Chromium complexes supported by bidentate thioether-imine [N,S] ligands: synthesis and ethylene oligomerization studies

Abstract

A series of new Cr(III) complexes of general formula [Cr{N,S}(THF)Cl₃] (2a, N,S = PhS-2-PhCH = N-^tBu; 2b, N,S = PhS-2-PhCH = N-C₆H₄-4-OMe; 2c, N,S = PhS-2-PhCH = N-C₆H₅; 2d, N,S = ^tBuS-2-PhCH = N-C₆H₅) were prepared and characterized by elemental analysis, IR spectroscopy and ESI-HRMS. DFT calculations confirm that the mer isomers are more stable by 23.9–34.3 kcal mol⁻¹ than the fac isomers. Upon activation with methylaluminoxane (MAO), chromium precatalysts 2a–2d showed moderate activity in ethylene oligomerization (TOF = 3.4–26.6 × 10³ (mol ethylene)(mol Cr)⁻¹ h⁻¹ at 80 °C) with a Schultz–Flory distribution of oligomers (K_C4–C12 ≈ 0.90) and production of polymer varying from 1.2 to 9.8 wt%. The catalytic activity is mainly controlled by electronic effects at the thioether/imine groups. In general, the precatalysts bearing electron-donating substituents (tert-butyl, p-C₆H₄OMe) (2a, 2b and 2d) showed higher activities in comparison to 2c containing an electron-withdrawing (i.e., phenyl) substituent. Notably, the MAO loading has a strong influence on the activity of 2a. Thus, the use of 2000 equiv. led to a remarkable increase in the catalytic activity [TOF = 971 100 (mol ethylene) (mol Cr)⁻¹ h⁻¹] with high selectivity to formation of oligomers (98.8 wt%).