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NMR and EPR Spectroscopy as a Tool for the Studies of Intermediates of Transition Metal–Catalyzed Oxidations
Published in Evgenii Talsi, Konstantin Bryliakov, Applications of EPR and NMR Spectroscopy in Homogeneous Catalysis, 2017
Evgenii Talsi, Konstantin Bryliakov
In spite of extensive studies of non-heme iron enzymes, there have been no unambiguous data on the structures and reactivities of ferric-superoxo complexes participating in their catalytic cycles. For synthetic non-heme iron complexes, the information on the structure and reactivity of iron-superoxo complexes is also very restricted. Hitherto, there have been two reports on synthetic non-heme dinuclear iron-superoxo complexes with the proposed diiron(μ-hydroxo) structures FeIIIFeIII–O2•− (22, Figure 3.8) [49] and FeIIFeIII–O2•− [50], and one report on mononuclear superoxo complex with the proposed structure FeIII–O2•− (23, Figure 3.8) [51]. Resonance Raman and Mössbauer spectroscopy were employed to unambiguously assign 23 as a species containing a mononuclear iron(III)-superoxide core. Complex 23 displays a resonance-enhanced vibration at 1125 cm−1 corresponding to the O─O bond of the superoxo moiety, and the Mössbauer spectrum corresponds to a high-spin FeIII center, exchange-coupled to the superoxo ligand. Complex 23 oxidizes dihydroanthracene to anthracene, supporting the assumption that FeIII–O2•− species can carry out H-atom abstraction [51]. The main drawback hampering more detailed studies of iron-superoxo complexes in non-heme iron enzymes and model systems is their low stability.
Kinetic investigations of the formation of iron(IV) oxido complexes
Published in Journal of Coordination Chemistry, 2022
Florian J. Ritz, Markus Lerch, Jonathan Becker, Siegfried Schindler
Non-heme iron enzymes play a key role in selective conversions of organic substrates and are capable of catalyzing a variety of reactions, including hydroxylation of aromatic and aliphatic hydrocarbons, desaturation, epoxidation, cis-dihydroxylation, halogenation etc. [1]. There has been increased interest in bioinorganic research, as evidenced by the growing number of reports of model complexes by oxygen activation in cytochrome P450 or similar enzymes [2]. The extensively studied dioxygen activation of these complexes occurs in many cases through, for example, superoxido, peroxido or hydroperoxido species as intermediates [3]. In biomimetic chemistry, high-valent oxido-Fe(IV) complexes (former nomenclature oxo-Fe) have been of interest for two decades due to their importance in enzymatic catalytic cycles [1, 4]. Since the first report of a full characterization of a nonheme-Fe(IV)-oxido complex with the macrocyclic ligand tetramethylcyclam (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane, 2, Scheme 1) by Rohde et al. in 2003 [5], a large number of new high valent Fe(IV)-species have been reported [6]. In a typical synthetic procedure the precursor complexes are treated with an excess of oxidant, mainly organoiodine compounds or aqueous solutions of hydrogen peroxide, at low temperatures to stabilize the often short-living and labile species [7]. In contrast, we have started to use ozone as an oxidant [8], which has rarely been used in the oxygen activation of coordination compounds [9]. Advantages of ozone are that it is introduced as a highly reactive gas into the solutions without solubility problems (iodosylbenzene) or introduction of water (aqueous hydrogen peroxide). Furthermore, it enables reactions at low temperatures that in many cases would not take place at all [8]. In enzymatic transformations, the O2 molecule has to be activated first (to overcome the spin forbidden reaction of the triplet ground state) to a superoxido complex that in further consecutive reactions leads to the oxido species. Therefore, only a few examples of a direct conversion with O2 to the Fe(IV)-oxido complex analogous to the biological models are known [1]. In this context, Ray and co-workers recently reported excellent work on formation of the Fe(IV)-oxido complex with cyclam (1,4,8,11-tetraaza-cyclotetradecane, 1, Scheme 1) in a direct reaction with oxygen [10].