Phosphinimino-amino Magnesium Complexes: Synthesis and Catalysis of Heteroselective ROP of <i>rac</i>-Lactide

Alkane elimination reactions of phosphinimino-amine ligands HL^1–8 ((2,6-Me₂-C₆H₃NH)­C­(Ph)CHPPh₂(NAr) (Ar = C₆H₅ (HL¹); 2,6-Me₂-C₆H₃ (HL²); 2,6-Et₂-C₆H₃ (HL³); 2,6-^<i>i</i>Pr₂-C₆H₃ (HL⁴); 2-OMe-C₆H₄ (HL⁵); 2-Cl-C₆H₄ (HL⁶); 3-CF₃-C₆H₄ (HL⁷); 4-MeO-C₆H₄ (HL⁸)) with Mg^<i>n</i>Bu₂, respectively, afforded a series of phosphinimino-amine-based complexes L^1–8Mg^<i>n</i>Bu­(THF) (<b>1</b>–<b>8</b>) by releasing butane. Complexes <b>1</b>–<b>8</b> are phosphinimino-amine-ligated THF-solvated mono­(alkyl)­s, among which <b>1</b>–<b>4</b> adopt twisted tetrahedral geometries, whereas <b>5</b> contains a trigonal bipyramido geometry core. Complexes <b>1</b>–<b>8</b> all display high activity for the ring-opening polymerization of <i>rac</i>-lactide. The molecular weights of the resulting PLA are close to the theoretic values, and the molecular weight distributions are narrow. Moreover, these complexes show medium to high heteroselectivity, which, interestingly, increases with the decrease of the ligand steric hindrance; thus, complex <b>1</b>, bearing a less bulky ligand, exhibits a heteroselectivity of <i>P</i>_r = 0.98, the highest value of a magnesium-based initiator achieved to date. The kinetics study showed that the polymerization rate is first-order dependent on both monomer and initiator concentrations, and the overall rate equation is −d­[LA]/d<i>t</i> = 3.78 M^–1 s^–1 [LA]­[Mg]