The affinity and selectivity of 1 and 2 towards various anions (as n-Pen4N + or n-Bu4N + salts) were studied in various solvents and solvent mixtures (Table ). In THF, 1 and 2 bind Cl -and Br -anions with affinities that exceed the detection limit of 1 H NMR titration. The Kas determined by UV titrations reach values as high as 1.3610 5 M -1 for Cl -binding by 2, and 6.910 4 M -1 by 1. Complexes of 2 with halides show slow kinetics of complexation on the 1 H NMR timescale, in line with their high stability. At ambient temperature, the signals of the interacting protons disappear (Fig. ) during 1 H NMR titrations and re-appear at different positions after >1 equiv. of Pen4NCl/Br/I is added (see SI). Upon lowering the temperature to 255 K, two sets of signals that correspond to free receptor 2 and its complex (2X -) become clearly visible throughout the entire titration (Fig. and). In the 1 H NMR spectrum of a solution containing an equimolar mixture of 2, Pen4NCl, Pen4NBr, and But4NI (6.7 mM each) at 255 K, there are three sets of signals corresponding to 2Cl -, 2Br -and 2I - , at ratios 0.47:0.42:0.11 (Fig. ). This selectivity order (Cl -Br ->>I -) corresponds well to the respective Ka values. It should be noted that the difference between the affinity to Cl -and Br -is small despite the much stronger hydrogen bond accepting ability of Cl -. This suggests that the soft nature of CH bond donors and geometrical features of 2 favor softer and larger Br -anion. ).