Photolabile Ru Model Complexes with Chelating Diimine Ligands for Light-Triggered Drug Release

A series of water-soluble photolabile model complexes of the general formula [Ru([9]aneS3)(chel)(py)]Cl2 ([9]aneS3 = 1,4,7-trithiacyclononane, chel = chelating diimine) was prepared and fully characterized. The photo-triggered release of pyridine with visible light as a function of the nature of the diimine (chel = 2,2′-bipyridine (6) 1,10-phenanthroline (7), 4,7-diphenil-1,10-phenanthroline (8), dipyrido-[3,2-a:2′,3′-c]phenazine (dppz, 9), 2,2′-biquinoline (bq, 10)) was investigated. Our aim is that of establishing if this type of complexes in the future might be realistically used in the photo-uncaging strategy of photoactivated chemotherapy (PACT). Compounds 6 – 9 present a MLCT absorption in the blue region of the visible spectrum. When irradiated with light at 470 nm, they rapidly and quantitatively release the coordinated pyridine. Complex 10 turned out to be quite different from to the others in the series. Structure-wise, in 10 the average plane of coordinated bq – owing to its steric demand – is remarkably tilted relative to the equatorial coordination plane (37.43 (4)°, with the “front” of the ligand pointing towards the axial py) and the orientation of py is ca. orthogonal compared to that found in 6 and 7 for minimizing steric clashes with bq. The low-lying acceptor orbitals of bq induce a red-shift of the MLCT absorption maximum to ca. 500 nm. Contrary to the expectations, complex 10 is more photo-stable compared to 6 – 9 and photo-dissociation of both py and bq, in ca. equal amounts, occurs. A detailed theoretical investigation was performed on 10 (and on 6 for comparison), for explaining its peculiar spectral features and photochemical behavior.