Relevant metal oxidation states of MAO-activated chromium catalysts for ethylene oligomerization

Chromium-catalyzed ethylene oligomerization is an industrially important reaction, but improving product specificity is an unsolved issue. In-depth spectroscopic and theoretical investigation of this catalytical reaction has 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 introduces high-frequency electron paramagnetic resonance spectroscopy as a powerful method for studying this catalytical 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. Secondly, no unambiguous evidence for the relevance of chromium(II) was found, in spite of the unequivocal ability of HFEPR to detect such species.