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

The
role of the central amine donor in a previously reported dinuclear
indium catalyst, [NMe2NHO)­InCl]2­(μ-Cl)­(μ-OEt)
(1), for the polymerization of lactide was investigated
through experimental methods. The solid state structural data of a
series of dimeric complexes related to 1, including the
previously reported bromide derivative [(NMe2NHO)­InBr]­(μ-Br)­(μ-OEt) (2) and
the newly synthesized methylated derivative [(NMe2NMeO)­InCl]2­(μ-Cl)­(μ-OEt)
(6), showed that weak hydrogen bonding may be present
within some of these complexes in the solid state. The polymerization
of rac-lactide with 2, 6, and a related achiral complex [(LH)­InCl]2­(μ-Cl)­(μ-OEt) (8) 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 1, 2, and 8, are 2 orders of magnitude
more reactive than those with central tertiary amine donors, such
as complex 6.