Oxidative Addition of Allylammonium BPh 4 - to Nickel(0): Synthesis, Crystal Structure, Fluxional Behavior, and Catalytic Activity of Chiral [( η 3 -allyl)(NH 3 )(PCy 3 )Ni]BPh 4

The synthesis, crystal structure, fluxional behavior, and reactivity of [( η 3 -C 3 H 5 )Ni(PCy 3 )(NH 3 )]BPh 4 ( 1 ) are described. Complex 1 was obtained by oxidative addition of allylammonium tetraphenylborate, (CH 2 CHCH 2 NH 3 )BPh 4, to Ni(0) [(Cy 3 P) 2 Ni( η 2 -CO 2 ) or (Cy 3 P) 2 NiNNNi(PCy 3 ) 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 [( η 3 -C 3 H 5 )Ni(PCy 3 )(NH 3 )] + ( 1 + ) is chiral as a result of the two possible orientations of the allyl group and the different ancillary ligands bound to the ( η 3 -C 3 H 5 )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 /H 2 has been compared with that exhibited by [ trans -(H)Ni(PCy 3 ) 2 ( η 1 ( N )-PhCH 2 N CMe 2 )]BPh 4 ( 3 ), easily obtained by reaction of [(PhCH 2 )HN CMe 2 ]BPh 4 with (Cy 3 P) 2 NiNNNi(PCy 3 ) 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.