Chromium Oxidation State Dictates Catalytic Function

The type of ethylene-conversion reaction mediated by chromium-based catalysts—polymerization, trimerization, or oligomerization—depends on the metal's oxidation state, according to researchers in Canada and the Netherlands (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200803434). Sandro Gambarotta of the University of Ottawa, Robbert Duchateau of Eindhoven University of Technology, and coworkers isolated catalytic species closely related to a commercial ethylene trimerization catalyst and found that a subtle variation in catalyst preparation altered the compound's structure, the oxidation state of its metal center, and its catalytic function. Chromium catalysts are used on a global scale to convert ethylene into polyethylene, 1-hexene, and other products. Scientists have proposed that the metal's oxidation state is related to its catalytic behavior, but the specifics of that relationship are hotly contested. Weighing in on the debate, Gambarotta, Duchateau, and coworkers showed that varying only the nature of the chromium salt counteranion leads to two distinct catalytic species: a square planar Cr(II) complex that polymerizes ethylene and a Cr(I) sandwich complex that trimerizes ethylene to 1-hexene. The team also found that a related Cr(III) compound oligomerizes ethylene nonselectively.