Features of Protonation of the Simplest Weakly Basic Molecules, SO 2 , CO, N 2 O, CO 2 , and Others by Solid Carborane Superacids

Caralumane Superacids of Lewis and Brønsted Character

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 CB₁₁ carborane core is replaced by a CAl₁₁ 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 η² 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.

Icosahedral Carborane Superacids and their Conjugate Bases Comprising H, F, Cl, and CN Substituents: A Theoretical Investigation of Monomeric and Dimeric Cages

Theoretical investigation of the H(CHB₁₁ X₁₁ ) (X=H, F, Cl, CN), H(CHB₁₁ X_n Y_11-n ) (X,Y=F, Cl; n=1,5), and dimeric (H(CHB₁₁ X₁₁ ))₂ (X=F, Cl) carborane superacids performed at the B3LYP/6-311++G(d,p) theory level revealed the similarity of their equilibrium structures and the possibility of nearly barrierless hydrogen atom migration among the substituents attached to one side of the icosahedral CB₁₁ cage. The vertical electron detachment energies predicted at the OVGF/6-311++G(3df,2pd) theory level for the conjugate bases (CHB₁₁ X₁₁ )^- were found to span the 5.82-9.00 ev range. The acid strengths (manifested by the Gibbs free deprotonation energies spanning the 213-266 kcal/mol range) predicted for the icosahedral H(CHB₁₁ X₁₁ ) carborane systems confirm their superacidic properties which might be increased even further by the attachment of the second carborane H(CHB₁₁ X₁₁ ) unit that leads to a dimeric structure mimicking a part of an experimentally observed H-bridged polymeric chain. The Gibbs free deprotonation energy of the dimeric (H(CHB₁₁ Cl₁₁ ))₂ acid was predicted to be smaller by 17 kcal/mol than that of the corresponding monomeric H(CHB₁₁ Cl₁₁ ) acid.

Lewis Acid-Mediated One-Electron Reduction of Nitrous Oxide

The one-electron reduction of nitrous oxide (N₂ O) was achieved using strong Lewis acids E(C₆ F₅ )₃ (E=B or Al) in combination with metallocenes. In the case of B(C₆ F₅ )₃ , electron transfer to N₂ O required a powerful reducing agent such as Cp*₂ Co (Cp*=pentamethylcyclopentadienyl). In the presence of Al(C₆ F₅ )₃ , on the other hand, the reactions could be performed with weaker reducing agents such as Cp*₂ Fe or Cp₂ Fe (Cp=cyclopentadienyl). The Lewis acid-mediated electron transfer from the metallocene to N₂ O resulted in cleavage of the N-O bond, generating N₂ and the oxyl radical anion [OE(C₆ F₅ )₃ ]^⋅- . The latter is highly reactive and engages in C-H activation reactions. It was possible to trap the radical by addition of the Gomberg dimer, which acts as a source of the trityl radical.