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

The effect of fluoro substitution on the C-C bondactivationof aromatic nitriles has been studied by reacting a variety of fluorinatedbenzonitriles with the nickel(0) fragment [Ni(dippe)] and by locatingthe reaction intermediates and transition-state structures on thepotential energy surface by using density functional theory calculationswith 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)] fragmentinitially formed an & eta;(2)-nitrile complex, which thenconverted to the C-CN bond activation product. Thermodynamicparameters for the equilibrium between these complexes have been determinedexperimentally in both a polar (tetrahydrofuran) and a nonpolar (toluene)solvent for 3-fluoro- and 4-fluorobenzonitrile. The stability of theC-C bond activation products is shown to be strongly dependenton the number of ortho-F substituents (-6.6kcal/mol per o-F) and only slightly dependent onthe number of meta-F substituents (-1.8 kcal/molper m-F).