Synthesis, spectral characterization and crystal structure of Ni(II) pyridoxal thiosemicarbazone complexes and their recyclable catalytic application in the nitroaldol (Henry) reaction in ionic liquid media

Abstract

The new square planar nickel(II) complexes 1, 2 and 3 have been synthesized from the reaction of [NiCl2(PPh3)2] with the tridentate Schiff base ligand, pyridoxal thiosemicarbazone (L1), pyridoxal N-methyl thiosemicarbazone (L2) and pyridoxal N-phenyl thiosemicarbazone (L3) respectively in ethanol. These complexes have been characterized by elemental analyses, IR, UV–Vis, 1H NMR, 31P NMR and ESI-MS spectroscopy. The molecular structure of the complex [Ni(L2)PPh3] (2) was determined by single-crystal X-ray diffraction, which reveals a distorted square planar geometry around the nickel(II) ion. The nitroaldol reaction was studied in detail using the nickel(II) complexes as catalysts in a homogeneous solution formed by an ionic liquid and methanol. The effect of solvent, ionic liquid, time, temperature, catalyst loading and substituent of the ligand moiety on the reaction was also studied. The β-nitroalcohol products were obtained in good yields of up to 97%. A two step substrate addition mechanism was tentatively proposed based on ESI-MS spectral monitoring of the reaction mass.