Superacidity
of <i>closo</i>-Dodecaborate-Based
Brønsted Acids: a DFT Study
- Publication date
- Publisher
Abstract
The structures and intrinsic gas-phase
acidities (GA) of some dodecaborane
acids, the derivatives of YB<sub>12</sub>H<sub>11</sub>H (Y <b>=</b> PF<sub>3</sub>, NH<sub>3</sub>, NF<sub>3</sub>, NMe<sub>3</sub>), B<sub>12</sub>H<sub>12</sub>H<sub>2</sub>, and B<sub>12</sub>H<sub>12</sub>H<sup>–</sup> (HA, H<sub>2</sub>A, and HA<sup>–</sup>, 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:
CF<sub>3</sub> < F < Cl and in case of anionic acids: CF<sub>3</sub> < Cl < F. The dodecatrifluoromethyl derivative of H<sub>2</sub>A, B<sub>12</sub>(CF<sub>3</sub>)<sub>12</sub>H<sub>1</sub>H<sub>2</sub>, 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, CB<sub>11</sub>(CF<sub>3</sub>)<sub>11</sub>H<sub>1</sub>H. The GA values of the respective monoanionic forms of the considered
acids all, but the (CF<sub>3</sub>)<sub>11</sub> derivative, remained
higher than the widely used threshold of superacidity. The HA derivatives’
(Y <b>=</b> PF<sub>3</sub>, NF<sub>3</sub>) GA’s were
approximately in the same range as the H<sub>2</sub>A acids’.
In the case Y <b>=</b> NH<sub>3</sub> or NMe<sub>3</sub> the
GA values were significantly higher. Also, the p<i>K</i><sub>a</sub> values of B<sub>12</sub>H<sub>12</sub>H<sub>2</sub>,
CB<sub>11</sub>H<sub>12</sub>H, 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><sub>a</sub> 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
B<sub>12</sub>H<sub>12</sub>H<sub>2</sub> are as a rule not significantly
weaker acids than the respective derivatives of CB<sub>11</sub>H<sub>12</sub>H. 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 CB<sub>11</sub>H<sub>12</sub><sup>–</sup> derivatives