Synthesis of Pure Enantiomers of Titanium(IV) Complexes with Chiral Diaminobis(phenolato) Ligands and Their Biological Reactivity

Racemic and enantiomerically pure titanium(IV) complexes with ortho-brominated or para-nitrated chiral diaminobis(phenolato) ligands were prepared with NH and NMe cyclohexyldiamino bridges through ligand to metal chiral induction. The hydrolytic behavior of the complexes was evaluated, identifying the N-methylated complex as the most stable. A representative NH complex hydrolyzed to first give a dimeric structure in solution as deduced by NMR diffusion measurements, followed by formation of clusters with higher nuclearity, as was supported by X-ray characterization of a tetranuclear cluster obtained in trace amounts following 30 days in water solutions. The cytotoxicity of the enantiomerically pure and racemic complexes was measured on HT-29 human colon cancer cell line based on the MTT assay; all stereochemical configurations of the N-methylated complex were inactive, whereas for the NH complexes, the racemic mixtures were mostly inactive but the pure enantiomers exhibited similarly high cytotoxicity, supporting a polynuclear active species. Analysis of the two enantiomers of the most active brominated complex for their cytotoxicity on human ovarian A2780, cisplatin resistant A2780cp and multi-drug-resistant A2780adr cell lines as well as for their apoptosis induction on the A2780 line revealed similar reactivity, supporting a similar mechanism for the two enantiomers.