Stéphane Ménage

Functional models of non-heme diiron enzymes

Abstract Diiron complexes are structural models of active centers of a variety of enzymes (methane monooxygenase, ribonucleotide reductase and purple acid phosphatase). More recently, they proved to have the potential to catalyze the oxidation of alkanes, alcohols, sulfides by peroxides or molecular oxygen (in the presence of electrons) and the hydrolysis of phosphodiesters. By selection of appropriate ligands, some reactions can be made highly selective, as shown from our report of the first enantioselective oxidation catalyzed by a chiral μ-oxo diiron complex.

H2O2‐Dependent Fe‐Catalyzed Oxidations: Control of the Active Species

Manipulation of the coordination sphere of an FeII ion can be used to tune the balance between different catalytic pathways for oxidation (OH. versus iron-based oxidant; see scheme). This reinvestigation of Fenton chemistry uses the iron complex shown as a mechanistic probe.

Dioxygen activation at a mononuclear Cu(I) center embedded in the calix[6]arene-tren core.

The reaction of a cuprous center coordinated to a calix[6]arene-based aza-cryptand with dioxygen has been studied. In this system, Cu(I) is bound to a tren unit that caps the calixarene core at the level of the small rim. As a result, although protected from the reaction medium by the macrocycle, the metal center presents a labile site accessible to small guest ligands. Indeed, in the presence of O2, it reacts in a very fast and irreversible redox process, leading, ultimately, to Cu(II) species. In the coordinating solvent MeCN, a one electron exchange occurs, yielding the corresponding [CalixtrenCu-MeCN](2+) complex with concomitant release of superoxide in the reaction medium. In a noncoordinating solvent such as CH2Cl2, the dioxygen reaction leads to oxygen insertions into the ligand itself. Both reactions are proposed to proceed through the formation of a superoxide-Cu(II) intermediate that is unstable in the Calixtren environment due to second sphere effects. The transiently formed superoxide ligand either undergoes fast substitution for a guest ligand (in MeCN) or intramolecular redox evolutions toward oxygenation of Calixtren. Interestingly, the latter process was shown to occur twice on the same ligand, thus demonstrating a possible catalytic activation of O2 at a single cuprous center. Altogether, this study illustrates the oxidizing power of a [CuO2](+) adduct and substantiates a mechanism by which copper mono-oxygenases such as DbetaH and PHM activate O2 at the Cu(M) center to produce such an intermediate capable of C-H breaking before the electron input provided by the noncoupled Cu(H) center.

Re-examination of the formation of dinitrosyl–iron complexes during reaction of S-nitrosothiols with Fe(II)

The reaction of S-nitrosothiol compounds with ferrous ions in solution has been investigated and the generated dinitrosyl–iron complexes have been characterized. During the reaction of S-nitrosocysteamine with Fe(II) in water solution in the presence of a twofold excess (with respect to iron) of cysteamine hydrochloride (CSH), an EPR-silent dinuclear iron complex (complex A of formula [Fe2(RS)2(NO)4]) was formed as the major species and was characterized by FAB MS + , UV–Vis, NMR and IR spectroscopies. In the presence of a large excess of CSH (CSH/Fe(II)= 20:1), a green paramagnetic mononuclear complex (complex B of formula [Fe(RS)2(NO)2] − ) was formed. From EPR and UV–Vis data, and also on the basis of the few crystallographic structures known for similar complexes, complex B is proposed to display a distorted tetrahedral geometry (C2), approaching a trigonal bipyramid with a missing ligand, with the unpaired electron mainly localized on the d z 2 orbital of the iron characterized by a d 9 electronic configuration.

Photocatalyzed sulfide oxygenation with water as the unique oxygen atom source.

In our research program aiming to develop new ruthenium-based polypyridine catalysts for oxidation we were interested in combining a photosensitizer and a catalytic fragment within the same complex to achieve catalytic light-driven oxidation. To respond to the lack of such conjugates, we report here a new catalytic system capable of using light to activate water molecules in order to perform selective sulfide oxygenation into sulfoxide via an oxygen atom transfer from H(2)O to the substrate with a TON of up to 197 ± 6. On the basis of electrochemical and photophysical studies, a proton-coupled electron-transfer process yielding to an oxidant Ru(IV)-oxo species was proposed. In particular, the synergistic effect between both partners in the dyad yielding a more efficient catalyst compared to the bimolecular system is highlighted.

μ-Oxo diferric complexes as oxidation catalysts with hydrogen peroxide and their potential in asymmetric oxidation

Abstract Non-heme diiron complexes, as models of methane monooxygenase, provide a new class of catalysts for oxidation reactions. We show here that such complexes have the potential for catalyzing stereoselective oxidations of sulfides by hydrogen peroxide.

A new chiral diiron catalyst for enantioselective epoxidation

The dinuclear chiral complex Fe(2)O(bisPB)(4)(X)(2)(ClO(4))(4) (X = H(2)O or CH(3)CN) catalyzes with high efficiency (up to 850 TON) and moderate enantioselectivity (63%) the epoxidation of electron deficient alkenes at 0 degrees C by a peracid.

A Dyad as Photocatalyst for Light-Driven Sulfide Oxygenation with Water As the Unique Oxygen Atom Source

With the objective to convert light energy into chemical oxidation energy, a ruthenium-based dyad constituted of the assembly of a photosensitizer and a catalytic fragment was synthesized. Upon irradiation with blue LEDs, and in the presence of an electron acceptor, the complex is able to catalyze selective sulfide oxygenation involving an oxygen atom transfer from water to the substrate. Electrochemical and photophysical studies highlighted a proton-coupled electron transfer (PCET) to access to a high valent oxidant Ru(IV) oxo species.

The Protein Environment Drives Selectivity for Sulfide Oxidation by an Artificial Metalloenzyme

Magic Mn–salen metallozyme: The design of an original, artificial, inorganic, complex‐protein adduct, has led to a better understanding of the synergistic effects of both partners. The exclusive formation of sulfoxides by the hybrid biocatalyst, as opposed to sulfone in the case of the free inorganic complex, highlights the modulating role of the inorganic‐complex‐binding site in the protein.

µ-Oxo-bridged diiron(III) complexes and H2O2: monooxygenase- and catalase-like activities

The µ-oxo-bridged diiron(III) complex [Fe2O(bipy)4(OH2)2][ClO4]41(bipy = 2,2′-bipyridine) was found to exhibit monooxygenase-like activity, using H2O2 as the oxidant. The system oxidizes alkanes to alcohols and ketones quite efficiently (46 mmol of cyclohexanol + cyclohexanone per rnmol complex in 10 min). In the case of adamantane, selectivity for the tertiary hydrogen was indicated by a high normalized C3:C2 ratio of 9:1. The same reaction yields and rates were obtained whether argon or dioxygen was bubbled through the solution. Dimethyl sulfide was transformed into dimethyl sulfoxide and dimethylsulfone and benzene into phenol. These results exclude O2 as a key reactant in this system and suggest that high-valent oxoiron intermediates and hydroxyl radicals are the active species. The potential of this system is strongly limited by the instability of the catalyst and by its strong catalase-like activity. Complex 1 is actually a very efficient catalyst for hydrogen peroxide dismutation, thus transforming 50% of the excess of H2O2 into O2 in 10 min.