A phosphino-oxazolineligand as a P,N-bridge in palladium/cobalt or P,N-chelate in nickel complexes: catalytic ethylene oligomerization

Abstract

The Pd(II) complex [PdCl₂(1)] [1 = ({oxazolin-2-yl}methyl)diphenylphosphine] was obtained by the 1 : 1 reaction of 1 with [PdCl₂(NCPh)₂]. Although this neutral complex is stable in the solid-state and in solution, it reacts with the dinuclear complex [CoCl₂(μ-1)]₂ to afford the heterometallic zwitterionic complex [{PdCl(1)}⁺(μ-1)(CoCl₃)⁻] (2). Under inert atmosphere, two equivalents of 1 reacted with [NiCl₂(dme)] to give trans-[NiCl₂(1)₂] (3) in CH₂Cl₂ but cis-[NiCl₂(1)₂] (4) in CHCl₃. When the latter reaction was performed in air, trans-[NiCl₂(5)₂] (6) [5 = ({oxazolin-2-yl}methyl)diphenylphosphine oxide] was obtained. All metal complexes, 2, 3, 4 and 6, have been structurally characterized by X-ray diffraction. Complexes 3, 4 and 6 have been evaluated as precatalysts for ethylene oligomerisation in the presence of AlEtCl₂ as cocatalyst. Complexes 3 and 6 yielded a turnover frequency (TOF) of 60 700 and 62 600 mol of C₂H₄/((mol of Ni)·h), respectively, in the presence of 10 equiv. of AlEtCl₂. In the presence of only 6 equiv. of cocatalyst, these Ni complexes yielded TOF values of 41 500 and 58 000 mol of C₂H₄/((mol of Ni)·h), respectively.