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 (LiOPr)-Pr-i to [(NON)-N-tBu]-ThCl5Li3 center dot DME ([(NON)-N-tBu](2-) = [((BuNSiMe2)-Bu-t)(2)O](2-)) in toluene gives [(NON)-N-tBu]Th((OPr)-Pr-i)(3)Li center dot DME (1-DME). Recrystallization of 1-DME from diethyl ether gives [(NON)-N-tBu]Th((OPr)-Pr-i)(3)Li center dot Et2O (1-Et2O). The addition of 2 equiv of (LiOPr)-Pr-i to {[(NON)-N-tBu]UCl2}(2) gives {[(NON)-N-tBu]U((OPr)-Pr-i)(2)}(2) (2). If (KOBu)-Bu-t is used instead, the product is {[(NON)-N-tBu]U((OBu)-Bu-t)}(2) (3). The reaction of 2 equiv of (KOBu)-Bu-t with [(NCOCN)-N-iPr2Ph]ThCl2 center dot DME ([(NCOCN)-N-iPr2Ph](2-) = [(2,6-(Pr2PhNCH2CH2)-Pr-i)(2)O](2-)) gives [(NCOCN)-N-iPr2Ph]Th((OBu)-Bu-t)(2) (4), while addition of 1 equiv of (KOBu)-Bu-t to [(NCOCN)-N-iPr2Ph]UCl3Li center dot THF results in [(NCOCN)-N-iPr2Ph]U((OBu)-Bu-t)Cl (5). Complexes 1-5 as well as the previously reported diamido actinide dialkyl complexes [(NON)-N-tBu]An(CH2SiMe3)(2) (An = Th (6), U (7)) and [(NCOCN)-N-iPr2Ph]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 degrees C, with moderate to good control over the molecular weight features (values of M-w/M-n 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 (P-r values up to 0.73 in THF).