Molecular complexes of simple anions with electron-deficient arenes: Spectroscopic evidence for two types of structural motifs for anion-arene interactions

Anion-π interactions between a π-acidic aromatic system and an anion are gaining increasing recognition in chemistry and biology. Herein, the binding features of an electron-deficient aromatic system (1,3,5-trinitrobenzene (TNB)) and selected anions (OH-, Br-, and I-) are examined in the gas phase by using the combined information derived from collision-induced dissociation experiments at variable energy, infrared multiple-photon dissociation spectroscopy, and quantum chemical calculations. We provide spectroscopic evidence for two different structural motifs of anionarene complexes depending on the nature of the anion. The TNB-OR - complexes (R = H, or alkyl groups which were studied earlier) adopt an anionic σ-complex structure whereby RO- attacks the aromatic ring with covalent bond formation, and develops a tetrahedral ring carbon bound to H and OR. The halide complexes rather conform to a structure in which the TNB moiety is hardly altered, and the halogen is placed on an unsubstituted carbon atom over the periphery of the ring at a C-X distance that is appreciably longer than a typical covalent bond length. The ensuing structural motif, previously characterized in the solid state and named weak a interaction, is now confirmed by an IR spectroscopic assay in the gas phase, in which the sampled species are unperturbed by crystal packing or solvation effects. © 2009 Wiley-VCH Verlag GmbH & Co.