Catalytic trimerization of bis-silylated diazomethane

Article abstract of DOI:10.1021/ja308104k

(Me₃Si)₂CNN isomerizes upon addition of traces of [Me₃Si]⁺ ions to give (Me₃Si)₂NNC, which then undergoes an unusual trimerization reaction to give exclusively 4-diazenyl-3-hydrazinylpyrazo

Full text of DOI:10.1021/ja308104k

Article
pubs.acs.org/JACS
Catalytic Trimerization of Bis-silylated Diazomethane
Muhammad Farooq Ibad,†,‡ Peter Langer,‡,§ Fabian Reiß,† Axel Schulz,*,†,§ and Alexander Villinger*,†
‡
†Abteilung Anorganische Chemie and Abteilung Organische Chemie, Institut fur Chemie, Universitat Rostock,
̈
̈
Albert-Einstein-Straße 3a, D-18059 Rostock, Germany
§Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock, Albert-Einstein-Straße 29a, D-18059 Rostock, Germany
̈
̈
S
* Supporting Information
ABSTRACT: (Me3Si)2CNN isomerizes upon addition of traces of
[Me3Si]+ ions to give (Me3Si)2NNC, which then undergoes an
unusual trimerization reaction to give exclusively 4-diazenyl-3-
hydrazinylpyrazole. As catalyst the isonitrilium ion, [(Me3Si)2NNC-
(SiMe3)]+, was identified and fully characterized. Experiments and
computations indicate a three-step reaction including isomerization of
diazomethane, a C−C or N−C coupling, and a formal cycloaddition
reaction. The kinetics and thermodynamics are discussed on the basis
of DFT calculations.
20
SiMe3]+ (X = F, Cl, Br, I;15 CN, N3, OCN, SCN;19 CF3SO3
)
1. INTRODUCTION
were generated and fully characterized using the super Lewis
acidic silylating media Me3Si−X and [Me3Si(solv)]+ salt.15 In
view of the success of the pseudo-halogen concept in super
Lewis acidic silylating media,15,19,20 we were intrigued by the
idea of utilizing the enormous Lewis acidity of the [Me3Si]+ ion
to activate small molecules such as bis-silylated diazomethane
(Me3Si)2CNN, aminoisonitrile (Me3Si)2NNC, and carbodii-
mide (Me3Si)NCN(SiMe3), which can be considered as 16-
electron-containing pseudo-chalcogens.21 In analogy to pseudo-
halogens, pseudo-chalcogens are species that form a homolo-
gous series H2Y, MYH, M2Y, in which alkali, alkaline earth, and
heavy metals are typical representatives for M, and Y = NCN,
CNN, NNC, CCO, CC, etc. Furthermore, the pseudo-
chalcogen group is completely integrated in the resulting
mesomeric bond system, and localization of the ionic charge at
the terminal atoms is observed. Pseudo-chalcogenide anions
can also act as bidentate ligands and are distinguished by their
ambident nature.21 To the best of our knowledge, salts
containing silylated [(Me3Si)2CNN(SiMe3)]+, [(Me3Si)2NNC-
(SiMe3)]+, and [(Me3Si)2NCN(SiMe3)]+ have not yet been
reported.
In this study, special attention was given to the diazomethane
compound, as it is an ambivalent reagent.22,23 While electro-
philes commonly attack at the nucleophilic C atom,24
nucleophiles prefer the terminal N atom of diazomethane.25
This ambivalent behavior was also observed in [3+2]
cycloaddition reactions with dipolarophiles.26 Herein, we report
what is, to the best of our knowledge, the first trimerization of a
bis-silylated diazomethane, which provides, based on the use of
[(Me3Si)2NNC(SiMe3)]+ as a catalyst, an efficient and facile
synthesis of 4-diazenyl-3-hydrazinyl-1H-pyrazoles. Pyrazoles
represent one of the most important classes of heterocyclic
More than a decade after the first isolation and characterization
of silylium ions,1 their chemistry has been an area of rapid
growth,2 since many applications have been found due to their
useful properties such as enormous Lewis acidity and catalytic
behavior.3−6 For example, Ozerov et al.4 and Muller et al.3j
̈
utilized silylium ions as reactive catalysts for the activation of
C−F bonds. The Ozerov group introduced a class of carborane-
supported, highly electrophilic silylium compounds that act as
long-lived catalysts for hydro-defluorination of perfluoroalkyl
groups by widely accessible silanes under mild conditions. The
reactions are completely selective for aliphatic carbon−fluorine
bonds in preference to aromatic carbon−fluorine bonds.4
Recently, Oestreich et al.3h demonstrated that a tamed,
ferrocene-based silylium ion catalyzes demanding Diels−Alder
reactions in an unprecedented temperature range.
Tri-coordinate silylium (also silylenium or silicenium)
ions,2,7 with their electron sextet and a vacant p orbital, are
electron-deficient species and therefore strong Lewis acids.
Even relatively weak Lewis bases such as π/σ-donor solvents
(toluene,8 CH3CN,9 etc.) form tetrahedral complexes with
silylium ions.8,10 In addition, intramolecular π coordination in
silylium ions containing a 2,6-diarylphenyl scaffold was
observed, which forms a Wheland-like complex.11 The first
well-documented examples of intramolecular π-stabilization in
silyl cations were silanorbornyl cations.12 The long search for a
“naked” [R3Si]+ cation,2,13 free of interactions with the
environment, was finally brought to an end with the isolation
and full characterization of [(Mes)3Si][CHB11Me5Br6]·C6H6
(Mes = 2,4,6-trimethylphenyl) by the groups of Lambert and
Reed in 2002.14
The silylium ion [Me3Si]+ might be regarded as a sterically
demanding big proton,15 and, similar to a proton, the bulky
+
silylium ion is always solvated, forming the [Me3Si(solv)
]
ion.8b,15−18 For example, the full series of salts containing the
Received: August 15, 2012
bis-silylated halonium/pseudo-halonium cations [Me3Si−X−
Published: September 22, 2012
© 2012 American Chemical Society
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compounds.27 Although a great variety of substituted pyrazoles
are known, only a few examples of diazenyl- or hydrazinylpyr-
azoles have been described in the literature so far.28 Likewise,
di(hydrazinylidene)pyrazoles have only been scarcely reported
in the literature.29 To the best of our knowledge, trimerization
reactions of diazomethane compounds have not yet been
described; however, dimerizations of ethyl diazoacetate30a,b and
of a (diazomethylene)phosphorane30c were previously reported
to yield 1,2,4,5-tetrazine derivatives. As early as 1947, Meerwein
reported the catalytic decomposition of diazomethane, yielding
polymethylene.31
the importance of steric hindrance was shown in the
stabilization of C,N-nitrilimines. For instance, the reaction of
n-BuLi with R(H)CN2 followed by addition of R−Cl yields the
bis-silylated diazomethane R2CNN for R = Me3Si,33 while the
analogous reaction for the bulkier substituent R = iPr3Si results
in the formation of the thermally stable nitrilimine R−CNN−R,
which only isomerizes to the carbodiimide R−NCN−R under
photolytic conditions.39
Besides 1 and 3, bis-silylated carbodiimde species 4 (Scheme
2)40 was studied for comparison. 4 can be regarded as a
constitutional isomer of 1−3 regarding the NCN unit. Only the
N,N′-substituted carbodiimide species 4 can be isolated and
structurally characterized,40 while N,N-bis-silylated carbodiimde
species 5 is unstable regarding the isomerization to species 4.
For the reaction with silylium ions, compounds 1, 2, and 4 were
utilized as starting materials. All three compounds are thermally
stable liquids and do not isomerize or show any other reactivity
in pure form at 298 K in the dark.
2. RESULTS AND DISCUSSION
For the molecule (Me3Si)2CNN, three acyclic constitutional
isomers with an NNC unit can be formulated (Scheme 1).32
Scheme 1. Isomerization of Bis-silylated Diazomethane
Catalyzed by [Me3Si−S]+ with S = Isomer 1, 2, or 332
2.1. Reaction of (Me3Si)2CNN, (Me3Si)2NNC, and
(Me3Si)NCN(SiMe3) with Silylium Ions and Other Strong
Lewis Acids. The reaction of bis(trimethylsilyl)diazomethane
(Me3Si)2CNN with a [Me3Si(solv)]+ source was studied in two
series of experiments (Schemes 3 and 4). At first, the reaction
Bis(trimethylsilyl)diazomethane (1)33 and bis(trimethylsilyl)-
aminoisonitrile ((Me3Si)2NNC, 3)34 are experimentally known,
but no structural data are available (Figure 1). Wiberg et al.
Scheme 3. Catalytic Trimerization of Bis-silylated
Diazomethane32
was carried out in neat (Me3Si)2CNN, generating a super Lewis
acidic silylating medium. However, upon addition of
[Me3Si(solv)]+ salt (solv = Me3Si−H), an immediate complex
reaction contrary to the analogous reaction with Me3Si−X (X =
halogen, pseudo-halogen) was observed, resulting in a deep red,
highly viscous reaction mixture. Thus n-pentane was added,
which makes the workup and isolation of the products much
easier due to a considerable decrease in the viscosity of the
reaction mixture. However, it should be noted that the reaction
in neat bis(trimethylsilyl)diazomethane yielded the same
product. In a typical reaction setup, to a stirred suspension of
[Me3Si(solv)][B(C6F5)4] in n-pentane was added a mixture of
liquid (Me3Si)2CNN (large excess) and n-pentane at −78 °C.
