Cu(I) complexes with thiosemicarbazides derived from p-toluenesulfohydrazide: Structural, luminescence and biological studies

Abstract

Reactions of thiosemicarbazide ligands (LR: R = cyclohexyl (Cy) or phenyl (Ph)) with [CuCl(PPh3)3] led to the formation of colorless Cu(I) complexes of composition [CuCl(PPh3)2(LR)]. Both complexes were characterized by spectroscopic methods and further studied by single crystal X-ray diffractometry. The crystal structures confirmed the tetrahedral geometry for the Cu(I) metal center, which is coordinated by a chlorido, two triphenylphosphane and by the thiosemicarbazide as a neutral S-donor monodentate ligand. Photophysical studies revealed that the complexes exhibit emission at room temperature with maxima around 480 nm. At 77 K, the emission is shifted to higher energy, a characteristic behavior of MLCT emitters. The main low lying molecular orbitals were calculated by TD-DFT and confirmed that the S0 → S1 transitions have mainly MLCT character as well as that the peripheral groups in the thiosemicarbazide ligand play an important role on this transition. Moreover, the emission bands are considerably suppressed in the presence of O2, indicating the triplet character of the emitter excited states. The intersystem crossing process is also suggested by the energy diagrams built through TD-DFT calculations. Overall, both experimental and theoretical analysis suggest 3MLCT radiative decays occurrence for the complexes. Furthermore, biological assays showed that the complexes are active on the intracellular amastigote form of Trypanosoma cruzi (Tulahuen Lac-Z strain), and present a significant cytotoxic effect against metastatic melanoma cells. Finally, no significant toxicity to LLC-MK2 cells was observed, resulting in a therapeutic index of about 30 for one of the complexes, in the same order of magnitude as that of the standard drug benznidazole.