Abstract
M.Sc. (Chemistry)
A series of new Schiff-base ligands (L1 to L5) bearing sulfonate and carboxylate water
solubilising groups were successfully synthesized. These N^O and N^S donor
chelating ligands were prepared via condensation of fufural, 2-
thiophencarboxaldehyde, and sulfonate- and carboxylate-bearing salicylaldehyde,
with 4-aminobenzoic acid.
The salicyladimine ligands (L1-L3) were treated with [RhCl(COD)]2 (1,5-
cyclocotadiene) in the presence of a base to yield new square planar rhodium(I)
complexes (C1 to C3), while the furyl- and thiophenyl-imine ligands (L4 and L5) were
reacted with the rhodium dimer, [RhCl(COD)]2, without the aid of a base to afford
complexes (C4 and C5).
All the ligands and complexes were characterized using multinuclear NMR and
infrared spectroscopy, elemental analyses and high resolution mass spectrometry. In
addition, crystals of the salicyladimine rhodium(I) complex (C3) were obtained and the
molecular structure solved using single crystal X-ray diffraction.
All complexes were active catalyst precursors for the hydroformylation on 1-octene
and resulted in excellent substrate conversions (>98%) with 100% chemoselectivities
to aldehydes, under mild conditions. No alkene or aldehyde hydrogenation products
were observed at the optimum conditions, of 8 hours, 40 bars and 85 °C. However,
poor regioselectivities were observed as a mixture of linear and branched aldehydes
were obtained, with no clear dominance of one aldehyde (n/iso ratio ranging between
0.7-1.2) over the other.
These water-soluble (pre)catalyst were recycled up to three times with no significant
loss in catalytic activity and selectivity.
Mercury poisoning tests were conducted during typical hydroformylation reactions in
order to establish the nature of the active species. This revealed that the conversion
of 1-octene into aldehydes was due to molecular active catalysts and not as a result
of colloidal particles that could have formed in situ through the decomposition of the
catalyst precursor. Molecular modelling was used to propose the structure of the
resting state of the complex and a mechanism of the reaction has been proposed.