Electron transfer properties of mono- and diferrocenyl based Cu complexes attached as self-assembled monolayers on gold electrodes by "self-induced" electroclick

International audienceTwo new Cu complexes bearing a 6-ethynyl bis-(methyl-pyridyl) amine (6eBMPA) moiety, as an electroclickable function linked to a ferrocenyl-based triazolyl arm (ligands 3 and 4) have been synthetized and characterized by UV-Visible, EPR spectroscopies and cyclic voltammetry in acetonitrile. Two different spacer groups between the terminal ferrocene and the triazolyl group were inserted: an hexyl chain in the case of the complex Cu-3, an ethenyl-bridged diferrocenyl system for the complex Cu-4. The monoelectronic oxidation of the diferrocenyl species yields a stable mixed-valence complex. NIR-Visible spectroscopic studies show a moderate interaction between ferrocenyl units (class II according to the Robin-Day classification). The immobilization of these systems as SAMs on an azidoundecanethiol modified gold electrode has been successfully operated by using the "self-induced electroclick" procedure. The voltammetric characterization of the surface-tagged Cu complexes indicates that good surface coverage was achieved, with moderately fast electron-transfer reaction between the electrode and the redox active immobilized systems (k0(Cu) = 2-4 s−1, k0(Fc) = 20-90 s−1). Remarkably, the rate of charge transport is significantly controlled by the nature of the spacer on the ferrocenyl triazole arm

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

International audienceThe 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

Models of metallo-enzyme active sites - Calixarenes and enzyme mimicry

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Determination of heterogeneous electron-transfer kinetics of decamethylferrocene at low temperatures (120 K < T < 200 K) by a.c. impedance

International audienceLow-temperature heterogeneous electron-transfer kinetics of decamethylferrocene obtained from a.c. impedance analysis are reported. The experiments have been performed for 120 K < T < 200 K with a gold working electrode in two different electrolyte solution mixtures containing equal amounts of decamethylferrocene and decamethylferricinium, tetrafluoroborate: 7:1:16 (v:v:v) tetrahydrofuran/2-methyltetrahydrofuran/ethylchloride with lithium tetrafluoroborate, and 1:2 (v:v) butyronitrile/ethylchloride with tetrabutylammonium perchlorate. Values of standard rate constants for each temperature have been evaluated by fitting the experimental a.c. impedance spectra with a simple RC parallel circuit in the middle frequency part of complex-plane plots. The analysis of Arrhenius plots, corrected from solvent dynamics (In k(0)T(1/2) vs T-1), for the range of temperatures studied yields the free activation energy Delta G* and the pre-exponential factor A' for the two solvents mixtures (respectively 18.22 kJ mol(-1) and 1140 cm s(-1) in the tetrahydrofuran-based solvent system, 22.53 U mol(-1) and 2618 cm s(-1) in the butyronitrile-based solvent system). The very slight discrepancies between the experimental results and theoretical values calculated from the solvent dielectric continuum model are discussed. (c) 2006 Elsevier B.V. All rights reserved

Supramolecular Modeling of Mono-copper Enzyme Active Sites with Calix[6]arene-based Funnel Complexes

Conspectus Supramolecular bioinorganic chemistry is a natural evolution in biomimetic metallic systems since it constitutes a further degree of complexity in modeling. The traditional approach consisting of mimicking the first coordination sphere of metal sites proved to be very efficient, because valuable data are extracted from these examples to gain insight in natural systems mechanisms. But it does not reproduce several specific aspects of enzymes that can be mimicked by the implementation of a cavity embedding the labile active site and thus controlling the properties of the metal ion by noncovalent interactions. This Account reports on a strategy aimed at reproducing some supramolecular aspects encountered in the natural systems.The cavity complexes described herein display a coordination site constructed on a macrocycle. Thanks to a careful design of the cavity-based ligands, complexes orienting their labile site specifically toward the inside of the macrocycle were obtained. The supramolecular systems are based on the flexible calix[6]arene core that surrounds the metal ion labile site, thereby constraining exogenous molecules to pass through the conic funnel to reach the metal center. Such an architecture confers to the metal ion very unusual properties and behaviors, which in many aspects are biologically relevant. Three generations of calix[6]-based ligands are presented and discussed in the context of modeling the monocopper sites encountered in some enzymes.A wide range of phenomena are highlighted such as the impact that the size and shape of the access channel to the metal center have on the selectivity and rate of the binding process, the possible remote control of the electronics through small modifications operated on the cavity edges, induced-fit behavior associated with host-guest association (shoe-tree effect) that affects the redox properties of the metal ion and the electron exchange pathway, consequences of forbidden associative ligand exchange allowing a redox switch to drive an "antithermodynamic" ligand exchange, drastic effects of the full control of the second coordination sphere, and dioxygen activation in a confined chamber conducted to a selective and unusual four-electron redox process. All these findings bring new clues for better understanding the control exerted by the proteic environment on a metal center, allow the identification of new reaction pathways, and lead to new proposals for enzymatic catalytic cycle (such as the formation of an alkylhydroperoxide intermediate for mononuclear Cu-hydroxylases). The supramolecular systems may also be exploited for designing highly selective and sensitive probes for molecules of particular function and shape or to design new selective catalysts.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Electrochemically Triggered Double Translocation of Two Different Metal Ions with a Ditopic Calix[6]arene Ligand

