The application of “PNP” aminodiphosphine complexes in the oxidation of n-octane and styrene.
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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%)