Probing the Role of Secondary versus Tertiary Amine Donor Ligands for Indium Catalysts in Lactide Polymerization

Abstract

The role of the central amine donor in a previously reported dinuclear indium catalyst, [N<sub>Me2</sub>N<sub>H</sub>O)­InCl]<sub>2</sub>­(μ-Cl)­(μ-OEt) (<b>1</b>), for the polymerization of lactide was investigated through experimental methods. The solid state structural data of a series of dimeric complexes related to <b>1</b>, including the previously reported bromide derivative [(N<sub>Me2</sub>N<sub>H</sub>O)­InBr]­(μ-Br)­(μ-OEt) (<b>2</b>) and the newly synthesized methylated derivative [(N<sub>Me2</sub>N<sub>Me</sub>O)­InCl]<sub>2</sub>­(μ-Cl)­(μ-OEt) (<b>6</b>), showed that weak hydrogen bonding may be present within some of these complexes in the solid state. The polymerization of <i>rac</i>-lactide with <b>2</b>, <b>6</b>, and a related achiral complex [(L<sub>H</sub>)­InCl]<sub>2</sub>­(μ-Cl)­(μ-OEt) (<b>8</b>) synthesized for this study indicates that hydrogen bonding may not influence the reactivity of these compounds. The nature of the central amine donor may play a role in tuning the reactivity of these types of catalysts. Catalysts with central secondary amine donors, such as complexes <b>1</b>, <b>2</b>, and <b>8</b>, are 2 orders of magnitude more reactive than those with central tertiary amine donors, such as complex <b>6</b>

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