b, the broad 31P signals at room temperature
suggested that the interconversion of the two stereoisomers
may be fast on the NMR timescale. Indeed, at 243 K the
Si2: 358.858), thus further confirming the “inverse polar-
=
ization” of the P1 Si2 bond. This is in line with the
planarization of both NMe2 groups by an apparent electron
donation of the lone pairs to the adjacent phosphorus and
silicon atoms (sum of angles: N1 359.58, N2 359.58).
signals sharpened sufficiently for the determination of the 2JP-
1
coupling constants of both isomers (at 243 K: E-5b JP-Si
=
The synthesis of stable 1-aza-3-phosphaallenes is usually
achieved by either intra- or intermolecular elimination of
disiloxanes or silanolates, respectively.[35] We reasoned that
the presence of a potentially mobile amino functionality as
the sole functionality at phosphorus might turn the phospha-
silene 5a into a suitable phosphinidene equivalent (PR) that
could be combined with isonitriles to afford a 1-aza-3-
phosphaallene without any systematically formed by-product.
Indeed, treatment of 5a with one equivalent of tert-butyl
isonitrile leads to full conversion into the 1-aza-3-phosphaal-
lene 6a after heating to 558C in toluene overnight, which was
isolated a pale yellow crystals from toluene in 77% yield. The
constitution of 6a was deduced by multinuclear NMR
analysis. Although 1-aza-3-phosphaallenes are typically
observed between about À140 to À60 ppm[35] in 31P NMR
spectra, the resonance of 6a at d = À214.5 ppm is in line with
a hitherto unknown P-silyl substituted derivative. Although
Si
187.6 Hz, 2JP-Si = 44.3 Hz; Z-5b 1JP-Si = 188.3 Hz, 2JP-Si
2
ꢀ 28 Hz). The larger JP-Si coupling that is due to the cis-
relationship of the lone pair at phosphorus and the b-silicon
atom allows for the unambiguous assignment of the major
isomer E-5b.[28]
Although E/Z isomerism has been observed by Driess
et al.[29] for unsymmetrically substituted phosphasilenes, the
high barrier prevented the coalescence of signals in the
observable temperature window.[30] This barrier is strongly
À
correlated to the strength of the P Si p-bond. In case of both
5a and 5b, the exchange is sufficiently fast at room temper-
ature, but owing to the more favorable ratio of isomers we
restricted a pulse-selective 31P NMR study[31–33] to 5b (see the
Supporting Information for details). The free energy of
activation for the conversion of E-5b into Z-5b was
calculated using the DynaFit3 program[34] to DG#300 = 59.5 Æ
0.8 kJmolÀ1 and for the reverse reaction to DG#300 = 55.5 Æ
the coupling constant of JPSi = 91.2 Hz to the a-silicon atom
1
À1
2.1 kJmol . The P Si p bond energy of a phosphasilene can
suggests single bonding, the 13C signal at d = 173.3 ppm with
1JPC = 87.0 Hz is in a typical range for the fairly ylidic
phosphaallene scaffold.
À
therefore be estimated for the first time on the basis of the
experimental data. The value of DG#347 ꢀ 64 kJmolÀ1
obtained from the coalescence of the 31P NMR signals of E/
Z-5b at 347 K roughly confirms these results and is only
marginally larger than the calculated value for predominantly
ylidic and therefore single-bonded phosphasilenes with nitro-
The molecular structure of 6a in the solid state confirmed
the structural assignment (Figure 3).[22] Compared to the
=
gen donors at the silicon end of the P Si bond recently
reported by the Driess group.[18]
Single crystals of E-5a were obtained from pentane. An
X-ray diffraction study confirms the constitution of a phos-
phasilene with dimethylamino groups bonded to phosphorus
and b-silicon, respectively (Figure 2).[22] The P1 Si2 bond
=
distance of 2.1187(7) ꢁ is fairly typical for phosphasilenes.
The tricoordinate Si2 is slightly pyramidalized (sum of angles
Figure 3. Molecular structure of 6a in the solid state (ellipsoids set at
50% probability, H atoms and disordered iPr groups omitted).
Selected bond lengths [ꢁ]: N3–C51 1.471(2), N3–C50 1.195(3), P1–C50
1.6739(19), Si2–P1 2.2934(7), Si2–N2 1.7357(17), Si1–Si2 2.5011(6).
crystallographically characterized 1-aza-3-phosphaallene
= =
reported by Yoshifujiꢀs group (Mes*P C NPh (7); Mes* =
tBu3C6H2),[35d] the P1–C50 bond distance is slightly elongated
(1.6739(19) for 6a vs. 1.651 ꢁ for 7) and the angle at the
phosphorus center significantly more acute (95.86(7)8 for 6a
vs. 99.28 for 7). In contrast, the C50–N3 bond is somewhat
shorter (1.195(3) for 6a vs. 1.209 ꢁ for 7) and the angle at the
imino nitrogen atom closer to linearity (142.0(2)8 for 6a vs.
