Ammonia, alkyl amines, and aryl amines are found to undergo rapid intermolecular N-H oxidative addition to a planar mononuclear Ï 3-phosphorus compound (1). The pentacoordinate phosphorane products (1·[H][NHR]) are structurally robust, permitting full characterization by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. Isothermal titration calorimetry was employed to quantify the enthalpy of the N-H oxidative addition of n-propylamine to 1 (nPrNH2 + 1 â† 1·[H][NHnPr], Î"H rxn 298 = -10.6 kcal/mol). The kinetics of n-propylamine N-H oxidative addition were monitored by in situ UV absorption spectroscopy and determination of the rate law showed an unusually large molecularity (Î= k[nPrNH2]3). Kinetic experiments conducted over the temperature range of 10-70 °C revealed that the reaction rate decreased with increasing temperature. Activation parameters extracted from an Eyring analysis (Î"Hâ§§ = -0.8 ± 0.4 kcal/mol, Î"Sâ§§ = -72 ± 2 cal/(mol·K)) indicate that the cleavage of strong N-H bonds by 1 is entropy controlled due to a highly ordered, high molecularity transition state. Density functional calculations indicate that a concerted oxidative addition via a classical three-center transition structure is energetically inaccessible. Rather, a stepwise heterolytic pathway is preferred, proceeding by initial amine-assisted N-H heterolysis upon complexation to the electrophilic phosphorus center followed by rate-controlling N â† P proton transfer.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry