Silylium-arene adducts: An experimental and theoretical study

Article abstract of DOI:10.1021/ja209693a

The solvent-coordinated [Me₃Si·arene][B(C ₆F₅)₄] salts (arene = benzene, toluene, ethylbenzene, n-propylbenzene, isopropylbenzene, o-xylene, m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene) are prepared and fully characterized. As an interesting decomposition product the formation of bissilylated fluoronium ion [Me₃Si-F-SiMe ₃]⁺ was observed and even cocrystallized with [Me ₃Si·arene][B(C₆F₅)₄] (arene = benzene and toluene). Investigation of the degradation of [Me ₃Si·arene][B(C₆F₅)₄] reveals the formation of fluoronium salt [Me₃Si-F-SiMe₃][B(C ₆F₅)₄], B(C₆F₅) ₃, and a reactive "C₆F₄" species which could be trapped with CS₂. Upon addition of CS₂, the formation of a formal S-heterocyclic carbene adduct, C₆F ₄CS₂-B(C₆F₅)₃, was observed. The structure and bonding of substituted [Me₃Si· arene][B(C₆F₅)₄] with arene = R _nC₆H_6-n (R = H, Me, Et, Pr, and Bu; n = 0-6) is discussed on the basis of experimental and theoretical data. X-ray data of [Me₃Si·arene][B(C₆F₅)₄] salts reveal nonplanar arene species with significant cation...anion interactions. As shown by different theoretical approaches (charge transfer, partial charges, trimethylsilyl affinity values) stabilizing inductive effects occur; however, the magnitude of such effects differs depending on the degree of substitution and the substitution pattern.

Full text of DOI:10.1021/ja209693a

ARTICLE
pubs.acs.org/JACS
SilyliumꢀArene Adducts: An Experimental and Theoretical Study
Muhammad Farooq Ibad,†,‡ Peter Langer,‡,§ Axel Schulz,*,†,§ and Alexander Villinger*,†
†Abteilung Anorganische Chemie and ‡Abteilung Organische Chemie, Institut f€ur Chemie, Universit€at Rostock,
Albert-Einstein-Strasse 3a, D-18059 Rostock, Germany
§Leibniz-Institut f€ur Katalyse e.V. an der Universit€at Rostock, Albert-Einstein-Strasse 29a, D-18059 Rostock, Germany
S Supporting Information
b
ABSTRACT: The solvent-coordinated [Me3Si arene][B(C6F5)4] salts (arene = benzene, toluene,
3
ethylbenzene, n-propylbenzene, isopropylbenzene, o-xylene, m-xylene, p-xylene, 1,2,3-trimethylben-
zene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene) are prepared and fully characterized. As an
interesting decomposition product the formation of bissilylated fluoronium ion [Me3SiꢀFꢀSiMe3]+
was observed and even cocrystallized with [Me3Si arene][B(C6F5)4] (arene = benzene and toluene).
3
Investigation of the degradation of [Me3Si arene][B(C6F5)4] reveals the formation of fluoronium
3
salt [Me3SiꢀFꢀSiMe3][B(C6F5)4], B(C6F5)3, and a reactive “C6F4” species which could be trapped
with CS2. Upon addition of CS2, the formation of a formal S-heterocyclic carbene adduct, C6F4CS2ꢀ
B(C6F5)3, was observed. The structure and bonding of substituted [Me3Si arene][B(C6F5)4] with
3
arene = RnC6H6ꢀn (R = H, Me, Et, Pr, and Bu; n = 0ꢀ6) is discussed on the basis of experimental and theoretical data. X-ray data of
[Me3Si arene][B(C6F5)4] salts reveal nonplanar arene species with significant cation anion interactions. As shown by different
3
3 3 3
theoretical approaches (charge transfer, partial charges, trimethylsilyl affinity values) stabilizing inductive effects occur; however, the
magnitude of such effects differs depending on the degree of substitution and the substitution pattern.
long-lived catalysts for hydrodefluorination of trifluoromethyl
and nonafluorobutyl groups by widely accessible silanes under
mild conditions. The reactions are completely selective for aliphatic
carbonꢀfluorine bonds in preference to aromatic carbonꢀfluorine
bonds.11b Recently, Oestreich et al.13 demonstrated that a tamed,
ferrocene-based silylium ion (Chart 2, species F) catalyzes
demanding DielsꢀAlder reactions in an unprecedented tempera-
ture range.
1. INTRODUCTION
Cations containing a tricoordinate silicon atom, R3Si+ (where
R is an alkyl or aryl group), are known as silylium (also silylenium
or silicenium) ions.1,2 A long debate concerning the existence of
“naked” R3Si+ cations (Chart 1, species A),3 free of interactions
with counterions (Chart 1, species B) and neighboring groups
or solvent (Chart 1, species C and D), was finally brought to an
end with the isolation and full characterization of [(Mes)3Si]-
Both the utilization of chemically robust weakly coordinating
anions17,18 and the steric shielding of the Lewis-acidic Si atom7,16
led to the structural determination of a silylium ion.4 The first struc-
turally characterized salt bearing a silylium cation, [Et3Si]+-
[HCB11Me5Br6] C6H6 (Mes = 2,4,6-trimethylphenyl) by the
3
groups of Lambert and Reed in 2002.4 The silylium ion in
[(Mes)3Si][HCB11Me5Br6] was shown to be three-coordinate,
planar, and well-separated from the carborane anions and benzene
solvate molecules by means of single-crystal X-ray studies. o-Methyl
groups of the bulky mesityl substituents shield the silicon atom
from the close approach of nucleophiles, while remaining innocent
as electron donors themselves.4
[B(C6F5)4]ꢀ 2(toluene), was reported by Lambert et al. in 1993.5
3
The crystal structure of [Et3Si]+[B(C6F5)4]ꢀ toluene revealed a
3
silyl cation with significant coordination to a toluene molecule,
which is the solvent for crystallization. The nature and extent of
this coordination were controversial.19,20 For a free R3Si+ cation,
all three substituents should lie in a plane, and the average bond
angle to the tricoordinate silicon should be 120ꢀ. However, for the
Et3Si+ ion, the average angle was only 114ꢀ, and thus, [Et3Si]+-
Silylium ions with their electron sextet and empty p orbitals
are electron-deficient species and thus strong Lewis acids. Even
relatively weak Lewis bases, such as π/σ donor solvents (e.g.,
toluene,5 CH3CN,6 etc.), form tetrahedral complexes with silylium
ions.5 In addition, intramolecular π coordination in silylium ions
containing a 2,6-diarylphenyl scaffold which adopt the C1-symmetric
geometry of a Wheland-like complex was observed (Chart 2,
species E).7 The first well-documented examples of intramole-
cular π-stabilization in silyl cations are silanorbornyl cations.8
As silylium ions are highly reactive Lewis acids, they are useful
reagents in chemical synthesis.9ꢀ15 Ozerov et al.11 and M€uller et al.12
have utilized silylium ions as reactive catalysts for the activation of
CꢀF bonds. The Ozerov group introduced a class of carborane-
supported, highly electrophilic silylium compounds that act as
[B(C6F5)4]ꢀ toluene should be regarded as a salt containing a
3
solvent complex as a cation of the type [Et3Si toluene]+ (Chart 1,
3
species C).
In contrast to the solid state, the free silylium cation in solution
seems to be an exception (e.g., Mes3Si+)21 due to interaction with
the solvent.22 The question is how much silylium cation character
(if any at all) can be retained in a solvent-coordinated silylium
Received: October 18, 2011
Published: November 15, 2011
r
2011 American Chemical Society
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Chart 1. Silylium Ionsa
aSpecies description: A, naked cation; B, ion pair with strong ca-
tionꢀanion interactions; C and D, solvent complexes as cation (Aꢀ
weakly coordinating anion, S = σ or π donor solvent).
=
Figure 1. ORTEP drawing of the molecular structure of [Me3Si
3
benzene]+. Thermal ellipsoids with 30% probability at 173 K.
Chart 2. Species E Showing Intramolecular π Coordination
in Silylium Ions (R1, R2 = H, Me) and Species F, a Ferrocene-
Based Silylium Ion Intramolecularly Stabilized by Electron-
Rich Fe
cation.23 There is computational evidence that even argon can be
a ligand to Me3Si+.3a
Ever since the isolation of [Et3Si toluene]+, no further solvent
3
complex bearing a silylium solvent complex as a cation of the type
[R3Si arene]+ (R = alkyl) has been isolated and structurally
3
Figure 2. ORTEP drawing of the molecular structure of [Me3Si mono-
characterized. Recently, salts containing [Me3SiꢀXꢀSiMe3]+
ions (X = halogen, pseudo-halogen), which can also be con-
sidered as solvent complexes of Me3SiꢀX and [Me3Si]+, have
been described.24 In these complexes the Me3Si fragment has
also almost completely lost its silylium character (strong devia-
tion from planarity), since a stable covalently bonded tetracoor-
dinated Si center is formed (Chart 1, species C with S = Me3-
SiꢀX).12,17b,24b,25
3
substituted_arene]+ (arene = toluene, ethylbenzene, n-propylbenzene,
and isopropylbenzene). Thermal ellipsoids with 30% probability at 173 K.
Besides salts bearing [R3Si arene]+ or [Me3SiꢀXꢀSiMe3]+
3
ions (X = halogen), a frequently used reagent in silylation
chemistry is [Et3Si+][B(C6F5)4], first reported by Lambert.5b,26
Only recently, Reed and Nava proved that the commonly used
triethylsilyl or trimethylsilyl perfluorotetraphenylborate salts,
[R3Si+][B(C6F5)4], were misidentified.27 All known alkyl-sub-
stituted, formal “[R3Si+][B(C6F5)4]” salts, prepared from R3SiꢀH
and [Ph3C][B(C6F5)4], form [B(C6F5)4] salts containing a hy-
dride-bridged silane adduct cation of the type [R3SiꢀHꢀSiR3]+.
