Oxidation of nitrogen monoxide by oxoiron(IV) complexes: mechanistic studies and related investigations with an iron nitrosyl complex

Reactions of the free radical nitrogen monoxide (NO) with metal–oxygen species of metalloproteins are relevant to NO metabolism and detoxification. For example, oxyhemoglobin and oxymyoglobin react with NO to form nitrate. The ferryl state of these globins also reacts with NO to reduce them to the Fe^III state, forming nitrite. This has led to the suggestion that the role of NO could be that of an antioxidant of oxoiron(IV) and oxoiron(IV) protein radicals to inhibit oxidative damage. In turn, the ferrylglobin-mediated oxidation of NO to nitrite may play a role in NO scavenging and detoxification. In the case of peroxidase enzymes, NO has been shown to increase the activity of some enzymes by accelerating reduction of compound II to the Fe^III state.

While synthetic examples do exist for the chemistry of superoxometal complexes and NO, knowledge of the fundamental reactivity between oxometal complexes and NO is limited. To gain insight into the reactivity of synthetic oxoiron(IV) complexes toward NO, the reaction of [Fe^IVO(tmc)(OAc)]⁺ with NO, where the Fe center is coordinated by the macrocyclic nitrogen-donor ligand 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc), has been investigated. This reaction caused reduction of the Fe^IV center to Fe^II and produced nitrite, which was identified in the form of [Fe^II(tmc)(ONO)]⁺. Mechanistic studies have been conducted to distinguish between two possible pathways involving either oxygen atom or oxide(·–) ion transfer from the Fe^IVO group to NO.

As a result of studying the reactivity of a different oxoiron(IV) complex, [Fe^IVO(N4Py)]²⁺, toward NO, the formation of Fe^II and nitrate was observed. Mechanistic studies have revealed a 2:1 stoichiometry between Fe^IV and NO. From these results, a mechanism can be proposed that includes an initial oxide(·#8211;) ion transfer from Fe^IVO group to NO to form nitrite, followed by an oxygen atom transfer from a second equivalent of [Fe^IVO(N4Py)]²⁺ to the nitrite intermediate to form nitrate. This second step chemistry was confirmed by independently studying the reaction of [Fe^IVO(N4Py)]²⁺ with nitrite to form nitrate.

There is also a biological inorganic chemistry in which metal nitrosyl species are oxidized to form innocuous nitrite or nitrate. In this context, the oxidation of the synthetic nitrosyl complex [Fe(tmc)(NO)]²⁺ has been studied, which also produced [Fe^II(tmc)(ONO)]⁺. The molecular structure of [Fe^II(tmc)(ONO)]⁺ determined by X-ray crystallography indicates a bidentate binding mode of the nitrito ligand via both oxygen atoms. The oxidation results are consistent with a net oxide(·–) ion transfer mechanism forming [Fe^II(tmc)(NO₂)]⁺, followed by a subsequent linkage isomerization. For comparison purposes, several related, independently synthesized [Fe^II(tmc)X]⁺ complexes (X = NO₂^–, NO₃^–, AcO^–) have been characterized by spectroscopic techniques, X-ray crystallography and differential pulse and cyclic voltammetry.

A final investigation involved studying the reactivity of a series of [Fe^IVO(tmc)X]⁺ (X = CF₃SO₃^–, CF₃CO₂^–, AcO^–) complexes toward organic substrates by oxygen atom transfer and hydrogen atom abstraction to construct a reactivity trend depending on the strength of the axial ligand X.