Cyclopentadienyl ligands functionalized by quinoline or N,N-dimethylaniline have an ideal and rigid geometry for the formation of half-sandwich compounds. As a result, stable chromium(III) complexes 5−8 with intramolecular nitrogen coordination have been obtained. Treatment with MAO leads to highly active, temperature-stable catalysts for the polymerization of ethylene.

Reaction of 2-lithio-N,N-dimethylaniline with 2,3,4,5-tetramethylcyclopentenone yields 1-[2-(N,N-dimethylaminophenyl)-2,3,4,5-tetramethylcyclopentadiene (2). As in 1-(8-quinolyl)-2,3,4,5-tetramethylcyclopentadiene (1), the N-donor is connected to the Cp ring by a rigid C 2 spacer. Co 2 (CO) 8 reacts with 1 or 2 to give the Co I -dicarbonyl half-sandwich complexes 7 and 3, respectively. Irradiation leads to the binuclear complexes 4 and 8 with a Co-Co double bond. Oxidation of 3 or 7a is possible with iodine. Whereas 7a directly forms the Co III complex 9 with coordination of the quinoline to the metal, 3 yields the monocarbonyldiiodo complex 5. Upon gentle heating in dichloromethane, 5 loses carbon monoxide to give the cobalt complex 6, displaying intramolecular nitrogen coordination. Reaction of the dimethylanilinesubstituted cyclopentadienide 10 with CoCl 2 yields the Co II complex 11. The Co III complex 6 and the Co II complex 11 were treated with methylaluminoxane (MAO) and gave low-activity catalysts for the polymerization of ethylene.