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

Time-resolved reaction calorimetry provides a measure of the turnover frequency (TOF), ca. 1.1 min(-1), and enthalpic driving force, ca. -73 kJ/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 298 K and 13.8 bar hydrogen (H-2) pressure. Lowering the H-2 pressure by a factor of two decreases the TOF (0.6 min(-1)), which is consistent with a pseudo first-order reaction in H-2. In the absence of imine, the heat flux measured in the calorimeter provides a measure of the enthalpy for heterolytic splitting of H-2, PBcat + H-2 -> PBCatH(2), Delta H ca. -43(4) kJ/mol. Solution phase F-19 nuclear magnetic resonance spectroscopy was used to determine the rate of heterolytic splitting of H-2 by PBCat, k = 0.7(.3) M(-1)s(-1)and the equilibrium constant for PBCat + H-2(soln) double left right arrow PBCatH(2), K-eq(295) = 2.2(.5) x 10(5) M, providing an estimate of the free energy for heterolytic splitting of H-2, Delta G ca. -29.8(1.3) kJ/mol at 295 K 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 H-2 activation by the Lewis acid-base pair is rate limiting). (C) 2015 Elsevier B.V. All rights reserved.