The resulting suspension was allowed to warm to ambient
temperature and stirred for 36 h. The reaction was followed by
1H NMR experiments, and it was obvious that diazomethane
(Me3Si)2CNN was involved in a more complex chemistry
under these extreme Lewis acidic conditions, beyond the simple
formation of [(Me3Si)2CNN(SiMe3)]+ (8) or [(Me3Si)2NNC-
(SiMe3)]+ (9) (Scheme 5). Furthermore, these 1H NMR
experiments showed the exclusive and quantitative formation of
pyrazole species 7 and the complete consumption of the
starting material bis(trimethylsilyl)diazomethane (Scheme 3).
The end of the reaction is indicated by deposition of the
catalyst, 9[B(C6F5)4], as a crystalline solid. Moreover, during
the course of the reaction, the color of the supernatant changed
gradually from yellow to dark red. Thus, both the precipitate
and the reaction solution were further studied. The supernatant
Figure 1. ORTEP drawing of the molecular structures of
(Me3Si)2CNN (left) and (Me3Si)2NNC (right) in the crystal. Thermal
ellipsoids with 50% probability at 173 K. Selected bond lengths (Å)
and angles (°): for (Me3Si)2CNN, N1−N2 1.135(2), N1−C1
1.312(2), C1−N1−N2 179.6(2); for (Me3Si)2NNC, N1−N2
1.366(2), N2−C1 1.152(2), C1−N1−N2 177.7(2).
have shown that nitrilimine isomer 2 cannot be isolated since it
undergoes rapid isomerization to carbodiimide 4 (Scheme 2).34
Ever since the discovery of the first C,N-nitrilimines by
Huisgen,35 the development of their chemistry has been
hampered because of their potential instability and the lack of
suitable preparative methods. Bertrand et al. established
efficient routes for preparing stable nitrilimines.36−38 Moreover,
Scheme 2. Isomerization of Bis-silylated Carbodiimide
Catalyzed by [Me3Si−S]+ (S = Solvent)
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Scheme 4. Catalytic Trimerization of Bis-silylated Diazomethane Utilizing 9[CHB11H5Br6] as Catalyst32
Scheme 5. Calculated Relative Gibbs Free Energies of
Since DFT calculations (see below) indicated the catalytic
formation of bis(trimethylsilyl)aminoisonitrile 3 in the first
reaction step (Schemes 2 and 3), in a second series of
experiments isomeric 3 was reacted with [Me3Si(solv)]+ under
the same reaction conditions as discussed before for bis-
(trimethylsilyl)diazomethane (isomer 1). Indeed, the reaction
of isomer 3 with [Me3Si(solv)]+ resulted also in the exclusive
formation of pyrazole species 7 along with 9[B(C6F5)4], thus
proving the computational results (see below). Since no
isomerization step is needed prior to the C−C coupling and
[3+2] cyclization, this reaction is faster (12 h for a complete
conversion). The long-term activity and reuse of the catalyst
was also studied (Table 1; see Supporting Information). Even
Isomers of Neutral (Me3Si)2CNN (1, 2, and 3) and Cationic
a
[(Me3Si)3CNN]+ Species (8 and 9)
a
Higher-lying cyclic isomers are omitted for clarity (see Supporting
Table 1. Details of the Catalytic Trimerization of
(Me3Si)2NNC
Information).32
mol%
cat.
time/
min
was removed by filtration, and the brownish residue was
washed with n-pentane. Recrystallization from a minimum of
toluene at −25 °C resulted in the deposition of 9[B(C6F5)4] as
colorless crystals (49% yield based on [Me3Si(solv)][B(C6F5)4];
Figure 2). Notably, exclusively 9[B(C6F5)4] was isolated
a
b
precatalyst
conversion
TON
[Me3Si−H−SiMe3][B(C6F5)4]
[Me3Si][CHB11H5Br5]
[(Et2O)2H][B(C6F5)4]
[Me3SiOEt2][B(C6F5)]
B(C6F5)3
1.0
2.0
1.2
1.3
1.9
0.9
4.9
1.3
1.0
0.92
0.90
0.84
0.74
0.84
0.89
0.00
0.00
0.70
1080
180
30
92
45
72
59
45
100
0
30
14400
240
780
780
30
d
[Ph3C][B(C6F5)4]
c
GaCl3
c
Ag[CHB11H5Br5]
0
[(Me3Si)2NCN(SiMe3)]
[B(C6F5)4]
71
a
Precatalyst that forms the catalyst [LA-CNN(SiMe3)2]+ (LA = Lewis
b
c
acid). TON = n(NNC)×conversion/n(precatalyst). No conversion
to 7, but slow decomposition to unidentified side products was
observed. Trityl ions do not catalyze the isomerization step.
d
Figure 2. ORTEP drawing of the molecular structure of the
[(Me3Si)2NCN(SiMe3)]+ ion in 6[B(C6F5)4] (left) and the
[(Me3Si)2NNC(SiMe3)]+ ion in 9[B(C6F5)4] (right) in the crystal.
Thermal ellipsoids with 50% probability at 173 K. Selected bond
lengths (Å) and angles (°): for 6, C1−N2 1.18(1), N1−C1 1.279 (2),
N1−Si2 1.835(1), N1−Si3 1.838(1), C1−N2−Si1 159.4(9), C1−N1−
Si2 115.01(8), C1−N1−Si3 116.04(8), Si2−N1−Si3 128.93(6), N1−
C1−N2 173.3(5); for 9, N1−N2 1.309(2), N2−C1 1.143(2), Si2−N1
1.818(1), Si3−N1 1.816(1), Si1−C1 1.897(2), C1−N2−N1 179.3(1),
N2−N1−Si3 113.76(8), N2−N1−Si2 114.89(8), N2−C1−Si1
174.6(1), Si3−N1−Si2 131.34(6).
after 14 days and several reuses, the catalyst was still active as
well as when the catalyst concentration was decreased to less
than 1 mol%. For instance, the reaction of aminoisonitrile 3
afforded in three runs isolated yields of pyrazole species 7
between 74 and 82% when 1 mol% catalyst was used, displaying
a constant activity over all runs. A TON of 230 was estimated
(36 h). After each cycle, the amount of catalyst remained
almost unchanged. The catalyst can always be recovered in
good yield as crystalline material at the end of the reaction.
For comparison, the reaction of carbodiimide 4 with 1 equiv
of [Me3Si(solv)][B(C6F5)4] (solv = Me3Si−H, toluene) dissolved
in toluene leads in a straightforward, almost quantitative
reaction (isolated yield 77%) to the formation of the silylated
species 6 with [B(C6F5)4]− as counterion (Scheme 2, Figure 2).
Interestingly, no isomerization to the N,N-bis-silylated
carbodiimde species 5 (Scheme 2) nor any other reactivity
was observed, even under reflux conditions.
instead of the expected [(Me3Si)2CNN(SiMe3)][B(C6F5)4].
From the combined solutions of the filtration and washing
processes, the silylated 4-diazenyl-3-hydrazinylpyrazole species
(7) was isolated as dark green crystals (Scheme 3; see below,
Figure 5 left). The overall isolated yield is about 51% (referring
to (Me3Si)2CNN, which was used in a 22-fold excess both as
reactant and as solvent instead of n-pentane) after 36 h at
ambient temperatures. Referring to [Me3Si(solv)][B(C6F5)4]),
this trimerization corresponds to a catalytic process with a
turnover number (TON) of about 10.8.
Variation of the Counteranion and the Lewis Acids. To
study the influence of the anion on the trimerization process,
we prepared trimethylsilylium closo-7,8,9,10,11,12-hexabromo-
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carboranate [Me3Si][CHB11H5Br6] and reacted it with (i)
stoichiometrically amounts of diazomethane 1 or amino-
isonitrile 3 in benzene and (ii) a large excess of 1 and 3
([Me3Si][CHB11H5Br6] as catalyst). As shown in Scheme 4,
pyrazole species 7 was generated with the same reactivity, and it
was also possible to isolate the catalytic species 9[CHB11H5Br6]
(Figure 3, Table 1).
stable when stored in a sealed tube and kept at ambient
temperature. All these compounds have been fully charac-
terized. All salts are extremely air- and moisture-sensitive but
stable under argon atmosphere over a long period as a solid.
Toluene is a good solvent for all species; however, the three
salts 6[B(C6F5)4], 9[B(C6F5)4], and 9[CHB11H5Br6] are poorly
soluble in non-aromatic organic solvents such as n-hexane or
slowly decompose in, e.g., CH2Cl2, in contrast to neutral 10
and 11. The [B(C6F5)4]− salts of 6 and 9 melt at 104 and 108
°C, respectively, while for 9[CHB11H5Br6] (206 °C) and 10
(136 °C) only decomposition is observed. The highest melting
point is found for the neutral compound 11, at 122 °C (Table
2).
