Design of supramolecular systems capable of recognizing anions uniquely by aliphatic C–H⋯anion hydrogen bonds: theoretical insights

Abstract

Anions perform several important functions in biological processes. Thus, structures that are able to interact with anions are of great relevance. Here, light is shed on the main characteristics of the bonds between the anions chloride (Cl⁻), bromide (Br⁻) and nitrite (NO₂⁻) and the –CH groups of trioxane structures non-substituted and polarized by the ligands (i) –CH₃, (ii) –CH₂Cl, (iii) –CHCl₂, and (iv) –CCl₃. There is a less attractive recognition of the anions Cl⁻, Br⁻ and NO₂⁻ using trioxane substituted by the –CH₃ groups compared to the receptors non-substituted due to the destabilization of the electrostatic interactions C–H⋯(Cl⁻, Br⁻ and NO₂⁻). The presence of electron withdrawing groups in the receptor structure improves the interaction with the anions through more favorable σ bonds between the –CH groups and anions Cl⁻, Br⁻ and NO₂⁻. The anion NO₂⁻ is recognized with (i) more favorable and (ii) similar energy and rate by receptors containing (i) –CH₃ and (ii) –CH₂Cl, –CHCl₂ or –CCl₃, compared to anions Cl⁻ and Br⁻. The lower values of the Pauli repulsion contribution in receptors⋯Cl⁻ bonds when compared to the interactions receptors⋯Br⁻ explain the more favorable recognition of the anion Cl⁻ in relation to Br⁻. The larger recognition rate of the anion Cl⁻ regarding to anion Br⁻ is determined by more favorable electrostatic and orbital interactions. These data provide relevant information about the recognition of anions using C–H bonds and can be used to guide the development of new receptors.