Stabilizing Halogen-Bonded Complex between Metallic Anion and Iodide

Halogen bonds (XBs) between metal anions and halides have seldom been reported because metal anions are reactive for XB donors. The pyramidal-shaped Mn(CO)₅⁻ anion is a candidate metallic XB acceptor with a ligand-protected metal core that maintains the negative charge and an open site to accept XB donors. Herein, Mn(CO)₅⁻ is prepared by electrospray ionization, and its reaction with CH₃I in gas phase is studied using mass spectrometry and density functional theory (DFT) calculation. The product observed experimentally at m/z = 337 is assigned as [IMn(CO)₄(OCCH₃)]⁻, which is formed by successive nucleophilic substitution and reductive elimination, instead of the halogen-bonded complex (XC) CH₃−I···Mn(CO)₅⁻, because the I···Mn interaction is weak within XC and it could be a transient species. Inspiringly, DFT calculations predict that replacing CH₃I with CF₃I can strengthen the halogen bonding within the XC due to the electro-withdrawing ability of F. More importantly, in so doing, the nucleophilic substitution barrier can be raised significantly, ~30 kcal/mol, thus leaving the system trapping within the XC region. In brief, the combination of a passivating metal core and the introduction of an electro-withdrawing group to the halide can enable strong halogen bonding between metallic anion and iodide.