Calix[4]azacrowns: self-assembly and effect of chain length and O-alkylation on their metal ion-binding properties

Both the consequences of 1,3-cyclization of calix[4]arenes by diamide formation, and the effect of 2- and 2,4-alkoxycarbonylmethylation of two 1,3-bridged calix[4]azacrowns on their metal ion-binding capacities have been studied by synthesis of the derivatives in their cone conformation and measurement of the stability constants for their complexation in a range of alkaline earth and transition metal cations. In these two cases, the conformation of the ligand in the solid state has been defined in detail by a crystal structure determination. In the first case, a tubular architecture performing methanol channels is displayed, however, in the second case a zigzag architecture generated by a chloroform network connecting calixarene molecules is present. Speciation in solution appears to be limited to the formation of 1:2 and/or 1:1 and/or 2:1 (M-L) complexes, depending on the system, with values of the stability constants determined by absorption spectroscopy in acetonitrile, lying in the ranges log β₁₂ ca. 10, log β₁₁ ca. 2-5, and log β₂₁ ca. 7-9. There is evidence for selectivity toward Cu(II) and for some unexpected anion effects. None of these ligands appear to be an efficient extractant for the metals as their picrate.