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Table 2: Partial charges (q) and charge transfer (QCT) in e, selected NBO
with an asymmetric fluorine bridge is the product of the
reaction of 1,8-bis(diphenylmethylium)naphthalenediyl di-
cation with fluoride, as shown by Gabaꢂ and co-workers. In
data, TMSA values (kcalmolꢀ1), and selected calculated structural data
(distances in ꢂ, angles in 8) of [Me3Si X SiMe3]+ ions at the pbe1pbe/
ꢀ ꢀ
aug-cc-pwCVDZ level of theory.[20]
ꢀ
this case, the fluorine atom is connected through a typical C
F
Cl
Br
I
F bond of 1.424(2) ꢀ and forms a long interaction of
2.444(2) ꢀ with the other methylium center.[13]
q[a]
ꢀ0.64
(ꢀ0.67)
0.18
0.28/0.96
1.29
ꢀ0.30
(ꢀ0.43)
0.35
0.40/0.92
2.88
ꢀ0.17
(ꢀ0.36)
0.42
0.43/0.90
4.16
+0.01
(ꢀ0.26)
0.51
0.48/0.88
5.34
ꢀ ꢀ
The heavier [Me3Si X SiMe3][B(C6F5)4] salts (X = Cl,
[b]
QCT
cSi/cX
Br, and I) crystallize isotypically in the orthorhombic space
[c]
group Pbca with eight formula units per cell. In contrast to the
hX =spl[c]
spl LP[d]
TMSA
ꢀ
fluoronium salt, one molecule of the solvent (Me3Si X) is
6.78
34.8
1.05
31.1
0.63
31.8
0.43
33.1
found along with a slightly distorted C2-symmetric cation and
the [B(C6F5)4]ꢀ ion in the asymmetric unit (Figure 1).
Comparing the structural data with respect to the Si-X-Si
angles and the deviation from planarity, the following trends
can be established (Table 1): The Si-X-Si angles decrease
along the series F, Cl, Br, I, while the distance between the Si
atom and the plane defined by the three methyl C atoms in
ꢀ
Si X
X-Si-X
1.830
148.4
2.276
117.3
2.422
112.3
2.630
106.8
ꢀ
[a] Values in parenthesis are the partial charges in Me3Si X. [b] Charge
+
ꢀ
ꢀ
transfer from Me3Si X to [Me3Si] . [c] Natural bond orbital: fNBO(Si X)=
cSi hSi +cX hX with the hybrid orbital h=spl. [d] Hybridization of the spl-
type lone pair; the second lone pair is composed of a pure p atomic
orbital.
ꢀ ꢀ
the Me3Si fragment increases. Since all three [Me3Si X
SiMe3][B(C6F5)4] salts (X = Cl, Br, and I) crystallize as solvate
ꢀ
with one molecule Me3Si X, the bridging Si-X distance (Cl:
2.238(5), Br: 2.385(3), I: 2.584(5) ꢀ) can easily be compared
Thus, small interactions of the cation with the environment
in the crystal can have a large influence on the Si-X-Si angle.
ꢀ
ꢀ
to the Si X distance in Me3Si X (Cl: 2.055(2), Br: 2.242(2), I:
2.479(2) ꢀ). As expected, the bridging Si X distance is
always larger, but interestingly the difference D(dSi-XSi
dSiꢀX) significantly decreases along Cl, Br, I (Cl: 0.183, Br:
ꢀ
The Si-X-Si angles reflect the steric repulsion of the Me3Si
+
ꢀ ꢀ
ꢀ
groups and suggest that in [Me3Si I SiMe3] the steric
repulsion is less pronounced, and the angle approaches the
structures of a p orbital bonding, in accord with the NBO data
0.143, I: 0.105 ꢀ).
