α-Iminocarboxamide Nickel Complexes: Synthesis and Uses in Ethylene Polymerization

A series of nickel complexes containing α-iminocarboxamide, η 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 α-iminocarboxamide framework were used to probe N,N- versus N,O -binding modes. When the steric bulk is sufficiently large, as in [ N -(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-κ 2 N,O ]Ni(η 1 -CH 2 Ph)(PMe 3 ) ( 1 ), [ N -(2,6-diethylphenyl)-2-(2,6-diethylphenylimino)propanamidato-κ 2 N, O ](η 1 -CH 2 Ph)(PMe 3 )nickel ( 2 ), and [ N -(2,6-methyl-isopropylphenyl)-2-(2,6-methylisopropylphenylimino)propanamidato-κ 2 N,O ](η 1 -CH 2 Ph)(PMe 3 )nickel ( 3 ), one observes N,O -binding. In the case of [ N -(2,6-dimethylphenyl)-2-(2,6-dimethylphenylimino)propanamidato]((η 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-κ 2 N, N ](η 1 -CH 2 Ph)(PMe 3 )nickel ( 5 ) and [ N -(2,6-diisopropylphenyl)-2-(phenylimino)propanamidato-κ 2 N,N ](η 1 -CH 2 Ph)(PMe 3 )nickel ( 6 ). Ethylene polymerization, upon activation with Ni(COD) 2, is observed only with the N,O -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 η 3 -benzyl fragment. Furthermore, the phosphine-free complex [ N -(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-κ 2 N,N ](η 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 N,O -bound α-iminocarboxamide ligands.