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Some useful results pertinent to the thermal decomposition of
metallacycloalkanes emerged from the current study. The rate of
decomposition of platinacycloalkanes in cyclohexane is increased
by adding dppp and dppe ligands respectively. The effect of the
additional ligands could be that of facilitating the reductive elimi-
nation of alkenes from a five-coordinated metalehydride inter-
mediate. The relative amount of 1-octene formed from the thermal
decomposition of platinacyclononanes is dependent on the
decomposition temperature and time. The formation of the total
products including 1-octene, 2-octenes, n-octane, 1,7-octadiene
and cyclooctane is first order for about the first 30% of the
decomposition. These results taken together demonstrate that
changes of decomposition medium, ring size, supporting ligands
and metal centres have significant effects on the decomposition
pathways; moreover, these factors seem to have a cooperative
effect in the decomposition of certain metallacycloalkanes. The
possible decomposition pathways involved in the thermolysis of
these metallacycloalkanes could be:
b-hydride transfer or an
intermolecular chain reaction to form 1-alkene; intermolecular or
intramolecular isomerisation for 2-alkenes; the formation of
n-alkane by an intermolecular hydrogen abstraction, or alterna-
tively an intramolecular hydrogen abstraction from supporting
ligands. The thermolysis data in this study suggest that the medium
to large metallacycloalkanes can be useful models for the inter-
mediates in selective catalytic oligomerisation reactions, particu-
larly ethylene trimerisation and tetramerisation reactions.
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Acknowledgements
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We thank University of Cape Town, Anglo Platinum, Johnson &
Matthey for the support and Prof. Luigi Nassimbeni for helpful
discussions. Dr. ASRK is highly grateful to VIT University for the
support.
[39]
%
Decomposition
¼
[total hydrocarbon products]/[Pt compound]
0 ꢀ 100% ¼ (total amount of hydrocarbon products)/(theoretical amount of
hydrocarbon products) ꢀ 100%; [100% decomposition means that the theo-
retical concentration of the total decomposition products should be equal to
the initial concentration of Pt compound].
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