Chemistry Reference
In-Depth Information
R'
R'
R'
R'
•
R'
R'
∆
∆
M
Al
Al
MM
R'
(M = Al)
•
M
M
R'
(R' = Si
t
Bu
3
)
R'
R'
R'
6a
(M = Al)
6b
(M = Ga)
5a
(M = Al)
5b
(M = Ga)
Na
(M = Ga)
6 NaR'
R'Br or
TCNE
6 NaR'
R'Br
(M = Al)
(M = Ga)
R'
−
Cl
−
R'
18-crown-6
Ga
2
Ga
Ga
M
R'
Ga
Ga
R'
Cl
Cl
R'
R'
R'
[Na(18-crown-6)(THF)
2
]
+
[Na(THF)
3
]
+
Scheme 10.4
as blue-black crystals from the reaction between NaR
and gallium trichloride (GaCl
3
)
in 3 : 1 molar
ratio (Scheme 10.4).
12a
The structure of
5b
exhibits a trigonal planar Si
2
Ga
II
Ga
I
unit and a slightly bent
Ga
II
Ga
I
Si chain (
Ga
II
6
◦
Ga
II
-Ga
I
-Si 170
3
◦
)
∠
359
.
and
∠
.
(the superscripts I and II refer to the formal
bond is formed by overlap of the sp
2
- and sp-
hybridized gallium atoms. The odd electron occupies a bonding
oxidation states of the gallium centers). The Ga-Ga
σ
molecular orbital formed by overlap of
the p
z
orbitals on the two gallium atoms resulting in a formal bond order of 1.5 for the Ga-Ga bond. The
EPR spectrum of
5b
shows a 64-line signal (
g
π
=
1
.
9947) arising from coupling of the unpaired electron
with the two inequivalent gallium centers (
69
Ga,
I
71
Ga,
I
2, 30.9 %).
12a
=
3
/
2, 60.1 %;
=
3
/
The two
different hfcc values (
a
1
(
69
/
71
Ga)
50/64 G and
a
2
(
69
/
71
Ga)
=
=
32/41 G) could not be assigned to the two-
and three-coordinate gallium atoms with certainty.
The three-membered ring [Ga
3
R
4
]
•
(R
=
(
6b
), the gallium analogue of
6a
, is synthesized either
by oxidation of the acyclic anion [R
2
Ga-GaR
-GaR
]
−
or by the reaction of [R
2
GaGaR
]
−
with R
Br
(Scheme 10.4).
12b
In contrast to the aluminum derivative
6a
, the neutral radical
6b
exhibits a Ga-Ga bond
length between the two three-coordinate galliums that is about 0.35 A longer than those involving the four-
coordinate R
2
Ga unit (2.879(1) vs 2.527(1) A), implying the onset of Ga-Ga bond cleavage. In solution,
the three-membered ring
6b
decomposes to give the EPR signal of the acyclic radical [R
2
GaGaR
]
•
(
5b
)
either after prolonged standing or upon warming; the initial EPR signal arising from
6b
is not sufficiently
resolved to be unambiguously analyzed.
12b
The dark crimson neutral radical [Al(AlR
3
)
2
]
•
(R
Si
t
Bu
3
)
(
7
) is isolated in 15 % yield from the
reaction of a metastable solution of aluminum chloride (AlCl) in toluene/diethyl ether with an equimolar
amount of LiR at
=
N(SiMe
2
Ph)
2
)
78
◦
C.
13
The structure of the
D
3
d
-symmetric Al
7
cluster is comprised of an aluminum
atom linked symmetrically to two Al
3
rings (Figure 10.1). The Al-Al bonds within the three-membered
rings are significantly shorter than those involving the central aluminum atom (2.61 vs 2.73 A). On the
basis of detailed bonding analysis of
7
and the corresponding monoanion [Al(AlR
3
)
2
]
−
(R
−
=
N(SiMe
3
)
2
)
(
8
), this fascinating cluster radical is referred to as a “metalloid” system.
13
10.2.2.2 Neutral radicals: heterocyclic systems
Two classes of heterocyclic radicals involving group 13 elements have been thoroughly investigated.
The first involves a series of neutral bis-boraamidinate (
bam
) radicals,
•
(M
B,
Al, Ga, In) (
9a
-
d
) (Scheme 10.5), which are obtained by one-electron oxidation of the corresponding
monoanions. The aluminum (
9b
) and gallium (
9c
) derivatives form thermally stable dark red and green
{
M[PhB(
µ
-N
t
Bu)
2
]
2
}
=
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