SINGLE-COMPONENT N-O CHELATED ARYLNICKEL(II) COMPLEXES AS ETHENE POLYMERIZATION AND CO ETHENE COPOLYMERIZATION CATALYSTS - EXAMPLES OF LIGAND-INDUCED CHANGES TO THE REACTION PATHWAY/

Arylnickel(II) phosphine complexes containing substituted N-O bidentat e ligands, of the type [NiR(N-O)L] [N-O = 4-nitropyridine-2-carboxylat e (4-NO2-pyca), R = o-tolyl, L = PPh3; N-O = 2-pyrazinecarboxylate (py zca), R = o-tolyl, L = PPh3; and N-O = 4-methoxypyridine-2-carboxylate (4-MeO-pyca), R = o-tolyl, L = PPh3] have been prepared and character ised. Single crystal X-ray studies of the complexes [Ni(o-tolyl)(pyca) PPh3], 1, and the isomorphous analogue [Ni(o-tolyl)(4-NO2-pyca)PPh3], 3, show the expected square planar coordination about the nickel centr es, with the pyridine nitrogens being trans to the phosphine ligand fo r both compounds. The coordination spheres of the two complexes are ve ry similar, no elongation of the Ni-N bond for complex 3, which contai ns the 4-NO2-pyca ligand, being evident. In complex 3 the o-tolyl liga nd is disordered over two sites indicating the presence, in the solid state, of two conformers in which the o-methyl groups of o-tolyl are l ocated to either side of the coordination plane. The complexes with su bstituted pyca ligands form single component catalysts for the convers ion of ethene to high molecular weight polyethene and for the copolyme risation of ethene and carbon monoxide to polyketone under mild condit ions. The nature of the product, whether predominantly high molecular weight polymer or a mixture of polymer and lower oligomer, is dependen t on the basicity of the N-O chelate ligand. From an NMR study of the effect of added ethene on the complex [Ni(o-tolyl)(4-NO2-pyca)PPh3], a mechanism involving alkene promoted ligand dissociation is suggested. (C) 1997 Elsevier Science S.A.