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

The complex [NiCl2{Ph2POCH2oxMe2}] (Ph2POCH2oxMe2 = 2-((diphenylphosphinooxy) methyl)-4,4- dimethyl-4,5-dihydrooxazole) 14 has been synthesized by reaction of solid [NiCl2(DME)] (DME ) 1,2- dimethoxyethane) with a CH2Cl2 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/CH2Cl2 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 AlEtCl2 or MAO as cocatalyst, or propylene, with MAO as cocatalyst. With AlEtCl2 as activator (6 equiv), the catalytic activities with ethylene were generally modest and [NiCl2{Ph2PCH2ox}] (Ph2PCH2ox = 2-((diphenylphosphinooxy)methyl)-4,5-dihydrooxazole) 1 was the most active, with turnover frequencies (TOF) up to 7.9 × 104 mol of C2H4 (mol Ni × h)-1, whereas 14a, activated with 2 equiv of AlEtCl2, 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 × 104 mol of C2H4 (mol Ni × h)-1. The highest selectivity for butenes was observed with complex [NiCl2{Ph2PCH2thiazMe2}] 3. With propylene, TOFs up to 3.3 × 104 mol of C3H6 (mol Ni × h)-1 were obtained with [NiCl2{Ph2POCH2pyridine}] 8 and selectivities for C6 products higher than 98.5% were observed with precatalysts 3, [NiCl2{t-Bu2POCH2pyridine}] 9, and 14a.