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

The mechanism of H₂ 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 °C. The experimental heat curves were modeled in Berkeley Madonna to obtain both kinetic and thermodynamics data simultaneously. The H₂ 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, Δ<i>H</i>_H₂ = −141 kJ/mol. An Eyring analysis gave activation parameters of Δ<i>H</i>^‡ = 13.6(9) kJ mol^–1 and Δ<i>S</i>^‡ = −204(85) J K^–1 mol^–1. Using D₂ gas in place of H₂ gas provided an opportunity to measure the relative rates of D₂ versus H₂ heterolysis to yield a the kinetic isotope effect, KIE = 1.1(1).