Nickel and Palladium Complexes with New Phosphinito-Imine Ligands and Their Application as Ethylene Oligomerization Catalysts

Phosphinito-imines, a new class of P,N donors, are readily generated by reaction of bulky arylamide anions [R²CONAr]⁻ (R² = Me or t-Bu; Ar = 2,6-i-Pr₂C₆H₄) with chlorophosphines ClP­(R¹)₂. In solution, free phosphinito-imines exist in equilibrium with the corresponding amidophosphine tautomers, containing a nitrogen-bound P­(R¹)₂ group. However, reacting the tautomer mixtures with metal precursor complexes, such as NiBr₂(dme) or PdCl₂(cod), selectively affords stable phosphinito-imine complexes MX₂(P-N) (M = Ni, Pd) in excellent yields. These complexes are diamagnetic and exhibit square-planar structures in the solid state, but in solution, the Ni derivatives exchange with a small amount of the corresponding high-spin tetrahedral isomers. On treatment with MMAO or DEAC, NiX₂(P-N) complexes become active ethylene oligomerization catalysts, affording mainly butenes along with smaller amounts of hexenes and octenes. The activity and the selectivity of these catalysts depend on the structure of the phosphinito-imine ligand and the cocatalyst used. When activated with DEAC, complexes containing the P­(i-Pr)₂ moiety are extremely active, achieving TOFs over 10⁶ mol C₂H₄/mol Ni·h and high selectivity for butenes.