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Issue 3, 2013
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A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane–amine Lewis pairs

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Abstract

Calorimetry is used to measure the reaction enthalpies of hydrogen (H2) 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 H2 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 H2 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, ΔHH2 = −31.5(0.2) kcal mol−1, to 1400 s with Lut, ΔHH2 = −23.4(0.4) kcal mol−1. The 11B 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 Keq of Lut + B(C6F5)3 ⇔ Lut–B(C6F5)3 as a function of temperature provides thermodynamic properties of the Lewis acid Lewis base adduct, ΔH = −17.9(1.0) kcal mol−1 and a ΔS = −49.2(2.5) cal mol−1 K, suggesting the Lut–B(C6F5)3 adduct is more stable in bromobenzene than in toluene.

Graphical abstract: A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane–amine Lewis pairs

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Publication details

The article was received on 20 Jul 2012, accepted on 29 Aug 2012 and first published on 20 Sep 2012


Article type: Communication
DOI: 10.1039/C2DT31628E
Citation: Dalton Trans., 2013,42, 615-619
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    A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane–amine Lewis pairs

    A. Karkamkar, K. Parab, D. M. Camaioni, D. Neiner, H. Cho, T. K. Nielsen and T. Autrey, Dalton Trans., 2013, 42, 615
    DOI: 10.1039/C2DT31628E

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