The structures adopted by a range of complexes AlH3·nL, (n = 1 or 2), have been explored in detail to identify the factors that determine the value of n, and whether a monomeric or dimeric arrangement is preferred for the 1 : 1 complexes. Single-crystal X-ray diffraction, vibrational and NMR spectroscopies, and thermal analysis data have been collected, DFT calculations have been performed for AlH3·nL species, and pKa values have been collated for a series of amine and phosphine ligands L. The pKa of the ligand L exerts an important influence on the type of complex formed: as the basicity of L increases, a monomeric structure is favoured over a dimeric arrangement. Dimeric amine complexes form if pKa < 9.76, while monomeric complexes are preferred when pKa > 9.99. The steric requirements of L also influence the structural preference: bulky ligands with large cone angles (>163°) tend to favour formation of monomers, while smaller cone angles (<125°) encourage the formation of dimeric or 1 : 2 adducts. The steric bulk of the ligand appears to be more important for phosphine complexes, with smaller phosphines being unable to stabilise the complex at ambient temperatures even through dimerisation. Raman spectroscopy and DFT calculations have been particularly helpful in elucidating the stoichiometric preferences of complexes that have been contentious; these include AlH3·NMe2Et, AlH3·NMe3 and AlH3·nEt2O.
一系列复合物
AlH3·nL (n = 1 或 2)采用的结构已被详细研究,以识别决定 n 值的因素,以及 1:1 复合物是偏向单体还是二聚体排列。研究收集了单晶 X 射线衍射、振动和 NMR 光谱以及热分析数据,并对 ·nL 物种进行了 DFT 计算。同时,还整理了一系列胺和
磷烷
配体 L 的 pKa 值。
配体 L 的 pKa 对形成的复杂类型产生重要影响:随着 L 的碱性增加,单体结构相较于二聚体结构更受青睐。当 pKa < 9.76 时,形成二聚胺复合物,而当 pKa > 9.99 时,单体复合物更为优选。
配体 L 的空间要求也会影响结构偏好:大锥角(>163°)的体积大
配体倾向于形成单体,而小锥角(<125°)则鼓励形成二聚体或 1:2 加合物。对于磷烷复合物而言,配体的空间体积似乎更加重要,较小的
磷烷在常温下即使通过二聚化也无法稳定复合物。拉曼光谱和 DFT 计算在阐明某些存在争议的复合物的计量偏好方面特别有帮助,这些复合物包括 ·NMe2Et、 ·NMe3 和 ·nEt2O。