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

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)S-R)CrCl₃] (R=C₁₀H₂₁) 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.