Reactivity of cationic α-diimine cyclopentadienyl nickel complexes towards AlEt 2 Cl: synthesis, characterisation and ethylene polymerisation

The electronically saturated and air-stable complexes [Ni(η 5 -C 5 H 5 )(Mes-BIAN)][PF 6 ] ( 1 ) and [Ni(η 5 -C 5 H 5 )(Mes-DAD)][PF 6 ] ( 2 ) were found to behave as efficient catalyst precursors for the polymerisation of ethylene under mild reaction conditions (temperature and pressure), when activated with excess AlEt 2 Cl (DEAC), exhibiting activities in the range 4 × 10 4 to 22 × 10 4 g PE (mol Ni) −1 h −1 bar −1 . Compound 2 was synthesised and fully characterised as reported hereby for the first time, and exhibits a catalytic activity of ca. 30 × 10 4 g PE (mol Ni) −1 h −1 bar −1 in the polymerisation of ethylene. The reactivity of these complexes towards DEAC was found to lead to the formation of diamagnetic cationic Ni( ii ) complexes ( 4 and 3 , respectively) containing the same initial cation [Ni(η 5 -C 5 H 5 )(α-diimine)] + and an [AlEtCl 3 ] − anion, whereby diimine = BIAN ( 1 and 4 ) and DAD ( 2 and 3 ). In the corresponding reaction of complex [Ni(η 5 -C 5 H 5 )(Mes-BIAN)][PF 6 ] ( 1 ), an unusual paramagnetic complex incorporating a cationic trinuclear α-diimine Ni cluster ( 5 ), containing five bridging Cl ligands and an [AlCl 4 ] − anion, was also isolated and fully characterised, in which the cyclopentadienyl ligand was absent. The intermediates 3 and 5 catalysed efficiently the polymerisation of ethylene when the DEAC cocatalyst was used in slight excess. The polyethylene produced ( ca. 10 × 10 4 to 30 × 10 4 g PE (mol Ni) −1 h −1 bar −1 , respectively) showed branching numbers ranging from 7 to 75 branches per 1000 carbon atoms according to 1 H NMR spectroscopy analysis. These findings are surprising given that the new 18-electron nickel precursors show remarkable properties towards ethylene polymerisation catalysis when activated with a small amount of aluminium activator under mild conditions, and also that some unusual intermediates responsible for the observed activity were isolated and characterised by X-ray crystallography.