Ferrocene-Substituted Bis(imino)pyridine Iron and Cobalt Complexes: Toward Redox-Active Catalysts for the Polymerization of Ethylene

Abstract

The synthesis, characterization, and ethylene polymerization behavior of a series of iron and cobalt halide complexes LMCl2 (M = Fe, Co) bearing chelating ferrocenyl-modified bis(imino)pyridyl ligands L [L = 2,6-(FcArNCMe)2C5H3N] are reported. X-ray diffraction studies show the geometry at the metal centers to be distorted square pyramidal. Electrochemical studies focus on an Fe (7) and a Co (10) complex and in each case show several quasi-reversible oxidation and reduction waves, the ferrocenyl iron atoms being easier to oxidize than the central iron (and cobalt) atoms. Chemical oxidation of these complexes is possible via addition of FcPF6, and good yields of the doubly charged iron and cobalt complexes were isolated. Treatment of the complexes 7 and 10 with methylaluminoxane (MAO) leads to highly active ethylene polymerization catalysts, converting ethylene to highly linear polyethylene (PE), with polymer molecular weights in the region of 900 000. As expected, the Fe catalyst is approximately an order of magnitude more active than the Co species, with activities of 6900 and 440 g mmol-1 bar-1 h-1, respectively. The effect of the polymerization activities by oxidation of the precatalysts to their ferrocenium counterparts was seemingly negated by the reducing properties of the alkyl aluminum activator (MAO).