Chromium Complexes Supported By Bidentate Thioether-Imine [N,S] Ligands: Synthesis And Ethylene Oligomerization Studies

A series of new Cr(iii) complexes of general formula [Cr{N,S}(THF)Cl-3] (2a, N,S = PhS-2-PhCH = N-Bu-t; 2b, N,S = PhS-2-PhCH = N-C6H4-4-OMe; 2c, N,S = PhS-2-PhCH = N-C6H5; 2d, N,S = (BuS)-Bu-t-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 x 10(3) (mol ethylene)(mol Cr)(-1) h(-1) at 80 degrees C) with a Schultz-Flory distribution of oligomers (KC4-C12 approximate to 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%).