Mechanistic Studies on the Nitrite-Catalyzed Reductive Nitrosylation of Highly Charged Anionic and Cationic Fe^III Porphyrin Complexes

The nitrosyl complexes formed during the binding of NO to the (Pⁿ)Fe^III(H₂O)₂ (n = 8+ and 8−) complexes, viz., (P^8-)Fe^II(H₂O)(NO⁺) and (P⁸⁺)Fe^II(H₂O)(NO⁺), undergo subsequent reductive nitrosylation reactions that were found to be catalyzed by nitrite, which was also produced during the reaction. The effect of the nitrite concentration, pH, temperature, and pressure on the nitrite-catalyzed reductive nitrosylation process was studied in detail for (P^8-)Fe^III(H₂O)₂, (P⁸⁺)Fe^III(H₂O)₂, and (P⁸⁺)Fe^III(OH)(H₂O), from which rate and activation parameters were obtained. On the basis of these data, we propose mechanistic pathways for the studied reactions. The available results favor the operation of an innersphere electron-transfer process between nitrite and coordinated NO⁺. By way of comparison, the cationic porphyrin complex (P⁸⁺)Fe^III(L)₂ (L = H₂O or OH^-) was found to react with NO₂^- to yield the nitrite adduct (P⁸⁺)Fe^III(L)(NO₂^-). A detailed kinetic studied revealed that nitrite binds to (P⁸⁺)Fe^III(H₂O)₂ according to a dissociative mechanism, whereas nitrite binding to (P⁸⁺)Fe^III(OH)(H₂O) at higher pH follows an associative mechanism, similar to that reported for the binding of NO to these complexes.