Bimetallic Dimethyl Aluminum for Ring-Opening Polymerization of ε-Caprolactone: Restricted Rotation for Enhanced Metal Cooperativity

A series of bimetallic aluminum complexes bearing constrained indanimine ligands have been developed. The dialuminum complexes were designed to have a short hydrazine bridge (2a) between the indanimine units to study their subunit interactions and effects on structure and catalytic activities compared to the longer ethylenediamine bridge (2b). The two subunits in 2a experience restricted N–N rotation, giving dialuminum complexes a structure akin to 1,1′-binaphthyl. Complex 2a bearing a rigid hydrazine bridging moiety displays a superior catalytic activity for the ring-opening polymerization of ε-caprolactone (CL), yielding poly(ε-caprolactone) (PCL) with narrow dispersity. A DFT calculation also unveiled the bimetallic cooperativity coherent with the short intermetallic distance which enhances catalytic performance.