Nickel Complexes with Phosphinito-Oxazoline Ligands: Temperature-Controlled Formation of Mono or Dinuclear Complexes and Catalytic Oligomerization of Ethylene and Propylene (original) (raw)

The complex [NiCl 2 {Ph 2 POCH 2 ox Me2 }] (Ph 2 POCH 2 ox Me2 ) 2-((diphenylphosphinooxy) methyl)-4,4dimethyl-4,5-dihydrooxazole) 14 has been synthesized by reaction of solid [NiCl 2 (DME)] (DME ) 1,2dimethoxyethane) with a CH 2 Cl 2 solution of the P,N ligand. X-ray diffraction studies on its red crystals established the mononuclear nature of the complex 14a, whose metal center has a distorted tetrahedral coordination geometry. Recrystallization of 14a from a toluene-pentane/CH 2 Cl 2 solution at temperatures below 253 K afforded green crystals of the dinuclear, chloride-bridged, formula isomer complex 14b. Conversion of 14b to 14a was observed as a function of temperature, both in the solid-state and in solution. Together with other Ni(II) complexes containing a chelating P,N ligand of the type phosphinoor phosphinito-oxazoline or phosphino-thiazoline, 14a has been evaluated as precatalyst in the oligomerization of ethylene, with AlEtCl 2 or MAO as cocatalyst, or propylene, with MAO as cocatalyst. With AlEtCl 2 as activator (6 equiv), the catalytic activities with ethylene were generally modest and [NiCl 2 {Ph 2 PCH 2 ox}] (Ph 2 PCH 2 ox ) 2-((diphenylphosphinooxy)methyl)-4,5-dihydrooxazole) 1 was the most active, with turnover frequencies (TOF) up to 7.9 × 10 4 mol of C 2 H 4 (mol Ni × h) -1 , whereas 14a, activated with 2 equiv of AlEtCl 2 , was the most selective for ethylene dimers (up to 96%) and 1-butene (up to 22%). With MAO as cocatalyst, complex 1 was again the most active, with TOF values up to 23 × 10 4 mol of C 2 H 4 (mol Ni × h) -1 . The highest selectivity for butenes was observed with complex [NiCl 2 {Ph 2 PCH 2 thiaz Me2 }] 3. With propylene, TOFs up to 3.3 × 10 4 mol of C 3 H 6 (mol Ni × h) -1 were obtained with [NiCl 2 {Ph 2 POCH 2 pyridine}] 8 and selectivities for C 6 products higher than 98.5% were observed with precatalysts 3, [NiCl 2 {t-Bu 2 POCH 2 pyridine}] 9, and 14a.