Lanthanide Borohydride Complexes of Bulky Guanidinate Ligands [(Me₃Si)₂NC(N‐Cy)₂]₂Ln(μ‐BH₄)₂Li(THF)₂ (Ln = Nd, Sm, Yb): Synthesis, Structure and Catalytic Activity in Lactide Polymerization

Abstract

Abstract A series of new lanthanide borohydride complexes supported by bulky guanidinate ligands [(Me 3 Si) 2 NC(N‐Cy) 2 ] 2 Ln(μ‐BH 4 ) 2 Li(THF) 2 [Ln = Nd ( 1 ), Sm ( 2 ), Yb ( 3 )] was synthesized by the reaction of related tris(borohydride)s Ln(BH 4 ) 3 (THF) 2 with a twofold molar excess of [(Me 3 Si) 2 NC(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 monoinitiators 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 < M w / M n < 1.77), provided moderate substrate‐to‐initiator ratios are used. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)