A new synthesis of triarylsilylium ions and their application in dihydrogen activation

Article abstract of DOI:10.1002/anie.201106582

Well-shuffled: An unexpected substituent distribution reaction via alkyldiarylsilylium ions leads to a distribution of substituents. Starting from alkyldiaryl silanes, this reaction provides a facile synthetic approach to sterically highly hindered triarylsilylium ions. These silylium ions can be applied in dihydrogen activation reactions. Copyright

Full text of DOI:10.1002/anie.201106582

Communications
DOI: 10.1002/anie.201106582
Silylium Ions
A New Synthesis of Triarylsilylium Ions and Their Application in
Dihydrogen Activation**
Andrꢀ Schꢁfer, Matti Reißmann, Annemarie Schꢁfer, Wolfgang Saak, Detlev Haase, and
Thomas Mꢂller*
One decade after the first reports on the synthesis and NMR
spectroscopic characterization of silylium ions,[1,2] the ingen-
ious use of their enormous electrophilicity for applications in
synthesis or catalysis has attracted attention.[3] In view of the
impressive success of the concept of frustrated Lewis pairs
(FLPs) developed and popularized by the groups of Stephan
and Erker,[4] we were intrigued by the idea to utilize the
enormous Lewis acidity of silylium ions, R3Si+, to activate
small molecules. The use of silylium ions in FLP chemistry
will lead to an increase of the reactivity of the FLP, and a shift
of the reactivity spectrum to stronger Lewis acids and weaker
Lewis bases can be expected.[5]
The only examples for triorganosubstituted silylium salts
in which the cation lacks coordination to solvent or counter-
anion are borates or carboranates of trimesitylsilylium (1a;
mesityl = 2,4,6-trimethylphenyl) and tridurylsilylium (1b;
duryl = 2,3,5,6-tetramethylphenyl). These sterically crowded
silylium ions were synthesized by applying the allyl leaving-
group method.[1] In an attempt to circumvent the severe
synthetic problems that arise from the high steric demands
made in this approach on the precursor silane Ar3SiC3H5, we
used diaryl (methyl)silanes Ar2(Me)SiH, 2, as starting mate-
rials for the classical Bartlett–Condon–Schneider hydride
transfer reaction to prepare diaryl(methyl) silylium ions or
their complexes with solvent molecules.[6] Quite unexpect-
edly, a substituent exchange reaction took place under these
conditions, and triarylsilylium ions Ar3Si+ and trimethylsi-
lane, Me3SiH, were formed (Scheme 1). This finding opens a
new, feasible synthetic route to triarylsilylium ions. Further-
more, the successful application of these triarylsilylium ions in
dihydrogen activation was demonstrated.
The reaction of dimesityl(methyl)silane (2a) with one
equivalent of [Ph3C][B(C6F5)4] in benzene at room temper-
ature gave a two-phase reaction mixture, which is typical for
solutions of salts of the [B(C6F5)4]À anion in aromatic
hydrocarbon solvents. NMR spectroscopic investigation of
the upper nonpolar phase revealed the complete consumption
of the starting silane 2a and the formation of triphenyl-
methane, Ph3CH. Inspection of the lower ionic phase by
29Si NMR spectroscopy indicated the formation of a single
silicon-containing species, which was characterized by a
29Si NMR signal at very low field (d(29Si) = 225.3 ppm). This
29Si NMR chemical shift is practically identical to the value
reported previously for Mes3Si+ (1a; d(29Si) = 225.5 ppm).[1]
1
Further comparison of the H and 13C NMR data with that
reported for 1a[1] confirmed that exclusively Mes3Si+ was
formed in the reaction.[7] Interestingly, the 1H and the
13C NMR data of the ionic phase clearly showed the presence
of excess trityl cation. The formation of trimesitylsilylium
(1a) was further substantiated by its derivatization applying
(nBu)3SnH and the subsequent detection of trimesitylsilane,
Mes3SiH. The only silicon-containing by-product of the
reaction, trimethylsilane, Me3SiH, was detected in the non-
