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

Abstract

The <i>N</i>,<i>O</i>-bidentate pyridyl functionalized alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (<b>L¹–H</b>) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (<b>L²–H</b>) have been prepared by treatment of acetone and benzophenone with monolithiated 2,6-lutidine. Deprotonolysis of the ligands <b>L¹–H</b> and <b>L²–H</b> with 1 equiv of Mg^<i>n</i>Bu₂ and ZnEt₂ in toluene by releasing butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl complexes [L¹Mg^<i>n</i>Bu]₂ (<b>1</b>), [L²Mg^<i>n</i>Bu]₂ (<b>2</b>), [L¹ZnEt]₂ (<b>3</b>), and [L²ZnEt]₂ (<b>4</b>). Complexes <b>1</b>–<b>4</b> were characterized by ¹H and ¹³C 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