Selective Ethylene Tri-/Tetramerization by in Situ-Formed Chromium Catalysts Stabilized by N,P-Based Ancillary Ligand Systems

A series of N,P-based ancillary ligands have been synthesized, and the corresponding catalysts, formed in situ by mixing one of the N,P-ligands, Cr­(acac)₃ and MAO, have been tested for ethylene oligomerization. Under standard ethylene oligomerization conditions (30 bar ethylene, 60 °C, methylcyclohexane solvent), all of the in situ-formed complexes show catalytic activity, producing oligomers together with varying amounts of polyethylene (PE). Of all these combinations, only the catalyst formed by mixing <i>N</i>-pyrrolyldiphenylphosphine with Cr­(acac)₃ and MAO is capable of <i>selectively</i> oligomerizing ethylene, producing a mixture of 1-hexene and 1-octene in varying ratios alongside a small amount of PE. Further investigations on this catalyst system revealed that the presence of a low concentration of toluene favors the production of 1-octene. However, in pure toluene as the solvent, the selectivity toward 1-hexene/1-octene is lost and a statistic mixture of α-olefins is produced. Moreover, the choice of the cocatalyst is found to dramatically influence the composition of the liquid products. By careful adjustment of the reaction conditions (temperature, ethylene pressure, catalyst loading, and ligand/Cr ratio), the 1-hexene/1-octene molar ratio can be tuned from 0.3 to 20 and a selectivity for 1-octene formation of up to 74% can be achieved

Chromium-Catalyzed CO₂–Epoxide Copolymerization

Iminopyrrole, aminopyrrole, and aminophosphine ligands were complexed with various chromium sources, producing eight complexes that were tested for their catalytic behavior toward epoxide–CO₂ copolymerization. As elucidated by MALDI-TOF-MS, copolymerizations afforded polycarbonates and poly­(ether-carbonates) exhibiting linear or cyclic topologies

Chromium-Catalyzed CO₂–Epoxide Copolymerization

Iminopyrrole, aminopyrrole, and aminophosphine ligands were complexed with various chromium sources, producing eight complexes that were tested for their catalytic behavior toward epoxide–CO₂ copolymerization. As elucidated by MALDI-TOF-MS, copolymerizations afforded polycarbonates and poly­(ether-carbonates) exhibiting linear or cyclic topologies

New Iminophosphonamide Chromium(II) Complexes as Highly Active Polymer-Free Ethylene Oligomerization Catalysts

The reactions of the deprotonated form of <i>cis</i>-{[(μ-N)(<i>t</i>-Bu)]₂PN(H)(<i>o</i>-OMeC₆H₄)]₂} (<b>a</b>) with either CrCl₂(THF)₂ or CrCl₃(THF)₃ afforded the corresponding dimeric Cr(II) and monomeric Cr(III) complexes {<i>cis-</i>[(μ-N)(<i>t</i>-Bu)]₂[PN(<i>o</i>-MeOC₆H₄)]₂Cr}₂ (<b>1a</b>) and <i>cis-</i>[(μ-N)(<i>t</i>-Bu)]₂[PN-2-MeOC₆H₄]₂CrCl (<b>2a</b>). By replacing the ligand’s <i>o</i>-OMeC₆H₄ groups with less-crowded CH₂CH₂N(<i>i</i>-Pr)₂ functionalities and reacting the deprotonated form of <i>cis</i>-{[(μ-N)(<i>t</i>-Bu)]₂[PN(H)CH₂CH₂N(<i>i</i>-Pr)₂)]₂} (<b>b</b>) with CrCl₂(THF)₂, the dimetallic, divalent {<i>cis-</i>[(μ-N)(<i>t</i>-Bu)]₂[PNCH₂CH₂N(<i>i</i>-Pr)₂)]₂Cr}₂ (<b>1b</b>) was obtained in crystalline form. Upon activation with MAO, both <b>1a</b> and <b>1b</b> afforded highly active, polyethylene-free ethylene oligomerization catalysts. Complex <b>2a</b> is instead catalytically inactive

Selective Ethylene Oligomerization with Chromium Complexes Bearing Pyridine–Phosphine Ligands: Influence of Ligand Structure on Catalytic Behavior

Chromium complexes bearing a series of pyridine–phosphine ligands have been synthesized and examined for their catalytic behavior in ethylene oligomerization. The choice of solvent, toluene versus methylcyclohexane, shows a pronounced influence on the catalytic activity for all these complexes. Variations of the ligand system have been introduced by modifying the phosphine substituents affecting ligand bite angles and flexibility. It has been demonstrated that minor differences in the ligand structure can result in remarkable changes not only in catalytic activity but also in selectivity toward α-olefins versus polyethylene and distribution of oligomeric products. Ligand PyCH₂N­(Me)­P^<i>i</i>Pr₂, in combination with CrCl₃(THF)₃ afforded selective ethylene tri- and tetramerization, giving 1-hexene and 1-octene with good overall selectivity and high purity, albeit with the presence of small amounts of PE