Oxidative addition of allylammonium BPh4- to nickel(0):: Synthesis, crystal structure, fluxional behavior, and catalytic activity of chiral [(η3-allyl)-(NH3) (PCy3)Ni]BPh4

The synthesis, crystal structure, fluxional behavior, and reactivity of C(eta (3)-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)(2)Ni(eta (2)-CO2) or (Cy3P)(2)NiNNNi(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 [(eta (3)-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 (eta (3)-C3H5)Ni+ moiety. In the solid state, cation It exists asa 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 pi -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/H-2 has been compared with that exhibited by [trans-(H)Ni(PCy3)(2)(eta (1)(N)-PhCH2N=CMe2)]Bph(4) (3), easily obtained by reaction of [(PhCH2)HN=CMe2]BPh4 with (Cy3P)(2)NiNNNi(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.