Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization (2017)

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<sub>3</sub>(THF)<sub>3</sub> and [(CH<sub>3</sub>CN)<sub>4</sub>Ag]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> afforded [L<sub>4</sub>Cr<sup>III</sup>Cl<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> (L = CH<sub>3</sub>CN or tetrahydrofuran (THF)), which was converted to [(PNP)CrCl<sub>2</sub>L<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>, where PNP is iPrN(PPh<sub>2</sub>)<sub>2</sub> (<b>1</b>) or [CH<sub>3</sub>(CH<sub>2</sub>)<sub>16</sub>]<sub>2</sub>CHN(PPh<sub>2</sub>)<sub>2</sub> (<b>2</b>). The molecular structures of [(THF)<sub>4</sub>Cr<sup>III</sup>Cl<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> and [<b>1</b>-CrCl<sub>2</sub>(THF)<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> were unambiguously determined by X-ray crystallography. The cationic (PNP)Cr<sup>III</sup> complexes paired with [B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> anions, that is, [(PNP)CrCl<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>, exhibited high activity in chlorobenzene when activated with common trialkylaluminum species (Me<sub>3</sub>Al, Et<sub>3</sub>Al, and iBu<sub>3</sub>Al). The activities and selectivity were comparable to those of the original MAO-based Sasol system (<b>1</b>-CrCl<sub>3</sub>/MAO). When activated with Et<sub>3</sub>Al or iBu<sub>3</sub>Al, the Cr complex, [<b>2</b>-CrCl<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>, 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 [<b>2</b>-CrCl<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>/Et<sub>3</sub>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<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> anion is compatible with trialkylaluminum species once it is not paired with a trityl cation. Hence, [(PNP)CrCl<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>/Et<sub>3</sub>Al exhibited a significantly higher activity than that of a previously reported system composed of [Ph<sub>3</sub>C]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>, that is, <b>1</b>/CrCl<sub>3</sub>(THF)<sub>3</sub>/[Ph<sub>3</sub>C]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>/Et<sub>3</sub>Al.