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

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 (NO 3 − ) anions and anthracene-squaramide conjugated compounds are elucidated considering the influence of the: (i) number, (ii) nature, and (iii) position of the substituents: trifluoromethyl (-CF 3 ) and nitro (-NO 2 ). Energy decomposition analysis (EDA) shows that the interactions between Cl − , Br − and NO 3 − 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 NO 3 − 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 NO 3 − 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 → -CF 3 or -NO 2 , favor the recognition of Cl − , Br − and NO 3 − due to the increase of the polarization in the receptor-NH anion interaction. The -NO 2 group promotes a larger effect relative to the -CF 3 ligand. The -NO 2 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. Anionic recognition appears in several biological processes. Here, the interaction between anthracene-squaramide conjugated compounds and Cl − , Br − and NO 3 − anions has been explored using density functional theory (DFT) calculations.