Water Soluble Iron-Nitroxyl Porphyrin Complexes: [FeII(TPPS)HNO]4? and [FeII(TPPS)NO]5?: pKa estimation and stability studies

AGOSTINA M. MAZZEO; JUAN PELLEGRINO; FABIO DOCTOROVICH

CABA

Simposio; Exploring the Frontiers of Chemistry: Challenges for the 21st Century; 2019

Interest in HNO (azanone according to IUPAC; usually nitroxyl) has increased in the last decade due to its remarkable chemical properties and increasing biological relevance.1 In addition, FeII-nitroxyl complexes are important intermediates in NO and nitrite-reducing enzymes in bacteria and fungi, which catalyze important processes related to the biogeochemical cycle of nitrogen. It has been considered that the protonated adduct is stabilized by amino acids,2 and bulky substituents.3 The stability reported for [Fe(CN)5(HNO)]3? 4 led us to consider that factors like H-bonds with water, could stabilize the Fe-HNO moiety. In this work, the heme water soluble nitroxyl complexes [FeII(TPPS)HNO]4? and [FeII(TPPS)NO]5? were obtained and characterized by UV-Vis spectroscopy. The pKa was estimated from both spectroscopic and electrochemical measurements. The protonated complex spontaneously reoxidizes to the nitrosyl species within minutes, much slower than in organic media.5 Kinetic measurements in different conditions, along with other experimental results and DFT calculations support an unimolecular reoxidation mechanism involving an homolytic rupture of the H-NO bond, via a phlorin intermediate also proposed and detected in similar systems.REFERENCES1.Doctorovich, F.; Bikiel, D.; Pellegrino, J.; Suárez, S. A.; Larsen, A.; Marí, M. A. Coord. Chem. Rev. 2011, 255, 2764-2784.2.Lin, R.; Farmer, P. J. J. Am. Chem. Soc. 2000, 122, 2393?2394.3.Goodrich, L. E.; Roy, S.; Alp, E. E.; Zhao, J.; Hu, M. Y.; Lehnert, N. Inorg. Chem 20134.Montenegro, A. C.; Amorebieta, V. T.; Slep, L. D.; Martín, D. F.; Roncaroli, F.; Murgida, D. H.; Bari, S. E.; Olabe, J. A. Angew. Chemie Int. Ed. 2009, 48. 4213−4216. 5.Pellegrino, J.; Bari, S. E.; Bikiel, D. E.; Doctorovich, F. J. Am. Chem Soc. 2010, 132. 989-9956.Solis, B. H.; Maher, A. G.; Dogutan, D. K.; Nocera, D. G.; Hammes-Schiffer, S. PNAS 2016, 113 (3), 485?492