Synthesis, characterization and single crystal X-ray studies of pincer type Ni(II)-Schiff base complexes: Application in synthesis of 2-substituted benzimidazoles

Abstract

Five new pincer type Ni(II)-Schiff base complexes of the general formula [NiL1(PPh3)] 1, [NiL2(PPh3)] 2, [NiL3(PPh3)] 3, [NiL4(PPh3)] 4 and [NiL4(4-MePy)] 5 [where H2L1 = 2-(2,3-dihydroxybenzylideneamino)phenol, H2L2 = N-(2,3-dihydroxybenzylidene)benzohydrazide, H2L3 = 2-(2,3-dihydroxybenzylidene)hydrazinecarbothioamide, H2L4 = 5-(diethylamino)-2-(2-hydroxybenzylideneamino)phenol, 4-MePy = 4-Methylpyridine] were synthesized by the reaction of the Ni(OAc)2.4H2O with the corresponding Schiff base ligand in methanol as coloured crystalline solids in high yields. All the five complexes were fully characterized by FT-IR, UV–Vis, 1H NMR, 13C{1H} NMR, 31P{1H} NMR, mass spectrometry and single crystal X-ray diffraction studies. The crystal structures of all five new complexes confirmed the tridentate nature of the pincer type Schiff base ligands (ONO and ONS) and distorted square planar geometry around the metal centre in all cases. The monodentate ligand (triphenylphosphine/4-Methylpyridine) occupied the fourth site at nickel. The catalytic potential of the complexes has been demonstrated in the synthesis of a series of 2-substituted benzimidazoles at room temperature using low catalyst loading (0.5 mol %), and without the use of any additives. All organic products were isolated in high yields (85–96%) and fully characterized by 1H and 13C{1H} NMR studies.