Nickel complexes with heterofunctionalized phosphine ligands. Catalytic oligomerization of ethylene with [Ni(C5Ph5){Ph2PCHC(O)Ph}]

Abstract

Reaction of [Ni(C₅Ph₅)Br(CO)] with Ph₂PCH₂C(O)Ph in the presence of 1,8-bis(dimethylamino)naphthalene (proton sponge) gives the chelate complex [Ni(C₅Ph₅){Ph₂PCHC(O)Ph}]1. The complexes [Ni(C₅Ph₅){Ph₂PCHC(O)(C₅H₄)Fe(C₅H₅)}]2, [Ni(C₅Ph₅){Ph₂PCHC(O)(1,3-C₆H₄)C(O)HCPPh₂}Ni(C₅Ph₅)]3, [Ni(C₅H₅){Ph₂PCHC(O)Ph}]4 and [Ni(C₅H₅){Ph₂PCHC(O)(C₅H₄)Fe(C₅H₅)}]5 have been synthesised using the appropriate phosphine ligand and applying a procedure similar to that for 1. Complex 1, activated with NaBH₄, exhibits catalytic activity in the low-pressure oligomerization of ethylene. At 130 °C, under 38 bar of ethylene, selectivities of up to 98% towards linear α-olefins were achieved. Protonation of the bimetallic complex 2 with HBF₄ gave the cationic complex [Ni(C₅Ph₅){Ph₂PCH₂C(O)(C₅H₄)Fe(C₅H₅)}]BF₄6. Crystais of 6 belong to the orthorhombic space group Pbca with a= 19.427(6), b= 38.595(11), c= 13.001(4)Å and Z= 8. The structure was refined to R= 0.049 (R′= 0.067). The bonding of C₅Ph₅ contrasts with the bonding modes usually found for cyclopentadienyl ligands. The ring contains two short [1.384(8) and 1.389(8)], two medium [1.444(8) and 1.443(8)], and one long [1.474(8)Å] C–C bonds so that the C₅Ph₅ may be viewed as an (alkyl, diene) ligand. This unusual bonding is likely to result from the ‘chemical’ dissymmetry of the P,O ligand and the low trans influence of the oxygen atom. The extremely low ν, (CO) frequency of the co-ordinated ketone (1525 cm^–1) reflects the strong electron-withdrawing effect of the Ni(C₅Ph₅)⁺ moiety.