Brzeski Jakub, Skurski Piotr, Simons Jack
Laboratory of Quantum Chemistry, Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, 80-308 Gdańsk, Poland.
Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States.
J Phys Chem A. 2021 Feb 4;125(4):999-1011. doi: 10.1021/acs.jpca.0c11014. Epub 2021 Jan 22.
Carborane Brønsted superacids have proven to be useful reagents in a variety of organic and inorganic synthetic processes. In this work, analogs in which the icosahedral CB carborane core is replaced by a CAl core are studied using ab initio electronic structure tools. Each so-called caralumane Brønsted acid is formed by adding HF, HCl, or HH to a corresponding caralumane Lewis acid possessing a vacant Al-centered orbital that acts to accept an electron pair from the HF, HCl, or HH. The Lewis acid strengths of the species involved, as measured by their F ion affinities, are all found to exceed the threshold for labeling them Lewis superacids. Also, the deprotonation Gibbs free energies of the Brønsted acids are found to be small enough for them to be Brønsted superacids. When HF or HCl is bound to a caralumane Lewis acid to form the Brønsted acid, the HF or HCl is bound datively to a single Al atom, and hydrogen bonds can be formed between this molecule's H atom and nearby F or Cl atoms attached to other Al atoms. In contrast, when HH is bound to the Lewis acid to form the Brønsted acid, two novel low-energy structures arise, both of which are Brønsted superacids. One has an essentially intact HH molecule attached to a single Al atom in a η fashion. In the other, the HH molecule is heterolytically cleaved to generate a hydride ion that attaches to a single Al atom and a proton that binds in a multicenter manner to other Al atoms. The structures and relative energies of a multitude of such caralumane Lewis and Brønsted superacids are provided and discussed.
碳硼烷布朗斯特超酸已被证明是各种有机和无机合成过程中有用的试剂。在这项工作中，使用从头算电子结构工具研究了二十面体CB碳硼烷核心被CAl核心取代的类似物。每个所谓的碳铝烷布朗斯特酸是通过将HF、HCl或HH添加到相应的具有空的以Al为中心的轨道的碳铝烷路易斯酸中形成的，该轨道用于接受来自HF、HCl或HH的电子对。通过它们的F离子亲和力测量的所涉及物种的路易斯酸强度都超过了将它们标记为路易斯超酸的阈值。此外，发现布朗斯特酸的去质子化吉布斯自由能足够小，足以使它们成为布朗斯特超酸。当HF或HCl与碳铝烷路易斯酸结合形成布朗斯特酸时，HF或HCl以配位方式与单个Al原子结合，并且该分子的H原子与附着在其他Al原子上的附近F或Cl原子之间可以形成氢键。相比之下，当HH与路易斯酸结合形成布朗斯特酸时，会出现两种新颖的低能量结构，它们都是布朗斯特超酸。一种结构是一个基本完整的HH分子以η方式附着在单个Al原子上。另一种结构是HH分子发生异裂，生成一个附着在单个Al原子上的氢化物离子和一个以多中心方式与其他Al原子结合的质子。提供并讨论了众多此类碳铝烷路易斯超酸和布朗斯特超酸的结构和相对能量。
