Tracking Reactive Intermediates by FTIR Monitoring of Reactions in Low-Temperature Sublimed Solids: Nitric Oxide Disproportionation Mediated by Ruthenium(II) Carbonyl Porphyrin Ru(TPP)(CO)

Interaction of NO (¹⁵NO) with amorphous layers of Ru­(II) carbonyl porphyrin (Ru­(TPP)­(CO), TPP^2‑ = <i>meso</i>-tetraphenylporphyrinato dianion) was monitored by FTIR spectroscopy from 80 K to room temperature. An intermediate spectrally characterized at very low temperatures (110 K) with ν­(CO) at 2001 cm^–1 and ν­(NO) at 1810 cm^–1 (1777 cm^–1 for ¹⁵NO isotopomer) was readily assigned to the mixed carbonyl–nitrosyl complex Ru­(TPP)­(CO)­(NO), which is the logical precursor to CO labilization. Remarkably, Ru­(TPP)-mediated disproportionation of NO is seen even at 110 K, an indication of how facile this reaction is. By varying the quantity of supplied NO, it was also demonstrated that the key intermediate responsible for NO disproportionation is the dinitrosyl complex Ru­(TPP)­(NO)₂, supporting the conclusion previously made from solution experiments

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)­(O₂) (Por = <i>meso</i>-tetra-phenyl and <i>meso</i>-tetra-<i>p</i>-tolylporphyrinato 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 ν­(O₂) band. Isotope labeling experiments with ¹⁸O₂, ¹⁵NO and N¹⁸O 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 ¹⁵NO, ¹⁸O₂ and N¹⁸O are used in the experiments and certainly belong to the 6-coordinate nitrato complexes (NH₃)­Co­(Por)­(η¹–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 Hb, 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