The IR and Raman data of all isonitrile species show a sharp
band in the expected region33,41 between 2209 and 2302 cm−1,
which can be approximately assigned to the stretching
frequency νCN (cf. 2041 cm−1 (IR) in (Me3Si)2CNN and
2102 cm−1 (Raman) in (Me3Si)2NNC)). It should be noted
that the entire CNN moiety vibrates in an asymmetric mode;
however, to a good approximation the small movement of the
amino nitrogen can be neglected. As previously shown, the
coordination of B(C6F5)3 to a R−CN species causes a
significant band shift to higher wave numbers; thus, for the
B(C6F5)3 adduct 11, a shift of Δν = 197 cm−1 was observed.42
A shift with a similar magnitude was also observed for all salts
containing cation 9 (Table 2). Hence, both IR and Raman
spectroscopy are particularly well suited to distinguish between
the starting material 3 and the silylated cation 9. The shift to
higher wave numbers upon adduct formation correlates nicely
with a smaller C−N distance (3, d(CN) = 1.152(2) Å vs
9[B(C6F5)4], 1.143(2); 9[CHB11H5Br6], 1.147(2); 10,
1.145(2); and 11, 1.145(4) Å; see X-ray structure elucidation,
Table 3). Interestingly, the CN distance in neutral adduct 11 is
almost the same as found in all cationic adducts 9 and 10.
Pyrazole species 7 (Figure 5 left) is neither very air- nor
moisture-sensitive, melts at 66 °C, and dissolves in almost all
common organic solvents. Depending on the solvent, several
solvates of 7 can be obtained, such as 7·n-hexane and
7·disiloxane (see below). Interestingly, one Me3Si group can
be selectively hydrolyzed by addition of 1 equiv of CF3SO3H or
water/n-hexane to give pyrazole species 12 (Scheme 6, Figure 5
right).43 It is interesting to note that crystals of 12 show
dichroism under polarized light which results in a violet or
orange appearance.
Figure 3. ORTEP drawing of the molecular structure of
9[CHB11H5Br6] in the crystal. Thermal ellipsoids with 50% probability
at 173 K. Selected bond lengths (Å) and angles (°): N1−N2 1.309(3),
N2−C1 1.147(4), Si2−N1 1.816(3), Si3−N1 1.809(3), Si1−C1
1.901(3), N2−N1−Si3 115.1(2), N2−N1−Si2 115.5(2), Si3−N1−
Si2,129.4(1), C1−N2−N1 178.6(4).
As listed in Table 1, in a next series of experiments several
neutral (GaCl3, B(C6F5)3) and cationic Lewis acids (Ag+,
[(Me3Si)2NCN(SiMe3)]+ (6), [Ph3C]+, [Me3SiOEt2]+, and
[(Et2O)2H]+) were tested. Except for Ag+ and GaCl3, all other
Lewis acids showed a significant reactivity and can be used for
the generation of pyrazole species 7. Interestingly, in contrast to
[Ph3C]+, the neutral Lewis acid B(C6F5)3 also catalyzes the
isomerization and trimerization process; however, the rates of
both processes are much slower. For both [Ph3C]+ and
B(C6F5)3, we were also able to isolate and fully characterize the
catalysts [(Me3Si)2NNC(CPh3)]+ (10) and [(Me3Si)2NNC(B-
(C6F5)3)]+ (11), respectively (Figure 4).
2.2. Properties and Spectroscopic Characterization.
All experimentally studied compounds (6[B(C6F5)4], 9[B-
(C6F5)4], 9[CHB11H5Br6], 10, and 11 as well as pyrazole
species 7 and 12) are easily prepared in large scale and are
Figure 4. ORTEP drawing of the molecular structures of 10 (left) and 11 (right) in the crystal. Thermal ellipsoids with 50% probability at 173 K.
Selected bond lengths (Å) and angles (°): for 10, N2−C1 1.145(4), N2−N1 1.334(4), C1−C8 1.480(4), N1−Si2 1.818(3), N1−Si1 1.823(3), C1−
N2−N1 179.1(3), N2−N1−Si2 111.9(2), N2−N1−Si1 113.3(2), Si2−N1−Si1 132.6(2); for 11, N2−C1 1.145(2), N2−N1 1.331(2), N1−Si1
1.805(1), N1−Si2 1.805(1), C1−B1 1.617(2), C1−N2−N1 177.7(1), N2−N1−Si1 114.91(6), N2−N1−Si2 111.52(6), Si1−N1−Si2 133.35(4).
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Table 2. Spectroscopic Details
6[B(C6F5)4]
9[B(C6F5)4]
9[CHB11H5Br6]
10
11
a
a
mp/°C
104
108
206
136
122
c
c
11B NMR
−16.64
2251(4)
−16.64
2209(9)
2215(m)
−20.22 (d, 5B, BH), −9.84 (s, 5B, BBr), −1.74 (s, 1B, 12-BBr)
−16.64
−21.27
2299(10), 2220(3),
2302(w)
b
b
Ra: νCN/cm−1
IR: νCN/cm−1
2287(w), 2243(m)
2236(w), 2165(w)
2285(w)
a
b
c
Decomposition temperature. Strong fluorescence. Slow decomposition in CD2Cl2
Table 3. Selected Bond Lengths (Å) and Angles (°) from X-ray Structure Analyses
d
compound
X−Y
Y−Z
Z−R
XYZ
NNC(SiMe3)2 (1)
1.135(2)
1.366(2)
1.361(2)
1.201(2)
1.279(2)
1.309(2)
1.309(3)
1.331(2)
1.342(2)
1.334(4)
1.312(2)
1.152(2)
1.153(2)
1.194(2)
1.18(1)
1.885(1)
179.6(2)
177.7(2)
178.4(2)
176.6(2)
173.3(5)
179.3(1)
178.6(4)
177.7(1)
177.7(2)
179.1(3)
b
b
(Me3Si)2NNC (3)
(Me3Si)2NNC (3)
−
−
c
(Me3Si)NCN(SiMe3) (4)
1.722(2)
[(Me3Si)2NCN(SiMe3)][B(C6F5)4]
[(Me3Si)2NNC(SiMe3)][B(C6F5)4]
[(Me3Si)2NNC(SiMe3)][CHB11H5Br6]
1.795(11)
1.897(2)
1.901(3)
1.617(2)
1.615(2)
1.480(4)
1.143(2)
1.147(4)
1.145(2)
1.143(2)
1.145(4)
a
(Me3Si)2NNC(B(C6F5)3) (11)
a
(Me3Si)2NNC(B(C6F5)3) (11)
[(Me3Si)2NNC(CPh3)][B(C6F5)4] (10)
a
b
c
d
Two slightly different data sets were obtained from different experiments. Two independent molecules in the unit cell. See ref 44. All
compounds can formally be written as R′X−Y−Z−R; e.g., for 1 and 3, X = N, Y = N, Z = C; for 4, X = N, Y = C, Z = N, etc.
and 12, were determined. Tables S1−S4 (see Supporting
Information) present the X-ray crystallographic data. X-ray-
quality crystals of all considered species were selected in
Fomblin YR-1800 (Alfa Aesar) at ambient temperature. All
samples were cooled to −100(2) °C during the measurement.
The molecular structures are shown in Figures 1−5, along with
selected bond lengths and angles. More details are found in the
Supporting Information, including the data for the solvates 7·n-
hexane and 7·disiloxane.
Structure of the Room-Temperature Liquids (Me3Si)2CNN
(1), (Me3Si)2NNC (3), and (Me3Si)NCN(SiMe3) (4). Crystals of 1
and 3 suitable for X-ray crystallographic analysis were obtained
Figure 5. ORTEP drawing of the molecular structures of pyrazole
by slow cooling of neat 1 and 3, respectively, to −80 °C over a
species 7 (left) and 12 (right) in the crystal. Thermal ellipsoids with
period of 8 h. Both species crystallized (space group for 1, P1;
̅
50% probability at 173 K. Except from H4, all other hydrogen atoms
are omitted for clarity. Selected bond lengths (Å): for 7, N1−C3
1.324(3), N1−N2 1.400(2), N2−C1 1.355(2), N3−N4 1.272(2),
N3−C2 1.389(3), N5−C3 1.387(2), N5−N6 1.455(2), C1−C2
1.398(3), C2−C3 1.425(3); for 12, N1−C3 1.381(3), N1−N2
1.433(3), N2−C1 1.314(3), N3−C2 1.310(3), N3−N4 1.348(3),
N5−C3 1.307(3), N5−N6 1.467(3), C1−C2 1.437(3), C2−C3
1.470(3).
for 3, Pbca; cf. for 4, P21/c44) with two independent molecules
per unit cell. Only very weak van der Waals interaction can be
assumed since large distances between molecules of 1, 3, and 4
are observed. There are only very small differences between the
structural parameters of the independent molecules. Hence
only one set of data is given for 1 and 3 in Figure 1 and Table 3.
