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

Abstract

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.