Mimicking the protein access channel to a metal center: effect of a funnel complex on dissociative versus associative copper redox chemistry.
ABSTRACT 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 calixarene 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.
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ABSTRACT: A new ligand with N8O2 donors containing three potential metal-binding sites () and its tricopper(ii) complex are synthesized. The tricopper species is found to be formed from a hypodentate dicopper(ii) complex in basic solutions. Complex may be isolated from the reaction of with a copper source under acidic conditions. Complex can undergo CO2-abstraction to yield an octacopper(ii) complex . The single crystal structures of complexes and are characterized by X-ray crystallography.Dalton Transactions 03/2014; · 4.10 Impact Factor
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ABSTRACT: 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.Dalton Transactions 03/2014; · 4.10 Impact Factor
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ABSTRACT: The first part of the study is devoted to the comparison between the doping effect of urea (a small molecule) and polyethylene glycol (PEG, a long-chain polymer) on the physical property of metallic gallium (Ga). The physical properties of the Ga composited in the two materials, Ga/urea and Ga/PEG, were investigated by scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, differential scanning calorimetry, superconducting quantum interference device, and surface-enhanced Raman scattering spectra and compared with our previous results for the effect of macrocyclic hosts (e.g., cyclodextrins, calixarenes) on the physical modification of metallic Ga. Our data provide new direct evidence that the modification of physical properties of Ga is highly dependent on the nature of dopants used. For example, the addition of a small amount of urea causes a fundamental change in the crystallization behavior of Ga, and the presence of PEG results in the occurrence of a weak paramagnetism of Ga at high fields, both of which are completely different from the effect of other dopants. The other part of the study is devoted to demonstrating whether there is a significant difference in the oxidation process of metallic Ga and its composites. Our result gives a strong positive answer to the question. β- and γ-gallium oxide nanocrystals were obtained by sintering the Ga/urea composite at different temperatures and exhibited distinctive photoluminescence and photocatalysis properties. These results gave a strong impression that the introduction of different dopants leads metallic Ga to generate different features in microstructure, physical property, and especially chemical reactivity. We believe that the findings of this study have important implications for the development of inorganic materials.The Journal of Physical Chemistry C 10/2012; 116(43):22859–22866. · 4.84 Impact Factor