Calorimetric Study of the Activation of Hydrogen by Tris(pentafluorophenyl)borane and Trimesitylphosphine

The mechanism of H-2 heterolysis by the frustrated Lewis pair of B(C6F5)(3) and P(mes)(3) was investigated by isothermal reaction calorimetry in the temperature range from 30 to 90 degrees C. The experimental heat curves were modeled in Berkeley Madonna to obtain both kinetic and thermodynamics data simultaneously. The H-2 activation reaction is treated as a single, termolecular step with a rate constant of 0.61 M-2 S-1 at 303 K with an exothermic enthalpy of reaction, Delta H-H2, = -141 kJ/mol. An Eyring analysis gave activation parameters of Delta H-double dagger = 13.6(9) kJ mol(-1) and Delta S-double dagger = -204(85) J K-1 mol(-1). Using D-2 gas in place of H-2 gas provided an opportunity to measure the relative rates of D-2 versus H-2 heterolysis to yield a the kinetic isotope effect, KIE = 1.1(1).