UnusuallyEfficient Pyridine Photodissociation fromRu(II) Complexes with Sterically Bulky Bidentate Ancillary Ligands

摘要

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)(Me2bpy)(py)]²⁺ (>2; Me2bpy = 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 CH3CN (λirr = 500 nm), ligand exchange occurs to produce the corresponding [Ru(tpy)(NN)(NCCH3)]²⁺ (NN = Me2bpy, biq) complex with quantum yields, Φ500, 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, Φ500 < 0.0001, under similar experimental conditions. The photolysis of >2 and >3 in H2O with low energy light to produce [Ru(tpy)(NN)(OH2)]²⁺ (NN = Me2bpy, biq) also proceeds rapidly (λirr > 590 nm). Complexes >1–>3 are stable in the dark in both CH3CN and H2O 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, Me2bpy 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 andtpy in >1, 83.34°. The steric bulk afforded by Me2bpy and biq also result in major distortions of the pyridineligand in >2 and >3, respectively, relativeto >1, which are believed to weaken its σ-bondingand π-back-bonding to the metal and play a crucial role in theefficiency 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 photochemotherapeuticagents for the delivery of drugs with pyridine binding groups.