Lanthanide borohydride complexes of bulky guanidinate ligands [(Me 3Si)2NC(N-Cy)2]2Ln(μ-BH 4)2Li(THF)2 (Ln = Nd, Sm, Yb): Synthesis, structure and catalytic activity in lactide polymerization

Abstract

A series of new lanthanide borohydride complexes supported by bulky guanidinate ligands [(Me3Si)2NC(N-Cy)2] 2Ln(μ-BH4)2Li(THF)2 [Ln = Nd (1), Sm (2), Yb (3)] was synthesized by the reaction of related tris(borohydride)s Ln(BH4)3(THF)2 with a twofold molar excess of [(Me3Si)2NC(N-Cy)2]Li in toluene at 65°C. The complexes were isolated after recrystallization from hexane in 66, 64, and 68% yield, respectively. X-ray diffraction studies revealed that compounds 1-3 are heterodimetallic complexes that have two borohydride ligands μ-bridging the lanthanide and lithium atoms. Those compounds, especially neodymium complex 1, act as mono-initiators for the ring-opening polymerization of racemic lactide, providing atactic polymers with a good degree of control, that is, controlled molecular weights and relatively narrow polydispersities (1.09 < 1.77), provided moderate substrate-to-initiator ratios are used. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.