Angewandte
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Table 1: Selected bond lengths [ꢃ] and angles [8],[55] melting points [8C],
which then, indeed, led to success. When exactly one
equivalent of OPMe3 is reacted with one equivalent of pure
T2SO4 in toluene, a crystalline trimethylsilylsulfate salt,
NMR data [ppm], charge (transfer) [e], and trimethylsilyl affinities
(TMSA) [kcalmolꢀ1].
[TSO4]ꢀ
T2SO4
[T3SO4]+
ꢀ ꢀ
[Me3P O T][TSO4], is obtained after concentration of the
solution in 77% yield (2, Scheme 2, Eq. 5, Figure 1). Only on
one occasion could we isolate from such a reaction mixture
one crystal of a side product, which was found to be the
[a]
ꢀ
S O
ꢀ
1.422–1.437
1.588
1.683
110.6–114.9
104.2–107.5
–
127.5
332.7
120
28.0
1.399–1.466
1.480–1.541
1.731–1.738
118.1
103.3–110.7
103.7
129.7–137.0
339.2–339.7
48
1.410
[b]
S O(-Si)
Si O
O-S-O
O-S-O(-Si)
(Si)-S-O(-Si)
1.488–1.503
1.761–1.782
–
111.3–114.8
104.7–106.4
133.1–135.6
342.4–344.4
114
ꢀ
ꢀ
doubly desilylated pyrosulfate, [Me3PO T]2[S2O7] (X-ray, see
SI).
O
The synthesis of
a
tris(trimethylsilyl)oxosulfonium
Si-O-S
S]Si
m.p.
[T3SO4]+ salt is achieved by reacting [T H T][B(C6F5)4]
with silylated sulfuric acid in an 1:1 ratio in toluene. Attempts
to crystallize the salt [T3SO4][B(C6F5)4] failed both at room
temperature and at lower temperatures such as 58C and
ꢀ208C. Attempts to remove the entire solvent in vacuum (1–
10ꢀ3 mbar) at 608C resulted in the decomposition of the salt,
which can be observed by the formation of a black insoluble
residue.[12] The addition of non-polar solvents such as n-
hexane to precipitate the salt also failed. Changing the solvent
from toluene to 1,2-dichlorobenzene was also unsuccessful.
For this reason, we changed the counterion, as we assumed
ꢀ ꢀ
d[29Si]
31.9
54.1
q(S)
2.572
2.639
2.702
[c]
q
ꢀ1.582
ꢀ1.350
ꢀ1.216
ðSO4
Þ
Dqtot
0.418
0.650
0.784
CT,T+
TMSA[d]
215.9
82.9
55.5
ꢀ
[a] Corresponds to d(S O) with O only bound to S. [b] Corresponds to
2ꢀ
ꢀ
ꢀ ꢀ
d(S O(-SiMe3)) with O in a S O SiMe3 unit. [c] Cf. ꢀ2 in [SO4]
,
DqtotCT =qðSO Þ—(ꢀ2). [d] Trimethylsilyl affinity (TMSA) of A(g) is defined
4
as the negative of the reaction enthalpy DH(g)8 in kcalmolꢀ1 at 298.15 K
for the reaction A(g) +T+(g)![AT]+(g), that is the TMSA values given is for
ꢀ
the conjugated acid–base pair A(g)/[AT]+
.
that the decomposition was initiated by a C F activation at
(g)
the borate anion. It is known that carborates are much more
chemically robust compared to the [B(C6F5)4]ꢀ anion.[4,12,51]
Indeed, when [Me3Si][CHB11Br6H5] is reacted with T2SO4 in
toluene, colorless crystals of the desired [T3SO4]+-salt are
obtained in 68% yield after 30 min ultrasound treatment at
608C and recrystallization (Scheme 2, Eq. 7). The formation
of the [T3SO4]+-ion with [CHB11Br6H5]ꢀ as counterion was
unequivocally proven by single-crystal X-ray studies
(Figure 1, bottom). It should be noted that although we
were able to generate the formal autosilylation products of
T2SO4 by separate synthesis routes, dissociation into [T3SO4]+
and [TSO4]ꢀ was not observed for T2SO4, but [T3SO4]+ and
[TSO4]ꢀ react to give two T2SO4 molecules immediately.
