Synthesis of the first persilylated ammonium ion, [(Me3Si) 3NSi(H)Me2]+, by silylium-catalyzed methyl/hydrogen exchange reactions

Article abstract of DOI:10.1021/om500519j

This work describes the unexpected synthesis and characterization of the first persilylated ammonium ion, [(Me₃Si)₃NSi(H)Me ₂]⁺, in the reaction of (Me₃Si)₃N with [Me₃Si-H-SiMe₃][B(C₆F₅) ₄]. NMR and Raman studies revealed a transition-metal-free silylium ion catalyzed substituent redistribution process when [Me₃Si-H- SiMe₃]⁺ was used as the silylating reagent. These observations were affirmed in the reaction with [Et₃Si-H-SiEt ₃][B(C₆F₅)₄]. A Lewis acid catalyzed scrambling process always occurs if an excess of silanes is present in the formation of silylium cations while employing the standard Bartlett-Schneider- Condon type reaction. Additionally, the thermodynamics of this process was accessed by DFT computations at the pbe1pbe/aug-cc-pVDZ level, indicating alkyl substituent exchange equilibria at the silane and preference of the formation of [(Me₃Si)₃NSi(H)Me₂]⁺ over [(Me ₃Si)₄N]⁺.

Full text of DOI:10.1021/om500519j

Communication
pubs.acs.org/Organometallics
Synthesis of the First Persilylated Ammonium Ion,
[(Me3Si)3NSi(H)Me2]+, by Silylium-Catalyzed Methyl/Hydrogen
Exchange Reactions
Rene Labbow,† Fabian Reiß,† Axel Schulz,*,†,‡ and Alexander Villinger†
́
†Institut fur Chemie, Abteilung Anorganische Chemie, Universitat Rostock, Albert-Einstein-Strasse 3a, 18059 Rostock, Germany
̈
̈
‡Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
̈
̈
S
* Supporting Information
ABSTRACT: This work describes the unexpected synthesis and
characterization of the first persilylated ammonium ion,
[(Me3Si)3NSi(H)Me2]+, in the reaction of (Me3Si)3N with
[Me3Si−H−SiMe3][B(C6F5)4]. NMR and Raman studies revealed
a transition-metal-free silylium ion catalyzed substituent redistrib-
ution process when [Me3Si−H−SiMe3]+ was used as the silylating
reagent. These observations were affirmed in the reaction with
[Et3Si−H−SiEt3][B(C6F5)4]. A Lewis acid catalyzed scrambling
process always occurs if an excess of silanes is present in the
formation of silylium cations while employing the standard
Bartlett−Schneider−Condon type reaction. Additionally, the
thermodynamics of this process was accessed by DFT computations at the pbe1pbe/aug-cc-pVDZ level, indicating alkyl
substituent exchange equilibria at the silane and preference of the formation of [(Me3Si)3NSi(H)Me2]+ over [(Me3Si)4N]+.
he rise of silylium cation chemistry was motivated by the
search for a three-coordinate silicon cation [R3Si]+ (where
dichlorobenzene) or can be even carried out in neat silanes
R3Si−H as solvent, yielding [R3Si−H−SiR3][B(C6F5)4] (2 for
R = Me) (Scheme 1).
T
R is an alkyl or aryl group) in analogy to the well-studied lighter
group 14 carbenium ions [R3C]+.1−3 In 2002 Reed and
Lambert solved the silylium ion problem by the synthesis and
full characterization of the salt [(Mes)3Si][CHB11Me5Br6]·
C6H6 (Mes = 2,4,6-trimethylphenyl) with a sterically
encumbered silylium cation.4 The major problem in the
synthesis of “naked” [R3Si]+ cations arises from their electron
sextet and an empty 3p valence orbital on the silicon, resulting
in super Lewis acidic character. This leads to the formation of
Lewis acid−base adducts even with weak Lewis bases (e.g.,
toluene,5−7 Me3Si−H8,9) or close cation−anion contacts with
weakly coordinating anions (wca; e.g., [CHB11H5Br6],10
[CHB11F11]11), respectively.
