High-valent Fe intermediates

High-valent iron intermediates have been proposed as the active species in OAT and C-H oxidation reactions for nonheme iron enzymes. In some cases, such intermediates have been trapped by rapid fireeze-quench studies and characterized. In ribonucleotide reductase from E. coli and MMO, intermediates X and Q with Fem-( l-0)2-Ferv and Ferv-( 0,-O)2-FeIV diamond core, respectively, have been characterized (Figure 3.11).35 Also, Fe,v oxo intermediates have been observed for mononuclear proteins such as taurine/2-oxoglutarate dioxygenase (TauD) (Figure 3.11).36... 

Regarding the iron coordination geometry, the most active catalysts exhibit two available cis-coordination sites that facilitate the activation of the oxidant. Conversely, relatively inactive complexes usually show only one available coordination site or have two available sites in a trans position. As stated before, iron-based enzymatic systems in nature involve the formation of high-valent metal intermediates within the catalytic cycle. Similarly, oxo-iron complexes have been proposed as the main intermediate for artificial oxidation processes. Depending on the iron complex, the type of oxidant and substrate and the mechanism involved in the oxidation process, this intermediate could be a Fe =0 or H0-Fe =0 species. Wieghard et al. 

Because there exist a number of reviews which deals with the structural and mechanistic aspects of high-valent iron-oxo and peroxo complexes [6,7], we focus in this report on the application and catalysis of iron complexes in selected important oxidation reactions. When appropriate we will discuss the involvement and characterization of Fe-oxo intermediates in these reactions. 

High-valent iron-imido complexes have also been proposed as reaction intermediates in several reactions of the iron catalysis. Que and coworkers have provided evidence for Fe(IV) imide as a reaction intermediate in the reaction of [(6-PhTPA)-Fe°(CH3CN)2] " with PhI=NTs. Borovik and coworkers have also reported the formation of an amide product involving the generation of a putative iron(IV) imide [36] (Scheme 7). 

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