| Abstrakt: |
A series of nickel complexes containing α-iminocarboxamide, η1-benzyl, and PMe3 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-κ2N,O]Ni(η1-CH2Ph)(PMe3) (1), [N-(2,6-diethylphenyl)-2-(2,6-diethylphenylimino)propanamidato-κ2N,O](η1-CH2Ph)(PMe3)nickel (2), and [N-(2,6-methyl-isopropylphenyl)-2-(2,6-methylisopropylphenylimino)propanamidato-κ2N,O](η1-CH2Ph)(PMe3)nickel (3), one observes N,O-binding. In the case of [N-(2,6-dimethylphenyl)-2-(2,6-dimethylphenylimino)propanamidato]((η1-CH2Ph)(PMe3)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-κ2N,N](η1-CH2Ph)(PMe3)nickel (5) and [N-(2,6-diisopropylphenyl)-2-(phenylimino)propanamidato-κ2N,N](η1-CH2Ph)(PMe3)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 η3-benzyl fragment. Furthermore, the phosphine-free complex [N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-κ2N,N](η3-CH2Ph)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. |
