Steric and Electronic Factors Associated with the Photoinduced Ligand Exchange of Bidentate Ligands Coordinated to Ru(II).

In an effort to create a molecule that can absorb low energy visible or near-infrared light for photochemotherapy (PCT), the new complexes Ru(biq)2 (dpb)2 (1, biq = 2,2'-biquinoline, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline) and (biq)2 Ru(dpb)Re(CO)3 Cl2 (2) were synthesized and characterized. Complexes 1 and 2 were compared to Ru(bpy)2 (dpb)2 (3, bpy = 2,2'-bipyridine) and Ru(biq)2 (phen)2 (4, phen = 1,10-phenanthroline). Distortions around the metal and biq ligands were used to explain the exchange of one biq ligand in 4 upon irradiation. Complex 1, however, undergoes photoinduced dissociation of the dpb ligand rather than biq under analogous experimental conditions. Complex 3 is not photoactive, providing evidence that the biq ligands are crucial for ligand photodissociation in 1. The crystal structures of 1 and 4 are compared to explain the difference in photochemistry between the complexes. Complex 2 absorbs lower energy light than 1, but is photochemically inert although its crystal structure displays significant distortions. These results indicate that both the excited state electronic structure and steric bulk play key roles in bidentate photoinduced ligand dissociation. The present work also shows that it is possible to stabilize sterically hindered Ru(II) complexes by the addition of another metal, a property that may be useful for other applications.