Synthesis of tripodal aza crown ether calix[4]arenes and their supramolecular chemistry with transition-, alkali metal ions and anions

Tripodal aza crown ether calix[4]arenes, 5a, 5b, 6a and 6b, have been synthesized. The structure of protonated 5a was elucidated by X-ray crystallography to be a self-threaded rotaxane. Complexation studies of 5a and 5b towards anions using Na+ as countercation were carried out by H-1 NMR titration in dimethylsulfoxide-d(6) and the mixture of chloroform-d and methanol-d(4), respectively. Ligands 5a and 5b were able to form 1:1 complexes with Br-, I- and NO3- and the complexation stability varied as follows: NO3- >I->Br-. The effect of countercation on anion complexation was also investigated. The results showed that the association constants of 5a towards Br- in the presence of various cations varied as K+>Bu4N+>Na+. The enhancement in anion complexation ability of 5a may result from the rearrangement of the tripodal ammonium unit in the presence of K+. The neutral forms, 6a and 6b, were able to form complexes with transition metal ions such as Co2+, Ni2+, Cu2+ and Zn2+. The stability of the complexes followed the sequence: Ni2+<Co(2+)much less thanCu(2+)much greater thanZn(2+). Compounds 6a and 6b may, therefore, potentially be used as either transition metal ion or anion receptors that can be controlled by pH of the solution. (C) 2002 Elsevier Science Ltd. All rights reserved.