Since the silylium ion became a focus of attention especially in
catalysis,9,11ꢀ13,15 and there is still a lack of data with respect to
silylium solvent complexes, we have studied in detail the struc-
ture, bonding, and interaction of the Me3Si+ ion with differently
substituted arenes of the type RnC6H6ꢀn, (R = H, Me, Et, Pr, and
Bu; n = 0ꢀ6). By changing the substitution pattern and the size
of the substituents (from small to bulky), we are able to discuss
the steric and electronic influence on the solvent complex for-
mation, which is, in addition, supported by computational data.
Furthermore, we show that disproportionation may occur upon
solvent complex formation when electron-rich t-Bu substituents
are used.
Figure 3. Short H1 Carene distances (Å) (C36, 3.009; C37, 3.199;
3 3 3
C38, 3.207; C39, 3.030; C40, 2.827; C41, 2.801) in the ethylbenzene
adduct indicating weak van der Waals interactions in η6 fashion with one
solvent molecule.
and was reacted with a large excess of arene solvent. These
Me3Si+ transfer reactions can be considered as Lewis acidꢀLewis
base reactions (see below). The solvent-coordinated [Me3Si
3
arene][B(C6F5)4] salts (arene = benzene, toluene, ethylbenzene,
n-propylbenzene, isopropylbenzene, o-xylene, m-xylene, p-xy-
lene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-tri-
methylbenzene, Figures 1ꢀ7) are easily obtained in 70ꢀ90%
yields by treatment of neat [Me3SiꢀHꢀSiMe3][B(C6F5)4]
with the corresponding arene solvent at ambient temperatures
(Scheme 1). Gently heating to 80 ꢀC affords a clear colorless
solution with an oiled out layer. Slow cooling to ambient tem-
peratures over a period of 1 h results in the deposition of colorless
2. RESULTS AND DISCUSSION
2.1. Synthesis of [Me3Si arene][B(C6F5)4] Salts. As the
3
Me3Si+ source, [Me3SiꢀHꢀSiMe3][B(C6F5)4] was always used
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Figure 4. ORTEP drawing of the molecular structure of [Me3Si disubstituted_arene]+ (arene = 1,2-dimethylbenzene, 1,3-dimethylbenzene, and 1,4-
3
dimethylbenzene). Thermal ellipsoids with 30% probability at 173 K.
degradation proceeds via abstraction of a Fꢀ ion by the reactive
Me3Si+ ion, leading finally to the formation of the fluoronium salt
[Me3SiꢀFꢀSiMe3][B(C6F5)4], B(C6F5)3 and a reactive “C6F4”
species. This assumption is supported by a trapping reaction
with CS2 as illustrated in Scheme 2. Upon addition of CS2, the
formation of a formal S-heterocyclic carbene (SHC) adduct,
SHCꢀB(C6F5)3, was observed besides [Me3SiꢀFꢀSiMe3]-
[B(C6F5)4] and B(C6F5)3. By fractional crystallization SHCꢀ
B(C6F5)3 could be isolated in small quantities and characterized
by single-crystal X-ray analysis. To the best of our knowledge 1,3-
dithiol-2-ylidenes (Chart 3, structure A), which can be regarded
as SHCs, are unknown since they immediately dimerize to well-
known tetrathiafulvalenes (structure B).28 The SHCꢀB(C6F5)3
Figure 5. Short H1 Carene,solvent distances (Å) (C37, 3.176; C38,
3 3 3
3.115; C39, 2.953; C40, 2.885; C41, 2.963; C42, 3.127) in the m-xylene
species is the first example of an S-heterocyclic carbene adduct
adduct indicating weak van der Waals interactions in η6 fashion with one
complex with a Lewis acid.29 Only recently Bertrand et al. re-
solvent molecule.
ported on metal complexes (structure E) of the hitherto un-
known 1,3-dithiol-5-ylidenes (structure D), which are isomers of
SHC (structure A).30
crystals. Removal of excess arene by decantation and drying in
All [Me3Si arene][B(C6F5)4] salts have been fully character-
ized by elemental analysis, Raman and IR spectroscopy, and
single-crystal structure elucidation.
vacuo gives the corresponding [Me3Si arene][B(C6F5)4] salt with
the solvent complex as the cation. It should be noted that it is
difficult to obtain crystals suitable for a single-crystal X-rayanalysis
3
3
due to the low solubility of [Me3Si arene][B(C6F5)4] salts in the
2.2. Disproportionation Catalyzed by Silylium Ions:
FriedelꢀCrafts Catalysis. While the synthetic protocol described
above worked nicely for benzene, toluene, ethylbenzene,
n-propylbenzene, isopropylbenzene, o-xylene, m-xylene, p-xylene,
1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, and 1,3,5-
trimethylbenzene, the same route yielded in the case of tert-
butylbenzene two major products, [Me3SiꢀFꢀSiMe3][B(C6F5)4],
which crystallizes first, and after concentration of the supernatant
solution 1,4-di-tert-butylbenzene. Both products were identified
by X-ray structure determination. This finding led to a detailed
study of this disproportionation reaction which can be referred to
as a FriedelꢀCrafts-type isomerization.31 In the course of more
than 120 years of FriedelꢀCrafts chemistry, two catalysts achieved
preeminence: (i) anhydrous aluminum trichloride, which was
introduced by Friedel and Crafts themselves, and (ii) boron
trifluoride or the more convenient etherateꢀBF3 complexes.32
Since the 1960s, some superacid catalysts such as antimony
pentafluoride gained significance.33 Furthermore, the catalytic
activity of superacids and metal triflate was intensively explored
by Olah et al.34 Also, FriedelꢀCrafts alkylations were already
observed in CꢀF bond activation chemistry, e.g., as shown by the
3
corresponding solvents. To avoid thermal decomposition, the
mixtures were carefully warmed with stirring until two clear col-
orless layers were obtained. Slow cooling to ambient temperature
resulted in the deposition of large crystals rather than needlelike
crystals or a crystalline slurry.
[Me3Si arene][B(C6F5)4] salts are air and moisture sensitive
3
but stable under an argon atmosphere over a long period as a solid
but slowly decompose in solution even at ambient temperatures.
Colorless crystals and solutions of [Me3Si arene][B(C6F5)4]
3
salts quickly turn yellow if traces of moisture are present. All
[Me3Si arene][B(C6F5)4] salts can be prepared in bulk and are
3
almost indefinitely stable when stored in a sealed tube. They are
thermally stable to over 80 ꢀC. Between 88 ꢀC (benzene) and
118 ꢀC (1,2,3-trimethylbenzene, hemimellitene), decomposition
occurs, which is presumably triggered by the formation of Me3SiꢀF.
As an interesting side product, the formation of bissilylated
fluoronium ion25 [Me3SiꢀFꢀSiMe3]+ was observed a few times
and was even cocrystallized with [Me3Si arene][B(C6F5)4] (arene =
3
benzene, toluene) depending on the crystallization conditions
(concentration, temperature, and time). Obviously, especially
the weakest bound solvent complexes with benzene and toluene
(see section 2.4) are reactive enough to slowly degrade the
[B(C6F5)4]ꢀ anion (Scheme 2) on gentle heating. A similar
degradation of the [B(C6F5)4]ꢀ ion has been reported before by
M€uller in naphthyl-based silylium ions.12 We assume that the
groups of M€uller and Siegel.35 Now, we can show that [Me3Si
3
arene][B(C6F5)4] salts are convenient and effective new Friedelꢀ
Crafts catalysts, which catalyze in the case of tert-butylbenzene
the disproportionation, affording 1,3-di-tert-butylbenzene (8.47%),
1,4-di-tert-butylbenzene (64.4%), and 1,3,5-tri-tert-butylbenzene
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Figure 6. ORTEP drawing of the molecular structure of [Me3Si trisubstituted_arene]+ (arene = 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, and
3
1,3,5-trimethylbenzene). Thermal ellipsoids with 30% probability at 173 K.
[B(C6F5)4] and SHCꢀB(C6F5)3 have been determined. Tables
S1ꢀS6 in the Supporting Information present the X-ray crystal-
lographic data. Selected molecular parameters are listed in Table 1.
X-ray-quality crystals of all considered species were selected in
Fomblin YR-1800 (Alfa Aesar) at ambient temperature. All samples
were cooled to 173 K during the measurement.
2.3.1. [Me3Si benzene][B(C6F5)4] crystallizes solvent free
3
from benzene in the monoclinic space group P2/c with four
formula units per unit cell. Interestingly, slightly different cell and
structural parameters are found for crystals from different
experiments (structures A and B, Table 1; Tables S1, S7, and
S8, Supporting Information). Depending on the crystallization
conditions (time and temperature), the degradation product
Figure 7. Short H1 Carene,solvent distances in 1,2,3-trimethylbenzene
(Å) (C37, 2.999; C38, 3.141; C39, 3.216; C40, 3.013; C41, 3.154; C42,
3.013) and 1,2,4-trimethylbenzene adduct (Å) (C37, 2.898; C38, 2.898;
C39, 3.067; C40, 3.228; C41, 3.228; C42, 3.067) indicating weak van der
Waals interactions in η6 fashion with one solvent molecule.
