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A heteroditopic ligand associated with a calix[6]arene scaffold bearing a tris(imidazole) coordinating site at its small rim and an amine/pyridine ligand at its large rim has been prepared, and its regioselective coordination to ZnII at the small rim and FeII in the amine/pyridine ligand has been achieved. The heterodinuclear complex obtained displays an overall cone conformation capped by the tris(imidazole)ZnII moiety and bears a non-heme FeII complex at its base. Each of the metal centers exhibits one labile position, allowing the coordination inside the cavity of a guest alkylamine at ZnII and the generation of reaction intermediates (FeIII(OOH) and FeIVO) at the large rim. A dependence between the chain length of the encapsulated alkylamine and the distribution of FeIII(OOH) intermediates and FeIII(OMe) is observed. In addition, it is shown that the generation of the FeIVO intermediate is enhanced by addition of the alkylamine guest. Hence, this supramolecular system gathers the three levels of reactivity control encountered in oxidoreductases: i) control of the FeII redox properties through its first coordination sphere, allowing us to generate high valent reactive species; ii) control of guest binding through a hydrophobic funnel that drives its alkyl chain next to the reactive iron complex, thus mimicking the binding pocket of natural systems; iii) guest-modulated reactivity of the FeII center towards oxidants. [ABSTRACT FROM AUTHOR]

A coordinatively unsaturated FeII complex bearing a pentadentate ligand (N,N′,N′-tris(2-pyridyl-methyl)-1,2-diaminoethane) functionalized with a cyclic disulfide group has been prepared in order to graft reactive metal entities as self-assembled monolayers (SAMs) on gold electrodes. Prior to grafting, exogenous ligand exchange has been investigated by cyclic voltammetry (CV) in solution, showing that the nature of the first coordination sphere (N5)FeII–X (X = Cl−, OTf−, MeCN, acetone) can be tuned, thanks to the control of the chemical conditions. The FeII complex has been immobilized on gold electrodes by spontaneous (passive) adsorption as well as by an electro-assisted method. The resulting SAMs were characterised by XPS and AFM analyses. CV experiments implementing these SAMs as working electrodes showed that the first coordination sphere of the grafted FeII complex can be controlled by adjusting the chemical conditions, similarly to the studies in a homogeneous solution. Finally, the supported FeII complex proved to be reactive with superoxide generated at the electrode surface by reduction of dissolved dioxygen. Under the employed conditions, leaking of the metal complex was not observed. [ABSTRACT FROM AUTHOR]

Light induced enantioselective oxidation of an organic molecule with water as the oxygen atom source is demonstrated in a system where chirality is induced by a protein, oxygen atom transfer by a manganese corrole, and photocatalysis by ruthenium complexes. [ABSTRACT FROM AUTHOR]

An artificial metalloenzyme based on the covalent grafting of a nonheme FeII polyazadentate complex into bovine β-lactoglobulin has been prepared and characterized by using various spectroscopic techniques. Attachment of the FeII catalyst to the protein scaffold is shown to occur specifically at Cys121. In addition, spectrophotometric titration with cyanide ions based on the spin-state conversion of the initial high spin ( S=2) FeII complex into a low spin ( S=0) one allows qualitative and quantitative characterization of the metal center's first coordination sphere. This biohybrid catalyst activates hydrogen peroxide to oxidize thioanisole into phenylmethylsulfoxide as the sole product with an enantiomeric excess of up to 20 %. Investigation of the reaction between the biohybrid system and H2O2 reveals the generation of a high spin ( S=5/2) FeIII(η2-O2) intermediate, which is proposed to be responsible for the catalytic sulfoxidation of the substrate. [ABSTRACT FROM AUTHOR]

Heme and nonheme monoxygenases and dioxygenases catalyze important oxygen atom transfer reactions to substrates in the body. It is now well established that the cytochrome P450 enzymes react through the formation of a high-valent iron(IV)-oxo heme cation radical. Its precursor in the catalytic cycle, the iron(III)-hydroperoxo complex, was tested for catalytic activity and found to be a sluggish oxidant of hydroxylation, epoxidation and sulfoxidation reactions. In a recent twist of events, evidence has emerged of several nonheme iron(III)-hydroperoxo complexes that appear to react with substrates via oxygen atom transfer processes. Although it was not clear from these studies whether the iron(III)-hydroperoxo reacted directly with substrates or that an initial OO bond cleavage preceded the reaction. Clearly, the catalytic activity of heme and nonheme iron(III)-hydroperoxo complexes is substantially different, but the origins of this are still poorly understood and warrant a detailed analysis. In this work, an extensive computational analysis of aromatic hydroxylation by biomimetic nonheme and heme iron systems is presented, starting from an iron(III)-hydroperoxo complex with pentadentate ligand system (L52). Direct CO bond formation by an iron(III)-hydroperoxo complex is investigated, as well as the initial heterolytic and homolytic bond cleavage of the hydroperoxo group. The calculations show that [(L52)FeIII(OOH)]2+ should be able to initiate an aromatic hydroxylation process, although a low-energy homolytic cleavage pathway is only slightly higher in energy. A detailed valence bond and thermochemical analysis rationalizes the differences in chemical reactivity of heme and nonheme iron(III)-hydroperoxo and show that the main reason for this particular nonheme complex to be reactive comes from the fact that they homolytically split the OO bond, whereas a heterolytic OO bond breaking in heme iron(III)-hydroperoxo is found. [ABSTRACT FROM AUTHOR]

