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Courtemanche M.-A.,Laval University | Legare M.-A.,Laval University | Maron L.,CNRS Laboratory of Physics and Chemistry of Nano-Objects | Fontaine F.-G.,Laval University
Journal of the American Chemical Society | Year: 2013

In this work, we report that organocatalyst 1-Bcat-2-PPh2-C 6H4 ((1); cat = catechol) acts as an ambiphilic metal-free system for the reduction of carbon dioxide in presence of hydroboranes (HBR2 = HBcat (catecholborane), HBpin (pinacolborane), 9-BBN (9-borabicyclo[3.3.1]nonane), BH3·SMe2 and BH 3·THF) to generate CH3OBR2 or (CH 3OBO)3, products that can be readily hydrolyzed to methanol. The yields can be as high as 99% with exclusive formation of CH 3OBR2 or (CH3OBO)3 with TON (turnover numbers) and TOF (turnover frequencies) reaching >2950 and 853 h-1, respectively. Furthermore, the catalyst exhibits "living" behavior: once the first loading is consumed, it resumes its activity on adding another loading of reagents. © 2013 American Chemical Society. Source

Liu B.,CNRS Chemistry Institute of Rennes | Roisnel T.,CNRS Chemistry Institute of Rennes | Maron L.,CNRS Laboratory of Physics and Chemistry of Nano-Objects | Carpentier J.-F.,CNRS Chemistry Institute of Rennes | Sarazin Y.,CNRS Chemistry Institute of Rennes
Chemistry - A European Journal | Year: 2013

The first solvent-free cationic complexes of the divalent rare-earth metals, [{RO}REII]+[A]- (REII= YbII, 1; EuII, 2) and [{LO}REII] +[A]- ([A]-=[H2N{B(C 6F5)3}2]-; RE II=YbII, 3; EuII, 4), have been prepared by using highly chelating monoanionic aminoether-fluoroalkoxide ({RO}-) and aminoether-phenolate ({LO}-) ligands. Complexes 1 and 2 are structurally related to their alkaline-earth analogues [{RO}AE] +[A]- (AE=Ca, 5; Sr, 6). Yet, the two families behave very differently during catalysis of the ring-opening polymerization (ROP) of L-lactide (L-LA) and trimethylene carbonate (TMC) performed under immortal conditions with excess BnOH as an exogenous chain-transfer agent. The ligand was found to strongly influence the behavior of the REII complexes during ROP catalysis. The fluoroalkoxide REII catalysts 1 and 2 are not oxidized under ROP conditions, and compare very favorably with their Ca and Sr congeners 5 and 6 in terms of activity (turnover frequency (TOF) in the range 200-400 molL-LA (molEu h-1)) and control over the parameters during the immortal ROP of L-LA (Mn,theor≈M n,SEC, Mw/Mn<1.05). The Eu II-phenolate 4 provided one of the most effective ROP cationic systems known to date for L-LA polymerization, exhibiting high activity (TOF up to 1 880 molL-LA×(molEu h)-1) and good control (Mw/Mn=1.05). By contrast, upon addition of L-LA the YbII-phenolate 3 immediately oxidizes to inactive RE III species. Yet, the cyclic carbonate TMC was rapidly polymerized by combinations of 3 (or even 1) and BnOH, revealing excellent activities (TOF=5000-7000 molTMC×(molEu h)-1) and unusually high control (Mn,theor≈Mn,SEC, M w/Mn<1.09); under identical conditions, the calcium derivative 5 was entirely inert toward TMC. Based on experimental and kinetic data, a new ligand-assisted activated monomer ROP mechanism is suggested, in which the so-called ancillary ligand plays a crucial role in the catalytic cycle. A coherent reaction pathway computed by DFT, compatible with the experimental data, supports the proposed scenario. In rare form: The first stable, solvent-free, cationic divalent rare-earth complexes are reported. The divalent Yb and Eu complexes are structurally related to their alkaline-earth (Ca, Sr) congeners, but display specific reactivity of their own and yield highly competent and redox-stable catalysts for the controlled immortal ring-opening polymerization (iROP) of cyclic esters and carbonates following a ligand-assisted activated monomer mechanism (see scheme). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Sarazin Y.,CNRS Chemistry Institute of Rennes | Liu B.,CNRS Chemistry Institute of Rennes | Roisnel T.,CNRS Chemistry Institute of Rennes | Maron L.,CNRS Laboratory of Physics and Chemistry of Nano-Objects | Carpentier J.-F.,CNRS Chemistry Institute of Rennes
Journal of the American Chemical Society | Year: 2011

