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6.6.5
Catalyst Decomposition
The exact mechanism of catalyst decomposition for these complexes is not rigor-
ously known. Several general points need to be described, however. First, the de-
composition of the ruthenium carbenes is second order in catalyst. Thermolytic
half-lives for some of these complexes have been measured and some of these
data is tabulated in Tab. 6.6.
Second, the decomposition rate is retarded in the presence of excess phosphine.
This behavior indicates that it is the phosphine-dissociated species
I
d
that under-
goes the majority of the decomposition, presumably by some kind of dimerization
reaction to produce unknown inorganic products [105] (Scheme 6.31). This also
indicates that the rate of phosphine dissociation is critical to the rate of decompo-
sition, consistent with the observation that the second-generation catalysts are
more stable than the first [96].
Again, the methylidene complex shows different behavior than the other car-
benes. The decomposition of the methylidene is not affected by the presence of
excess phosphine and is first order in catalyst. The decomposition in this case ap-
pears to be due to activation of C-H bonds in the L ligand.
Tab. 6.6
Thermolytic half-lives at 55
C in benzene-
d
6
for selected complexes [96, 105].
Complex
a)
Half-life
6
8d
7
40 min
[Ru] =CHEt
8 h
[Ru]*=CH
2
6h
a)
[Ru] =RuCl
2
(PCy
3
)
2
; [Ru]*=RuCl
2
(PCy
3
)(SIMes).
Scheme 6.31
Decomposition processes: (a) general carbene (b) methylidene complex.
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