3 3 3
[Me3SiꢀFꢀSiMe3][B(C6F5)4] cocrystallizes with [Me3Si
3
3
benzene][B(C6F5)4], forming mixed crystals of the type 0.76[Me3Si
benzene][B(C6F5)4] 0.24[Me3SiꢀFꢀSiMe3][B(C6F5)4]. Here,
3
the position of one [Me3Siꢀbenzene]+ cation was found to be
partially occupied by a [Me3SiꢀFꢀSiMe3]+ ion. The occupancy
of each part was refined freely (0.525(2)/0.475(2)). Partial sub-
Scheme 1. Synthesis of [Me3Si arene][B(C6F5)4] Salts
3
stitution of [Me3Si benzene]+ by [Me3SiꢀFꢀSiMe3]+ ions
3
leads to a change in the space group to P21/c and eight formula
units in the unit cell.
Although in all three structures the cations are well-separated
from the [B(C6F5)4] anions, there are numerous very weak
Hmethyl,cation CꢀFanion and Harene,cation CꢀFanion interac-
3 3 3
3 3 3
(27.2%) besides benzene as determined by GC/MS. The overall
isolated yield is about 5.3% (referring to tert-butylbenzene) after
3 h at ambient temperatures or about 560% (referring to [Me3Siꢀ
HꢀSiMe3][B(C6F5)4]). This corresponds to a turnover number
(TON) of about 7.0 (referring to [Me3SiꢀHꢀSiMe3][B(C6F5)4],
3 h of reaction time). The long-term stability was also studied. Even
after three days the catalyst was still active. Interestingly, also
at ꢀ80 ꢀC disproportionation was observed. The observation of
predominant formation of the 1,4-di-tert-butylbenzene (64.4%)
isomer can be explained only by intermolecular isomerization
according to Scheme 3. Computations indicated that the silylium
ion preferentially attacks at the para position (see section 2.4).
Disproportionation was only observed for the electon-rich
tert-butylbenzene as the free tert-butyl species is more stable than
other alkyl cations corresponding to the other substituted ben-
zenes. For instance, in the case of n-propylbenzene and isopro-
pylbenzene, no disproportionation was observed even after
refluxing for several days at high temperatures in a sealed tube
(T = 160 ꢀC).
tions. For instance, 21 such contacts are found for structure A
(Table 1), all between 2.4 and 3.0 Å (cf. ∑rvdW(H F) = 2.9 Å).36
3 3 3
The silicon atom in the cations is tetracoordinated with bonding
angles around the Si atoms between 341.7ꢀ and 343.1ꢀ, display-
ing a strong deviation from planarity (360.0ꢀ) as well as from the
value for an ideal tetrahedral environment (328.4ꢀ). Such rela-
tively large ∑—(Si) values37 were reported for complexes between
silylium ions and solvent molecules (341.4(5)ꢀ and 342.6(5)ꢀ for
[Et3Si toluene][B(C6F5)4])6 and anions (345.0(10)ꢀ and
3
349.0(9)ꢀ for [Et3Si][Br6CB11H6])38 and for the bissilylated
halonium ions (345ꢀ348ꢀ for [Me3SiꢀXꢀSiMe3][B(C6F5)4],
X = halogen).24b A second interesting aspect of the structure is
the intriguingly large distance between silicon and the fourth
coordination site, the solvent benzene (Figure 1). The coordina-
tion mode of this interaction is clearly η1 rather than η2 or η6 (cf.
d(SiꢀC1) = 2.174(2) Å vs d(SiꢀC2) = 2.758(2) Å, d(SiꢀC3) =
3.558(2) Å, d(SiꢀC4) = 3.884 Å, d(SiꢀC5) = 3.562 Å, and
d(SiꢀC6) = 2.758 Å). The three slightly different SiꢀC1
distances (structures AꢀC, Table 1) both illustrate the huge
influence of the environment due to a very flat potential energy
surface and manifest the error of structure elucidation. The
observed value of 2.169(3)ꢀ2.183(4) Å is considerably larger
than the sum of the C and Si covalent radii (1.91 Å;39 cf. 2.18 Å
2.3. X-ray Crystallography. The structures of [Me3Si arene]-
3
[B(C6F5)4] salts (arene = benzene, toluene, ethylbenzene, n-
propylbenzene, isopropylbenzene, o-xylene, m-xylene, p-xylene,
1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethy-
lbenzene) and the decomposition products [Me3SiꢀFꢀSiMe3]-
in [Et3Si toluene][B(C6F5)4])5 but still much shorter than the
3
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Scheme 2. Degradation Reaction of [Me3Si arene][B(C6F5)4] and Trapping of C6F4 by SHC Adduct Formation upon Addition
3
of CS2
position for H1 as displayed by the H1ꢀC1ꢀC2ꢀC6 dihedral
Chart 3. Structural Framework of Unknown SHCs = 1,3-
angle (Table 1).
Dithiol-2-ylidenes (A), Which Dimerize to Known Tetra-
thiafulvalenes (B), Adducts with Lewis Acids (C), and the
Unknown 1,3-Dithiol-5-ylidene Isomer (D) and Its Known
Adducts (E) (LA = Lewis Acid)
2.3.2. [Me3Si RC6H5][B(C6F5)4] (R = Me, Et, n-Pr, i-Pr)
3
crystallizes from RC6H5 in the orthorhombic space group Pbca
(R = Me) or the monoclinic space groups P21/n (Et) and P21/c
(n-Pr, i-Pr) with either four (R = Et, i-Pr) or eight (R = Me, n-Pr)
formula units per unit cell. Again, for the toluene species it was
possible to isolate mixed crystals of the type 0.92[Me3Si
3
toluene][B(C6F5)4] 0.08[Me3SiꢀFꢀSiMe3][B(C6F5)4] besides
3
pure [Me3Si toluene][B(C6F5)4]. Both sorts of crystals have
3
almost identical cell data.
In all four alkyl-substituted benzene adducts (Figure 2), the
silylium cation attacks in the para position to the alkyl substituent
and slightly shorter Si C1 distances compared to those of the
3 3 3
unsubstituted [Me3Si benzene]+ ion are observed in accord with
3
the theoretical results (see section 2.1). In the case of the
derivatives with longer alkyl side chains, the β C atom of the
alkyl chain always adopts a cis position with respect to the silyl
group. Similar to the [Me3Si benzene]+ ion, also all four alkyl-
Scheme 3. Me3Si+-Catalyzed Isomerization Reaction Lead-
ing to the 1,4-Substituted Arene (R = t-Bu; Counterion =
[B(C6F5)4]ꢀ)
3
substituted benzene cations display very weak Hmethyl,cation
3 3 3
CꢀFanion and Harene,cation CꢀFanion interactions. Among
3 3 3
these four salts only the ethyl derivative crystallizes with one
solvent molecule (EtC6H5) per cation. As can be seen from
Figure 3, the solvent molecule is closely arranged to the cation
and clearly directed toward the H1 proton in η6-type coordina-
tion mode with Harene,cation Carene,solvent distances between
3 3 3
2.80 and 3.20 Å (cf. ∑rvdW (H C) = 3.1 Å).36 This solvent
3 3 3
3 3 3
Harene,cation interaction is further supported by a significantly
larger displacement of the H1 proton from the arene ring plane
within the cation as indicated by the H1ꢀC1ꢀC2ꢀC6 dihedral
angle (ꢀ147.4ꢀ vs <ꢀ155ꢀ for all other species). Furthermore,
natural population analysis (NPA) partial charge calculations
reveal that H1 carries the largest positive charge (Table 2) with
0.32e (cf. 0.23eꢀ0.27e for all other arene protons) and even
the protons of the Me3Si unit are less positive (0.27ꢀ0.29e).
In the uncoordinated solvent the charges of all arene protons
are all very similar and in the range of 0.23eꢀ0.24e, displaying
especially for H1 a large positive charge accumulation upon
adduct formation. Comparison of the averaged CareneꢀCarene
distances in the ethylbenzene cation and the uncoordinated
solvent molecule displays a shortening of these distances by
ca. 0.025 Å. An even stronger effect is found for the CβꢀCγ
distance of the ethyl group, increasing by 0.049 Å in the cation,
which might partly be attributed to a stronger hypercon-
gative effect of the CβꢀCγ σ bond with the π* bond system
of the arene upon attack of the silylium cation in the para
position.
sum of the van der Waals radii (3.8 Å).36 As a consequence of the
Si C1 interaction, the trigonal planar environment around C1
3 3 3
changes to strongly distorted tetrahedral, leading to an out-of-plane
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Table 1. Selected Structural Data of Experimentally Observed [Me3Si arene]+ Ions
3
arene
d(Si1 C1), Å
∑—(Si), deg
—(H1ꢀC1ꢀC2ꢀC6), deg
3 3 3
C6H6 (benzene)
Aa
Ba
Cb
A
2.174(2)
2.169(3)
2.183(4)
2.135(5)
2.120(2)
2.140(3)
2.137(2)
2.169(2)
2.137(3)
2.148(2)
2.167(5)
2.129(5)
2.121(3)
2.139(2)
2.171(6)c
341.7
341.8
343.1
341.0
340.7
341.5
340.9
342.1
341.4
338.3
341.2
339.2
336.2
334.2
336.5c
ꢀ157.8
ꢀ157.7
ꢀ162.1
ꢀ156.0
ꢀ158.9
ꢀ147.4
ꢀ159.4
ꢀ155.9
ꢀ158.0
ꢀ148.9
ꢀ155.6
ꢀ154.2
ꢀ155.2
ꢀ150.0
MeC6H5 (toluene)
Bb
EtC6H5
n-PrC6H5
i-PrC6H5
1,2-Me2C6H4 (o-xylene)
1,3-Me2C6H4 (m-xylene)
1,4-Me2C6H4 (p-xylene)
1,2,3-Me3C6H3
1,2,4-Me3C6H3
1,3,5-Me3C6H3
a Two slightly different data sets. b Mixed crystals with [Me3SiꢀFꢀSiMe3]+ ions. c Two independent molecules in the unit cell.
