Oxidative Addition of Allylammonium BPh₄^- to Nickel(0): Synthesis, Crystal Structure, Fluxional Behavior, and Catalytic Activity of Chiral [(<i>η</i>³-allyl)(NH₃)(PCy₃)Ni]BPh₄

The synthesis, crystal structure, fluxional behavior, and reactivity of [(η3-C3H5)Ni(PCy3)(NH3)]BPh4 (1) are described. Complex 1 was obtained by oxidative addition of allylammonium tetraphenylborate, (CH2CHCH2NH3)BPh4, to Ni(0) [(Cy3P)2Ni(η2-CO2) or (Cy3P)2NiNNNi(PCy3)2], under mild conditions, through selective activation of the N−C-allyl bond. Complex 1 is a unique example of an allyl−Ni cationic complex with three different ligands and donor atoms. It has been fully characterized both in the solid state and in solution. The X-ray study shows that the cation [(η3-C3H5)Ni(PCy3)(NH3)]+ (1+) is chiral as a result of the two possible orientations of the allyl group and the different ancillary ligands bound to the (η3-C3H5)Ni+ moiety. In the solid state, cation 1+ exists as a racemic mixture of the two enantiomers 1a+ and 1b+. In solution, complex 1+ is involved in a slow fluxional process that causes the left-to-right exchange of syn and anti protons of the allyl group and leads to the interconversion of the two enantiomeric forms 1a+ and 1b+. The thermal stability of 1, in solution, has also been investigated. At 323 K, in THF, complex 1 is poorly stable and decomposes with formation of organic products, among which are benzene and allylbenzene. The formation of allylbenzene represents the first example of Ni-promoted phenyl transfer from tetraphenylborate to a π-allyl ligand. In the presence of dihydrogen (0.1 MPa) as cocatalyst, 1 promotes the selective head-to-tail oligomerization of methylacrylate (MA) to give dimethyl methyleneglutarate (DMG) and the trimer 2,4,6-tri(carbomethoxy)-1-hexene, through the intermediacy of a Ni−H intermediate. The catalytic activity of the system 1/H2 has been compared with that exhibited by [trans-(H)Ni(PCy3)2(η1(N)-PhCH2NCMe2)]BPh4 (3), easily obtained by reaction of [(PhCH2)HNCMe2]BPh4 with (Cy3P)2NiNNNi(PCy3)2, which also promotes the selective head-to-tail oligomerization of MA to afford the same products. The formation of a Ni−H species is demonstrated to be the key step in the di- and trimerization process.