Trifluoromethyl substitution enhances photoinduced activity against breast cancer cells but reduces ligand exchange in Ru(ii) complex

Abstract

A series of five ruthenium complexes containing triphenyl phosphine groups known to enhance both cellular penetration and photoinduced ligand exchange, cis-[Ru(bpy)₂(P(p-R-Ph)₃)(CH₃CN)]²⁺, where bpy = 2,2'-bipyridine and P(p-R-Ph)₃ represent para-substituted triphenylphosphine ligands with R = -OCH₃ (1), -CH₃ (2) -H (3), -F (4), and -CF₃ (5), were synthesized and characterized. The photolysis of 1-5 in water with visible light (λ _irr ≥ 395 nm) results in the substitution of the coordinated acetonitrile with a solvent molecule, generating the corresponding aqua complex as the single photoproduct. A 3-fold variation in quantum yield was measured with 400 nm irradiation, Φ ₄₀₀, where 1 is the most efficient with a Φ ₄₀₀ = 0.076(2), and 5 the least photoactive complex, with Φ ₄₀₀ = 0.026(2). This trend is unexpected based on the red-shifted metal-to-ligand charge transfer (MLCT) absorption of 1 as compared to that of 5, but can be correlated to the substituent Hammett para parameters and pK _a values of the ancillary phosphine ligands. Complexes 1-5 are not toxic towards the triple negative breast cancer cell line MDA-MB-231 in the dark, but 3 and 5 are >4.2 and >19-fold more cytotoxic upon irradiation with blue light, respectively. A number of experiments point to apoptosis, and not to necrosis or necroptosis, as the mechanism of cell death by 5 upon irradiation. These findings provide a foundation for understanding the role of phosphine ligands on photoinduced ligand substitution and show the enhancement afforded by -CF₃ groups on photochemotherapy, which will aid the future design of photocages for photochemotherapeutic drug delivery.

Fig. 1. Structural representation of complexes 1–5.

Fig. 2. Thermal ellipsoid plots of (a) 1 and (b) 2; H atoms, PF₆⁻ ions, and co-crystallized solvent molecules omitted for clarity (ellipsoids drawn at 50% probability).

Fig. 3. Electronic absorption spectra of 1 (green), 2 (purple), 3 (black), 4 (red) and 5 (blue) in CH₃CN.

Fig. 4. Cyclic voltammograms of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5 in CH₃CN vs. Ag/AgCl (0.1 M TBAPF6, scan rate = 100 mV s⁻¹).

Fig. 5. Calculated energies of the HOMO and LUMO for 1–5.

Fig. 6. Changes to the electronic absorption spectrum of 1 upon irradiation (t_irr = 0–80 s) in water (<5% acetone, λ_irr > 395 nm).

Fig. 7. Hammett parameter plot of the relative values of Φ₄₀₀ for 1, 2, 4, and 5, Φ_x, relative to that of 3, Φ₀, as a function of σ_p values.

Fig. 8. Viability of MDA-MB-231 cells treated with 5 (2 μM) alone and co-treated with necrostatin (NEC), Z-VAD-FMK, N-acetyl cysteine (NAC) followed by irradiation (t_irr = 20 min, λ_irr = 460–470 nm, 56 J cm⁻²). P values are vs. 2 μM 5; ***P < 0.01 **P < 0.05 *P < 0.10.

Fig. 9. Flow cytometric analysis of MDA-MB-231 cells after 72 h treatment using Annexin V/propidium iodide staining, showing (a) vehicle followed by irradiation, (b) vehicle without irradiation, (c) [Ru(thd)(tpy)(py)]PF₆ (6 μM), (d) H₂O₂ (500 mM); (e) 5 (10 μM) without irradiation and (f) 5 (10 μM) with irradiation. Data are indicative of three independent experiments.