Titanocene-Phosphine Derivatives as Precursors to Cytotoxic Heterometallic TiAu2 and TiM (M = Pd, Pt) Compounds. Studies of Their Interactions with DNA

A series of tri- and bimetallic titanium-gold, titanium-palladium, and titanium-platinum derivatives of the general formulas [Ti{eta(5)-C5H4(CH2)(n)PPh2(AuCl)}(2)]center dot 2THF [n = 0 (1); n = 2 (2); n = 3 (3)] and [TiCl2{eta(5)-C5H4 kappa-(CH2)(n)PPh2}(2)-(MCl2)]center dot 2THF [M = Pd, n = 0 (4); n = 2 (5); n = 3 (6); M = Pt, n = 0 (7); n = 2 (8); n = 3 (9)] have been synthesized and characterized by different spectroscopic techniques and mass spectrometry. The molecular structures of compounds 1-9 have been investigated by means of density functional theory calculations. The calculated IR spectra of the optimized structures fit well with the experimental IR data obtained for 1-9. The stability of the heterometallic compounds in deuterated solvents [CDCl3, dimethyl sulfoxide (DMSO)-d(6), and mixtures 50:50 DMSO-d(6)/D2O and 1:99 DMSO-d(6)/D2O at acidic and neutral pH] has been evaluated by P-31 and H-1 NMR spectroscopy showing a higher stability for these compounds than for Cp2TiCl2 or precursors [Ti{eta(5)-C5H4(CH2)(n)PPh2}(2)]. The new compounds display a lower acidity (1-2 units) than Cp2TiCl2. The decomposition products have been identified over time. Complexes 1-9 have been tested as potential anticancer agents, and their cytotoxicity properties were evaluated in vitro against HeLa human cervical carcinoma and DU-145 human prostate cancer cells. TiAu2 and TiPd compounds were highly cytotoxic for these two cell lines. The interactions of the compounds with calf thymus DNA have been evaluated by thermal denaturation (1-9) and by circular dichroism (1, 3, 4, and 7) spectroscopic methods. All of these complexes show a stronger interaction with DNA than that displayed by Cp2TiCl2 at neutral pH. The data are consistent with electrostatic interactions with DNA for TiAu2 compounds and for a covalent binding mode for TiM (M = Pd, Pt) complexes.