High-valent iron

Abstract In this review, recent developments of iron-catalyzed oxidations of olefins (epoxidation), alkanes, arenes, and alcohols are summarized. Special focus is given on the ligand systems and the catalytic performance of the iron complexes. In addition, the mechanistic involvement of high-valent iron-oxo species is discussed. 

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). 

High-valent iron can occur in a wide variety of electronic configurations. Figure 8.25 (a-c, e-i) presents a summary of the corresponding one-electron crystal-field states for the 3(/, 3J, and 3J electron configurations, allocated to HS and LS states in distorted octahedral and tetrahedral symmetry. Part d, in addition, depicts the case of low-low-spin iron(IV) found in some trigonal... 

High-valent iron also occurs in -nitrido bridged dimers with linear [Fe °-N=Fe" ]" and [Fe =N=Fe ] " cores [209, 210] (and references therein). Such compounds have been prepared first by thermolysis [247] or photolysis [248] of iron(III)-porphyrin complexes with an azide ligand, (N3). Mixed-valent iron-nitrido porphyrin dimers exhibit valence delocalization as can be inferred from the... 

I. M.C.M., Reversible formation of high-valent-iron-oxo prophyrin intermediates in heme-based catalysis revisiting the kinetic model for horseradish peroxidase, Inorg. Chim. Acta, 275/276, 98-105, 1998. 

Atkins, W.M. and Sligar, S.G. (1988) Deuterium isotope effects in norcamphor metabolism by cytochrome P-450cam kinetic evidence for the two-electron reduction of a high-valent iron-oxo intermediate. Biochemistry, 27 (5), 1610-1616. 

Fe catalytically decomposes H2O2, imfortimately much less attention has been paid to this reaction and the possible mechanism varies according to the authors. Those that support the free radical OH approach, agree with the classical radical chain mechanism represented by equations (9, 10, 11 and 12). However, on the other side, high valent iron-oxo complex has also been proposed in Fe -H202 interaetions Kremer 1985 Gozzo 2001). 

See the special issue High valent iron intermediates in biology J. Inorg. Biochem. 2006,100, 419—880. 

The structural similarity of the catalytic domains of the enzymes of the AAH family, together with the identical reaction that they catalyze (i.e., hydroxylation of aromatic substrates) and the common dependency on BH4 and 02 (Section I), suggests that the mechanisms by which these enzymes operate are similar. It is assumed that the general AAH reaction mechanism follows a two-step reaction route in which a high-valent iron-oxo (FeIV=0) complex is formed in the first step, and that this intermediate is responsible for the hydroxylation of the aromatic amino acid substrate in the second step (15,26-28,50). The first step starts with 02 binding and activation and proceeds via a Fe-0-0-BH4 bridge and a subsequent heterolytic cleavage of the... 

Weiss, R. Gold, A. Trautwein, A. X. Terner, J. High-valent iron and manganese complexes of porphyrins and related macrocycles, The Porphyrin Handbook. Volume 4. Biochemistry and Binding Activation of Small Molecules , Eds. Kadish, . M. Smith, . M. Guilard, R. Academic Press San Diego, 2000, pp. 65-96. 

Nam, W., High-valent iron(IV) -oxo complexes of heme and non-heme ligands in oxygenation reactions. Acc. Chem. Res. 2007, 40, 522-531. 

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