The synthesis and application of novel Ni(II) N-alkyl dipyridylaldiminato complexes as selective ethylene oligomerisation catalysts.
A series of N-alkyl 2,2'-dipyridylamine ligands of general formula (2-C5H3NR)2NR', (a): R = H, R' = Me; (b): R = H, R' = benzyl; (c): R = H, R' = methylcyclohexyl; (d): R = H, R' = neopentyl; (e): R = Me, R' = Me) were prepared by a modified method involving base-mediated N-alkylation with the respective alkyl halide. Reaction of the ligands, a-e, with NiCl2(DME) allowed for the isolation of μ-Cl Ni(ii) complexes: [Ni(μ-Cl){a}Cl]2 (1a); [Ni(μ-Cl){b}Cl]2 (1b); [Ni(μ-Cl){c}Cl]2 (1c); [Ni(μ-Cl){d}Cl]2 (1d) and [Ni(μ-Cl){e}Cl]2 (1e). The complexes were characterised by FT-IR spectroscopy, magnetic susceptibility measurements, mass spectrometry, elemental analyses and in the case of 1a, SCD analysis. In the case of complex 1e, an acid-mediated hydrolysis process was identified. The product of hydrolysis, the protonated ligand and a tetrachloronickelate salt (1e-A), was characterised by SCD analysis. Activation of 1a-1e with alkyl aluminium reagents generated highly active catalysts for the oligomerisation of ethylene, with activities of up to 864 kgoligomers molNi(-1) h(-1) and high selectivity toward the formation of butenes. In general, trans 2-butene was observed as the major isomer, with the exception of 1e. In the case of 1e, the selectivity for 1-butene was 98%, thereby demonstrating the significant effect that the introduction of a low degree of steric pressure in the coordination sphere of the catalyst has on selectivity.
Stellenbosch University, Department of Chemistry and Polymer Science, Private Bag X1, Matieland, 7602, Stellenbosch, South Africa. 15428958@sun.ac.za smapolie@sun.ac.za.
Andrew J
AJ
Swarts
Selwyn F
SF
Mapolie