Unusually Efficient Pyridine Photodissociation from Ru(II) Complexes with Sterically Bulky Bidentate Ancillary Ligands

[Image: see text] The introduction of steric bulk to the bidentate ligand in [Ru(tpy)(bpy)(py)](2+) (1; tpy = 2,2′:2′,6″-terpyridine; bpy = 2,2′-bipyridine; py = pyridine) to provide [Ru(tpy)(Me(2)bpy)(py)](2+) (2; Me(2)bpy = 6,6′-dimethyl-2,2′-bipyridine) and [Ru(tpy)(biq)(py)](2+) (3; biq = 2,2′-biquinoline) facilitates photoinduced dissociation of pyridine with visible light. Upon irradiation of 2 and 3 in CH(3)CN (λ(irr) = 500 nm), ligand exchange occurs to produce the corresponding [Ru(tpy)(NN)(NCCH(3))](2+) (NN = Me(2)bpy, biq) complex with quantum yields, Φ(500), of 0.16(1) and 0.033(1) for 2 and 3, respectively. These values represent an increase in efficiency of the reaction by 2–3 orders of magnitude as compared to that of 1, Φ(500) < 0.0001, under similar experimental conditions. The photolysis of 2 and 3 in H(2)O with low energy light to produce [Ru(tpy)(NN)(OH(2))](2+) (NN = Me(2)bpy, biq) also proceeds rapidly (λ(irr) > 590 nm). Complexes 1–3 are stable in the dark in both CH(3)CN and H(2)O under similar experimental conditions. X-ray crystal structures and theoretical calculations highlight significant distortion of the planes of the bidentate ligands in 2 and 3 relative to that of 1. The crystallographic dihedral angles defined by the bidentate ligand, Me(2)bpy in 2 and biq in 3, and the tpy ligand were determined to be 67.87° and 61.89°, respectively, whereas only a small distortion from the octahedral geometry is observed between bpy and tpy in 1, 83.34°. The steric bulk afforded by Me(2)bpy and biq also result in major distortions of the pyridine ligand in 2 and 3, respectively, relative to 1, which are believed to weaken its σ-bonding and π-back-bonding to the metal and play a crucial role in the efficiency of the photoinduced ligand exchange. The ability of 2 and 3 to undergo ligand exchange with λ(irr) > 590 nm makes them potential candidates to build photochemotherapeutic agents for the delivery of drugs with pyridine binding groups.