Control of Conformations of Piperazidine-Bridged Bis(phenolato) Groups: Syntheses and Structures of Bimetallic and Monometallic Lanthanide Amides and Their Application in the Polymerization of Lactides

A series of bimetallic and monometallic lanthanide amides stabilized by a piperazidine-bridged bis­(phenolato) ligand were successfully prepared, and the factors controlling the formation of these lanthanide amides were elucidated. Reactions of Ln­[N­(TMS)2]3(μ-Cl)­Li­(THF)3 (TMS = SiMe3; THF = tetrahydrofuran) with a piperazidine-bridged bis­(phenol), H2[ONNO]­[4-bis­(2-hydroxy-3,5-di-tert-butylbenzyl)­piperazidine], in a 2:1 molar ratio in THF at 60 °C gave the anionic bimetallic bis­(phenolato) lanthanide amido complexes [ONNO]­{Ln­[N­(TMS)2]2­(μ-Cl)­Li­(THF)}2 [Ln = Y (1), Er (2), Eu (3), Sm (4)], whereas the same reactions conducted at room temperature gave the monometallic bis­(phenolato) lanthanide amides [ONNO]­LnN­(TMS)2(THF) [Ln = Y (5), Sm (6)]. Complex 1 can be transformed to a neutral bimetallic bis­(phenolato) yttrium amido complex, [ONNO]­{Y­[N­(TMS)2]2}2 (7), by heating a toluene solution to 80 °C. Complex 7 can also be conveniently prepared by the reaction of the yttrium amide Y­[N­(TMS)2]3 with H2[ONNO] in a 2:1 molar ratio at 60 °C. For neodymium and praseodymium, only the monometallic lanthanide amido complexes [ONNO]­LnN­(TMS)2(THF) [Ln = Nd (8), Pr (9)] were isolated, even when the reactions were conducted at 60 °C. Furthermore, reaction of H2[ONNO] with the less bulky lanthanide amides Ln­[N­(SiMe2H)2]3(THF)2 in a 2:1 molar ratio at 60 °C gave the monometallic lanthanide amido complexes [ONNO]­Ln­[N­(SiMe2H)2]­(THF) [Ln = Yb (10), Y­(11), Nd (12)] as neat products; no bimetallic species were formed. All of these complexes were characterized by IR, elemental analyses, and single-crystal X-ray diffraction. Complexes 1, 5, 6, 7, and 11 were further confirmed by NMR spectroscopy. These complexes are highly efficient initiators for the ring-opening polymerization of l-lactide. In addition, complexes 1, 3, 5, 7, and 11 can initiate rac-lactide polymerization with high activity to give heterotactic-rich polylactides.