New neutral and cationic dialkylaluminium complexes bearing imino-amide or imino-phenoxide ligands: synthesis, characterization and reactivity with olefins

The synthesis and the characterization of some new aluminium complexes carrying bidentate monoanionic iminoamide or imino-phenoxide ligands are described. Reaction of 1-(o,o9-diisopropylphenylamino)-6-(o,o9-diisopropylphenylimino) cyclohexene with AlMe3 proceeded by methane elimination to produce the yellow compound dimethylaluminium N-(o,o9-diisopropylphenyl)-6-[N-(o,o9-diisopropylphenyl)- imido]-1-cyclohexen-1-amide (1). In contrast, reaction of the same ligand with AlEt3 under the same experimental conditions involved the 1,4-addition of AlEt3 to the á,â-unsaturated imine, and led to the highly crystalline diethylaluminium N- (o,o9-diisopropylphenyl)-2-[(o,o9-diisopropylphenyl)amino]- 3-ethyl-1-cyclohexen-1-amide (2). The structures of compounds 1 and 2 were determined by single-crystal X-ray dif- Introduction Since the disclosure by Ziegler of the ‘‘aufbau’’ reaction, alkylaluminium compounds have been known as olefin oligomerization catalysts, because of the displacement reaction competing with a ‘‘smooth stepwise addition of ethylene’’.[ 1] In 1992 Martin demonstrated that the growth reaction of ethylene at bis(dichloroaluminium)ethane and trialkylaluminium produces, with low activity, polyethylene of high molecular weight and thermoplastic character.[2] More recently, Sen et al. reported that simple alkylaluminium compounds, after reaction with the activators commonly used in the homogeneous olefin polymerization catalysis {i.e. B(C6F5)3, [(C6H5)3C][B(C6F5)4] or [(C6H5)N(CH3)2H]- [B(C6F5)4]} are able to catalyse the polymerization of ethylene and propene, although with low activity.[3] The authors hypothesize that neutral aluminium species, such as [(R)(C6F5)2Al]x, are the active catalysts operating in these systems.[3] Actually, the reaction of AlMe3 with [(C6H5)3C][B(C6F5)4] gives the transient species [a] Dipartimento di Chimica, Universita` di Salerno, 84081 Baronissi, Salerno, Italy Fax: (internat.) 1 39-089/965296 E-mail: pappalardo@chem.unisa.it [b] Facolta` di Scienze MM. FF. NN., Universita` del Sannio, Via Port’Arsa 11, 82110, Benevento, Italy Eur. J. Inorg. Chem. 2002, 6212628 ÓWILEY-VCH Verlag GmbH, 69451Weinheim, Germany, 2002 143421948/02/030320621 $ 17.501.50/0 621 fraction. Thermolysis of 2 gave rise to ethane elimination, and led to the oily bis(amido) monoethyl derivative. Treatment of the salicylaldimine ligands 3-tBu-2-(OH)C6H3CH= N-R with AlMe3 yielded the dimethylaluminium compounds {3-tBu-2-(O)C6H3CH=N-R}AlMe2 [R = C6H5 (3); 2,6-iPr2C6H3 (4); and C6F5 (5)]. Compounds 1, 3, and 5 underwent methyl abstraction reactions with B(C6F5)3; trapping of the cationic species was accomplished in the presence of THF in dichloromethane solution. Preliminary polymerization tests were carried out for the synthesized Al complexes. Toluene solutions of 3, 4, and 5, when activated with 1 equiv. of B(C6F5)3, polymerised ethylene (1 atm) to solid polyethylene with low activity.