Heterobimetallic Pt ^II -Au ^I Complexes Comprising Unsymmetrical 1,1-Bis(diphenylphosphanyl)methane Bridges: Synthesis, Photophysical, and Cytotoxic Studies

In the present work, a series of aryl-cycloplatinated(II) complexes with general formula [Pt(C ^{^} N)(Ar)(κ ¹ -dppm)], 1, [C ^{^} N = 7,8-benzoquinolinyl (bzq) or 2-phenylpyridinyl (ppy); Ar = C ₆ F ₅ or p-MeC ₆ H ₄ , dppm = 1,1-bis(diphenylphosphanyl)methane] was employed in the reaction with AuCl(SMe ₂ ) in order to generate heterobimetallic Pt ^II -Au ^I complexes, [Pt(C ^{^} N)(Ar)(µ-dppm)Au(Cl)], 2, featuring a dppm bridge between the metal centers. The expectation was to induce metallophilic character into the excited state and to reduce non-radiative deactivation pathways of the dangling auxiliary κ ¹ -dppm ligand through molecular motions, to improve the photophysical properties. After characterization of the new complexes by means of NMR spectroscopy and X-ray crystallography technique, the photophysical properties of all the complexes were investigated by UV/Vis and photoluminescence spectroscopy. Both of the monometallic complexes and heterobimetallic products have shown to be luminescent in different states and temperature conditions. However, by addition of Au ^I , the impact on the photophysics of the heterobimetallic products in relation to the precursors with dangling dppm is minimal, a finding which can be attributed to the absence of a Pt ^II -Au ^I bond in these compounds. Indeed, the character of the excited states of the monomer Pt ^II complexes and their corresponding bimetallic Pt ^II -Au ^I ones are similar, as confirmed by density functional theory (DFT) and time resolved DFT (TD-DFT) calculations. The cytotoxic activities of the compounds along with that of [ClAu(µ-dppm)AuCl] were evaluated against human breast cancer (MCF-7), human lung cancer (A549), human ovarian cancer (SKOV3) and non-tumorigenic epithelial breast (MCF-10A) cell lines. The highest activity was found for the heterometallic Pt-Au species, suggesting a cooperative effect of both metallic fragments. The most cytotoxic compound, i.e. [Pt(bzq)(p-MeC ₆ H ₄ )(µ-dppm)Au(Cl)], 2b, effectively causes cell death in MCF-7 cancer cell line by inducing apoptosis. Fluorescence microscopy experiments for 2a were performed.