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Reactivity of Hydrogen Sulfides (H2S/HS-) with [IrCl5(NO)]-. Stabilization of Nitrososulfide Ligand.
MAURICIO LLAVER; ANA FOI; FLORENCIA DI SALVO; FABIO DOCTOROVICH
Encuentro; IV Latin American Meeting on Biological Inorganic Chemistry - V WOQUIBIO; 2014
INTRODUCTION Nitric oxide (NO) and dihydrogen sulfide (H2S) are two gaseous transmitters that regulate numerous physiological functions. Although NO chemistry has been widely studied since its identification as an endogenous molecule, H2S has only recently been recognized as an important physiological compound, with effects similar to those of NO.1 Also, S-nitrosothiols have been recognized in biological systems for over two decades,2 but the existence of the smallest S-nitrosothiol, thionitrous acid (HSNO), was recently reported.3 HSNO can act as NO+, NO* and NO- donor, all relevant from a physiological point of view,3 which makes the study of the reaction between H2S and NO in the context of coordination chemistry quite interesting. For instance, an interesting example is the reaction of H2S with sodium nitroprusside (SNP = Na2[Fe(CN)5NO]). In this metal nitrosyl, the NO is coordinated to the Fe(II) as NO+ and it is suggested as a NO donor in the organism,4 thus its use as vasodilator. The results for the above mentioned reaction, demonstrated the presence of the adduct [Fe(CN)5N(O)SH]3- in the first stage, and [Fe(CN)5N(O)S]4- obtained by its deprotonation. These two and other minor species where characterized in solution and studied by electronic structure calculations.5 As well as SNP, [IrCl5(NO)]- can be used as a model to study the NO-SH2 interaction. The NO+ in this Ir(III) complex is the most electrophilic nitrosyl known to date, very likely due to a very weak Ir-NO backbonding. However, the inertness of iridium, is such, that the Ir-N(O)X bonds are very strong in almost every case studied. Therefore, the addition of nucleophiles to the nitrosyl ligand in [IrCl5(NO)]- is a potent tool for the facile obtaining of coordinated Ir-N(O)X species (X = NH, S, C and R = alkyl, aryl) that are unstable in free form.7 In this work we studied the reaction between the aforementioned Ir(III) complex and sulfur species. The products are obtained as brown-red solids from the reaction mixtures in very good yields. FT-IR, ESI-MS and NMR results, suggests the presence of [IrCl4(SV)NOSH]- (SV = acetonitrile) and/or [IrCl5NOSH]2- as the main products. DFT (Density Functional Theory) computational calculations were also performed in order to obtain further insight on these results. EXPERIMENTAL METHODS All reactions were carried out in strict nitrogen atmosphere using Schlenk techniques, and under light protection. Except from K[IrCl5(NO)], all reactants were used without further purification; anhydrous methanol, acetonitrile and mixtures of these two were used as solvents. In all cases the sulfur reactant was added in small excess (~1.5:1). For low temperature experiments, a -30ºC Slush bath (ethanol/liquid nitrogen) was used. Reaction products were analyzed by UV-Vis and FT-IR spectroscopy, 1H-NMR, Elemental Analysis and ESI-MS. For comparison with experimental data, isotopic patterns were calculated using the MassLynx software.DFT computational calculations were performed with the Gaussian 09 suit of programs, using the B3LYP functional, with Ahlrichs-pVDZ basis set for N, O, Cl, S, H, and LANL2DZ functional and basis pseudopotential for Ir. Structure optimization and energy calculations were performed for both the anti and syn conformation of the NOSH unit coordinated to the Ir complex, in gas phase. RESULTS AND DISCUSSION The reaction between K[IrCl5NO] and NaHS was carried out at both 25 and -30ºC, in different solvents, yielding a brown-red powder instantaneously after mixing the reactants in all cases. Using Na2S instead of NaHS resulted in a product with the same characteristics as described before, except at low temperature, where a green powder was obtained. Based on previous experimental evidence, we suggest that the color difference observed is a consequence of the coordination of a solvent molecule to the trans position at room temperature.8 The reasons why this difference is not observed for the NaHS reactions are yet under study. Reaction products were analyzed by ESI-MS, showing evidence of the presence of the [IrCl4NOSH]- ion (m/z exptl. 395.840, m/z calc. 395.816). Further details are presented in Table 1. This result confirms the presence of a NOSH unit, but does not provide information about the remaining ligand, which could be either Cl- or a solvent molecule. FT-IR spectra signals assignments are presented in Table 2. Such results correlate well with the ESI-MS ones, and are in fairly good agreement with theoretical calculations. CONCLUSION The presence of [IrCl4NOSH]- detected by high resolution ESI-MS shows the stabilization of the NOSH specie by Ir(III). Characteristic IR signals are also in good agreement with this result. Reaction temperature appears to be an important factor which affects the ligand in the trans position stabilization, favoring its labilization and replacement by a solvent molecule at room temperature. Further investigations concerning the reactivity (particularly the ability to act as NO donor), stability in different solvents and potential links to biological processes are the next steps to be pursued in the near future. REFERENCES 1. Yang, G. et al. Science 2008, 322. 587-590 2. Stamler, J. S.; Singel, D. J.; Loscalzo, J. Science 1992, 258, 1898-1902. 3. Filipovic, M. R. et al. J. Am. Chem. Soc. 2012, 134, 12016-12027. 4. Roncaroli, F.; Videla, M.; Slep, L. D.; Olabe, J. A. Coord. Chem. Rev. 2007, 251, 1903-1930 5. Quiroga, S. L. ; Almaraz, A. E.; Amorebieta, V. ; Perissinotti, L. L. ; Olabe, J. Chem. Eur. J. 2011, 17, 4145 - 4156. 6. Filipovic, M. R.; Eberhardt, M.; Prokopovic, V.; Mijuskovic, A.; Orescanin-Dusic, Z.; Reeh, P.; Ivanovic-Burmazovic, I. J. Med. Chem., 2013, 56 (4), pp 1499-1508. 7. Doctorovich, F.; Di Salvo, F. Acc. Chem. Res. 2007, 40 985. 8. Perissinotti, L.L.; Leitus, G.; Shimon, L.; Estrin, D.A.; Doctorovich, F. Inorg. Chem. 2008, 47, 4723. ACKNOWLEDGMENTS The authors would like to thank the Consejo Nacional de Investigaciones Científicas (CONICET, Grant no: PICT 2012 1335) for providing financial support to this project.