Quantified Structure–Performance Relationships for Fe(III)K(I) Heterodinuclear Catalysts in Cyclohexene Oxide/CO2 Ring-Opening Copolymerization

Abstract Catalyst design and understanding are improved by quantified structure–performance relationships, but these are underutilized in polymerization catalysis. In epoxide/carbon dioxide ring-opening copolymerization (ROCOP), an important route to make polymers from bio- and waste-based feedstocks, significant success has been achieved in understanding cobalt catalysts, but understanding of other metal catalysts is much less developed. This work targets earth-abundant bimetallic catalysts featuring iron and potassium. A series of seven Fe(III)K(I) catalysts, coordinated by different ancillary ligands, are compared with analogues Co(III)K(I) catalysts; the iron catalysts demonstrate both high activity (TOF = 675 h–1) and polymer selectivity (98%) in cyclohexene oxide/carbon dioxide copolymerization (20 bar CO2 pressure, 100 °C). The Fe(III)K(I) catalysts also show a linear free energy relationship between the copolymerization rates and the catalyst ligand-to-metal charge transfer (LMCT) wavelength, with a volcano relationship identifying the best catalysts. The most active catalysts have an Fe(III) center with an intermediate LMCT wavelength, and these features are consistent with the proposed copolymerization mechanism. In this field of catalysis, quantified structure-performance relationships are very rare; thus, the findings are fundamentally interesting and should help to accelerate the design of better-performing iron-based polymerization catalysts.