This is an in-depth study into the role hemilabile ligands play in tuning stability and reactivity in cationic indium complexes.

Overcoming entropic constraints to selectively produce high-molecular-weight cyclic poly­(lactide) (c-PLA) is a challenge. In this work, we use an air-stable cationic alkyl indium complex featuring a hemilabile pyridine pedant arm to reproducibly synthesize c-PLA exclusively with low dispersity (Đ ∼ 1.30) and high molecular weights (up to 416,000 g mol–1). The complex remains active after extended periods of exposure to high-humidity air. The process allows the formation of high-molecular-weight c-PLLA, c-PDLA, and their stereocomplex with no epimerization. We propose that polymerization occurs through a cooperative Lewis-pair-based coordination–insertion mechanism where the monomer coordination to the cationic indium center and ring-opening of lactide by the hemilabile pyridine donor group are essential steps.

Cationic indium complexes were used as catalysts for the copolymerization of epoxides and racemic lactide (rac-LA) via sequential addition to form high-molecular weight block copolymers. Mechanistic studies and control experiments indicate that the epoxide is polymerized by a cationic mechanism to yield a neutral alkoxide indium species that subsequently polymerizes the lactide by a coordination-insertion mechanism with no significant interference of the two mechanisms under polymerization conditions. The thermal and tensile properties of different block copolymers were studied, revealing mostly amorphous materials. We were able to control the ductility and stiffness of the copolymers by tuning the nature and chain length of the blocks.

Group 13 metal complexes have emerged as powerful catalysts for transforming CO2 into added-value products. However, direct comparisons of reactivity between Al, Ga, and In catalysts are rare. We report aluminum (1), gallium (2), and indium (3) complexes supported by a half-salen H­[PNNO] ligand with a pendent phosphine donor and investigate their activity as catalysts for the copolymerization of CO2 and cyclohexene oxide. In solution, the P-donor is dissociated for the Al and Ga complexes while for the In complex it exhibits hemilabile behavior. The indium complex shows higher conversion and selectivity than the Al or Ga analogues. The mechanism of the reaction was studied by NMR and FTIR spectroscopy experiments as well as structural characterization of off-cycle catalytic intermediate indium trichloride complex [(PNNO)­InCl3]­[TBA] (4). This study highlights the impact of a hemilabile phosphine group on group 13 metals and provides a detailed analysis of the initiation step in CO2/epoxide copolymerization reactions.

We report the alternating copolymerization of caprolactone and epoxide through the in situ formation of pre-sequenced spiroorthoester monomer.