Metal-free activation of EH3 (E=P, As) by an ylide-like silylene and formation of a donor-stabilized arsasilene with a HS=AsH subunit

Article abstract of DOI:10.1002/anie.201005903

Twice, and faster: Reaction of the zwitterionic silylene 1 with AsH ₃ occurs stepwise at ambient temperature to give the first crystalline, donor-stabilized arsasilene 3 via its 1,1-addition product (silylarsane 2). In contrast, the activation

Full text of DOI:10.1002/anie.201005903

Communications
DOI: 10.1002/anie.201005903
Bond Activation
Metal-Free Activation of EH3 (E = P, As) by an Ylide-like Silylene and
=
Formation of a Donor-Stabilized Arsasilene with a HSi AsH Subunit**
Carsten Prꢀsang, Miriam Stoelzel, Shigeyoshi Inoue, Antje Meltzer, and Matthias Driess*
In memory of Hans Georg von Schnering
Compounds containing homoleptic and heteroleptic multiple
bonds between heavier main-group elements are a highly
interesting current topic of research owing to their intriguing
structure reactivity features. In the case of silicon, the
isolation and characterization of the first stable disilene[1]
and the first crystalline silylene 1 (Scheme 1)[2] are milestones
sites.[8] Because of the yilde-like (zwitterionic) electronic
structure of silylene 2[9] (Scheme 1), its chemistry is distinc-
tively different from that of other known NHSi molecules.
Compound 2 exhibits an electron-rich butadiene moiety in
the backbone that can be utilized for the metal-free activation
ꢀ
of E H bonds or the addition of Lewis acids. NHSi 2 thus
features three reactive sites instead of two: The basic silicon
lone pair, a formally empty (acidic) 3p orbital at silicon, and a
basic butadiene moiety. The straightforward formation of
silathioformamide 3 from 2 and H2S[10] is the only example to
date where all three of these sites have been involved in a
single reaction. However, it is possible to selectively trigger
one or two of the donor–acceptor functional groups if the
right choice of reagent is made. For example, strong Lewis
and Brønsted acids add to the exocyclic electron-rich
methylene group[11] (compounds 4a,b, Scheme 1), whereas
the silicon(II) lone pair can be utilized as a strong s donor for
the preparation of silylene–metal complexes, as shown for
complex 5 in which the zwitterionic character of the ligand is
retained.[12] The 1,1-addition to the silicon center, which is
also well-documented for other NHSis,[13] and the 1,4-addition
across the C3N2Si heterocycle are the most common reactivity
features of 2. Both pathways have even been observed
simultaneously during the reaction of water with two
equivalents of NHSi 2 to yield donor-stabilized siloxysilylene
6 (Scheme 1).[14]
Scheme 1. Isolable N-heterocyclic silylenes 1 and 2 and adducts 3–6
derived from silylene 2. LA=H, (C6F5)3Bꢀ; Ar=2,6-iPr2C6H3.
that have inspired and encouraged many research groups to
gain access to novel types of silicon-containing functional
groups.[3] This work has recently enabled the discovery of new
ꢀ
reactivity patterns, such as the metal-free activation of N H
It has been shown that 2 and even the {Ni(CO)3} complex
bonds of ammonia or amines with multiply bonded silicon.[4]
Apart from one example published by Kira et al.,[5] all
known isolable silylenes are stabilized by adjacent nitrogen
donor(s) and can be classified as N-heterocyclic silylenes[6]
(NHSi molecules) or donor-stabilized silicon(II) com-
pounds.[7] The reactivity of NHSis is comparable to that of
well-known N-heterocyclic carbenes (NHCs), with the silicon
5 are capable of activating the N H bonds of ammonia and
ꢀ
hydrazines,[15] C H bonds of alkynes and arenes, and C F
bonds of fluoroarenes[17] to eventually yield the corresponding
thermodynamic 1,1-addition products. As part of our ongoing
investigations on ylide-like silylene 2, we were curious about
its reactivity towards the more Brønsted acidic heavier
Group 15 hydrides EH3 (E = P, As). This reactivity could
[16]
ꢀ
ꢀ
ꢀ
=
center and the Si N bonds being the predominant reactive
possibly lead to unprecedented donor-stabilized Si E multi-
ple bonds akin to the formation of silathioformamide 3
formed from 2 and H2S. Herein we present the strikingly
different reactivity of 2 towards PH3 and AsH3. Remarkably,
whilst PH3 leads merely to the 1,1-addition product, the
activation of AsH3 furnishes a donor-stabilized arsasilene
(silylidenearsane, Si As) with a HSi = AsH subunit in a two-
step process, which could be isolated in the form of deep blue
crystals.
