Lactide polymerization activity of alkoxide, phenoxide, and amide derivatives of yttrium(III) arylamidinates

In quest of new, single-site catalysts for cyclic ester polymerizations, a series of mononuclear yttrium(III) complexes of N,N′-bis(trimethylsilyl)benzamidinate ([LTMS]−) and hindered N,N′-bis-(2,6-dialkylaryl)toluamidinates ([LEt]−, aryl = Et2C6H3, and [LiPr]−, aryl = iPr2C6H3) were synthesized and characterized by X-ray diffraction: LY(μ-Cl)2Li(TMEDA) (1), LY(OC6H2tBu2Me) (2), LY(OC6H3Me2)2Li(THF)4 (3), LY(μ-OtBu)2Li(THF) (4), LiPrY[N(SiMe2H)2]2(THF) (5), LY(THF)(Cl)(μ-Cl)Li(THF)3 (6), and LY[N(SiMe2H)2] (7). Coordination numbers ranging from five to seven were observed, and they appeared to be controlled by the steric bulk of the supporting amidinate and alkoxide, phenoxide, or amide coligands. Complexes 2–5 and 7 are active catalysts for the polymerization of D,L-lactide (e.g., with 2 and added benzyl alcohol, 1000 equiv of D,L-lactide were polymerized at room temperature in less than 1 h, with polydispersities less than 1.5). The neutral complexes 2, 5, and 7 were more effective than the anionic complexes 3 and 4. In addition, the presence of the more hindered amidinate ligands [LEt]− and [LiPr]− on yttrium-amides slowed the polymerizations (7 < 5 < Y[N(SiMe2H)2]3).