Synthesis, characterization and catalytic activity of copper thiosemicarbazone complexes and their oxides in the reduction of nitroaromatic compounds

Abstract

This study involves the synthesis, characterization and evaluation of the catalytic activity of copper thiosemicarbazone complexes and their copper oxides derivatives in the reduction of nitroaromatic compounds. A series of copper complexes of thiosemicarbazone ligands have successfully been synthesized, in which six of them are copper(I) complexes; 2- acetylpyridine-N(4)-(R)-thiosemicarbazone-tris- (triphenylphosphine)copper(I) nitrate {R = methoxyphenyl (19), R = methylphenyl (20), R = phenyl (21) and pyrrole-2-carboxaldehyde- N(4)-(R)-thiosemicarbazonebis( triphenylphosphine)copper(I) nitrate {where R = phenyl (22), R= methoxyphenyl (23), R = methylphenyl (24)} and another six copper(II) complexes; 2- acetylpyridine- N(4)-(R)-thiosemicarbazone-triphenylphosphine-copper(II) chloride; {R = methoxyphenyl (25), R = methylphenyl (26), R = phenyl (27)} and pyrrole-2- carboxaldehyde- N(4)-(R)-thiosemicarbazone-triphenylphosphine-copper(II) chloride (R = phenyl (28), R = methoxyphenyl (29), R = methylphenyl (30)}. These complexes were characterized using Fourier transform infrared (FTIR), UV-visible (UV-Vis) and proton nuclear magnetic resonance (1H-NMR) spectroscopic techniques. Single crystal X-ray diffraction analysis on complex 23 showed that the complex adopted a distorted tetrahedral geometry, where the central copper(I) ion is bonded to nitrogen and sulphur atoms of thiosemicarbazone bidentate ligand and two phosphorus atoms from two triphenylphosphine monodentate ligands while a nitrate ion acted as the counter ion. Molar conductivity value of the complex indicated a 1:1 electrolytic nature which supported the single crystal X-ray diffraction data. The copper complex 23 was converted into copper oxide by means of thermal decomposition. Evaluation of the catalytic performance of the copper(I) complex and copper oxide in the reduction of 4- nitrophenol (4-NP) to 4-aminophenol (4-AP) shows that copper oxide has a higher catalytic activity (98.7%) compared to the copper(I) complex (78.2%). Optimization of the catalyst loading revealed that 1.0 mol% of the catalyst was the most optimized amount with the highest conversion (98.7%). Reproducibility and recyclability tests of the copper oxide catalyst proved that the catalyst exhibits consistent catalytic performances and could be reused four times without a significant decrease. On the other hand, the copper(I) complex required a more prolonged reduction time and a higher amount of catalyst loading due to its insolubility in an aqueous solution. The product from the catalytic reduction, 4-AP was isolated, purified and characterized using FTIR and 1H-NMR spectroscopic techniques. The catalytic activity of the copper oxide catalyst was also evaluated in the reduction of other nitroaromatic compounds with various substituent groups. In these reactions, the copper oxide maintained its excellent catalytic activity and showed consistent results.