[*] Dr. C. Prꢀsang, M. Stoelzel, Dr. S. Inoue, Dr. A. Meltzer,
Prof. Dr. M. Driess
Technische Universitꢀt Berlin
Institute of Chemistry: Metalorganics and Inorganic Materials
Sekretariat C2, Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax: (+49)30-314-29732
E-mail: matthias.driess@tu-berlin.de
Homepage: http://www.driess.tu-berlin.de
=
The reaction of 2 with a twentyfold excess of PH3
furnishes merely the 1,1-addition product, that is, silylphos-
phane 7a, even at low temperatures (Scheme 2). The progress
of the PH3 addition can easily be monitored by the slow
disappearance of the characteristic yellow color of 2 in
[**] We thank the Deutsche Forschungsgemeinschaft (DR 226-17/1, the
Cluster of Excellence “UniCat” (EXC 314/1), and the JSPS (S.I.) for
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005903.
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AsH3 was added to solutions of 2 in toluene at ꢀ788C. Upon
warming to room temperature, the color of the reaction
mixtures changed from yellow to deep blue; the arsasilene 8
could be isolated from this solution in a 48% yield (Figure 2).
Scheme 2. Reaction of silylene 2 with PH3 and AsH3.
toluene solutions. The silylphosphane 7a is the only product,
1
as shown by H NMR spectroscopy. The 31P NMR spectrum
reveals a triplet of doublets at d = ꢀ257.4 ppm (1JP, H
=
2
187.1 Hz, JP, H = 23.5 Hz), which clearly shows the formation
of the 1,1-insertion product. Accordingly, the exocyclic
1
methylene group is still present in 7a, as shown by the H
and 13C NMR spectra, and the 29Si chemical shift (d =
ꢀ18.5 ppm, 1JSi,P = 8.9 Hz, 1JSi,H = 240.2 Hz) is in the range
ꢀ
ꢀ
typical of tetracoordinate silicon centers, with Si H and Si P
single bonds but significantly more deshielded than the
analogous 1,1-insertion product of silylene 2 and ammonia
(d = ꢀ45.0 ppm).[15a] The diastereotopic PH2 protons are
Figure 2. Molecular structure of 8. Hydrogen atoms are omitted except
for those at As1, Si1, C1, C3, and C5). Thermal ellipsoids are set at
50% probability. Selected distances [pm] and angles [8]: Si1–As1
221.78(1), Si1–N1 182.17(1), Si1–N2 180.26(1); As1-Si1-N1 119.1,
As1-Si1-N2 116.2, N1-Si1-N2 96.4, Si-C3N2 (mean plane) 66.9, As-C3N2
(mean plane) 17.3.
1
observed as an AB spin system in the H NMR spectrum at
d = 0.76 and 0.81 ppm, each with a multiplet pattern and a
1
geminal H,1H coupling constant of 12.1 Hz.
The structure of 7a was confirmed by single-crystal X-ray
analysis[18] (Figure 1). Suitable crystals were obtained from a
concentrated n-hexane solution at ꢀ308C. The atoms of the
The 1H NMR spectrum of 8 is consistent with its higher
symmetry as compared to the precursor 2 or the possible 1,1-
addition product 7b. Only six distinct resonances are
observed for the protons of the isopropyl groups, and the
exocyclic methyl groups appear as one singlet at d = 1.42 ppm.
