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

Abstract

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: L2TMSY(μ-Cl)2Li(TMEDA) (1), L2TMSY(OC6H2tBu2Me) (2), L2TMSY(OC6H3Me2)2Li(THF)4 (3), L2TMSY(μ-OtBu)2Li(THF) (4), LiPrY[N(SiMe2H)2]2(THF) (5), L2EtY(THF)(Cl)(μ-Cl)Li(THF)3 (6), and L2EtY[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).