Preparation and characterization of [CF3SO3(SiMe 3)2]+[B(C6F5) 4]-

Article abstract of DOI:10.1039/c0cc00013b

Using tetrakis(pentafluorophenyl)borate, [B(C₆F ₅)₄]^-, as weakly coordinating counterion, the bis(trimethylsilyl)trifluoromethylsulfonium cation, [CF₃SO ₃(Me₃Si)₂]⁺, is readily isolated for X-ray, NMR and IR structural characterization.

Full text of DOI:10.1039/c0cc00013b

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Preparation and characterization of [CF3SO3(SiMe3)2]+[B(C6F5)4]ꢀw
Axel Schulz,*ab Johannes Thomasa and Alexander Villinger*a
Received 18th February 2010, Accepted 26th March 2010
First published as an Advance Article on the web 14th April 2010
DOI: 10.1039/c0cc00013b
Using tetrakis(pentafluorophenyl)borate, [B(C6F5)4]ꢀ, as weakly
coordinating counterion, the bis(trimethylsilyl)trifluoromethyl-
sulfonium cation, [CF3SO3(Me3Si)2]+, is readily isolated for
X-ray, NMR and IR structural characterization.
Scheme 1
The salt, [CF3SO3(SiMe3)2]+[B(C6F5)4]ꢀ (1) was prepared
in a good yield (83%) in a one step synthesis according to
Scheme 1.
Recently it was shown by Kornath et al.1 that sulfurtrioxide
reacts with the superacidic solutions2–5 XF/SbF5 (X = H, D)
to form the corresponding salts [X2SO3F]+[SbF6]ꢀ, which
are the protonated forms of fluorosulfonic acid. On the
Hammett2,3,6 acidity scale fluorosulfonic acid reaches an H0
value of ꢀ15.1, which is in the region of a superacid. By
definition, acids with a lower H0 value than that of sulfuric
acid (H0 = ꢀ12.1) are superacids. Trifluoromethanesulfonic
acid, CF3SO3H, represents a similarly strong superacid with
an H0 value of ꢀ14.1.7 The silylated species trifluoromethyl-
sulfonato trimethylsilane (also known as trimethylsilyl trifluoro-
methanesulfonate), CF3SO3SiMe3, is frequently used as Me3Si+
transfer reagent.
The bulky trimethylsilicenium ion [Me3Si]+ may be regarded
as a sterically demanding big proton.8 Thus, the reaction
mixture Me3Si–Y/[Me3Si][wca] (wca = weakly coordinating
anion9 such as [B(C6F5)4]ꢀ and Y = weakly basic moiety such
as a halogen) can be considered as super Lewis acid silylating
medium.8,10,11 By applying this approach the full series of salts
containing the bissilylated halonium cations [Me3Si–Y–SiMe3]+
(Y = F, Cl, Br, and I) have been generated and isolated in the
super Lewis acid silylating medium Me3Si–Y (as solvent) and
[Me3Si][B(C6F5)4] as silylating reagent.8
Beside carborane anions the [B(C6F5)4]ꢀ ion is amongst the
least coordinating,9 least basic and most chemically inert
anions presently known. Even the least coordinating anions
presently known form four-coordinate species of the type
R3Si–wca,12 which, although mostly ion-like, nevertheless
retain some degree of covalence. Weakly basic solvents such
as Me3Si–Y (Y = halogen),8 acetonitrile13 or toluene12,14 have
been shown to displace the wca’s, forming isolable salts of the
type [R3Si–solvent]+[wca]ꢀ with a four-coordinate silicon
species. In the present communication, we show that a solvent
as weakly coordinating as trifluoromethylsulfonato trimethyl-
