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).