Roosevelt University, Department of Biological, Chemical, and Physical Sciences, Schaumburg, IL 60173
Halogen bonding is an attractive intermolecular interaction, R-X•••D, between an electron-poor halogen atom, X, and an electron-rich center, D, where R is an organic molecule residue or another halogen atom. This interaction has been recognized recently as an important factor in molecular recognition, rational drug design, crystallization, and synthesis involving halogenated molecules. Halogen bonding is commonly related to the electrostatic attraction of nucleophiles to the areas of positive charge on the surface of electrophilic halogen atoms. Many recent studies also suggested the importance of other factors, such as intermolecular charge-transfer. However, there are very few works in the scientific literature in which the spectral and structural properties of halogen bonded complexes are experimentally characterized. The lack of experimental data, especially in solutions, hinders clarification of the nature of halogen bonding. In our work, we studied the interaction between the bromine-containing electron acceptors (R-Br), such as CBr4, CBr3H, CBr3F, CBr3NO2, CBr¬3CN, CBr3COOH, CBr3CONH2, CBr3COCBr3, and C3Br2F6 with the Cl-, Br-, I-, SCN-, NCO-, and N3-, anions (A-) taken as tetraalkylammonium salts. UV-Vis spectral measurements have shown that addition of any of these anions to a solution of the bromosubstituted electrophile in dichloromethane results in formation of an absorption band between 230 to 370 nm which is absent in the spectra of individual components. Jobs method indicated that these new bands are related to the formation of new 1:1 [R-Br, A-] complexes. Analysis of the dependence of the intensity of absorption on the concentration of reagents led to extinction coefficients (ε ~ 5-20 x 103 M-1cm-1) and equilibrium constants of complex formation (K ~ 0.2 – 10 M-1). Most notably, the energies of the absorption bands of the [R-Br, A-] complexes were directly related to the differences of the energies E = ELUMO - EHOMO between the lowest unoccupied molecular orbital of R-Br (ELUMO) and the highest occupied orbital of the anion (EHOMO). Furthermore, the attraction between R-Br and A- is sufficiently strong for co-crystallization of such dissimilar species as neutral organic molecules and ionic salts. In fact, slow diffusion of hexane into dichloromethane ¬ solutions of electrophiles and nucleophiles at low temperature afforded single crystals suitable for X-ray measurements. Crystallographic studies revealed that these crystals contained 2D- and 3D- networks involving R-Br molecules and A- anions, with the voids occupied by the alkylammonium counter-ions. The R-Br … A- distances between electrophilic bromine atoms and electron-rich halide, nitrogen or sulfur centers are about 10-15% shorter than the sum of their van der Waals radii (which would be expected for the non-interacting species). Also, the R-Br-A angles in all cases were close to 180 deg. Such geometric features are common to halogen bonding. Thus, structural studies indicated that electrophilic R-Br molecules form halogen bonds with electron rich halide and pseudo-halide anions, A- . Solution-phase studies reveal that the spectral and thermodynamic features of the [R-Br, A-] complexes, especially Mulliken correlation between the energy of absorption bands and ELUMO/EHOMO difference, E, indicate the importance of the charge-transfer component in the halogen. Authors thank the National Science Foundation (grant CHE-1112126) for financial support of this work and Charlotte Stern (Northwestern University) for crystallographic assistance.
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