A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane–amine Lewis pairs
作者:Abhi Karkamkar、Kshitij Parab、Donald M. Camaioni、Doinita Neiner、Herman Cho、Thomas K. Nielsen、Tom Autrey
DOI:10.1039/c2dt31628e
日期:——
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, ÎHH2 = â31.5(0.2) kcal molâ1, to 1400 s with Lut, ÎHH2 = â23.4(0.4) kcal molâ1. 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 pyridineâB(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 LutâB(C6F5)3 adduct appear in equilibrium. A measure of the change in Keq of Lut + B(C6F5)3 â LutâB(C6F5)3 as a function of temperature provides thermodynamic properties of the Lewis acid Lewis base adduct, ÎH = â17.9(1.0) kcal molâ1 and a ÎS = â49.2(2.5) cal molâ1 K, suggesting the LutâB(C6F5)3 adduct is more stable in bromobenzene than in toluene.
量热法用于测量氢气(H2)通过2,6-路易狄辛(Lut)、2,2,6,6-四甲基哌啶(TMP)、N-甲基-2,2,6,6-四甲基哌啶(MeTMP)和三特丁基膦(TBP)与三(五氟苯基)硼烷(BCF)活化反应的反应焓。在6.6巴氢气压力下,294 K时溴苯中路易斯酸-路易斯碱对转化为相应离子对的转化在60分钟内定量完成。从时间上积分FLP与H2反应的热释放,不仅得到氢化焓,还得到了相对氢化速率。氢化半衰期从TMP的230秒(ΔH_H2 = -31.5(0.2) kcal mol-1)到Lut的1400秒(ΔH_H2 = -23.4(0.4) kcal mol-1)不等。11B核磁共振(NMR)谱显示,溴苯中的B(C6F5)3具有三种不同特性,取决于路易斯碱的空间位阻:(1)存在吡啶时,仅观察到配位键加合物吡啶-B(C6F5)3;(2)存在TMP和MeTMP时,仅观察到游离的B(C6F5)3;(3)存在Lut时,游离的B(C6F5)3和Lut-B(C6F5)3加合物处于平衡状态。以温度为函数测定的Lut + B(C6F5)3 ⇌ Lut-B(C6F5)3平衡常数变化,提供了路易斯酸-路易斯碱加合物的Thermodynamic properties,即ΔH = -17.9(1.0) kcal mol-1,ΔS = -49.2(2.5) cal mol-1 K,表明Lut-B(C6F5)3加合物在溴苯中比在甲苯中更稳定。