This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol. Cite this: DOI: 10.1039/c3cy00334e Tandem ethylene oligomerisation and Friedel–Crafts alkylation of toluene catalysed by bis-(3,5-dimethyl- pyrazol-1-ylmethyl)benzene nickel(II) complexes and ethylaluminium dichloride† Asheena Budhai, a Bernard Omondi,z a Stephen O. Ojwach,y a Collins Obuah, a Emmanuel Y. Osei-Twum b and James Darkwa* a Three ligands, 1,2-bis(3,5-dimethylpyrazol-1-ylmethyl)benzene (L1), 1,3-bis(3,5-dimethylpyrazol-1-ylmethyl)- benzene (L2) and 1,4-bis(3,5-dimethylpyrazol-1-ylmethyl)benzene (L3), were reacted with either nickel(II) chloride or nickel(II) bromide to produce four nickel complexes, Ni(L1)Br 2 (1), Ni(L1)Cl 2 (2), Ni(L2)Br 2 (3), and Ni(L1)Br 2 (4). The complexes were either mononuclear, 1 and 2, or polymeric, 3 and 4, depending on the positions of the pyrazolyl units on the benzene linker in the ligand. This was established from the crystal structures of 1, 2 and 3. All four complexes upon activation with ethylaluminium dichloride produced a tandem catalyst system that oligomerised ethylene to mainly 1-butene and 1-hexene and subsequently used the olefins present in the reaction medium to alkylate toluene that was used as solvent in the reactions. This led to mono-, di- and tri-alkyltoluenes with ethylene, butene and hexene. Introduction Late transition nitrogen-donor metal complexes continue to be investigated as ethylene transformation catalysts since Brookhart 1 demonstrated in the mid-1990s their robust nature to withstand oxo-containing monomers. In the last decade nitrogen-donor nickel catalysts in particular have received great attention; initially as ethylene polymerization catalysts, 2 but lately as ethylene oligomerization catalysts. 3 Much of the nickel oligomerization work, mainly carried out with P^N 4 and N^N 5 donor ligands, lead principally to dimerization and trimerization of ethylene; where 1-butene and 1-hexene are the dominant products. But in these dimerization and trimerization reactions, the role of aluminium co-catalysts has only recently been brought into focus following a report by Dyer and co-workers 6 in 2008. Using (P^N)nickel(II) complexes and ethylaluminium dichloride as co-catalysts, they observed ethylene oligomeriza- tion for all four nickel complexes; but only one of these nickel precursors was found to perform Friedel–Crafts alkylation of toluene, the solvent used in the oligomerization reaction. 6 Another report by Gao and co-workers 7 appeared soon after the Dyer report; showing that ethylene oligomerization products, from another (P^N)nickel(II) complex and MAO as a co-catalyst, perform Friedel–Crafts alkylation of toluene to alkyltoluenes but only at 50 1C. It is clear from the above two reports that reaction conditions, the ligand on the nickel catalysts, and the aluminium co-catalyst may all play a role in promoting the Friedel–Crafts alkylation reaction observed in these reports, because there are several examples 4,5 where (P^N)nickel(II) complexes with MAO or ethylaluminium dichloride as co-catalysts produce only ethylene oligomers in toluene. It is also clear that where ethylene oligomerization is followed by Friedel–Crafts alkylation the two reactions occur in tandem and are promoted by a tandem catalytic system. The role ligands play in this tandem may be more important because our recent reports 8 on the use of pyrazolyl nickel complexes and ethylaluminium dichloride invariably lead to large amounts of mono-, di- and tri-butyltoluenes and other alkyltoluene products. The present report is indeed further demonstration that ethylaluminium dichloride and nickel com- plexes, supported on bis(pyrazol-1-yl) ligands, provide the best tandem catalyst systems that perform ethylene oligomerization and post oligomerization olefin alkylation of toluene. a Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, South Africa. E-mail: jdarkwa@uj.ac.za b Saudi Basic Industries Corporation, P. O. Box 42503, Riyadh 11551, Saudi Arabia † Electronic supplementary information (ESI) available. CCDC 942278 (1), 942279 (2) and 942280 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cy00334e ‡ Current address: School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa. § Current address: School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01, Scottsville 3209, South Africa. Received 14th May 2013, Accepted 8th June 2013 DOI: 10.1039/c3cy00334e www.rsc.org/catalysis Catalysis Science & Technology PAPER Published on 10 June 2013. Downloaded on 04/07/2013 10:43:18. View Article Online View Journal