Table 2. Selected Structural Data of Experimentally Observed [Me3SiꢀFꢀSiMe3]+ Ions
[Me3SiꢀFꢀSiMe3][B(C6F5)]
d(SiꢀF), Å
—(SiꢀFꢀSi), deg
∑—(Si), deg
[Me3Si C6H6][B(C6F5)]a
1.708(7), 1.741(7)
1.73(2), 1.73(2)
1.753(9)
159.0(6)
158(2)
347.8, 347.9
348.3, 348.0
348.0
3
[Me3Si MeC6H5][B(C6F5)]b
3
pure salt c
163.0(3)
a Cocrystallized bis(trimethylsilyl)fluoronium ion taken from structure C in Table 1. b Cocrystallized bis(trimethylsilyl)fluoronium ion taken from
structure B in Table 1. c Taken from ref 25a.
2.3.3. [Me3Si Me2C6H4][B(C6F5)4]. All three possible xylene
ortho and meta positions are feasible (C1, C3, C5, and C6 are
3
derivatives (ortho, meta, and para) were synthesized (Figure 4).
While o-xylene (1,2-dimethylbenzene) and m-xylene (1,3-
dimethylbenzene) adducts crystallize in monoclinic space groups
P21/n and P21/c with eight and four formula units, respectively,
p-xylene (1,4-dimethylbenzene) crystallizes in the orthorhombic
space group Pbca with eight molecules per unit cell. In the case of
the ortho and para species, solvent molecules are included in the
unit cell. However, only for the ortho species the η6 coor-
equivalent, Figure 4). As a result the longest Si C1 distance
3 3 3
was found for p-xylene (2.167(5) Å, Table 1). The small differ-
ence between o- and m-xylene might be explained by the fact that
in o-xylene the one para position and one meta position are
energetically favored over one para position and one ortho
position.
2.3.4. [Me3Si Me3C6H3][B(C6F5)4]. Both the 1,2,3-trimethyl-
3
benzene (hemimellitene) adduct and the 1,2,4-trimethylbenzene
(pseudocumene) adduct salts crystallize in the monoclinic space
group P21/c with four formula units, while the 1,2,4-trimethyl-
benzene (mesitylene) adduct salt crystallizes in P1 with four
dination mode with the solvent—as described for [Me3Si EtC6H5]-
3
[B(C6F5)4] (Figure 3)—was observed, again with a stronger
displacement of the H1 proton and fairly short H1arene,cation
3 3 3
Carene,solvent distances (Figure 5). For the p-xylene species, no
formula units and two independent [Me3Si Me3C6H3][B(C6F5)4]
3
such η6 coordination arrangement was found and the shortest
species (Figure 6). Always one solvent molecule per [Me3Si
3
H1 Cmethyl distance amounts to 3.592 Å, which was observed
Me3C6H3][B(C6F5)4] moiety is included in the unit cell of all
three salts. For the 1,2,3- and the 1,2,4-trimethylbenzene adduct
species, the solvent is again in the vicinity of the H1 proton as
3 3 3
between H1 and one methyl carbon atom of the solvent molecule
(cf. the shortest H1arene,cation Carene,solvent distance, 4.417 Å).
3 3 3
A comparison of the Si C1 distances with those of the
observed for [Me3Si EtC6H5][B(C6F5)4] and the 1,2-substi-
3 3 3
3
benzene or monosubstituted species is not straightforward, since
addition of the silyl group in the para position is not feasible in
p-xylene. Thus, the most interesting question is the influence of
tuted [Me3Si Me2C6H4][B(C6F5)4] (Figure 7). In the mesity-
3
lene species the solvent molecules are well separated from the
cations, but as found for all other species weak Hmethyl,cation
3 3 3
the substitution pattern on the Si C1 distance within the
CꢀFanion and Harene,cation
CꢀFanion interactions are also
3 3 3
3 3 3
group of xylene species. For the 1,2-substituted species, both
(equivalent) para positions (C1/C6 in Figure 4) are energeti-
cally favored over all other possibilities. A similar situation is
found for the 1,3-substituted cation, where the position at C1
(equivalent to C5) represents the para position (Figure 4). In the
case of the 1,4-substituted species, adduct formation in the para
position is rather unlikely due to steric repulsion with one methyl
group in accord with theory (see section 2.4). Hence, only the
observed.
In the case of 1,2,3-trimethylbenzene, three para positions are
available (C1 equivalent to C5, and C6). The silylium ion prefers
for energetic reasons (see section 2.4) to attack position C1/C5
(one para, one ortho, and one meta C atom) rather than C6 with
one para and two meta carbon atoms. For 1,2,3-trimethylben-
zene there are three different adduct ions possible: (i) attached to
C1 with one para, one ortho, and one meta position, (ii) attached
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of d(S1ꢀC1) = 1.669(2) Å and d(S2ꢀC1) = 1.691(2) Å are
determined for the bonds to the carbene carbon atom, while (ii)
slightly larger distances (d(S1ꢀC3) = 1.728(2) Å and d(S2ꢀC2) =
1.735(2) Å) are found for the two other SꢀCring bonds. All four
SꢀC bond lengths are considerably shorter than the sum of the
covalent radii (∑rcov(SꢀC) = 1.78 Å),39 thus indicating partial
double bond character within the five-membered C3S2 heterocycle.
2.4. Computations. Since very flat potential energy surfaces
are observed for the systems [Me3Si arene]+ with respect to
3
CꢀSiꢀC and SiꢀC1ꢀH1 angles and the Si C1 distance,
3 3 3
consistent trends are only obtained for isolated species in the gas
phase when environmental effects are excluded. All calculations
were carried out with the Gaussian 03 package of molecular
orbital programs.43 The structures of silyliumꢀarene adducts
and the free arenes were optimized within the DFT approach at
the pbe1pbe level with an aug-cc-pVDZ basis set.44 Vibrational
frequencies were also computed to include zero-point vibrational
energies and thermal corrections in thermodynamic parameters
and to characterize all structures as minima on the potential
energy surface. A natural bond orbital (NBO) analysis45 was per-
formed at the same level to study the charge distribution, bond
polarization, and hybridization effects.
Figure 8. ORTEP drawing of the molecular structure of SHCꢀ
B(C6F5)3 in the crystal. Thermal ellipsoids with 30% probability at 173 K.
Selected bond lengths (Å) and angles (deg): S1ꢀC1, 1.669(2); S1ꢀC3,
1.728(2); S2ꢀC1, 1.691(2); S2ꢀC2, 1.735(2); C1ꢀB1, 1.660(2);
C2ꢀC7, 1.390(3); C2ꢀC3, 1.393(2); C3ꢀC4, 1.394(2); C8ꢀB1,
1.652(3); C14ꢀB1, 1.654(3); C20ꢀB1, 1.639(3); C1ꢀS1ꢀC3,
97.82(9); C1ꢀS2ꢀC2, 97.18(8); B1ꢀC1ꢀS1, 123.0(1); B1ꢀC1ꢀS2,
121.0(1); S1ꢀC1ꢀS2, 115.5(1); C7ꢀC2ꢀC3, 120.0(2); C7ꢀ
C2ꢀS2, 125.3(1); C3ꢀC2ꢀS2, 114.6(1); C2ꢀC3ꢀC4, 119.9(2);
C2ꢀC3ꢀS1, 114.8(1); C4ꢀC3ꢀS1, 125.2(1); C3ꢀS1ꢀC1ꢀB1,
173.5(1); C3ꢀS1ꢀC1ꢀS2, 0.7(2); C2ꢀS2ꢀC1ꢀB1, ꢀ174.0(1);
C2ꢀS2ꢀC1ꢀS1, ꢀ1.1(1); C1ꢀS2ꢀC2ꢀC7, 179.0(2); C1ꢀS2ꢀ
C2ꢀC3, 1.2(2).
2.4.1. Structure and Isomers. It is common knowledge that a
tetracoordinated Si atom is tetrahedral and its tricoordinated
species mostly trigonal planar.1ꢀ17 However, the question arises
as to the structure of a silylium derivative in which one coor-
dination site is significantly more weakly bound, being in the
range of a transition between a covalent bond and a van der Waals
interaction. Of special interest is the effect of delocalization and
substitution on the structure and energetics of the different
to C3 with no para but two ortho positions and one meta position,
and (iii) attached to C6 with one ortho and two meta positions.
Since para position attack is preferred, the most stable isomer is
the one where the silyl group attacks C1 in accord with theory
(see below and the Supporting Information). For mesitylene
only one isomer is possible since all three hydrogen-substituted
arene carbon atoms are equivalent. Note: Adduct formation at an
arene carbon atom attached to a methyl group is always unfavorable.
2.3.5. Molecular Structure of [Me3SiꢀFꢀSiMe3]+ Ions Co-
crystallized in [Me3Si benzene][B(C6F5)4] and [Me3Si toluene]-
possible isomers for the studied system [Me3Si arene]+. Se-
3
lected experimental and computed structural data (of the lowest
lying isomers) are given in Tables 1 and 3, respectively.
