A series of 2-(1-arylimino)ethyl-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridine derivatives was synthesized and fully characterized, and thereafter reacted with iron dichloride to form their corresponding iron(II) complexes. The single crystals of representative organic and iron complex compounds were obtained and analyzed by the X-ray diffraction analysis, indicating the distorted bipyramidal geometry around the iron core. Moreover, DFT calculations were performed on selected species to determine their structural features. On treatment with either MAO or MMAO, all iron complex pre-catalysts showed high activities (up to 1.56 × 10(7) gPE mol(-1)(Fe) h(-1)) toward ethylene polymerization. Regarding the nature of the ligands and reaction parameters, their catalytic activities and the characters of the obtained polyethylenes have been carefully investigated. The ring strain of the fused-cycloheptane of the ligands within iron complexes was considered to affect their catalytic performance in ethylene polymerization. The active species were activated and controlled by using a co-catalyst of MMAO preferred over MAO, and the obtained polyethylenes with MMAO showed narrower molecular polydispersity than the corresponding polyethylenes with MAO.