Superacidity of <i>closo</i>-Dodecaborate-Based Brønsted Acids: a DFT Study

Abstract

The structures and intrinsic gas-phase acidities (GA) of some dodecaborane acids, the derivatives of YB₁₂H₁₁H (Y <b>=</b> PF₃, NH₃, NF₃, NMe₃), B₁₂H₁₂H₂, and B₁₂H₁₂H^– (HA, H₂A, and HA^–, respectively) have been computationally explored with DFT B3LYP method at the 6-311+G** level of theory as new possible directions of creating superstrong Brønsted acids. Depending on the nature and number of the substituents different protonation geometries were investigated. In general, the GA values of the neutral systems varied according to the substituents in the following order: CF₃ < F < Cl and in case of anionic acids: CF₃ < Cl < F. The dodecatrifluoromethyl derivative of H₂A, B₁₂(CF₃)₁₂H₁H₂, emerges as the strongest among the considered acids and is expected to be in the gas phase at least as strong as the undecatrifluoromethyl carborane, CB₁₁(CF₃)₁₁H₁H. The GA values of the respective monoanionic forms of the considered acids all, but the (CF₃)₁₁ derivative, remained higher than the widely used threshold of superacidity. The HA derivatives’ (Y <b>=</b> PF₃, NF₃) GA’s were approximately in the same range as the H₂A acids’. In the case Y <b>=</b> NH₃ or NMe₃ the GA values were significantly higher. Also, the p<i>K</i>_a values of B₁₂H₁₂H₂, CB₁₁H₁₂H, and their perfluorinated derivatives in 1,2-dichloroethane (DCE) were estimated with SMD and cluster-continuum model calculations. The obtained estimates of p<i>K</i>_a values of the perfluorinated derivatives are by around 30 units lower than that of trifluoromethylsulfonylimide, making these acids the strongest ever predicted in solution. The derivatives of B₁₂H₁₂H₂ are as a rule not significantly weaker acids than the respective derivatives of CB₁₁H₁₂H. This is important for expanding practical applicability of this type of acids and their anions, as they are synthetically much more accessible than the corresponding CB₁₁H₁₂^– derivatives