Solvent-Driven Structural Changes in Anion−π Complexes

Among the π interactions, the anion-π interaction has been a novel type of interaction. In the cases of halide-π complexes, which are the most typical examples of the anion-π interaction, the theoretically predicted and experimentally observed structures in the gas phase are quite different from the most frequently observed crystal structures. We here investigate the structural changes in complexation of the F(-)/Cl(-) ion with triazine (TAz) as the number of water/acetonitrile molecules increases from 1 to 4. Both the covalent bonding type for F(-)-TAz and the hydrogen-bonding type for Cl(-)-TAz, which are the lowest-energy structures in the gas phase, change to the solvent-mediated anion-π-type or displaced anion-π-type complexes. This study explains why the (displaced) anion-π-type complexes with some flexible orientations are most common in many crystal structures.