In general, in both isoelectronic species 1 and 3 the NNC
moiety is almost linear, with short NN and CN bond lengths
displaying partial double- and triple-bond character. For
diazomethane species 1 a very short NN distance, indicating
triple-bond character (N1−N2 1.135(2) Å, cf. ∑rcov(NN) =
1.08 Å),45 is found, while the CN bond is much longer, at
1.312(2) Å. This situation is exactly the other way around in 3,
with a CN triple bond (1.152(2) Å, ∑rcov(CN) = 1.14 Å)
2.3. X-ray Structural Analysis. As far as we know, there
are no structural data available for the starting materials 1 and
3, while the structures of bis-silylated carbodiimide 4 and its
tetramer were reported in 1994 by Riedel et al.44 The structures
of silylium cation-containing species 6[B(C6F5)4], 9[B(C6F5)4],
and 9[CHB11H5Br6], as well as 10 and 11, besides pyrazoles 7
Scheme 6. Selective Hydrolysis of 7
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Figure 6. Left: Energies of the intrinsic isomerization process without a [Me3Si]+ ion as catalyst. Right: Calculated transition states for the intrinsic
1,3-Me3Si shift in 1 (distances in Å).
and a rather long NN bond, at 1.366(2) Å (cf. ∑rcov(NN) =
1.20 Å and ∑rcov(N−N) = 1.42 Å).45 For comparison, in the
carbodiimide species 4 the CN bond lengths amount to
1.201(2) and 1.194(2) Å, and the NCN unit is also almost
linear (176.6(2)°).44
Si angles (113.76(8) and 114.89(8)°) and one large Si3−N1−
Si2 angle (131.34(6)°).
[(Me3Si)2NNC(SiMe3)][CHB11H5Br6] (9[CHB11H5Br6]). As dis-
played in Table 3 and Figure 3, the structural parameters (bond
lengths and angles) of the cation are identical (within the
standard deviation) with those of 9[B(C6F5)4], indicating that
both borate anions are true weakly coordinating anions
(shortest distances between C1cation···Branion = 3.734 Å; cf.
C1cation···Fanion = 3.632 Å in 9[B(C6F5)4]).
[(Me3Si)2NNC(CPh3)][B(C6F5)4] (10) and [(Me3Si)2NNC(B-
(C6F5)3)] (11). The formal change of the Lewis acid (Me3Si+ in
9), as in 10 (Ph3C+) and 11 (B(C6F5)3), leads to small
differences in the NN distance, which are a little longer (10,
1.334(4), and 11, 1.342(2) Å vs 9[CHB11H5Br6], 1.309(3), and
9[B(C6F5)4], 1.309(2) Å), while the CN bond lengths are
identical to those in 9[CHB11H5Br6] and 9[B(C6F5)4]) (Figure
4, Table 3). The donor−acceptor bond lengths are in the
expected range, at 1.480(4) (C1−C8) and 1.615(2) (C1−B1),
respectively (∑rcov(C−C) = 1.50 Å and ∑rcov(C−B) = 1.60
Å).45
[(Me3Si)2NCN(SiMe3)][B(C6F5)4] (6[B(C6F5)4]). The X-ray
diffraction analyses (space group P21/c) revealed a disordered
cation with a di-coordinated and tri-coordinated (planar)
nitrogen center (sum of the angles around N1 = 360°, Figure
2 left). The strong interaction of carbodiimide with the formally
added, electron-deficient [Me3Si]+ center is clearly apparent
from the Si−N bond lengths (N1−Si2 1.835(1), N1−Si3
1.838(1), and N2−Si1 1.794(1) Å), which are slightly shorter
than the sum of the covalent radii (1.87 Å).45 Due to the di-
and tri-coordination, respectively, two significantly different CN
bond lengths (N1−C1 1.279(2), N2−C1 1.18(1) Å) are
observed, in contrast to the neutral parent molecule 4
(1.201(2) and 1.194(2) Å).44 The NCN unit is slightly bent,
with 173.3(5)°, and all Si atoms are not part of the plane
formed by these three atoms. Only weak interactions are found
between the ions.
Pyrazole Species 7 and 12. Both pyrazole species crystallize
[(Me3Si)2NNC(SiMe3)][B(C6F5)4] (9[B(C6F5)4]). Compound 9
crystallizes in the monoclinic space group P21/c with four units
per cell. As depicted in Figure 2 (right), the cation adopts a C1-
symmetric structure with an almost linear CNN unit (C1−N2−
N1 179.3(1)°, cf. 179.6(2) in (Me3Si)2CNN and 177.7(2)° in
(Me3Si)2NNC, Figure 1), and a slightly bent Si1−C1−N2
angle of 174.6(1)° is found. The C−N bond length amounts to
1.143(2) Å, which nicely agrees with the sum of the covalent
radii for a CN triple bond (∑rcov(CN) = 1.14 Å;45 cf.
1.152(2) Å in (Me3Si)2NNC or 1.157(2) Å in [Me3Si−CN−
SiMe3]+).19 The N1−N2 bond is rather long, at 1.309(2) Å (cf.
∑rcov(NN) = 1.20 Å and ∑rcov(N−N) = 1.42;45 1.135(2) Å
in (Me3Si)2CNN and 1.366(2) Å in (Me3Si)2NNC), indicating
partial double-bond character. In accord with natural bond
orbital analysis,46 thus the best Lewis representation is that with
a CN triple bond and an NN single bond: [Me3Si−CN+−
N(SiMe3)2]+. The tri-coordinated N1 atom sits in a trigonal
planar environment (∑∠N = 360.0°) with two small N2−N1−
in the triclinic space group P1 with two molecules per unit cell.
̅
As illustrated in Figure 5, the major difference in the molecular
structure of both species arises from an intramolecular
hydrogen bond in 12, which forces a change of the
configuration along the diazenyl substituent (cf. 7, N4 trans
to C3 vs 12, N4 cis to C3), allowing the formation of a six-
membered ring closed by the H-bridge. Furthermore, due to
the hydrogen bond, the amino nitrogen atom of the (Me3Si)2N
moiety pyramidalizes (∑∠N = 338.1°) since the nitrogen lone
pair cannot be delocalized by hyperconjugation. Hyper-
conjugative effects are known to be responsible for the
planarization of the amino nitrogen atom, as found in 7.47
Both N atoms of the hydrazine substituent in 12 have a
distorted trigonal-planar geometry (∑∠N = 359.4 and 359.7°),
and both trigonal planes are almost perpendicular to each other
(∠Si4−N5−N6−Si6 = 93.8°). Both (amino)silyl groups adopt
a staggered configuration in contrast to 12, for which an
eclipsed configuration is observed. While the N5−N6 bond
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Figure 7. Left: Energies of the intrinsic isomerization process with a [Me3Si]+ ion as catalyst. Right: Calculated transition state for the intrinsic 1,3-
Me3Si shift in 8 (distances in Å).
lengths are similar in 7 (1.455(2) Å) and 12 (1.467(3) Å), the
N3−N4 bond length of the diazenyl unit increases upon
hydrogen bond formation in 12 (cf. 1.272(2) vs 1.348(3) Å;
∑rcov(NN) = 1.20 Å, ∑rcov(N−N) = 1.42 Å;45 1.247(3) Å
in Ph−NN−Ph).48 The central five-membered pyrazole ring
is planar (deviation from planarity less than 2°), with bond
lengths and angles in the expected ranges.49
isomerization process in neutral 1 at ambient temperatures is
rather unlikely, since activation barriers larger than 56 kcal/mol
need to be overcome. These barriers for a 1,3-shift considerably
decrease once the [Me3Si]+ ion is added. However, they are still
too large (ca. 40.5 kcal/mol, Figure 7) to allow such an intrinsic
process at ambient or even low temperatures. Therefore, it can
be assumed that bimolecular processes such as illustrated in
Scheme 7 have to be considered. For the equilibrium of the
2.4. Theoretical Aspects of the Trimerization Process.
The astonishing isolation of [(Me3Si)2NNC(SiMe3)]+ instead
of isomeric [(Me3Si)2CNN(SiMe3)]+ as well as the isolation of
the trimerization product 7 prompted us to carry out quantum
chemical calculations at the pbe1pbe/aug-cc-pwCVDZ level of
theory to gain insight into the thermodynamics and kinetics of
this complex reaction. In any case, the formation of pyrazole
species 7, starting from diazomethane, includes several
successive reaction steps, including (i) a C−C or C−N
coupling and (ii) a [3+2] or [4+1] cycloaddition reaction.
Experimentally, it is known that pure (Me3Si)2CNN is
thermally stable for a long time. However, it is rearranged to
bis(trimethylsilyl)carbodiimide, (Me3Si)NCN(SiMe3), when it
is heated in the presence of suitable catalysts such as Cu2+.33,41a
Thus, we studied the potential energy surface of neutral
(Me3Si)2CNN and cationic [(Me3Si)3CNN]+ species, always
retaining the CNN unit. As depicted in Scheme 5, the
diazomethane isomer 1 is favored by 5.0 and 5.1 kcal/mol over
nitrilimine 2 and aminoisonitrile 3, respectively (cf. (Me3Si)-
NCN(SiMe3) lies 42.1 kcal/mol below 1). Upon [Me3Si]+
addition this situation switches. Now isomer 9 generated from
2 and 3 represents the lowest-lying isomer, separated by 10.5
kcal/mol from cation 8, in accord with the experimental
observation (cf. [(Me3Si)2NCN(SiMe3)]+ lies 49.7 kcal/mol
below 9).
