The application of “PNP” aminodiphosphine complexes in the oxidation of n-octane and styrene.
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Date
2015
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Abstract
The oxidation of hydrocarbons provides a cost effective method of converting cheap starting
material to bulk chemicals and more importantly in the synthetic transformation to fine
chemicals. Transition metals effectively catalyze these oxidation reactions. However, the use
of a good ligand system is imperative in controlling the activity of the metal complexes.
Aminodiphosphine or “PNP” ligands have been used extensively in ethylene oligomerisation
with chromium as the active metal.
In this study six PNP ligands were synthesized and the substituent on the nitrogen atom was
varied by making use of alkyl substituents such a cyclohexyl, iso-propyl and pentyl, as well as
phenyl and substituted phenyl (chlorophenyl and methoxy phenyl) substituents. The ligands
were complexed to the transition metals Co, Rh, Ir and Ru. These new bidentate complexes
were fully characterized by NMR analysis, IR spectroscopy, HRMS and melting point
determination. X-ray quality crystals were grown for eight of the metal complexes (all novel,
R% < 10).
These complexes were then compared in the oxidation of styrene and n-octane. This includes
the comparison of two structural types of "PNP" cobalt complexes having the cyclohexyl, isopropyl
and pentyl substituents on the nitrogen atom. In the oxidation of n-octane, the complex
with the flexible ligand backbone showed higher activity. The ketones were the dominant
product with highest selectivity to 2-octanone (34%). In the oxidation of styrene under
optimum conditions, the complexes bearing the rigid ligand backbone were most active with
good yields to benzaldehyde (25%).
In the oxidation of styrene, of the six Ir and Rh complexes investigated, the Ir complexes were
slightly more active than the Rh complexes, with the complex bearing the chlorophenyl
substituent on the nitrogen atom being the most active (88% conversion). Higher yields to
benzaldehyde than styrene oxide were obtained. In the oxidation of n-octane, the ketones were
the dominant product formed over both the Ir and Rh catalysts. For both studies the catalysts
were recovered and reused over 3 cycles.
Ruthenium catalysts bearing the alkyl substituents were also applied in both oxidation studies.
In the oxidation of styrene, > 80% conversion was obtained with a greater yield to
benzaldehyde. In the oxidation of n-octane, the alcohols were the dominant product with good
selectivity to 2 and 3-octanol (> 23%)
Description
Doctor of Philosophy in Chemistry. University of KwaZulu-Natal, Durban, 2015.
Keywords
Theses - Chemistry.