Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

The synthesis of the first alkoxide-bridged indium complex supported by a chiral dinucleating ligand platform (<b>1</b>), along with its zinc analogue (<b>2</b>), is reported. Both complexes are synthesized in a one-pot reaction starting from a chiral dinucleating bis­(diamino)­phenolate ligand platform, sodium ethoxide, and respective metal salts. The dinucleating indium analogue (<b>7</b>) based on an achiral ligand backbone is also reported. Indium complexes bearing either the chiral or achiral ligand catalyze the ring-opening polymerization of racemic lactide (<i>rac</i>-LA) to afford highly heterotactic poly­(lactic acid) (PLA; <i>P</i>_r > 0.85). The indium complex bearing an achiral ligand affords essentially atactic PLA from <i>meso</i>-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

The impact of the metal size and Lewis acidity on the polymerization activity of group 13 metal complexes was studied, and it was shown that, within the same ligand family, indium complexes are far more reactive and selective than their gallium analogues. To this end, gallium and aluminum complexes supported by a tridentate diaminophenolate ligand, as well as gallium complexes supported by <i>N</i>,<i>N</i>′-ethylenebis­(salicylimine)­(salen) ligands, were synthesized and compared to their indium analogues. Using the tridentate ligand set, it was possible to isolate the gallium chloride complexes <b>3</b> and (±)-<b>4</b> and the aluminum analogues <b>5</b> and (±)-<b>6</b>. The alkoxygallium complex (±)-<b>2</b>, supported by a salen ligand, was also prepared and characterized and, along with the three-component system GaCl₃/BnOH/NEt₃, was tested for the ring-opening polymerization of lactide and ε-caprolactone. The polymerization rates and selectivities of both systems were significantly lower than those for the indium analogues. The reaction of (±)-<b>2</b> with 1 equiv of lactide forms the first insertion product, which is stable in solution and can be characterized at room temperature. In order to understand the differences of the reactivity within the group 13 metal complexes, a Lewis acidity study using triethylphosphine oxide (the Gutmann–Beckett method) was undertaken for a series of aluminum, gallium, and indium halide complexes; this study shows that indium halide complexes are less Lewis acidic than their aluminum and gallium analogues. Density functional theory calculations show that the Mulliken charges for the indium complexes are higher than those for the gallium analogues. These data suggest that the impact of ligands on the reactivity is more significant than that of the metal Lewis acidity