Conjugated tri-nuclear salen-Co complexes for the copolymerization of epoxides/CO2: cocatalyst-free catalysis

Abstract

A novel trinuclear salen-Co(III) complex 1 was designed for carbon dioxide (CO₂) and propylene oxide (PO) copolymerization. Due to its conjugated structure, the trinuclear complex 1 system showed a synergistic effect with high activity at low catalyst loadings. In contrast, the corresponding mononuclear complex 2 system almost lost its activity under the same conditions. Unexpectedly, a conjugated effect in the absence of a nucleophilic cocatalyst was discovered, in which complex 1 showed unusual activity even under 0.1 MPa pressure of CO₂. This is the first example of PO/CO₂ copolymerization using salen-Co(III) as the catalyst without the aid of a cocatalyst as the stabilizing cation. Moreover, complex 1 showed great tolerance to H₂O in the reaction system, and the activity increased efficiently with the addition of H₂O. The induction period was scarcely observed in the copolymerization by in situ spectroscopy. The copolymerization mechanism for complex 1 without any cocatalyst was postulated based on rationally designed experiments, and the pathway provided new insight into the nature of the true initiating intermediate. Complex 1 could also catalyze the copolymerization of lactide (LA), PO and CO₂ without a nucleophilic cocatalyst to produce multiblock copolymers. The stabilization and distinct catalytic activity of trinuclear complex 1 could be attributed to the change of the electrophilicity of the Co(III) center generated from the specific electronic interactions among the three salen-Co(III) subunits.