Synthesis, Electronic Structure, and Ethylene Polymerization Activity of Bis(imino)pyridine Cobalt Alkyl Cations

Abstract

A new spin on polymers: the title cations comprise low-spin Co(II) centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single-component ethylene polymerization catalysts and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.

Methyl Complexes of the Transition Metals

“…An example is (IMes) 2 CoMe 2 , which has square‐planar coordination around the metal, as shown by X‐ray diffraction and is analogous to the above‐mentioned Fe II complexes of Glorius . Another series of recent examples of cobalt complexes structurally comparable to their iron counterparts are a number of neutral and cationic cobalt methyl complexes with bis(imino)pyridine ligands, the latter studied by Chirik et al . as catalysts in ethylene polymerization reactions.…”

“…; b) electrophilic attack on an electron‐rich metal complex using a Me δ+ ‐X reagent; c) oxidative addition of a Me−X reagent (X=halide or pseudo‐halide). Selected recent examples of each method are shown in Scheme …”

Sequential Reduction and Alkyl Exchange Reactions of Bis(imino)pyridine Dialkyliron(II) with Trimethylaluminum

“…Calcd for C 37 H 54 N 3 FeSi·0.5C 7 H 7 : C, 72.51; H,8.71;N,6.26. Found: C,72.04;H,8.17;N,6. [AlEt 2 ( iPr BIP)], 5. A solution of complex 1b (1.60 g, 2.25 mmol) in toluene (30 mL), stirred at room temperature, was treated with 4.5 mL of a 1 M solution of TEA in the same solvent (4.5 Eq "AlEt 3 " ).…”

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

“…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][11][12][13][14][15][16][17][18][19][20][21] Our group has reported the synthesis of cationic bis(imino)pyridine iron [22][23][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.…”

“…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.…”