Oxorhenium(V) and Oxotechnetium(V) [NN][S]₃ Complexes of 2-Phenylbenzothiazole Derivatives

The reaction of 2-(2‘-pyridyl)benzothiazole, [NN], with the ReO(V)³⁺ and TcO(V)³⁺ cores in the presence of thiophenols, [S] (RC₆H₄SH, R = H, 4-CH₃, 4-OCH₃), as coligands led to the isolation of hexacoordinated complexes of the MO[NN][S]₃ type (M = Re, Tc). In all cases, two geometric mer isomers were formed, as evidenced by NMR spectroscopy and confirmed by X-ray crystallography. In both isomers, the coordination geometry about the metal ion is a distorted octahedral defined by the two nitrogen atoms of the bidentate ligand, the three sulfur atoms of the monodentate thiols, and the oxygen atom of the oxo group. The apical positions of the octahedron are occupied by the oxygen of the oxo group and, in one of the isomers, the nitrogen of the pyridyl moiety of 2-(2‘-pyridyl)benzothiazole, while, in the second isomer, the imine nitrogen of 2-(2‘-pyridyl)benzothiazole. The complexes are stable, neutral, and lipophilic. Complete ¹H and ¹³C NMR assignments are reported for all complexes. The synthetic reaction was also successfully transferred at the technetium-99m tracer level by ligand exchange reaction using ^99mTc−glucoheptonate as precursor in the presence of 2-(2‘-pyridyl)benzothiazole and 4-CH₃C₆H₄SH. The structure of the technetium-99m complex was established by high-performance liquid chromatographic comparison with the analogous oxotechnetium and oxorhenium complexes. The 2-(2‘-pyridyl)benzothiazole ligand serves as a preliminary model for 2-(4-aminophenyl)benzothiazole, which possesses interesting properties for the development of technetium and rhenium radiopharmaceuticals for tumor imaging and/or radiotherapy as well as in vivo diagnosis of Alzheimer's disease.