The reaction of the iron(ii) complex supported by the ligand L52aH (2,2-dimethyl-N-[6-({[2-(methyl-pyridin-2-ylmethyl-amino)-ethyl]-pyridin-2-ylmethyl-amino}-methyl)-pyridin-2-yl]-propionamide) with dioxygen, in the presence of HClO4and NaBPh4yields the corresponding low spin (S = 1/2) FeIII(OOH) complex. This reaction, where the anion BPh4−acts as an electron donor, is reminiscent of the reductive activation of O2by enzymatic systems. Under these specific experimental conditions, the hydroperoxoiron(iii) complex evolves in an unexpected way to yield a high spin (S = 5/2) green species. This transformation is shown to arise from the reaction between the hydroperoxoiron(iii) intermediate and BPh3, a side product of BPh4−. [ABSTRACT FROM AUTHOR]

Two new mononuclear FeII complexes, [(L5²aH)FeII](PF6)2 (1-(PF6)2) and [(L5²a)FeII]BPh4 (2-(BPh4)) have been synthesized with the new aminopyridyl ligand bearing a pivaloylamido arm L5²aH (2,2-dimethyl-N-[6-({[2-(methyl-pyridin-2-ylmethyl-amino)-ethyl]-pyridin-2-ylmethyl-amino}-methyl)-pyridin-2-yl]-propionamide), or its deprotonated form L5²a-. The structures of the ferrous complexes have been determined by X-ray analysis. The mononuclear FeII is in a pseudo-octahedral environment in both complexes, the six positions around the metal center being occupied by five nitrogen atoms and one oxygen atom from the ligand. Whatever the protonation state of the amide function, the structures are very similar, the FeII being 6-fold coordinated by the two amines, three pyridines, and the oxygen atom from the ligand. These two complexes exhibit an acid/base equilibrium in solution that has been studied by UV-vis spectroscopy and cyclic voltammetry in acetonitrile. The reactivity of 1-(PF6)2 with H2O2 in methanol affords the formation of a new low-spin FeIII(OOH) intermediate in which the oxygen atom is retained in the coordination sphere of the metal. [ABSTRACT FROM AUTHOR]

Three new FeIII complexes have been synthesized with the tridentate ligand salEen (salEen=N-ethyl-N-(2-aminoethyl)salicylaldiminate). Two of them are neutral dinuclear complexes and their structures have been resolved. Their formulae are [Fe(salEen)(Cl)(μ-CH3O)]2 and [Fe(salEen)(DBC)]2 (DBC=3,5 di-tert-butylcatecholate), respectively. These complexes exhibit a ‘diamond-core’ geometry with the ferric ions antiferromagnetically coupled by the bridging oxygens of the methoxy or DBC ligands, respectively. The last complex is a mononuclear one of formula [Fe(HsalEen)2Cl2]Cl, with the protonated ligand acting as a bidentate one. This compound is actually a mixture of two complexes with the same formula but with different coordination of the HsalEen fragments. [Copyright &y& Elsevier]

Abstract: Two iron(II) complexes with ligands related to TPEN, N,N,N′,N′-tetrakis-(2-pyridylmethyl)ethane-1,2-diamine, were prepared, characterized and compared as catalysts for the hydroxylation of aromatic compounds by H2O2. The (TPEN)FeII complex gave the best yields ever reported for such iron polyazadentate catalysts with yields up to 86, 59, 59 and 38% for the hydroxylation of anisole, benzene, chlorobenzene and ethylbenzene, respectively. Experiments using H2 18O2 and H2 18O clearly showed that the oxygen atom incorporated in the aromatic substrates almost exclusively comes from H2O2. Preliminary experiments on the effects of O2 and spin-traps on the reactions suggested a minor contribution of uncontrolled radicals such as the presently available data, a tentative mechanism based on the hydroxylation of substrates by a FeIIIOOH intermediate or, more probably, by a species resulting from the O–O bond cleavage of this intermediate, such as [FeIVO+ roposed. [Copyright &y& Elsevier]

We report an in-depth study of the reductive activation of O 2 by the nonheme [Fe II (L25)(MeCN)] 2+ complex carried out by cyclic voltammetry. Experimental evidence is obtained for the slow coordination of dioxygen to the ferrous center yielding an Fe II /O 2 adduct with a strong Fe II -O 2 character rather than an Fe III -superoxo one. Electron injection in the Fe II -O 2 species occurs at a potential of ca. -700 mV vs. SCE, i.e. 200 mV above the O 2 to O 2 ˙ - reduction, leading to the formation of a Fe III -peroxo intermediate and then Fe III -hydroperoxo upon protonation by residual water. The experimental CVs recorded at variable scan rate or variable Fe II concentration are well simulated taking into account a detailed mechanism initiated by the competitive reduction of O 2 and the Fe II -O 2 adduct. Analysis of the concentration of the reaction intermediates generated as a function of the applied potential indicates that the Fe III -peroxo intermediate significantly accumulates at a potential of -650 mV. Oxidative bromination of anisole is assayed under electrolytic conditions at this potential to yield bromoanisole products. The low faradaic yields observed reveal that deleterious reactions such as direct reduction of reaction intermediates likely occur. Based on the detailed mechanism elucidated, a number of improvements to achieve more efficient catalytic reactions can be proposed.