Six examples of α,α'-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridine-cobalt(II) chlorides, [2,3:5,6-{C4H8C(NAr)}2C5HN]CoCl2 (Ar = 2-(C5H9)-6-MeC6H3 Co1, 2-(C6H11)-6-MeC6H3 Co2, 2-(C8H15)-6-MeC6H3 Co3, 2-(C5H9)-4,6-Me2C6H2 Co4, 2-(C6H11)-4,6-Me2C6H2 Co5, 2-(C8H15)-4,6-Me2C6H2 Co6), containing N-aryl groups that differ in either the ring size of the ortho-cycloalkyl substituents or the para-R group (R = H, Me), have been synthesized using a one-pot template approach. The molecular structure of Co1 highlights the ring puckering of both the ortho-cyclopentyl substituents and the two pyridine-fused seven-membered rings; a squarebased pyramidal geometry is conferred about the metal center. On activation with either methylaluminoxane (MAO) or modified MAO (MMAO), all six complexes afforded strictly linear polyethylene (all Tm's > 130 ºC) with high molecular weight (Mw up to 64.3 kg mol-1). Furthermore, all precatalysts displayed high activities (up to 2 × 10 6 g PE mol-1 (Co) h-1) at temperatures between 20-60 ºC with the catalytic activities correlating with the type of ortho-cycloalkyl substituent: cyclohexyl (Co2, Co5) > cyclopentyl (Co1, Co4) > cyclooctyl (Co6, Co3) for either R = H or Me. The narrow unimodal distributions of the resulting polymers are consistent with single-site active species for the catalysts.
This review focuses on recent progress made using well-defined molecular chromium complexes that, upon suitable activation, can catalyze the tri-, tetra, oligo-and/or polymerization of ethylene. In particular, emphasis will be placed on the tuning of the performance characteristics of these homogeneous catalysts through structural modifications made to the multidentate ligand manifold (e.g., donor atoms, charge, backbone and strain) and the effects these changes have on the resulting ethylene derivatives. While the ability of these catalysts to mediate the formation of high molecular weight linear polyethylene continues to see many developments, their capacity to form polyethylene waxes and oligomers has witnessed some major advances. Moreover, the impressive selectivity of some chromium systems to generate commercially important 1-hexene and more recently 1-octene has seen the implementation of this technology at the industrial level. The types of precatalysts to be discussed will be divided broadly on the basis of their ability to generate either polymers/oligomers or short chain αolefins; the effects of co-catalyst and reaction conditions (e.g., temperature, pressure, solvent) on catalytic activity and selectivity, will be also developed. In addition, current proposals as to the mechanistic details displayed by these versatile chromium catalysts will be highlighted.
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