Efficient ring-opening polymerization (ROP) of ϵ-caprolactone catalysed by isomeric pyridyl β-diketonate iron(III) complexes

A series of Fe(iii) complexes of β-diketonate ligands, 1-(2-pyridyl)-3-(3-pyridyl)-1,3-propanedione (L1), 1-(2-pyridyl)-3-(2-pyridyl)-1,3-propanedione (L2), 1-(2-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L3), 1-(3-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L4), 1-(3-pyridyl)-3-(3-pyridyl)-1,3-propanedione (L5) and 1-(4-pyridyl)-3-(4-pyridyl)-1,3-propanedione (L6), viz. [Fe(L1)₃] (1), [Fe(L2)₃] (2), [Fe(L3)₃] (3), [Fe(L4)₃] (4), [Fe(L5)₃] (5) and [Fe(L6)₃] (6) have been structurally characterized. All but one complex (1) catalyzed the ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) in near quantitative yield at 110 °C to give polymers with relatively narrow polydispersities (PDI). The comparison of in situ reaction and a reaction with preformed 1 indicated that the latter was a better catalyst, giving a higher molecular weight. Complex 2 catalyzed this reaction in a more modest yield reflecting its greater thermal stability, shorter Fe-O bonds and minimal distortion in fold angle among the isomeric complexes, suggesting that ligand dissociation is important for catalytic activity.