Réactivité vis-à-vis de l’oxygène des nouveaux modèles dinucléaires au cuivre : études électrochimiques et spectroscopiques

Methane has the strongest C-H bond of any hydrocarbon (BDE = 104 kcal mol-1); its oxidation under mild conditions remains a great challenge. The particulate Methane Monooxygenase (pMMO) is a copper enzyme that oxides methane (CH4) to methanol (CH3OH). In the active site of the enzyme, two copper ions are located at a short distance (2.6 Å). Recent researches have suggested a mixed-valent Cu2III,II/O2 cluster as a key intermediate in the catalytic cycle. The main objective of this work was the synthesis and characterization of new mixed-valent CuIIICuII bis(μ-oxo) and (μ-OH, μ-O) dinuclear complexes. For this purpose we designed promising symmetrical and unsymmetrical complexes based on specific and distinct scaffolds for each side of the structure. Two families of coordination pattern have been used, polypyridyle or polyamide; the two sites are shortly and rigidly bridged by phenoxo, alkoxo or naphthyridine linkers. New complexes have been characterized by electrochemistry, UV-vis and EPR spectroscopies, and by theoretical calculations. A new cryo-UV-Vis-NIR spectroelectrochemical set up, developed in parallel during this work, has allowed the spectroscopic identification of these transient intermediate species, known to be unstable at room temperature. New mixed-valence Cu2 III,II(μ-OH, μ-O) and Cu2 III,IIbis(μ-OH) complexes have been characterized. These results expand the recent knowledge on the only mixed valent CuIII(μ-OH)CuII species described so far.La molécule de méthane possède la liaison C-H la plus forte parmi les hydrocarbures (BDE = 104 kcal mol-1) : son oxydation en conditions douces représente un challenge d'importance. La Méthane Monoxygénase particulaire (pMMO) est une enzyme à cuivre qui catalyse l'oxydation du méthane (CH4) en méthanol (CH3OH). Le site actif de l'enzyme est composé d'atomes de cuivre séparés par 2.6 Å. Des recherches récentes suggèrent qu'un cluster Cu2 III,II/O2 à valence mixte soit un intermédiaire-clé du cycle catalytique. L'objectif de ce travail vise à la synthèse et caractérisation de nouveaux complexes dinucléaires à valence mixte de type bis(µ-oxo)Cu2 III,II ou (µ-OH, µ-O)Cu2 III,II. Deux familles de motifs coordinants ont été mises en oeuvre, polypyridyle ou polyamide ; les deux sites sont assemblés par des ponts courts et rigides, phenoxo, naphthyridine ou alkoxo. De nouveaux complexes ont été caractérisés par électrochimie, spectroscopies UV-visible et RPE, et par des calculs théoriques. Un dispositif original de cryo-spectroélectrochimie UV-vis-NIR a été développé en parallèle de cette étude : il permet l'identification spectroscopique d'intermédiaires transitoires, réputés très instables à température ambiante. De nouveaux composés à valence mixte, Cu2 III,II(μ-OH, μ-O) et Cu2 III,IIbis(μ-OH) ont été identifiés. Ces résultats élargissent le champ des données de cette famille d'intermédiaires instables limitée jusqu'ici à un seul exemple

Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

International audienceA series of copper/nitrosoarene complexes was created that mimics several steps in biomimetic O2 activation by copper(I). The reaction of the copper(I) complex of N,N,N′,N′-tetramethypropylenediamine with a series of para-substituted nitrosobenzene derivatives leads to adducts in which the nitrosoarene (ArNO) is reduced by zero, one, or two electrons, akin to the isovalent species dioxygen, superoxide, and peroxide, respectively. The geometric and electronic structures of these adducts were characterized by means of X-ray diffraction, vibrational analysis, ultraviolet–visible spectroscopy, NMR, electrochemistry, and density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the ArNO moiety: κN for ArNO, mononuclear η2-NO and dinuclear μ-η2:η1 for ArNO•–, and dinuclear μ-η2:η2 for ArNO2–. 15N isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm–1 for κN-ArNO, 1226 cm–1 for η2-ArNO•–, 1133 cm–1 for dinuclear μ-η2:η1-ArNO•–, and 875 cm–1 for dinuclear μ-η2:η2 for ArNO2–). The 15N NMR signal disappears for the ArNO•– species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O2 analogues. DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion

Influence of Asymmetry on the Redox Properties of Phenoxo- and Hydroxo-Bridged Dicopper Complexes: Spectroelectrochemical and Theoretical Studies