All three silylated sulfuric acid species [TSO4]ꢀ, T2SO4
and [T3SO4]+ were studied by different 13C, 17O, 29Si, and
31P NMR techniques in solution (see SI) as well as IR/Raman
spectroscopy. As expected, the 29Si resonance of [T3SO4]+
(d[29Si] = 54.1) was shifted by 22.2 ppm to lower field com-
pared to T2SO4 (d[29Si] = 31.9), while a small high-field shift
by 3.9 ppm was observed for [TSO4]ꢀ (d[29Si] = 28.0, cf. 32
[Me-CN-SiMe3]+,[13] 35.6 [T4PO4]+,[37] and computed 385 ppm
without decomposition, while decomposition occurs only
above 1608C.
Crystallization of all three silylated sulfuric acid species
ꢀ
from either n-pentane (T2SO4) or toluene ([Me3PO T][TSO4]
and [T3SO4][CHB11Br6H6]) yielded colorless crystals
(Figure 1). T2SO4 crystallized in the monoclinic space group
C2/c, while [Me3POT][TSO4] and [T3SO4][CHB11Br6H6] crys-
tallized in the orthorhombic space group Pbca and P212121,
respectively. For all three compounds, there are only rela-
ꢀ
tively weak intermolecular O···H C interactions (Figures S2–
S4, SI), but these are found in each case for the non-silylated
O atom of the SO4 core within the silylated species. That is, for
[TSO4]ꢀ with three non-silylated O atoms one finds such
interactions with three neighboring [Me3POT]+ cations (Fig-
ure S3), for T2SO4 correspondingly with two neighboring
T2SO4 molecules (Figure S2) and in [T3SO4]+ exactly one such
interaction (Figure S4), however, with one adjacent cation. In
ꢀ
the latter case, interestingly, weak Branion···H Ccation interac-
ꢀ
tions are added. Likewise, weak Branion···H Canion interactions
for naked [Me3Si]+
,
[52,53] see SI, Table S3). As the 29Si NMR
are found between the H atom attached to the C atom of one
(g)
ꢀ
chemical shifts can be used as an indicator for the silylium ion
character (and the deviation from planarity, see below),[52–54]
that is, for the strength of the [Me3Si]+ interaction with the
solvent T2SO4, it can be assumed that T2SO4 is a rather strong
coordinating solvent utilizing the scale by Cremer et al. (ꢀ50
to 90 ppm, cf. 90–190 weakly coordinating, 200–370 weakly
interacting, 370–385 noncoordinating solvents and 385 ppm
gas phase).[52]
Crystals of all three silylated sulfuric acid species are
moisture sensitive but thermally considerably stable with
defined melting points (Table 1; T2SO4: 48, [Me3PO-T]-
[TSO4]: 1208C, and [T3SO4][CHB11Br6H6]: 1148C). Interest-
ingly, while [T3SO4][B(C6F5)4] begins to decompose upon
concentration in solution at ambient temperatures, [T3SO4]
[CHB11Br6H6] can be isolated in substance and even melts
carborate anion and the Br atom in para-position to the C H
bond atom of an adjacent second carborate anion (Figure S4).
This leads to a zig–zag chain of carborate anions in the solid.
The [T3SO4]+ cations coordinate with this chain via the above-
ꢀ
mentioned weak Branion···H Ccation interactions.
As depicted in Figure 1, the central SO4 core always
adopts a highly distorted tetrahedral geometry, with two
different S O bond lengths (Table 1). In accord with electro-
ꢀ
static consideration, with increasing number of Me3Si groups,
ꢀ
ꢀ
the Si O bond lengths are elongated along [TSO4] <
T2SO4 < [T3SO4]+. Similarly, the Si-O-S angles (127.5 to
134.28 (averaged)) and the sum of the angles around the Si
atoms (from 332 to 3448) increase, indicating the largest
silylium ion character in [T3SO4]+ > T2SO4 > [TSO4]ꢀ.
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
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