Solvent polarity is the main parameter that modulates the stability of complexes. For complexes that are based on hydrogen bonds, the affinity decreases with increasing solvent polarity (G  1/). Although the correlation is not linear in the whole range of , the trend is typically preserved. Water is one of the most strongly competing solvents due to its high  and both strong hydrogen bond donating and accepting ability, which implies effective solvation of both anions and receptors. On the other extreme, for receptors that bind lipophilic anions through classical and nonclassical hydrophobic effects, organic solvents are proven to be destructive. We found that the solvent dependence for 2 is different and that the typical trends mentioned above are violated (Table ). For example, the Ka (2, Cl -) in acetone ( = 20.1) is higher than in THF ( = 7.6), while in DCM ( = 8.9) it is three orders of magnitude lower. Furthermore, anion binding in chloroform, which is the least polar among the tested solvents ( = 4.8), is negligible. Particularly remarkable is the strength and selectivity of complexation in THF:water 9:1. This mixture of organic solvent and water should be destructive for anion binding regardless of the binding mechanism. We found that the addition of water indeed decreases the binding constant for the most hydrophilic anions, but the value of Ka (2, Cl -) = 500 M -1 remains remarkably high considering such a competitive environment and a receptor based only on weak CH hydrogen bond donors. At the other extreme, the affinity for hydrophobic I -, Ka (2, I -) > 10 4 M -1 , is also unprecedented, considering the 90% content of the organic solvent in the mixture. To explain the solvent dependence and the high affinity of 2 towards anions, we have to consider its unique structural features: hydrophobic binding site positioned in the hydrophobic, semi-open pocket and the overall high dipole moment of the molecule. Thus, the high dipole moment provides an attractive force that drives anions into the binding site through a lower rim binding funnel, composed of CH groups possessing diffuse partial positive charges. This hydrophobic binding funnel makes polar molecules (water) and molecules with a significant fraction of positively polarized H atoms on their surfaces (THF, acetone, DMSO) unwanted guests in the cavity, which explains the relative resistance of anion binding by 2 to these solvents. On the contrary, chloroform, which has a diffuse negative charge at the surface originating from three C-Cl bonds, is highly competitive. The semi-open cavity may also be beneficial because binding may not require full desolvation of the anions (the most energy-demanding is the removal of the last water molecules from the solvation sphere). The anion binding characteristics of 1, which is devoid of the hydrophobic pocket, agree with the above reasoning, and, contrary to 2, the solvent dependence of anion binding by 1 is more typical (e.g., lowering of Ka values for all anions was observed in THF:water 9:1, acetone and DMSO). Particularly illustrative are the opposite effects of chloroform and DMSO on chloride binding by 1 and 2 (Fig. ). In chloroform, Cl -binding by 2 is strongly diminished (due to competitive interactions with hydrophobic walls), while for 1 strong binding of Cl -is still observed. In DMSO, the effects are opposite: Cl -binding by 2 is effective because DMSO cannot compete for the alkyl chainsburied binding site, whereas for 1, DMSO reduces Cl -binding to less than 20 M -1 , because it can easily access the binding site.