On the recognition of chloride, bromide and nitrate anions by anthracene–squaramide conjugated compounds: a computational perspective

Abstract

Anion recognition is widely used in several biological fields. Squaramide derived compounds appear as potential structures to recognize anions. Here, the bond mechanisms between the chloride (Cl⁻), bromide (Br⁻) and nitrate (NO₃⁻) anions and anthracene–squaramide conjugated compounds are elucidated considering the influence of the: (i) number, (ii) nature, and (iii) position of the substituents: trifluoromethyl (–CF₃) and nitro (–NO₂). Energy decomposition analysis (EDA) shows that the interactions between Cl⁻, Br⁻ and NO₃⁻ and anthracene–squaramide have an attractive interaction energy supported predominantly by electrostatic energy followed by orbital contribution. Molecular electrostatic potential (MEP) surfaces imply electrostatic interactions between Cl⁻, Br⁻ and the oxygen atom from NO₃⁻ and the hydrogen atoms from N–H and C–H bonds present in the squaramide structure, and an aryl group, respectively. Cl⁻ interacts with the receptors more strongly than Br⁻. The NO₃⁻ recognition is less attractive than those presented by Cl⁻ and Br⁻, in agreement with the hardness–softness features of these anions. Importantly, one and, mostly, two group substitutions, –H → –CF₃ or –NO₂, favor the recognition of Cl⁻, Br⁻ and NO₃⁻ due to the increase of the polarization in the receptor–NH⋯anion interaction. The –NO₂ group promotes a larger effect relative to the –CF₃ ligand. The –NO₂ ligand positioned at the largest distance conceivable to the benzene–NH group promotes the lowest interference in the N–H⋯Cl⁻ interaction. These results provide information to design receptors with a larger capability to recognize anions.