Browsing by Author "Naicker, Dunesha."

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The application of “PNP” aminodiphosphine complexes in the oxidation of n-octane and styrene.
(2015) Naicker, Dunesha.; Friedrich, Holger Bernhard.
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%)
The oxidation of n-octane by iridium and cobalt PNP complexes.
(2011) Naicker, Dunesha.; Friedrich, Holger Bernhard.
Paraffin activation has practical implications in the replacement of current petrochemical feedstocks (olefins), by utilizing economical and easily accessible alkanes, which may result in more efficient strategies for fine chemical synthesis and the proficient use of energy. However, the chemical inertness of paraffins limits their conversion to more valuable products. Several pincer chelate complexes are utilized in stoichiometric and catalytic C–H activation. These pincer ligands have attained much interest in that they are part of a system, which displays high stability, activity and variability. In this study four aminodiphosphine (PNP) pincer ligands were successfully synthesized and characterized by NMR, IR and HRMS. To investigate the steric effects on the metal center, four different functional groups on the nitrogen atom were used, a cyclic ring (cyclohexyl (3.1)) branched chain (iso-propyl (3.2)); straight chain (pentyl (3.3)); and aromatic ring (benzyl (3.4)). The ligands were successfully complexed to the transition metals iridium and cobalt and characterized by elemental analyses, IR, HRMS and thermogravimetric measurements. The thermal behaviour of the ligands showed that ligands 3.1-3.3 displayed similar decomposition patterns. Similar fragmentation patterns were observed for the iridium and cobalt complexes containing ligands 3.1 and 3.3. The complexes were tested in the oxidation of n-octane in two solvent systems, DCM and MeCN with H2O2 and t-BuOOH as the oxidants. The optimum substrate to oxidant ratio was found to be 1:5. No conversion was observed with H2O2. The conversion in DCM for the iridium catalysts was much higher than that of the cobalt catalyst. However, higher conversion was obtained in MeCN for the cobalt catalysts. No conversion was observed for the iridium catalyst in MeCN. The selectivity to ketones was much higher than to the alcohols, with only the C(1) position being most selective to the alcohols. The in situ, single pot testing of n-octane using a ruthenium precursor and ligand 3.1- 3.4 undertaken in DCM showed no conversion, whilst in MeCN a conversion of 17% was observed. The selectivity was similar to that obtained by the cobalt catalysts in MeCN. All testing showed that the catalyst containing ligand 3.1 was the most active giving the highest conversions in different solvent systems, which is attributed to the bite angle effect.