Switchable Chromium(II) Complexes of a Chelating Amidophosphine (N−P) for Selective and Nonselective Ethylene Oligomerization

Abstract

Treatment of the lithium salt of t-BuN(H)PPh 2 in THF with CrCl 2 3 (THF) 2 afforded a dinuclear, [(t-BuNPPh 2 )Cr 2 (μ-t-BuNPPh 2 ) 3 ] 3 (toluene) 1.5 (1), and a tetrameric cluster, [(t-BuNPPh 2 )Cr(μ-t-BuNPPh 2 ) 2 Cr(μ-Cl)] 2 3 (toluene) 2 (5), depending on the reagents' stoichiometric ratio. Complex 1 is dimeric with a long intermetallic distance and the ligands adopting an asymmetric and distorted bridging-chelating bonding mode. Complex 5 is instead a symmetry-generated tetramer with two identical dimetallic units, each closely related in geometry to 1, linked by two bridging chlorine atoms. The reactions of 1 with alkyl aluminum activators afforded a series of divalent complexes, [{(μ-AlMe 3 )- 4), containing organo-aluminum residues. Similarly, reaction of 5 with AlMe 3 gave [{(μ-AlMe 2 )(t-BuNPPh 2 ) 2 }Cr(μ-Cl)] 2 3 (toluene) 1.9 (6), also characterized by an X-ray crystal structure. Finally, the trivalent complex [(t-BuNPPh 2 ) 3 Cr] (7) was readily prepared via reaction of the lithiated ligand with CrCl 3 (THF) 3 . Upon treatment with AlMe 3 or Et 2 AlCl, complexes 2 and [{(μ-AlEtCl 2 )(t-BuNPPh 2 )} 2 Cr] 3 (toluene) (8) were isolated and fully characterized. In turn, this indicated that reduction to the divalent state is the primary stage of the activation process of the trivalent species. The catalytic behavior of all of these complexes has been assessed in the presence and absence of cocatalyst and with different solvents. The result showed a pronounced solvent effect, allowing switching from nonselective oligomerization to selective trimerization.

Activity Enhancement of a Catalyst System for the Selective Trimerization of Ethene to 1‐Hexene by Modification of the Chromium to Chloride to Aluminium Ratio

“…However, in any case the activity of the trimerization system [CrA C H T U N G T R E N N U N G (acac) 3 In conclusion, the important role of chloride in the described chromium catalyst system for selective trimerization of ethene to form 1-hexene is elucidated. As predicted [5] and in accordance with results of Gambarotta and co-workers [14] for related systems, it is most likely that the active catalytic species contains chloride ( Figure 5).…”

“…Average activity Predicted [5] and by Gambarotta and co-workers [14] isolated structures containing N-Al-Cl-Cr interactions.…”

Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

“…In conclusion, the important role of chloride in the described chromium catalyst system for selective trimerization of ethene to form 1‐hexene is elucidated. As predicted5 and in accordance with results of Gambarotta and co‐workers14 for related systems, it is most likely that the active catalytic species contains chloride (Figure 5).…”

“… Predicted5 and by Gambarotta and co‐workers14 isolated structures containing N‐Al‐Cl‐Cr interactions. …”

A Highly Selective Ethylene Tetramerization Catalyst

“…[12,13] To study how the nuclearity of the catalysts may affect the catalytic behavior, we have now linked two N/P-units with both ethylenic and propylenic bridges in order to stimulate the formation of dimetallic species. Herein, we describe the first catalytic system that has a good activity and is capable of producing pure 1-octene (up to 91 % purity) with little or no polymer side products.…”