Photochemical nitric oxide precursors: Synthesis, photochemistry, and ligand substitution kinetics of ruthenium salen nitrosyl and ruthenium salophen nitrosyl complexes

Abstract

Described are syntheses, characterizations, and photochemical reactions of the nitrosyl complexes Ru(salen)(ONO)(NO) (I, salen = N,N′-ethylenebis(salicylideneiminato) dianion), Ru(salen)(Cl)(NO) (II), Ru(tBu4salen)(Cl)(NO) (III, tBu4salen = N,N′-ethylenebis(3,5-di-tert-butylsalicylideneiminato) dianion), Ru(tBu4salen)(ONO)(NO) (IV), Ru(tBu2salophen)(Cl)(NO) (V, tBu2salophen = N,N′-1,2-phenylenediaminebis(3-tert -butylsalicylideneiminato) dianion), and Ru(tBu4salophen)(Cl)(NO) (VI, tBu4salophen = N,N′ -1,2-phenylenebis(3,5-di-tert-butylsalicylideneiminato) dianion). Upon photolysis, these Ru(L)(X)(NO) compounds undergo NO dissociation to give the ruthenium(III) solvento products Ru(L)(X)(Sol). Quantum yields for 365 nm irradiation in acetonitrile solution fall in a fairly narrow range (0.055-0.13) but decreased at longer λirr. The quantum yield (λirr = 365 nm) for NO release from the water soluble complex [Ru(salen)(H2O)(NO)]Cl (VII) was 0.005 in water. Kinetics of thermal back-reactions to re-form the nitrosyl complexes demonstrated strong solvent dependence with second-order rate constants kNO varying from 5 × 10-4 M-1 s-1 for the re-formation of II in acetonitrile to 5 × 108 M-1 s-1 for re-formation of III in cyclohexane. Pressure and temperature effects on the back-reaction rates were also examined. These results are relevant to possible applications of photochemistry for nitric oxide delivery to biological targets, to the mechanisms by which NO reacts with metal centers to form metal-nitrosyl bonds, and to the role of photochemistry in activating similar compounds as catalysts for several organic transformations. Also described are the X-ray crystal structures of I and V.