Cobalt (II) and Nickel (II) Complexes of Substituted (Pyrazolyl)amine Ligands: Structural Characterization and Ethylene Oligomerization Studies

ABSTRACT The cobalt (II) and nickel (II) complexes of the general formula [Co( L1 )Cl 2 ] ( Co1 ), [Co( L2 )Cl 2 ] ( Co2 ), [Co( L3 )Cl 2 ] ( Co3 ), [Ni( L1 )Br 2 ] ( Ni1 ), and [Ni( L2 )Br 2 ] ( Ni2 ) were prepared by the reaction of either CoCl 2 or [NiBr 2 (DME)] metal salts with the (pyrazolyl)amine ligands, N ‐benzyl‐2‐(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)ethan‐1‐amine ( L1 ), 2‐(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)‐ N ‐(2‐methyl‐benzyl)ethan‐1‐amine ( L2 ) and 2‐(3,5‐diphenyl‐1H‐pyrazol‐1‐yl)‐ N ‐(2‐methylbenzyl)ethan‐1‐amine ( L3 ). The new compounds were characterized using NMR spectroscopy (for ligands), IR spectroscopy, mass spectrometry, elemental analysis and single crystal X‐ray crystallography. The bidentate coordination modes of ligands L1 ‐ L3 to form monometallic complexes was confirmed by the solid‐state structures of Co1 , Co2 , Co3 , and Ni1 . Activation of these complexes with either EtAlCl 2 or MAO resulted in the formation of highly active ethylene oligomerization catalysts (up to 10 3 kg [oligomers] mol −1 [catalyst]·h −1 ), to afford predominantly C 4 and C 6 oligomers. The nature and coordination chemistry of the complexes was found to influence the catalytic performance. In general, nickel (II) complexes were more active than the corresponding cobalt (II) analogues. Significantly, cobalt (II) complexes were highly selective (99%) towards the formation of C 4 oligomers.