Carbocyclic-fused N,N,N-pincer ligands as ring-strain adjustable supports for iron and cobalt catalysts in ethylene oligo-/polymerization

Recent progress in the application of homogeneous iron and cobalt catalysts in ethylene oligo-/polymerization is reviewed with particular emphasis placed on the tuning of catalyst performance through the introduction of controlled amounts of ring strain to the ligand frame. While new examples of catalysts bearing the prototypical bis(arylimino)pyridine continue to emerge in the literature, the last decade has witnessed a number of key advances concerned with the fusion of carbocyclic units to the N,N,N-pincer manifold with a view to enhancing both the catalytic activities and thermal stablility of their resultant catalysts. Some notable examples include iron complexes containing aryl-fused imino-phenanthroline ligands, which have proved highly active catalysts for ethylene oligomerization and indeed have shown considerable industrial promise on the pilot plant scale. Elsewhere, bis(arylimino)pyridines incorporating singly or doubly fused cycloalkyl units with the ring sizes anywhere between five- and eight-membered have been systematically developed and have proved versatile supports for both metal centers. More significantly, clear correlations between structure and activity as well as oligo-/polymer properties are a feature of these strain-adjustable catalysts. In many cases, linear vinyl-polyethylenes are accessible which are in demand for the production of long-chain branched copolymers, functional polymers as well as coating materials.