Organometallics: Materials and Catalysis, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France.
Chemistry. 2013 Mar 18;19(12):3986-94. doi: 10.1002/chem.201204340. Epub 2013 Feb 5.
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 =H2 N{B(C6 F5 )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 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 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 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 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.
二价稀土金属的首例无溶剂阳离子配合物 {RO}RE(II) A (RE(II) =Yb(II), 1; Eu(II), 2) 和 {LO}RE(II) A (A =H2 N{B(C6 F5 )3 }2 ; RE(II) =Yb(II), 3; Eu(II), 4)，已通过使用高螯合的单阴离子氨基醚-氟代醇盐 ({RO}(-) ) 和氨基醚-酚盐 ({LO}(-) ) 配体制备。配合物 1 和 2 在结构上与它们的碱土金属类似物 {RO}AE A (AE=Ca, 5; Sr, 6) 有关。然而，在使用过量的 BnOH 作为外源链转移剂在永生条件下进行 L-丙交酯 (L-LA) 和三亚甲基碳酸酯 (TMC) 的开环聚合 (ROP) 催化过程中，这两个系列的行为非常不同。研究发现，配体强烈影响 ROP 催化过程中 RE(II) 配合物的行为。氟代醇盐 RE(II) 催化剂 1 和 2 在 ROP 条件下不被氧化，并且与 Ca 和 Sr 同类物 5 和 6 相比，在活性（在 200-400 molL-LA (molEu h(-1) ) 范围内的周转率频率（TOF））和对 L-LA 永生 ROP 过程中参数的控制（Mn，理论≈Mn，SEC ，Mw / Mn <1.05）方面表现非常出色。Eu(II) -酚盐 4 为目前为止已知的用于 L-LA 聚合的最有效的 ROP 阳离子体系之一，表现出高活性（TOF 高达 1880 molL-LA ⋅(molEu h)(-1) ）和良好的控制（Mw / Mn =1.05）。相比之下，当添加 L-LA 时，Yb(II) -酚盐 3 立即氧化为非活性 RE(III) 物种。然而，环状碳酸酯 TMC 可通过 3（甚至 1）和 BnOH 的组合迅速聚合，显示出优异的活性（TOF=5000-7000 molTMC ⋅(molEu h)(-1) ）和异常高的控制（Mn，理论≈Mn，SEC ，Mw / Mn <1.09）；在相同条件下，钙衍生物 5 对 TMC 完全没有反应活性。基于实验和动力学数据，提出了一种新的配体辅助活性单体 ROP 机制，其中所谓的辅助配体在催化循环中起着关键作用。通过 DFT 计算的一致反应途径，与实验数据兼容，支持所提出的方案。
