Acidity and photolability of ruthenium salen nitrosyl and aquo complexes in aqueous solutions
Graphical abstract
Photolyses of Ru(salen)(NO)(OH2)+ and Ru(salen)(NO)Cl in aqueous solution at different pH’s result in the same photoproduct, , with NO release. Photolysis of the chloro complex results also in chloride release via sequential photoreactions. The acidity of the coordinated water of Ru(salen)(NO)(OH2)+ (pKa = 4.5) indicates that the ruthenium should have a Ru(III) character in this complex.
Introduction
Nitric oxide (NO) has numerous important roles in mammalian biology including activity in blood pressure regulation, as a neurotransmitter and as a toxic agent formed in immune response to pathogens [1], [2], [3], [4], [5]. The physiological functions of NO are complicated and depend on thermodynamic, kinetic and concentration considerations [6], [7]. NO can be either beneficial or harmful, depending on the circumstances [1], [3], [4], [6], [7], [8], [9]. For instance, at high concentration NO can induce cell death, but in low concentrations NO reduces metastasis in pulmonary tissues [8], [10], [11], [12], [13], [14]. The importance of NO as a bioregulator suggests potential applications for controlled and site specific NO delivery agents. In this regard, several NO donors and scavengers, many of these being metal complexes, have been reported [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. Of parallel interest are potential applications of photochemistry in the treatment of various disease states, including cancer [45], [46], [47], [48]. Such photochemotherapy would involve irradiation-induced production of a biologically active species in elevated concentrations at a specific locale from photoactive compounds introduced into the patient. Similar concepts should be able to generate NO photochemically.
Ruthenium nitrosyl ammine complexes such as the trans-Ru(NO)(NH3)4L3+ cations have been shown to have biological activity and low toxicity [49], [50], [51], [52], [53]. They also release NO photochemically (Eq. (1)). Net quantum yields for this reaction are dependent on the irradiation wavelength and the solution pH. Although these complexes are not very photoactive to visible irradiation, near-UV excitation may be effective for topical applications or in a solid-state implant [54], [55], [56].
Ruthenium salen nitrosyls of the type Ru(salen-Y)(X)(NO) (X = Cl− or ONO, salen-Y = various substituted salen dianions, where salen = N,N′-ethylenebis(salicylideneiminato) dianion) have also been shown to be photoactive toward NO labilization in various solvents (Eq. (2)) [33], [57]. However, with regard to potential medical applications, it is important to understand the aqueous media reactivities, so in this context, we report the photochemistry of the water-soluble complexes Ru(salen)(NO)Cl (I) and Ru(salen)(NO)(OH2)+ (II) in aqueous solution.
We also report the pKa values for several new ruthenium aquo complexes and compare these to the pKa values of other known ruthenium complexes, and discuss these in terms of the relative electron acceptor/donor characters of the nitrosyl and other ligands.
Section snippets
Reagents
All chemicals used were reagent grade or of comparable purity. All manipulations were carried out under inert atmosphere, except where noted. Milli-Q grade or doubly distilled water was used throughout the work. Commercially available argon (Air Liquid) was passed through a drying column and through a column of MnO for deoxygenation prior to use. Dimethylformamide (Aldrich), methanol (Synth), dichloromethane (Synth), ethylenediamine (Merck), salicylaldehyde (Merck), acetonitrile (Acros) were
Results and discussion
The aqueous solution electronic absorption spectra of the Ru(salen)(NO)(OH2)+ cation and Ru(salen)(NO)Cl, each display a relatively strong (ε ≈ 104 M−1 cm−1) and broad absorption band below 300 nm (Fig. 1), that can be assigned to an intra-ligand π → π∗ transition by comparison with the free salen spectrum and with related complexes [40]. These aqueous solution spectra also display an absorption band of lower intensity around 360 nm and are similar to those of related complexes in organic solvents [33]
Conclusions
In summary, photolyses of Ru(salen)(NO)(OH2)+ and Ru(salen)(NO)Cl in aqueous solution at different pHs lead to NO release and the Ru(III) photoproducts, and its conjugate base forms. Photolysis of the chloro complex results in both NO and chloride release via sequential photoreactions. In addition, comparing the pKa values measured for the coordinated water of trans-Ru(salen)(NO)(OH2)+ (4.5) and of (5.9) indicates that representing the Ru–NO bonding in
Acknowledgements
The authors thank grants and fellowships from FAPESP, CAPES, and CNPq. J.B. thanks a CAPES Ph.D. fellowship. P.C.F. acknowledges support from the US National Science Foundation.
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