Ni (II) complexes stabilized by tripodal pyrazole-containing ligands with additional donor atoms in the catalysis of the ethylene oligomerization

In the present work, several dibromonickel (II) complexes ligated by tetradentate NNNX and tridentate NNN pyrazole-containing compounds were synthesized and compared in detail as precatalysts for ethylene oligomerization. All nickel complexes, after activation by DEAC or EASC (diethylaluminium chloride or ethylaluminium sesquichloride), showed moderate activity (110 to 1100 kg_PE/(mol_Ni h)), producing predominantly short-chain olefins (C₄ and C₆). The introduction of additional donor atoms (X) to the tridentate NNN 3,5-dimethylpyrazole-containing ligand leads to an increase in catalytic activity in the sequence of the nickel (II) complexes activated by 300 eq. DEAC with NNN << NNNN < NNNO << NNNS (55 << 225 < 280 << 505 kg_PE/(mol_Ni h)) ligands. For the dibromonickel (II) complex with an additional nitrogen donor (X = N), the optimal Al/Ni ratio was established as 75 eq. (1010 and 250 kg_PE/(mol_Ni h), activated by DEAC and EASC, respectively). The complexes bearing tetradentate NNNN ligands with unsubstituted and 4‑bromo-3,5-dimethylpyrazole rings were also tested under these conditions. The structure of the latter complex was established by X-ray diffraction; it is shown that all four nitrogen atoms coordinate with the metal and form three five-membered chelate cycles.