New Type of Aromatic pi-Systems for Anion Recognition: Strong Anion-pi and C-H...Anion Interactions Between Halides and Aromatic Ligands in Half-Sandwich Compounds

Half-sandwich compounds of benzene, cyclopentadienyl, pentamethylcyclopentadienyl, and indenyl were studied as a new type of aromatic pi-systems for interactions with halide anions. Although uncoordinated benzene forms only C-H...anion interactions, and hexafluorobenzene forms only anion-pi interactions, aromatic ligands in half-sandwich compounds can form both types of interactions, because their entire electrostatic potential surface is positive. These aromatic ligands can form stronger anion-pi interactions than organic aromatic molecules, as a consequence of more pronounced dispersion and induction energy components. Moreover, C-H...anion interactions of aromatic ligands are stronger than anion-pi interactions, and significantly stronger than C-H...anion interactions of benzene. Our study shows that transition-metal coordination can make aromatic moieties suitable for strong interactions with anions, and gives insight into the design of new anion receptors.