Scheme 1. General Synthesis of Silylium Ions
Furthermore, especially [Me3Si·X][wca] salts (X = Me3SiH,
arene; wca = [B(C6F5)4]−, [CHB11H5Y6]− (Y = Br, Cl))
allowed access to homoleptic onium ions, for example bis-
silylated halonium and pseudohalonium cations of the type
[Me3Si−X−SiMe3]+ (X = F, Cl, Br, I;14 X = CN, OCN, SCN,
NNN),15 which could be isolated and fully characterized as
their [B(C6F5)4]− salts. The persilylated chalcogonium ions
[(Me3Si)3O]+ and [(Me3Si)3S]+ were prepared and inves-
tigated by Olah et al.16,17 Recently, we reported the fully
characterized pseudochalcogonium salts [(Me3Si)2NCN-
(SiMe3)][B(C6F5)4] and [(Me3Si)2NNC(SiMe3)][B-
(C6F5)4].18 In 2001 Driess et al. presented the first structurally
characterized persilylated phosphonium [(Me3Si)4P]+ and
arsonium [(Me3Si)4As]+ ions (pnictonium).19 All of these
examples of onium ions can be obtained in a general silylation
reaction combining the super Lewis acidic20 media of [Me3Si·
In 1963 Corey and West12 used the Lewis acid assisted
hydrogen/halogen exchange Bartlett−Schneider−Condon13
reaction for the first time in silicon chemistry. Thirty years
later Lambert used a borate ([B(C6F5)4]−) as a wca in this
reaction type and published a general synthetic approach to
trialkylsilylium cations [R3Si]+ for the first time.6 However,
Nava and Reed9 showed that the commonly used triethylsilyl
perfluorotetraphenylborate salt, [Et3Si][B(C6F5)4], was mis-
identified. As prepared, the cation is the hydride-bridged silane
adduct [R3Si−H−SiR3]+. The effective combination of a H-
silane (R3Si−H) and a triphenylmethylium (tritylium) salt with
a wca (e.g., [Ph3C][B(C6F5)4] (1)) has been shown to be a
well-established method, which was used to synthesize a variety
of silylium cations on numerous occasions. This reaction is
tolerant toward different solvents (benzene, toluene, 1,2-
Received: May 16, 2014
© XXXX American Chemical Society
A
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Communication
(solvent)][B(C6F5)4] and the neutral silylated group 15−17
compound in suitable solvents, as depicted in Scheme 2.
a
Scheme 2. Synthesis of Homoleptic Onium Cations
a
n = 1: E = F, Cl, Br, I, CN, OCN, SCN, NNN with X = Me3SiH. n =
2: E = O, S, NCN, NNC with X = Me3SiH. n = 3: E = P, As with X =
benzene.
Figure 1. ORTEP drawing of the molecular structure of the cation
[(Me3Si)3NSi(H)Me2]+ in the crystal. Thermal ellipsoids are drawn
with 50% probability at 153 K, and the [B(C6F5)4]− anion is omitted
for clarity. Selected distances (Å) and angles (deg): N−Si1 1.889(2),
N−Si2 1.899(2), N−Si3 1.891(2), N−Si4 1.870(2); Si1−N−Si2
112.27(2), Si1−N−Si3 110.27(2), Si3−N−Si2 107.92(2) Si4−N−Si1
111.24(2) Si4−N−Si2 106.32(2) Si4−N−Si3 108.69(2).
We have long been interested in the preparation of the
missing homoleptic tetrakis(trimethylsilyl)ammonium cation,
[(Me3Si)4N]+. Herein we describe the attempted synthesis of
the [(Me3Si)4N]+ cation and show that a [(Me3Si)3NSi(H)-
Me2]+ cation is formed instead. In addition, while studying the
problem with the synthesis of [(Me3Si)4N]+ we discovered a
hidden silylium-catalyzed methyl/hydrogen exchange at the
silane Me3SiH, which finally explained the loss of one methyl
group.
