Synthesis, Stability, and Complexation Behavior of Isolable Salen-Type N₂S₂ and N₂SO Ligands Based on Thiol and Oxime Functionalities^†

The new salen-type N₂S₂ tetradentate ligands, H₂L¹ and H₂L², which have a donor set comprising oxime and thiol groups, were synthesized. These ligands are obtained as isolable colorless crystals, whereas the imine analogues are too unstable to be isolated. The unsymmetrical N₂SO ligands, H₂L³ and H₂L⁴, were also obtained as stable compounds. When ligands H₂L¹−H₂L⁴ are heated above the melting points, they mainly decompose via cleavage of the N−O bonds of a thiosalicylaldoxime moiety to give 1,2-benzisothiazole derivatives. The complexation of the N₂S₂ ligands (H₂L¹ and H₂L²) with nickel(II) acetate afforded square-planar mononuclear complexes [Ni(L¹)] and [Ni(L²)], respectively. In contrast, the complexation of the N₂SO ligand H₂L³ with nickel(II) acetate resulted in cleavage of the N−O bond, giving a tetranuclear complex having a cubane-type Ni₄O₄ core. The N−O bonds of H₂L¹−H₂L⁴ are more readily cleaved when the ligands are allowed to react with copper(II) acetate. In these cases, the alkoxo-bridged dinuclear complexes having a Cu−O−Cu−O four-membered ring are obtained. On the other hand, mononuclear complexes can be obtained by complexation of the ligands (H₂L¹ or H₂L³) with palladium(II) acetate without N−O bond cleavage.