Comparison of our gas-phase geometry with the crystal struc-
ture shows a general agreement, within experimental errors. For
3
3
[B(C6F5)4]. While the molecular structure of the pure salt is ideal
C2 symmetric, the symmetry is decreased to C1 for the fluor-
onium cations in the mixed crystals (Table 2). Thus, slightly
different SiꢀF bond distances are observed which range from
1.708(7) to 1.73(2) Å (cf. 1.753(9) Å for the pure salt). The larg-
est difference is found for the SiꢀFꢀSi angles, which are some-
what smaller for the cations in the mixed crystals (158ꢀ/159ꢀ vs
163ꢀ), and this can be attributed to a very flat energy potential
for the variation of the SiꢀFꢀSi angle.
instance, according to Table 1, the difference in the Si C1
3 3 3
bond length scatters about 0.015 Å, the angle sum around Si
about 1.4ꢀ, and the dihedral angle H1ꢀC1ꢀC2ꢀC6 about 4.4ꢀ.
All considered [Me3Si arene]+ groups are nonplanar with
3
Si C1 distances between 2.196 Å ([Me3Si benzene]+) and
3 3 3
3
2.112 Å ([Me3Si 1,3,5-i-Pr3C6H3]+). More sensitive with re-
3
spect to the substitution pattern is the angle sum around Si
(∑—(Si)) ranging between 342.4ꢀ ([Me3Si benzene]+) and
3
333.2ꢀ ([Me3Si 1,3,5-i-Pr3C6H3]+). In the case of ([Me3Si
3
3
2.3.6. SHC-B(C6F5)3 crystallizes in the monoclinic space
group P21/n with eight formula units per unit cell and two inde-
pendent molecules. The planar SHC and the B(C6F5)3 group
(Figure 8) are connected by means of a strong BꢀC donorꢀ
acceptor bond that amounts to 1.660(2) Å (cf. ∑rcov(BꢀC) =
1.60 Å39 and 1.641(2) Å in (1,3,4,5-tetramethylimidazol-2-
ylidene)tris(pentafluorophenyl)borane40 or 1.649(3) Å in the
carbene adduct of 1,3-di-tert-butylimidazolin-2-ylidene with tris-
(pentafluorophenyl)borane),41 which is slightly longer than
C6Me6]+) an even smaller angle sum (∑—(Si)) of 331.2ꢀ is com-
puted due to steric repulsion between the methyl group attached
to the arene C1 atom and the three methyl groups of the Si atom.
In all other species the silylium ion is always attached to a C1
atom bearing a hydrogen atom. Only very minor changes are
observed for the C1ꢀH1 distances, which only slightly increase
upon substitution (1.095ꢀ1.099 Å).
In general, with increasing degree of substitution, the Si C1
3 3 3
bond lengths decrease (C6H6, 2.196 Å; Me1C6H5, 2.132 Å;
Me2C6H4, 2.132 Å;Me3C6H3, 2.125 Å; Me4C6H2, 2.123 Å;
Me5C6H1, 2.113 Å; Table 3), ∑—(Si) decreases (cf. 342.4ꢀ,
341.0ꢀ, 338.5ꢀ, 335.0ꢀ, 334.5ꢀ, and 334.0ꢀ), while the C1ꢀH1
bond lengths are almost not affected by the higher degree of
substitution (cf. 1.0952, 1.0955, 1.0964, 1.0970, 1.0970, and 1.0971 Å).
In Me6C6 the situation changes significantly since the C1 arene
ring atom is now attached to a methyl group, introducing steric
those found for the BꢀC
rings (d(C8ꢀB1) = 1.652(3) Å,
C6F5
d(C14ꢀB1) = 1.654(3) Å, d(C20ꢀB1) = 1.639(3) Å).42 The
boron atom of B(C6F5)3 is tetracoordinated, while the carbene
carbon atom sits in an almost trigonal planar environment
(—(B1ꢀC1ꢀS1) = 123.0(1)ꢀ, —(B1ꢀC1ꢀS2) = 121.0(1)ꢀ,
—(S1ꢀC1ꢀS2) = 115.5(1)ꢀ). The coordination geometry
around boron in the BC4 core is slightly distorted with the smallest
angle 102.2(2)ꢀ and the largest 114.7(1)ꢀ. Two sets of different
SꢀC bond distances are found: (i) Two rather short bond lengths
strain, which leads to a longer Si C1 bond but a smaller value
3 3 3
for ∑—(Si).
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Table 3. Theoretically Obtained Selected Structural Data (Distances, Å; Angles, deg) of Substituted [Me3Si arene]+ and
3
[Me3SiꢀHꢀSiMe3]+ Ions Along with Partial Charges (e) and the Overall Charge Transfer (e)
a
cationd
qSi
QCT
qH1
d(Si C1)
d(C H1)
—(Si)
3 3 3
3 3 3
Me3SiꢀHꢀSiMe3
1.816
1.922
1.910
1.912
1.907
1.915
1.909
1.908
1.902
1.909
1.904
1.910
1.905
1.923
1.910
1.906
1.909
1.910
1.903
1.908
1.909
0.330
0.275
0.297
0.301
0.309
0.296
0.311
0.313
0.320
0.316
0.322
0.312
0.327
0.318
0.299
0.320
0.300
0.300
0.328
0.302
0.301
ꢀ0.340
0.315
0.318
0.318
0.315
0.313
0.314
0.315
0.309
0.314
0.309
0.308
0.308
348.1
342.4
341.0
340.5
338.5
338.7
338.0
337.7
335.0
337.4
334.5
334.8
334.0
331.2
340.7
334.2
340.7
340.7
333.2
340.6
340.4
C6H6_TMSb
2.1962
2.1464
2.1421
2.1323
2.1587
2.1296
2.1253
2.1258
2.1204
2.1230
2.1424
2.1130
2.1534
2.1437
2.1222
2.1422
2.1419
2.1119
2.1381
2.1394
1.0952
1.0955
1.0956
1.0964
1.0962
1.0961
1.0964
1.0970
1.0964
1.0970
1.0970
1.0971
1Me_C6H5_4TMSc
1,2Me2_C6H4_4TMSc
1,3Me2_C6H4_4TMSc
1,4Me2_C6H4_2TMSc
1,2,3Me3_C6H3_4TMSc
1,2,4Me3_C6H3_5TMSc
1,3,5Me3_C6H3_2TMSc
1,2,3,4Me4_C6H2_5TMSc
1,2,3,5Me4_C6H2_4TMSc
1,2,4,5Me4_C6H2_3TMSc
1,2,3,4,5Me5_C6H1_6TMSc
1Me6_C6_1TMS
1Et_C6H5_4TMSc
0.318
0.308
0.318
0.318
0.309
0.318
0.318
1.0955
1.0973
1.0955
1.0955
1.0982
1.0956
1.0955
1,3,5Et3_C6H3_2TMSc
1-n-Pr_C6H5_4TMSc
1-i-Pr_C6H5_4TMSc
1,3,5-i-Pr3_C6H3_2TMSc
1-n-Bu_C6H5_4TMSc
1-t-Bu_C6H5_4TMSc
a QCT = 1 ꢀ ∑qi(SiMe3). b TMS = trimethylsilyl. c Only the lowest lying isomer is considered. d Notation: xRn_C6Hn_yTMS with x and y = numerals
describing the positions in the arene.
Substitution of the methyl group by ethyl, n-propyl, isopropyl,
or n-butyl groups only marginally affects the structural data
respect to the para-substituted species, while the para-substi-
tuted species is favored by about 2 kcal molꢀ1 over the meta/ortho
species.
(Table 2); e.g., the Si C1 distances slightly decrease along
3 3 3
H (2.1962 Å) < Me (2.1464 Å) < Et (2.1437 Å) < n-Pr (2.1422 Å)
< n-Bu (2.1394 Å).
2.4.2. Energies and Charge Distribution. [Me3Si arene]+ ions
3
can be considered as solvent complexes between arene and
[Me3Si]+. In these complexes the Me3Si fragment has almost
completely lost its silylium character (strong deviation from
planarity, Tables 1ꢀ3), since a stable bonded tetracoordinated Si
center is formed. In this context and in analogy to the proton
affinity, a trimethylsilylium affinity (TMSA) can be defined as the
enthalpy change associated with the dissociation of the conju-
gated acid:25a,46
While the interaction of the silylium ion with benzene and
hexamethyl benzene gives only one species, in the case of all
other species of the type MenC6H6ꢀn (n = 0ꢀ6), in principle, at
least two different isomers should be observed since silylation of
the corresponding substituted arene might occur at the carbon
arene atom attached to either a hydrogen atom or a methyl
(alkyl) group. Additionally, silylation can also occur in the ortho,
meta, or para position of the carbon ring atom bearing a methyl
group, with the para-substituted isomer always being the lowest
lying isomer. For example, for toluene four different isomers have
been calculated (see the Supporting Information). The para-
substituted isomer is energetically preferred over the ortho and
meta compounds by ΔG298 = 2.88 and 2.65 kcal molꢀ1, respec-
½Me3Si Bꢁþ f B þ ½Me3Siꢁþ ðΔH298
Þ
ð1Þ
ðgÞ
3
ðgÞ
ðgÞ
TMSA values (ΔH(gas,298 K)) describe the energetics of the
desilylation reaction of a trimethylsilylium ion donor in the gas
phase at 298 K, and small gas-phase TMSA values in comparison
with that of unsubstituted benzene can be regarded a measure of
stabilization in substituted benzenes. Furthermore, with the help
of TMSA values, it is possible to decide if silylation transfer
tively, in accord with the experimentally observed [Me3Si
3
p-toluene]+ species (Figure 2). The energy difference between
both ortho and meta isomers is rather small (0.23 kcal molꢀ1).
