Square-Planar N₂S₂Ni^II Complexes with an Extended π-Conjugated System

The reaction of a mixture of 2-(1-naphthyl)benzothiazoline (HL¹) and 2,6-diphenylbenzo[1,2-d:4,5-d‘]bisthiazoline (H₃L²) with nickel(II) acetate tetrahydrate yielded three kinds of square-planar nickel(II) complexes:  one nickel(II) complex with innocent ligands ([Ni(L¹)₂] (1c)) and two nickel(II) complexes with non-innocent ligands ([Ni(L¹−L¹)] (1a) and [Ni(L¹−L²)] (1b)). The complex 1c has two bidentate-N,S ligands, which are formed via ring opening of HL¹. On the other hand, the two complexes 1a and 1b contain a tetradentate-N₂S₂ ligand, which is created via ring opening of HL¹ and H₃L², followed by bond formation between imino carbon atoms. Complexes 1a and 1b show very intense absorptions in the near-infrared (NIR) region, characteristic of square-planar complexes with non-innocent ligands. The third nickel(II) complex having a non-innocent tetradentate-N₂S₂ ligand ([Ni(L²−L²)] (2)) was prepared from H₃L² and nickel(II) acetate tetrahydrate. The electronic spectrum of 2 exhibits a very intense absorption at 981 nm (ε = 3.6 × 10⁴ M^-1 cm^-1), which is significantly red-shifted compared with those of 1a (837 nm and 4.4 × 10⁴ M^-1 cm^-1) and 1b (885 nm and 4.5 × 10⁴ M^-1 cm^-1), indicating the presence of an extended π delocalization. The reaction of 2,6-bis(3,5-dichlorophenyl)benzo[1,2-d:4,5-d‘]bisthiazoline (H₃L³) with nickel(II) acetate tetrahydrate also led to the formation of a nickel(II) complex with a non-innocent ligand ([Ni(L³−L³)] (3)). While complex 3 is analogous to 2, its electrical conductivity is much higher than that of 2. The molecular structures of 1b, 1c, 2, and 3 were determined by X-ray crystallography.