Spectroscopic and electrochemical study of iron porphyrins and iron chlorins in the presence of nitrite in nonaqueous solvents

The aim of this work was: (1) to characterize the complex formed between iron porphyrins and iron chlorins with nitrite; and (2) to study the electrochemical reduction of this complex in order to understand the mechanism of the electrocatalytic reduction of nitrite to NO by the nitrite reductase model compounds in non-aqueous solvents. The reaction between Fe$\sp{\rm III}$(P)Cl and Fe$\sp{\rm III}$(P)(NO$\sb3$) (where P = tetraphenyl porphyrin (TPP), tetraphenyl chlorin (TPC) and octaethyl porphyrin (OEP)) with nitrite in non-aqueous solvents was studied by uv/visible, proton NMR aned infrared spectroscopy and by electrochemistry. Quantitative analysis of the visible spectra generated by the titration of ferric-porphyrin complexes with nitrite showed the appearance of mono- and bis-nitrite complexes. The appearance of new waves and the shift in E$\sb{1/2}$ with the added nitrite was consistent with the formation of mono- and bis-nitrite complexes. The formation constants calculated voltammetrically were consistent with the spectroscopically obtained values. These complexes were unstable at room temperature but stable at O$\sp\circ$C in methylene chloride for the electrochemical study. Both these complexes are low spin and are Fe-N bonded nitro complexes as characterized by proton NMR and IR spectroscopy. In methylene chloride only, following the reduction of Fe$\sp{\rm III}$/Fe$\sp{\rm II}$ in the presence of nitrite, a new wave was observed which was characterized to be due to iron nitrosyl. Electrolysis of Fe$\sp{\rm III}$/Fe$\sp{\rm II}$ in the presence of nitrite produced iron nitrosyl involving coulometric n-value equal to 2 electrons. Nitrite reacted with Fe$\sp{\rm II}$ porphyrins giving iron nitrosyl and ferric species in methylene chloride while in DMF a mixture of Fe(P)-(NO$\sb2)\sp{-}$ and ferric species were observed. It is proposed in this dissertation that in non-aqueous solvents: (1) nitrite forms mono and bis nitro complexes with ferric porphyrins and chlorins; (2) nitrite is electrocatalytically reduced to NO by iron porphyrins in non-coordinating solvent, the reduction being preceded by the coordination of nitrite to Fe$\sp{\rm II}$ when an electron from Fe$\sp{\rm II}$ is transferred to the coordinated nitrite and (3) the ferrous nitrite complex is stable in DMF on the voltammetric scale