C1-symmetrical titanium(IV) complexes of salan ligands with differently substituted aromatic rings: enhanced cytotoxic activity.

Diaminobis(phenolato) ("salan") titanium(IV) complexes of differently substituted aromatic rings were synthesized, and their hydrolytic stability and cytotoxicity were analyzed and compared to those of the C2-symmertrical analogues and their equimolar mixtures. The hydrolytic stability of the asymmetrical complexes was in between those of the symmetrical analogues, implying an additive influence of the ligand structural parameters. Most mixed halogenated/nitrated complexes showed a marked improvement of cytotoxic activity relative to the symmetrical analogues and their mixtures, with IC50 values as low as <1 μM corresponding to activity exceeding that of cisplatin by up to 30-fold. In contrast, asymmetrical complexes with substitutions of similar properties revealed an added influence of both, with cytotoxicity in between those of the symmetrical analogues. With the presumption that the active species is generally a polynuclear hydrolysis product kept in mind, it is overall evident that particular ligand design and fine-tuning of the parameters of influence including hydrophilicity and hydrophobicity are essential for maximizing biological efficiency.