Monomeric and Dimeric Nickel Complexes Derived from a Pincer Ligand Featuring a Secondary Amine Donor Moiety

Reaction of NiBr2(CH3CN) x with the unsymmetrical pincer ligand m-(i-Pr2PO)(CH2NHBn)C6H4 (Bn = CH2Ph) gives the complex (R,S)-κ P ,κ C ,κ N -{2-(i-Pr2PO),6-(CH2NHBn)-C6H3}NiIIBr, 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)-C6H3}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 C 2-symmetric dimer involving four Ni−N interactions and a Ni2N2 core featuring a short Ni−Ni distance (2.51 Å). 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.