Diversity in Zwitterionic Metal Ammonium Tris(phenolate)s for the Controlled Immortal Polymerization of Lactide: Dramatic Activity Enhancement and Mechanistic Insight on Expansion beyond Zirconium

Reaction of tris-(2,4-dimethylbenzyl)-amine, H₃L^Me, with tris- or tetrakis-(alkoxide)-s of large metals consistently affords, respectively, <i>pseudo</i>-homoleptic and homoleptic zwitterionic compounds [M-(III)-(HL^Me)-(H₂L^Me)] (M = Yb-(III), Y-(III), Pr-(III), La-(III), Sc-(III), Sm-(III)) and [M-(IV)-(HL^Me)₂] (M = Zr-(IV), Hf-(IV), Ce-(IV)). The Zr-(IV) congener is known to be a robust and efficient catalyst for the ring-opening polymerization of lactides under industrially relevant solvent-free conditions, exhibiting some heteroselectivity in the polymerization of the racemic monomer. The generality of the synthetic route, encompassing various metals, permits exploration of the role of metal center size and other subtle structural variations in influencing catalytic activity and selectivity. Kinetic studies have revealed all M-(III) compounds assessed (M = Yb-(III), Y-(III), Pr-(III), La-(III)) to be significantly more active than the Zr-(IV) system, exhibiting a clear correlation between ionic radius and reaction rate, while generally retaining a high degree of control. The La-(III) compound, in particular, offers both remarkable activity (>20 × Zr-(IV) at 120 °C, 50 wt %/vol monomer in PhCl) and resilience under challenging, industrially relevant conditions (180 °C, solvent-free, 2×10^-3-5×10^-3 mol % catalyst). Comprehensive structural analyses have, additionally, afforded insight into the unusual mechanism favored by these catalysts. Although only the Zr-(IV) and Hf-(IV) systems exhibit appreciable stereoselectivity, variable-temperature ¹H NMR spectroscopic and crystallographic methods have illuminated trends regarding the conformational chirality of the ligand systems in the compounds of interest, the facility of inversion of which we propose underpins much of the variation in their catalytic properties. Additionally, whereas the Ce-(IV) system, despite its greater metal size, did not tend to outperform Zr-(IV), in situ reduction to the anionic [Ce-(III)-(HL^Me)₂]^- provided an activity enhancement assessed to exceed 2 orders of magnitude. Accordingly, Ce-(III) offers a similarly dramatic rate enhancement when benchmarked against Zr-(IV).