The starting point of this project was the attempted direct
silylation of (Me3Si)3N, as depicted in Scheme 2. Although
Driess and Wiberg21 failed in similar experiments, we decided
to reinvestigate the preparation of the missing persilylated
ammonium ion. The gas-phase reaction (1) of (Me3Si)3N with
solvents followed by uncontrolled decomposition. The IR
spectrum of 5 showed a weak mode at 2192 cm−1, which could
be assigned to stretches of the Si−H unit. This is in reasonable
agreement with the calculated value for νSiH(calcd) 2232 cm−1
(pbe1pbe/aug-cc-pVDZ level; see the Supporting Informa-
tion). The existence of ammonium salt 5 was unequivocally
proven by X-ray diffraction analysis (Figure 1). 5 crystallizes in
the monoclinic space group P21/c with four formula units per
cell. It is interesting to note that crystallization at higher
temperatures (3 °C) resulted in the crystallization of another
(Me3Si)3N(g) + [Me3Si]+(g) → [(Me3Si)4N]+(g)
(1)
[Me3Si]+ calculated at the pbe1pbe/aug-cc-pVDZ level of
theory (ΔH298 = −427.1 kJ mol−1) also strongly supported the
idea of direct silylation (cf. −127.2 kJ mol−1 for toluene, −131.0
kJ mol−1 for Me3Si−H and −227.6 kJ mol−1 for Me3Si−
CN).14,15
modification of 5 (triclinic space group P1; see the Supporting
̅
Information), which, however, showed a highly disordered
cation. Therefore, only the ordered structure is discussed.
There are neither significant cation−anion nor anion−anion
contacts. The observed molecular structure exhibits the
expected nearly tetrahedral coordination environment around
the nitrogen atom with Si−N−Si angles between 106.32(2) and
112.27(2)° (cf. [(Me3Si)4P]+, Si−P−Si 107.30(7)−111.06(7)°;
[(Me3Si)4As]+, Si−As−Si 108.8(1)−110.0(1)°). The N−Si
bonds of 5 (N−Si1 1.889(2) Å, N−Si2 1.899(2) Å, N−Si3
1.891(2) Å, N−Si4 1.870(2) Å) are in good agreement with
those observed in the persilylated diazonium cation
[(Me3Si)2NN(SiMe3)]+ (cf. 1.829(1), 1.894(1), 1.899(1) Å,22
∑rcov(Si−N) = 1.87 Å).23 It should be mentioned that the
silicon−nitrogen bond of the dimethylsilyl group (N−Si4) is
significantly shortened in comparison to those of the three
trimethylsilyl groups (N−Si1, N−Si2, N−Si3) as a conse-
quence of less steric hindrance (cf. N−Si1 1.889(2) Å, N−Si2
1.899(2) Å, N−Si3 1.891(2) Å, N−Si4 1.870(2) Å).
We repeated the reactions reported by Driess19 and co-
workers, who attempted to use [Me3Si·(toluene)][B(C6F5)4]
(3) as the silylation agent with no success. However, in an
earlier work we noticed that the salt 3 slowly decomposes
during drying in vacuo and isolation, which was indicated by a
gradual color switch from colorless to yellow.7 Therefore, we
decided to prepare 3 in situ. [Me3Si−H−SiMe3][B(C6F5)4] (2)
was treated with an excess of toluene. Then trimethylsilane (bp
6.7 °C) was removed with a freeze−pump−thaw procedure,
affording colorless crystals of [Me3Si·(toluene)][B(C6F5)4] (3).
The resulting suspension was reacted with 1 equiv of
(Me3Si)3N (4) and heated to 70 °C, where a characteristic
biphasic system was observed. Slow cooling of this solution to
−40 °C resulted in the deposition of colorless crystals of the
unexpected salt [(Me3Si)3NSi(H)Me2][B(C6F5)4] (5) in a
moderate isolated yield of 46% (Figure 1).
