A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane-amine Lewis pairs

Calorimetry is used to measure the reaction enthalpies of hydrogen (H-2) activation by 2,6-lutidine (Lut), 2,2,6,6-tetramethylpiperidine (TMP), N-methyl-2,2,6,6-tetramethylpiperidine (MeTMP), and tri-tert-butylphosphine (TBP) with tris(pentafluorophenyl) borane (BCF). At 6.6 bar H-2 the conversion of the Lewis acid Lewis base pair to the corresponding ionic pair in bromobenzene at 294 K was quantitative in under 60 min. Integration of the heat release from the reaction of the Frustrated Lewis Pair (FLP) with H-2 as a function of time yields a relative rate of hydrogenation in addition to the enthalpy of hydrogenation. The half-lives of hydrogenation range from 230 s with TMP, Delta H-H2 = -31.5(0.2) kcal mol(-1), to 1400 s with Lut, Delta H-H2 = -23.4(0.4) kcal mol(-1). The B-11 nuclear magnetic resonance (NMR) spectrum of B(C6F5)(3) in bromobenzene exhibits three distinct traits depending on the sterics of the Lewis base; (1) in the presence of pyridine, only the dative bond adduct pyridine-B(C6F5)(3) is observed; (2) in the presence of TMP and MeTMP, only the free B(C6F5)(3) is observed; and (3) in the presence of Lut, both the free B(C6F5)(3) and the Lut-B(C6F5)(3) adduct appear in equilibrium. A measure of the change in K-eq of Lut + B(C6F5)(3) double left right arrow Lut-B(C6F5)(3) as a function of temperature provides thermodynamic properties of the Lewis acid Lewis base adduct, Delta H = -17.9(1.0) kcal mol(-1) and a Delta S = -49.2(2.5) cal mol(-1) K, suggesting the Lut-B(C6F5)(3) adduct is more stable in bromobenzene than in toluene.