Nitric Oxide Dioxygenation Reaction by Oxy-Coboglobin Models: In-situ Low-Temperature FTIR Characterization of Coordinated Peroxynitrite

The oxy-cobolglobin models of the general formula (NH3)Co(Por)(O-2) (Por = meso-tetra-phenyl and meso-tetra-p-tolylpoiphyrinato dianions) were constructed by sequential low temperature interaction of NH3 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. Upon warming the layers from 80 to 120 K a set of new IR bands grows with correlating intensities along with the consumption of the nu(O-2) band. Isotope labeling experiments with O-18(2), (NO)-N-15 and (NO)-O-18 along with DFT calculations provides a basis for assigning them to the six-coordinate peroxynitrite complexes (NH3)Co(Por)(OONO). Over the course of warming the layers from 140 to 170 K these complexes decompose and there are spectral features suggesting the formation of nitrogen dioxide NO2. Upon keeping the layers at 180-210 K the bands of NO2 gradually decrease in intensity and the set of new bands grows in the range of 1480, 1270, and 980 cm(-1). These bands have their isotopic counterparts when (NO)-N-15, O-18(2) and (NO)-O-18 are used in the experiments and certainly belong to the 6-coordinate nitrato complexes (NH3)Co(Por)(eta(1)-ONO2) demonstrating the ability of oxy coboglobin models to promote the nitric oxide dioxygenation (NOD) reaction similar to oxy-hemes. As in the case of lib, Mb and model iron-porphyrins, the six-coordinate nitrato complexes are not stable at room temperature and dissociate to give nitrate anion and oxidized cationic complex Co(III)(Por)(NH3)(1,2).