Scheme 7. Isomerization Equilibria of Bis-silylated
Diazomethane Catalyzed by Silylium Ions
isomerization process, which includes both the neutral and the
cationic species as catalyst according to Scheme 7, the ΔG298
values are estimated to be −5.5 and 0.1 kcal/mol, respectively,
displaying [Me3Si]+ exchange equilibria. The barriers for
[Me3Si]+ exchange are dramatically decreased under solvation
conditions, as can be seen by studying the bimolecular reaction
paths (Scheme 7). Now, the exchange of a [Me3Si]+ ion
between cations 8 or 9 and the neutral isomers 1, 2, and 3
occurs almost barrier-free (less than 10 kcal/mol, see
Supporting Information) at ambient temperatures, resulting
in monosolvate formation of the type [Me3Si]+·2S adduct (S =
1, 2, and 3). In the cases of 1 and 3, these solvate adducts
feature a trigonal planar [Me3Si]+ ion that is almost
symmetrically stabilized by two S donor molecules (Figure
8). From these computational data, which are in agreement
with experimental data, it can be concluded that the initial
reaction step for the trimerization is the catalytic isomerization
of diazomethane 1 and the cation 8 in a bimolecular process, as
Two questions were addressed with respect to our
experimental findings: (i) Why does diazomethane 1 isomerize
to isonitrile 3 (which then trimerizes) upon addition of traces
of [Me3Si]+, and (ii) why is cation 9 exclusively formed from
diazomethane species 1 upon adding stoichiometric amounts of
[Me3Si]+? To answer these questions we have calculated the
transition states for the instrinsic 1,3-shift of a Me3Si group in
the neutral species 1 (Figure 6) and the cationic species 8
(Figure 7). In accord with our experimental data, the intrinsic
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Figure 8. Calculated structures of stable [Me3Si]+·2S adducts: left, S = (Me3Si)2CNN (1); right, S = (Me3Si)2NNC (3). Distances in Å.
shown in Scheme 7, generating the reactive species 2 and 3,
respectively, besides cation 9.
derivatives, which could also be extended to the aminoisonitrile
isomer. This transformation can be regarded as a domino
reaction.51 From a practical viewpoint, the chemistry reported
herein provides a facile approach to novel hydrazine-diazene-
substituted pyrazoles that are of pharmacological relevance and
not readily available by other methods.52,53
The trimethylsilylium affinity, which describes the enthalpy
change associated with the dissociation of the conjugated
acid,50 is largest for isomer 2 (71.1 kcal/mol), followed by 3
(69.0 kcal/mol) and 1 (53.1 kcal/mol) (cf. Me3Si−H, 31.3;
Me3Si−CN, 54.4; and (Me3Si)NCN(SiMe3) 71.7 kcal/mol),19
indicating that [(Me3Si)2NNC(SiMe3)]+ (9) is the most stable
ion in the reaction mixture, in accord with experiment. Once
the isomerization is triggered by the action of [(Me3Si)2NNC-
(SiMe3)]+, probably a dimerization via C−C or N−C coupling
reaction of aminoisonitrile 3 occurs prior to a formal [3+2] or
even [4+1] cycloaddition, as illustrated in Scheme 8. It is also
3. EXPERIMENTAL DETAILS
3.1. General Information. All manipulations were carried out
under oxygen- and moisture-free conditions under argon using
standard Schlenk or drybox techniques.
Toluene and diethyl ether were dried over Na/benzophenone. n-
Hexane and n-pentane were dried over Na/benzophenone/tetraglyme.
Carbon tetrachloride, dichloromethane, and hexamethyldisiloxane
(Me3Si)2O were dried over P4O10. Acetone was dried by distillation
from K2CO3 followed by storage over 3 Å molecular sieves for 24 h.
All solvents were freshly distilled prior to use. n-Heptane,
chlorotrimethylsilane Me3SiCl (98%, Merck), and N,N-bis-
(trimethylsilyl)carbodiimide (Me3Si)NCN(SiMe3) (97%, abcr) were
freshly distilled prior to use. Lithium (99.9%, Merck), gallium
trichloride GaCl3 (abcr, 99.9%), chloromethyltrimethylsilane
Me3SiCH2Cl (97%, abcr), n-BuLi (2.5 M, Acros), and magnesium
sulfate MgSO4 (98%, VWR) were used as received. Alumina
(aluminum oxide, basic, type T, Merck), was activated with
triethylamine (98%, Merck) and dried in an oven at 120 °C for 36
h. p-Toluenesulfonylazide MePhSO2N3,54 bis(trimethylsilyl)-
hydronium tetrakis(pentafluorophenyl)borate [Me3Si−H−SiMe3][B-
(C6F5)4],8b,15,55 tris(pentafluorophenyl)borane B(C6F5)3,55 tri-
(phenyl)methylium tetrakis(pentafluorophenyl)borate [Ph3C][B-
(C6F5)4],55 trimethylsilyl(diethyl)oxonium tetrakis(pentafluoro-
phenyl)borate [Me3SiOEt2][B(C6F5)],20,56 silver closo-7,8,9,10,11,12-
hexabromocarboranate Ag[CHB11H5Br6],57 trimethylsilylium closo-
7,8,9,10,11,12-hexabromocarboranate [Me3Si][CHB11H5Br6],57 and
bis(diethyloxo)hydronium tetrakis(pentafluorophenyl)borate
[(Et2O)2H][B(C6F5)4]58 were prepared as previously reported.
Trimethylsilyldiazomethane Me3SiCHNN, bis(trimethylsilyl)-
diazomethane (Me3Si)2CNN, N,N-bis(trimethylsilyl)aminoisonitrile
dichloride (Me3Si)2NNCCl2, and N,N-bis(trimethylsilyl)-
aminoisonitrile (Me3Si)2NNC have been reported in the literature
and were prepared by slightly modified procedures.33,34,59
Scheme 8. Retrosynthetic Analysis of 7: (i) C−C Coupling
Prior to [3+2] Cycloaddition, (ii) C−C Coupling Prior to
[4+1] Cycloaddition, and (iii) C−N Coupling Prior to [4+1]
Cycloaddition
possible that species 2 and 3 undergo dimerization. Since
aminoisonitrile 3 is thermally stable in pure state and reacts
only when the catalyst 9 is present, it can be concluded that
also the dimerization reaction is catalyzed by strong Lewis acids
(e.g., silylium ions [(Me3Si)2NNC(SiMe3)]+ + (Me3Si)2NNC).
All three reactions steps are accompanied by formal Me3Si
shifts, which can easily occur upon addition of Me3Si+ ions
(vide supra). Interestingly, according to in situ NMR experi-
ments (see above), there is no experimental proof for the
generation of a dimer or any other species involved in the
proposed formal [3+2] or [4+1] cycloaddition. The fact that
only pyrazole species 7 is observed besides the starting material
indicates either a fast reaction on the NMR time scale or a very
low concentration of the intermediates. The overall trimeriza-
tion process according to Scheme 3 is exothermic/exergonic,
with −67.0/−35.1 kcal/mol in the gas phase.
2.5. Conclusions. We present an efficient and facile
trimerization reaction of bis-silylated diazomethane, triggered
by the action of silylium ions to give exclusively 4-diazenyl-3-
hydrazinylpyrazole. As catalyst the isonitrilium ion
[(Me3Si)2NNC(SiMe3)]+, a formal pseudo-chalcogonium ion,
was identified and fully characterized for the first time. The
reaction is described by an isomerization process followed by a
C−C or N−C coupling reaction and a formal cycloaddition to
give finally the pyrazole derivative. From a mechanistic
viewpoint, we report the first trimerization of diazomethane
NMR. 29Si INEPT, 13C{1H}, 13C DEPT, and 1H NMR spectra were
obtained on a Bruker AVANCE 250 or 300 spectrometer and were
referenced internally to the deuterated solvent (13C: CDCl3, δreference
=
77 ppm, CD2Cl2, δreference = 54 ppm, C6D6, δreference = 128 ppm) or to
protic impurities in the deuterated solvent (1H: CHCl3, δreference = 7.26
ppm, CDHCl2, δreference = 5.31 ppm, C6D5H, δreference = 7.16 ppm).
CDCl3 and CD2Cl2 were dried over P4O10; C6D6 was dried over Na/
benzophenone and freshly distilled prior to use.
IR. A Nicolet 6700 FT-IR spectrometer with a Smart Endurance
ATR device was used.
Raman. A Bruker VERTEX 70 FT-IR spectrometer with a RAM II
FT-Raman module, equipped with a Nd:YAG laser (1064 nm), was
used.
CHN Analyses. A C/H/N/S-Mikronalysator TruSpec-932 instru-
ment from Leco was used.
