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

Abstract

AbstractIn 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 (ΦNOs) 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‐NNO 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 ΦNOs 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.