title: Understanding the anion-π interactions with tetraoxacalix[2]arene[2]triazine.
abstract: 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.
authors.affiliation: Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Laboratory for Computational Physical Science, Department of Chemistry, Fudan University, Shanghai, 200433, People's Republic of China. xxchem@fudan.edu.cn.
authors.forename: Jinyang
authors.initials: J
authors.lastname: Xi
authors.forename: Xin
authors.initials: X
authors.lastname: Xu