Efficient and Heteroselective Heteroscorpionate Rare-Earth-Metal
Zwitterionic Initiators for ROP of <i>rac</i>-Lactide: Role
of σ‑Ligand
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Abstract
A series of oxophosphine (3,5-Me<sub>2</sub>Pz)<sub>2</sub>CHP(R)<sub>2</sub>O (Pz = pyrazole; R = <sup><i>t</i></sup>Bu (HL<sup>1</sup>), Cy (HL<sup>2</sup>))
and iminophosphine (3,5-Me<sub>2</sub>Pz)<sub>2</sub>CHP(R)<sub>2</sub>NAr (R = Cy, Ar = Ph (HL<sup>3</sup>); R = Ph, Ar = Ph (HL<sup>4</sup>), Ar = 2,6-Me<sub>2</sub>-phenyl
(HL<sup>5</sup>)) heteroscorpionate ligands were synthesized. Abstraction
of the methide proton of these ligands by rare-earth-metal tris(alkyl)s,
Ln(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub>, afforded
the corresponding zwitterionic bis(alkyl) complexes L<sup>1–5</sup>Ln(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF) (L<sup>1</sup>, Ln = Y (<b>1a</b>), Lu (<b>1b</b>); L<sup>2</sup>,
Ln = Y (<b>2a</b>), Lu (<b>2b</b>); L<sup>3</sup>, Ln
= Y (<b>3a</b>), Lu (<b>3b</b>); L<sup>4</sup>, Ln = Y
(<b>4a</b>), Lu (<b>4b</b>); L<sup>5</sup>, Ln = Y (<b>5a</b>), Lu (<b>5b</b>), while metathesis reaction of the
lithium salts of LiL<sup>3</sup> and LiL<sup>4</sup> with YCl<sub>3</sub>(THF)<sub>2</sub> or YBr<sub>3</sub>(THF)<sub>2</sub> followed
by treatment with LiCH<sub>2</sub>SiMe<sub>3</sub> and KN(SiHMe<sub>2</sub>)<sub>2</sub>, respectively, afforded the first heteroscorpionate
yttrium mixed halogen/alkyl or amido complexes L<sup>3–4</sup>Y(Cl)(CH<sub>2</sub>SiMe<sub>3</sub>)(THF) (L<sup>3</sup> (<b>6a</b>), L<sup>4</sup> (<b>7a</b>)), L<sup>3–4</sup>Y(Cl)(N(SiHMe<sub>2</sub>)<sub>2</sub>)(THF) (L<sup>3 </sup>(<b>8a</b>), L<sup>4</sup> (<b>9a</b>)), L<sup>4</sup>Y(Br)(CH<sub>2</sub>SiMe<sub>3</sub>)(THF) (<b>10a</b>), and
L<sup>4</sup>Y(Br)(N(SiHMe<sub>2</sub>)<sub>2</sub>)(THF) (<b>11a</b>). The structures of these complexes were well-defined, and the molecular
structures of <b>1a</b>, <b>2a</b>, <b>3b</b>, <b>4b</b>, <b>5a</b>, and <b>7a</b> were further characterized
by single crystal X-ray diffraction analysis. Complexes <b>1</b>–<b>5</b> showed similar high activity toward the ROP
of <i>rac</i>-LA at room temperature, and both the alkyl
species participated in initiation, of which the lutetium complexes
exhibited slightly higher selectivity than their yttrium analogues
(<i>P</i><sub>r</sub> = 0.85–0.89 vs 0.80–0.84)
despite the bulkiness of the ligands. Interestingly, the mixed halogen
complexes <b>6a</b>–<b>11a</b> were single-site
initiators, where the σ-halogen moiety remaining on the central
metal showed, for the first time, facilitating the heteroselectivity
up to <i>P</i><sub>r</sub> = 0.98. This result sheds new
light on designing specifically selective catalytic precursors
