Nitric oxide interaction with oxy-coboglobin models containing trans-pyridine ligand: two reaction pathways

Area of Science:

• Coordination Chemistry
• Organometallic Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Cobalt porphyrin complexes are studied for their ability to bind dioxygen, mimicking biological systems.
• Previous research on ammonia cobalt porphyrin dioxygen complexes established a dioxygenation pathway with nitric oxide.
• Understanding reactions of these complexes with nitrogen oxides is crucial for catalysis and bioinorganic chemistry.

Purpose of the Study:

• To investigate the reaction of pyridine cobalt porphyrin dioxygen complexes with nitric oxide (NO).
• To elucidate the reaction mechanisms, including intermediate formation and product distribution.
• To compare the reactivity with related ammonia cobalt porphyrin systems.

Main Methods:

• Construction of oxy-cobalt porphyrin models ((Py)Co(Por)(O2)) using pyridine and dioxygen with cobalt porphyrins.
• Cryogenic temperature reactions with incremental addition of nitric oxide (NO).
• Monitoring reactions using FTIR and UV-visible spectroscopy, including isotopic labeling ((18)O2, (15)NO, N(18)O, (15)N(18)O).
• Supporting mechanistic insights with Density Functional Theory (DFT) calculations.

Main Results:

• Nitric oxide dioxygenation occurred, forming thermally unstable nitrato complexes ((Py)Co(Por)(η(1)-ONO2)) via peroxynitrite adducts ((Py)Co(Por)(OONO)).
• Weaker dioxygen binding in pyridine systems led to autoxidation of NO by O2, forming N2O3 and N2O4.
• Formation of nitrosyl ((Py)Co(Por)(NO)) and nitro ((Py)Co(Por)(NO2)) complexes was observed, with nitro complexes being stable.
• Isotopic labeling confirmed the formation of a caged radical pair from homolytic O-O bond cleavage in peroxynitrite.
• Nitrato complexes decomposed to nitrate and oxidized cobalt complexes in vacuo, but favored nitro complexes in excess NO.

Conclusions:

• The reaction proceeds through peroxynitrite intermediates, with subsequent pathways influenced by dioxygen binding strength and NO excess.
• The pyridine cobalt porphyrin system exhibits distinct reactivity compared to ammonia systems, particularly in autoxidation pathways.
• The study provides a detailed mechanistic understanding of nitrogen oxide interactions with cobalt porphyrin dioxygen complexes, highlighting oxo-transfer reactivity.