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Previously synthesized and spectroscopically characterized mononuclear nonheme, low-spin iron(III)-iodosylbenzene complex bearing a bidentate pyridyl-benzimidazole ligands has been investigated in alkane and aldehyde oxidation reactions. The in situ generated Fe(III) iodosylbenzene intermediate is a reactive oxidant capable of activating the benzylic C-H bond of alkane. Its electrophilic character was confirmed by using substituted benzaldehydes and a modified ligand framework containing electron-donating (Me) substituents. Furthermore, the results of kinetic isotope experiments ( KIE ) using deuterated substrate indicate that the C-H activation can be interpreted through a tunneling-like HAT mechanism. Based on the results of the kinetic measurements and the relatively high KIE values, we can conclude that the activation of the C-H bond mediated by iron(III)-iodosylbenzene adducts is the rate-determining step.

In this paper we describe a detailed mechanistic studies on the [Fe II (PBO) 2 (CF 3 SO 3 ) 2 ] ( 1 ), [Fe II (PBT) 2 (CF 3 SO 3 ) 2 ] ( 2 ), and [Fe II (PBI) 3 ](CF 3 SO 3 ) 2 ( 3 )-catalyzed (PBO = 2-(2'-pyridyl)benzoxazole, PBT = 2-(2'-pyridyl)benzthiazole, PBI = 2-(2'-pyridyl)benzimidazole) Baeyer-Villiger oxidation of cycloketones by dioxygen with cooxidation of aldehydes and peroxycarboxylic acids, including the kinetics on the reactivity of (μ-1,2-peroxo)diiron(III), acylperoxo- and iodosylbenzene-iron(III) species as key intermediates.

Heme and nonheme-type flavone synthase enzymes, FS I and FS II are responsible for the synthesis of flavones, which play an important role in various biological processes, and have a wide range of biomedicinal properties including antitumor, antimalarial, and antioxidant activities. To get more insight into the mechanism of this curious enzyme reaction, nonheme structural and functional models were carried out by the use of mononuclear iron, [Fe II (CDA-BPA*)] 2+ ( 6 ) [CDA-BPA = N , N , N ', N '- tetrakis -(2-pyridylmethyl)-cyclohexanediamine], [Fe II (CDA-BQA*)] 2+ ( 5 ) [CDA-BQA = N , N , N ', N '- tetrakis -(2-quinolilmethyl)-cyclohexanediamine], [Fe II (Bn-TPEN)(CH 3 CN)] 2+ ( 3 ) [Bn-TPEN = N -benzyl- N , N ', N '-tris(2-pyridylmethyl)-1,2-diaminoethane], [Fe IV (O)(Bn-TPEN)] 2+ ( 9 ), and manganese, [Mn II (N4Py*)(CH 3 CN)] 2+ ( 2 ) [N4Py* = N , N -bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine)], [Mn II (Bn-TPEN)(CH 3 CN)] 2+ ( 4 ) complexes as catalysts, where the possible reactive intermediates, high-valent Fe IV (O) and Mn IV (O) are known and well characterised. The results of the catalytic and stoichiometric reactions showed that the ligand framework and the nature of the metal cofactor significantly influenced the reactivity of the catalyst and its intermediate. Comparing the reactions of [Fe IV (O)(Bn-TPEN)] 2+ ( 9 ) and [Mn IV (O)(Bn-TPEN)] 2+ ( 10 ) towards flavanone under the same conditions, a 3.5-fold difference in reaction rate was observed in favor of iron, and this value is three orders of magnitude higher than was observed for the previously published [Fe IV (O)(N2Py2Q*)] 2+ [ N , N -bis(2-quinolylmethyl)-1,2-di(2-pyridyl)ethylamine] species.

Heme-type catalase is a class of oxidoreductase enzymes responsible for the biological defense against oxidative damage of cellular components caused by hydrogen peroxide, where metal-oxo species are proposed as reactive intermediates. To get more insight into the mechanism of this curious reaction a non-heme structural and functional model was carried out by the use of a mononuclear complex [Fe II (N4Py*)(CH 3 CN)](CF 3 SO 3 ) 2 (N4Py* = N , N -bis(2-pyridylmethyl)- 1,2-di(2-pyridyl)ethylamine) as a catalyst, where the possible reactive intermediates, high-valent Fe IV =O and Fe III -OOH are known and spectroscopically well characterized. The kinetics of the dismutation of H 2 O 2 into O 2 and H 2 O was investigated in buffered water, where the reactivity of the catalyst was markedly influenced by the pH, and it revealed Michaelis-Menten behavior with K M = 1.39 M, k cat = 33 s -1 and k 2 (k cat /K M ) = 23.9 M -1 s -1 at pH 9.5. A mononuclear [(N4Py)Fe IV =O] 2+ as a possible intermediate was also prepared, and the pH dependence of its stability and reactivity in aqueous solution against H 2 O 2 was also investigated. Based on detailed kinetic, and mechanistic studies (pH dependence, solvent isotope effect (SIE) of 6.2 and the saturation kinetics for the initial rates versus the H 2 O 2 concentration with K M = 18 mM) lead to the conclusion that the rate-determining step in these reactions above involves hydrogen-atom transfer between the iron-bound substrate and the Fe(IV)-oxo species., Competing Interests: The authors declare no conflict of interest.

