Reactivity of cationic α-diimine cyclopentadienyl nickel complexes towards AlEt2Cl: synthesis, characterisation and ethylene polymerisationElectronic supplementary information (ESI) available. NMR data of complexes and polyethylenes, thermograms of polyethylenes, and CCDC 1546750 for 3, 1546751 for 5, and 1546752 for 6. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cy00875a

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. Cationic α-diimine cyclopentadienyl nickel complexes transform into intermediates upon reaction with AlEt 2 Cl giving rise to active ethylene polymerisation catalysts.