Efficient protocols for the syntheses of well-defined, solvent-free cations of the large alkaline-earth (Ae) metals (Ca, Sr, Ba) and their smaller Zn and Mg analogues have been designed. The reaction of 2,4-di-tert-butyl-6- (morpholinomethyl)phenol ({LO1}H), 2-{[bis(2-methoxyethyl)amino] methyl}-4,6-di-tert-butylphenol ({LO2}H), 2-[(1,4,7,10-tetraoxa-13- azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO3}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3- hexafluoropropan-2-ol ({RO3}H) with [H(OEt2) 2]+[H2N{B(C6F5) 3}2]- readily afforded the doubly acidic pro-ligands [{LO1}HH]+[X]- (1), [{LO 2}HH]+[X]- (2), [{LO3}HH] +[X]- (3), and [{RO3}HH]+[X] - (4) ([X]- = [H2N{B(C6F 5)3}2]-). The addition of 2 to Ca[N(SiMe3)2]2(THF)2 and Sr[N(SiMe3)2]2(THF)2 yielded [{LO2}Ca(THF)0.5]+[X]- (5) and [{LO2}Sr(THF)]+[X]- (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO 2}CaN(SiMe3)2 (7) with [H(OEt2) 2]+[X]-. Complexes [{LO3}M] +[X]- (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO3}M]+[X]- (M = Zn, 13; Mg, 14; Ca, 15; Sr, 16; Ba, 17) were synthesized in high yields (70-90%) by reaction of 3 or 4 with the neutral precursors M[N(SiMe3)2]2(THF) x (M = Zn, Mg, x = 0; M = Ca, Sr, Ba, x = 2). All compounds were fully characterized by spectroscopic methods, and the solid-sate structures of compounds 1, 3, 7, 8, 13, 14, {15}4•3CD2Cl 2, {16}4•3CD2Cl2, and {{17}4•EtOH}•3CD2Cl2 were determined by X-ray diffraction crystallography. Whereas the complexes are monomeric in the case of Zn and Mg, they form bimetallic cations in the case of Ca, Sr and Ba; there is no contact between the metal and the weakly coordinating anion. In all metal complexes, the multidentate ligand is κ6-coordinated to the metal. Strong intramolecular M•••F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}4•3CD2Cl2, {16} 4•3CD2Cl2, and {{17}4• EtOH}•3CD2Cl2. VT 19F{1H} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae•••F energies of stabilization of 25-26 (Ca, Ba) and 40 kcal•mol-1 (Sr) were calculated by NBO analysis on DFT-optimized structures. The identity and integrity of the cationic complexes are preserved in solution in the presence of an excess of alcohol (BnOH, iPrOH) or L-lactide (L-LA). Efficient binary catalytic systems for the immortal ring-opening polymerization of L-LA (up to 3-000 equiv) are produced upon addition of an excess (5-50 equiv) of external protic nucleophilic agents (BnOH, iPrOH) to 8-12 or 13-17. PLLAs with Mn up to 35-000 g•mol-1 were produced in a very controlled fashion (Mw/Mn ≈ 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn < Ca < Sr ≈ Ba; also, Ae complexes supported by the aryloxide ligand are more active than their parents supported by the fluorinated alkoxide ancillary, possibly owing to the presence of Ae•••F interactions in the latter case. The rate law -d[L-LA]/dt = k p•[L-LA]1.0•[16]1.0•[BnOH] 1.0 was established by NMR kinetic investigations, with the corresponding activation parameters ΔH⧧ = 14.8(5) kcal•mol-1 and ΔS⧧ = -7.6(2.0) cal•K-1•mol-1. DFT calculations indicated that the observed order of catalytic activity matches an increase of the L-LA coordination energy onto the cationic metal centers with parallel decrease of the positive metal charge. © 2011 American Chemical Society. Source

Carrey J.,INSA Toulouse | Connord V.,CNRS Laboratory of Physics and Chemistry of Nano-Objects | Respaud M.,CNRS Laboratory of Physics and Chemistry of Nano-Objects
Applied Physics Letters | Year: 2013

We show theoretically that, in an inhomogeneous alternating magnetic field of frequency f, due to the alternating gradient, magnetic nanoparticles oscillate mechanically and generate ultrasound waves. This effect is maximized and better controlled if a static magnetic field is superimposed to an alternating gradient. It makes possible the generation of ultrasounds intracellularly and might also explain recent experiments in magnetic hyperthermia in which cells have been killed without any global temperature increase. Combined to an efficient targeting, it could permit ultrasound therapy with an unprecedented spatial resolution. © 2013 AIP Publishing LLC. Source

Lassagne B.,CNRS Laboratory of Physics and Chemistry of Nano-Objects | Ugnati D.,CNRS Laboratory of Physics and Chemistry of Nano-Objects | Respaud M.,CNRS Laboratory of Physics and Chemistry of Nano-Objects
Physical Review Letters | Year: 2011

We describe herein how a nanoelectromechanical system based on a carbon nanotube used as a force sensor can enable one to assess the magnetic properties of a single and very small nano-object grafted onto the nanotube. Numerical simulations performed within the framework of the Euler-Bernoulli theory of beams predict that the magnetic field dependence of the nanotube mechanical resonance frequency is a direct probe for the nano-object magnetic properties and that a sensitivity around a few (few hundreds) Bohr magnetic moments at low temperature (room temperature) can be expected. © 2011 American Physical Society. Source

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