α-Alkylation of Nitriles with Alcohols Catalyzed by NNN′ Pincer Ru(II) Complexes Bearing Bipyridyl Imidazoline Ligands

A series of unsymmetrical NNN′ ruthenium(II) complexes supported by a tridentate bipyridyl imidazoline ligand with variable steric hindrance (2a–c; R1 = tBu, iPr, or Bn) and electronic effect (2d–h; R2 = H, CH3, OCH3, Br, or NO2) were prepared. The molecular structures of ligands 1f and 1g, and Ru complex 2a were further determined by X-ray single-crystal diffraction. The catalytic activity of these eight complexes for α-alkylation of nitriles with alcohols was evaluated, which could be controlled by the substituents on the imidazoline moiety. Ru complex 2h bearing a strong electron-withdrawing group (R2 = NO2) demonstrated the highest catalytic activity, with alkylated nitriles achieved in up to 97% yield.