Ortho-Fluoro Effect on the C–C Bond Activation of Benzonitrile Using Zerovalent Nickel

The effect of fluoro substitution on the C–C bond activation of aromatic nitriles has been studied by reacting a variety of fluorinated benzonitriles with the nickel(0) fragment [Ni(dippe)] and by locating the reaction intermediates and transition-state structures on the potential energy surface by using density functional theory calculations with the [Ni(dmpe)] fragment (dippe = 1,2-bis(diisopropylphosphino)ethane, dmpe = 1,2-bis(dimethylphosphino)ethane). As in the previous reports, the reaction of fluorinated benzonitriles with the [Ni(dippe)] fragment initially formed an η²-nitrile complex, which then converted to the C–CN bond activation product. Thermodynamic parameters for the equilibrium between these complexes have been determined experimentally in both a polar (tetrahydrofuran) and a nonpolar (toluene) solvent for 3-fluoro- and 4-fluorobenzonitrile. The stability of the C–C bond activation products is shown to be strongly dependent on the number of ortho-F substituents (?6.6 kcal/mol per o-F) and only slightly dependent on the number of meta-F substituents (?1.8 kcal/mol per m-F).