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

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{Ph(2)PCH(2)ox(Me2)}] (Ph(2)PCH(2)ox(Me2) = 2-[(diphenylphosphanyl)-methyl]-4,4-dimethyl-4,5-dihydro-oxazole) ( 8) and the phosphinito-oxazoline complexes [CoCl2{Ph(2)POCH(2)ox(Me2)}] (Ph2POCH2oxMe2 = 1-[ 4,4-dimethyl-2{1-oxy( diphenylphosphino)-1-methyl}]-4,5-dihydro-oxazole) ( 9) and [CoCl2{Ph(2)POCMe(2)ox(Me2)}] (Ph(2)POCMe(2)ox(Me2) = 1-[ 4,4-dimethyl-2[ 1-oxy( diphenylphosphino)-1-methylethyl]]-4,5-dihydrooxazole) ( 10) are mononuclear, the phosphino-oxazoline complexes [ CoCl2{mu-i-Pr(2)PCH(2)ox}](2) ( i-Pr(2)PCH(2)ox = 2-[( diisopropyl-phosphanyl)-methyl]-4,5-dihydro-oxazole) ( 6) and [ CoCl2{mu-Ph(2)PCH(2)ox}](2) (Ph(2)PCH(2)ox = 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

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(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 C-6-C-12 oligomers were produced.