International audienceThe redox properties and electronic structures of a series of phenoxo- and hydroxo-bridged dicopper(II) complexes have been explored. Complexes (1a–c)2+ are based on symmetrical ligands with bis(2-methylpyridyl)aminomethyl as complexing arms bearing different substituting R groups (CH3, OCH3, or CF3) in the para position of the phenol moiety. Complex 2a2+ is based on a symmetrical ligand with bis(2-ethylpyridyl)aminomethyl arms and R = CH3, while complex 3a2+ involves an unsymmetrical ligand with two different complexing arms (namely bis(2-ethylpyridyl)aminomethyl and bis(2-methylpyridyl)aminomethyl). Investigations have been done by electrochemical and spectroelectrochemical means and correlated to theoretical calculations as this series of complexes offers a unique opportunity of an in-depth comparative analysis. The voltammetric studies have shown that the redox behavior of the dicopper complexes is not influenced by the nature of the solvent. However, the increase of the spacer chain length and the unsymmetrical design induce significant modifications of the voltammetric responses for both oxidation and reduction processes. DFT calculations of the redox potentials using a computational reference redox couple calculated at the same level of theory to reduce systematic errors confirm these results. Ligand contributions to the electronic structure of the different species have been analyzed in detail. The good agreement between experimental and theoretical results has validated the developed calculation method, which would be used in the following to design new dinuclear copper complexes. These studies demonstrate that subtle modification of the ligand topology can significantly affect the redox and spectroscopic properties. In particular, the unsymmetrical design allows the formation of a transient mixed-valent Cu(II)-Cu(III) phenoxo complex detected upon spectroelectrochemical experiments at room temperature, which evolves toward a dicopper (II,II) phenoxyl complex. The latter displays an intense π → π* transition band at 393 nm in the UV–vis spectrum compared to the less intense ligand to metal charge transfer band at 518 nm observed for the mixed-valent Cu(II)-Cu(III) phenoxo complex

Characterization of a Dinuclear Copper(II) Complex and Its Fleeting Mixed-Valent Copper(II)/Copper(III) Counterpart

International audienceThe synthesis of a dinuclear copper(II) complex, supported by a 1,3-diamino-2-propanol-based tetraamide ligand, is reported. Structural properties in the solid state and in solution, by means of XRD analysis and NMR spectroscopy, respectively, provide evidence of a highly flexible complex that can display several conformations, leading to the image of the wings of a butterfly. The complex was fully characterized and the redox properties were investigated. Room-temperature spectro-electrochemistry was used to monitor the formation of a metastable mono-oxidized product that displayed an absorption band centered at λ=463 nm. EPR investigation of the low-temperature, chemically generated, mono-oxidized product reveals the presence of an intermediate described as a mixed-valent CuIICuIII species, which is a model of the possible highly oxidizing intermediate in particulate methane monooxygenase

Effect of ligand exchange on the one-electron oxidation process of alkoxo or phenoxo bridged binuclear copper(II) complexes

International audienceIn the present paper, we report on the influence of methanol/methoxide on the structure and the mono-electronic oxidation process of two different dicopper(II,II) complexes: a binuclear µ-alkoxo µ-acetato dicopper(II,II) complex obtained with a tetraamide-containing ligand (1) or a new unsymmetric µ-phenoxo, µ-hydroxo dicopper(II,II) complex based on a ligand bearing one bis(2-pyridylmethyl)aminomethyl arm (BPA) and one thiosemicarbazone (2). Structural data obtained from X-ray diffraction analysis showed that, in the case of complex 2, bridging hydroxide was replaced by a methoxide bound in a bridging mode (2-OMe). Concerning complex 1, redox properties were investigated by electrochemistry and rationalized using theoretical calculations. Our studies indicate that (i) the acetate bridge of 1 is decoordinated from the copper ions after mono-oxidation (ii) in presence of methoxide, acetate is released and a complex with two pendant exogen ligands (one methoxide and one solvent, 1-OMe-DMF) is a plausible structure although bridging methoxide cannot be excluded. In the case of 2 the anodic potentials are not influenced by the nature of the bridge (hydroxo in 2 or methoxo in 2-OMe). Finally, the computed redox potentials using DFT calculations are in good agreement with the experimental ones and indicates that in the case of the complexes derived from 1, oxidation is copper-centered therefore leading to Cu(II)Cu(III) species although oxidation is ligand-centered in the case of 2 and 2-OMe

Effect of Monoelectronic Oxidation of an Unsymmetrical Phenoxido-Hydroxido Bridged Dicopper(II) Complex

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Room-Temperature Characterization of a Mixed-Valent μ-Hydroxodicopper(II,III) Complex

International audienceBis(μ-hydroxo)dicopper(II,II) bearing a naphthyridine-based ligand has been synthesized and characterized in the solid state and solution. Cyclic voltammetry at room temperature displays a reversible redox system that corresponds to the monoelectronic oxidation of the complex. Spectroscopic and time-resolved spectroelectrochemical data coupled to theoretical results support the formation of a charge-localized mixed-valent CuII,III2 species