Bénédicte Douziech

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.

Mimicking the protein access channel to a metal center: effect of a funnel complex on dissociative versus associative copper redox chemistry.

The control of metal-ligand exchange in a confined environment is of primary importance for understanding thermodynamics and kinetics of the electron transfer process governing the reactivity of enzymes. This study reveals an unprecedented change of the Cu(II)/Cu(I) binding and redox properties through a subtle control of the access to the labile site by a protein channel mimic. The cavity effect was estimated from cyclic voltammetry investigations by comparison of two complexes displaying the same coordination sphere (tmpa) and differing by the presence or absence of a calix[6]arene cone surrounding the metal labile site L. Effects on thermodynamics are illustrated by important shifts of E(1/2) toward higher values for the calix complexes. This is ascribable to the protection of the labile site of the open-shell system from the polar medium. Such a cavity control also generates specific stabilizations. This is exemplified by an impressively exalted affinity of the calixarene system for MeCN, and by the detection of a kinetic intermediate, a noncoordinated DMF guest molecule floating inside the cone. Kinetically, a unique dissymmetry between the Cu(I) and Cu(II) ligand exchange capacity is highlighted. At the CV time scale, the guest interconversion is only feasible after reduction of Cu(II) to Cu(I). Such a redox-switch mechanism results from the blocking of the associative process at the Cu(II) state, imposed by the calixarene funnel. All of this suggests that the embedment of a reactive redox metal ion in a funnel-like cavity can play a crucial role in catalysis, particularly for metallo-enzymes associating electron transfer and ligand exchange.

Supramolecular Assembly with Calix[6]arene and Copper Ions − Formation of a Novel Tetranuclear Core Exhibiting Unusual Redox Properties and Catecholase Activity

The supramolecular biomimetic chemistry based on calix[6]arene N-ligands has been further explored. A tris(imidazole)CuI complex was treated with 1 mol-equiv. of cuprous ion under dioxygen to produce a tetranuclear cupric species. X-ray structural determination of this novel Cu4 complex revealed that the self-inclusion of an imidazolyl coordinating arm into the hydrophobic calixarene cavity provides the base of coordination for a {ClImCu(OH)2CuIm2}2 assembly. The Cu4 core is maintained in solution and is stable even in a coordinating solvent such as acetonitrile. Magnetic susceptibility measurements evidenced a strong antiferromagnetic coupling in each Cu(OH)2Cu subunit with J = −408 cm−1. The complex displayed catecholase activity in the presence of 3,5-di-tert-butylcatechol behaving as a four-electron hole with, however, a sluggish CuI4 ⇄ CuII4 regeneration through O2 autooxidation. Finally, electrochemical studies revealed two oxidative reversible processes that successively gave rise to a {CuIICuIII}{CuII2} and a {CuIICuIII}2 mixed-valence species that could be characterized by UV/Vis and EPR spectroscopy. The overall structure and behavior of this tetranuclear complex is reminiscent of multicopper enzymes. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Supramolecular control of an organic radical coupled to a metal ion embedded at the entrance of a hydrophobic cavity

A novel N3ArO-calix[6]arene-based system is presented. It allows the formation of an aryloxy radical bound to a metal ion (CuII or ZnII) that presents a free coordination site in a concave cavity. Its oxidative activity appears highly controlled by the supramolecular system hence providing a good model for radical enzymes such as Galactose oxidase.

Insights into water coordination associated with the Cu(II)/Cu(I) electron transfer at a biomimetic Cu centre.

The coordination properties of the biomimetic complex [Cu(TMPA)(H2O)](CF3SO3)2 (TMPA = tris(2-pyridylmethyl)amine) have been investigated by electrochemistry combined with UV-Vis and EPR spectroscopy in different non-coordinating media including imidazolium-based room-temperature ionic liquids, for different water contents. The solid-state X-ray diffraction analysis of the complex shows that the cupric centre lies in a N4O coordination environment with a nearly perfect trigonal bipyramidal geometry (TBP), the water ligand being axially coordinated to Cu(II). In solution, the coordination geometry of the complex remains TBP in all media. Neither the triflate ion nor the anions of the ionic liquids were found to coordinate the copper centre. Cyclic voltammetry in all media shows that the decoordination of the water molecule occurs upon monoelectronic reduction of the Cu(II) complex. Back-coordination of the water ligand at the cuprous state can be detected by increasing the water content and/or decreasing the timescale of the experiment. Numerical simulations of the voltammograms allow the determination of kinetics and thermodynamics for the water association-dissociation mechanism. The resulting data suggest that (i) the binding/unbinding of water at the Cu(I) redox state is relatively slow and equilibrated in all media, and (ii) the binding of water at Cu(I) is somewhat faster in the ionic liquids than in the non-coordinating solvents, while the decoordination process is weakly sensitive to the nature of the solvents. These results suggest that ionic liquids favour water exchange without interfering with the coordination sphere of the metal centre. This makes them promising media for studying host-guest reactions with biomimetic complexes.

Synthesis and crystal structure of highly soluble ansa-titano- and zirconocene dichloride complexes [Me2Si(η5-C5H2(SiMe3)2]2MCl2 (M=Ti, Zr)

Abstract The synthesis and X-ray characterization of ansa-metallocene dichloride titanium and zirconium complexes of the type [Me2Si(η5-C5H2(SiMe3)2)2]MCl2 (M=Zr (1), Ti (2)) are reported. The complexes have been tested for ethylene polymerization.

A zirconium hydride enolate from carbonylation of the alkyl compound [Zr{C5H2(SiMe3)3-1,2,4}2(Me)2]

Reaction of the hindered, substituted electron-withdrawing cyclopentadienyl zirconium complex [Zr{C5H2(SiMe3)3}2(Me)2] with CO leads, via the acyl intermediate [Zr{C5H2(SiMe3)3}2(COMe)(Me)] detected during the course of the reaction, to the metal hydride enolate [Zr{C5H2(SiMe3)3}2(H){OC(Me)CH2}].

An efficient synthesis of the first electroactive phosphorus-containing bisferrocene macrocycles

Condensation reactions of the phosphodihydrazide XP(Ph)(NMeNH2)2(X = O 1a, S 1b) with ferrocene-1,1′-dicarboxaldehyde 2 afford in good yield the first examples of phosphorus ferrocenyl macrocycles Fe[C5H4CHNNMeP(X)PhNMeNCHC5H4]2Fe (X = O 3a, S 3b) and 3b can be converted to the new compounds Fe[C5H4CH2NHNMeP(S)PhNMeNHCH2C5H4]2Fe 5 and [Fe{C5H4CHNNMeP(S)(Me)PhNMeNCHC5H4}2Fe][CF3SO3]26 by reaction with, respectively, LAH and CF3SO3Me; 5 represents a novel prototype of an anion receptor which electrochemically recognises the H2PO4–, HSO4– and Cl– anions.