Synthesis and Characterization of Luminescent Zinc(II) and Cadmium(II) Complexes with N,S-Chelating Schiff Base Ligands

Abstract

The reactions of zinc(II) acetate with a variety of 2-substituted benzothiazolines afforded tetrahedral mononuclear complexes with a N₂S₂ donor set, [Zn(R—Ph—C(H)═N—C₆H₄—S)₂]. The obtained zinc(II) complexes can be divided into three groups based on the characteristics of the absorption spectra; Group 1 (R = 2,4,6-triMe (1), 2,6-diCl (2)) showing an intense band at 250−300 nm and a weak band at 400−450 nm, Group 2 (R = 4-Cl (3), H (4), 4-Et (5), 4-OMe (6)) showing two intense bands at 250−300 nm and a weak band at 400−450 nm, and Group 3 (R = 4-NMe₂ (7), 4-NEt₂ (8)) showing an intense band at 250−300 nm and two very intense bands at 350−450 nm. The Group 2 and Group 3 complexes exhibited a strong emission on irradiating with ultraviolet light while the Group 1 complexes were not emissive at room temperature. However, all the zinc(II) complexes were luminescent in CH₂Cl₂/toluene glass at 77 K, and their emission peak energies were found to correlate with the Hammett constant of the substituent at para position of a pendent phenyl ring in each complex. Similar reactions of cadmium(II) acetate with 2-substituted benzothiazolines were also carried out to synthesize corresponding cadmium(II) complexes. While [Cd(R—Ph—C(H)═N—C₆H₄—S)₂] (R = 2,4,6-triMe (9)) with bulky substituents at ortho positions of a pendent phenyl ring had a tetrahedral mononuclear structure, other cadmium(II) complexes [Cd₂(R—Ph—C(H)═N—C₆H₄—S)₄] (R = 4-Et (10), 4-OMe (11), 4-NMe₂ (12)) possessed S-bridged dinuclear structures. These cadmium(II) complexes, which are assumed to have a mononuclear structure in solution, showed photophysical properties similar to those of the corresponding zinc(II) complexes.