Complexes of Group 3 Metals and Lanthanides That Contain Siloxane-Bridged Bisaminopyridinato Ligands: Synthesis, Structure, and Application in the Ring-Opening Polymerization of Lactones

Abstract

The reaction of one equivalent of dilithiated O(SiMe2-Ap-H)(2) [Ap-H = -N(2-amino-4-methylpyridine)] (1), generated in situ, with LnCl(3) (Ln = Y, Sm) in THF affords O(SiMe2-Ap)(2)YCl(THF)(2) (2) or O(SiMe2-Ap)(2)SmCl(THF)(3) (3). In contrast, the reaction of one or two equivalents of dilithiated 1, again generated in situ, with LaBr3 in THF affords O(SiMe2-Ap)(2))(2)LaLi(THF)(3) (4). An X-ray structural analysis of 2 and 3 reveal the O(SiMe2-Ap)(2-) ligand to bind in a planar tetradentate manner. Equivalent Sm-N distances in 3 indicate a delocalized binding mode. Compound 2 reacts with Bu4NBH4, NaBH4 or LiCH(SiMe3)(2) to give the corresponding ate complexes O(SiMe2-Ap)(2)Y(BH4)Cl(THF) Bu4N (5), O(SiMe2-Ap)(2)Y(BH4)(2)Na(THF)(2) (6) and O(SiMe2-Ap)(2)Y(CH(TMS)(2))(2)Li(THF)(3) (7), respectively. The steric demand of the O(SiMe2-Ap)(2-) ligand is not large enough to stabilize monoalkyl or monoborohydride complexes. Complex 4 has been used as an initiator for the ring-opening polymerization of epsilon-caprolactone or delta-valerolactone. In both cases an almost linear relation between the monomer-to-initiator ratio and the molecular weight of the obtained polyester is observed. By conducting the polymerization in neat epsilon-caprolactone at room temperature a solid polyester block is formed after 3 min (300000 g.mol(-1), M-w/M-n 2.3).