Investigating the Active Species in a [(R‐SN(H)S‐R)CrCl₃] Ethene Trimerization System: Mononuclear or Dinuclear?

Abstract

Cr-catalyzed ethene trimerization is an industrially important process to produce 1-hexene. Despite its industrial relevance, the changing oxidation state and the structural rearrangements of the metal center during the catalytic cycle remain unclear. In this study, we have investigated the active species in a [(R-SN (H)SÀ R)CrCl 3 ] (R = C 10 H 21 ) catalyzed ethene trimerization system using a combination of spectroscopic techniques (XAS, EPR and UV/VIS) and DFT calculations. Reaction of the octahedral Cr III complex with modified methylaluminoxane (MMAO) in absence of ethene gives rise to the formation of a square-planar Cr II complex. In the presence of ethene (1 bar), no coordination was observed, which we attribute to the endergonic nature of the coordination of the first ethene molecule. Employing an alkyne as a model for ethene coordination leads to the formation of a dinuclear cationic Cr III alkyne complex. DFT calculations show that a structurally related dinuclear cationic Cr III ethene complex could form under catalytic conditions. Comparing a mechanism proceeding via mononuclear cationic Cr II /Cr IV intermediates to that proceeding via dinuclear cationic Cr II /Cr III intermediates demonstrates that only the mechanism involving mononuclear cationic Cr II /Cr IV intermediates can correctly explain the observed product selectivity.[a] B.

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

“…[ 27,108,109 ] The role of Al activators on catalytic performance of chromium catalysts in homogeneous ethylene oligomerization has been recently studied by Tromp, Brückner and co‐workers. [ 110,111 ] With regards to product distribution within the liquid fraction, Cr3 /PMAO‐IP catalytic system leads to a more balanced distribution of oligomers, with a drastic decreases in α‐C 4 (30.20 mol %) and increase in α‐C 8 (16.80 mol %) and α‐C 10 (10.80 mol %) fractions (Figures S9 and S10).…”

Alkene oligomerization via metallacycles: Recent advances and mechanistic insights

“…[72] Recently, Tromp used Cr K-Edge XAS and EPR experiments combined with DFT calculations to propose Cr II species as the active species in the trimerization of ethylene. [73,74] Square planar Cr II complexes were observed after activation with MMAO (400eq.) in two different cases (Scheme 30).…”

Role of the ligand and activator in selective Cr–PNP ethene tri- and tetramerization catalysts – a spectroscopic study

“…In a previous study, we have shown for the [(R-SN(H)S-R)CrCl 3 ] trimerization system, that coordination of the first ethene molecule is endergonic under 1 bar of ethene pressure. 27 Coordination of substrates could be achieved by using more electron-rich substrates (e.g. alkynes).…”