Five-coordinate iron(II) complexes bearing tridentate nitrogen donor ligands as catalysts for atom transfer radical polymerisation

A series of five-coordinate complexes of iron(II) containing tridentate nitrogen donor ligands has been synthesised and evaluated for the atom transfer radical polymerisation of styrene. The molecular structures of two derivatives are described. Cyclic voltammetric studies show that the redox potential for the Fe(II)/Fe(III) couple strongly depends on the donor capacity of the complexing ligand, and styrene polymerisations reveal that ligands derived from alkyl amine or pyridine groups are the most active for ATRP, although slower than for their four-coordinate relatives due to unfavourable steric interactions. In general, catalyst activity decreases in the order of donor group: alkyl amine ≈ pyridine > alkyl imine ≫ aryl imine > aryl amine, with the aryl derivatives being almost completely inactive in ATRP. The trend in polymerisation activity correlates with the redox potential of the corresponding complexes. Complexes derived from aryl-imine or -amine ligands afford catalytic chain transfer rather than ATRP catalysis.