Catalytic reduction of polar substrates without metals: A thermodynamic and kinetic study of heterolytic activation of hydrogen by vacancies in frustrated Lewis pairs

Abstract

Time-resolved reaction calorimetry provides a measure of the turnover frequency (TOF), ca. 1.1min−1, and enthalpic driving force, ca. −73kJ/mol, for the metal-free catalytic reduction of an organic imine, tert-butylbenzaldimine, (tBu-IM) with a frustrated Lewis acid-base pair, [2-(dimesitylphosphino)ethyl] bis(pentafluorophenyl)borane (PBCat), at 298K and 13.8bar hydrogen (H2) pressure. Lowering the H2 pressure by a factor of two decreases the TOF (0.6min−1), which is consistent with a pseudo first-order reaction in H2. In the absence of imine, the heat flux measured in the calorimeter provides a measure of the enthalpy for heterolytic splitting of H2, PBcat+H2 --> PBCatH2, ΔH ca. −43(4)kJ/mol. Solution phase 19F nuclear magnetic resonance spectroscopy was used to determine the rate of heterolytic splitting of H2 by PBCat, k=0.7(.3)M−1s−1 and the equilibrium constant for PBCat+H2(soln) ⇔ PBCatH2, Keq(295)=2.2(.5)×105M, providing an estimate of the free energy for heterolytic splitting of H2, ΔG ca. −29.8(1.3)kJ/mol at 295K in toluene. Deconvolution of the instrument time constant from the heat flux using the Tian equation shows the concentration of imine decreases linearly in time (i.e., the substrate imine is not involved in the rate limiting step, suggesting that H2 activation by the Lewis acid-base pair is rate limiting).