Dinuclear iron(iii) complexes bearing phenylene-bridged bis(amino triphenolate) ligands as catalysts for the copolymerization of cyclohexene oxide with carbon dioxide or phthalic anhydride

Abstract

A series of dinuclear complexes, in which two iron(III) amino triphenolate moieties are bridged by a phenylene backbone, were synthesized. Catalysis for the alternating copolymerization of cyclohexene oxide (CHO)/CO₂ and CHO/phthalic anhydride (PA) by the dinuclear iron(III) complexes and by their corresponding mononuclear analogue in the presence of a bis(triphenylphosphine)iminium chloride (PPNCl) co-catalyst was systematic investigated. Generally, the catalytic activities of these dinuclear iron(III) complexes increased in the order of para < meta < ortho. In the case of CHO/CO₂ copolymerization, complex 1b displayed superior catalytic activity under extremely low catalyst concentrations compared to the monometallic analogue, thereby suggesting the existence of intramolecular cooperativity between the two metal centers. Notably, complex 1b was found to produce a perfectly alternating poly(cyclohexene carbonate) (PCHC) with a turnover frequency (TOF) of 37 h⁻¹ at only 0.1 MPa CO₂. In contrast, the dinuclear iron(III) complexes have no advantages compared to the mononuclear analogue in CHO/PA copolymerization. However, both complexes 1b and 4 were highly active towards polyester formation with TOFs up to 1100 h⁻¹ at 100 °C in neat CHO.