International audienceA ditopic ligand based on a calix[6]arene with three imidazoles (Im) appended at the small rim and three triazoles (Tria) at the large one is able to form selectively two stable heterodinuclear complexes with Zn(Im)(II)/Cu(Tria)(I) and Cu(Im)(II)/Zn(Tria)(II). In the Cu(I) case, the zinc cation is preferentially coordinated at the Im site while the copper is bound at the Tria site. The situation is the opposite when Cu(II) is used. The position of the two cations within the complex can be electrochemically switched via the oxidation reduction of the copper cation between oxidation states +I and +II. The presence of the zinc cation is crucial (i) to control the bistability of the system by an allosteric structuring role and (ii) to promote the metal switch since the monocopper complex exhibits reversible behavior with Cu located at the imidazole site in both oxidation states. This represents the first example of a double translocation of two different metal cations

Tris(triazolyl) Calix[6]arene-Based Zinc and Copper Funnel Complexes: Imidazole-like or Pyridine-like? A Comparative Study

International audienceHuisgen dipolar cycloaddition leads straightforwardly to new funnel complexes based on the calix[6]arene macrocycle bearing three functionalized triazoles as coordinating units at the small rim. Coordination to Zn(II) and Cu(I) cations was studied using (1)H NMR and IR spectroscopies and cyclic voltammetry. The nature of the substituents on the triazole ring affects the behavior of the ligands and their coordinating ability and controls the host guest properties of the metal receptors for exogenous substrates. Depending on their substitution pattern but also on the metal ion and the guest ligand, the triazole-based systems behave either imidazole-like or pyridine-like. The ease of preparation and the versatility of 1,4-disubstituted-1,2,3-triazoles with tunable steric and electronic properties make them promising candidates for further applications from biology to materials

Funnel complexes with CoII and NiII: New probes into the biomimetic coordination ability of the calix[6]arene-based tris(imidazole) system

cited By 30International audienceThe coordination properties of the calix[6]arene-based tris-(imidazole) ligand X6Me3Imme3 were further explored with CoII and NiII. This imidazole system stabilizes tetrahedral mononuclear CoII complexes with an exchangeable fourth exogenous ligand (water, alcohol, amide) located at the heart of the hydrophobic calixarene cavity. With a weak donor ligand such as a nitrile, both four-coordinate tetrahedral and five-coordinate trigonal bipyramidal complexes were obtained. The latter contains a second nitrile molecule trans to the included guest nitrile. These complexes were characterized in solution as well as in the solid state. The NiII complexes are square-based pyramidal five-coordinate edifices with a guest nitrile inside the cavity and a water molecule outside. A comparison with previously described ZnII and CuII complexes emphasizes the flexibility of this ligand. A comparison with carbonic anhydrase, a mononuclear zinc enzyme with a tris(histidine) coordination core, shows that X6Me 3Imme3 displays many structural features of this enzyme except for the cis coordination of the exogenous ligands. © Wiley-VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004

Chemoselective guest-triggered shaping of a polynuclear Cu II calix[6]complex into a molecular host

International audienceA new calix[6]arene scaffold bearing a tris-imidazole binding site at the small rim and three tetradentate aza ligands at the large rim was synthesized. The system binds three CuII ions at the large rim sites and is unable to bind a fourth one, which remains in solution. The charge repulsion between the complexes, together with the flexibility of the scaffold, disorganizes the small rim site for binding and prevents its use for host–guest studies. Although the presence of MeCN or DMF guests does not alter this state, the addition of a heptylamine guest, which further displays Brønsted basicity, restores its receptor ability by stabilizing the extra CuII ion at the tris-imidazole site with concomitant guest encapsulation and binding of an exo hydroxo ligand. This chemoselective nuclearity switch yields a tetranuclear complex in which the guest backbone is preorganized in front of three potentially reactive Cu(II) complexes, reminiscent of polynuclear CuII enzyme active sites

Synthesis of an unsymmetrical N-functionalized triazacyclononane ligand and its Cu(II) complex

International audienceThe unsymmetrical 1,4-bis(2-aminophenyl)-7-(pyridin-2-ylmethyl)-1,4,7-triazacyclononane ligand (L3) has been prepared and characterized by NMR spectroscopy. The L3 ligand is based on the triazamacrocycle ring bearing one flexible 2-pyridylmethyl linked to the macrocycle group via the methyl group, and two rigid 2-aminophenyl pendant donor groups linked to the macrocycle via the aromatic carbon atoms. Reaction of this ligand with Cu(ClO4)(2)center dot 6H(2)O afforded the corresponding complex [Cu(L3)](ClO4)(2)center dot H2O (4) which was structurally characterized both in solid state and in solution. The crystal structure of 4 consists of a discrete monomeric [Cu(L3)](2+) in which the Cu(II) ion is six coordinated with three nitrogen atoms of the macrocycle ring, two of the aminophenyle and one of the pyridine appended functions. The triazacyclonane macrocycle ring is facially coordinated and the N-donor atoms of the three pendant groups (two aniline and one pyridine groups), are disposed in the same side of the basal macrocyclic ring, leading to a distorted and elongated CuN4N2 octahedron. UV-Vis spectroscopy of complex 4 in acetonitrile displays a d-d transition band at lambda = 673 nm. The voltammetric studies show that the [Cu(L3)](2+) cation can be reduced quasi-reversibly and oxidized irreversibly, both process being monoelectronic. (C) 2014 Elsevier B.V. All rights reserved