130.58 for 7). These cumulative findings suggest a significant
ylidic contribution to the ground state of 6a along the lines of
Figure 2. Structure of E-5a in the solid state (ellipsoids set at 50%
probability, H atoms, co-crystallized pentane and disordered iPr
groups omitted). Selected bond lengths [ꢁ]: Si2–P1 2.1187(7), Si1–Si2
2.3895(7), P1–N1 1.6872(18), Si1–N2 1.7399(16).
À
+
À
À ꢁ
À
R P C N tBu (R = disilanyl moiety).
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[12] G. R. Whittell, M. D. Hager, U. S. Schubert, I. Manners, Nat.
In conclusion, we have demonstrated that with sufficiently
small amino substituents at phosphorus, phosphasilenes are
favored over isomeric phosphino disilenes. The amino groups
are rather mobile in this system and therefore addition of an
isonitrile as relatively strong donor induces the shift of the
second amino group from phosphorus (5a) to silicon (6a),
affording a 1-aza-3-phosphaallene in a novel by-product free
approach to a heterocumulene. Along with this kinetic
argument, the gradual strengthening of the double bonds
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=
=
=
from Si Si via Si P to P C is without doubt an important
contributor to the increasing thermodynamic stability. Inves-
tigations concerning the scope of this reactivity as well as
further manipulations of the residual functionality are
currently in progress.
Received: September 30, 2013
Revised: November 21, 2013
Published online: January 23, 2014
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Keywords: disilenes · heavier multiple bonding ·
heterocumulenes · phosphorus · silicon
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[22] Crystal structure determination at T= 152(2) K on Bruker
SMART APEX II CCD diffractometer with a Mo-Ka radiation.
4c: yellow blocks from pentane; C57H97N2PSi2, Mr = 897.52,
monoclinic, space group P21/n, a = 12.4858(7), b = 13.5009(8),
c = 34.718(2) ꢁ, b = 98.565(3)8, V= 5787.2(6 ꢁ3; Z=4, 1calcd
=
1.030 gcmÀ3
;
crystal dimensions: 0.51 ꢅ 0.37 ꢅ 0.20 mm3; F-
(000) = 1984, m = 0.123 mmÀ1; 2Vmax = 52.748; 105610 reflec-
tions, 11818 independent (Rint = 0.0506), R1 = 0.0610 (I > 2s(I))
and wR2(all data) = 0.1715, GooF = 1.032, max/min residual
electron density: 1.8842/À0.467 eꢁÀ3. 5a: yellow blocks from
pentane; C49H81N2PSi2·C5H12 Mr = 857.45, monoclinic, space
group
18.2234(10) ꢁ, b = 112.709(2)8, V= 5396.0(6) ꢁ3; Z = 4, 1calcd
1.055 gcmÀ3 crystal dimensions: 0.72 ꢅ 0.51 ꢅ 0.33 mm3; F-
P21/n,
a = 17.5336(11),
b = 18.3070(11),
c =
=
;
(000) = 1896, m = 0.130 mmÀ1; 2Vmax = 55.788; 50097 reflections,
12837 independent (Rint = 0.0480), R1 = 0.0504 (I > 2s(I)) and
wR2(all data) = 0.1360, GooF = 1.026, max/min residual electron
density: 0.732/À0.492 eꢁÀ3. 6: colorless cuboids from pentane;
C54H90N3PSi2, Mr = 868.44, monoclinic, space group P2(1), a =
12.2643(4), b = 15.9882(4), c = 13.6518(3) ꢁ, b = 91.927(2)8, V=
2675.39(12) ꢁ3; Z = 2, 1calcd = 1.078 gcmÀ3; crystal dimensions:
0.56 ꢅ 0.45 ꢅ 0.16 mm3; F(000) = 956, m = 0.132 mmÀ1; 2Vmax
=
55.668; 26092 reflections, 10486 independent (Rint = 0.0350),
R1 = 0.0359 (I > 2s(I)) and wR2(all data) = 0.0841, GooF =
1.026, max/min residual electron density: 0.322/À0.218 eꢁÀ3
.
CCDC 963244 (4c), 963245 (5a), and 963246 (6) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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