silane, CF3SO3SiMe3, also forms a stable adduct-cation with
the Me3Si+ ion analogous to its lighter congener FSO3H.
[Me3Si][B(C6F5)4] undergoes immediate reaction with
one equivalent of Me3Si–X yielding a clear colorless solu-
tion of 1 dissolved in the excess of CF3SO3SiMe3 (1 mmol
of [Me3Si][B(C6F5)4] in a 10 to 20 fold molar excess of
CF3SO3SiMe3 when gently heated to 60 1C). The resulting clear
colourless solution is concentrated to incipient crystallisation.z
Storage at 5 1C for ten hours results in the deposition of
colourless crystals. Removal of the supernatant by decantation,
washing with 2 mL of neat trimethylsilyl-trifluoromethyl-
sulfonate and drying in vacuo yields 1 as colourless crystals.
[CF3SO3(SiMe3)2][B(C6F5)4] (1) is extremely air and moisture
sensitive but stable under argon atmosphere over a long period
as solid and in CF3SO3SiMe3 solutions. Compound 1 is easily
prepared in bulk and is thermally stable up to 127 1C.
Decomposition starts at these temperatures upon melting.
[CF3SO3(SiMe3)2][B(C6F5)4] crystallizes in the triclinic space
ꢀ
group P1 with two units per cell. The molecular structure is
shown in Fig. 1, confirming an essentially Cs symmetrical
cation that is well separated from the borate anion. The
Cs symmetric SO3 core is structurally characterized by
two longer S–O bonds (d(S–O1) = 1.477(1) and d(S–O2) =
1.477(1) A) and one considerably shorter S–O bond
(d(S–O3) = 1.411(1); cf. [H2SO3F]+: dlong(S–O) = 1.475(3)/
1.485(3) A and dshort(S–O) = 1.395(3) A),1 which indicates
double bond character for all S–O bonds (cf. Srcov(S–O) =
1.7, Srcov(SQO) = 1.50).15 It should be mentioned that polar
covalent bonds tend to be shorter than would be expected on
the basis of the sum of covalent radii, and it can be assumed
that the bonding along the S–O–Si fragment is highly polar
(see below).15 Hence, SQO double bond character can be
questioned.16 As expected the S–O bond lengths increase upon
silylation. The Si–O bond lengths (d(Si1–O1) = 1.813(2),
d(Si2–O2) = 1.824(1) A) are significantly elongated compared
to those found in Me3Si–O–SiMe3 (1.626(2) A).17
The coordination geometry around both Si atoms in 1 is
P
a Universitat Rostock, Albert-Einstein-Str. 3a, 18059 Rostock,
¨
Germany. E-mail: axel.schulz@uni-rostock.de;
strongly distorted tetrahedral as displayed by +C–Si–C =
347.41 (cf. Me3Si–O–SiMe3: 330.41,17 Me3Si–F: 334.51,18
Me3Si–F–SiMe3: 348.01,8 [Me3Si]+: 354.41 as [HCB11F11]-salt).19
The Si–O–S angles with 132.16(7)1 and 135.01(8)1, respec-
tively, are widened due to steric strain. The coordination
geometry around the sulfur is also distorted tetrahedral since
the steric demand of the short SQO double bond requires
Fax: +49 (0)381-498-6382; Tel: +49 (0)381-498-6400
¨
b Leibniz Institut fur Katalyse e.V, Albert-Einstein-Str. 29,
18059 Rostock, Germany
w Electronic supplementary information (ESI) available: Experi-
mental and computational details. CCDC 766935. For ESI and
crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc00013b
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3696 | Chem. Commun., 2010, 46, 3696–3698

