Neutral and cationic chiral NCN pincer nickel(II) complexes with 1,3-bis(2′-imidazolinyl)benzenes: synthesis and characterization

Chiral 1,3-bis(2′-imidazolinyl)benzenes 1a–e easily undergo direct nickelation at the C2 position of the central benzene ringvia the C–H bond activation in the reaction with anhydrous NiCl2 giving neutral NCN pincer nickel(II) complexes 2a–e in 40–87% yields. Treatment of the nickel pincers 2a or 2c with AgBF4 in CH3CN–CH2Cl2 afforded the cationic nickel pincers 3a or 3c in good yields. All the complexes were characterized by elemental analysis, 1H, 13C NMR, and IR spectra. Molecular structures of the neutral complexes 2a, 2b and 2c as well as the cationic complex 3c have been determined by X-ray single-crystal diffraction. The cationic nickel pincers 3 are found to be effective catalysts for the Michael addition of ethyl 2-cyanopropionate to methyl vinyl ketone in the presence of i-Pr2NEt base with a catalyst loading of 5 mol% even at −78 °C, producing the adduct in >99% yield after 24 h albeit with no ee.