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

Abstract

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.

Olefin oligomerization, homopolymerization and copolymerization by late transition metals supported by (imino)pyridine ligands

“…The vast majority of 2-organylimino-pyridyl ligands are straightforwardly prepared by condensing 2-acetyl or 2-formyl-pyridine systems with an equivalent amount of the proper amine [13][14][15][16][17][18] or aniline [2,3,[5][6][7]16,[19][20][21][22][23][24][25][26][27][28][29][30], possibly in the presence of an acid cocatalyst (Scheme 2a). The synthesis of 6-organyl-2-iminopyridyl ligands generally requires a more complex procedure and the Schiff-base condensation step just concludes the synthetic path.…”

“…To date, however, due to the complexity of the ligand design and synthesis, late metal centers tend to be positioned at the extremities of the manifold ligand, thus limiting any potential cooperative metal-metal interaction. Aimed at forcing two late transition metal centers into close proximity, novel molecular architectures, coupling two iminopyridyl moieties (Scheme 4) [27], or combining and iminopyridyl fragment with a bis(imino)pyridine unit (Scheme 5) [28,29], have been recently proposed as effective scaffolds for the preparation of mono and/or binuclear oligomerization/polymerization catalysts.…”

“…Scheme 5 illustrates the stepwise procedures proposed by Bianchini and Giambastiani to prepare new symmetrical and dissymmetrical N 5 ligands in good yield [28,29]. The symmetrical triketone L 2 has been straightforwardly synthesized by reacting the lithiated form of the bromo(pyridyl)ketal I with methylchloroformate, followed by acid cleavage of the O,O-ketal groups [41].…”

“…A description of this procedure is briefly given below. The intermediate II was conveniently prepared in a three step synthesis by transformation of 2,5-dibromopyridine into the 5-bromo-2-trimethylsilyl acetylene derivative via a regioselective Sonogashira's protocol, followed by a mercury(II)-catalyzed hydrolysis and protection of the resulting ketone as O,O-ketal [29,41].…”

Iron-Based and Cobalt-Based Olefin Polymerisation Catalysts

“…While the 2,6-bis(imino)pyridine ligand frame has continued to lead the way, the past 10 years has also seen the development of alternative ligand sets that can act as compatible supports for iron and cobalt ethylene oligomerisation/polymerisation catalysts ( Table 4 ) [46,50,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209] ; active catalysts based on bimetallic iron and cobalt precatalysts have also started to emerge ( Table 5 ) [54,210,211,212,213,214,215] . In the main, these systems show lower activities than the prototype bis(imino)pyridine-based catalysts, although several of these systems have significantly started to approach their catalytic performances (e.g.…”