Cationic indium catalysts for ring opening polymerization: tuning reactivity with hemilabile ligands

This is a comprehensive study of the effects of rationally designed hemilabile ligands on the stability, reactivity, and change in catalytic behavior of indium complexes. We report cationic alkyl indium complexes supported by a family of hemi-salen type ligands bearing hemilabile thiophenyl (2a), furfuryl (2b) and pyridyl (2c) pendant donor arms. Shelf-life and stability of these complexes followed the trend2a<2b<2c, showing direct correlation to the affinity of the pendant donor group to the indium center. Reactivity towards polymerization of epichlorohydrin and cyclohexene oxide followed the trend2a>2b>2cwith control of polymerization following an inverse relationship to reactivity. Surprisingly,2cpolymerizedracemiclactide without an external initiator, likely through an alkyl-initiated coordination-insertion mechanism.