Coordination chemistry of sterically encumbered pyrrolyl ligands to chromium(ii): mono(pyrrolyl)chromium and diazachromocene formation

Homo- and heterobimetallic transition metal cluster derived from [Cp*Fe(η5-E5)] (E = P, As) - unprecedented structural motifs of the resulting polypnictogen ligands

A general synthetic procedure to neutral homo- and heterobimetallic cage compounds exhibiting various structural motifs of the polypnictogen ligands starting from [Cp*Fe(η5-E5)] (E = P (1), As (2); Cp* = C5Me5) is reported. The impact of the implemented transition metal precursors {Cp'''M} (M = Cr, Mn, Fe, Ni; Cp''' = 1,2,4-tBu3C5H2) emphasises the variability of the isolated complexes exhibiting a broad variety of structural motifs of the pnictogen ligands. Spectroscopic, crystallographic, and theoretical investigations provide insight into the structure of the partially unprecedented polypnictogen ligands.

Spectroscopic Investigation of the Activation of a Chromium-Pyrrolyl Ethene Trimerization Catalyst

1-Hexene is an important α-olefin for polyethylene production and is produced from ethene. Recent developments in the α-olefin industry have led to the successful commercialization of ethene trimerization catalysts. An important industrially applied ethene trimerization system uses a mixture of chromium 2-ethylhexanoate, AlEt₃, AlEt₂Cl, and 2,5-dimethylpyrrole (DMP). Here, we have studied the activation of this system using catalytic and spectroscopic experiments (XAS, EPR, and UV–vis) under conditions employed in industry. First, chromium 2-ethylhexanoate was prepared and characterized to be [Cr₃O(RCO₂)₆(H₂O)₃]Cl. Next, the activation of chromium 2-ethylhexanoate with AlEt₃, AlEt₂Cl, and DMP was studied, showing immediate reduction (<5 ms) of the trinuclear Cr(III) carboxylate and formation of a neutral polynuclear Cr(II) carboxylate complex. Over time, this Cr(II) carboxylate complex is partially converted into a chloro-bridged dinuclear Cr(II) pyrrolyl complex. In cyclohexane, small quantities of an unknown Cr(I) complex (∼1% after 1 h) are observed, while in toluene, the quantity of Cr(I) is much higher (∼23% after 1 h). This is due to the formation of cationic bis(tolyl)Cr(I) complexes, which likely leads to the observed inferior performance using toluene as the reaction solvent. Catalytic studies allow us to exclude some of the observed Cr(I) and Cr(II) complexes as the active species in this catalytic system. Using this combination of techniques, we have been able to structurally characterize complexes of this selective Cr-catalyzed trimerization system under conditions which are employed in industry.

Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization

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 CrCl₃(THF)₃ and [(CH₃CN)₄Ag]⁺[B(C₆F₅)₄]^- afforded [L₄Cr^IIICl₂]⁺[B(C₆F₅)₄]^- (L = CH₃CN or tetrahydrofuran (THF)), which was converted to [(PNP)CrCl₂L₂]⁺[B(C₆F₅)₄]^-, where PNP is iPrN(PPh₂)₂ (1) or [CH₃(CH₂)₁₆]₂CHN(PPh₂)₂ (2). The molecular structures of [(THF)₄Cr^IIICl₂]⁺[B(C₆F₅)₄]^- and [1-CrCl₂(THF)₂]⁺[B(C₆F₅)₄]^- were unambiguously determined by X-ray crystallography. The cationic (PNP)Cr^III complexes paired with [B(C₆F₅)₄]^- anions, that is, [(PNP)CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)₄]^-, exhibited high activity in chlorobenzene when activated with common trialkylaluminum species (Me₃Al, Et₃Al, and iBu₃Al). The activities and selectivity were comparable to those of the original MAO-based Sasol system (1-CrCl₃/MAO). When activated with Et₃Al or iBu₃Al, the Cr complex, [2-CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)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-CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)₄]^-/Et₃Al 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(C₆F₅)₄]^- anion is compatible with trialkylaluminum species once it is not paired with a trityl cation. Hence, [(PNP)CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)₄]^-/Et₃Al exhibited a significantly higher activity than that of a previously reported system composed of [Ph₃C]⁺[B(C₆F₅)₄]^-, that is, 1/CrCl₃(THF)₃/[Ph₃C]⁺[B(C₆F₅)₄]^-/Et₃Al.

Ethylene oligomerization promoted by chromium complexes bearing pyrrolide–imine–amine/ether tridentate ligands

Chromium complexes bearing pyrrolide–imino-amine/ether tridentate ligands have been synthesized and their catalytic behavior in ethylene oligomerization has been investigated. Drastic changes in selectivity have been observed upon slight variation of the MAO-to-Cr ratio.