A nitric oxide releaser based on the μ-oxo-hexaacetate-bis(4-methylpyridine)triruthenium nitrosyl complex
Graphical abstract
NO release has been observed by light irradiation of the trinuclear cluster, either in solution, solid state or in PVA films, allowing one to monitor the UV-B light exposure intensities from the absorbance changes accompanying the photochemical process.
Introduction
In recent years, a great number of reports dealing with NO activity in biological systems have been published [1], [2], [3], [4], [5], [6], [7], [8] and ruthenium complexes have been studied as scavengers of NO, or reciprocally, their nitrosyl complexes have been investigated as NO delivery systems toward biological targets [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. In this field, important contributions have been made by Shepherd and co-workers [25], [26], [27], [28], [29], [30], [31], [32], specially in the design and evaluation of a number of new transition metal complexes as NO carriers for biological applications.
It should be noticed that nitric oxide is a non-innocent ligand which can coordinate to a metal center yielding either a linear or a bent MNO geometry [33], [34], [35], [36], assuming three possible oxidation states: NO+, NO0 or NO− [37]. Therefore, the NO behavior is rather puzzling, since in the NO+ form the ligand is a very strong π-acceptor and would prefer metal ions in low oxidation states, but in the NO0 form it can combine with metal ions either in intermediate or high oxidation states, remaining formally in the neutral form or converting into NO+ by reducing the metal center. On the other hand, NO can also be converted into NO− species, under more reducing conditions.
Because of the systematic changes in the electronic properties for the several successive oxidation states, the triangular ruthenium acetate clusters [Ru3O(CH3CO2)6L3]n (L = H2O, N-heterocyclic, etc.) [38] provide rather unique species to interact with NO, as we reported in our preliminary work on the [Ru3O(CH3CO2)6(py)2(NO)]PF6 complex [39]. In this system, the strong interaction between the unpaired π* electron from NO and the unpaired electron from the π-orbitals of the (RuIII,III,III)3O unity is responsible for its pronounced NO0 character, as evidenced by the vibrational and spectroelectrochemical data. Here, in order to enhance the NO0 character further, the pyridine ligand was replaced by the 4-methyl pyridine (picoline) analogue (Fig. 1), and a full characterization of the complex was carried out by means of mass spectrometry, spectroscopy, electrochemistry, spectroelectrochemistry and scanning probe microscopy. Due to its higher basicity, the picoline ligand is expected to decrease the electron acceptor properties of the (RuIII,III,III)3O unity, diminishing the extent of electron donation from the NO ligand. In addition, NO release was observed under UV–Vis light irradiation, changing the complex from original violet color into deep blue. This effect was also probed at the nanoparticle level by means of MAC mode atomic force microscopy, revealing intriguing morphology changes accompanying the NO release.
Section snippets
Materials
Tetraethylammonium perchlorate (TEAClO4) [40] and the starting complex [Ru3O(CH3CO2)6(pic)2(CH3OH)]PF6 were prepared as previously described in the literature [41]. Acetonitrile (HPLC grade, Aldrich) was kept in the presence of 3 Å molecular sieves.
Synthesis of [Ru3O(CH3CO2)6(pic)2(NO)]PF6
Dichloromethane, 50 mL, containing 0.52 g [Ru3O(CH3CO2)6(pic)2(CH3OH)]PF6 was saturated with Ar for 20 min, then with NO for 35 min and finally with Ar for 60 min. After this, the product was precipitated by adding petroleum ether. The violet solid
ESI spectra and CID pattern
The positive ion ESI mass spectrum of the [Ru3O(CH3CO2)6(pic)2(NO)]PF6 complex in methanol:water solution is shown in Fig. 2(a). In the electrospray positive ion mode, the complex can be directly transferred by ESI to the mass spectrometer and can be detected as a multicomponent structure of isotopomeric ions; the most abundant ion being that of m/z 891. Ru possesses seven isotopes: 104Ru (18.7%), 102Ru (31.6%), 101Ru (17.0%), 100Ru (12.6%), 99Ru (12.7%), 98Ru (1.88%), and 96Ru (5.52%).
The CID
Conclusion
Extending our previous work on the interaction of the pyridine–ruthenium–acetate clusters with NO [39], we have confirmed the existence of a strong Ru–NO electronic coupling, responsible for the loss of the metal and NO ligand identities in the EPR, cyclic voltammetry and FT-IR spectroelectrochemistry results. Such conclusion has been corroborated by semi-empirical theoretical calculations, supporting a predominantly NO0 character for the nitrosyl ligand in the [Ru3O(CH3CO2)6(C5H5N)2(NO)]PF6
Acknowledgments
Financial support from FAPESP, CNPq, RENAMI and IM2C is gratefully acknowledged.
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2018, Journal of Inorganic BiochemistryCitation Excerpt :This removes the electronic pairing verified in the ground state, to reduce interaction between the species and consequently prompt NO release. The [Ru3O] center becomes paramagnetic after NO release, as probed by the photoproduct typical spectral profile (Figs. S10 and S11) [17,19,20]. Therefore, when the complex is in the excited state, the metallic core formal charge is indeed +1, which is more stabilized by interaction with a σ-donor ancillary ligand such as 4-tbpy as compared to a more π-acid ligand like 4-acpy [17].
Studying the interaction between trinuclear ruthenium complexes and human serum albumin by means of fluorescence quenching
2016, Journal of LuminescenceCitation Excerpt :Such as iron, ruthenium ion has low toxicity due to their various oxidation states available in physiological environment (II, III, IV) [17]. Also, their complexes can act both as NO scavengers or releasers, increasing the interest on the development of a variety of nitrosyl ruthenium complexes [17–26]. Of interest to this work, trinuclear ruthenium complexes of general formula [Ru3O(CH3COO)6(L)3]PF6, with L=N-heterocycles, have been widely studied in recent decades due to its rich mixed-valence chemistry and catalytic properties [27].