No abstract

Operando EPR under elevated ethylene pressure supported by in situ XAS was for the first time applied to discriminate between active and deactivating Cr species in ethylene tetramerization. Starting from Cr(III) in the Cr(acac)3 precursor, a (PNP)Cr(II)(CH3)2 complex is most likely formed upon adding PNP and MMAO, which is regarded as the active species that converts ethylene to 1-octene by passing a reversible redox cycle, while reduction to Cr(I) leads to deactivation.

The performance of certain olefin metathesis reactions catalyzed by Grubbs catalysts has been enhanced by the simple addition of phenol. Addition of phenol to self-metathesis reactions catalyzed by 1 produced very small quantities of unwanted byproducts and allowed for room-temperature metathesis at high substrate:catalyst loadings. The efficiency of cross-metathesis reactions between methyl acrylate and 1-decene catalyzed by 2 was also significantly increased by addition of p-cresol to the reaction mixture. Mechanistic studies, including NMR spectroscopy and DFT calculations, established that phenol is playing a number of positive roles in the active metathesis cycle, including altering the relative rates of phosphine loss and rebinding, activating the carbene carbon for reaction with olefinic substrate, and hemilabile stabilization of the key 14-electron intermediate species.

For the first time, a Cr(acac) 3 /PNP mixture (PNP ) Ph 2 PN(i-Pr)PPh 2 ) and a [(PNP)CrCl 2 (µ-Cl)] 2 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 2+ . Fast decomposition of [(PNP)CrCl 2 (µ-Cl)] 2 to [Cr(η 6 -CH 3 C 6 H 5 ) 2 ] + is a probable reason for low activity.