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trans-1,2-Diphenylethylene Linked Isoindoline–Salicylaldiminato Nickel(II) Halide Complexes: Synthesis, Structure, Dehydrogenation, and Catalytic Activity toward Olefin Homopolymerization



Tridentate ligands 4-R1-6-R2-2-{[trans-2-(isoindolin-2-yl)-1,2-diphenylethylimino]methyl}phenol [R1 = R2 = H, H(La); R1 = R2 = tert-butyl, H(Lb); R1 = Ph, R2 = H, H(Lc)] in which a substituted salicylaldimine moiety and an isoindoline are linked by a trans-1,2-diphenylethylene moiety have been prepared. Deprotonation of these tridentate ligands H(La)–H(Lc) by NaH at room temperature in tetrahydrofuran (THF), followed by treatment with 1 equiv. of [(PPh3)2NiX2] (X = Cl, Br, I) at room temperature afforded the desired nickel complexes [(L)NiX] (X = Cl, L = La, 1; X = Cl, L = Lb, 2; X = Cl, L = Lc, 3; X = Br, L = Lb, 4; X = I, L = Lb, 5) in moderate yields. The structures of 2 and 4 were unequivocally confirmed by single-crystal X-ray diffraction in the solid state. The metal atom has a distorted square-planar coordination geometry and is coordinated to the two nitrogen atoms and one oxygen atom from the tridentate ligand and one halide atom. Dehydrogenation of the chelate tridentate ligand (Lb) in the presence of oxygen afforded the corresponding nickel complexes [(Lb*)NiX] (X = Br, 6; X = I, 7). X-ray analysis showed that in these complexes the two bridged benzylic carbon atoms of (Lb) were oxidized and a cis-stilbene moiety, which includes a C=C double bond (1.205–1.268 Å) was formed. These nickel complexes with such trans-1,2-diphenylethylene bridged tridentate ligands proved to be active catalysts for ethylene oligomerization in the presence of methylaluminoxane (MAO) and produced butene and hexene with catalytic activities of 0.49 × 105–3.25 × 105 g mol–1 Ni h–1 at 30 °C, under 20 bar of ethylene, and with an MAO/Ni ratio of 250. They were also active catalysts for the homopolymerization of norbornene and styrene and displayed activities of up to 1.89 × 105 and 7.36 × 105 g mol–1 Ni h–1, respectively.