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 = 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 2 H) 2 ] 3 (THF) 2 in a 2:1 molar ratio at 60 °C gave the monometallic lanthanide amido complexes [ONNO]Ln[N(SiMe 2 H) 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.