Nickel complexes with heterofunctionalized phosphine ligands. Catalytic oligomerization of ethylene with [Ni(C5Ph5){Ph2PCHC(O)Ph}]

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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.
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