Unsymmetrical pyrazole-based PCN pincer Ni^II halides: Reactivity and catalytic activity in ethylene oligomerization

The reactivity of unsymmetrical pyrazole-based PCN pincer Ni(II) halides has been tested in the presence of copper(II) halides as an oxidizing agent as well as with NaBH and LiAlH as a hydride source. While the reaction with CuX resulted into the generation of metastable PCN pincer Ni(III) species, the treatment of (PCN)NiCl with NaBH led to the preparation and isolation of the borohydride complex (PCN)Ni(BH). Attempts to isolate nickel-hydride species upon treatment of (PCN)NiCl with either NaBH or LiAlH invariably led to the formation of free PCN(H) ligand and metallic Ni(0) nanoparticles after an in situ reductive elimination from the (metastable) hydride derivative (PCN)NiH. Finally, the halide complexes (pre-activated by MMAO) have been tested as homogeneous catalysts in ethylene oligomerization showing moderate activity (~ 14 × 10 mol?mol?h) with the formation of even-numbered olefins (mainly C-C fractions) as products.