Cationic Pyridine(diimine) Iron Tethered Alkene Complexes: Synthetic Models For Elusive Intermediates In Iron-Catalyzed Ethylene Polymerization

Abstract

Cationic bis(imino)pyridine iron alkene complexes, key intermediates in the polymerization and oligomerization of ethylene, have been targeted to understand important physical properties such as spin state of the metal and the role of the redox-active supporting ligand. The allyl derivative, (^MePDI)Fe(h³-C₃H₅) was synthesized but proved unstable toward one electron oxidation with ferrocenium reagents. Tethered alkoxide-alkene complexes were explored as more well behaved alternatives and (^MePDI) Fe(OC(Ph)₂CH₂(h²-CHCH₂) was synthesized, crystallographically characterized and determined to be a high spin Fe(II) complex with a bis(imino)pyridine radical anion. One electron oxidation with [Cp₂Fe][BArF₂₄] (BArF₂₄ = C₆H₃-3,5-(CF₃)₂) generated the desired cationic iron complex, [(^MePDI)Fe(OC(Ph)₂(h²-C₃H₅))][BArF₂₄]. The solid-state structure confi rmed alkene coordination and provided the metrical parameters used to establish a neutral bis(imino)pyridine chelate. The observation of a high spin Fe(II) complex demonstrates that the iron does not change spin state upon olefi n coordination.