polar phase. The presence of excess trityl cation resulted in
the ionization of trimethylsilane and the isolation of [Me3Si-
(C7H8)][B(C6F5)4] in the form of colorless crystals from
toluene.[7]
Three other diaryl(methyl) silanes 2c–e were found to
yield triarylsilylium ions 1c–e upon reaction with trityl cation
at room temperature (Scheme 1). In the case of the precursor
silanes 2c and 2e, the reaction was accomplished in 60 min,
while in the case of the sterically more hindered 2,4,6-
triisopropylphenyl (Tipp)-substituted silane 2d, the reaction
required 5 h for the complete consumption of silane 2d. The
identity of each cation, 1c–e, was confirmed by 29Si, 13C, and
1H NMR spectroscopy.[7] Most characteristic for the forma-
tion of silylium ions 1 is the 29Si resonance of the positively
charged silicon atom, which is shifted downfield significantly
(d(29Si) = 216–230 ppm; Table 1). The 29Si NMR chemical
shifts measured for silylium ions 1c–e are independent of
the arene solvent used (Table 1), indicating no significant
interaction between the silylium ion and the solvent mole-
cules. In the case of silylated arenium ions [R3SiArH]+,
chemical shift differences Dd(29Si) between the benzenium
and the toluenium species of more than Dd(29Si) = 10 ppm
have been reported.[8] The substituent exchange is not
restricted to methyl groups; that is, ethyldimesitylsilane
3(Ar=Mes) cleanly undergoes the transformation to give
Scheme 1. Synthesis of triarylsilylium ions 1 from diaryl(methyl)
silanes 2. [a: Ar=2,4,6-trimethylphenyl (Mes); c: Ar=2,6-dimethyl-
phenyl (Xylyl); d: Ar=2,4,6-tri-iso-propylphenyl (Tipp); e: Ar=2,3,4,5,6-
pentamethylphenyl (Pemp)].
[*] A. Schꢀfer, M. Reißmann, Dr. A. Schꢀfer, W. Saak, D. Haase,
Prof. Dr. T. Mꢁller
Institute for Pure and Applied Chemistry
Carl von Ossietzky University Oldenburg
Carl von Ossietzky-Strasse 9–11, 26111 Oldenburg (Germany)
E-mail: thomas.mueller@uni-oldenburg.de
[**] This work was supported by the DFG (Mu-1440/7-1).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106582.
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Table 1: 29Si NMR chemical shifts of silylium ions Ar2Ar’Si+ (1) and
Ar2EtSi+ (4).[a]
solid state for several weeks. In arene solution, however, slow
decomposition occurs even at À108C. This severely hampered
all attempts for crystallization, because only decomposition
products such as perfluorinated tetraaryl borates of proton-
ated arenes were isolated. For example, mesitylenium tetra-
kispentafluorophenylborate was isolated from a benzene
solution of 1a[B(C6F5)4] after several days at room temper-
ature.[7]
Ion
Ar/Ar’
d(29Si)
Ion
Ar/Ar’
d(29Si)
1a[b]
1a[c]
1a[d]
1c[b]
Mes/Mes
Mes/Mes
Mes/Mes
Xylyl/Xylyl
225.3
223.8
225.5
229.9
1d[b]
1e[b]
1 f[b]
4[b]
Tipp/Tipp
Pemp/Pemp
Mes/Tipp
Tipp
229.8
216.2
217.0
244.7
[a] Spectra recorded at 305 K. [b] In [D6]benzene. [c] In [D8]toluene. [d] In
[D5]chlorobenzene.
Our mechanistic proposal for the intermolecular aryl–
alkyl exchange reaction is based on previous results from
Lickiss et al.[9] and our group.[10,11] Intramolecular alkyl–aryl
exchange reactions for silyl cations of type 5 and 6 have been
known since the work of Eaborn and co-workers.[12,13]
Disilylated arenium ions 7 and 8, which might be regarded
as key intermediates in these ligand exchange reactions
involving aryl groups, were recently isolated.[9,10] Quite
recently, we established a mechanism that rationalized the
reversible isomerisation of disilyl areniumions 9 and 10
(Scheme 3).[11] DFT computations suggested that in this
trimesitylsilylium 1a. However, when ethylbis(triisopropyl-
phenyl)silane 3(Ar=Tipp) was used as starting material no
substituent exchange took place and the ethylbis(triisopro-
pylphenyl) silylium ion 4 was obtained (Scheme 2). Silylium
ion 4 is characterized by an even stronger deshielded silicon
Scheme 2. Hydride transfer reaction with diaryl (ethyl)silanes 3.
