Visible Light-Induced Release of Nitrogen Monoxide from a Nitrosylrhodium Complex

The important roles that nitric oxide (NO) plays in biological environments, and the need for precise and targeted delivery of NO for medicinal and other purposes have led to intense research in the area of metal nitrosyl complexes as thermal and photochemical sources of NO. Complexes with a good combination of chemical stability and high quantum yield for photochemical release of NO upon irradiation with visible light in aqueous solutions are rare. Here we report that a simple macrocyclic nitrosylrhodium complex [L-2(H2O)Rh(NO)](2+) (L-2=Me-6[14]aneN(4)) exhibits unique chemical and photochemical properties that make it an excellent photochemical precursor of NO. The complex is highly soluble in water, thermally stable, and resistant toward O-2. Irradiation in the 648 nm band generates NO and [L-2(H2O)Rh](2+) in aqueous solutions with a quantum yield of 1.00 +/- 0.07, the highest ever reported for a nitrosyl complex under any conditions. In the absence of O-2, the two fragments combine to regenerate [L-2(H2O)Rh(NO)](2+), but in O-2-containing solutions, [L-2(H2O)RhOO](2+) is formed as determined in spectral and kinetic measurements. The kinetics of the reaction of this superoxo complex with NO were measured by laser flash photolysis, k=(3.9 +/- 0.4) x 10(7) M(-1)s(-1). Steady-state photolysis of [L-2(H2O)Rh(NO)](2+) under O-2 yielded [L-2(H2O)Rh(ONO2)](2+), a long-lived nitrato intermediate that can also be generated in a direct reaction between NO and genuine [L-2(H2O)RhOO](2+). Thus, visible-light photolysis of the [L-2(H2O)Rh(NO)](2+)/O-2 system converts it to the [L-2(H2O)RhOO](2+)/NO combination.