Molecular Complexes of Simple Anions with Electron‐Deficient Arenes: Spectroscopic Evidence for Two Types of Structural Motifs for Anion–Arene Interactions

Abstract

Anion-pi interactions between a pi-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 anion-arene complexes depending on the nature of the anion. The TNB-OR- complexes (R=H, or alkyl groups which were studied earlier) adopt an anionic sigma-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 sigma 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.

Substituent Effects on Non‐Covalent Interactions with Aromatic Rings: Insights from Computational Chemistry

“…That interactions of simple anions with phenyl hydrogens are preferred energetically over the face of fluorinated benzenes has been demonstrated spectroscopically. [81] Similarly, both Mascal and coworkers [82] and Hay et al [83] showed computationally that interactions of simple anions with aryl hydrogens are more enthalpically favorable than anion/p complexes. Further complicating discussions of anion/p interactions is the existence of favorable complexes in which the anion forms a covalent (or partially covalent) bond with an aryl carbon (Meisenheimer complexes), which are often competitive with anion/p interactions.…”

Gas Phase Reactions of 1,3,5-Triazine: Proton Transfer, Hydride Transfer, and Anionic σ-Adduct Formation

“…Gas phase spectroscopic evidence of these two types of structural motifs supports this relationship [23]. The amount of interaction between the electrons of the anion and the л-system is enhanced and stabilized by electron deficiencies in the aromatic ring.…”

Nitrosyl–heme and anion–arene complexes: structure, reactivity and spectroscopy

“…Thus, IRMPD spectroscopy has provided for the first time conclusive evidence for the existence of Meisenheimer complexes in the gas phase (see Fig. 2 with X = H and Y = OH, OCH 3 , OC 2 H 5 ) [23,24].…”

“…X-ray diffraction data have been reported showing that in the crystal structure a halide is placed over the periphery of the ring of strongly electron deficient arenes. Geometrical features that characterize the TNB-Brcomplex 4 are an elongated C-Br bond (2.67 Å) if compared with a typical covalent C-Br bond length (1.93 Å) and a H-C-Br angle close to 90° while the carbon atom closest to Br is not appreciably distorted from the original sp 2 -type geometry [23]. Forcing this carbon to attain a tetrahedral coordination, as in a strongly covalent anionic σ-complex 5, and constraining the C-Br distance to 1.93 Å yields a structure noticeably higher in energy relative to 4 that is not a stationary point on the potential energy surface.…”

“…The first complex isolated by Meisenheimer in 1902 and characterized by the crystal structure responds to X = Y = OCH 3 . A relatively novel, structurally diagnostic tool has recently allowed us to ascertain the bonding features of the gaseous adducts of simple anions (OH -, OCH 3 -, OC 2 H 5 -, F -, Cl -, Br -, I -, CN -) with a π-acidic arene (1,3,5-trinitrobenzene, TNB) [21][22][23][24].…”