Comparative study of new chromium-based catalysts for the selective tri- and tetramerization of ethylene

Two new ligands, Ph2PN(iPr)P(Ph)N(c-Hex)(CH3) (2) (Ph = phenyl, iPr = isopropyl, c-Hex = cyclohexyl) and Ph2PN(c-Hex)P(Ph)NEt(CH3) (Et = ethyl) (3) were synthesized. 2 was characterized by X-ray analysis. To compare their applicability with the previously known Ph2PN(iPr)P(Ph)N(iPr)H (1) these ligands were investigated together with chromium(III) acetylacetonate, different co-catalysts and solvents for the catalytic oligomerization of ethylene. The in situ prepared catalysts proved to be highly active, yielding highly pure 1-octene as the main product and 1-hexene as the main byproduct. It was possible to reach productivities of several hundred kg product per gram chromium and hour, up to almost 65 wt% 1-octene with about 99% purity and overall 1-hexene plus 1-octene yields of more than 85 wt%.