This study reports the kinetics and mechanism of Fe(III)-catalyzed oxidative decarboxylation and deamination of a series of acyclic (α-aminoisobutyric acid, α-(methylamino)isobutyric acid, alanine, norvaline, and 2-aminobutyric acid) and cyclic (1-aminocyclopropane-1-carboxylic acid, 1-amino-1-cyclobutanecarboxylic acid, 1-aminocyclopentanecarboxylic acid, and 1-aminocyclohexanecarboxylicacid) amino acids using hydrogen peroxide, t-butyl hydroperoxide, iodosylbenzene, m-chloroperbenzoic acid, and peroxomonosulphate as oxidant in 75% DMF-25% water solvent mixture. Model complex [Fe(IV)O(SALEN)](•+) (SALENH2: N,N'-bis(salicylidene)ethylenediamine) was generated by the reaction of Fe(III)(SALEN)Cl and H2O2 in CH3CN at 278 K as reported earlier. This method provided us high-valent oxoiron species, stable enough to ensure the direct observation of the reaction with amino acids., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The mononuclear complex, Fe(III)(O-bs)(salen) (salenH(2)=1,6-bis(2-hydroxyphenyl)-2,5-diaza-hexa-1,5-diene; O-bsH=O-benzoylsalicylic acid) was synthesized as synthetic enzyme-depside complex, and characterized by spectroscopic methods and X-ray crystal analysis. The dioxygenation of flavonol (flaH) and 3-hydroxy-4-quinolone (quinH(2)) derivatives in the presence of catalytic amounts of Fe(III)(O-bs)(salen) results in the oxidative cleavage of the heterocyclic ring to give the corresponding O-benzoylsalicylic and anthranilic acid derivatives with concomitant release of carbon monoxide. These reactions can be regarded as biomimetic functional models with relevance to the iron-containing flavonol and the cofactor-independent 3-hydroxy-4(1H)-quinolone 2,4-dioxygenases., (Copyright © 2011 Elsevier Inc. All rights reserved.)

In this communication we describe the first example of a biomimetic mononuclear iron complex, [Fe(III)(Salen)Cl] (Salen = N,N'-bis(salicylidene)-ethylenediaminato), that highly selectively and efficiently catalyzes the oxidation of 1-aminocyclopropane-1-carboxylic acid (ACCH), α-aminoisobutyric acid (AIBH), and alanine (ALAH) to ethylene or the corresponding carbonyl compounds, mimicking the action of the non-heme iron enzyme 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO).

Mononuclear iron(iii) flavonolate was synthesized as synthetic enzyme-substrate complex, and its direct and carboxylate-enhanced dioxygenation as biomimetic functional models with relevance to flavonol 2,4-dioxygenase are briefly described.

With the goal of gaining insight into the structures of peroxo intermediates observed for oxygen-activating nonheme diiron enzymes, a series of metastable synthetic diiron(III)-peroxo complexes with [Fe(III)(2)(mu-O)(mu-1,2-O(2))] cores has been characterized by X-ray absorption and resonance Raman spectroscopies, EXAFS analysis shows that this basic core structure gives rise to an Fe-Fe distance of approximately 3.15 A; the distance is decreased by 0.1 A upon introduction of an additional carboxylate bridge. In corresponding resonance Raman studies, vibrations arising from both the Fe-O-Fe and the Fe-O-O-Fe units can be observed. Importantly a linear correlation can be discerned between the nu(O-O) frequency of a complex and its Fe-Fe distance among the subset of complexes with [Fe(III)(2)(mu-OR)(mu-1,2-O(2))] cores (R = H, alkyl, aryl, or no substituent). These experimental studies are complemented by a normal coordinate analysis and DFT calculations.

Mononuclear manganese(II) and iron(III) flavonolates were synthesized as synthetic enzyme-substrate complexes, and their oxygenation reactions as biomimetic functional models with relevance to flavonol 2,4-dioxygenases are briefly described.

Nonheme oxoiron(IV) complexes of two pentadentate ligands, N4Py (N,N-bis(2-pyridylmethyl)-bis(2-pyridyl)methylamine) and Bn-tpen (N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane), have been generated and found to have spectroscopic properties similar to the closely related tetradentate TPA (tris(2-pyridylmethyl)amine) complex reported earlier. However, unlike the TPA complex, the pentadentate complexes have a considerable lifetime at room temperature. This greater thermal stability has allowed the hydroxylation of alkanes with C-H bonds as strong as 99.3 kcal/mol to be observed at room temperature. Furthermore, a large deuterium KIE value is found in the oxidation of ethylbenzene. These observations lend strong credence to postulated mechanisms of mononuclear nonheme iron enzymes that invoke the intermediacy of oxoiron(IV) species.