The isomer with the silylium ion in the 1-position (methyl and
Me3Si attached at C1, isomer 4) is always the highest lying isomer
(7.64 kcal molꢀ1). The energetically preferred species always
reactions are feasible, e.g., between RꢀX and [Me3Si arene]+
3
(X = H, halogen, any basic center). Table 4 summarizes TMSA
and Gibbs free energies of all considered arene species (lowest
lying isomer) at 298 K along with those of B (= Me3SiꢀX; X = H,
halogen, and pseudo-halogen) for comparison. The TMSA value
of benzene amounts to 25.83 kcal molꢀ1, which is, astonishingly,
smaller than that of Me3SiꢀH, 31.30 kcal molꢀ1 (cf. TMSAhalogen
show the smallest Si C1 distances (para, 2.146 Å; meta, 2.173 Å;
3 3 3
ortho, 2.178 Å; isomer 4, 2.283 Å).
A similar picture is found for all other RnC6H6ꢀn (n = 0ꢀ6) (R =
alkyl) species, which we do not want to discuss here in detail. In
Tables 3 and 4 only data of the lowest lying isomers are presented
(a complete set of data is listed in the Supporting Information).
The isomers with the Me3Si group attached to a ring carbon atom
bearing a methyl group is unfavored by 4ꢀ7 kcal molꢀ1 with
between 31 and 35 kcal molꢀ1). This means that [Me3Si
3
benzene]+ is a stronger silylating agent than [Me3SiꢀHꢀSiMe3]+
in the gas phase when solvent effects (liquid phase) or solid-state
effects (solid phase) are impossible. However, it can be assumed
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Table 4. TMSA Values (ΔH298) Along with ΔE0 and ΔG298
by about 161 kcal molꢀ1 than the corresponding proton affinities
(cf. TMSA (kcal molꢀ1)/PA (kcal molꢀ1): 25.8/183.3, benzene;
30.9/189.8, toluene; 33.2/195.9, m-xylol; 32.5/193.3, o-xylol;
31.3/192.0, p-xylol; 35.3/200.7, mesitylene).47
Values (kcal molꢀ1) of Substituted [Me3Si arene]+ and
3
[Me3SiꢀXꢀSiMe3]+ Ions (X = H, Halogen, Pseudo-Halogen)
cationa
ΔE0
ΔH298
ΔG298
The Si C1 bond in [Me3Si arene]+ ions might be regarded
3 3 3
3
a donorꢀacceptor bond that can be characterized by the charge
TMSꢀHꢀTMS
34.53
38.02
34.43
35.11
36.23
58.20
47.80
42.16
44.49
46.63
29.37
33.78
35.94
36.80
34.93
37.09
38.79
39.01
40.64
40.58
38.35
42.29
40.61
34.35
40.04
34.93
34.92
40.81
35.41
35.49
31.30
34.79
31.05
31.78
33.07
54.44
44.23
39.34
41.14
43.29
25.83
30.92
32.48
33.21
31.25
33.37
35.21
35.27
37.07
36.87
34.75
38.59
36.51
30.81
36.19
31.40
31.30
37.44
31.80
31.91
23.23
26.79
21.60
22.62
25.15
45.65
32.90
27.82
31.49
34.84
15.63
19.31
22.33
21.96
19.56
18.72
24.13
21.02
25.98
24.45
23.91
27.13
22.71
20.83
23.81
21.53
21.47
23.45
21.48
21.70
transfer from the arene into the Me3Si+ ion (Table 3), which
TMSꢀFꢀTMS
becomes less positive. For the [Me3Si benzene]+ ion an overall
TMSꢀClꢀTMS
3
charge transfer of 0.275e is found. The hydrogen atom attached
TMSꢀBrꢀTMS
to C1 suffers the largest loss of electron density upon complex
TMSꢀIꢀTMS
formation (C6H6, qH1 = 0.237e; [Me3Si benzene]+, qH1,cation
=
3
TMSꢀCNꢀTMS
0.315e; cf. qH2ꢀ6,cation between 0.269e and 0.271e). A closer look
into the charge transfer displays that the overall charge transfer
can mainly be attributed to the arene hydrogen atoms (89.5%).
With increasing degree of substitution, the charge transfer
slightly increases (C6H6, 0.275e; Me1C6H5, 0.297e; Me2C6H4,
0.309e; Me3C6H3, 0.320e; Me4C6H2, 0.322e; Me5C6H1, 0.327e).
Substitution with a longer alkyl chain also only marginally in-
creases the overall charge transfer (Me1C6H5, 0.297e; Et1C6H5,
0.299e; n-Pr1C6H5, 0.300e; n-Bu1C6H5, 0.302e).
TMS2ꢀN3
TMSꢀNCOꢀTMS
TMSꢀNCSꢀTMS
TMSꢀNCSeꢀTMS
C6H6_TMS
1Me_C6H5_4TMS
1,2Me2_C6H4_4TMS
1,3Me2_C6H4_4TMS
1,4Me2_C6H4_2TMS
1,2,3Me3_C6H3_4TMS
1,2,4Me3_C6H3_5TMS
1,3,5Me3_C6H3_2TMS
1,2,3,4Me4_C6H2_5TMS
1,2,3,5Me4_C6H2_4TMS
1,2,4,5Me4_C6H2_3TMS
1,2,3,4,5Me5_C6H1_6TMS
1Me6_C6_1TMS
It is interesting to mention that for the [Me3SiꢀHꢀSiMe3]+
ion the charge transfer of 0.330e exclusively stems from the
Me3Si moiety of the Me3SiꢀH fragment. Moreover, the bridging
H atoms become even more negative upon complex formation
(Me3SiꢀH, qH = ꢀ0.200e; [Me3SiꢀHꢀSiMe3]+, qH,cation
=
ꢀ0.340e), which in turn means that the hydride character in
[Me3SiꢀHꢀSiMe3]+ is increased compared to that in Me3SiꢀH.
This also means that the Me3Si moiety of the Me3SiꢀH fragment
decreases its charge by 0.47e (= 0.33e + 0.14e = QCT + ΔqH)
upon complexation.
2.5. Conclusions. A simple synthetic route to solvent-coordi-
1Et_C6H5_4TMS
nated [Me3Si arene][B(C6F5)4] salts (arene = benzene, toluene,
1,3,5Et3_C6H3_2TMS
1-n-Pr_C6H5_4TMS
1-i-Pr_C6H5_4TMS
1,3,5-i-Pr3_C6H3_2TMS
1-n-Bu_C6H5_4TMS
1-t-Bu_C6H5_4TMS
3
ethylbenzene, n-propylbenzene, isopropylbenzene, o-xylene,
m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene,
1,3,5-trimethylbenzene) starting from [Me3SiꢀHꢀSiMe3]-
[B(C6F5)4] has been described. This formal Lewis acidꢀLewis
base reaction allows preparation of large quantities in good
yields. [Me3Si arene][B(C6F5)4] salts are air and moisture sen-
3
a Notation: xRn_C6Hn_yTMS with x and y = numerals describing the
positions in the arene. TMS = trimethylsilyl, and TMSA = trimethylsilyl
affinity.
sitive but stable under an argon atmosphere over a long period as
solids but slowly decompose in solution even at ambient tem-
peratures. They are thermally stable up to over 80 ꢀC. Between
88 ꢀC (benzene) and 118 ꢀC (1,2,3-trimethylbenzene, hemi-
mellitene), decomposition occurs, which is triggered by the for-
that in solution as well in the solid state interactions with the
environment (as discussed before) stabilize [Me3Si benzene]+
mation of Me3SiꢀF. Investigation of the degradation of [Me3Si
3
3
relative to [Me3SiꢀHꢀSiMe3]+. Taking the TMSA value for
benzene as a reference, all considered substituted benzene species
possess larger TMSA values ranging between 30 and 39 kcal
molꢀ1. The small TMSA of benzene and also toluene (TMSA =
30.92 kcal molꢀ1) may explain why fast degradation leading to
the formation of a [Me3SiꢀFꢀSiMe3]+ salt is observed (see
section 2.1). Upon increasing substitution, the TMSA value
arene][B(C6F5)4] revealed the formation of the fluoronium salt
[Me3SiꢀFꢀSiMe3][B(C6F5)4], B(C6F5)3, and a reactive “C6F4”
species which could be trapped by CS2. Upon addition of CS2,
the formation of a formal S-heterocyclic carbene adduct,
C6F4CS2ꢀB(C6F5)3, was observed. The synthetic protocol
described above does not work for tert-butylbenzene. Here the
formation of [Me3SiꢀFꢀSiMe3][B(C6F5)4] and 1,4-di-tert-bu-
tylbenzene was observed, which can be referred to as a Friedelꢀ
Crafts-type isomerization. Computations and X-ray structure
increases by at least 5 kcal molꢀ1 (C6H6, 25.83 kcal molꢀ1
Me1C6H5, 30.92 kcal molꢀ1; Me2C6H4, 33.21 kcal molꢀ1
Me3C6H3, 35.27 kcal molꢀ1; Me4C6H2, 37.07 kcal molꢀ1
;
;
;
elucidation reveal a tetracoordinated Si atom with a long Si
3 3 3
Me5C6H1, 38.59 kcal molꢀ1). Due to steric reasons in Me6C6,
the TMSA value (36.51 kcal molꢀ1) decreases compared to that
of Me5C6H1. This trend nicely corresponds to the trend discussed
C1arene distance and an angle sum at Si considerably smaller than
360ꢀ. The Si C1 coordination mode is always η1 rather than
3 3 3
η2 or η6. Due to very flat potential energy surfaces, the molecular
for the Si C1 distances. Only small changes (30.92ꢀ31.80
structure parameters (e.g., d(SiꢀC1), ∑—(Si)) of the [Me3Si
3 3 3
3
kcal molꢀ1) are computed when the methyl group is substituted
by ethyl, n-propyl, isopropyl, n-butyl or tert-butyl. Finally, it might be
of interest to compare the TMSA values with proton affinities
(PAs). The TMSA values of benzene and its derivatives are lower
arene]+ ion strongly depend on the magnitude of interactions
with the environment, such as anionꢀcation or cationꢀsolvent
interactions. If solvent molecules are in the proximity of the
[Me3Si arene]+ ion, the solvent molecule is closely arranged to
3
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Journal of the American Chemical Society
ARTICLE
the cation and clearly directed toward the H1cation ring proton
3.1.4. CHN Analyses. A C/H/N/S-Mikronalysator TruSpec-932 from
Leco was used.
