The C-I...X super(-) halogen bonding of tetraiodoethylene with halide anions in solution and cocrystals investigated by experiment and calculation

The selection of the halogen bonding (XB) donor is an important factor in molecular recognition of halides by XB. Here XB complexes between tetraiodoethylene (TIE) as another donor instead of iodoperfluorobenzene and halides are investigated using UV-Vis, Raman, FT-IR, XRD, PXRD and calculations. A 1 : 1 stoichiometry of TIE with halide anions in dilute solution is confirmed. Comparatively, the TIE...Cl super(-) complex possesses greater bonding constant and molar extinction coefficient than TIE...Br super(-)/I super(-), probably due to Cl super(-) having the most negative electrostatic potential, or higher electronic density due to its small size. The XRD reveals that TIE/Cl super(-) and TIE/I super(-) cocrystals always keep the stoichiometry of 3 : 1 and 1 : 1, respectively, regardless of which solvent is used for preparing them. But the intermediate TIE/Br super(-) cocrystals swing between 3 : 1 and 1 : 1. These results indicate that the TIE...Br super(-) complex should be influenced more easily by solvent nature and the stoichiometries of interaction between TIE and halide anions are different in solution and cocrystals. Moreover, the calculation of energies of XB interaction indicates that the XB strength of C-I...X super(-) is much stronger in the gas-phase and cocrystals than in solution, and much stronger than C-I... pi and C-I...I-C contacts in cocrystals. The study will be of benefit in anion recognition and new material design using XB.