Synthesis of a new polydentate ligand obtained by coupling 2,6-bis(imino)pyridine and (imino)pyridine moieties and its use in ethylene oligomerization in conjunction with iron(II) and cobalt(II) bis-halides

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 I 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 four-coordinate. 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 alpha-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.