Mono- and dinuclear cobalt complexes with chelating or bridging bidentate P,N phosphino- and phosphinito-oxazolineligands: synthesis, structures and catalytic ethylene oligomerisation

Abstract

Cobalt(II) complexes of the type [CoCl₂(P,N)], where P,N represents a heterobidentate phosphino- or phosphinito-oxazoline-type ligand, have been synthesised and characterised by infrared spectroscopy and elemental analysis. Their molecular structures were established by single-crystal X-ray diffraction in the solid state. Whereas the phosphino-oxazoline complex [CoCl₂{Ph₂PCH₂ox^Me2}] (Ph₂PCH₂ox^Me2 = 2-[(diphenylphosphanyl)-methyl]-4,4-dimethyl-4,5-dihydro-oxazole) (8) and the phosphinito-oxazoline complexes [CoCl₂{Ph₂POCH₂ox^Me2}] (Ph₂POCH₂ox^Me2 = 1-[4,4-dimethyl-2{1-oxy(diphenylphosphino)-1-methyl}]-4,5-dihydro-oxazole) (9) and [CoCl₂{Ph₂POCMe₂ox^Me2}] (Ph₂POCMe₂ox^Me2 = 1-[4,4-dimethyl-2- [1-oxy(diphenylphosphino)-1-methylethyl]]-4,5-dihydrooxazole) (10) are mononuclear, the phosphino-oxazoline complexes [CoCl₂{μ-i-Pr₂PCH₂ox}]₂ (i-Pr₂PCH₂ox = 2-[(diisopropyl-phosphanyl)-methyl]-4,5-dihydro-oxazole) (6) and [CoCl₂{µ-Ph₂PCH₂ox}]₂ (Ph₂PCH₂ox = 2-[(diphenyl-phosphanyl)-methyl]-4,5-dihydro-oxazole) (7) are dinuclear compounds and contain two bridging phosphino-oxazoline ligands which form a 10-membered ring. In the course of this work, the zwitterionic complex [CoCl₃{Ph₂PCH₂C(O)OCH₂CMe₂NH₃] (11) was obtained and characterised by X-ray diffraction in which the oxazoline ring has been opened. Air-oxidation of the phosphine function of the mononuclear P,N chelate complex 8 yielded the blue N,O-bridged, centrosymmetric dinuclear complex [CoCl₂{μ-OPPh₂CH₂CNCMe₂CH₂O}]₂ (12) which contains a 12-membered ring. All these complexes are paramagnetic and their magnetic moments in solution were measured by the Evans method. Complexes 6–10 were evaluated in the catalytic oligomerisation of ethylene with AlEtCl₂ or methylaluminoxane (MAO) as cocatalysts and provided moderate activities. In the presence of AlEtCl₂ (6–14 equiv.), the selectivities for ethylene dimers were higher than 92% and complex 8 showed the highest turnover frequency with 14 equiv. of AlEtCl₂. When MAO was used as cocatalyst, the catalytic activities were similar to those with AlEtCl₂ but significant amounts of C₆–C₁₂ oligomers were produced.