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excess of diethyl ether, which yields in a straightforward
reaction [Et2OSiMe3]+[B(C6F5)4]ꢀ (2) in almost quantitative
yield. Compound 2 was fully characterized (see ESIw). The
structure was already reported by Driess et al.23
In conclusion, we have shown that bissilyl(trifluoromethyl)-
sulfonic acid 1 is easily prepared in very good yields analogous
to the protonated acid and can be utilized as ‘‘soft’’ silylating
agent. In addition the better solubility of 1 in organic solvents
compared to pure Me3Si+-salts might be useful in Me3Si+
transfer reactions.
Notes and references
z To neat trimethylsilicenium tetrakis(pentafluorophenyl)borate
[Me3Si][B(C6F5)4] (0.752 g, 1.0 mmol), a minimum of trifluoromethyl
sulfonyl trimethylsilane (10 to 20 fold molar excess) was added
dropwise at ambient temperatures with stirring, followed by gently
heating to 60 1C. The resulting clear colourless solution is concen-
trated to incipient crystallisation. Storage at 5 1C for ten hours results
in the deposition of colourless crystals. Removal of the supernatant by
decantation, washing with 2 mL of neat (trimethylsilyltrifluoromethyl-
sulfonate and drying in vacuo yields 0.811 g (0.832 mmol, 83%)
bis(trimethylsilyl)trifluoro-methylsulfonium tetrakis pentafluorophenyl-
borate [(SiMe3)2CF3SO3][B(C6F5)4] (1) as colourless crystals. Mp
127 1C (dec.). Anal. Calc. % (found): C 38.21 (40.71); H 1.86 (1.80);
Fig. 1 ORTEP drawing of the cation of 1 in the crystal. Thermal
ellipsoids with 50% probability at 173 K. Selected bond lengths (A)
and angles (1): S–O3 1.411(1), S–O1 1.477(1), S–O2 1.477(1), S–C1
1.830(2), Si1–O1 1.813(2), Si2–O2 1.824(1); O1–Si1–C3 99.67(6),
O1–Si1–C4 102.33(7), O1–Si1–C2 104.08(7), C3–Si1–C4 115.91(9),
C3–Si1–C2 115.65(9), C4–Si1–C2 115.8(1), S–O1–Si1 132.16(7),
S–O2–Si2 135.01(8), O3–S–O1 116.77(8), O3–S–O2 116.76(8),
O1–S–O2 106.64(8).
1H NMR (25 1C, C6D6, 250.13 MHz):
d
=
ꢀ0.06 (s, 18H,
more space as depicted by the two significantly enlarged
O–S–O angles (O3–S–O1 116.77(8), O3–S–O2 116.76(8) vs.
O1–S–O2 106.64(8)1).
[((CH3)3Si)2CF3SO3]+). 11B NMR (25 1C, C6D6, 96.3 MHz): d =
ꢀ15.9. 13C{1H} NMR (25 1C, C6D6, 75.5 MHz): d = ꢀ0.92 (s,
[((CH3)3Si)2CF3SO3]+), 117.7 (q, [((CH3)3Si)2CF3SO3]+, J(13C–19F) =
1
321 Hz), 125 (br, ipso-C), 136.9 (dm, m-CF, 1J(13C–19F) = 246 Hz),
Besides the C–Si–C angles, the 29Si NMR chemical shift can
also be used as a measure for the silicenium ion character. The
29Si NMR chemical shift of d(29Si) = 75.4 (d6-benzene), which
is detected for the cation, [CF3SO3(SiMe3)2]+, indicates con-
siderable charge accumulation at the silicon atoms. This
chemical shift is characteristic for bissilylated cations, and
similar values are reported for e.g. [iPr3Si(SO2)][CHB11H5Br6]
with 85 or 77.2 for a disilylfluoronium ion with a naphthyl
backbone.12,20
138.8 (dm, p-CF, 1J(13C–19F)
= 246 Hz), 149.0 (dm, o-CF,
1J(13C–19F) = 242 Hz). 19F{1H} NMR (25 1C, C6D6, 282.4 MHz):
d = ꢀ167.1 (m, 8F, m-CF), ꢀ163.2 (m, 4F, p-CF), ꢀ132.1 (m, 8F,
o-CF), ꢀ74.1 (s, 3F, [((CH3)3Si)2CF3SO3]+). 29Si{1H} NMR (25 1C,
C6D6, 59.6 MHz): d = 75.4 (s, [((CH3)3Si)2CF3SO3]+). X-Ray crystal
data for 1 (CCDC 766935): C31H18BF23O3SSi2, M = 974.50, triclinic,
ꢀ
P1; a = 10.319(6), b = 13.300(7), c = 15.086(9) A, a = 68.29(1), b =
82.48(1), g = 76.86(2)1. V = 1871(2) A3, Z = 2, r = 1.730, m = 0.300 mmꢀ1
T = 173(2) K, measured reflections 38916, independent reflections
10715, Rint. = 0.0259, R1 = 0.0360, wR2 = 0.1035, GooF = 1.053.
,
According to NBO21 analysis both Si–O bonds are highly
polarized in [CF3SO3(SiMe3)2]+ and in agreement with the
computed partial charges (q(Si) = +1.998e, q(O) = ꢀ0.983e).
The silylation is associated with an overall charge transfer
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Q
CT = 0.238e corresponding to a Me3Si-group charge of 0.762e.
The cation [CF3SO3(SiMe3)2]+ can be regarded as solvent
complexes between CF3SO3(SiMe3) and [Me3Si]+. Analogous
to the proton affinity, the trimethylsilicenium affinity (TMSA)
is defined as the enthalpy change associated with the dissocia-
tion of the conjugated Lewis acid as illustrated in eqn (1):8,22
[CF3SO3(SiMe3)2]+ - CF3SO3(SiMe3)(g)
(g)
+ [Me3Si]+ + DH298
(1)
(g)
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(g)
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similar TMSA were computed for both silylated species which
amount to 41.3 and 44.3 kcal molꢀ1. These TMSA are
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.
The reactivity of [CF3SO3(SiMe3)2]+[B(C6F5)4]ꢀ (1) as
silylating agent has been shown in the reaction of 1 with
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