Diamido-Ether Actinide Complexes as Initiators for Lactide Ring-Opening Polymerization

The synthesis and characterization of a series of new
diamido-ether actinide­(IV) alkoxide complexes
are reported. Addition of 2 equiv of LiOiPr to [tBuNON]­ThCl5Li3·DME ([tBuNON]2– = [(tBuNSiMe2)2O]2–) in toluene gives [tBuNON]­Th­(OiPr)3Li·DME
(1-DME). Recrystallization of 1-DME from
diethyl ether gives [tBuNON]­Th­(OiPr)3Li·Et2O (1-Et2O). The addition of 2 equiv
of LiOiPr to {[tBuNON]­UCl2}2 gives {[tBuNON]­U­(OiPr)2}2 (2). If KOtBu is
used instead, the product is {[tBuNON]­U­(OtBu)}2 (3). The reaction
of 2 equiv of KOtBu with [iPr2PhNCOCN]­ThCl2·DME ([iPr2PhNCOCN]2– = [(2,6-iPr2PhNCH2CH2)2O]2–) gives [iPr2PhNCOCN]­Th­(OtBu)2 (4), while addition of 1 equiv
of KOtBu to [iPr2PhNCOCN]­UCl3Li·THF results in [iPr2PhNCOCN]­U­(OtBu)Cl (5). Complexes 1–5 as well as the previously reported diamido actinide dialkyl
complexes [tBuNON]­An­(CH2SiMe3)2 (An = Th (6), U (7)) and [iPr2PhNCOCN]­An­(CH2SiMe3)2 (An = Th (8), U (9)) were evaluated as initiators for the ring-opening polymerization
(ROP) of l-lactide (l-LA) and racemic lactide (rac-LA). It was established that complexes 1–8 were capable of producing poly­(l-LA)
(PLLA) under mild conditions within relatively short periods of time.
Diisopropoxide complex 2 polymerized up to 500 equiv
of l-LA in 90 min at 30 °C, with moderate to good control
over the molecular weight features (values of Mw/Mn with this complex are typically
1.5 or below). The hydrocarbyl complexes 6–8 also proved active under the same conditions, with 7 showing a similar ability to control the ROP with monomer
loadings up to 500 equiv. PLLAs prepared with initiators 1–5 and 6–8 have
been characterized by end-group analysis via NMR and MALDI-TOF MS,
which indicate clearly that the diamido ligand does not participate
as an initiating group in the ROP process and that the alkoxide and
alkyl moieties, respectively, are the only initiating groups. When
it was applied to mediate the ROP of rac-LA, 2 yielded heterotactically enriched PLA (Pr values up to 0.73 in THF).

本文报道了一系列新型二酰胺醚锕系(IV)醇盐配合物的合成与表征。将2当量异丙醇锂（LiOiPr）加入甲苯中的[tBuNON]·ThCl5Li3·DME（[tBuNON]²⁻ 即[(tBuNSiMe2)2O]²⁻）体系中，可得到[tBuNON]·Th(OiPr)₃Li·DME（1-DME）。将1-DME从乙醚中重结晶，即可分离得到[tBuNON]·Th(OiPr)₃Li·Et₂O（1-Et2O）。将2当量异丙醇锂加入{[tBuNON]·UCl₂}₂中，得到{[tBuNON]·U(OiPr)₂}₂（2）；若改用叔丁醇钾（KOtBu），则目标产物为{[tBuNON]·U(OtBu)}₂（3）。将2当量叔丁醇钾与[iPr2PhNCOCN]·ThCl₂·DME（[iPr2PhNCOCN]²⁻ 即[(2,6-iPr2PhNCH2CH2)2O]²⁻）反应，得到[iPr2PhNCOCN]·Th(OtBu)₂（4）；而向[iPr2PhNCOCN]·UCl₃Li·THF中加入1当量叔丁醇钾，则可得到[iPr2PhNCOCN]·U(OtBu)Cl（5）。本研究以配合物1~5及此前已报道的二酰胺锕系二烷基配合物[tBuNON]·An(CH2SiMe3)2（An=Th(6)、U(7)）和[iPr2PhNCOCN]·An(CH2SiMe3)2（An=Th(8)、U(9)）作为引发剂，评价其对L-丙交酯（l-LA）与外消旋丙交酯（rac-LA）的开环聚合（ring-opening polymerization, ROP）的催化性能。研究结果表明，配合物1~8可在温和条件下于较短时间内合成聚(L-丙交酯)(PLLA)。其中二异丙氧基配合物2在30℃、90分钟内可聚合多达500当量的L-丙交酯，对聚合物的分子量分布具有中等至良好的控制效果（该配合物对应的Mw/Mn值通常不超过1.5）。烷基配合物6~8在相同条件下同样表现出催化活性，其中配合物7在单体投料量高达500当量时，仍可实现对开环聚合的类似控制效果。通过核磁共振（NMR）与基质辅助激光解吸/电离飞行时间质谱（MALDI-TOF MS）对以引发剂1~5和6~8制备得到的PLLA进行端基分析，结果明确显示二酰胺配体并未参与开环聚合的引发过程，而醇盐与烷基片段分别为唯一的引发活性位点。当将配合物2用于介导外消旋丙交酯的开环聚合时，可得到具有间同富集特性的聚乳酸（PLA），在四氢呋喃（THF）中其Pr值最高可达0.73。