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DSC. A DSC 823e instrument from Mettler-Toledo (heating rate, 5
°C/min) was used. Melting points (and decomposition points) were
corrected. In addition, clearing points are given in brackets.
1374 (m), 1340 (w), 1261 (m), 1167 (m), 1081 (s), 976 (s), 923 (w),
907 (w), 855 (s), 822 (s), 773 (m), 768 (m), 756 (s), 726 (m), 700
(w), 683 (m), 660 (m), 643 (m) 626 (m), 611 (m), 602 (m), 573
(m). Raman (1500 mW, 25 °C, 800 scans, cm−1): 2970 (4), 2910
(10), 2209 (9), 1643 (3), 1598 (1), 1576 (1), 1536 (1), 1513 (1),
1454 (1), 1417 (1), 1375 (2), 1314 (1), 1276 (1), 1263 (1), 1125 (1),
1106 (1), 976 (1), 960 (1), 945 (1), 862 (1), 845 (1), 820 (3), 771
(1), 757 (1), 733 (1), 700 (1), 685 (1), 661 (1), 643 (5), 628 (5), 584
(7), 509 (2), 491 (5), 476 (4), 448 (6), 422 (4), 394 (5), 371 (1), 359
(1), 348 (1), 320 (1), 287 (1), 265 (1), 244 (1), 206 (1), 186 (2), 157
(1), 119 (1). MS (CI+, isobutane): 259 [(SiMe3)3CN2]+, 187
[(SiMe3)2CN2 + H]+. Crystals suitable for X-ray crystallographic
analysis were obtained from toluene as depicted above.
MS. A Finnigan MAT 95-XP spectrometer from Thermo Electron
was used (CI+, isobutene; EI+, T = 200 °C, 70.0 V).
HRMS. A 6210 time-of-flight LC/MS instrument from Agilent
Technologies (MeOH/0.1% HCOOH in H2O 90:10) was used.
3.2. Synthesis of 6[B(C6F5)4]. To a stirred suspension of [Me3Si−
H−SiMe3][B(C6F5)4] (0.1 mmol, 0.083 g) in toluene (0.5 mL) was
added neat (Me3Si)NCN(SiMe3) (0.1 mmol, 0.019 g) at ambient
temperature. The resulting colorless solution was concentrated in
vacuo, resulting in a colorless oil. Storage at −25 °C for 24 h led to the
deposition of colorless crystals. Removal of supernatant by decantation
and drying in vacuo yields 0.072 g (0.077 mmol, 77%) of 6[B(C6F5)4].
Mp: 104 °C (106 °C). Anal. Calcd (found): C, 43.51 (43.52); H, 2.90
(2.98); N, 2.98 (2.95). IR (ATR, 16 scans): 3467 (w), 3408 (w), 3381
(w), 3327 (w), 2962 (w), 2912 (w), 2359 (w), 2320 (m), 2287 (w),
2243 (m), 1643 (m), 1558 (w), 1512 (s), 1456 (s), 1412 (m), 1383
(m), 1374 (m), 1317 (m), 1263 (s), 974 (s), 907 (m), 853 (s), 823
(s), 768 (s), 755 (s), 726 (m), 700 (w), 683 (m), 661 (s), 634 (m),
610 (m), 603 (m), 573 (m). Raman (190 mW, 25 °C, 400 scans,
cm−1): 2982 (4), 2973 (4), 2913 (10), 2251 (4), 1648 (4), 1516 (1),
1468 (1), 1422 (1), 1380 (2), 1322 (2), 1277 (1), 1266 (1), 1127 (1),
1110 (1), 1098 (1), 1088 (1), 1056 (1), 980 (1), 824 (3), 774 (2), 760
(1), 710 (1), 687 (1), 664 (1), 645 (8), 587 (6), 531 (1), 494 (5), 479
(5), 452 (6), 425 (5), 398 (5), 381 (1), 361 (1), 350 (1), 290 (1), 248
(2), 210 (1), 190 (2), 161 (1), 124 (1), 76 (1). MS (CI+, isobutane):
259 [(SiMe3)3CN2]+, 187 [(SiMe3)2CN2 + H]+, 115 [(SiMe3)CN2 +
2H]+. Crystals suitable for X-ray crystallographic analysis were
obtained from slowly cooling a saturated toluene solution of
6[B(C6F5)4] to 5 °C over a period of 12 h.
3.3. Synthesis of 9[B(C6F5)4] and Catalytic Reaction to 7.
Procedure 1: Reaction with (Me3Si)2CNN. To a stirred suspension of
[Me3Si−H−SiMe3][B(C6F5)4] (0.05 mmol, 0.040 g) in n-pentane (2
mL) was added a solution of (Me3Si)2CNN (1.06 mmol, 0.197 g) in n-
pentane (2 mL) at −78 °C. The resulting suspension was allowed to
warm to ambient temperature. Stirring for 36 h resulted in the
deposition of crystals while the color of the supernatant changed
gradually from yellow to dark red. The supernatant was removed by
filtration (F4), and the brownish residue was washed three times with
n-pentane (3 mL). Recrystallization from a minimum of toluene at
−25 °C resulted in the deposition of colorless crystals. Removal of
supernatant by decantation and drying in vacuo yielded 0.022 g (0.02
mmol, 49% yield based on 0.05 mmol of [Me3Si−H−SiMe3][B-
(C6F5)4]) of 9[B(C6F5)4] as colorless crystals. The solutions from
filtration and washing were combined, and the solvent was removed in
vacuo, resulting in a dark red solution. Storage at −25 °C led to the
deposition of greenish crystals. Removal of supernatant and drying in
vacuo yielded 0.097 g (0.17 mmol, 51%) of 7 as dark green crystals.
Procedure 2: Reaction with (Me3Si)2NNC. To a stirred suspension
of [Me3Si−H−SiMe3][B(C6F5)4] (0.05 mmol, 0.041 g) in n-hexane (4
mL) was added neat (Me3Si)2NNC (1.0 mmol, 0.186 g) at −78 °C.
The resulting suspension was allowed to warm slowly to ambient
temperature. Stirring for 12 h resulted in the deposition of yellowish
crystals while the color of the supernatant changed gradually from
yellow to dark red. The supernatant was removed by filtration (F4),
and the yellowish residue was washed with n-hexane (3 mL).
Recrystallization from a minimum of toluene at 5 °C resulted in the
deposition of colorless crystals. Removal of supernatant by decantation
and drying in vacuo yielded 0.027 g (0.03 mmol, 58% yield based on
0.05 mmol [Me3Si−H−SiMe3][B(C6F5)4]) of 9[B(C6F5)4] as color-
less crystals. The solutions from filtration and washing were combined
and concentrated in vacuo, resulting in a dark red solution. Storage at
−25 °C led to the deposition of greenish crystals. Removal of
supernatant and drying in vacuo yielded 0.120 g (0.21 mmol, 68%) of 7
as dark green crystals.
Data for 7. Mp: 66 °C (70 °C). Anal. Calcd (found): C, 45.10
1
(45.15); H, 9.73 (9.44); N, 15.03 (15.05). H NMR (25 °C, CD2Cl2,
300.13 MHz): 0.15 (s, 18H, Si(CH3)3), 0.16 (s, 9H, Si(CH3)3), 0.28
(s, 9H, Si(CH3)3), 0.32 (s, 9H, Si(CH3)3), 0.48 (s, 9H, Si(CH3)3). 1H
NMR (25 °C, C6D6, 300.13 MHz): 0.23 (s, 9H, Si(CH3)3), 0.35 (s,
18H, Si(CH3)3), 0.37 (s, 9H, Si(CH3)3), 0.44(s, 9H, Si(CH3)3),
0.48(s, 9H, Si(CH3)3). 13C{1H} NMR (25 °C, CD2Cl2, 62.89 MHz):
−3.35 (s, Si(CH3)3), −0.21 (s, Si(CH3)3), 0.08 (s, (Si(CH3)3)2), 0.11
(s, Si(CH3)3), 0.29 (s, Si(CH3)3), 137.29 (C), 146.91(C), 158.15 (C).
