Electronic Influence of the Thienyl Sulfur Atom on the Oligomerization of Ethylene by Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis

Abstract

The position of the sulfur atom in the thienyl group of 6-(thienyl)-2-(imino)pyridine ligands strongly affects the catalytic activity of the corresponding tetrahedral high-spin dihalide CoII complexes in the oligomerization of ethylene to R-olefins upon activation with methylaluminoxane (MAO). Complexes with the sulfur atom in the 3-position of the thienyl ring catalyze the selective conversion of ethylene to 1-butene, while catalysts containing thien-2-yl groups give C4-C14 R-olefins. In situ EPR experiments showed the occurrence of a spin state changeover with the formation of low-spin CoII species upon activation of the catalyst precursors by MAO. DFT calculations suggest that only thien-2-yl rings allow for the coordination of the sulfur atom to the cobalt center in the MAO-activated systems

Bi- and tri-dentate imino-based iron and cobalt pre-catalysts for ethylene oligo-/polymerization

“…Both the size and the electro-negativity of these substituents played an important role in determining activity. For small halogen substituents (F, Cl, Br), electro-negativity was the decisive factor (highest activity for the fluoro complex), whereas iodo substituents 30 again led to a higher polymerization activity along with an increased content of higher molecular weight olefins. 53 When sterically demanding alkynyl substituted cyclopentadienyl/fluorenyl moieties were introduced, via Sonogashira coupling reactions, the resulting complexes 35 produced exclusively polymers.…”

“…c Determined by GC ; ∑C denotes the total amounts of oligomers. 30 activity as well as a high selectivity for ethylene dimerization. The presence of an additional para methyl group also led to improved activities.…”

“…Given this, the search for new catalyst systems remains of great interest in the 30 academic community and pivotal to the plastics industry. plastic production; however, the search for cheaper and more active catalytic systems, particularly those that will operate at high temperatures remains an area of immense interest.…”

“…21 Inspired by catalytic systems which revealed high activities at high temperature, but which 25 were inert at room temperature, our group revisited the bis(imino)pyridylmetal (Fe or Co) complexes derived from benzhydryl-substituted anilines, 17,22,23 and confirmed high activity for both iron and cobalt systems during ethylene polymerization. As well as active pre-catalysts bearing N^N^N 30 tri-dentate ligation at the metal (Fe or Co), a number of N^N bi-dentate metal (Fe or Co) Chart 2. Number of publications per year concerned with the study of 35 iron and cobalt olefin polymerization/oligomerization catalysts since their discovery in 1998 (*as of 05/06/13; patent applications not included).…”

“…Such low activity is probably due to the 25 formation of unstable active species, which given the greater open space available at the metal, tend to be more prone to coordinating with other species present in the system (leading to deactivation). The result of this poor performance is that less attention has been paid to bi-dentate iron/cobalt complex 30 pre-catalysts, although readily available iminopyridine derivatives have been used to form cobalt complexes, [27][28][29][30] which were found to exhibit high activity with selectivity toward dimerization. Following on from this, recent investigations involving the design of new N^N bi-dentate 35 ligands and the metal complexes thereof, have revealed promising results for ethylene polymerization.…”