Cytotoxic Vanadium Complexes of Branched [ONNO]-Type Diamine Bis(phenolato) Ligands

Vanadium complexes are attractive potential alternatives to platinum-based anticancer drugs. Two vanadium(V) complexes, based on a common chelating tetradentate diaminobis(phenolato) ligand featuring a branched connectivity but differing in their labile ligands, were synthesized and characterized. Whereas the isopropoxido complex was obtained as a mixture of cis and trans isomers with regard to the orientation of the labile group vs. the amine sidearm, the salicylato-containing complex was obtained as a single trans isomer. X-ray structures of the complexes featured octahedral geometry for both. The two complexes exhibited high cytotoxic activity toward different cancer cells, often higher than that of cisplatin, including toward cisplatin-resistant ovarian cancer cells. These complexes demonstrated rapid hydrolysis, releasing the labile ligand within several minutes, with no indications of release of free chelating ligand after water exposure, suggesting that polynuclear hydrolysis products are involved in the cellular activity.