Effect of Electron-donating Group on NO Photolysis of {RuNO}⁶ Ruthenium Nitrosyl Complexes with N₂O₂ Lgands Bearing π-Extended Rings

In this study, we introduced the electron-donating group (−OH) 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, and IR spectroscopies under white room light. Spectroscopic results indicated that the complexes were diamagnetic Ru(II)−NO⁺ species which were converted to low-spin Ru(III) species (d⁵, S=1/2) and released NO radicals by photons. The conversion was also confirmed by determining the single-crystal structure of the photoproduct of 1. The photochemical quantum yields (Φ_NOs) of the photolysis were determined to be 0>1, 2, 3 at both the visible and UV excitations. Femtosecond (fs) time-resolved mid-IR spectroscopy was employed for studying NO-releasing dynamics. The geminate rebinding (GR) rates of the photoreleased NO to the photolyzed complexes were estimated to be 0≃1, 2, 3. DFT and TDDFT computations found that the introduction of the hydroxyl groups elevated the ligand π-bonding orbitals (π (salophen)), resulting in decrease of the HOMO-LUMO gaps in 1–3. The theoretical calculations suggested that the Ru-N_NO bond dissociations of the complexes were mostly initiated by the ligand-to-ligand charge transfer (LLCT) of π(salophen)→π*(Ru−NO) with both the visible and UV excitations and the decreasing Φ_NOs 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.