Effect of electron-donating group on NO photolysis of {RuNO}6 ruthenium nitrosyl complexes with N2O2 ligands bearing π-extended rings.

We introduced OH groups to the aromatic rings of Ru(salophen)(NO)Cl (0) (salophenH2 = N,N'-(1,2-phenylene)bis(salicylideneimine)) to investigate the influence of the substitution on NO photolysis and NO-releasing dynamics. Three derivative complexes, Ru((o-OH)2-salophen)(NO)Cl (1), Ru((m-OH)2-salophen)(NO)Cl (2), and Ru((p-OH)2-salophen)(NO)Cl (3) were developed and their NO photolysis was monitored by using UV/Vis, EPR, NMR, IR, and X-ray crystallography. Spectroscopic results indicated that the complexes were diamagnetic Ru(II)-NO+ species which were converted to low-spin Ru(III) species and released NO radicals by photons. The photochemical quantum yields (PCQYs) of the photolysis were determined to be 0 > 1, 2, 3 at both the visible and UV excitations. The geminate rebinding rates of the photoreleased NO were estimated to be 0 ≃ 1, 2, 3. Theoretical computations found that the introduction of the OH groups elevated the ligand π-bonding orbitals, resulting in decrease of the HOMO-LUMO gaps. The theoretical calculations suggested that the Ru-NNO bond dissociations of the complexes were mostly initiated by the ligand-to-ligand charge transfer and the decreasing PCQYs could be explained by the changes of the electronic structures in which the photoactivable bands of 1-3 have relatively less contribution of transitions related with Ru-NO bond than those of 0.