a) Ph3C+, room temperature, benzene, Ar=Tipp; b) Ph3C+, room
temperature, benzene, Ar=Mes.
atom (d(29Si) = 244.7 ppm), and in this case the 1H/
29Si HMQC spectrum clearly indicated the presence of the
aryl and the ethyl substituent at the central silicon atom
(Figure 1). This result suggests that the formation of triar-
ylsilylium ions according to Scheme 1 is significantly influ-
enced by steric effects. On the other hand, aryl substituents
smaller than xylyl groups did not initiate the reaction. For
instance, methyldiphenylsilane formed the corresponding
arenium ion [Ph2MeSi(C6H6)]+ upon reaction with trityl
cation. This was indicated by a 29Si NMR resonance at a
frequency typical for silylarenium ions (d(29Si) = 74.0 ppm).[8]
Triarylsilylium borate 1[B(C6F5)4] and diaryl(ethyl) sily-
lium borate 4[B(C6F5)4] are stable at room temperature in the
Scheme 3. Isomerization reactions in arenium ions 9 and 10.[11]
case, the methyl group transfer proceeds via a methonium-
like transition state TS9/10.
On the basis of these previous results, the following
mechanistic scenario seems plausible: the first step, the
Figure 1. 1H/29Si HMQC spectrum of 4[B(C6F5)4] in [D6]benzene at
+
*
305 K ( unreacted Ph3C , + TippH). For a more detailed figure, see
Scheme 4. Suggested reaction course for the formation of triarylsily-
lium ions 1 from alkyldiarylsilanes 2 (R=alkyl, Ar=aryl).
the Supporting Information, Figure S16.
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www.angewandte.org 12637

Communications
hydride transfer reaction [Scheme 4, Eq. (1)] is rate-deter-
atmosphere at room temperature resulted in no detectable
formation of products. In contrast, when trimesityl phosphane
15 was applied, the FLP underwent irreversible dihydrogen
mining (k1 < k2). Therefore, alkyl diaryl silylium ions 11
cannot be detected. They undergo a fast alkyl–aryl exchange
reaction with neutral starting silane 2 to form more stable
triaryl silylium ions 1, and the less sterically hindered dialkyl
aryl silane 12 [Eq. (2)]. As silane 12 is less sterically
congested, the reaction described by Equation (3) is even
faster (k3 > k2), and only the products triaryl silylium 1 and
trialkylsilane 13 can be detected.
Scheme 6. Dihydrogen activation by a silylium/phosphane Lewis pair
(conditions: 0.1013 MPa, H2, RT, benzene).
activation (Scheme 6) and yielded silane 1e(H) quantitatively
according to NMR spectroscopy (d(29Si) = À38.9 ppm, 1JSi,H
=
195 Hz) and trimesitylphosphonium borate [Mes3PH][B-
(C6F5)4] (16; d(31P) = À27.2 ppm, 1JP, H = 478 Hz). The identity
of both compounds was further confirmed by complete
analysis including XRD.[7]
Received: September 16, 2011
Published online: November 9, 2011
Scheme 5. Proposed reaction mechanism for the alkyl–aryl exchange.
Keywords: dihydrogen activation · frustrated Lewis pairs ·
Lewis acids · NMR spectroscopy · silylium ions
.
The key step, the alkyl–aryl exchange [Eq. (2), Scheme 4]
is believed to progress via disilylated arenium ions 14
(Scheme 5), which show close resemblance to disilyl cation
10 from our previous work.[11] The aryl exchange then
proceeds and the products are formed via alkonium-like
TS14. Although this mechanistic proposal is based on
conclusions by analogy, there are experimental facts which
[1] a) J. B. Lambert, Y. Zhao, Angew. Chem. 1997, 109, 389; Angew.
Chem. Int. Ed. Engl. 1997, 36, 400; b) J. B. Lambert, Y. Zhao, H.
