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

A series of new lanthanide borohydride complexes supported by bulky guanidinate ligands [(Me3Si)(2)NC(N-Cy)(2)](2)Ln(mu-BH4)(2)Li(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)(2)NC(N-Cy)(2)]Li in toluene at 65 degrees 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 mu-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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).