1398 | Chem. Commun., 2014, 50, 1398--1407 This journal is © The Royal Society of Chemistry 2014 Cite this: Chem. Commun., 2014, 50, 1398 Ethylene oligomerization using iron complexes: beyond the discovery of bis(imino)pyridine ligands Adrien Boudier, a Pierre-Alain R. Breuil,* a Lionel Magna, a He ´ le ` ne Olivier-Bourbigou* a and Pierre Braunstein b Since the discovery that bis(imino)pyridine ligands are able to confer high activities in ethylene oligomerization and polymerization to their iron complexes, considerable attention has been focused on catalyst design for these reactions and this research constitutes an ever-growing area in molecular catalysis. The tuning of the ligand structures and properties, and thus of catalysts, generally represents the basis for subsequent work contributing to process development and industrialization. Significant effort is therefore devoted to generate structural diversity in order to access the required catalyst stability and selectivity. This feature article outlines nitrogen-containing ligands that have been developed for the iron-catalyzed oligomerization of ethylene since the seminal discovery of the properties of bis(imino)pyridine ligands. Introduction Initial discovery of the so-called ‘‘nickel effect’’ by Ziegler leading to ethylene polymerization 1 and to oligomerization 2 triggered great efforts to develop new homogeneous late transition metal catalysts enabling the transformation of olefins and led to worldwide processes 3 and significant changes in our society. Ligand design plays a pivotal role in the development of more active and selective catalysts for future industrial processes. Monoanionic (P,O) ligands coordinated to nickel complexes and developed by Keim and coworkers and the Shell company proved to be excellent one-component model catalysts for the oligomerization of ethylene 4 and resulted in the successful Shell Higher Olefin Process (SHOP) (Scheme 1) for the production of linear olefins. 5 Neutral (P,O) ligands leading to cationic nickel catalysts active in ethylene oligomerization in the presence of MAO (methylaluminoxane) were later reported 6 and Brookhart et al. developed new palladium(II) 7 and nickel(II) 8 complexes chelated by a-diimine ligands (Scheme 1) as very active catalysts for a-olefin oligomerization and polymerization. In the 1990s, the groups of Bennett, Brookhart and Gibson reported that tridentate 2,6-bis(imino)pyridine (BIP) ligands yielded the most active ethylene oligomerization catalysts to date, once coordi- nated to an iron centre and after activation by MAO (Scheme 1), leading to a wide range of linear alpha olefins (C4–C30). 9–11 Since then, much work has been devoted to modifications of these ligands and understanding of the chemistry of their metal derivatives. Bianchini, Gibson and co-workers independently reviewed these results. 12–14 These iron complexes represent a remark- able new generation of ethylene oligomerization catalysts that have extended our understanding of the influence of electronic and steric properties of the ligands in controlling transition metal-catalyzed olefin polymerization and oligomerization. However, several limita- tions still need to be addressed for further development of a catalytic system such as the stability of the catalyst, the proper control of the reaction parameters related to the tuning of the selectivity or the limitation of side-products, i.e. mainly polyethylene. 15 Circumventing these limitations is of considerable interest for iron-based olefin transformations and would allow the use of an abundant, inexpen- sive and environmentally friendly metal in additional industrial processes for the production of linear alpha olefins. Regarding bis(imino)pyridine-based iron catalysts, the nature of the catalytic reaction, i.e., ethylene oligomerization Scheme 1 Examples of homogeneous olefin transformation precatalysts. a IFP Energies nouvelles, Rond point de l’e ´changeur de Solaize, 69360 Solaize, France. E-mail: pierre-alain.breuil@ifpen.fr, helene.olivier-bourbigou@ifpen.fr b Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite ´ de Strasbourg, 4 rue Blaise Pascal, F-67081 Strasbourg Cedex, France Received 11th October 2013, Accepted 2nd December 2013 DOI: 10.1039/c3cc47834c www.rsc.org/chemcomm ChemComm FEATURE ARTICLE Published on 03 December 2013. Downloaded by Université de Strasbourg, Service Commun de la Documentation on 16/01/2014 06:51:45. View Article Online View Journal | View Issue