Synthesis, structures and ion selectivity of homocalix[3]arene thioketals derived from homocalix[3]arene ketones

The H-1 NMR spectrum of [3.3.3] metacyclophane triketone 1a showed its flexible structure even at -80 degreesC in CDCl3-CS2 (1:3 v/v). In contrast, the cyclic thioketal derivative 2a is fixed with a "partial-cone" conformation below 10 degreesC by the observation of the splitting pattern for the benzylic protons (J = 14.2 Hz), besides other changes in both the aromatic and aliphatic regions of the H-1 NMR spectrum. The coalescence temperature (T-c) for the benzylic methylene protons is 0 degreesC and the free energy of activation for ring inversion is estimated as 13.2 kcal mol(-1). Titration of the cyclic thioketal derivative 2a with AgSO3CF3 in acetone-d(6)-CDCl3 (3:1 v/v) monitored by H-1 NMR clearly demonstrates that a 1:1 complex is formed with retention of the original symmetry. A 1: 1 complexation of 2a with Ag+ was also confirmed by the experiment using the mole ratio method. The two-phase solvent extraction data indicated that the cyclic thioketal derivatives 2a and 2b show a strong Ag+ affinity and a high Ag+ selectivity was observed. On the contrary, the triketone 1a shows a poor Ag+ affinity. Thus, the synergism of the cyclophane moiety and two or three ethanedithia groups play a significant role on the complexation of tris- and bis-thioketals 2a and 2b with Ag+ cation.