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

The nickel complexes ((PCN)-P-BzTz)NiX ((PC)-P-BzTz(H)N = 2-(3-((di-tert-butylphosphino)methyl)phenoxy)benzo[d] thiazole; X = F, Br) containing the unsymmetrical pincer k(3)-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 x 10(3) mol C2H4 center dot molNi(-1).h(-1)), with formation of even-numbered olefins (mainly C-4-C-10 fractions) as products. The comparison of their performance with the results obtained for more rigid pyrazole-based analogues ((PCN)-P-Pyr)NiX ((PC)-P-Pyr(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 (BzTzPCN)NiX by MMAO was studied by UV-Vis and P-31 NMR spectroscopies combined with time-dependent density functional theory (TD-DFT), revealing the formation of Ni-CH3 species. Finally, DFT calculations were also performed to explore the mechanism of ethylene oligomerization catalyzed by the methyl analogues ((PCN)-P-Pyr)Ni(CH3) and ((PCN)-P-BzTz)Ni(CH3). (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Nickel complexes with unsymmetrical pincer ligands were tested as homogeneous catalysts in ethylene oligomerization, showing higher activity when the ligand flexibility was increased. Their activation by Modified Methylaluminoxane (MMAO) and the mechanism of ethylene oligomerization were studied using spectroscopic techniques and density functional theory (DFT) calculations.