Understanding the anion–π interactions with tetraoxacalix[2]arene[2]triazine

Anion-π interaction is a new type of non-covalent interaction. It has attracted growing interest in recent years both theoretically and experimentally. However, the nature of bonding between an anion and an electron-deficient aromatic system has remained elusive. To understand the bonding nature in depth, we have carried out a systematic computational study, using model systems that involve tetraoxacalix[2]arene[2]triazine 1, an electron-deficient macrocyclic host, and four anions, X(-) (X(-) = SCN(-), NO3(-), BF4(-), and PF6(-)), of varied sizes and shapes. The geometries for the 1·X(-) complexes were optimized using the extended ONIOM (XO) method. The good agreements with the X-ray experimental results provide a validation of our theoretical schemes. The nature of the non-covalent interactions was analyzed with the help of the AIM (atoms in molecules), RDG (reduced density gradient) and LMO-EDA (local molecular orbital-energy decomposition analysis) methods. The results clearly reveal the involvement of anion-π bonding, as well as a weak, yet significant, hydrogen bonding interaction between the benzene C-H on 1 and the anion of NO3(-) or PF6(-). The bonding energies of 1·X(-) were calculated with the XYG3 functional, and the results were compared with those from MP2, M06-2X and some other functionals with non-covalent interaction corrections (e.g., B3LYP-D3, and ωB97X-D). We conclude that the binding strengths follow the order of 1·NO3(-) > 1·SCN(-) > 1·BF4(-) > 1·PF6(-), where the difference between 1·SCN(-) and 1·BF4(-) is less significant. The strongest interaction in 1·NO3(-) comes from: (1) the effective electronic interaction between NO3(-) and the triazine rings on 1; and (2) the weak hydrogen bonding interaction between the benzene C-H on 1 and nitrate, which cooperates with the anion-π interactions.