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)(mu-Cl)mu-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](mu-Br)(mu-OEt) (2) and the newly synthesized methylated derivative [(NMe2NMeO)InCl](2)(mu-Cl)(?-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 [(L-H)InCl](2)(mu-Cl)(mu-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.