Unsymmetric PCN Pincer Palladium and Nickel Complexes: Synthesis, Characterization and Reactivity

New palladium and nickel complexes based on unsymmetric PCN pincer ligands have been synthesized and fully characterized using different techniques. The substituents on the nitrogen side arm of the ligand have great influence on the cyclometallation and the reactivity of the new complexes particularly in case of nickel. The less sterically hindered PCNMe ligand enabled formation of the trivalent nickel halide complexes through a straightforward reaction with anhydrous copper halide salts. The trivalent nickel halide complexes were isolated and characterized using EPR, magnetic moment and elemental analysis. The reactivity of the PCN nickel halide complexes in Kharasch addition was tested.
The PCN palladium and nickel complexes relevant to CO2 insertion reactions were synthesized, and their reactivity towards CO2 were investigated giving facile insertion reactions at room temperature in case of the hydroxo and amido complexes. Insertion of CO2 into the nickel methyl bond was conducted under mild reaction conditions using the more electron donating and the sterically hindered PCNi-Pr ligand. Insertion of CO2 into the palladium phenyl acetylide complex offered the corresponding hydrogen carbonate complex. Investigation of the identity of the later complex led to the development of catalytic decarboxylative cross coupling reaction.
A short synthetic route was developed to prepare the PCNPy nickel complexes including pyridine as a nitrogen side arm. The new PCNPy allowed C-H activation at room temperature and enhanced the stability of the alkyl nickel complexes. The PCN nickel complexes mediate the catalytic Kumada coupling reaction.