Benzothiazole- vs. pyrazole-based unsymmetrical (PCN) pincer complexes of nickel(II) as homogeneous catalysts in ethylene oligomerization

The nickel complexes (PCN)NiX (PC(H)N = 2-(3-((di-tert-butylphosphino)methyl)phenoxy)benzo[d]thiazole; X = F, Br) containing the unsymmetrical pincer k-tridendate ligand with an oxo-bridged benzothiazole side-arm have been tested as homogeneous catalysts in ethylene oligomerization after preliminary activation by Modified Methylaluminoxane (MMAO). They showed high activity in the process (up to 200 × 10 mol?mol?h), with formation of even-numbered olefins (mainly C-C fractions) as products. The comparison of their performance with the results obtained for more rigid pyrazole-based analogues (PCN)NiX (PC(H)N = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole; X = F, Br) demonstrates a positive effect stemming from the increased ligand flexibility. Moreover, the activation of (PCN)NiX by MMAO was studied by UV-Vis and P NMR spectroscopies combined with time-dependent density functional theory (TD-DFT), revealing the formation of Ni-CH species. Finally, DFT calculations were also performed to explore the mechanism of ethylene oligomerization catalyzed by the methyl analogues (PCN)Ni(CH) and (PCN)Ni(CH).