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首页> 外文期刊>Chemistry: A European journal >Can an amine be a stronger acid than a carboxylic acid? the surprisingly high acidity of amine-borane complexes
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Can an amine be a stronger acid than a carboxylic acid? the surprisingly high acidity of amine-borane complexes

机译：胺能比羧酸强吗？胺-硼烷配合物的高酸度

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摘要

The gas-phase acidity of a series of amine-borane complexes has been investigated through the use of electrospray mass spectrometry (ESI-MS), with the application of the extended Cooks kinetic method, and high-level G4 ab initio calculations. The most significant finding is that typical nitrogen bases, such as aniline, react with BH_3 to give amine-borane complexes, which, in the gas phase, have acidities as high as those of either phosphoric, oxalic, or salicylic acid; their acidity is higher than many carboxylic acids, such as formic, acetic, and propanoic acid. Indeed the complexation of different amines with BH_3 leads to a substantial increase (from 167 to 195 kJ mol~(-1)) in the intrinsic acidity of the system; in terms of ionization constants, this increase implies an increase as large as fifteen orders of magnitude. Interestingly, this increase in acidity is almost twice as large as that observed for the corresponding phosphine-borane analogues. The agreement between the experimental and the G4-based calculated values is excellent. The analysis of the electron-density rearrangements of the amine and the borane moieties indicates that the dative bond is significantly stronger in the N-deprotonated anion than in the corresponding neutral amine-borane complex, because the deprotonated amine is a much better electron donor than the neutral amine. On the top of that, the newly created lone pair on the nitrogen atom in the deprotonated species, conjugates with the BN bonding pair. The dispersion of the extra electron density into the BH_3 group also contributes to the increased stability of the deprotonated species. It's all relative: The complexation of boranes to amines leads to a dramatic increase in the acidity of the amine. Thus, typical nitrogen bases, such as aniline, give amine-borane complexes, which are nitrogen-based acids that are as acidic as phosphoric acid in the gas phase.
机译：通过使用电喷雾质谱（ESI-MS），扩展的Cooks动力学方法和高水平的G4从头计算，研究了一系列胺-硼烷配合物的气相酸性。最重要的发现是典型的氮碱（例如苯胺）与BH_3反应生成胺-硼烷配合物，该配合物在气相中的酸度与磷酸，草酸或水杨酸的酸度相同。它们的酸度高于许多羧酸，例如甲酸，乙酸和丙酸。实际上，不同的胺与BH_3的络合会导致系统的固有酸度大幅增加（从167 kJ mol〜（-1）从167 kJ mol〜（-1））。就电离常数而言，这种增加意味着增加了十五个数量级。有趣的是，这种酸度的增加几乎是相应的膦-硼烷类似物观察到的酸度的两倍。实验值与基于G4的计算值之间的一致性非常好。对胺和硼烷部分的电子密度重排的分析表明，N-去质子化阴离子中的配位键比相应的中性胺-硼烷络合物中的键强得多，因为去质子化的胺比其更好的电子供体中性胺。最重要的是，在去质子化物种的氮原子上新创建的孤对与BN键对共轭。额外的电子密度分散到BH_3基团中也有助于增加去质子化物质的稳定性。都是相对的：硼烷与胺的络合导致胺的酸度急剧增加。因此，典型的氮碱，例如苯胺，得到胺-硼烷络合物，其为基于氮的酸，其在气相中的酸性与磷酸一样。

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