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

Anion-pi 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-pi 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 omega B97X-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-pi interactions.