Synthesis, spectroscopic analysis and photolabilization of water-soluble ruthenium(III)-nitrosyl complexes

In this paper, the synthesis, structural and spectroscopic characterization of a series of new Ru(III)-nitrosyls of {RuNO}(6) type with the coligand TPA (tris(2-pyridylmethyl)amine) are presented. The complex [Ru(TPA)Cl-2(NO)]ClO4 (2) was prepared from the Ru(III) precursor [Ru(TPA)Cl-2]ClO4 (1) by simple reaction with NO gas. This led to the surprising displacement of one of the pyridine (py) arms of TPA by NO (instead of the substitution of a chloride anion by NO), as confirmed by X-ray crystallography. NO complexes where TPA serves as a tetradentate ligand were obtained by reacting the new Ru(II) precursor [Ru(TPA)(NO2)(2)] (3) with a strong acid. This leads to the dehydration of nitrite to NO+, and the formation of the {RuNO}(6) complex [Ru(TPA)(ONO)(NO)](PF6)(2) (4), which was also structurally characterized. Derivatives of 4 where nitrite is replaced by urea (5) or water (6) were also obtained. The nitrosyl complexes obtained this way were then further investigated using IR and FT-Raman spectroscopy. Complex 2 with the two anionic chloride coligands shows the lowest N-O and highest Ru-NO stretching frequencies of 1903 and 619 cm(-1) of all the complexes investigated here. Complexes 5 and 6 where TPA serves as a tetradentate ligand show nu(N-O) at higher energy, 1930 and 1917 cm(-1), respectively, and nu(Ru-NO) at lower energy, 577 and 579 cm(-1), respectively, compared to 2. These vibrational energies, as well as the inverse correlation of nu(N-O) and nu(Ru-NO) observed along this series of complexes, again support the Ru(II)-NO+ type electronic structure previously proposed for {RuNO}(6) complexes. Finally, we investigated the photolability of the Ru-NO bond upon irradiation with UV light to determine the quantum yields (phi) for NO photorelease in complexes 2, 4, 5, and additional water-soluble complexes [Ru(H(2)edta)(Cl)(NO)] (7) and [Ru(Hedta)(NO)] (8). Although {RuNO}(6) complexes are frequently proposed as NO delivery agents in vivo, studies that investigate how phi is affected by the solvent water are lacking. Our results indicate that neutral water is not a solvent that promotes the photodissociation of NO, which would present a major obstacle to the goal of designing {RuNO}(6) complexes as photolabile NO delivery agents in vivo.