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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2011-01, Vol.40 (17), p.4687-4696
• Main Authors: Otero, Antonio, Lara-Sánchez, Agustín, Fernández-Baeza, Juan, Alonso-Moreno, Carlos, Márquez-Segovia, Isabel, Sánchez-Barba, Luis F, Castro-Osma, José A, Rodríguez, Ana M
• Format: Article
• Language: English
• Subjects: Activation; Amides - chemistry; Catalysis; Coordination Complexes - chemical synthesis; Coordination Complexes - chemistry; Crystallography, X-Ray; Esters; Heterocyclic Compounds - chemistry; Initiators; Kinetics; Lanthanoid Series Elements - chemistry; Ligands; Metals, Rare Earth - chemistry; Molecular Conformation; Neodymium; Neodymium - chemistry; Polymerization; Rare earth compounds; Rare earth metals; Stereoisomerism
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c0dt01678k

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(SiHMe(2))(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(SiHMe(2))(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(SiHMe(2))(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.