Abstract
M.Sc. (Chemistry)
This study describes the synthesis of new pyrazolyl(pyridyl) Ru(II) and Rh(I/III) complexes and their application in the catalytic transfer hydrogenation of furfural and levulinic acid.
The transition metal complexes were synthesized by complexation of the tridentate N^N^N ligands (E)-N-(pyridin-2-methylene)-2-(1H-pyrazol-1-yl) ethanamine (L1), 2,6-bis((3,5-diphenyl-1H-pyrazol-1-yl)methyl)pyridine (L2), 2,6-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)pyridine (L3) and 2,6-bis((1H-pyrazol-1-yl)methyl)pyridine (L4) to the dimeric metal precursors [Rh(CO)2Cl]2 or [Ru(CO)3Cl2]2. Complexation of L1 led to the isolation of bis-ligand octahedral complexes (C1, C1A and C2), while complexes C3–C8 were afforded after reacting [Rh(CO)2Cl]2 or [Ru(CO)3Cl2]2 with ligands L2 - L4. All the complexes are new and have been fully characterized by NMR spectroscopy 1H, 13C{1H}, 31P{1H}, 19F{1H}, FT-IR spectroscopy, high resolution mass spectrometry, elemental analysis and single crystal X-ray diffraction (C2, C3 and C7A).
When evaluated for their efficacy in hydrogenation of furfural and levulinic acid, C1, C1A and C2 were found to have poor activity (turnover frequencies: 33.3, 34.2 and 15.4; respectively), possibly due to the stable octahedral coordination around the metal centres. Complexes C3–C8 were also evaluated as catalyst precursors in a base promoted hydrogenation of furfural and levulinic acid using formic acid as a hydrogen source. All the ruthenium(II) complexes were generally more active compared to the rhodium(I) counterparts. Complex C6 was found to be the most active; resulting in turnover frequencies as high as 550 h-1 in the hydrogenation of levulinic acid. Levulinic acid was completely converted to γ-valerolactone selectively at a catalyst loading of 0.1 mol% at 150 ℃ in 4 h. Full conversion of furfural to furfural alcohol and difurfuryl ether was also achieved using a 0.1 mol% catalyst loading at 130 ℃ over 4 h...