13C{1H} NMR (25 °C, C6D6, 62.89 MHz): −0.64 (s, Si(CH3)3), 2.58
(s, Si(CH3)3), 2.75 (s, Si(CH3)3), 3.02 (s, 2 Si(CH3)3), 140.26 (C),
149.72 (C), 161.15 (C). 29Si{1H} NMR (25 °C, CD2Cl2, 49.69 MHz):
−13.39 (m, 1Si, J(1H−29Si) = 6.82 Hz), 6.22 (m, 1Si, J(1H−29Si) =
6.79 Hz), 8.94 (m, 2Si, J(1H−29Si) = 6.65 Hz), 11.31 (m, 1Si,
J(1H−29Si) = 6.59 Hz), 17.15 (m,1Si, J(1H−29Si) = 6.71 Hz). 29Si{1H}
NMR (25 °C, C6D6, 49.69 MHz): −13.71 (m, 1Si, J(1H−29Si) = 6.54
Hz), 6.00 (m, 1Si, J(1H−29Si) = 6.54 Hz), 9.09 (m, 2Si, J(1H−29Si) =
6.54 Hz), 11.40 (m, 1Si, J(1H−29Si) = 6.54 Hz), 17.17 (m, 1Si,
J(1H−29Si) = 6.54 Hz). IR (ATR, 16 scans): 2956 (m), 2899 (m),
2864 (w), 1603 (w), 1574 (w), 1565 (w), 1521 (w), 1510 (w), 1490
(m), 1454 (m), 1388 (m), 1363 (m), 1298 (w), 1279 (m), 1245 (s),
1178 (m), 1142 (w), 1103 (w), 1074 (m), 1022 (w), 1004 (w),
981(w), 939 (m), 875 (m), 832 (s), 822 (s), 753 (s), 726 (m), 890
(m), 632 (m), 621 (m), 561 (w), 532 (m). Raman (1500 mW, 25 °C,
1209 scans, cm−1): 2957 (3), 2903 (10), 1490 (2), 1453 (2), 1388 (7),
1364 (9), 1282 (5), 1245 (1), 1103 (1), 1080 (1), 1006 (1), 964 (2),
946 (1), 878 (1), 838 (1), 748 (1), 726 (1), 693 (3), 610 (3), 535 (1),
472 (1), 454 (1), 407 (1), 376 (1), 351 (1), 340 (1), 308 (1), 256 (1),
204 (1), 182 (1), 114 (1). MS (CI+, isobutane): 558 [M]+, 543 [M −
CH3]+, 486 [M − SiMe3]+. UV−vis (25 °C, CH2Cl2, nm): 556, 393,
321, 255. Crystals suitable for X-ray crystallographic analysis were
obtained by slowly cooling a saturated solution of 7 in either n-
pentane, n-hexane, hexamethyldisiloxane, or acetone to −25 °C over a
period of up to 12 h. Crystals obtained from n-hexane and
hexamethyldisiloxane solutions could be characterized as n-hexane
hemisolvate (7·n-hexane) or hexamethyldisiloxane hemisolvate (7·dis-
iloxane), while crystals from n-pentane or acetone solutions did not
contain solvent. However, the n-hexane was easily removed upon
drying at ambient temperature for a few minutes.
3.4. Synthesis of 9[CHB11H5Br6]. To a stirred brownish
suspension of [Me3Si][CHB11H5Br6] (0.1 mmol, 0.069 g) in toluene
(3.0 mL) was added neat (Me3Si)2NNC (0.6 mmol, 0.120 g) at
ambient temperature. The resulting colorless suspension was allowed
to stir for an additional 1 h. Removal of supernatant by syringe and
drying in vacuo yielded 0.068 g (0.0785 mmol, 79%) of
9[CHB11H5Br6] as pale brownish microcrystalline solid. Mp (dec):
206 °C. Anal. Calcd (found): C, 15.08 (14.88); H, 3.80 (4.06); N, 3.20
(3.50). 1H NMR (25 °C, CD2Cl2, 300.13 MHz): 0.47 (s, 18H,
J(1H−29Si) = 6.6 Hz, J(1H−13C) = 121 Hz), 0.64 (s, 9H, J(1H−29Si) =
7.18 Hz, J(1H−13C) = 124 Hz), 1.12−3.57 (m, 5H, BH, J(1H−11B) =
159 Hz), 2.59 (s, 1H, CH-carboranate). 11B NMR (25 °C, CD2Cl2,
96.29 MHz) −20.22 (d, 5B, BH), −9.84 (s, 5B, BBr), −1.74 (s, 1B, 12-
BBr). 13C{1H} NMR (25 °C, CD2Cl2, 75.48 MHz): −1.32 (s,
Si(CH3)3), −0.15 (s, 2Si(CH3)3). 29Si{1H} NMR (25 °C, CD2Cl2,
59.62 MHz): 4.78 (1Si), 29.01 (2Si). IR (ATR, 16 scans): 3051 (w),
2958 (w), 2902 (w), 2606 (m), 2236 (w), 2165 (w), 1682 (w), 1417
(w), 1309 (w), 1256 (s), 1113 (m), 1054 (m), 1002 (m), 991 (m),
Data for 9[B(C6F5)4]. Mp: 108 °C (109 °C). Anal. Calcd (found):
C, 43.51 (43.75); H, 2.90 (2.85); N, 2.98 (2.86). IR (ATR, 16 scans):
3374 (w), 3224 (w), 2962 (w), 2912 (w), 2327 (w), 2215 (m), 1643
(m), 1600 (w), 1556 (w), 1512 (s), 1460 (s), 1413 (m), 1382 (m),
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Article
953 (m), 933 (m), 855 (s), 819 (s), 746 (s), 702 (m), 633 (s). MS
(CI−, isobutane): 616 m/e [CHB11H5Br6]−. Crystals suitable for X-ray
crystallographic analysis of 9[CHB11H5Br6] were obtained from the
saturated catalyst phase of the benzene reaction mixture after 12 h.
3.5. Synthesis of [(Me3Si)2NNC(CPh3)]B(C6F5)4] (10). To a
stirred orange solution of [Ph3C][B(C6F5)4] (0.5 mmol, 0.427 g) in
dichloromethane (3 mL) was added neat (Me3Si)2NNC (0.5 mmol,
0.093 g) at ambient temperature. The mixture was allowed to stir for 1
h while the color of the solution changed within 20 min from orange
to yellow. Slow addition of n-hexane (5 mL) resulted in the deposition
of a yellow greenish, microcrystalline solid. The yellow supernatant
was removed by syringe, and the residue was re-dissolved in
dichloromethane (2 mL) and again precipitated by addition of n-
hexane (4 mL). This procedure was repeated a further two times.
Removal of supernatant by decantation and drying in vacuo yielded
0.324 g (0.29 mmol, 58%) of [(Me3Si)2NNC(CPh3)]B(C6F5)4] as
microcrystalline solid. Mp (dec): 136 °C. Anal. Calcd (found): C,
54.16 (54.03); H, 3.00 (3.09); N, 2.53 (2.62). 1H NMR (25 °C,
CD2Cl2, 300.13 MHz): 0.38 (s, 18H, J(1H−29Si) = 6.8 Hz, J(1H−13C)
= 121 Hz), 7.08−7.53 (m, 15H, CH). 19F{1H} NMR (25 °C, CD2Cl2,
282.4 MHz): δ = −167.5 (m, m-CF), −163.6 (m, p-CF), −133.0 (m,
o-CF). 11B NMR (25 °C, CD2Cl2, 96.29 MHz) −16.64. 13C{1H}
NMR (25 °C, CD2Cl2, 75.48 MHz): 0.10 (s, Si(CH3)3), 124.59 (br,
ipso-C), 129.11 (s, CH), 130.43 (s, CH), 130.75 (s, p-CH), 136.91
(dm, m-CF, 1J(13C−19F) = 247.59 Hz), 138.90 (dm, p-CF, 1J(13C−19F)
= 232.18 Hz), 148.76 (dm, o-CF, 1J(13C−19F) = 240.44 Hz). 29Si{1H}
NMR (25 °C, CD2Cl2, 59.62 MHz): 30.87. IR (ATR, 16 scans): 2959
(w), 2913 (w), 2285 (w), 1643 (m), 1599 (w), 1584 (w), 1512 (s),
1493 (w), 1460 (s), 1456 (s), 1411 (m), 1383 (w), 1373 (w), 1359
(w), 1265 (m), 1186 (w), 1171 (w), 1160 (w), 1145 (w), 1084 (s),
1033 (w), 1001 (w), 974 (s), 942 (m), 902 (m), 854 (s), 823 (s), 773
(m), 756 (s), 748 (s), 726 (m), 698 (s), 683 (s), 660 (s), 635 (m), 610
(m), 603 (m), 573 (m). MS (CI+, isobutane): 431 (40) [M + H], 521
(10) [B(C6F5)3], 357 (10) [Me3SiN(H)NCCPh3], 167 (78) [C6F5],
187 (10) [(SiMe3)2NNC + H], 270 (38) [Ph3CCN + H]. Crystals
suitable for X-ray crystallographic analysis were obtained from slow
diffusion of n-hexane onto the saturated dichloromethane solution of
10 at ambient temperature over a period of 36 h.
3.6. Synthesis of (Me3Si)2NNC(B(C6F5)3) (11). Procedure 1:
Reaction with (Me3Si)2NNC. To a stirred solution of B(C6F5)3 (0.5
mmol, 0.256 g) in dichloromethane (2.5 mL) neat (Me3Si)2NNC (0.5
mmol, 0.094 g) was added at 0 °C. The stirred reaction mixture was
allowed to warm to ambient temperature within 1 h. The pale yellow
solution was filtered (F4), concentrated approximately to 0.7 mL and
stored in the refrigerator (5 °C) for 12 h. The resulting colorless
Crystals were washed with dichloromethane (0.5 mL). Removal of
supernatant by syringe and drying in vacuo yielded 0.250 g (0.358
mmol, 72%) of (Me3Si)2NNC(B(C6F5)3) (11).
