“…Therefore the low barriers observed for 38 − 41 may be attributed to stability imparted to the transition state by delocalization of π-electron density in the ring. It should be noted, however, that the −SiMe 3 substituents in these anions may also play a role in lowering the barrier to inversion . This possibility will be addressed in future experiments intended to broaden the database for such inversion barriers. 7 Reaction coordinate diagram for inversion in the complex [Li(12-crown-4) 2 ][C 4 Et 4 GeSiMe 3 ] ( 40 ).…”
Synthetic routes to the metallole species
C4Me4E(H)R (9, E = Si, R
= Si(SiMe3)3; 10, E = Si, R
= Mes
(mesityl); 11, E = Ge, R =
Si(SiMe3)3; 12, E = Ge, R =
Mes),
C4R4E(SiMe3)2
(13, E = Si, R = Me; 14, E =
Ge,
R = Me; 19, E = Si, R = Et; 20, E = Ge, R
= Et), and
C4Me4E(R)E(R)Me4C4
(15, E = Si, R = SiMe3; 16, E
=
Si, R = Me; 17, E = Ge, R = SiMe3;
18, E = Ge, R = Me) are described. In the presence
of 18-crown-6, dihalides
1 and 2 are reduced by potassium in
tetrahydrofuran to give crystalline samples of the silole dianion
[K(18-crown-6)+]2[C4Me4Si2-]
(21) and the germole dianion
[K4(18-crown-6)3][C4Me4Ge]2
(22). Compound 21 adopts an
inverse-sandwich geometry, while 22 is a dimer with a bridging
[K(18-crown-6)K]2+ group and η5-binding
modes for all of
the potassium atoms. The metallole dianions in these structures
appear to possess delocalized π-systems, as evidenced
by nearly equivalent C−C bond lengths in the five-membered rings.
Silolyl and germolyl anions have been obtained
by various methods involving nucleophilic cleavage of bonds to
germanium and silicon. Deprotonation of 11
and
12 in the presence of a crown ether gave the anions
[K(18-crown-6)][C4Me4GeR]
(23, R = Si(SiMe3)3;
24, R =
Mes) and
[Li(12-crown-4)2][C4Me4GeR]
(25, R = Si(SiMe3)3;
26, R = Mes). NMR parameters for these
species,
and X-ray structures for 25 and 26, indicate that
the anionic rings possess pyramidal germanium centers and
bond
localization in the diene portion of the ring. Spectroscopic and
X-ray crystallographic data for
[Na(15-crown-5)][C4Me4GeMe] (28),
prepared by reductive cleavage of the Ge−Ge bond in 18,
reveal a similar structure for the
germolyl ring. The latter compound possesses a Na···Ge
interaction in the solid state. Silolyl and germolyl
anions
M[C4Me4E(SiMe3)]
(30, E = Si, M = Li; 31, E = Si, M = K;
32, E = Si, M = Li(12-crown-4)2;
33, E = Si, M
= K(18-crown-6); 34, E = Ge, M = K; 35, E
= Ge, M = K(18-crown-6)) have been prepared by
nucleophilic
cleavage of the E−SiMe3 bond in
C4Me4E(SiMe3)2
with MCH2Ph (M = Li, K). By similar methods,
the monoanionic
species
[K(18-crown-6)][C4Me4E(SiMe3)C4Me4E]
(36, E = Si; 37, E = Ge) were obtained.
A crystal structure
determination for 33 revealed a highly pyramidalized Si
center (the angle between the C4Si plane and the
Si−Si
bond is 99.6°) and pronounced double bond localization in the ring.
Interaction between the
[K(18-crown-6)]+
cation and the anion is rather weak, as indicated by the
K···Si distance (3.604(2) Å) and the atomic position
for K.
By variable-temperature 1H NMR spectroscopy, inversion
barriers for the compounds
[Li(12-crown-4)2][C4Et4ESiMe3]
(38, E = Si; 40, E = Ge) and
K[C4Et4ESiMe3]
(39, E = Si; 41, E = Ge) were estimated.
Barriers for the germolyl
anions 40 and 41 (10.5(1) and 9.4(1)
kcal mol-1, respectively) are distinctly higher than
those for the corresponding
silolyl anions 38 and 39, as might be expected
from periodic trends. The silolyl anions exhibited
coalescence
temperatures below the freezing point of tetrahydrofuran (165 K), but
upper limits to the invers...
“…Therefore the low barriers observed for 38 − 41 may be attributed to stability imparted to the transition state by delocalization of π-electron density in the ring. It should be noted, however, that the −SiMe 3 substituents in these anions may also play a role in lowering the barrier to inversion . This possibility will be addressed in future experiments intended to broaden the database for such inversion barriers. 7 Reaction coordinate diagram for inversion in the complex [Li(12-crown-4) 2 ][C 4 Et 4 GeSiMe 3 ] ( 40 ).…”
Synthetic routes to the metallole species
C4Me4E(H)R (9, E = Si, R
= Si(SiMe3)3; 10, E = Si, R
= Mes
(mesityl); 11, E = Ge, R =
Si(SiMe3)3; 12, E = Ge, R =
Mes),
C4R4E(SiMe3)2
(13, E = Si, R = Me; 14, E =
Ge,
R = Me; 19, E = Si, R = Et; 20, E = Ge, R
= Et), and
C4Me4E(R)E(R)Me4C4
(15, E = Si, R = SiMe3; 16, E
=
Si, R = Me; 17, E = Ge, R = SiMe3;
18, E = Ge, R = Me) are described. In the presence
of 18-crown-6, dihalides
1 and 2 are reduced by potassium in
tetrahydrofuran to give crystalline samples of the silole dianion
[K(18-crown-6)+]2[C4Me4Si2-]
(21) and the germole dianion
[K4(18-crown-6)3][C4Me4Ge]2
(22). Compound 21 adopts an
inverse-sandwich geometry, while 22 is a dimer with a bridging
[K(18-crown-6)K]2+ group and η5-binding
modes for all of
the potassium atoms. The metallole dianions in these structures
appear to possess delocalized π-systems, as evidenced
by nearly equivalent C−C bond lengths in the five-membered rings.
