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