Cyclometalated and NNN Terpyridine Ruthenium Photocatalysts and Their Cytotoxic Activity

The cyclometalated terpyridine complexes [Ru(η²-OAc)(NC-tpy)(PP)] (PP = dppb 1, (R,R)-Skewphos 4, (S,S)-Skewphos 5) are easily obtained from the acetate derivatives [Ru(η²-OAc)₂(PP)] (PP = dppb, (R,R)-Skewphos 2, (S,S)-Skewphos 3) and tpy in methanol by elimination of AcOH. The precursors 2, 3 are prepared from [Ru(η²-OAc)₂(PPh₃)₂] and Skewphos in cyclohexane. Conversely, the NNN complexes [Ru(η¹-OAc)(NNN-tpy)(PP)]OAc (PP = (R,R)-Skewphos 6, (S,S)-Skewphos 7) are synthesized in a one pot reaction from [Ru(η²-OAc)₂(PPh₃)₂], PP and tpy in methanol. The neutral NC-tpy 1, 4, 5 and cationic NNN-tpy 6, 7 complexes catalyze the transfer hydrogenation of acetophenone (S/C = 1000) in 2-propanol with NaOiPr under light irradiation at 30 °C. Formation of (S)-1-phenylethanol has been observed with 4, 6 in a MeOH/iPrOH mixture, whereas the R-enantiomer is obtained with 5, 7 (50-52% ee). The tpy complexes show cytotoxic activity against the anaplastic thyroid cancer 8505C and SW1736 cell lines (ED₅₀ = 0.31-8.53 µM), with the cationic 7 displaying an ED₅₀ of 0.31 µM, four times lower compared to the enantiomer 6.