3.1.5. Differential Scanning Calorimetry (DSC). A DSC 823e from
Mettler-Toledo (heating rate 5 ꢀC/min) was used.
3.2. General Procedure for the Synthesis of Trimethylsili-
in η6-type coordination mode with Hcation,arene Csolvent,arene
3 3 3
distances between 2.80 and 3.20 Å. This solventꢀcation inter-
action is further supported by a significantly larger displacement
of the H1cation,arene proton from the arene ring plane as indicated
by the H1ꢀC1ꢀC2ꢀC6 dihedral angle (ꢀ147ꢀ vs ꢀ155ꢀ for all
other nonsolvate species). Furthermore, NPA partial charge
calculations reveal that H1cation,arene always carries the largest
positive charge within the ring (Table 3), about 0.32e (cf.
0.23eꢀ0.27e for all other arene protons), and even the protons
of the Me3Si unit are less positive (0.27eꢀ0.29e).
ceniumꢀArene Salts [Me3Si arene][B(C6F5)4] (Arene =
3
Benzene, Toluene, Ethylbenzene, n-Propylbenzene, Isopro-
pylbenzene, o-Xylene, m-Xylene, p-Xylene, 1,2,3-Trimethyl-
benzene, 1,2,4-Trimethylbenzene, and 1,3,5-Trimethylbenzene).
To neat bis(trimethylsilyl)hydronium tetrakis(pentafluorophenyl)borate
([Me3SiꢀHꢀSiMe3][B(C6F5)4], structure C) (0.413 g, 0.5 mmol)
was added a minimum of the corresponding arene (3ꢀ5 mL) at ambient
temperature with stirring, followed by gentle heating to 80 ꢀC until a
clear colorless solution and an oiled-out layer was obtained. Slow cooling
to ambient temperature over a period of 1 h resulted in the deposition of
colorless crystals. Removal of excess arene by decantation and drying
in vacuo gave the corresponding trimethylsiliceniumꢀarene tetrakis-
Since very flat potential energy surfaces are observed for the
systems [Me3Si arene]+ with respect to CꢀSiꢀC and SiꢀC1ꢀH1
3
angles and the donorꢀacceptor bond (Si C1 distance),
3 3 3
consistent structural trends are only obtained for isolated species
in the gas phase when environmental effects are excluded. A
systematic study of the influence of the arene substitution pattern
(pentafluorophenyl)borate ([Me3Si arene][B(C6F5)4] (arene = ben-
3
in [Me3Si arene][B(C6F5)4] (arene = RnC6H6ꢀn, R = H, Me, Et,
3
zene, toluene, ethylbenzene, n-propylbenzene, isopropylbenzene, o-xylene,
m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-
trimethylbenzene)) as a colorless solid in good yield (70ꢀ90%).
Pr, and Bu; n = 0ꢀ6) shows the following general trends: (i) para
substitution with respect to the alkyl group is always favored over
ortho or meta isomers (by ca. 2 kcal molꢀ1). (ii) With increasing
3.2.1. [Me3Si C6H6][B(C6F5)4] (Benzene). Mp: 88 ꢀC (dec. Anal.
3
degree of substitution, the shorter the Si C1 distance (between
3 3 3
Calcd for [Me3Si C6H6][B(C6F5)4] (Found): C, 47.73 (45.98); H,
3
2.196 Å [Me3Si benzene]+ and 2.113 Å [Me3Si Me5C6H1]),
1.82 (1.45). IR (ATR, 16 scans, cmꢀ1): 3113 (w), 3092 (w), 3034 (w),
2996 (w), 2914 (w), 1643 (m), 1600 (w), 1588 (w), 1556 (w), 1513 (s),
1455 (s), 1412 (m), 1383 (m), 1372 (m), 1342 (m), 1321 (m), 1271
(m), 1180 (w), 1164 (w), 1082 (s), 1034 (w), 1022 (w), 972 (s), 912
(w), 869 (m), 856 (w), 813 (m), 770 (m), 755 (m), 737 (m), 728 (w),
698 (m), 683 (m), 662 (s), 623 (m), 611 (w), 603 (w), 573 (m).
3
3
the larger the overall charge transfer (between 0.275e [Me3Si
3
benzene]+ and 0.327e [Me3Si Me5C6H1]) and the larger the
3
calculated TMSA value (between 25.83 kcal molꢀ1 [Me3Si
3
benzene]+ and 38.59 kcal molꢀ1 [Me3Si Me5C6H1]). The TMSA
3
values can be regarded a measure of stabilization in substituted
benzenes. Furthermore, with the help of a TMSA scale, it is
possible to decide if silylation transfer reactions are feasible, e.g.,
3.2.2. [Me3Si C7H8][B(C6F5)4] (Toluene). Mp: 108 ꢀC dec. Anal.
3
Calcd for [Me3Si C7H8][B(C6F5)4] (Found): C, 48.36 (48.01); H,
3
between RꢀX and [Me3Si arene]+ (X = H, halogen, any basic
2.03 (1.76). IR (ATR, 16 scans, cmꢀ1): 3092 (w), 3014 (w), 2979 (w),
2914 (w), 2042 (w), 2016 (w), 1987 (w), 1644 (m), 1598 (w), 1556
(w), 1513 (s), 1456 (s), 1413 (m), 1380 (m), 1321 (m), 1271 (m), 1261
(m), 1216 (w), 1190 (w), 1179 (w), 1145 (w), 1082 (s), 1022 (w), 998
(m), 972 (s), 918 (w), 865 (m), 820 (s), 799 (s), 774 (s), 755 (s), 735
(w), 727 (w), 694 (m), 683 (m), 659 (s), 624 (m), 610 (w), 603 (w),
573 (m).
3
center). From this scale, it can be concluded that in the gas phase
the strongest Me3Si+ transfer reagent is [Me3Si benzene]+, even
3
stronger than [Me3SiꢀHꢀSiMe3]+ (TMSA = 31.30 kcal molꢀ1).
3. EXPERIMENTAL DETAILS
3.2.3. [Me3Si C8H10][B(C6F5)4] C8H10 (Ethylbenzene). Mp: 112 ꢀC
3.1. General Information. All manipulations were carried out
under oxygen- and moisture-free conditions under argon using standard
Schlenk or drybox techniques.
3
3
dec. Anal. Calcd for [Me3Si C8H10][B(C6F5)4] C8H10 (Found): C,
3
3
53.54 (53.35); H, 3.03 (3.13). IR (ATR, 16 scans, cmꢀ1): 3084 (w),
3063 (w), 3028 (w), 2969 (w), 2936 (w), 2913 (w), 2877 (w), 1643
(m), 1615 (w), 1597 (w), 1562 (w), 1556 (w), 1512 (s), 1456 (s), 1412
(m), 1382 (m), 1374 (m), 1341 (w), 1327 (w), 1270 (m), 1263 (m),
1186 (w), 1082 (s), 1037 (w), 1031 (w), 973 (s), 923 (w), 907 (w), 863
(m), 809 (m), 773 (s), 770 (s), 755 (s), 726 (m), 700 (m), 683 (m), 660
(s), 623 (m), 610 (m), 603 (m), 573 (m), 558 (m).
Benzene (99.7%, Sigma-Aldrich), toluene (99.7%, Sigma-Aldrich),
ethylbenzene (VEB Berlin, Berlin Adlershof), n-propylbenzene (98%,
Aldrich), isopropylbenzene (97%, Ferak Chemikalien, Berlin), tert-
butylbenzene (97%, Fluka), m-xylene (97%, Reachim), p-xylene (97%,
VEB Teerdestilation and Chemische Fabrik, Erkner), o-xylene (97%,
VEB Petrolchemisches Kombinat Schwedt, BT Erkner), 1,2,3-trimethyl-
benzene (95%, Fluka), 1,2,4-trimethylbenzene (techn., VEB Teerdesti-
lation and Chemische Fabrik, Erkner), and 1,3,5-trimethylbenzene
(98%, Merck) were dried over Na/benzophenone and freshly distilled
prior to use. n-Heptane was freshly distilled prior to use. Bis(trimethylsilyl)-
hydronium tetrakis(pentafluorophenyl)borate ([Me3SiꢀHꢀSiMe3]-
[B(C6F5)4], structure C) was prepared as previously reported.5b,24b
3.1.1. NMR Spectroscopy. 13C{1H}, 13C DEPT (distortionless en-
hancement by polarization transfer), and 1H NMR spectra were obtained
on a Bruker AVANCE 300 spectrometer and were referenced internally
to the deuterated solvent (13C, CDCl3, δreference = 77 ppm) or to protic
impurities in the deuterated solvent (1H, CHCl3, δreference = 7.26 ppm).
CDCl3 was dried over P4O10 and freshly distilled prior to use.
3.1.2. IR Spectroscopy. A Nicolet 6700 FT-IR spectrometer with a
Smart Endurance attenuated total reflectance (ATR) device was used.
