Heteroscorpionate rare-earth initiators for the controlled ring-opening polymerization of cyclic esters

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Abstract
A series of neutral rare-earth metal amides containing different achiral and chiral heteroscorpionate ligands was synthesized and characterized and these compounds were employed in the polymerization of cyclic esters. Thus, treatment of [Ln{N(SiHMe2)2}3(thf)2] (Ln = Nd, Sm) with acetamide or thioacetamide heteroscorpionate ligands for 2 h at 0 °C afforded the α-agostic silylamido dimeric rare-earth compounds [Ln{N(SiHMe2)2}(NNE)]2 (Ln = Nd and Sm; NNE = heteroscorpionate ligands, E = O, S) (1–8), some as enantiopure complexes. Complexes 1–8 contain dianionic heteroscorpionate pseudoallyl ligands resulting from C–H activation of the bridging methine group of the bis(pyrazol-1-yl)methane moiety and subsequent coordination to the metal center. However, when the reaction was carried out for 1 h at lower temperature new bis(silylamido) dimeric lanthanide compounds [Ln{N(SiHMe2)2}2(NNE)]2 (Ln = Nd and Sm; E = O) (9 and 10) were obtained. The structures of the complexes were determined by spectroscopic methods and the X-ray crystal structures of 1, and 4 were also established. Neodymium complexes are active initiators for the ring-opening polymerization (ROP) of lactide (LA) and lactones, giving rise to medium-high molar mass polymers under mild conditions and with narrow polydispersities. These complexes were well suited for achieving well-controlled polymerization through an insertion-coordination mechanism. Achiral and racemic complexes did not affect stereocontrol in the polymerizarion of rac-LA but the enantiomerically pure complex 1 was found to exhibit a homosteric preference for one of the two enantiomers of rac-LA at low conversions.

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