The oxidation of n-octane by iridium and cobalt PNP complexes.
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Date
2011
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Abstract
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.
Description
Thesis (M. Sc.)-University of KwaZulu-Natal, Durban, 2011.
Keywords
Alkanes., Chemical reactions., Iridium., Transition metals., Cobalt., Theses--Chemistry.