The design of anion–π interactions and hydrogen bonds for the recognition of chloride, bromide and nitrate anions

The role of anions in several biochemical processes has given rise to enormous interest in the identification/exploration of compounds with the potential ability to recognize anions. Here, an anthracene–squaramide conjugated compound, O2C4[NH(C14H10)][(NH(C6H6)], has been modified through the substitutions (i) H → F and (ii) H → OH at the anthracene and benzene rings to improve the capabilities of these structures for recognizing chloride, bromide, and nitrate anions. Through an energy decomposition analysis method, the recognition of the anions is chiefly identified as a non-covalent process. H → F substitutions at the benzene ring and, principally, the anthracene ring favor anion recognition, since H → F substitutions create a π-acid region in the aromatic ring, as indicated based on the molecular electrostatic potential surfaces. Similarly, H → OH substitutions also improve the recognition of anions, which is related to the establishment of partly covalent chemical bonds of the form O–H⋯(Cl−, Br− and O−), which are verified based on the quantitative analysis of the maximum and minimum values of the molecular electrostatic potential surfaces and the quantum theory of atoms in molecules method. The presence of large electron density has a key role in the recognition of Cl− anions, and the more favorable electrostatic interactions between the anthracene structure and Br− anions, relative to NO3− anions, mean that receptor⋯Br− interactions are more attractive than receptor⋯NO3− ones. These data can contribute to the design of structures with the relevant abilities to interact with anions.