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

Abstract

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)]ClO(4) (2) was prepared from the Ru(III) precursor [Ru(TPA)Cl(2)]ClO(4) (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)(NO(2))(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)](PF(6))(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 ν(N-O) at higher energy, 1930 and 1917 cm(-1), respectively, and ν(Ru-NO) at lower energy, 577 and 579 cm(-1), respectively, compared to 2. These vibrational energies, as well as the inverse correlation of ν(N-O) and ν(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 (φ) 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 φ 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.

The versatile ruthenium(II/III) tetraazamacrocycle complexes and their nitrosyl derivatives

“…[105]. Lehnert and co-workers also point out that water may be an important player in the photochemistry [105].…”

“…[105]. Lehnert and co-workers also point out that water may be an important player in the photochemistry [105]. The fact that in general UV light (<400 nm) is required for photoactivation is a disadvantage, but for topical applications this is not a limitation [40], and to overcome it, Ford [39,180] devised several strategies.…”

“…The absence of an EPR signal for trans-[Ru II (NO + )Cl(cyclam)](PF 6 ) 2 has been used to assign the canonical form [Ru II NO + ] to the complex [34,104]. Similarly, absorption bands appearing at ∼1900 cm −1 support assignment of a linear [Ru II NO + ] group [34,105]. For instance, Lehnert and co-workers recently reported that [Ru(NO)(urea)(tpa)] 3+ ( Fig.…”

“…For instance, Lehnert and co-workers recently reported that [Ru(NO)(urea)(tpa)] 3+ ( Fig. 9) and [Ru(NO)(OH 2 )(tpa)] 3+ have (N O) stretching frequencies from 1930 to 1903 cm −1 [105].…”

Proton-Assisted Mechanism of NO Reduction on a Dinuclear Ruthenium Complex

“…Merkle et al recently reported that strong π-back bonding causes a distinct lowering of the NO stretching frequency in Ru(II)NO + complexes bearing tris(2-pyridylmethyl)amine as an ancillary ligand. 59 On the basis of all the calculated results, the electronic structure of the Ru 2 (NO) 2 moiety in the closed-shell singlet state of C can be described as structure IV, [Ru(II)(NO + )] 2 , in Chart 1.…”