Stabilization of gold(I) and gold(III) complexes by pyridil bis{3-hexamethylene-iminylthiosemicarbazone}: spectroscopic, structural and computational study

Abstract

The reaction of pyridil bis{3-hexamethylene-iminylthiosemicarbazone} (H₂Plhexim) with dichloro[2-(dimethylaminomethyl)phenyl-C¹,N]-gold(III) ([Au(damp-C¹,N)Cl₂]) led to the isolation of the complexes [Au₂(H₂Plhexim)(Hdamp-C¹)₂Cl₄]Cl₂·2H₂O (1) and [Au(H₂Plhexim)]₂Cl₂·S₈·6H₂O (2). The spectroscopic data for 1 suggests that the bisthiosemicarbazone is coordinated to two different Au(III) atoms through the sulfur atoms. The fourfold coordination of the gold atoms is completed by one protonated Hdamp ligand, with the corresponding cleavage of an Au–N bond, and by two chloro ligands. The coordination mode and structure has been rationalized by means of DFT calculations. The crystal structure of 2 shows the neutral bisthiosemicarbazone coordinated through the sulfur atoms in a linear S–Au–S geometry. Gold–gold interactions are not observed and this is due to the steric hindrance of the bulky groups present in the ligand, which prevent the approach of two neighboring gold atoms. During the reaction, the reduction of Au(III) to Au(I) takes place with the bisthiosemicarbazone oxidized to the elemental sulfur, S₈, which is present in the complex. (H₂O)₁₂clusters are also located in the void spaces. The in vitro antitumor activity against the HeLa-299 cell line was also investigated. The IC₅₀ values are slightly higher than that of cisplatin. In comparison to the value for cisplatin, the maximum antiproliferative activity is higher in the Au(I) complex and similar in Au(III) complex, thus, the Au(I) complex exhibits greater activity against the human cervix carcinoma cell line.