Abstract
Schiff base ligands 4,4'-((1Z)-butane-2,3-diylidenebis(azanylylidene))dibenzoic acid (L1), (E)-4-(((4-carboxyphenyl)imino)methyl)-3-hydroxybenzoic acid (L2), E)-4-((4-carboxybenzylidene)amino)-3-hydroxybenzoic acid (L3) and (1E,1'E)-1,1'-(1,3-phenylene)bis(N-propylmethanimine (L4) were synthesised and fully characterized. New imine platinum group metals (PGM) complexes (C1-C10) of RuII, IrIII, RhIII and PdII were synthesised from L1-L4 and [RuCl2(p-cymene)]2 and [Cp*MCl2]2 (where M = Ir and Rh) precursors. The ligands and complexes were characterized by 1H and 13C{1H} NMR spectroscopy, FT-IR spectroscopy, elemental analysis, HR-ESI mass spectrometry and single crystal X-ray crystallography among other techniques. These new complexes were evaluated for the homogeneous hydrogenation of CO2 to formate. New cationic α-diimine complexes of RuII (C1), RhIII (C2), IrIII (C3) complexed to 4,4'-((1Z)-butane-2,3-diylidenebis(azanylylidene))dibenzoic acid, ligand L1 were synthesized, fully characterized and tested as pre-catalysts for the catalytic hydrogenation of CO2. All three catalyst precursors hydrogenated CO2 to formate in the presence of a biphasic solvent mixture of THF/H2O and KOH as a base at 120 oC. C1 was the best performing pre-catalyst with a TON of 322 and TOF of 35 h-1 achieved in 2 h. Complexes C1-C3 showed excellent chemo-selectivity in conversion of CO2 to formate while stability studies revealed that C1 could be excellently reused at least three times. In view of the findings from this study, the newly synthesized N^N complexes prove to be effective pre-catalysts and offers extraordinary selectivity in conversion of CO2 to formate under moderate conditions in homogeneous phase. A plausible mechanism supported by density function theory (DFT) calculations postulates formation of a 16-electron ruthenium complex to form a ruthenium hydride active species which can hydrogenate CO2...
Ph.D. (Chemistry)