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

Abstract 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 1 H 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 NO 3 − and the complexation stability varied as follows: NO 3 − >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 + >Bu 4 N + >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 Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ . The stability of the complexes followed the sequence: Ni 2+ 2+ ⪡Cu 2+ ⪢Zn 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.