Nickel(II) and copper(II) complexes of tetradentate unsymmetrical Schiff base ligands: First evidence of positional isomerism in such system

The 1:1 condensation of 2,4-pentanedione and 1,2-diaminopropane gives a mixture two positional isomers of tridentate mono-condensed product 7-amino-4-methyl-5-aza-3-octene-2-one (HAMAO) and 7-amino-4,6-dimethyl-5-aza-3-heptene-2-one (HADAH) that reacted readily with Ni(II) thiocyanate to yield exclusively a single product, [Ni(AMAO)NCS] (1) in which the methyl substituent of diamine is ‘remote’ from the imino nitrogen. The mixture of terdentate ligands has been used for further condensation with pyridine-2-carboxaldehyde or 2-acetylpyridine to obtain the unsymmetrical tetradentate Schiff base ligands. The tetradentate ligands formed by the condensation of it and pyridine-2-carboxaldehyde readily yielded complexes with Cu(II) and Ni(II) (2 and 3, respectively). Crystal structure analysis shows that in 2 the condensation site of the diamine with 2,4-pentanedione is the same as in 1 but that in 3 is different (the methyl group of the diamine is located in the vicinity of 2,4-pentanedione), i.e., the tetradentate ligand is in two different isomeric forms in complexes 2 and 3. Another tetradentate ligand, obtained by the condensation of the tridentate ligands and 2-acetylpyridine yielded a Ni(II) complex (4) where the methyl group is in the vicinity of 2,4-pentanedione as in 3. The isomerization in the Ni(II) complexes has been studied by NMR spectroscopy.