New photo-activated NO donors based on cyclometalated ruthenium nitrosyls

Based on [Ru(dfppy)(bpy)(CH 3 CN)NO](PF 6 ) 2 (complex 1 , where Hdfppy= 2-(2,4-difluorophenyl)pyridine, bpy=2,2′-bipyridine), three new cyclometalated ruthenium nitrosyl complexes [Ru(dfppy)(L)(CH 3 CN)NO](PF 6 ) 2 ( 2 − 4 , where L = 2,2’-bipyridine derivatives) were synthesized by grafting methoxy or dimethyl ester groups onto the 2,2’-bipyridine ligand. These complexes were fully characterized by NMR, IR, MS and single-crystal X-ray diffraction spectra, confirming the {Ru II −NO + } 6 electronic configuration for all complexes. Together with the pronounced changes in their UV−Vis absorption spectra under irradiation, the MS and 1 HNMR spectra of the photolysis products suggested a photoinduced homolysis-reduction process. ESR spectra exhibited the photorelease of NO and 1 O 2 . Notably, complex 3 with dimethyl esters at the 4,4’-position of bipyridine demonstrated the shortest N−O bond length of 1.129(13) Å and the longest Ru−N(O) bond length of 1.767(9) Å, indicating a weakened Ru–NO linkage that facilitates NO release upon irradiation. Consistently, complex 3 displayed the highest photolysis rate ( k NO =3.1 × 10 −2 min −1 ) in the series and the most efficient NO release, reaching a concentration of 5.32 μmol/L after 30 min of irradiation (365 nm LED, 13 mW/cm 2 ) as determined by the Griess assay. Furthermore, CCK8 assays indicated that all the complexes exhibit low dark toxicity and desirable phototoxicity towards 4T1 cells. Among them, complex 3 displayed the strongest phototoxicity due to its excellent photo-sensitive NO release capacity.