FULL PAPER Dalton www.rsc.org/dalton New iron bis(imino)pyridyl complexes containing dendritic wedges for alkene oligomerisation† Matthew J. Overett,‡ Reinout Meijboom§ and John R. Moss* Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa. E-mail: jrm@science.uct.ac.za; Fax: +27 (0)21 689 7499; Tel: +27 (0)21 689 2535 Received 2nd August 2004, Accepted 19th November 2004 First published as an Advance Article on the web 21st December 2004 The synthesis, characterisation and catalytic behaviour of new iron bis(imino)pyridyl complexes containing dendritic wedges, as well as the synthesis of bis(para-hydroxyphenylimino)pyridines is described. The hydroxyl functionality of the bis(para-hydroxyphenylimino)pyridines was used to attach dendritic wedges of the carbosilane type as well as the benzylphenyl ether type. After attachment of the dendritic wedges, complexation of these ligands to iron(II) chloride was achieved. The resulting dendritically functionalised bis(imino)pyridyl iron complexes were tested in the catalytic oligomerisation of ethene. Introduction The oligomerisation of ethene is one of the primary industrial processes for the production of linear 1-alkenes. 1 Oligomers in the range C 6 –C 20 are used as co-monomers in the polymerisation of ethene to give linear low-density polyethene (LLDPE), or for the preparation of detergents and synthetic lubricants. Catalysts currently used in industry for the Shell Higher Olefin Process (SHOP) 2 contain Ni(II) complexes bearing bidentate mono- anionic ligands. 1,2 Cationic Ni(II) a-diimine complexes were also reported to be effective ethene oligomerisation catalysts, 3 while iron-based bis(imino)pyridyl complexes were described as highly active compounds in the catalytic oligomerisation of ethene in combination with the co-catalyst MAO or MMAO. 4,5 Several modifications of the bis(imino)pyridine backbone, which have already been described in the literature, 6 mostly lead to a decrease in catalytic activity. Recently, fluoro-substituted bis(imino)pyridine complexes and their catalytic reactivity in ethene oligomerisations were described, 7 as were bis(imino)- pyridine complexes containing methoxy and CF 3 groups. 8 Since the independent reports by Gibson 4,6 and Brookhart 9,10 on iron and cobalt bis(imino)pyridyl catalysts and nickel and palladium a-diimine catalysts for the polymerisation and oligomerisation of 1-alkenes, much attention has been focused on these late transition metal catalysts as alternatives to estab- lished technologies. It is known that the position and steric bulk of the substituents on the aryl rings of the bis(imino)pyridyl catalysts (Fig. 1) play a crucial role in determining the selectivity of the catalyst. The ortho substituents (R 1 ) have been identified as being particularly important. If both R 1 groups on each ring are non-hydrogen substituents, and preferably bulky alkyl substituents such as iso- propyl groups, then the catalysts are selective for the production of high molecular weight polymers from ethene. If only one of the ortho substituents R 1 on each ring is an alkyl group, and the other a hydrogen, then the catalyst is selective for the oligomerisation of ethene to linear 1-alkenes with a Schultz– Flory chain length distribution, as well as for the dimerisation of longer chain 1-alkenes (such as 1-hexene). The effect of substituents in the para-position (R 2 ) has been less studied. Our interest in dendritic molecules, 11 their †Electronic supplementary information (ESI) available: Synthetic de- tails. See http://www.rsc.org/suppdata/dt/b4/b411884g/ ‡ Current address: Sasol Technology R&D, P.O. Box 1, Sasolburg 1947, South Africa. § Current address: Department of Chemistry and Biochemistry, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa. Fig. 1 Iron bis(imino)pyridyl precatalyst for the oligomerisation of 1-alkenes. applications in catalysis and as new materials, led us to consider the possibility of functionalising late transition metal oligomeri- sation catalysts with dendritic components. This could take the form of attaching dendritic wedges to the catalysts, thus creating a catalyst at the core of a dendrimer. A catalytic site at the core of a dendrimer makes it possible to control the microenvironment around the catalytic centre and thus allows modifications of the catalytic selectivity. 12 Incorporation of a catalytically active site in a dendritic macromolecule has the added potential advantage of enabling one to separate the catalyst from the product stream by means of ultra-filtration methods. Functionalising a bis(imino)pyridyl ligand with dendritic wedges to create a catalytic ligand at the core of a dendrimer is likely to be synthetically achievable. The aryl components of the ligand are particularly amenable to dendritic functionalisation, as a wide range of anilines with different functionalities for substitution are commercially available. These may be reacted in standard Schiff-base type reactions with 2,6-diacetylpyridine to form the bis(imino)pyridyl framework with aryl rings suitable for attachment of dendritic wedges. Dendritic functionalisation at the para-position is most favourable. The direct steric control around the catalytic centre may be controlled by appropriate ortho-substituents (R 1 , Fig. 1), thereby ensuring an oligomeri- sation catalyst. The heteroatomic functionality, for attachment of the dendritic wedges, is introduced on the para-position (R 2 , Fig. 1) and thus removed from the active catalytic centre in order to minimise unfavourable interactions. Results and discussion Preparation of ligands Appropriate ligands would be bis(imino)pyridyl ligands with either both ortho-substituents (R 1 ) being hydrogen, or one of the substituents R 1 on each ring being a methyl group and the other a hydrogen. Substituent R 2 was chosen to be a hydroxyl group for ease of attachment of alkylbromide functionalised dendritic wedges, using the Williamson ether synthesis. Thus DOI: 10.1039/b411884g This journal is © The Royal Society of Chemistry 2005 Dalton Trans. , 2005, 551–555 551