Monitoring structure and valence state of chromium sites during catalyst formation and ethylene oligomerization by in situ EPR spectroscopy

For the first time, a Cr(acac)3/PNP mixture (PNP = Ph2PN(i-Pr)PPh2) and a [(PNP)CrC12(,U-CI)12 complex were monitored by in situ EPR in cyclohexane and toluene, respectively, during oligomerization of ethylene. For Cr(acac)3/PNP, Cr 3 + reduction is faster than low-spin Cr+ formation, suggesting that the major active species is EPR-silent, possibly an antiferromagnetic Cr+ dimer and/or Cr 21 Fast decomposition of [(PNP)CrC12(U-CI)12 to [Cr07 6 -CH3C6H5)21-+- is a probable reason for low activity.