The SiH and AsH moieties each give a doublet (d = 6.77 and
3
ꢀ2.22 ppm, JH,H = 6.7 Hz); the remarkably high shielding of
[20]
ꢀ
ꢀ
the As H proton is unprecedented for As H compounds
and is presumably due to relativistic effects (spin–orbit
coupling). Owing to the dative N!Si coordination, the 29Si
resonance of 8 (d = 17.6 ppm) appears much further upfield
than those of arsasilenes containing three-coordinate silicon
atoms (d = 179–187 ppm).[21] DFT calculations of a model
system (see below) confirm that the intense blue color of
solutions of arsasilene 8 is attributed to a p!p* transition
(lmax(toluene) = 590 nm, e = 246, very broad, TD-DFT:
Figure 1. Molecular structure of 7a. Hydrogen atoms are omitted
except for those at P, Si. Thermal ellipsoids are set at 50% probability.
Selected distances [pm] and angles [8] are mean values and ranges of
four crystallographically independent molecules in the unit cell: Si–P
223.8(2)–224.2(2), Si–N 171.7(3)–173.2(3), Cring–N 171.7; P-Si-N 119.1,
N-Si-N 96.4, Si-C3N2 (mean plane) 50.0–57.8, P-C3N2 (mean plane)
26.0–49.9.
=
lcalcd = 634 nm) of the Si As bond (HOMO) and mainly the
ligand C3N2 backbone (LUMO; see Figure 4).
Suitable crystals for single-crystal X-ray analysis[18] were
obtained from a concentrated toluene solution at ꢀ308C. As
in the case of silylphosphane 7a, the ligand backbone of
arsasilene 8 exhibits an almost planar arrangement and the
ꢀ
AsH moiety occupies an equatorial position. The Si As
distance of 221.78(1) pm is rather short and between the
C3N2 backbone of 7a are nearly co-planar. The silicon center
and the PH2 group are located on opposite sides of the C3N2
mean plane. The PH2 moiety occupies an equatorial position,
whilst the hydrogen atom at silicon is located in an axial
values observed for single (236 pm) and double bonds
(216 pm).[21] This elongation results from dative N!Si AsH
=
interaction similar to the situation observed for related N-
ꢀ
position. The Si P distance (av. 224 pm) is in the character-
donor-stabilized silanones and silanthiones such as 3.[3e,f,22]
[19]
ꢀ
ꢀ
istic range of Si P single bonds.
Accordingly, a significant lengthening of the N Si bonds in
arsasilene 8 (180.26(1), 182.17(1) pm) as compared to 7a
(171.7–173.2 pm) is observed owing to the Si As multiple-
In contrast to the addition of PH3, reaction of 2 with AsH3
proceeds much faster, and no excess of the latter is necessary
to complete the conversion. On preparative scales, gaseous
=
[22]
ꢀ
bond character that weakens the Si N p interactions.
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Remarkably, dissolution of pure 8 in benzene leads to an
equilibrium with its tautomer (1,1-addition product), silylar-
1
sane 7b, as shown by H NMR spectroscopy. The concen-
tration of 7b reaches its maximum after about four hours. The
equilibrium was also utilized to determine the 29Si chemical
shift of compound 7b by dissolving 8 in C7D8, keeping this
sample at room temperature for four hours, and then cooling
to ꢀ508C. This sample contained compounds 7b and 8 in the
1
molar ratio of 2:8 according to H NMR analysis. The same
sample shows a new resonance in the 29Si NMR spectrum at
d = ꢀ18.8 ppm, which is almost identical to that of silylphos-
phane 7a (d = ꢀ18.5 ppm) and thus could be assigned to 7b.
