Halogen bonds are noncovalent interactions in which covalently bound halogens act as electrophilic species. The utility of halogen bonding for controlling self-assembly in the solid state is evident from a broad spectrum of applications in crystal engineering and materials science. Until recently, it has been less clear whether, and to what extent, halogen bonding could be employed to influence conformation, binding or reactivity in the solution phase. This tutorial review summarizes and interprets solution-phase thermodynamic data for halogen bonding interactions obtained over the past six decades and highlights emerging applications in molecular recognition, medicinal chemistry and catalysis.

Studies of a series of urea-based anion receptors designed to probe the potential for anion recognition through combinations of hydrogen and halogen bonding are presented. Proton- and fluorine-NMR spectroscopy indicates that the two interactions act in concert to achieve binding of certain anions, a conclusion supported by computational studies. Replacement of the halogen-bond donating iodine substituent by fluorine (which does not participate in halogen bonding) enables estimation of the contribution of this interaction to the free energy of anion binding. Evidence for attractive contacts between anions and electron-deficient arenes arising from the use of perfluoroarene-functionalized ureas as control receptors is also discussed. The magnitude of the free energy contribution of halogen bonding depends both on the geometric features of the group linking the hydrogen- and halogen-bond donor groups and on the identity of the bound anion. The results are interpreted in relation to fundamental features of the halogen-bonding interaction, including its directionality and unusual preference for halides over oxoanions. Cooperation between two distinct noncovalent interactions leads to unusual effects on receptor selectivity, a result of fundamental differences in the interactions of halogen- and hydrogen-bond donor groups with anions.

Zinc trifluoromethanesulfonate promotes efficient condensations of anilines with squarate esters, providing access to symmetrical and unsymmetrical squaramides in high yields from readily available starting materials. Efficient access to electron-deficient diaryl squaramides has enabled a systematic investigation of the colorimetric anion-sensing behavior of a p-nitro-substituted squaramide. Its behavior differs in dramatic and unexpected ways from that of structurally similar p-nitroaniline-based ureas, an effect that highlights the remarkable differences in acidity between the squaramide and urea functional groups. Computational studies illustrating the enhanced hydrogen bond donor ability and acidity of squaramides in comparison to ureas are presented.

Correlations between experimental, solution-phase thermodynamic data and calculated gas-phase energies of interaction are investigated for noncovalent halogen bonding interactions between electron-deficient iodo compounds and Lewis bases. The experimental data consist of free energies of interaction spanning roughly 7 kcal/mol; they encompass halogen bonds involving both organic (iodoperfluoroarene or iodoperfluoroalkane) and inorganic (I(2), IBr, ICN) donors with nitrogen- and oxygen-based acceptors and are divided into two sets according to the identity of the solvent in which they were determined (alkanes or CCl(4)). Adiabatic energies of halogen bonding were calculated using a variety of methods, including 22 DFT exchange-correlation functionals, using geometries optimized at the MP2/6-31+G(d,p) level of theory. Certain DFT functionals, particularly the B97-1, B97-2, and B98 family, provide outstanding linear correlations with the experimental thermodynamic data, as assessed by a variety of statistical methods.

Association constants between diphenylborinic acid and representative analytes capable of reversible two-point covalent binding (diols, catechols, and hydroxy acids) were determined using an indicator-displacement assay. Unlike boronic acids, which have been studied in great detail as receptors for diols and related compounds, borinic acids have effectively been ignored as candidates for such applications. The results of this study indicate that diphenylborinic acid displays high affinity for certain analytes of this type in aqueous solution. Of particular interest are differences between the selectivity of the borinic acid and that of a boronic acid of similar pKa towards the series of analytes studied: the borinic acid displays an unusually high level of discrimination for catechols over carbohydrates. The distinct selectivity observed, and the unique opportunities for steric and electronic tuning of diarylborinic acids, suggest that these compounds hold significant potential for applications in aqueous-phase molecular recognition.