Discrete Divalent Rare-Earth Cationic ROP Catalysts: Ligand-Dependent Redox Behavior and Discrepancies with Alkaline-Earth Analogues in a Ligand-Assisted Activated Monomer Mechanism

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 200400molL-LA (molEuh1)) and control over the parameters during the immortal ROP of L-LA (Mn,theor approximate to 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 1880molL-LA(molEuh)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=50007000 molTMC(molEuh)1) and unusually high control (Mn,theor approximate to 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.