Nickel(II) Schiff base complexes: Synthesis, characterization and catalytic activity in Kumada–Corriu cross-coupling reactions

Abstract

The syntheses of ten new Ni(II) complexes 5a–5j with H2L tridentate (ONO) Schiff-base ligand (2-((2-oxybenzylidene)amino)phenolate) have been described and fully characterized by means of elemental analysis, FT–IR, electronic, 1H NMR and 13C NMR spectroscopy and single-crystal X-ray diffraction. In all [Ni(L)B] complexes (where B are imidazole, 4-benzylpyridine, 2-methyl-5-ethylpyridine, 3-hydroxymethylpyridine, 2-methylbenzimidazole, 2,6-dimethylpyridine, 4-methylpyridine, 2-isopropylbenzimidazole, 4-methylpiperidine or triphenylphosphine) the Schiff base completely deprotonates and coordinates to the metal ion as a dianionic tridentate ligand via the donor oxygens and nitrogen atoms. The coordination number of Ni(II) atoms is four with distorted square-planar stereochemistry. This is in a good agreement with the ligand field band position in their electronic spectra, as well as with the X-ray structure analysis for all complexes under study. Six selected complexes are used as catalysts for Kumada–Corriu cross-coupling reactions and exhibit a moderate to good catalytic activity in the synthesis of biaryl derivatives.