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

摘要

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 = SiMe _3; THF = tetrahydrofuran) with a piperazidine-bridged bis(phenol), H _2[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 H _2[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 H _2[ONNO] with the less bulky lanthanide amides Ln[N(SiMe _2H) _2] _3(THF) _2 in a 2:1 molar ratio at 60 °C gave the monometallic lanthanide amido complexes [ONNO]Ln[N(SiMe _2H) _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.