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- Title
Discrete Divalent Rare-Earth Cationic ROP Catalysts: Ligand-Dependent Redox Behavior and Discrepancies with Alkaline-Earth Analogues in a Ligand-Assisted Activated Monomer Mechanism.
- Authors
Liu, Bo; Roisnel, Thierry; Maron, Laurent; Carpentier, Jean ‐ François; Sarazin, Yann
- Abstract
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(C6F5)3}2]−; REII=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≈ Mn,SEC, Mw/ Mn<1.05). The EuII-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 REIII 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, Mw/ 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.
- Publication
Chemistry - A European Journal, 2013, Vol 19, Issue 12, p3986
- ISSN
0947-6539
- Publication type
Article
- DOI
10.1002/chem.201204340