In silico studies toward the recognition of fluoride ion by substituted borazines

The substituted borazines have been computationally investigated as new type of receptors for the recognition of fluoride ion. Fluorine, methyl and phenyl groups have been selected as electron-withdrawing, electron-releasing and aromatic substituents for the study employing DFT (B3LYP/6-311+G**) and ab initio (MP2/6-311+G**) levels of calculations. N-substituted borazines have shown higher fluoride ion affinity than their corresponding B-substituted borazines. In the case of fluorinated borazines, the binding affinity of fluoride is enhanced with the increasing number of substitutions. The F⁻ and Cl⁻ ions, generally, prefer to bind with the boron atom of substituted borazine rings, whereas, Br⁻ ion prefers to bind with NH hydrogens of the borazine receptor units. Phenyl derivatives of borazine also showed analogous behavior with halide anions. The binding affinities of halides with fluorinated and phenyl derivatives of borazine have been found to be much higher than the simple borazine receptor molecule. The NBO analyses performed for the complexation of F⁻ ion with fluorinated borazines suggest that the Lewis energy contribution in the total SCF energy enhanced with increasing the substitutions. However, in the case of methylated borazines, the delocalization energy is responsible for the stabilization of F⁻ ion complexes. The N-trifluoroborazine showed much higher fluoride ion affinity (30.9 kcal/mol) in aqueous phase than the simple borazine.