| Summary: | Numerous efforts have been made to achieve “on-purpose” 1-octene production since Sasol discovered a Cr-based selective ethylene tetramerization catalyst in the early 2000s. By preparing a series of bis(phosphine) ligands iPrN(PAr<sub>2</sub>)<sub>2</sub> where the Ar contains a bulky –SiR<sub>3</sub> substituent (Ar = −C<sub>6</sub>H<sub>4</sub>-<i>p</i>-Si(nBu)<sub>3</sub> (<b>1</b>), −C<sub>6</sub>H<sub>4</sub>-<i>p</i>-Si(1-hexyl)<sub>3</sub> (<b>2</b>), −C<sub>6</sub>H<sub>4</sub>-<i>p</i>-Si(1-octyl)<sub>3</sub> (<b>3</b>), −C<sub>6</sub>H<sub>4</sub>-<i>p</i>-Si(2-ethylhexyl)<sub>3</sub> (<b>4</b>), −C<sub>6</sub>H<sub>4</sub>-<i>p</i>-Si(3,7-dimethyloctyl)<sub>3</sub> (<b>5</b>)), we obtained an extremely active catalyst that meets the criteria for commercial utilization. The Cr complexes [iPrN(PAr<sub>2</sub>)<sub>2</sub>−CrCl<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>–</sup>, obtained by reacting ligands <b>1</b>–<b>5</b> with [(CH<sub>3</sub>CN)<sub>4</sub>CrCl<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>–</sup>, showed high activity exceeding 6000 kg/g-Cr/h, when combined with the inexpensive iBu<sub>3</sub>Al, thus avoiding the use of expensive modified methylaluminoxane (MMAO). The bulky –SiR<sub>3</sub> substituents played a key role in the success of catalysis by blocking the formation of inactive species (Cr complexes coordinated by two iPrN(PAr<sub>2</sub>)<sub>2</sub> ligands, that is, [(iPrN(PAr<sub>2</sub>)<sub>2</sub>)<sub>2</sub>−CrCl<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>–</sup>). Among the complexes prepared, [<b>3</b>−CrCl<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>–</sup> exhibited the highest activity (11,100 kg/g-Cr/h, 100 kg/g-catalyst) with high 1-octene selectivity (75 wt%) and, moreover, mitigated the generation of undesired > C10 fractions (10.7 wt%). A 10-g-scale synthesis of <b>3</b> was developed, as well as a facile and low-cost synthetic method for [(CH<sub>3</sub>CN)<sub>4</sub>CrCl<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>–</sup>.
|
|---|