All of these results highlighted the difficulties in the synthesis
of the intended homoleptic [(Me3Si)4N]+ ion but gave no
satisfactory explanation for the loss of a methyl group at one
silyl group. For this reason, we took a closer look at our starting
materials and their preparation. The analytical data of 1, 2, and
4 showed no appreciable impurities. Therefore, we decided to
examine the stability of 2 in solution. After the treatment of
[Ph3C][B(C6F5)4] (3 mol %) with an excess of Me3SiH and
storage over a period of 12 h (Scheme 1), we recondensed the
As an isolated solid, 5 is thermally surprisingly stable up to
152 °C, while it immediately decomposes in CH2Cl2 solution at
ambient temperature, which is indicated by a switch from
colorless to deep orange. This prompted us to carry out NMR
experiments at −80 °C in CD2Cl2. However, only decom-
position products of the cation could be detected. Remarkably,
the borate anion is not involved in the decomposition, as
visualized by its characteristic 19F{1H} NMR resonance pattern
and 11B NMR shift of −17 ppm. Actually, the use of o-
dichlorobenzene as the solvent in NMR experiments failed to
identify the cation. This observation is in contrast to the case
for the analogous phosphonium and arsonium cations, which
remain unchanged in CH2Cl2 solution. These results gave
evidence for the dissociation of 5 even in weak Lewis basic
1
excess silane into a sealed NMR tube. The H NMR data
revealed, in addition to the expected signals for Me3SiH (δ(1H)
0.92 (d), 4.87 (dec)), additional signals which could be
assigned to Me4Si (δ(1H) 0.86 (s)), Me2SiH2 (δ(1H) 0.97 (t),
4.67 (sep)), and MeSiH3 (δ(1H) 4.41 (q)).24 The composition
of the mixture (0.14 (MeSiH3):0.88 (Me2SiH2):1
(Me3SiH):0.70 (Me4Si)) was rather unexpected to us (Figure
B
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Organometallics
Communication
2), but the formation of this methylsilane mixture now
Scheme 4. Alkyl Exchange Reactions (R = Me, Et)
explained the presence of one Me2SiH group in 5.
In further experiments we studied the influence of the wca.
For this reason we added catalytic amounts of [Ph3C]-
[CHB11H5Cl6] to neat Me3SiH, leading also to the formation
of a silane mixture, as depicted in Schemes 3 and 4.
Additionally, the reaction of 1 as precatalyst (0.34 mol %)
and Et3SiH was examined by a 29Si-INEPT NMR study, which
showed signals for Et4Si (δ(29Si) 8.33), Et3SiH (δ(29Si) 0.52)
and Et2SiH2 (δ(29Si) −22.56) after an equilibration time of 70
days (see Supporting Information, Figure S18). Again the in
situ formed silylium ion (here [Et3Si−H−SiEt3][B(C6F5)4]
(6)) catalyzes the ethyl/hydrogen exchange reactions, for
which also very small enthalpies were computed (equilibrium
A, 0.3 kJ mol−1; equilibrium B, −0.1 kJ mol−1; see Scheme 4
with R = Et). Interestingly, no signal for the EtSiH3 species was
observed, which might be due to the less sensitive 29Si-INEPT
NMR method. The estimated turnover numbers (TON) and
turnover frequencies (TOF) were calculated at the 50% state of
equilibration and are depicted in Table 1. The higher activity of
1
Figure 2. H NMR spectrum of the recovered silane mixture (25 °C,
C6D6 (external), 300.13 MHz): δ 0.86 (s, (CH3)4Si, 2J(1H−29Si) = 6.4
Hz), 0.92 (d, (CH3)3SiH, 3J(1H−1H) = 3.59 Hz), 0.97 (t, (CH3)2SiH2,
3J(1H−1H) = 4.15 Hz). 4.41 (quart, (CH3)SiH3, J(1H−1H) = 4.72
3
Hz), 4.67 (sep, (CH3)2SiH2), 4.87 (dec, (CH3)3SiH).
To gain more insight into the formation of these mixed
methylsilanes, we treated 0.26 mol % of 1 with Me3SiH as
reactant and solvent at ambient temperature, resulting in the
precipitation of [Me3Si−H−SiMe3][B(C6F5)4] (2), which
represented the catalytically active species in this reaction.