+
ꢀ ꢀ
[Me3Si X SiMe3] ions can be considered as solvent
(see below and Table 2) and with computed results of the
+
+
ꢀ
ꢀ ꢀ
complexes between Me3Si X and [Me3Si] . In these com-
series of [Me X Me] ions, where this effect is even more
plexes the Me3Si fragment has almost completely lost its
silicenium character (strong deviation from planarity,
Table 1), as a stable covalently bonded tetracoordinated Si
center is formed. In this context, and in analogy to the proton
affinity, a trimethylsilicenium affinity (TMSA) can be defined
as the enthalpy change associated with the dissociation of the
conjugated acid [Eq. (1), B = base].[1,2,19]
pronounced (Cl: 1058, Br: 101.18, I: 97.88).[22] As expected, all
ꢀ
Si X bonds are highly polarized, but this polarization
decreases considerably for the heavier halogens (q in
Table 2), in accord with the computed partial charges. In
contrast to the negatively charged F, Cl, and Br atoms in
+
ꢀ ꢀ
[Me3Si X SiMe3] ions, for iodine a small positive net charge
is found. Upon silylation, the fluorine partial charge does not
+
ꢀ ꢀ
change
significantly
(DqX = qX(Me3Si X SiMe3] )ꢀqX-
½BꢀSiMe3ꢂþ ! BðgÞ þ ½Me3Siꢂþ DH298
ꢀ
ð1Þ
(Me3Si X); DqF = 0.03 e) but increase along Cl (0.13 e), Br
(0.19 e), and I (0.25 e). Compared to the overall charge
transfer QCT (see Table 2), the contribution of the F atom is
ðgÞ
ðgÞ
Using the pbe1pbe and MP2 levels of theory and an aug-
cc-pwCVDZ basis set,[20] we have computed the TMSA and
Gibbs free energies of all considered species at 298 K (B =
negligible, and thus QCT stems mainly from the other Me3Si
+
ꢀ ꢀ
group, while in the heavier analogue [Me3Si I SiMe3] , the
ꢀ
ꢀ
Me3Si X; X = F, Cl, Br, and I). Along the series of Me3Si X
I atom contributes almost half to the overall charge transfer
ꢀ
bases, the largest TMSA value was found for Me3Si F
(Table 2).
(pbe1pbe: 34.8 kcalmolꢀ1), which decreased to 31.1 kcal
The NBO Lewis picture[20] shows two equivalent Si X
ꢀ
molꢀ1 for Me3Si Cl. Interestingly, from Cl to I the TMSA
ꢀ
s bonds and two lone pairs (LPs) localized at the halogen
atom (a pure p-type LP and a spl-hybridized LP). A detailed
study of the NBO data reveals that the heavier the halogen is,
the larger is the p contribution in the hybrid orbitals of the
slightly increases (Br: 31.8, I: 33.1 kcalmolꢀ1; for MP2 and
DG298 values see Table S3 in the Supporting Information).[20]
In agreement with computational results for dimethyl
halonium ions[21,22] at different DFT levels and the exper-
imental data (see above), all C2-symmetric structures are
bent, with the Si-X-Si angles decreasing in the series F, Cl, Br,
I (Table 1 and Table 2); however, there are fairly large
deviations between theory and experiment, especially for the
fluoronium ion. The reason is that the energy potentials for
the variation of the Si-X-Si angle are very flat. For example, in
the case of the fluoronium ion the difference between the
linear C3h (eclipsed) and bent C2 (staggered) species amounts
only to 0.5 kcalmolꢀ1 (DE0) at the pbe1pbe level of theory,
ꢀ
halogen forming the Si X s bond. In contrast, the s character
of the spl-hydridized LP significantly increases along F < Cl <
Br< I (Table 2).
In conclusion, we present herein a straightforward, facile
synthetic procedure to form bissilylated halonium salts. By
means of this approach it was possible to prepare in high
yields the first example of an acyclic bissilylated fluoronium
cation with a slightly bent Si-F-Si unit, which can be regarded
+
ꢀ ꢀ
as the silyl analogue of the well-known [H F H] ion. The
ꢀ
fact that Me3Si F acts as both solvent and reactant combined
with the easy handling of Me3Si F (b.p. 198C, moisture- and
while DH298 favors the linear species by 1.4 kcalmolꢀ1 [23]
.
ꢀ
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 7444 –7447