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

Abstract

Four ruthenium nitrosyls, namely [(bpb)Ru(NO)(Cl)] (1), [(Me(2)bpb)Ru(NO)(Cl)] (2), [(Me(2)bpb)Ru(NO)(py)](BF(4)) (3), and [(Me(2)bqb)Ru(NO)(Cl)] (4) (H(2)bpb = 1,2-bis(pyridine-2-carboxamido)benzene, H(2)Me(2)bpb = 1,2-bis(pyridine-2-carboxamido)-4,5-dimethylbenzene, H(2)Me(2)bqb = 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 nu(NO) in the range 1830-1870 cm(-)(1) indicating the [Ru-NO](6) configuration. Clean (1)H NMR spectra in CD(3)CN (or (CD(3))(2)SO) 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 (lambda(max) = 302 nm). Electron paramagnetic resonance (EPR) spectra of the photolyzed solutions display anisotropic signals at g approximately 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 (lambda(max)) of 4 is significantly red shifted (455 nm in DMF) compared to the lambda(max) 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 (lambda > 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.

Photo-Controlled Release of NO and CO with Inorganic and Organometallic Complexes

“…Having more carboxamide groups resulted in a higher bathochromic effect, higher quantum yields, and a better stability under physiological conditions [90,107,108]. An example is the tetradentate anionic ligand bpb -containing two carboxamide groups (H 2 bpb = 1,2-bis(pyridine-2-carboxamido)benzene) from which they prepared the ruthenium complexes Ru(bpb)(NO)(X) Interestingly, the more sterically crowded Ru((OMe) 2 bQb)(NO)(Cl) (Figure 11) is less photoactive than the isoquinoline analog Ru((OMe) 2 IQ1)(NO)(Cl) [32] …”

Nitrogen Monoxide and Nitrous Oxide Binding and Reduction

“…The nitrite to NO conversion can be effected by either a heme or copper enzyme (vide infra) and the NO to N 2 O transformation also involves hemes (see below). Further, relevant syn- thetic/chemical research includes the study of the catalytic reduction of NO (or other nitrogen oxides) (mentioned below) and as mentioned above the development of (metallo) reagents as drugs designed to thermally or photochemically release NO [28][29][30][31][32][33][34][35][36][37][38].…”

“…The use of metal ion chemistry for such purposes is a very active area of research, i.e. to sense biological NO [23][24][25][26][27], deliver it as drug [28][29][30][31][32][33][34][35][36][37][38] or modulate NO concentrations (as in septic shock therapy [39,40]). Another form of cellular signaling is thought to also involve NO, but with biological thiols [41], consisting of nitrosylation-denitrosylation chemistry, i.e.…”

Synthesis, Characterization and Reaction of New Ruthenium Aromatic Thioamide Nitrosyls with some Nitrogen Donor Ligands

“…Ruthenium nitrosyls are particularly attractive in this regard owing to their thermal stability and NO release upon exposure to light. [7][8][9] The NO release, in most of the octahedral ruthenium nitrosyls having {Ru II NO + } moiety, can be induced by one electron reduction by photolysis or PDT. In general, it has been found that NO release depends markedly on the p-acceptor strength of the ligand coordinated trans to NO and increase in plane ligand field strength.…”