Furthermore, we learned that 7b is the initial product from
the reaction of 2 with AsH3. Accordingly, if the reactants are
combined at low temperature in C7D8 solutions, the formation
of 7b takes place starting at ꢀ508C, and the formation of
arsasilene 8 can be observed above ꢀ308C. A maximum
molar fraction of 0.70 of compound 7b was observed by
1H NMR spectroscopy when the above sample was warmed to
ambient temperature and measured immediately. After this
sample was kept at room temperature for five minutes, the
ratio of 7b to 8 was 3:7.
DFT calculations on model systems in which the bulky
aryl groups were replaced by phenyl substituents at nitrogen
were performed to rationalize the differences in the course of
the reaction between silylene 2 and the different Group 15
hydrides NH3, PH3, and AsH3. We calculated a series of
possible products, namely the analogous model compounds
Figure 4. HOMO (left) and LUMO (right) of model compound 8b’.
position, unlike the structure observed experimentally for 7a.
This can be explained by the steric hindrance of the isopropyl
groups in the 2,6-position of the aromatic rings, which force
the EH2 moieties into an equatorial position. The geometric
parameters for the calculated model compounds 7a’ and 8b’
are in good agreement with the experimental values of
silylphosphane 7a and arsasilene 8, respectively. The calcu-
lations predict that the hypothetical aminosilylene 10c’ and its
tautomeric iminosilanes 8c’ and 9c’ are significantly higher in
energy than the corresponding aminosilane 7c’. In other
words, the experimental observation of such species seems
very unlikely, and this is in accordance with recent results.[15a]
In contrast to the nitrogen systems, the calculated relative
energies of the phosphasilenes 8a’, 9a’ and arsasilenes 8b’, 9b’
are about equal or even lower than those of the corresponding
tautomeric 1,1-addition products 7a’ and 7b’, respectively.
Nevertheless, no experimental evidence could be found for
the formation of the corresponding phosphasilene if 2 was
allowed to react with PH3, even at low temperatures.
=
7a’–c’ (1,1-addition products, EH2 group axial), 8a’–c’ (Si E
=
subunit equatorial), 9a’–c’ (Si E subunit axial), and 10a’–c’
(1,4-addition products) containing different Group 15 ele-
ments E (E = N, P, As; Figure 3).
The energies of 8a’–c’, 9a’–c’, and 10a’–c’ are referenced
to 7a’, 7b’, and 7c’ (E = 0 kcalmolꢀ1), respectively. It is worth
mentioning that no minimum could be located on the
hypersurface for 7a’–c’ with the EH2 group in equatorial
=
As expected, the Si As bond character of 8 as represented
=
by the HOMO (Figure 4) is weaker with respect to Si As
systems with three-coordinate silicon atom owing to the
dative N!Si interaction. Accordingly, MO analysis of the
corresponding model system 8b’ revealed a reduced Wiberg
bond index (WBI = 1.465) in comparison with diaminoarsa-
=
silene (H2N)2Si AsH as a reference system (WBI = 1.643).
Our calculations further show that the deep blue color of 8 is
=
due to the HOMO (Si As p)!LUMO (ligand p*) transition
(Figure 4).
ꢀ
In conclusion, double metal-free As H bond activation
occurs readily by reaction of the ylide-like silylene 2 with
AsH3, yielding the unique donor-stabilized, deep-blue arsa-
=
silene 8 with a HSi AsH subunit. This subunit undergoes
reversible tautomerization to give the corresponding silylar-
sane 7b. In contrast, addition of PH3 to 2 proceeds much more
slowly and leads merely to silylphosphane 7a. This drastic
difference can be explained by the higher Brønsted acidity of
ꢀ
AsH3 versus PH3, which facilitates As H activation by the
zwitterionic silylene 2.
Received: September 20, 2010
Published online: November 19, 2010
Figure 3. Calculated relative energies (kcalmolꢀ1) of model compound
7a’–c’, 8a’–c’, 9a’–c’, and 10a’–c’. Dashed lines connect isomers that
exhibit the same element E and do not suggest possible reaction
pathways.
Keywords: arsanes · density functional calculations ·
.
N-heterocyclic silylenes · phosphanes · zwitterions
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