Synthesis, Characterization and HNO Binding Properties of Metalocorroles

MIGUEL A. MORALES; NICOLÁS I. NEUMAN; JUAN PELLEGRINO; FABIO DOCTOROVICH

Chascomús

Workshop; 4th Latin American Meeting on Biological Inorganic Chemistry; 2014

POSTER Synthesis, Characterization and HNO Binding Properties of Metalocorroles Miguel A. Morales,1 Nicolás I. Neuman,1,2 Juan Pellegrino,1 and Fabio Doctorovich1 1 INQUIMAE/DQIAyFQ, FCEN, Universidad de Buenos Aires, Argentina 2 Departamento de Física, FBCB, Universidad Nacional del Litoral, Argentina mmorales@qi.fcen.gov.ar INTRODUCTION Corroles are contracted tetrapyrrolic macrocycles which possess a direct pyrrole-pyrrole bond. Corrole synthesis was first inspired by natural occurence of vitamin B12, which is a derivatized cobalt corrin (reduced corrole).1 In 1999 two facile syntheses of corroles were reported,2,3 which led to a geometrical increase in the number of studies of these compounds and their properties. Corroles have several interesting properties. They stabilize metals in high oxidation states, which has applications in catalysis, have several photochemical applications, such as singlet oxygen generation (ref) and bind small ligands such as O2, NO and HNO (azanone). Our group´s interest in HNO was intensified lately due to the finding that several biologically relevant reductants are capable to reduce NO to HNO in physiological conditions (see poster by M. Muñoz et al.) In this work we will sinthesize corroles and study the reductive binding of HNO by Co(III) and Fe(III) corroles, which could lead to new useful HNO sensors. EXPERIMENTAL METHODS Corroles were synthesized according to refs. 4 and 5, by acid-catalyzed condensation of substituted benzaldehyde and pyrrole, followed by oxidative ring-closure of a tetrapyrrolic intermediate using p-chloranyl. The resulting corroles (deep green) were purified by column chromatography using silica-gel and dichloromethane (DCM)/hexanes mixtures. Metallation was achieved by refluxing a DCM/MeOH solution of corrole and a metal salt such as Fe(SO4)2 or Co(Acetate)2. All products were characterized by UV-visible and NMR spectroscopy. Their electrochemical properties were studied by cyclic voltammetry and UV-vis spectroelectrochemistry (results not shown here). HNO binding was measured by mixing iron or cobalt corrole with HNO donor Piloty's acid (PA)6 and 2,6-lutidine (base) and following the UV-vis spectral changes. RESULTS AND DISCUSSION Figure 1 shows the UV-vis spectra of free base tris-meso-(4-nitrophenyl)- (NO2C), tris-meso-(4-methoxyphenyl)- (MeOC), tris-meso-(phenyl)- (PhC), and tris-meso-(2,6-dichlorophenyl)corrole (diClC). The spectra of PhC, MeOC and diClC show a split Soret band in the 400-420 nm region and two to four Q-bands in the 500-700 nm region. NO2C shows a broader Soret band at 455 nm. Fig. 1. UV-vis spectra of free base corroles. Free base corroles are unstable, and they have to be kept in the dark during purification and stored in the dark under inert gas atmosphere. Upon metallation the corroles become much more stable, but still the oxidation state of the metal may change if not stored under inert atmosphere. Figure 2 shows the UV-vis spectra of Co(IV)MeOC, Co(III)MeOC, and the product of the reaction Co(III)MeOC with HNO donor Piloty´s acid. Reaction of the reduced cobalt corrole with PA is very fast, and in EtOH doesn´t require addition of a base to initiate decomposotion of PA to produce HNO. Fig. 2. UV-vis spectra of Co(IV)MeOC, Co(III)MeOC, and Co(III)MeOC plus Piloty´s Acid. Alternatively, Co(IV)MeOC reaction with Piloty´s acid as a function of time was followed spectrophotometrically and required hours to completion. A proposed mechanism, based on previous work in cobalt porphyrins,7 is the following Instead, Co(IV)MeOC is likely coordinated with an oxo- or hidroxo- group which slows the direct metal-HNO reaction. Further characterization of the reaction mechanism is underway. CONCLUSIONS We have synthesized and characterized a series of corroles and cobalt metallocorroles using UV-vis spectroscopy, NMR and electrochemical methods. We have also observed that Co(III)MeOC binds HNO at a considerable rate, and is thus promising in the development of an HNO sensing system. REFERENCES 1. Aviv-Harel I. et al., Chem.Eur.J. (2009), 15:8382-8394 2. Paolesse R. et al., Chem. Comm. (1999), 1307-1308 3. Gross Z. et al., Angew. Chem. Int. Ed. Engl. (1999), 38(10):1427-1429 4. Gryko D.T. et al., OBC (2003), 1(2):350-357 5. Koszarna B. et al., JOC (2006), 71(10):3707-3717 6. Bonner F.T. et al. IC (1992), 31:2514:2519 7. Suárez, S. et al., AC (2013), 85:10262-10269 ACKNOWLEDGMENTS The authors would like to thank CONICET, UBA and ANPCyT for providing financial support to this project. MAM and NIN thank CONICET for a fellowship grant. FD is a member of CONICET.