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

Abstract

Aryl-substituted pyridine(diimine) iron and cobalt dihalide complexes, when activated with excess methylaluminoxane (MAO) in presence of ethylene, exhibit high activity for the production of linear polyethylene. 1,2,3,4 Alteration of the tridentate ligand from pyridine(diimine) to modified ad ii min e s w it h pendant phosphine donors (P N N) resulted in efficient iron and cobalt catalysts for ethylene oligomerization. 5-7 The results are notable as deviation from the pyridine(diimine) scaffold usually results in catalysts with diminished performance. 8,9 Considerable effort has been devoted to understanding the identity and nature of the propagating species in olefin polymerization, including the spin state of the first row transition metal and the role of the potentially redox-active chelate. 10-21 Our group has reported the synthesis of cationic bis(imino)pyridine iron 22-24 and cobalt 25 alkyl complexes that serve as single component catalysts for the polymerization of ethylene (Figure 1). In each case examined, neutral pyridine(diimine) chelates were observed suggesting that redox chemistry with the supporting ligand is not a necessary component for catalytic performance. 24,25 More recently, we have discovered that addition of the neutral, Lewis acidic borane, B(C 6 F 5) 3 to both bis(imino)pyridine and PNN-supported iron butadiene complexes resulted in C-B bond formation to yield the corresponding borate betaine derivatives that are also active for ethylene polymerization and oligomerization, respectively, without the need for an additional activator. 26 Elucidation of the electronic structures of the bis(imino)pyridine derivative established a high spin Fe(II) ion engaged in antiferromagnetic coupling to both chelate (S = ½) and allyl (S = ½) radical anions. Fig. 1 Single component iron and cobalt catalysts for ethylene polymerization and oligomerization. These observations suggested that while the pyridine(diimine) chelate adopted its neutral form in cationic iron alkyl complexes, the electronic structure may change upon olefin coordination. To probe this effet, we targeted the synthesis of 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, (Me PDI)Fe(h 3-C 3 H 5) was synthesized but proved unstable toward one electron oxidation with ferrocenium reagents. Tethered alkoxide-alkene complexes were explored as more well behaved alternatives and (Me PDI) Fe(OC(Ph) 2 CH 2 (h 2-CHCH 2) 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 [Cp 2 Fe][BArF 24 ] (BArF 24 = C 6 H 3-3,5-(CF 3) 2) generated the desired cationic iron complex, [(Me PDI)Fe(OC(Ph) 2 (h 2-C 3 H 5))][BArF 24 ]. 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.