Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization

[Image: see text] 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.