Pyrrolide-Ligated Organoyttrium Complexes. Synthesis, Characterization, and Lactide Polymerization Behavior
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posted on 2007-01-29, 00:00 authored by Yi Yang, Shihui Li, Dongmei Cui, Xuesi Chen, Xiabin JingThe <i>N,N</i>-bidentate ligand 2-{(<i>N</i>-2,6-diisopropylphenyl)iminomethyl)}pyrrole (<b>L</b><b><sup>1</sup></b><sup></sup>) and the <i>N,N,P</i>-tridentate ligand 2-{(<i>N</i>-2-diphenylphosphinophenyl)iminomethyl)}pyrrole (<b>L</b><b><sup>2</sup></b><sup></sup>) have been prepared. Their
reactions with homoleptic yttrium tris(alkyl) compound Y(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> have been investigated.
Treatment of Y(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> with 1 equiv of <b>L</b><b><sup>1 </sup></b><sup></sup>generated a THF-solvated bimetallic (pyrrolylaldiminato)yttrium mono(alkyl) complex (<b>1</b>) of central symmetry. In this process, <b>L</b><b><sup>1</sup></b><sup></sup> is deprotonated by
metal alkyl and its imino CN group is reduced to C−N by intramolecular alkylation, generating dianionic
species that bridge two yttrium alkyl units in a unique η<sup>5</sup>/η<sup>1</sup>:κ<sup>1</sup> mode. The pyrrolyl ring behaves as a
heterocyclopentadienyl ligand. Reaction of Y(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> with 2 equiv of <b>L</b><b><sup>1</sup></b><sup></sup> afforded the
monomeric bis(pyrrolylaldiminato)yttrium mono(alkyl) complex (<b>2</b>), selectively. Amination of <b>2</b> with
2,6-diisopropylaniline gave the corresponding yttrium amido complex (<b>3</b>). In <b>3</b> the pyrrolide ligand is
monoanionic and bonds to the yttrium atom in a η<sup>1</sup>:κ<sup>1</sup> mode. The homoleptic tris(η<sup>1</sup>:κ<sup>1</sup>-pyrrolylaldiminato)yttrium complex (<b>4</b>) was isolated when the molar ratio of <b>L</b><b><sup>1</sup></b><sup></sup> to Y(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> increases to 3:1.
Reaction of <b>L</b><b><sup>2</sup></b><sup></sup> with equimolar Y(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> afforded an asymmetric binuclear complex (<b>5</b>).
The dianionic <i>N,N,P</i> tridentate moieties derived from <b>L</b><b><sup>2</sup></b><sup></sup> coordinate to yttrium atoms in η<sup>5</sup>/η<sup>1</sup>:κ<sup>2</sup> modes,
generating tetrahedron and trigonal/pyramidal geometry around the two metal centers, respectively. Both
alkylation of the imino CN group of ligand <b>L</b><b><sup>2</sup></b><sup></sup> and the pyrrole's ability to act similar to a
heterocyclopentadienyl moiety were also found in complex <b>5</b>. Complexes <b>1</b>−<b>3</b> and <b>5</b> initiated
polymerizations of d,l-lactide to give atactic polylactides with high molecular weights and narrow
molecular weight distributions.
