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-<i>tert</i>-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 (<b>1</b>), Er (<b>2</b>), Eu (<b>3</b>), Sm (<b>4</b>)], whereas the same reactions conducted at room temperature gave the monometallic bis­(phenolato) lanthanide amides [ONNO]­LnN­(TMS)₂(THF) [Ln = Y (<b>5</b>), Sm (<b>6</b>)]. Complex <b>1</b> can be transformed to a neutral bimetallic bis­(phenolato) yttrium amido complex, [ONNO]­{Y­[N­(TMS)₂]₂}₂ (<b>7</b>), by heating a toluene solution to 80 °C. Complex <b>7</b> 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 (<b>8</b>), Pr (<b>9</b>)] 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 (<b>10</b>), Y­(<b>11</b>), Nd (<b>12</b>)] 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 <b>1</b>, <b>5</b>, <b>6</b>, <b>7</b>, and <b>11</b> were further confirmed by NMR spectroscopy. These complexes are highly efficient initiators for the ring-opening polymerization of l-lactide. In addition, complexes <b>1</b>, <b>3</b>, <b>5</b>, <b>7</b>, and <b>11</b> can initiate <i>rac</i>-lactide polymerization with high activity to give heterotactic-rich polylactides