Synthesis and characterization of luminescent zinc(II) and cadmium(II) complexes with N,S-chelating schiff base ligands

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 rim. 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 [Cd-2(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..