Halogen Bonding between Anions and Iodoperfluoroorganics: Solution‐Phase Thermodynamics and Multidentate‐Receptor Design

Abstract

The interactions of iodoperfluoroarenes and -alkanes with anions in organic solvent were studied. The data indicates that favorable halogen-bonding interactions exist between halide anions and the monodentate model compounds C(6)F(5)I and C(8)F(17)I. These data served as a basis for the development of preorganized multidentate receptors capable of high-affinity anion recognition. Several new receptor architectures were prepared, and the multidentate-iodoperfluorobenzoate-ester design, as described in a preliminary communication, was evaluated in more detail. Computation was employed to better interpret the structure-activity relationships arising from these studies. Investigations of the thermodynamics of anion binding (by van't Hoff analysis) and solvent effects reveal details of these halogen bonding interactions.

Anion Recognition in Solution via Halogen Bonding

“…Values of the critical dissociation energy E 0 were determined by master equation modeling of the BIRD data to be 100 AE 7 and 81.5 AE 6 kJ/mol, respectively, for the Br À Á Á ÁIC 8 F 17 and I À Á Á ÁIC 8 F 17 complexes. The preferred binding of the more charge-dense Br À over I À is consistent with the data from Table 2, and is in line with the trend of calculated energies of interaction between the halides and iodoperfluorobutane at the B3LYP and MP2 levels of theory [25]. Several types of monodentate, organic halogen bond donor interact favorably enough with anions to enable determinations of binding constants in organic solvent.…”

“…The association constant between Bu 4 N + Cl À and C 8 F 17 I is solvent-dependent: K a values of 1.3 Â 10 2 , 32, and 31 M-1 were determined in acetone, DMSO, and dichloromethane, respectively [25]. The halogen bond donor ability of tetraiodoethylene towards bromide is similar to those of the fluorinated organics discussed above.…”

“…For certain donors, changes in UV-vis spectra accompany addition of anionic acceptors, enabling spectrophotometric titrations: for example, for electron-deficient bromomethanes, anion complexation is often signaled by the appearance of a low-energy charge transfer band [26][27][28][29][30]. In the case of fluorinated donors, upfield changes in 19 F chemical shift of nearby fluorine substituents are characteristic of halogen bonding, and permit determinations of binding constants by nuclear magnetic resonance (NMR) titrations [25,31]. Representative data for interactions of tetraalkylammonium bromides with diverse classes of bromo-and iodo-based monodentate organic donors are assembled in Table 3.…”

“…Iodoperfluoroorganics interact favorably with halide ions in acetone or acetonitrile solvent, as probed by 19 F NMR titrations [25,31] or UV-vis spectroscopy [29] (Table 3). The superior donor ability of iodoperfluorooctane relative to iodoperfluorobenzene which can be inferred from the binding constants with Br À also holds for uncharged acceptors [33].…”

Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques

“…More recently, XB found applications also in solution, mostly for anion recognition [22][23][24] and catalysis [25][26][27], with the difference that structural characterization in solution is much more complicated than in the solid state, since most of the experimental techniques are less "direct" than X-ray crystallography. The general method, in the case of an intermolecular interaction, is to monitor a property of the system, which often (but not necessarily) is a spectroscopic observable, in the absence and the presence of that interaction.…”