By unsymmetrical derivatization of acetylacetone with S-ethyl-dithiocarbazide, a new tetradentate chelate ligand (N, N, S, S) has been prepared and studied. The compound can exist in two isomeric forms: an open tetradentate chelating form and a cyclic dihydropyrazole form. The cyclic form is obtained in the solid state and is the only one detected even in solution. However, in the presence of metal ions, a competition between intramolecular cyclization and metal complexation of the open form is active, and monochelate square planar complexes can be obtained through transformation of the cyclized into the open ligand form. We have proved this with metal ions prone to square planar coordination such as Ni(II) and Pd(II). On the other hand, with VO(II), Pb(II), UO2(II), La(III), Ce(III), no complex was obtained and the cyclized form was recovered from the reaction mixture. This indicates some selectivity of the ligand towards specific metal ions.

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