A series of 2,8-bis(1-aryliminoethyl)quinolines [2,8-(2,6-R12-4-R2C6H2N═CCH3)2C9H5N (L), R1 = Me, Et, or i-Pr, R2 = H or Me] is synthesized and used in the reaction with ferrous or cobaltous chlorides. The corresponding complexes LMCl2 are limitedly formed with ligands containing ortho-methyl groups of the imino phenyl rings (R = H, M = Fe (Fe1), M = Co (Co1); R = Me, M = Fe (Fe2), M = Co (Co2). The X-ray diffraction study reveals a distorted pyramidal geometry of Co1 around the cobalt atom. These complexes, activated with methylaluminoxane (MAO), show unique properties toward ethylene polymerization: no activity observed at low temperature, but high activity achieved at temperatures higher than 80 °C (up to 7.61 × 106 g·mol−1·h−1 at 100 °C). Moreover, the polyethylenes obtained are high molecular weight with narrow molecular distribution. This is the first example of iron and cobalt procatalysts with high activity for ethylene polymerization at high temperature.