Nitric Oxide Interaction with Oxy–Coboglobin Models Containing trans-Pyridine Ligand: Two Reaction Pathways

Abstract

The oxy-cobolglobin models of the general formula (Py)Co(Por)(O2) (Por = meso-tetraphenyl- and meso-tetra-p-tolylporphyrinato dianions) were constructed by sequential low-temperature interaction of Py and dioxygen with microporous layers of Co-porphyrins. At cryogenic temperatures small increments of NO were introduced into the cryostat and the following reactions were monitored by the FTIR and UV-visible spectroscopy during slow warming. Similar to the recently studied (NH3)Co(Por)(O2) system (Kurtikyan et al. J. Am. Chem. Soc., 2012, 134, 13671-13680), this interaction leads to the nitric oxide dioxygenation reaction with the formation of thermally unstable nitrato complexes (Py)Co(Por)(η(1)-ONO2). The reaction proceeds through the formation of the six-coordinate peroxynitrite adducts (Py)Co(Por)(OONO), as was demonstrated by FTIR measurements with the use of isotopically labeled (18)O2, (15)NO, N(18)O, and (15)N(18)O species and DFT calculations. In contrast to the ammonia system, however, the binding of dioxygen in (Py)Co(Por)(O2) is weaker and the second reaction pathway takes place due to autoxidation of NO by rebound O2 that in NO excess gives N2O3 and N2O4 species adsorbed in the layer. This leads eventually to partial formation of (Py)Co(Por)(NO) and (Py)Co(Por)(NO2) as a result of NO and NO2 reactions with five-coordinate Co(Por)(Py) complexes that are present in the layer after the O2 has been released. The former is thermally unstable and at room temperature passes to the five-coordinate nitrosyl complex, while the latter is a stable compound. In these experiments at 210 K, the layer consists mostly of six-coordinate nitrato complexes and some minor quantities of six-coordinate nitro and nitrosyl species. Their relative quantities depend on the experimental conditions, and the yield of nitrato species is proportional to the relative quantity of peroxynitrite intermediate. Using differently labeled nitrogen oxide isotopomers in different stages of the process the formation of the caged radical pair after homolytic disruption of the O-O bond in peroxynitrite moiety is clearly shown. The composition of the layers upon farther warming to room temperature depends on the experimental conditions. In vacuo the six-coordinate nitrato complexes decompose to give nitrate anion and oxidized cationic complex Co(III)(Por)(Py)2. In the presence of NO excess, however, the nitro-pyridine complexes (Py)Co(Por)(NO2) are predominantly formed formally indicating the oxo-transfer reactivity of (Py)Co(Por)(η(1)-ONO2) with regard to NO. Using differently labeled nitrogen in nitric oxide and coordinated nitrate a plausible mechanism of this reaction is suggested based on the isotope distribution in the nitro complexes formed.