3.1.3. Raman Spectroscopy. A Bruker VERTEX 70 FT-IR spectrom-
eter with an RAM II FT-Raman module equipped with a Nd:YAG laser
(1064 nm) was used.
3.2.4. [Me3Si C9H12][B(C6F5)4] (n-Propylbenzene). Mp: 87 ꢀC dec.
3
Anal. Calcd for [Me3Si C9H12][B(C6F5)4] (Found): C, 49.56 (47.46);
3
H, 2.43 (2.00). IR (ATR, 16 scans, cmꢀ1): 3089 (w), 3009 (w), 2973
(w), 2939 (w), 2879 (w), 1644 (m), 1644 (w), 1610 (w), 1595 (w),
1563 (w), 1556 (w), 1513 (s), 1456 (s), 1412 (m), 1381 (m), 1375 (m),
1322 (m), 1271 (m), 1188 (w), 1180 (w), 1162 (w), 1144 (w), 1082 (s),
1028 (w), 1011 (w), 972 (s), 921 (m), 909 (w), 861 (m), 811 (m), 774
(s), 769 (s), 755 (s), 726 (m), 711 (w), 683 (m), 661 (s), 623 (m), 611
(m), 603 (m), 573 (m).
3.2.5. [Me3Si C9H12][B(C6F5)4] (Isopropylbenzene, Cymene). Mp:
3
95 ꢀC dec. Anal. Calcd for [Me3Si C9H12][B(C6F5)4] (Found): C,
3
49.56 (49.12); H, 2.43 (2.13). IR (ATR, 16 scans, cmꢀ1): 3099 (w),
3014 (w), 3013 (w), 2974 (w), 2936 (w), 2914 (w), 2875 (w), 1644
(m), 1611 (w), 1595 (w), 1557 (w), 1557 (w), 1512 (s), 1457 (s), 1413
(m), 1381 (m), 1375 (m), 1367 (m), 1325 (w), 1271 (m), 1261 (m),
1192 (w), 1164 (w), 1080 (s), 1048 (w), 1029 (w), 998 (m), 974 (s),
922 (m), 908 (w), 865 (m), 853 (m), 834 (w), 808 (s), 774 (s), 768 (s),
21025
dx.doi.org/10.1021/ja209693a |J. Am. Chem. Soc. 2011, 133, 21016–21027

Journal of the American Chemical Society
ARTICLE
756 (s), 725 (m), 702 (w), 683 (m), 660 (s), 622 (m), 611 (m), 603 (m),
573 (m), 563 (w), 538 (m).
3.2.6. [Me3Si C8H10][B(C6F5)4] (1,2-Dimethylbenzene, o-Xylene).
’ AUTHOR INFORMATION
Corresponding Author
axel.schulz@uni-rostock.de; alexander.villinger@uni-rostock.de
3
Mp: 91 ꢀC (106 ꢀC dec). Anal. Calcd for [Me3Si C8H10][B(C6F5)4]
3
(Found): C, 48.97 (48.40); H, 2.23 (1.80). IR (ATR, 16 scans, cmꢀ1):
3013 (w), 2975 (w), 2915 (w), 2874 (w), 1644 (m), 1595 (w), 1556
(w), 1512 (s), 1455 (s), 1412 (m), 1381 (m), 1374 (m), 1321 (w), 1270
(m), 1262 (m), 1188 (w), 1179 (w), 1161 (w), 1081 (s), 1035 (w), 972
(s), 938 (m), 895 (w), 858 (m), 823 (m), 801 (m), 773 (s), 768 (s),
755 (s), 727 (m), 696 (m), 683 (s), 661 (s), 622 (m), 610 (m), 603 (m),
573 (w).
’ ACKNOWLEDGMENT
We are indebted to Nick Hartmann and Dr. Jeanette Stelter
(University of Rostock, Entsorgungshof) for the kind gift of
chemicals. Martin Ruhmann (University of Rostock) is acknowl-
edged for the measurement of the Raman spectra. Financial
support by the Deutscher Akademischer Austausch Dienst (DAAD;
German Academic Exchange Service) (scholarship for M.F.I.) is
gratefully acknowledged. Financial support by the Deutsche
Forschungsgemeinschaft (DFG; German Research Foundation)
(Grant 1170/6-1) is gratefully acknowledged.
3.2.7. [Me3Si C8H10][B(C6F5)4] C8H10 (1,3-Dimethylbenzene, m-
3
3
Xylene). Mp: 104 ꢀC dec. Anal. Calcd for [Me3Si C8H10][B(C6F5)4]
3
(Found): C, 48.97 (50.64); H, 2.23 (2.59). IR (ATR, 16 scans, cmꢀ1):
3013 (w), 2950 (w), 2918 (w), 2864 (w), 2734 (w), 1644 (m), 1620
(w), 1601 (w), 1558 (w), 1512 (s), 1455 (s), 1412 (m), 1374 (m), 1341
(w), 1325 (w), 1300 (w), 1270 (m), 1205 (w), 1176 (w), 1158 (w),
1144 (w), 1082 (s), 1031 (w), 998 (m), 972 (s), 924 (m), 907 (m), 862
(m), 818 (m), 808 (m), 773 (s), 756 (s), 727 (m), 715 (w), 692 (m), 683
(s), 660 (s), 624 (m), 610 (m), 603 (m), 574 (m), 543 (w), 530 (w).
3.2.8. [Me3Si C8H10][B(C6F5)4] C8H10 (1,4-Dimethylbenzene, p-Xylene).
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3
3
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S. E.; Chung, W. J. Org. Chem. 2008, 73, 4582.
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Mp: 95 ꢀC dec. Anal. Calcd for [Me3Si C8H10][B(C6F5)4] (Found): C,
3
48.97 (48.75); H, 2.23 (2.29). IR (ATR, 16 scans, cmꢀ1): 2995 (w),
2974 (w), 2961 (w), 2923 (w), 2872 (w), 1643 (m), 1622 (w), 1613
(w), 1600 (w), 1556 (w), 1512 (s), 1456 (s), 1412 (m), 1380 (m), 1375
(m), 1323 (w), 1270 (m), 1211 (w), 1183 (w), 1145 (w), 1081 (s), 1038
(w), 1030 (w), 995 (m), 974 (s), 924 (m), 904 (m), 861 (m), 821 (m),
802 (s), 773 (s), 755 (s), 725 (m), 703 (w), 682 (s), 659 (s), 622 (m),
610 (m), 602 (m), 573 (m), 553 (w), 544 (w).
3.2.9. [Me3Si C9H12][B(C6F5)4] C9H12 (1,2,3-Trimethylbenzene,
Hemimellitene). Mp: 118 ꢀC dec. Anal. Calcd for [Me3Si C9H12]-
3
3
3
[B(C6F5)4] C9H12 (Found): C, 54.45 (54.25); H, 3.35 (2.61). IR
3
(ATR, 16 scans, cmꢀ1): 3066 (w), 3041 (w), 3013 (w), 2944 (w),
2916 (w), 2871 (w), 2732 (w), 1643 (m), 1606 (w), 1586 (w), 1512 (s),
1456 (s), 1412 (m), 1375 (m), 1328 (w), 1270 (s), 1176 (w), 1082 (s),
1033 (w), 973 (s), 926 (m), 907 (w), 857 (m), 816 (m), 802 (m), 773
(s), 756 (s), 726 (m), 708 (m), 683 (m), 659 (s), 624 (m), 610 (m), 602
(m), 573 (m), 538 (m).
3.2.10. [Me3Si C9H12][B(C6F5)4] C9H12 (1,2,4-Trimethylbenzene,
Pseudo-Cymene). Mp: 115 ꢀC dec. Anal. Calcd for [Me3Si C9H12]-
3
3
3
[B(C6F5)4] C9H12 (Found): C, 54.45 (53.19); H, 3.35 (2.61). IR
3
(ATR, 16 scans, cmꢀ1): 2962 (w), 2943 (w), 2925 (w), 2873 (w),
2735 (w), 1643 (m), 1620 (w), 1604 (w), 1556 (w), 1512 (s), 1457 (s),
1413 (m), 1375 (m), 1320 (w), 1271 (s), 1259 (m), 1154 (w), 1082 (s),
1030 (w), 974 (s), 924 (w), 908 (w), 872 (m), 853 (m), 815 (s), 773 (s),
756 (s), 726 (m), 695 (w), 683 (s), 660 (m), 622 (m), 610 (m), 602 (m),
573 (m), 540 (m).
3.2.11. [Me3Si C9H12][B(C6F5)4] C9H12 (1,3,5-Trimethylbenzene,
Mesitylene). Mp: 89 ꢀC (116 ꢀC dec). Anal. Calcd for [Me3Si C9H12]-
3
3
3
[B(C6F5)4] C9H12 (Found): C, 53.34 (53.47); H, 3.14 (2.61). IR
3
(ATR, 16 scans, cmꢀ1): 3011 (w), 2953 (w), 2916 (w), 2862 (w),
1643 (m), 1622 (w), 1600 (m), 1556 (w), 1512 (s), 1457 (s), 1412 (m),
1381 (m), 1374 (m), 1338 (w), 1293 (w), 1272 (m), 1259 (m), 1161
(w), 1153 (w), 1082 (s), 1032 (w), 999 (m), 975 (s), 933 (m), 917 (m),
853 (m), 841 (m), 815 (s), 773 (s), 755 (s), 726 (m), 683 (s), 660 (s),
631 (m), 611 (m), 603 (m), 573 (m), 537 (m).
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S
Supporting Information. Experimental and computa-
b
tional details, crystallographic information (CIF), further experi-
mental and theoretical data of all considered species, and com-
plete ref 43. This material is available free of charge via the
Internet at http://pubs.acs.org.
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