Procedure 2: Reaction with (Me3Si)2CNN. To a stirred solution of
B(C6F5)3 (0.5 mmol, 0.256 g) in dichloromethane (2.5 mL) was
added neat (Me3Si)2CNN (0.6 mmol, 0.120 g) at 0 °C. The stirred
reaction mixture was allowed to warm to ambient temperature within
1 h. The pale yellow solution was filtered (F4), concentrated
approximately to 0.7 mL, and stored in the refrigerator (5 °C) for
12 h. The resulting colorless crystals were washed with dichloro-
methane (0.5 mL). Removal of supernatant by syringe and drying in
vacuo yielded 0.178 g (0.255 mmol, 51%) of (Me3Si)2NNCB(C6F5)3
(11).
(s), 1134 (m), 1127 (m), 1095 (s), 1029 (w), 979 (s), 968 (s), 928
(w), 890 (m), 880 (m), 854 (s), 826 (s), 800 (s), 764 (m), 747 (m),
728 (m), 700 (m), 680 (m), 672 (m), 647 (m), 630 (m), 577 (w).
Raman (460 mW, 25 °C, 200 scans, cm−1): 2974 (5), 2913 (10), 2797
(4), 2539 (3), 2299 (10), 2220 (3), 1650 (4), 1474 (1), 1422 (1),
1394 (2), 1279 (2), 1138 (1), 1098 (1), 861 (1), 803 (2), 766 (1), 753
(1), 710 (1), 685 (1), 676 (1), 652 (6),635 (1), 622 (1), 585 (10), 510
(2), 496 (4), 471 (8), 450 (7), 419 (4), 398 (6), 357 (1), 344 (1), 288
(1), 271 (1), 244 (2), 188 (2), 159 (2), 91 (3). MS (EI, 70 eV): 698
(10) [M], 531 (14) [M − C6F5], 512 (100) [B(C6F5)3], 364 (13)
[M − 2C6F5], 186 (11) [(SiMe3)2NNC], 73 (83) [(SiMe3)]. Crystals
suitable for X-ray crystallographic analysis were obtained by slowly
cooling a saturated dichloromethane solution of 11 to 5 °C over a
period of 12 h.
3.7. Synthesis of 12. Procedure 1. To a stirred solution of 7 (0.48
mmol, 0.271 g) in n-hexane (5 mL) was added neat CF3SO3H (0.50
mmol, 0.075 g) dropwise at ambient temperature with stirring. The
resulting suspension was stirred for 2 h while the color of the
supernatant changed gradually from brown red to yellowish orange,
and the yellowish precipitate was dissolved completely. Concentration
to an approximate volume of 0.5 mL and storage at 5 °C resulted in
the deposition of yellow crystals. Removal of supernatant, washing
with a few drops of n-hexane, and drying in vacuo yielded 0.140 g
(59%) of 12 as yellow crystals.
Procedure 2. 7 (0.179 mmol, 0.1 g) was dissolved in distilled (but
not anhydrous) n-hexane (3 mL) at ambient temperature with stirring.
The resulting orange solution was stirred for 15 min. Concentration to
an approximate volume of 0.5 mL and storage at 5 °C resulted in the
deposition of yellowish crystals. Removal of supernatant and drying in
vacuo yielded 0.085 g (97%) of 12 as yellow crystals.
Data for 12. Mp: 94 °C (101 °C). Anal. Calcd (found): C, 44.39
1
(44.57); H, 9.523 (9.33); N, 17.26 (17.25). H NMR (25 °C, C6D6,
500.13 MHz): 10.15 (s, 1H, NH), 0.52 (s, 9H, Si(CH3)3, N-1), 0.41
(s, 9H, Si(CH3)3, C-1), 0.27 (s, 9H, Si(CH3)3, N-4), 0.15 (s, 18H, 2
Si(CH3)3, N-6). 13C{1H} NMR (25 °C, C6D6, 128.0 MHz): 161.9,
142.1 (C-2, C-3), 157.3 (C-1), 0.3 (s, 2 Si(CH3)3, N-6), −0.5 (s,
Si(CH3)3, N-1), −0.6 (s, Si(CH3)3, N-4), −1.5 (s, Si(CH3)3, C-1).
29Si{1H} NMR (25 °C, C6D6, TMS = 0, 99.3 MHz): 10.1, 7.4
(SiMe3(N-1), SiMe3(N-4)), 6.6 (2, SiMe3(N-6)), −8.6 (SiMe3(C-1)).
1H NMR (25 °C, CD2Cl2, 300.13 MHz): 0.10 (s, 18H, N(Si(CH3)3)2,
J(1H−29Si) = 6.6 Hz), 0.24 (s, 9H, Si(CH3)3, J(1H−29Si) = 6.9 Hz,
J(1H−13C) = 121 Hz), 0.29 (s, 9H, Si(CH3)3), 0.37 (s, 9H, Si(CH3)3,
J(1H−29Si) = 6.9 Hz, J(1H−13C) = 120 Hz), 9.97 (s, 1H, NH).
13C{1H} NMR (25 °C, CD2Cl2, 62.89 MHz): −1.36 (s, Si(CH3)3),
−0.32 (s, Si(CH3)3), −0.21 (s, Si(CH3)3), 0.56 (s, (Si(CH3)3)2), 141.9
(C), 157.7 (C), 161.9 (C). 29Si{1H} NMR (25 °C, CD2Cl2, 99.3
MHz): −8.79, 6.91, 7.91, 9.91. IR (ATR, 16 scans): 2956 (m), 2899
(m), 2855 (w), 1573 (m), 1563 (m), 1520 (m), 1465 (m), 1405 (m),
1345 (w), 1325 (w), 1300 (m), 1245 (s), 1179 (s), 1076 (m), 1050
(m), 982 (m), 961 (m), 930 (m), 898 (m), 870 (m), 828 (s), 818 (s),
753 (s), 693 (m), 662 (m), 628 (s), 601 (m). Raman (1500 mW, 25
°C, 800 scans, cm−1): 2960 (5), 2900 (10), 1562 (1), 1522 (1), 1466
(4), 1411 (1), 1345 (1), 1300 (1), 1250 (1), 1179 (1), 1076 (1), 983
(1), 963 (1), 941 (1), 898 (1), 843 (1), 763 (1), 694 (1), 663 (1), 634
(2), 601 (1), 531 (1), 462 (1), 435 (1), 357 (1), 318 (1), 303 (1), 235
(1), 177 (1), 117 (1). MS (CI+, isobutane): 486 [M]+, 471 [M −
CH3]+, 413 [M − SiMe3]+. UV−vis (25 °C, CH2Cl2, nm): 412, 294.
Data for 11. Mp: 122 °C (124 °C). Anal. Calcd (found): C, 42.99
ASSOCIATED CONTENT
* Supporting Information
Experimental and computational details, crystallographic
information, and further experimental and theoretical data of
all considered species. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
■
(43.04); H, 2.60 (2.84); N, 4.01 (3.75). H NMR (25 °C, CD2Cl2,
300.13 MHz): 0.29 (s, 18H, J(1H−29Si) = 6.8 Hz, J(1H−13C) = 121
Hz). 19F{1H} NMR (25 °C, CD2Cl2, 282.4 MHz): δ = −164.5 (m, m-
CF), −157.9 (m, p-CF), −133.4 (m, o-CF). 11B NMR (25 °C, CD2Cl2,
96.29 MHz): −21.27. 13C{1H} NMR (25 °C, CD2Cl2, 75.48 MHz):
−0.36 (s, Si(CH3)3), 116.02 (br, ipso-C), 137.63 (dm, m-CF,
S
1
1J(13C−19F) = 229.43 Hz), 140.77 (dm, p-CF, J(13C−19F) = 214.03
Hz), 148.58 (dm, o-CF, 1J(13C−19F) = 240.99 Hz). 29Si{1H} NMR (25
°C, CD2Cl2, 59.62 MHz): 25.46. IR (ATR, 16 scans): 2972 (w), 2911
(w), 2302 (w), 1645 (m), 1557 (w), 1514 (s), 1470 (s), 1466 (s),
1456 (s), 1418 (m), 1392 (m), 1382 (s), 1287 (m), 1282 (m), 1261
AUTHOR INFORMATION
Corresponding Author
axel.schulz@uni-rostock.de
■
17766
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Journal of the American Chemical Society
Article
(13) Maerker, C.; Kapp, J.; Schleyer, P. von, R. In Organosilicon
Chemistry II; Auner, N., Weis, J., Eds.; VCH: Weinheim, Germany,
1996; pp 329−358.
Notes
The authors declare no competing financial interest.
(14) Kim, K.-C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.;
Lin, L.; Lambert, J. B. Science 2002, 297, 825−827.
(15) Lehmann, M.; Schulz, A.; Villinger, A. Angew. Chem., Int. Ed.
2009, 48, 7444−7447.
ACKNOWLEDGMENTS
■
Martin Ruhmann (University Rostock) is acknowledged for the
measurement of Raman spectra. Financial support by the DFG
is gratefully acknowledged.
(16) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. Commun. 1993,
383−384.
(17) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993, 262, 402−
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