2 research outputs found
Nitric Oxide Interaction with Oxy–Coboglobin Models Containing <i>trans</i>-Pyridine Ligand: Two Reaction Pathways
The oxy–cobolglobin
models of the general formula (Py)ÂCoÂ(Por)Â(O<sub>2</sub>) (Por = <i>meso</i>-tetraphenyl- and <i>meso</i>-tetra-<i>p</i>-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 (NH<sub>3</sub>)ÂCoÂ(Por)Â(O<sub>2</sub>) system (Kurtikyan et al. <i>J. Am. Chem.
Soc.</i>, <b>2012</b>, <i>134</i>, 13671–13680),
this interaction leads to the nitric oxide dioxygenation reaction
with the formation of thermally unstable nitrato complexes (Py)ÂCoÂ(Por)Â(η<sup>1</sup>-ONO<sub>2</sub>). The reaction proceeds through the formation
of the six-coordinate peroxynitrite adducts (Py)ÂCoÂ(Por)Â(OONO), as
was demonstrated by FTIR measurements with the use of isotopically
labeled <sup>18</sup>O<sub>2</sub>, <sup>15</sup>NO, N<sup>18</sup>O, and <sup>15</sup>N<sup>18</sup>O species and DFT calculations.
In contrast to the ammonia system, however, the binding of dioxygen
in (Py)ÂCoÂ(Por)Â(O<sub>2</sub>) is weaker and the second reaction pathway
takes place due to autoxidation of NO by rebound O<sub>2</sub> that
in NO excess gives N<sub>2</sub>O<sub>3</sub> and N<sub>2</sub>O<sub>4</sub> species adsorbed in the layer. This leads eventually to partial
formation of (Py)ÂCoÂ(Por)Â(NO) and (Py)ÂCoÂ(Por)Â(NO<sub>2</sub>) as a
result of NO and NO<sub>2</sub> reactions with five-coordinate CoÂ(Por)Â(Py)
complexes that are present in the layer after the O<sub>2</sub> 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<sup>III</sup>(Por)Â(Py)<sub>2</sub>. In the presence of NO excess, however, the nitro–pyridine
complexes (Py)ÂCoÂ(Por)Â(NO<sub>2</sub>) are predominantly formed formally
indicating the oxo-transfer reactivity of (Py)ÂCoÂ(Por)Â(η<sup>1</sup>-ONO<sub>2</sub>) 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