Magnesium and Zinc Complexes
Supported by <i>N</i>,<i>O</i>-Bidentate Pyridyl
Functionalized Alkoxy
Ligands: Synthesis and Immortal ROP of ε-CL and l-LA
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Abstract
The <i>N</i>,<i>O</i>-bidentate pyridyl
functionalized
alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (<b>L<sup>1</sup>–H</b>) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol
(<b>L<sup>2</sup>–H</b>) have been prepared by treatment
of acetone and benzophenone with monolithiated 2,6-lutidine. Deprotonolysis
of the ligands <b>L<sup>1</sup>–H</b> and <b>L<sup>2</sup>–H</b> with 1 equiv of Mg<sup><i>n</i></sup>Bu<sub>2</sub> and ZnEt<sub>2</sub> in toluene by releasing
butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl
complexes [L<sup>1</sup>Mg<sup><i>n</i></sup>Bu]<sub>2</sub> (<b>1</b>), [L<sup>2</sup>Mg<sup><i>n</i></sup>Bu]<sub>2</sub> (<b>2</b>), [L<sup>1</sup>ZnEt]<sub>2</sub> (<b>3</b>), and [L<sup>2</sup>ZnEt]<sub>2</sub> (<b>4</b>).
Complexes <b>1</b>–<b>4</b> were characterized
by <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy analysis, and
the molecular structures of <b>1</b>, <b>3</b>, and <b>4</b> were further confirmed by X-ray diffraction analysis. The
investigation of the catalytic behavior of these complexes toward
ε-caprolactone (ε-CL) and l-lactide (l-LA) polymerizations showed that the Mg-based complexes gave higher
activity than those attached to zinc metal, probably owing to the
greater ionic character of the magnesium metal. Remarkably, the magnesium
complex <b>2</b> exhibited a striking “immortal”
nature in the presence of primary alcohols where up to 500 PCL chains
grew from each Mg active center when benzyl alcohol was employed,
while, in particular, in the presence of triethanolamine, complex <b>2</b> also displayed an immortal mode for the polymerization of l-LA