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Discrete Divalent Rare‐Earth Cationic ROP Catalysts: Ligand‐Dependent Redox Behavior and Discrepancies with Alkaline‐Earth Analogues in a Ligand‐Assisted Activated Monomer Mechanism
Author(s) -
Liu Bo,
Roisnel Thierry,
Maron Laurent,
Carpentier JeanFrançois,
Sarazin Yann
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201204340
Subject(s) - cationic polymerization , chemistry , ligand (biochemistry) , catalysis , divalent , chelation , polymerization , medicinal chemistry , alkaline earth metal , monomer , ring opening polymerization , polymer chemistry , stereochemistry , inorganic chemistry , polymer , metal , receptor , organic chemistry , biochemistry
The first solvent‐free cationic complexes of the divalent rare‐earth metals, [{RO}RE II ] + [A] − (RE II =Yb II , 1 ; Eu II , 2 ) and [{LO}RE II ] + [A] − ([A] − =[H 2 N{B(C 6 F 5 ) 3 } 2 ] − ; RE II =Yb II , 3 ; Eu II , 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 RE II complexes during ROP catalysis. The fluoroalkoxide RE II 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 mol L‐LA (mol Eu h −1 )) and control over the parameters during the immortal ROP of L ‐LA ( M n,theor ≈ M n,SEC , M w / M n <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 mol L‐LA ⋅(mol Eu h) −1 ) and good control ( M w / M n =1.05). By contrast, upon addition of L ‐LA the Yb II ‐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 mol TMC ⋅(mol Eu h) −1 ) and unusually high control ( M n,theor ≈ M n,SEC , M w / M n <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.