Chromium(III) complexes based on phenoxy-imine ligands with pendant N- and O-donor groups as precatalysts for ethylene oligomerization: synthesis, characterization, and DFT studies

Abstract

Chromium complexes of general formula [Cr{ZNO}(THF)Cl2] [2a, ZNO = C9H6N-8-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2b, ZNO = Ph(NH)-C2H4-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2c, ZNO = 2-MeO-Ph-CH2-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2d, ZNO = 2-PhO-Ph-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2e, ZNO = PhO-C2H4-(N=CH)-2,4-tert-butyl-2-(OC6H2)] and the bis(ligand) complex [Cr{C9H6N-8-NH2}2Cl2]Cl (4) were synthetized and characterized by elemental analysis, IR spectroscopy, and by X-ray crystallography for 4. In the solid state, 4 is monomeric with two 8-amino-quinoline acting as bidentate ligands and two chloride ligands in cis position. The DFT calculations showed slightly higher HOMO energy for 2d. In addition, the energy levels of the LUMO are slightly influenced by pendant O- and N-donor group. Particularly, the LUMOs for complexes 2a and 2d show a small contribution from Cr and Cl atoms as compared to other chromium complexes (2b, 2c and 2e), and the orbitals are almost entirely delocalized over the phenoxy-imine unit. Upon activation with methylaluminoxane (MAO), chromium precatalysts 2a-2e showed good activity in ethylene oligomerization (TOF = 22.0 – 52.7 × 103 (mol ethylene)(mol Cr)−1⋅h−1 at 80°C) with Schultz-Flory distribution of oligomers (KC4-C10 ≈ 0.92), and production of polymer varying from 2.9 to 22.3 wt.%. The catalytic performance is mainly controlled by electronic effects at the phenoxy-imine ligands. The bis(ligand) chromium complex 4 showed good activity in ethylene oligomerization (TOF = 39,400 (mol ethylene)(mol Cr)−1 h−1), producing mostly oligomers (95.2 wt% of total products) with high selectivity for α-olefins. The highest activity among the six precatalysts screened was reached with 2c (TOF = 52,700 mol(ethylene)•mol(Cr)−1•h−1).