Structure and energetics of gas phase halogen-bonding in mono-, bi-, and tri-dentate anion receptors as studied by BIRD.

Complexes of mono-, bi- (RB), and tridentate (RT) receptors with a range of anions (Cl(-), Br(-), I(-), NO3(-), H2PO4(-), HSO4(-), and tosylate (TsO(-))) have been studied in the gas phase by both experimental and theoretical methods. Temperature dependent blackbody infrared radiative dissociation (BIRD) experiments were performed on complexes of C8F17I with Br(-) and I(-), RB with I(-), NO3(-), HSO4(-), H2PO4(-), and TsO(-), and RT with I(-), HSO4(-) and TsO(-) and the observed Arrhenius parameters are reported here. Master equation modeling of the BIRD kinetics data was carried out to determine threshold dissociation energies. Geometry optimizations and thermochemistry calculations were performed using the B3LYP/6-31+G(d,p) level of theory. Additional single point energies were calculated using MP2/6-311++G(2d,p). Results were examined in terms of the binding order of various anions as well as the added binding strength from additional halogen bonding (XB) interactions. The relative binding energies of ions were generally consistent with the ordering previously reported from solution phase experiments; however, the relatively strong binding of H2PO4(-) to the bidentate receptor contrasted the solution phase observation of oxoanions having weaker interactions when compared to halides. An increase in the energy required to remove the same anion from the tridentate receptor when compared to the bidentate and monodentate receptors is explained as being due to the increase in halogen bonding interactions. The possibility of mixed halogen and hydrogen bonded complexes were considered.

Halogen bonding in solution: thermodynamics and applications.

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.

Correlations between computation and experimental thermodynamics of halogen bonding.

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.

Anion receptors composed of hydrogen- and halogen-bond donor groups: modulating selectivity with combinations of distinct noncovalent interactions.

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.

N,N'-diarylsquaramides: general, high-yielding synthesis and applications in colorimetric anion sensing.

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.