Complexation of anions with tripodal ammonium capped calix[4]arenes

Abstract

25,26,27-N,N',N''-tri((2-ethoxy)benzyl)ethylenetetraamine-p-tert-butylcalix[4]arene.4HCl, 6a, has been synthesized. The compound 6a was crystallized in a monoclinic C2/c space group. Complexation studies of 6a and 25,26,27-N,N',N''-tri((4-ethoxy)benzyl)ethylenetetraamine-p-tert-butylcalix[4]arene.4HCl, 6b, towards anions such as F-, Br-, r-, NO3-, CO3 2-, SO4 2-, PO4 3-, AsO2-, HPO4 2- and H2PO4- using Na+ as countercation were carried out by 1H-NMR titration in dimethylsulfoxide-d6 and the mixture of chloroform-d and methanol-d4, respectively. Ligands 6a and 6b were able to form 1:1 complexes with Br- (log K = 2.23, 2.28), I- (log K = 2.62. 2.44) and NO3- (log K = 2.47, 2.34) and the complexation ability varied as follows: I->NO3->Br-. The effect of countercations towards anion complexation was also investigated. For a non-metal countercation, nBu4N+, the association constants (log K's) of 6a and 6b towards Br- and I- were 2.13, 2.38 and 1.78, 2.40,respectively. However, the association constants (6a and 6b) when K+ was employed as countercation were 2.14, 2.22 and 2.19, 2.35 towards Br- and I-, respectively. The results showed that the association constants of 6a towards Br- and I- in the presence of various cations varied as Na+>K+>nBu4N+. On the other hand, the association constants of 6b towards anions were comparable no matter what countercation was used. The enhancement in anion complexation ability of 6a may result from the rearrangement of the tripodal ammonium unit in the presence of alkali cations.