Silolyl and germolyl anions have been obtained
by various methods involving nucleophilic cleavage of bonds to
germanium and silicon. Deprotonation of 11
and
12 in the presence of a crown ether gave the anions
[K(18-crown-6)][C4Me4GeR]
(23, R = Si(SiMe3)3;
24, R =
Mes) and
[Li(12-crown-4)2][C4Me4GeR]
(25, R = Si(SiMe3)3;
26, R = Mes). NMR parameters for these
species,
and X-ray structures for 25 and 26, indicate that
the anionic rings possess pyramidal germanium centers and
bond
localization in the diene portion of the ring. Spectroscopic and
X-ray crystallographic data for
[Na(15-crown-5)][C4Me4GeMe] (28),
prepared by reductive cleavage of the Ge−Ge bond in 18,
reveal a similar structure for the
germolyl ring. The latter compound possesses a Na···Ge
interaction in the solid state. Silolyl and germolyl
anions
M[C4Me4E(SiMe3)]
(30, E = Si, M = Li; 31, E = Si, M = K;
32, E = Si, M = Li(12-crown-4)2;
33, E = Si, M
= K(18-crown-6); 34, E = Ge, M = K; 35, E
= Ge, M = K(18-crown-6)) have been prepared by
nucleophilic
cleavage of the E−SiMe3 bond in
C4Me4E(SiMe3)2
with MCH2Ph (M = Li, K). By similar methods,
the monoanionic
species
[K(18-crown-6)][C4Me4E(SiMe3)C4Me4E]
(36, E = Si; 37, E = Ge) were obtained.
A crystal structure
determination for 33 revealed a highly pyramidalized Si
center (the angle between the C4Si plane and the
Si−Si
bond is 99.6°) and pronounced double bond localization in the ring.
Interaction between the
[K(18-crown-6)]+
cation and the anion is rather weak, as indicated by the
K···Si distance (3.604(2) Å) and the atomic position
for K.
By variable-temperature 1H NMR spectroscopy, inversion
barriers for the compounds
[Li(12-crown-4)2][C4Et4ESiMe3]
(38, E = Si; 40, E = Ge) and
K[C4Et4ESiMe3]
(39, E = Si; 41, E = Ge) were estimated.
Barriers for the germolyl
anions 40 and 41 (10.5(1) and 9.4(1)
kcal mol-1, respectively) are distinctly higher than
those for the corresponding
silolyl anions 38 and 39, as might be expected
from periodic trends. The silolyl anions exhibited
coalescence
temperatures below the freezing point of tetrahydrofuran (165 K), but
upper limits to the invers...
“…Early theoretical studies have been carried out on various organosilicenium ions such as substituted silicenium and silylenium ions. [3][4][5] Apeloig and Schleyer's studies showed that organosilicenium ions are more stable than their carbocation equivalents because silicon is larger and more electropositive than carbon and can thus accommodate effectively positive charge, although such conclusions have also been disputed (vide infra). 3 Whereas several groups have claimed to have experimentally observed persistent silicenium ions in strong acids, their reported results have been for long incorrect.…”
Section: Introductionmentioning
confidence: 99%
“…Silicenium ions are known to form through fragmentation of silyl compounds in mass spectrometry and as intermediates in hydride transfer and other S N 1-type reactions or rearrangements. , Attention has been given to the generation of long-lived organosilicenium ions in solution or in the solid state. Early theoretical studies have been carried out on various organosilicenium ions such as substituted silicenium and silylenium ions. − Apeloig and Schleyer’s studies showed that organosilicenium ions are more stable than their carbocation equivalents because silicon is larger and more electropositive than carbon and can thus accommodate effectively positive charge, although such conclusions have also been disputed (vide infra) . Whereas several groups have claimed to have experimentally observed persistent silicenium ions in strong acids, their reported results have been for long incorrect.…”
The C(3h) conformation of the trimethylsilicenium ion 1 was established to be the preferred global energy minimum structure based on energy calculations. Because C-H hyperconjugation occurs least favorably in this conformation of the analogous tert-butyl cation, it may not contribute in large part to the stabilization of this cation, especially given the ineffectiveness of the 3p-2sp(3) overlap that would need to be involved. This is in contrast with the preferred C(s) global energy conformers of the tert-butyl cation. The C(2v) structure 4 and C(2) enantiomers 6 and 7 are the preferred conformations of the dimethylsilicenium ion based on energy comparison. None of these structures have C-H bonds ideally oriented for hyperconjugation with the empty p orbital of the cationic silicon, indicating that it does not likely stabilize the ion to any significant extent. The computed IR spectra and (29)Si, (13)C, and (1)H NMR chemical shifts of the isomers were also discussed. Whereas the studied alkylsilicenium ions are thermodynamically stable, their observation as persistent ions in solution is much more difficult because of their kinetic instability toward varied electron-donating solvents.
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