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In-Depth Information
100
(SiO)Zr(CCMe
3
)
3
80
d
n
9
r
4
n
g
|
1
(SiO)
2
Zr(CCMe
3
)
2
60
40
(SiO)
3
Zr(CCMe
3
)
20
0
150
200
250
300
350
400
450
500
T(°C)
Figure 1.2 Relative proportions of (
SiO)
x
Zr[CH
2
C(CH
3
)
3
]
4
x
, where x
¼
1-3, after
reaction of tetraneopentyl zirconium with silica dehydroxylated at vari-
ous temperature.
species.
62
This reaction is also very important in catalysis, for depolymer-
ization and hydrogenolysis or metathesis of alkanes. This point will be
discussed in more details in the catalysis part.
g-H abstraction has been proposed to occur during the thermolysis of
some tetraneopentyl complexes of group 4.
63
This reaction leads to the
evolution of neopentane and the formation of a metallacyclobutane. Even if
it is relatively uncommon, it has been observed during the thermolysis of
neopentyl hafnium complexes supported on silica.
64
.
1.4.3 Reaction with Hydrogen
When looking at all above surface complexes only those containing the
alkylidene ligand can be used in catalysis, for olefin metathesis. However,
highly active species for the activation of alkanes can be prepared by
hydrogenolysis. Treatment under hydrogen at moderate temperature (ca.
150 1C or below) of the alkyl, alkylidene and alkylidyne surface complexes
leads to the formation of surface hydrides which are highly electron deficient
and so potentially highly active catalysts. The expected reaction can simply
be written by replacing one alkyl ligand by one hydrogen, one alkylidene one
by two hydrogens and one alkylidyne one by three hydrogens with evolution
of the corresponding alkane. However, the resulting species are so electron
deficient and so reactive that they will react with neighboring siloxane
bridges
Si-O-Si
leading to the formation of M-O-Si
bonds with the
surface and the formation of a Si-H group. Thermodynamically this reaction
is highly favored. The result is a digrafted metal hydride which can further
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