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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

In this paper are described the synthesis, characterization, and coordinating properties of a new potentially pentadentate nitrogen ligand, CyAr2 N 5 , that combines in the same molecular structure 2,6-bis(imino)-pyridine and (imino)pyridine moieties. This ligand reacts with 1 or 2 equiv of anhydrous MCl 2 (M ) Fe, Co) to give paramagnetic mononuclear or homodinuclear complexes of the formula CyAr2 N 5 MCl 2 and CyAr2 N 5 M 2 Cl 4 . In the dinuclear complexes, one metal center is five-coordinate, while the other is fourcoordinate. Ligand and metal complexes have been characterized, both in the solid state and in solution, by a variety of techniques, including single-crystal X-ray diffraction analyses, magnetic susceptibility determinations, IR, vis-NIR, 1 H NMR, and X-band EPR spectroscopies. On activation by methylaluminoxane (MAO) in toluene, the Fe II and Co II complexes generate effective catalysts for the oligomerization of ethylene to R-olefins with productivities and Schulz-Flory parameters depending on the type and number of the coordinated metals. In an attempt to rationalize the surprisingly high activity of the Co II precursors, and in particular that of the dinuclear derivative CyAr2 N 5 Co 2 Cl 4 , which is 4 times higher than that of the mononuclear analogue CyAr2 N 5 CoCl 2 , a Co II complex has been synthesized where the supporting ligand is sterically similar to CyAr2 N 5 , yet it contains only the three nitrogen donor atoms of the 2,6-bis-(imino)pyridine moiety. From this study, it is concluded that all five nitrogen atoms of CyAr2 N 5 play an active role under catalytic conditions, even when the precursor contains a free (imino)pyridine moiety.