Insertion into the nickel-carbon bond of N-O chelated arylnickel(II) complexes. The development of single component catalysts for the oligomerisation of ethylene

A series of arylnickel phosphine complexes containing chelating N-O ligands, of the type [NiAr(N-O)L] [N-O=pyridine carboxylate (pyca); Ar = o-tolyl; L = PPh(3), P(CH(2)Ph)(3), PMePh(2), PMe(2)Ph, PCy(3): R = p-tolyl; L = PPh(3): Ar = phenyl; L = PPh(3): R - mesityl; L = PMePh(2): N-O = pyridine acetate (pyac); Ar = mesityl; L = PMePh(2)] have been prepared, providing complexes with chelate ring sizes of five and six. Crystal structures for the complexes indicate that both complexes have ''square planar'' coordination about the nickel centre, with the nitrogen of the pyridine being trans to the phosphine in each case. Whereas significant bending and buckling of the six-membered chelate ring is evident for [Ni(mesityl)(pyac)PMePh(2)], the five-membered ring of [Ni(mesityl)(pyca)PMePh(2)] is essentially planar. On warming the complexes readily insert ethylene into the Ni-aryl bond, forming single component catalysts for the conversion of ethylene into higher oligomers with low to moderate activity. Products are linear with generally 60-80% having the double bond in the alpha-position. Catalyst activities and product distributions are markedly dependent on the phosphine present, Addition of excess PPh(3) to the catalyst [Ni(o-tolyl)(pyca)PPh(3)] leads to a marked change in the product distribution with a less dramatic change in catalyst activity. A possible mechanism in which ethylene insertion occurs via an associative pathway from a five coordinate intermediate, [NiAr(CH2=CH2)(pyca)PR(3)], is suggested.