In an α-olefin manufacturing process by the oligomerization of ethylene using an iron complex of a diimine of a 2,6-diacylpyridine or a 2,6-pyridinecarboxaldehyde, in which certain substituted iminoaryl groups are present, less higher molecular weight unwanted products are produced when the diimine and/or its precursor arylamine does not have impurities with substitution on a second ortho position to the imino group. This leads to less fouling of the process apparatus and higher yields of desired α-olefins.