Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization

Ethylene tetramerization catalyst systems comprising a Cr(III) complex containing PNP ligands and methylaluminoxane (MAO) are useful for the production of 1-octene. However, a concern with these systems is the use of expensive MAO in excess. Herein, we report a catalytic system that avoids the use of MAO. Metathesis of CrCl 3 (THF) 3 and [(CH 3 CN) 4 Ag] + [B(C 6 F 5 ) 4 ] - afforded [L 4 Cr III Cl 2 ] + [B(C 6 F 5 ) 4 ] - (L = CH 3 CN or tetrahydrofuran (THF)), which was converted to [(PNP)CrCl 2 L 2 ] + [B(C 6 F 5 ) 4 ] - , where PNP is iPrN(PPh 2 ) 2 ( 1 ) or [CH 3 (CH 2 ) 16 ] 2 CHN(PPh 2 ) 2 ( 2 ). The molecular structures of [(THF) 4 Cr III Cl 2 ] + [B(C 6 F 5 ) 4 ] - and [ 1 -CrCl 2 (THF) 2 ] + [B(C 6 F 5 ) 4 ] - were unambiguously determined by X-ray crystallography. The cationic (PNP)Cr III complexes paired with [B(C 6 F 5 ) 4 ] - anions, that is, [(PNP)CrCl 2 (CH 3 CN) 2 ] + [B(C 6 F 5 ) 4 ] - , exhibited high activity in chlorobenzene when activated with common trialkylaluminum species (Me 3 Al, Et 3 Al, and iBu 3 Al). The activities and selectivity were comparable to those of the original MAO-based Sasol system ( 1 -CrCl 3 /MAO). When activated with Et 3 Al or iBu 3 Al, the Cr complex, [ 2 -CrCl 2 (CH 3 CN) 2 ] + [B(C 6 F 5 ) 4 ] - , which bears long alkyl chains, showed high activity in the more desirable methylcyclohexane solvent (89 kg/g-Cr/h) and much higher activity in cyclohexene (168 kg/g-Cr/h). Other advantages of the [ 2 -CrCl 2 (CH 3 CN) 2 ] + [B(C 6 F 5 ) 4 ] - /Et 3 Al system in cyclohexene were negligible catalyst deactivation, formation of only a negligible amount of polyethylene side product (0.3%), and formation of fewer unwanted side products above C10. The [B(C 6 F 5 ) 4 ] - anion is compatible with trialkylaluminum species once it is not paired with a trityl cation. Hence, [(PNP)CrCl 2 (CH 3 CN) 2 ] + [B(C 6 F 5 ) 4 ] - /Et 3 Al exhibited a significantly higher activity than that of a previously reported system composed of [Ph 3 C] + [B(C 6 F 5 ) 4 ] - , that is, 1 /CrCl 3 (THF) 3 /[Ph 3 C] + [B(C 6 F 5 ) 4 ] - /Et 3 Al.