Phosphinimino-amino Magnesium
Complexes: Synthesis
and Catalysis of Heteroselective ROP of <i>rac</i>-Lactide
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Abstract
Alkane
elimination reactions of phosphinimino-amine ligands HL<sup>1–8</sup> ((2,6-Me<sub>2</sub>-C<sub>6</sub>H<sub>3</sub>NH)C(Ph)CHPPh<sub>2</sub>(NAr) (Ar = C<sub>6</sub>H<sub>5</sub> (HL<sup>1</sup>); 2,6-Me<sub>2</sub>-C<sub>6</sub>H<sub>3</sub> (HL<sup>2</sup>); 2,6-Et<sub>2</sub>-C<sub>6</sub>H<sub>3</sub> (HL<sup>3</sup>); 2,6-<sup><i>i</i></sup>Pr<sub>2</sub>-C<sub>6</sub>H<sub>3</sub> (HL<sup>4</sup>);
2-OMe-C<sub>6</sub>H<sub>4</sub> (HL<sup>5</sup>); 2-Cl-C<sub>6</sub>H<sub>4</sub> (HL<sup>6</sup>); 3-CF<sub>3</sub>-C<sub>6</sub>H<sub>4</sub> (HL<sup>7</sup>); 4-MeO-C<sub>6</sub>H<sub>4</sub> (HL<sup>8</sup>)) with Mg<sup><i>n</i></sup>Bu<sub>2</sub>, respectively,
afforded a series of phosphinimino-amine-based complexes L<sup>1–8</sup>Mg<sup><i>n</i></sup>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><sub>r</sub> = 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<sup>–1</sup> s<sup>–1</sup> [LA][Mg]
