# 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

## TL;DR

This paper explores new zwitterionic metal compounds that improve the efficiency of lactide polymerization, offering insights into how metal choice affects catalytic performance.

## Contribution

The study introduces a new class of zwitterionic metal ammonium tris(phenolate)s that significantly enhance lactide polymerization activity and provides mechanistic insights.

## Key findings

- M(III) compounds show significantly higher activity than Zr(IV) systems, with La(III) being over 20 times more active under industrial conditions.
- Ce(III) compounds, when reduced from Ce(IV), demonstrate a catalytic activity enhancement exceeding two orders of magnitude compared to Zr(IV).
- Structural and spectroscopic analyses reveal conformational chirality trends in ligand systems affecting catalytic properties.

## Abstract

Reaction of tris­(2,4-dimethylbenzyl)­amine,
H3LMe, with tris- or tetrakis­(alkoxide)­s of
large metals consistently
affords, respectively, pseudo-homoleptic and homoleptic
zwitterionic compounds [M­(III)­(HLMe)­(H2LMe)] (M = Yb­(III), Y­(III), Pr­(III), La­(III), Sc­(III), Sm­(III))
and [M­(IV)­(HLMe)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)­(HLMe)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).