Heterolytic Cleavage of H2 by Frustrated B/N Lewis Pairs

Treatment of the Lewis acid B(C6F5)3 with the Lewis base 2,6-dimethylpiperidine (DMP) resulted in the formation of the classical Lewis acid·base adduct DMP-B(C6F5)3, 1a, which was anticipated to undergo thermal dissociation to the “unquenched” Lewis centers. The free Lewis pair was able to form a frustrated Lewis pair (FLP), which induced heterolytic splitting of H2, affording the ionic product [DMPH][HB(C6F5)3], 1b. FLPs, derived from B(C6F5)3 and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP) and 1,2,2,6,6-pentamethylpiperidine (PMP), could also heterolytically activate H2, affording the salts [TMPH][HB(C6F5)3], 2, and [PMPH][HB(C6F5)3], 3, respectively. In a VT NMR study the TMP/B(C6F5)3 reaction was studied in greater detail, trying to trace intermediates. The supposed most prominent intermediate, the TMP/H2/B(C6F5)3 complex, could, however, not be detected. The combination of B(C6F5)3 with the even more sterically demanding Lewis base 1-ethyl-2,2,6,6,-tetramethylpiperidine (Et-TMP) displayed FLP reactivity with H2, but required the high temperature of 110 °C, forming [2,2,6,6-(CH3)4C5H6NH(CH2CH3)][HB(C6F5)3], 4a. In the absence of H2 the combination of B(C6F5)3 and Et-TMP generated at room temperature a mixture of 4a and [2,2,6,6-(CH3)4C5H6NCHCH2-B(C6F5)3], 4b. 4b was formed via consecutive hydride and proton abstractions with Et-TMP as the base, generating 4a. 2,4,6-Tri-tert-butylpyridine (TTBP), exhibiting reduced Lewis basicity as compared to piperidine derivatives, showed FLP reactivity with B(C6F5)3, which gave in the presence of H2 the [TTBPH][HB(C6F5)3], 5, salt as the only product after several hours. The steric demand of the Lewis bases was evaluated by aid of DFT calculations on borane adducts, which roughly correlated with the reaction temperature of H2 splitting. 1a, 1b, 3, 4a, and 4b were studied by single-crystal X-ray diffraction analyses.将路易斯酸（Lewis acid）B(C6F5)3与路易斯碱（Lewis base）2,6-二甲基哌啶（DMP）进行反应，合成得到经典的路易斯酸碱加合物DMP-B(C6F5)3（即化合物1a）。该加合物理论上可热解离为“未淬灭”的路易斯活性中心，其游离的路易斯对可形成受阻路易斯酸碱对（frustrated Lewis pair, FLP），该受阻路易斯酸碱对能够诱导氢气（H2）发生异裂，生成离子型产物[DMPH][HB(C6F5)3]（即化合物1b）。由B(C6F5)3与位阻型路易斯碱2,2,6,6-四甲基哌啶（TMP）、1,2,2,6,6-五甲基哌啶（PMP）衍生得到的受阻路易斯酸碱对，同样可异裂活化氢气，分别得到盐类产物[TMPH][HB(C6F5)3]（化合物2）与[PMPH][HB(C6F5)3]（化合物3）。研究团队通过变温核磁共振（VT NMR）对TMP/B(C6F5)3反应体系开展了更为细致的探究，尝试追踪反应中间体，但理论上最为关键的TMP/H2/B(C6F5)3复合物并未被检测到。将B(C6F5)3与位阻效应更强的路易斯碱1-乙基-2,2,6,6-四甲基哌啶（Et-TMP）混合，该体系同样展现出针对氢气的受阻路易斯酸碱对反应活性，但需在110 ℃的高温下进行反应，最终生成[2,2,6,6-(CH3)4C5H6NH(CH2CH3)][HB(C6F5)3]（化合物4a）。在无氢气存在的条件下，B(C6F5)3与Et-TMP在室温下混合可得到4a与[2,2,6,6-(CH3)4C5H6N=CHCH2-B(C6F5)3]（化合物4b）的混合物。4b通过以Et-TMP为碱的连续氢负离子与质子攫取过程生成，同时伴随4a的形成。与哌啶类衍生物相比，碱性减弱的2,4,6-三叔丁基吡啶（TTBP）同样可与B(C6F5)3形成受阻路易斯酸碱对，在氢气存在下反应数小时后，仅得到盐类产物[TTBPH][HB(C6F5)3]（化合物5）。研究团队通过对硼烷加合物进行密度泛函理论（DFT）计算，对各类路易斯碱的位阻效应进行了评估，该位阻参数与氢气异裂反应的反应温度大致呈相关性。通过单晶X射线衍射分析对化合物1a、1b、3、4a与4b进行了结构表征。