Institut für Chemie und Biochemie/Anorganische Chemie, Freie Universität Berlin, Fabeckstrasse 36a,14195 Berlin, Germany.
J Phys Chem A. 2010 Sep 23;114(37):10185-96. doi: 10.1021/jp100995n.
The electronic characteristics of the dative N−B bond in three Lewis acid−base adducts, hydrazine borane, hydrazine bisborane, and ammonia trifluoroborane, are analyzed by an approach combining experimental electron density determination with a broad variety of theoretical calculations. Special focus is directed to the weak dihydrogen contacts in hydrazine borane. The Atoms In Molecules partitioning scheme is complemented by additional methods like the Source Function, and the Electron Localizability Indicator. For the multipole-free theoretical models of hydrazine borane and hydrazine bisborane, a weak charge donation from Lewis base to acid of about 0.05 e is found, whereas multipole refinement of theoretical and experimental structure factors resulted in opposite signs for the Lewis acid and base fragments. For ammonia trifluoroborane, the donation from Lewis base to acid is slightly larger (about 0.13 e) in the multipole-free models, and the charges obtained by multipole refinement retain the direction of the charge donation but show quite large variations. The natural population analysis charges predict larger charge donations (0.35 e) from the Lewis bases to the acids for the three title complexes. Although the three compounds exhibit intermolecular interactions of different types and strengths, including classical hydrogen bonds, F···H contacts and the already mentioned dihydrogen bonds, almost no charge transfer is detected between different molecules within the crystal environment. The main electronic effect of the formation of the Lewis acid-base adducts and of the crystallization is an increase in the charge separation within the ammonia/hydrazine fragments, which is supported by all investigated bond and atomic properties. The nature of the dative N-B bond is found to be mainly electrostatic, but with a substantial contribution of covalency. The F-B bonds show similarities and differences from the N-B bonds, which makes a distinction of coordinative (or dative) bonds from polar covalent interactions possible.
三种路易斯酸碱加合物（肼硼烷、二肼二硼烷和氨三氟化硼）中供体 N−B 键的电子特性通过结合实验电子密度测定和多种理论计算的方法进行分析。特别关注肼硼烷中较弱的双氢键接触。分子中的原子划分方案通过源函数和电子局域化指标等附加方法进行补充。对于无多极矩的肼硼烷和二肼二硼烷理论模型，发现路易斯碱向酸的弱电荷捐赠约为 0.05 e，而理论和实验结构因子的多极矩细化导致路易斯酸和碱片段的符号相反。对于氨三氟化硼，无多极矩模型中路易斯碱向酸的捐赠略大（约 0.13 e），多极矩细化得到的电荷保留了电荷捐赠的方向，但变化较大。自然布居分析电荷预测在三个标题配合物中，路易斯碱向酸的电荷捐赠更大（0.35 e）。尽管这三种化合物表现出不同类型和强度的分子间相互作用，包括经典氢键、F···H 接触和前面提到的双氢键，但在晶体环境中几乎没有检测到不同分子之间的电荷转移。路易斯酸碱加合物的形成和结晶的主要电子效应是增加氨/肼片段内的电荷分离，所有研究的键和原子性质都支持这一点。供体 N-B 键的性质主要是静电的，但具有相当大的共价贡献。F-B 键与 N-B 键具有相似性和差异性，这使得区分配位（或供体）键和极性共价相互作用成为可能。
