Zinc(II) complexes derived from the di-Schiff-base ligand N,N′-bis[1(pyridin-2-yl)ethylidene]ethane-1,2-diamine (LA) and its hydrolytic-cleavage product N-[1-pyridin-2-yl)ethylidene]ethane-1,2-diamine (L):: preparation, characterization and crystal structure of the 5-coordinate species [ZnLCl2]

The reactions of ZnX2 (X=Cl, Br) with the di-Schiff-base ligand N,N'-bis[1-(pyridin-2-yl)ethylidene]ethane-1,2-diamine (L-A) were studied. The chemical identity of the products is solvent-dependent. The reactions in tetrahydrofuran give complexes [ZnLACl2] (I) and [ZnLABr2] (2). Treatment of L-A with an equimolar, or excess, amount of ZnX2 in ethanol yields the five-coordinate complexes [ZnLCl2] (3) and [ZnLBr2] (4), where L is the mono-Schiff base N-[1-pyridin-2-yl)ethylidene]-ethane-1,2-diamine complexes 3 and 4 have resulted from hydrolytic cleavage of the precursor di-Schiff base L-A. These two complexes can also be prepared in one step and in good yield by the reactions of ZnX2 with 2 acetylpyridine and ethane-1,2-diamine in refluxing methanol. The X-ray structure of 3 demonstrates that L behaves as a tridentate chelate with the ligated atoms being the pyridine, methine and amino nitrogens; two terminal, equatorial chlorides complete five-coordination at zinc(II). The new complexes 1-4 were characterized by elemental analyses and spectroscopic (IR, far-IR,H-1 NMR) techniques. It is proposed that L-A behaves as a tetradentate chelate in complexes 1 and 2, for which a monomeric pseudo-octahedral stereochemistry with trans chloro ligands is assigned in the solid state. All data are discussed in terms of the nature of bonding and known or assigned structures. (C) 1999 Elsevier Science Ltd. All nights reserved.