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c) J. B. Lambert, L. Lin, J. Org. Chem. 2001, 66, 8537.
[2] Reviews: a) V. Y. Lee, A. Sekiguchi, Organometallic Com-
pounds of Low-Coordinate Si, Ge, Sn and Pb, Wiley, Chichester,
2010; b) T. Mꢀller, Adv. Organomet. Chem. 2005, 53, 155.
[3] H. F. T. Klare, M. Oestreich, Dalton Trans. 2010, 39, 9176.
[4] a) D. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50; Angew.
Chem. Int. Ed. 2010, 49, 46; b) D. W. Stephan, Dalton Trans.
2009, 3129.
support this mechanism: 1) According to our proposal, only
2
= equivalents of trityl cation is needed for a complete
3
consumption of silane 2 (Schemes 1 and 4), which is in
agreement with the detection of excess trityl cation in our first
experiments when a 1:1 stoichiometry was applied; 2) In a
cross experiment which uses silanes 2a and 2d, only three
different triarylsilylium ions are formed upon reaction with
trityl cation. Apart from silylium ions 1a and 1d, Mes2TippSi+
(1 f) was obtained as the only cross-product.[14] In particular,
no MesTipp2Si+ (1g) was formed. The silanes 2a and 2d show
a significantly different reactivity versus the trityl cation. The
sterically less-hindered silane 2a is consumed much more
rapidly than 2d. Consequently, at short reaction times, only
silylium ions 1a and 1 f are formed. At the time the
concentration of the intermediate silylium ion Tipp2MeSi+
becomes significant, no dimesityl(methyl)silane (2a) is avail-
able, and therefore only Tipp3Si+ (1d) is formed as a third
species.
[5] T. A. Rokob, A. Hamza, I. Papai, J. Am. Chem. Soc. 2009, 131,
10701.
[6] a) P. D. Bartlett, F. E. Condon, A. Schneider, J. Am. Chem. Soc.
1944, 66, 1531; b) J. Y. Corey, J. Am. Chem. Soc. 1975, 97, 3237.
[7] For further details, see the Supporting Information.
[8] J. B. Lambert, S. Zhang, S. M. Ciro, Organometallics 1994, 13,
2430.
[9] N. Choi, P. D. Lickiss, M. McPartlin, P. D. Masangane, G. L.
Veneziane, Chem. Commun. 2005, 6023.
[10] a) R. Meyer, K. Werner, T. Mꢀller, Chem. Eur. J. 2002, 8, 1163;
b) R. Panisch, M. Bolte, T. Mꢀller, Organometallics 2007, 26,
3524.
[11] N. Lꢀhmann, H. Hirao, S. Shaik, T. Mꢀller, Organometallics
2011, 30, 4087.
´
[12] C. Eaborn, P. D. Lickiss, S. T. Najim, W. A. Stanczyk, J. Chem.
Soc. Chem. Commun. 1987, 1461, and references therein.
À
[13] Si C bond cleavage reactions involving GaCl3 seem to be also
Triarylsilylium ions 1 form frustrated Lewis pairs with
bulky triaryl phosphanes. This is indicated by the unchanged
NMR spectroscopic parameter when triarylsilylium borates
are mixed with triaryl phosphanes in benzene. For effective
dihydrogen activation, however, the properties of the phos-
phane base must be adjusted. For example, stirring a 1:1
mixture of the silylium borate 1e[B(C6F5)4] with tris(penta-
fluorphenyl)phosphane in benzene under a dihydrogen
related; see, for example: a) H. Schmidbaur, W. Findeiss, Angew.
Chem. 1964, 76, 752; b) B. Luo, V. G. Young, W. L. Gladfelter, J.
Organomet. Chem. 2002, 649, 268.
[14] Cation 1 f was identified in the mixture by its NMR data. In
particular, the relative intensities of the proton signals at d(1H) =
2.85 (1H, p-CH(CH3)2), 2.41 (2H, o-CH(CH3)2), 2.04 (6H, p-
CH3), 1.98 ppm (12H, o-CH3) were decisive in its identification.
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Angew. Chem. Int. Ed. 2011, 50, 12636 –12638