Zinc(II) complexes derived from the di-Schiff-base ligand N,N '-bis[1(pyridin-2-yl)ethylidene]ethane-1,2-diamine (L-A) 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 chemica l identity of the products is solvent-dependent. The reactions in tetrahydr ofuran give complexes [ZnLACl2] (I) and [ZnLABr2] (2). Treatment of L-A wit h an equimolar, or excess, amount of ZnX2 in ethanol yields the five-coordi nate complexes [ZnLCl2] (3) and [ZnLBr2] (4), where L is the mono-Schiff ba se 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. Thes e two complexes can also be prepared in one step and in good yield by the r eactions 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 trident ate chelate with the ligated atoms being the pyridine, methine and amino ni trogens; two terminal, equatorial chlorides complete five-coordination at z inc(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 beh aves 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 a nd known or assigned structures. (C) 1999 Elsevier Science Ltd. All nights reserved.