Relevant Metal Oxidation States of MAO-Activated Chromium Catalysts for Ethylene Oligomerization

Chromium-catalyzed ethylene oligomerization is an industrially important reaction, but improving the product specificity remains essential. In-depth spectroscopic and theoretical investigations of this catalytic reaction have allowed a great deal of insight into it. However, fundamental issues, such as the oxidation states relevant to the catalysis, are still unclear. This study makes the case for high-frequency electron paramagnetic resonance spectroscopy as a powerful method for studying this catalytic reaction, profiting from high g-value resolution and access to large energy splittings. The results confirm the occurrence of chromium(I) species but also show that such species are not necessarily dead ends in the catalytic cycle. Second, no unambiguous evidence for the relevance of chromium(II) was found, in spite of the unequivocal ability of HFEPR to detect such species. X-ray Absorption Spectroscopy (XAS) and TD-DFT calculations enabled the structural and electronic ground-state characterization of the dimeric, chloride-bridged Cr(III) precatalyst Cr-NHC-N as a precondition for its following HFEPR investigation.