N-NO bond dissociation energies of N-nitroso diphenylamine derivatives (oranalogues) and their radical anions: Implications for the effect of reductive electron transfer on N-NO bond activation and for the mechanisms of NO transfer to nitranions

The heterolytic and homolytic N-NO bond dissociation energies [i.e., Delta H-het(N-NO) and Delta H-homo-(N-NO)] of 12 N-nitroso-diphenylamine derivati ves (1-12) and two N-nitrosoindoies (13 and 14) in acetonitrile were determ ined by titration calorimetry and from a thermodynamic cycle, respectively. Comparison of these two sets of data indicates that homolysis of the N-NO bonds to generate NO. and nitrogen radical-is energetically much more favor able (by 23.3-44.8 kcal/mol) than the corresponding heterolysis to generate a pair of ions, giving hints for the driving force and possible mechanism of NO-initiated chemical and biological transformations. The first (N-NO)(- .) bond dissociation energies [i.e., Delta H(N-NO)(-.) and Delta H'(N-NO)(- .)] of radical anions 1(-.)-14(-.) were also derived on the basis of approp riate cycles utilizing the experimentally measured Delta H-het(N-NO) and el ectrochemical data. Comparisons of these two quantities with those of the n eutral N-NO bonds indicate a remarkable bond activation upon a possible one -electron transfer to the N-NO bonds, with an average bond-weakening effect of 48.8 +/- 0.3 kcal/mol for heterolysis and 22.3 +/- 0.3 kcal/mol for hom olysis, respectively. The good to excellent linear correlations among the e nergetics of the related heterolytic processes [Delta H-het(N-NO), Delta H( N-NO)(-.), and pK(a)(N-H)] and the related homolytic processes [Delta H-hom o(N-NO), Delta H'(N-NO)(-.), and BDE(N-H)] imply that the governing structu ral factors for these bond scissions are similar. Examples illustrating the use of such bond energetic data jointly with relevant redox potentials for analyzing various mechanistic possibilities for nitrosation of nitranions are presented.