Boron-based Super-electrophiles as Metal-Free Catalysts*

Using density functional theory, we show that modified dodecaborate anion, [B₁₂H₁₂]^2-, with hydrogen atoms replaced by halogens (F) and superhalogens (CN) moieties can be effective metal-free catalysts, once one of their ligands is removed. Studies of the structure, stability, and electronic properties of [B₁₂X₁₁]^- and [B₁₂X₁₁]^2- (X = H, F, CN) show that they can simultaneously bind a range of gas molecules such as H₂, O₂, N₂, CO₂, CO, OH, and NH₃ with energies intermediate between physisorption and chemisorption and activate them. In some cases, [B₁₂X₁₁]^2- (X = F, CN) species, that contain one electron more than required by Wade’s rule, are not only more stable than [B₁₂X₁₁]^- but also can activate gas molecules more strongly than their monoanion counterpart. When compared with the interaction of these clusters with noble gas atoms Ne, Ar, and Kr, these results further illustrate the different roles played by the dipole moment and electron distribution. The strong dipole moment of [B₁₂X₁₁]^-, arising due to the large positive charge on the naked boron atom that has lost a ligand and the distribution of the extra electron over the remaining ligands, is responsible for binding noble gas atoms at room temperature. However, in [B₁₂X₁₁]^2-, the second electron proceeds to neutralize much of the positive charge on the naked boron, thus, substantially reducing its dipole moment. Consequently, the ability of [B₁₂X₁₁]^2- to bind noble gas atoms strongly is compromised. On the other hand, the ease with which the second electron from [B₁₂X₁₁]^2- can be removed allows these moieties to interact strongly with gas molecules, activating them in the process. These studies demonstrate the potential of modified dodecaborates as metal-free catalysts for a range of reaction products.