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-H2L1 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(L-1)] and [Ni(L-2)], 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.