DALTON COMMUNICATION DOI: 10.1039/b002631j J. Chem. Soc., Dalton Trans., 2000, 1969–1971 1969 This journal is © The Royal Society of Chemistry 2000 A five-coordinate chromium alkyl complex stabilised by salicylaldiminato ligands Vernon C. Gibson,* Sergio Mastroianni, Claire Newton, Carl Redshaw, Gregory A. Solan, Andrew J. P. White and David J. Williams Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY. E-mail: V.Gibson@ic.ac.uk Received 3rd April 2000, Accepted 22nd May 2000 Published on the Web 14th June 2000 The five-coordinate bis(salicylaldiminato)chromium(III) complex [3,5-(t-Bu) 2 (O)C 6 H 2 CHN(2,6-i-Pr 2 C 6 H 3 )] 2 CrCl (2) reacts with triethylaluminium in THF at room temperature to give the air sensitive ethyl species [3,5-(t-Bu) 2 - (O)C 6 H 2 CHN(2,6-i-Pr 2 C 6 H 3 )] 2 CrEt (3); in the presence of aluminium activators, 2 and 3 are moderately active ethylene polymerisation catalysts. Much research effort has been devoted to the study of half-sandwich chromium complexes 1 as model systems for the Unipol family of silica-supported chromium catalysts 2 which have been commercially exploited for the production of high density polyethylene. More recently, a number of non- cyclopentadienyl ligand sets have been investigated including imido, 3 β-diketiminates 4 and reduced Schiff base [N,O] chelate derivatives, 5 with a view to generating new, tunable molecular chromium polymerisation systems. The simple and readily accessible salicylaldiminato ligand family has played an important role in the development of tran- sition metal coordination chemistry, and recently they have been shown to support highly active polymerisation centres for both the early 6 and late 7 transition metals. There are, however, no examples of olefin polymerisation catalysts bearing these ligands for the mid transition metals, and indeed, as far as we are aware, there have been no reports of chromium alkyl species stabilised by Schiff base [N,O] chelate ligands.† We therefore decided to target such species as potentially viable ethylene polymerisation catalysts. Here, we report the synthesis and characterisation of a five-coordinate chromium alkyl species bearing two bulky salicylaldiminato ligands and its perform- ance, along with its precursor and a closely related derivative, as ethylene polymerisation catalysts. The bis(salicylaldiminato)chromium() complex [3,5-(t-Bu) 2 - (O)C 6 H 2 CHN(2,6-i-Pr 2 C 6 H 3 )] 2 CrCl (2) is prepared in high yield by treatment of [CrCl 3 (THF) 3 ] with two equivalents of the lithium salt of 1 in THF (Scheme 1). Prolonged standing at -10 °C of a pentane solution gave 2 as a pale yellow microcrystalline solid. Complex 2 is presumed to be a monomeric, square-based pyramidal complex with an apical chlorine atom (by com- parison with the structure of 3 – vide infra); 8 microanalysis ‡ and the magnetic moment (3.94 μ B , Evans Balance) are consistent with its formulation. The FAB + mass spectrum of 2 shows a molecular ion peak along with a fragmentation peak corre- sponding to loss of one chlorine atom. Attempts at preparing a mono(salicylaldiminato) chromium complex by reacting [CrCl 3 (THF) 3 ] with one equivalent of the lithium salt of 1 also resulted in the isolation of 2 albeit in lower yield. Treatment of 2 with triethylaluminium at room temperature in THF afforded the air sensitive green ethyl complex [3,5-(t- Bu) 2 (O)C 6 H 2 CHN(2,6-i-Pr 2 C 6 H 3 )] 2 CrEt (3).‡ Crystals of 3 suit- able for a single crystal X-ray determination§ were grown from a saturated light petroleum (bp 40–60 °C) solution. The complex has crystallographic C 2 symmetry with a consequent trans dis- position of the coordinated atoms of the chelating ligands; the apical ethyl substituent is disordered about this axis (Fig. 1). Scheme 1 Reagents and conditions (i) n-BuLi, -78 °C, THF; (ii) [CrCl 3 (THF) 3 ], -78 °C, THF, 71%; (iii) AlEt 3 , THF, 70%; (iv) MeCN, 89%.