α-Iminocarboxamide nickel complexes:: Synthesis and uses in ethylene polymerization

A series of nickel complexes containing alpha-iminocarboxamide, eta(1)-benzyl, and PMe(3) ligands were synthesized to identify structural features of neutral Ni complexes that are employed in ethylene polymerization and ethylene/functionalized norbornene copolymerizations. Variations in steric bulk on aryl substituents in the alpha-iminocarboxamide framework were used to probe NN-versus N,O-binding modes. When the steric bulk is sufficiently large, as in [N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-kappa(2)N,O]Ni(eta(1)-CH(2)Ph)(PMe3) (1), [N-(2,6-diethylphenyl)-2-(2,6-diethylphenylimino)propanamidato-kappa(2)N,O](eta(1)-CH(2)Ph)(PMe(3))nickel (2), and [N-(2,6-methyl-isopropylphenyl)-2-(2,6-methylisopropylphenylimino)propanamidato-kappa(2)N,O](eta(1)-CH(2)Ph)(PMe3)nickel (3), one observes NO-binding. In the case of [N-(2,6-dimethylphenyl)-2-(2,6-dimethylphenylimino)propanamidatol ((eta(1)-CH(2)Ph)(PMe(3))nickel (4), both, N,O- and N,N-bound modes can be obtained and isolated; the N,N structure is the thermodynamic product. N,N products are observed with smaller ligands, as in [N-phenyl-2-(2,6-diisopropylphenylimino)propanamidato-kappa(2)N,N](eta(1)-CH(2)Ph)(PMe(3))nickel (5) and [N-(2,6-diisopropylphenyl)-2-(phenylimino)propanamidato-kappa(2)N,N](eta(1)-CH(2)Ph)(PMe(3))nickel (6). Ethylene polymerization, upon activation with Ni(COD)(2), is observed only with the NO-bound species. NMR spectroscopy shows that addition of Ni(COD)2 to 5 and 6 results in removal of the phosphine and the formation of an eta(3)-benzyl fragment. Furthermore, the phosphine-free complex [N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-kappa(2)N,N](eta(3)-CH(2)Ph)nickel (7) is also inactive toward ethylene polymerization. These observations suggest that ethylene polymerization is preferentially initiated by nickel complexes with NO-bound alpha-iminocarboxamide ligands.