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

A series of Fe(III) complexes of beta-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 epsilon-caprolactone (epsilon-CL) in near quantitative yield at 110 degrees 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.