Chemistry Reference
In-Depth Information
a
(
23
Na)
a
(
29
Si
b
)
a
(
29
Si
b
)
a
(
29
Si
a
)
a
(
29
Si
a
)
a
(
29
Si
a
)
a
(
29
Si
a
)
a
(
29
Si
b
)
a
(
29
Si
b
)
(a)
(b)
Figure10.5
EPRspectrumof(a)solvent-separatedionpair[R
2
Si]
•
−
[Na
+
(THF)
4
](
29
)inDME(a(
29
Si
α
)
=
29.1G
anda(
29
Si
β
)
10.2G),and(b)contactionpair[R
2
SiNa(15-crown-5)]
•
(
28
)intoluene(a(
29
Si
α
)
29.1G,a(
29
Si
β
)
=
=
10.2G and a(
23
Na)
=
=
1.9G); g
=
2
.
0074,R
=
SiMetBu
2
. (Reprinted with permission from [38a]. Copyright
2007AmericanChemicalSociety.)
solv
, via a two-electron reduction process
(Scheme 10.10).
38
The possibility of obtaining neutral radicals
28
or anionic paramagnetic species
29
(the anion radical of a silylene) was first noted when the EPR spectra of [R
2
Si]
•
Na(15-crown-5)
n
were
recorded both in polar (THF, DME) and non-polar (toluene) solvents.
38a
It was apparent from the EPR
spectra (Figure 10.5) that in polar solvents the paramagnetic species exists as an ion-separated salt
29
with
a
(
29
Si
α
)
=
=
crown ether (n
=
2),THF,DME(n
=
4); R
=
SiMe
t
Bu
2
)
29
Si
β
)
=
10.2 G, whereas in the non-polar solvent the coupling to the Na
+
(
29.1 G and
a
cation
23
Na)
23
Na,
I
(
=
=
/
was also observable for the contact ion pair
28
(
a
2, 100 %). Subsequently,
the controlled formation of
28
and
29
, both in solid state and in solution, was achieved by using the
appropriate amount of crown ether according to the reactions (iv) and (v) in Scheme 10.10.
38
The solvent-separated ion pair
29
(M
1.9 G;
3
=
=
12-crown-4, Scheme 10) is converted
to the contact ion pair
28
by removal of the crown ether with lithium bromide. The product exists
as a monomer in solution, as shown by the EPR signal with hfc constants of
a
Li, crown ether
29
Si
α
)
=
(
34.0 G,
29
Si
β
)
=
7
Li)
1.6 G (
7
Li,
I
a
(
9.7 G and
a
(
=
=
3
/
2, 92.5 %), but dimerizes in the solid state to the
2
]
••
(
30
)(R
diradical [(
(Scheme 10.10), as proven by the X-ray crystal
structure.
38
The diradical
30
was produced earlier by UV irradiation of the frozen hexane solution (150
K) of the
gem
-dilithiosilane, [(R
2
SiLi
2
µ
-Li
·
THF)
2
(SiR
2
)
=
SiMe
t
Bu
2
)
, and its triplet state nature was
determined from the EPR spectrum, which consists of a signal with four
)
(R
2
HSiLi)
2
](R
=
SiMe
t
Bu
2
)
29
Si sidebands arising from
2 transition.
39
M
s
=
1 transitions and a signal at half-field (1675 G) resulting from the forbidden
M
s
=
In contrast to linear alkynes RC
≡
CR, the heavier group 14 congeners, RE
≡
ER (E
=
Si, Ge, Sn), assume
a
trans
-bent geometry with respect to the triple bond.
40,41
Similarly to the heavy alkene analogs R
2
E
=
ER
2
(E
Si, Sn) described above, this results in a relatively low energy level for the LUMO in the heavy
alkyne compounds. By exploiting the resulting lowered potential for reduction, the disilyne radical anion,
[RSi
=
SiR]
•
−
(
31
), is obtained directly from the neutral precursor by one-
electron reduction with KC
8
(Scheme 10.11).
40
≡
(R
=
Si(
i
Pr)[CH(SiMe
3
)
2
]
2
)
The analogous germylene and stannylene radical anions,
ER]
•
−
[RE
≡
(M
=
Ge, R
=
2,6-Tripp
2
C
6
H
3
(
32a
)or2,6-Dipp
2
C
6
H
3
(
32b
); E
=
Sn, R
=
2,6-Tripp
2
C
6
H
3
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