The upper silane phase was investigated in a series of 1H NMR
and Raman experiments. This long-term analysis showed an
unambiguous catalytic reaction, as depicted in Scheme 3 and
Figure 3 (see also the Supporting Information).
Table 1. Estimated Parameters for the Catalyzed Alkyl/
Hydrogen Exchange Reaction
a
b
c
silane
amt of PC, mol %
TON50
TOF50
Me3SiH
Et3SiH
0.26
0.34
63
39
1.28
0.41
a
b
a
PC = precatalyst = [Ph3C][B(C6F5)4]. Referenced to R4Si species at
Scheme 3. Catalytic Exchange Reaction of Me3SiH
c
50% of equilibration state. TOF50 = TON/time (h).
the smaller “[Me3Si]+” silylium ion is impressively demon-
strated by a 3 times larger TOF and should be taken into
account when preparing 2 or homoleptic species bearing Me3Si
groups.
a
PC = precatalyst = [Ph3C][B(C6F5)4].
Since the increase of Me2SiH2 and Me4Si was accompanied
by the decrease of Me3SiH (Figure 3, left), the reaction might
be regarded as a methyl/hydrogen redistribution reaction
(Scheme 4, reaction A with R = Me). Moreover, a second much
slower equilibrium reaction interfered with the aforementioned
exchange reaction. This equilibrium could be attributed to the
substituent redistribution of Me2SiH2 into the methylsilane
MeSiH3 and the starting material Me3SiH, as depicted in
Scheme 4 (reaction B with R = Me) and Figure 3 (right). The
small enthalpy changes along reactions A (−0.6 kJ mol−1) and
B (−1.9 kJ mol−1) support this assumption.
These results can be compared to earlier observations by
Muller and co-workers25,26 and Oestreich et al.,27 who
̈
described substituent scrambling in the formation of the
arene silylium cation [(Mes)3Si]+ and the ferrocene-substituted
species [iPrSi(Fc)2]+, respectively. As these reports focused on
the substituent scrambling at the silylium cation center, there
was no report of catalytic substituent redistribution at the silane
compounds. Brookhart et al. reported on a reaction of
Me2EtSiH with the transition-metal complex Et3Si(H)2Ir(μ-
SiEt2)2Ir(H)2SiEt3 as catalyst in the presence of hydrogen and
observed a substituent redistribution, affording Et2MeSiH,
1
Figure 3. Time-dependent relative integrals taken from H NMR spectra: (left) high-field region MexSi; (right) low-field region SiHx.
C
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Me2EtSiH, Me3SiH, and Et3SiH.28 Astonishingly, the transient
metal complex seemed to be restricted in the catalysis of
redistribution on the substitution pattern R3SiH without
forming R4Si and R2SiH2 species. Nonetheless, this work
represented the first catalytic alkyl redistribution reaction via an
assumed silyl−silylene intermediate. Just recently, Heinekey
and co-workers described the structural characterization of
[Et3Si−H−SiEt3][B(C6F5)4] (6).29 When 6 was dissolved in
benzene or toluene, the evolution of hydrogen gas and the
formation of tetraethylsilane was observed. The latter
observation clearly indicated a substituent redistribution.
Thus, Heinekey proposed “This suggests that the highly
reactive, Lewis acidic silylium species are promoting redis-
tribution of the ethyl groups. Further, the reaction appears to
be catalytic.”29
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.S.: axel.schulz@uni-rostock.de.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Dr. Dirk Michalik is gratefully acknowledged for the attempted
low-temperature NMR experiments. Financial support by the
■
Universitat Rostock and the LIKAT is gratefully acknowledged.
̈
REFERENCES
■
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ASSOCIATED CONTENT
* Supporting Information
ACS Catal. 2012, 2, 307−316.
(29) Connelly, S. J.; Kaminsky, W.; Heinekey, D. M. Organometallics
2013, 32, 7478−7481.
■
S
Text, tables, figures, and CIF and xyz files giving detailed
information about experiments and computations. This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.
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