Monomeric and dimeric nickel complexes derived from a pincer ligand featuring a secondary amine donor moiety

Article abstract of DOI:10.1021/ic100914x

Reaction of NiBr₂(CH₃CN)_x with the unsymmetrical pincer ligand m-(i-Pr₂PO)(CH₂NHBn)C ₆H₄ (Bn = CH₂Ph) gives the complex (R,S)-κ^P,κ^C,κ^N-{2-(i-Pr ₂PO),6-(CH₂NHBn)-C₆H₃}Ni ^IIBr, 1, featuring an asymmetric secondary amine donor moiety. Deprotonation of the latter with methyl lithium gave a dark brown compound that could not be characterized directly, but fully characterized derivatives prepared from this compound indicate that it is the LiBr adduct of the 14-electron amido species [κ^P,κ^C, κ^N-{2-(i-Pr₂PO),6-(CH₂NBn)-C ₆H₃}Ni], 2. Thus, 2·LiBr reacts with water to regenerate 1, while reaction with excess benzyl or allyl bromide gave the POCN-type pincer complexes 3 and 4, respectively, featuring tertiary amine donor moieties. On the other hand, heating 2·LiBr at 60 °C led to loss of LiBr and dimerization to generate the orange crystalline compound [μ^N;κ^P,κ^C,κ^N- {2-(i-Pr₂PO),6-(CH₂NBn)-C₆H₃}Ni] ₂, 5. Solid state structural studies show that 1, 3, and 4 are monomeric, square planar complexes involving one Ni?N interaction, whereas complex 5 is a C₂-symmetric dimer involving four Ni?N interactions and a Ni₂N₂ core featuring a short Ni?Ni distance (2.51 A). Preliminary reactivity tests have shown that 5 is stable toward weak nucleophiles such as acetonitrile but reacts with strong nucleophiles such as CO or 2,6-Me₂(C₆H₃)NC. Reactions with protic reagents showed that phthalimide appears to break the dimer to generate a monomeric species, whereas alcohols appear to leave the dimer intact, giving rise instead to adducts through N...H...O interactions. These ROH adducts of 5 were found to be active precatalysts for the alchoholysis of acrylonitrile with up to 2000 catalytic turnover numbers.