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 CrCl3(THF)3 and [(CH3CN)4Ag]+[B­(C6F5)4]− afforded [L4CrIIICl2]+[B­(C6F5)4]− (L = CH3CN or tetrahydrofuran (THF)), which was converted to [(PNP)­CrCl2L2]+[B­(C6F5)4]−, where PNP is iPrN­(PPh2)2 (1) or [CH3(CH2)16]2CHN­(PPh2)2 (2). The
molecular structures of [(THF)4CrIIICl2]+[B­(C6F5)4]− and [1-CrCl2(THF)2]+[B­(C6F5)4]− were
unambiguously determined by X-ray crystallography. The cationic (PNP)­CrIII complexes paired with [B­(C6F5)4]− anions, that is, [(PNP)­CrCl2(CH3CN)2]+[B­(C6F5)4]−, exhibited high activity
in chlorobenzene when activated with common trialkylaluminum species
(Me3Al, Et3Al, and iBu3Al). The activities
and selectivity were comparable to those of the original MAO-based
Sasol system (1-CrCl3/MAO). When activated
with Et3Al or iBu3Al, the Cr complex, [2-CrCl2(CH3CN)2]+[B­(C6F5)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-CrCl2(CH3CN)2]+[B­(C6F5)4]−/Et3Al 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(C6F5)4]− anion is compatible
with trialkylaluminum species once it is not paired with a trityl
cation. Hence, [(PNP)­CrCl2(CH3CN)2]+[B­(C6F5)4]−/Et3Al exhibited a significantly higher activity than
that of a previously reported system composed of [Ph3C]+[B­(C6F5)4]−, that is, 1/CrCl3(THF)3/[Ph3C]+[B­(C6F5)4]−/Et3Al.