Photolabile Ruthenium Nitrosyls with Planar Dicarboxamide Tetradentate N4 Ligands: Effects of In-Plane and Axial Ligand Strength on NO Release

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Abstract
Four ruthenium nitrosyls, namely [(bpb)Ru(NO)(Cl)] (1), [(Me2bpb)Ru(NO)(Cl)] (2), [(Me2bpb)Ru(NO)(py)](BF4) (3), and [(Me2bqb)Ru(NO)(Cl)] (4) (H2bpb = 1,2-bis(pyridine-2-carboxamido)benzene, H2Me2bpb = 1,2-bis(pyridine-2-carboxamido)-4,5-dimethylbenzene, H2Me2bqb = 1,2-bis(quinaldine-2-carboxamido)-4,5-dimethylbenzene; H is the dissociable amide proton), have been synthesized and characterized by spectroscopy and X-ray diffraction analysis. All four complexes exhibit νNO in the range 1830−1870 cm-1 indicating the {Ru−NO}6 configuration. Clean 1H NMR spectra in CD3CN (or (CD3)2SO) confirm the S = 0 ground state for all four complexes. Although the complexes are thermally stable, they release NO upon illumination. Rapid NO dissociation occurs when solutions of 1−3 in acetonitrile (MeCN) or DMF are exposed to low-intensity (7 mW) UV light (λmax = 302 nm). Electron paramagnetic resonance (EPR) spectra of the photolyzed solutions display anisotropic signals at g ≈ 2.00 that confirm the formation of solvated low-spin Ru(III) species upon NO release. The ligand trans to bound NO namely, anionic Cl- and neutral pyridine, has significant effect on the electronic and NO releasing properties of these complexes. Change in the in-plane ligand strength also has effects on the rate of NO release. The absorption maximum (λmax) of 4 is significantly red shifted (455 nm in DMF) compared to the λmax values of 1−3 (380−395 nm in DMF) due to the extension of conjugation on the in-plane ligand frame. As a consequence, 4 is also sensitive to visible light and release NO (albeit at a slower rate) upon illumination to low-intensity visible light (λ > 465 nm). Collectively, the photosensitivity of the present series of ruthenium nitrosyls demonstrates that the extent of NO release and their wavelength dependence can be modulated by changes of either the in-plane or the axial ligand (trans to bound NO) field strength.

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