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 ([L^TMS]^-) and hindered N, N′-bis-(2,6-dialkylaryl)toluamidinates ([L^Et]^-, aryl = Et₂C₆H₃, and [L^iPr]^-, aryl = ⁱPr₂C₆H₃) were synthesized and characterized by X-ray diffraction: L₂^TMSY(μ-Cl)₂Li(TMEDA) (1), L₂^TMSY(OC₆H₂^tBu₂Me) (2), L₂^TMSY(OC₆H₃Me₂)₂Li(THF)₄ (3), L₂^TMSY(μ-O^tBu)₂Li(THF) (4), L^iPrY[N(SiMe₂H)₂]2(THF) (5), L₂^EtY(THF)(Cl)(μ-Cl)Li(THF)₃ (6), and L₂^EtY[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 [L^Et]^- and [L^iPr]^- on yttrium-amides slowed the polymerizations (7<5<Y[N(SiMe₂H)₂]₃).