In the last decade N-heterocyclic carbene (NHC) ligands have become important in organometallic
chemistry and homogeneous catalysis, rivalling the well established phosphines. Most of the
current attention to date has focused on the NHC complexes of the platinum group metals (rhodium,
palladium and nickel) plus ruthenium based system, but the chemistry of NHC systems of iron
which is relatively inexpensive and environmentally friendlier is considerably less developed.
Thus, this project involves the design, synthesis, characterization and application in catalytic
transfer hydrogenation of NHC ligands and their iron(II) complexes.
The motivation for the choice of NHC as a ligand stems from the ability to systematically tune the
ligand both electronically and sterically in addition to the stability and robustness of the ligand to
stabilize metal centres in various environments. In this research imidazolium based NHCs are
generated. Thus, three different series of imidazolium salts were synthesized and their iron(II)
complexes was obtained. All the compounds were characterized by spectroscopic and
crystallographic methods. These are: (a) 1,3-dialkylimidazolium salts (b) 1,3-diarylimidazolium
salts and (c) ferrocenylimidazolium salts bearing methyl and phenyl spacers between the ferrocenyl
and the imidazolium moieties. A total of 20 novel compounds were synthesized and are reported in this thesis. Furthermore, the application of the new compounds as transfer hydrogenation catalysts was
investigated using 17 saturated and unsaturated ketones as substrates, in the presence of KOH as the
base and 2-propanol as the hydrogen source. The dialkylated NHC iron(II) complexes showed
excellent yields, and TON values of up to 200 were achieved under the optimized reaction
conditions. Without complexation with iron, the 1,3-diarylimidazolium and ferrocenylimidazolium
series of salts were also found to be active catalysts for the transfer hydrogenation reaction of
ketones in alcoholic media. In the case of ferrocenylimidazolium salts a TON value up to 1880 was
achieved. Notably, two of the unsaturated ketones were successfully converted at a high yield with
a high selectivity to the corresponding saturated ketones only.
In addition, the stability of NHC ligands to moisture was investigated, since an understanding of the
stability of various deprotonated NHC-based imidazolium cations to attack by moisture resulting in
hydrolysis products is very important to understanding the coordination chemistry of the ligands on
to metal centres. Four novel ionic diamino aldehyde compounds were obtained by moisture attack
on saturated NHC ligands. The route to the formation of the hydrolysed compounds is formulated
to occur via an imidazolinium ring opening process. On the other hand the unsaturated counterparts
were more stable towards hydrolysis yielding adducts with the iron(II) precursors.
Finally, the electrochemical properties of the ferrocenylimidazolium salts were investigated using
cyclic voltametry. By comparing the relative shifts in the formal electrode potentials of the
ferrocene/ferrocenium coupled with the ferrocenylimidazolium salts, it was easy to evaluate the
influence of the substituents on the carbene containing imidazolium moiety on the electrochemical
properties of the iron centres. The formal electrode potential of the ferrocenylimidazolium salts
shifted to higher positive potentials as compared to ferrocene, indicating a high electron
withdrawing effect of the imidazolium salts. This makes the metal centres more vulnerable to
attack by nucleophiles. The electrochemical studies have enabled a structure-activity correlation to
be drawn for the various ferrocenylimidazolium salts. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10079 |
| Date | 27 November 2013 |
| Creators | Ikhile, Monisola Itohan. |
| Contributors | Bala, Muhammad D., Nyamori, Vincent O. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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