Dye-Tethered Ruthenium Nitrosyls Containing Planar Dicarboxamide Tetradentate N4 Ligands: Effects of In-Plane Ligand Twist on NO Photolability

To examine the steric effects of the in-plane ligands in dye-sensitized {RuNO}6 nitrosyls on their NO photolability, two new ligands, namely, 1,2-Bis(pyridine-2-carboxamido)-4,5-dimethoxybenzene (H2(OMe)2bpb) and 1,2-Bis(Isoquinoline-1-carboxamido)-4,5-dimethoxybenzene (H2(OMe)2IQ1, H's are dissociable carboxamide protons) have been designed and synthesized. The syntheses and spectroscopic properties of {RuNO}6 nitrosyls derived from these two ligands, namely, [((OMe)2bpb)Ru(NO)(Cl)] (4-Cl), [((OMe)2IQ1)Ru(NO)(Cl)] (5-Cl), [((OMe)2bpb)Ru(NO)(Resf)] (4-Resf), and [((OMe)2IQ1)Ru(NO)(Resf)] (5-Resf), are reported. The structures of 5-Cl, 4-Resf, and 5-Resf have been determined by X-ray crystallography. Removal of the in-plane ligand twist in the quinoline-based R2bQb2− ligand frame (because of steric interactions between the extended quinoline ring systems) in both R2bpb2− and R2IQ12− (pyridine and 1-isoquinoline rings, respectively, instead of quinoline rings in the equatorial plane) results in enhanced solution stability, as well as higher quantum yield values for NO photorelease upon exposure to 500 nm light. Both dye-tethered {RuNO}6 nitrosyls 4-Resf and 5-Resf exhibit greater sensitivity to visible light compared to the chloro-bound species 4-Cl and 5-Cl. In addition, the dye-tethered nitrosyls are fluorescent and hence can be used as trackable NO donors in cellular studies.